Opioid Modulation of Major Neurotransmitter Systems: Acute, Chronic, and Withdrawal Effects.

Opioid Modulation of Major Neurotransmitter Systems: Acute, Chronic, and Withdrawal Effects

ABSTRACT

Opioids are widely used in clinical practice for the management of moderate‐to‐severe acute and chronic pain. However, their use is associated with the development of tolerance and physical dependence, often followed by a withdrawal syndrome upon discontinuation. The mechanisms underlying opioid dependence and withdrawal are highly complex and remain incompletely understood. These processes involve multiple regions of the central nervous system and are accompanied by widespread alterations in neuronal signaling pathways. Neurotransmitters, which are essential for communication between neurons, play a critical role in opioid addiction and withdrawal. In this review, we provide a comprehensive overview of the major neurotransmitter systems, their involvement, and the changes that occur during acute and chronic opioid exposure. Particular attention is also given to the consequences of opioid discontinuation. Overall, this review summarizes current knowledge of the molecular mechanisms underlying both the acute and long‐term effects of opioids on key neurotransmitter systems in the brain.

Keywords: acetylcholine, dopamine, GABA, glutamate, neurotransmitters, opioid dependence, opioid withdrawal, opioids, serotonin

Opioids are widely used but also frequently misused drugs that affect multiple neurotransmitter systems, including dopaminergic, glutamatergic, GABAergic, serotonergic, and cholinergic pathways. Opioid‐induced neuroadaptations depend on the duration of drug exposure, with distinct changes also emerging following drug discontinuation. Dysregulation of these neurotransmitter systems contributes to the development and maintenance of opioid use disorder. This review summarizes current knowledge of the effects of acute and chronic opioid exposure, as well as withdrawal, on major neurotransmitter systems in the brain.

Abbreviations

ACadenylyl cyclase

AcCoAacetyl‐coenzyme A

AChacetylcholine

AChEacetylcholinesterase

AMPARα‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid receptor

AMYamygdala

CBcerebellum

Chcholine

ChTcholine transporter

ChATcholine acetyltransferase

CNScentral nervous system

COMTcatechol‐O‐methyltransferase

CPucaudate putamen

Ctxcerebral cortex

DAdopamine

DAGdiacylglycerol

DATdopamine transporter

DβHdopamine β‐hydroxylase

DOPAC3,4‐dihydroxyphenylacetic acid

DORδ‐opioid receptor

DRNdorsal raphe nucleus

EAATexcitatory amino acid transporter

GABAγ‐aminobutyric acid

GADglutamate decarboxylase

GATGABA transporter

GIRKG protein–activated inwardly rectifying potassium channel

GDHglutamate dehydrogenase

GLNglutamine

GLSglutaminase

GLTglutamate transporter

GLUglutamate

GPCRG protein–coupled receptor

GSglutamine synthetase

HIPhippocampus

HVAhomovanillic acid

iGluRsionotropic glutamate receptors

IP₃inositol 1,4,5‐trisphosphate

KORκ‐opioid receptor

LClocus coeruleus

L‐DOPAL‐3,4‐dihydroxyphenylalanine

LDTglaterodorsal tegmental nucleus

MAOmonoamine oxidase

MAPKmitogen‐activated protein kinase

mGluRsmetabotropic glutamate receptors

MORμ‐opioid receptor

NAcnucleus accumbens

NMDARN‐methyl‐D‐aspartate receptor

NEnorepinephrine

ORopioid receptor

OUDopioid use disorder

PAGperiaqueductal gray

PFCprefrontal cortex

PKAprotein kinase A

PKCprotein kinase C

PLCphospholipase C

SERTserotonin transporter

TCAtricarboxylic acid

TPHtryptophan hydroxylase

VGATvesicular GABA transporter

VTAventral tegmental area

5‐HIAA5‐hydroxyindoleacetic acid

5‐HT5‐hydroxytryptamine (serotonin)

αKGα‐ketoglutarate

1.Introduction

Communication between individual cells is a fundamental feature of multicellular organisms. In the nervous system, this function is primarily mediated by neurotransmission. Neurotransmitters are endogenous signaling molecules that transmit information from presynaptic neurons to target postsynaptic cells (Rizo 2018). These molecules are typically small and include acetylcholine (ACh), amino acids such as glutamate—the principal excitatory neurotransmitter—and their derivatives, such as γ‐aminobutyric acid (GABA), the primary inhibitory neurotransmitter. In addition, neurotransmitters comprise monoaminergic mediators, which are classified into catecholamines (dopamine [DA], norepinephrine [NE], and epinephrine), synthesized from tyrosine and characterized by a catechol moiety, and indolamines (tryptamines), synthesized from tryptophan and containing an indole structure, with serotonin as a key representative (Bowery and Smart 2006; Kawai et al. 2007; Ko and Strafella 2012; Rapport et al. 1948). Interestingly, most neurotransmitters, including ACh, DA, NE, serotonin, glutamate, and GABA, can also function as neuromodulators. Unlike classical neurotransmitters, which typically produce rapid excitatory or inhibitory effects in target cells and thereby enable fast communication within neural circuits, neuromodulators primarily influence the excitability of neurons, the strength of synaptic transmission, and the way neural networks respond to incoming signals. Their effects are often more prolonged and can extend beyond the immediate synaptic environment. Whether a signaling molecule functions primarily as a neurotransmitter or a neuromodulator depends on several factors, including its site of release, the receptors it activates, the spatial and temporal dynamics of signaling and the extent of its effects within the surrounding neural network. Thus, the same signaling molecule may function as either a neurotransmitter or a neuromodulator depending on the cellular and physiological context (Özcete et al. 2024). It is worth noting in this context that endogenous opioids are neuropeptides that function primarily as neuromodulators, whereas opioid drugs exert similar neuromodulatory effects by mimicking the actions of endogenous opioids. By modulating neuronal activity and synaptic transmission, opioids can substantially influence brain function (Reeves et al. 2022).

Opioids are best known for their potent analgesic effects, acting via opioid receptors (ORs) in the central nervous system (CNS) to suppress pain transmission. Although opium has been known since ancient Sumerian times, the significance of opioids in clinical pharmacology increased following the isolation of morphine from the opium poppy in 1806 (Brook et al. 2017). Today, opioids remain an integral component of acute and chronic pain management. They can be administered via multiple routes, including epidural and intrathecal delivery, and are used not only in anesthesia but also in the treatment of conditions such as diarrhea and cough. Despite their therapeutic benefits, opioids are widely misused due to their psychotropic effects (Volkow et al. 2018). One of the most serious consequences of opioid use is addiction, characterized by compulsive drug‐seeking behavior and withdrawal symptoms upon cessation, including insomnia, anxiety, and depression (Ghasemi et al. 2015; Kosten and Baxter 2019; Listos et al. 2013). The development of tolerance often necessitates escalating doses to achieve the same therapeutic effect (Al‐Hasani and Bruchas 2011). Moreover, the risk of respiratory depression—a potentially fatal adverse effect—varies among opioids due to differences in their pharmacokinetic and pharmacodynamic properties. Highly potent opioids pose the greatest risk for overdose and dependence (Burgraff et al. 2023). Importantly, prolonged opioid exposure induces complex, brain region–specific molecular alterations that are highly variable and may persist long after drug discontinuation (Drastichova et al. 2021; Hejnova et al. 2024; Ujcikova et al. 2021). Although the effects of opioids on neurotransmission have been extensively studied, their impacts on individual neurotransmitter systems have not yet been systematically integrated. A comprehensive understanding of these interactions is essential for developing more effective therapeutic strategies for opioid use disorder (OUD), particularly for addressing opioid addiction and withdrawal symptoms.

2.Opioid Pharmacology and Mechanisms of Action

2.1.Opioids and the Opioid System

Opioid substances can be classified into endogenous opioids—such as endorphins, enkephalins, dynorphins, and endomorphins—and exogenous opioids. The endogenous opioid system plays a critical role in regulating reward processing, mood, motivation, learning and memory, gastrointestinal function, immune responses, and analgesia (Higginbotham et al. 2022). Endogenous opioids are present in both the central and peripheral nervous systems. Exogenous opioids, in contrast, are administered externally and bind to the same receptors as endogenous opioids. These compounds are further categorized into naturally occurring opioid alkaloids (e.g., morphine, codeine, and thebaine) and semisynthetic or synthetic opioids (e.g., heroin, oxycodone, and fentanyl) (Drewes et al. 2013). Opioids exert their effects primarily through three classical OR subtypes: mu (μ, MOR), delta (δ, DOR), and kappa (κ, KOR). Each receptor subtype contributes uniquely to the modulation of analgesia, reward, and addictive behaviors. Activation of these receptors alters neuronal excitability and neurotransmission, thereby influencing a wide array of physiological and psychological functions (Reeves et al. 2022; Stevens 2009).

2.2.ORs and Mechanism of Action

The concept of ORs was first proposed in the 1950s (Beckett and Casy 1954), with stereospecific binding sites subsequently identified in the brain during the 1970s. These studies demonstrated that both exogenous ligands and naturally occurring opioid‐like peptides exert their effects by binding to distinct ORs (Snyder and Childers 1979). The classical ORs—MOR, DOR, and KOR—are members of the G protein–coupled receptor (GPCR) family. A fourth receptor, the nociceptin/orphanin FQ receptor, is considered a nonclassical member of the OR family (Dhawan et al. 1996). Agonist binding to these receptors triggers intracellular signaling cascades that induce cellular hyperpolarization, ultimately reducing neuronal excitability and modulating pain perception (Cox et al. 2015; Pathan and Williams 2012).

Structurally, ORs contain seven transmembrane domains, three extracellular and three intracellular loops, an extracellular N‐terminal domain, and an intracellular C‐terminal domain. While these receptors share a high degree of homology, each subtype exhibits a distinct pharmacological profile (Raynor et al. 1994). ORs are widely expressed throughout the central and peripheral nervous systems. Initial localization studies in 1973 revealed their presence in brain regions involved in pain transmission, including the thalamus, rostroventral medulla, and periaqueductal gray (PAG), as well as in key components of the reward circuitry, such as the nucleus accumbens (NAc) and ventral tegmental area (VTA) (Pert and Snyder 1973). ORs are also found in additional brain regions, including the hypothalamus, amygdala (AMY), ventral pallidum, globus pallidus, raphe nuclei, and hippocampus (HIP) (George et al. 1994; Kitchen et al. 1997; Mansour et al. 1995). The distribution of OR subtypes varies across brain regions. MOR is highly expressed throughout the forebrain, midbrain, and hindbrain, with dense expression in the cerebral cortex (Ctx), caudate putamen (CPu), NAc, thalamus, HIP, AMY, colliculi, nucleus tractus solitarius, spinal trigeminal nucleus, and dorsal horn (Mansour et al. 1987). KOR is concentrated in the CPu, NAc, AMY, hypothalamus, and nucleus tractus solitarius (Clarke et al. 2001; Jomary et al. 1988), whereas DOR is less abundant in the CNS, with prominent expression in the olfactory bulb, Ctx, CPu, NAc, and AMY (Goody et al. 2002). In peripheral tissues, ORs are present in the gastrointestinal and respiratory systems (Wittert et al. 1996).

Binding of an opioid ligand induces conformational changes in the associated heterotrimeric G protein, composed of α, β, and γ subunits. Upon activation, the Gα subunit exchanges GDP for GTP and dissociates from the Gβγ dimer. Both components contribute to intracellular signaling pathways (Childers et al. 1979; Svoboda et al. 2004). While stimulatory G proteins (Gs) activate adenylyl cyclase (AC) and increase intracellular cAMP levels, inhibitory G proteins (Gi) suppress AC activity, resulting in reduced cAMP production. This decrease leads to diminished activation of protein kinase A (PKA) and reduced phosphorylation of the cAMP response element‐binding protein (CREB) (Fleming et al. 1992).

OR activation engages two principal signaling pathways: the G protein–dependent pathway and the β‐arrestin pathway. The β‐arrestin pathway regulates receptor desensitization and internalization (Bohn et al. 2000; Whistler et al. 1999; Zhang et al. 1998), whereas the G protein pathway mediates classical intracellular signaling events that vary depending on receptor subtype (Ho et al. 2018; Mafi et al. 2021). These pathways influence multiple downstream targets, including AC, phosphatidylinositol 3‐kinase, MAPK cascades, calcium channels, and G protein–activated inwardly rectifying potassium (GIRK) channels (Al‐Hasani and Bruchas 2011). OR predominantly couples to Gi/o proteins, thereby exerting inhibitory effects. Activation of Gi inhibits AC, while the Gβγ subunits can directly interact with GIRK channels, promoting potassium efflux and neuronal hyperpolarization (Montandon et al. 2016). Additionally, the Gβγ complex can inhibit voltage‐gated calcium channels, reducing calcium influx into the cell (Xie et al. 2007).

3.Overview of Major Neurotransmitter Systems

3.1.The Dopaminergic System

Dopamine (DA) is one of the brain's key neurotransmitters, regulating behavior, reward, learning, and memory processes (Ranjbar‐Slamloo and Z. Fazlali 2020). It belongs to the catecholamine group, a class of biogenic amines characterized by a 3,4‐dihydroxyphenyl (catechol) structure. Its biosynthesis originates from L‐tyrosine, which is first hydroxylated to L‐3,4‐dihydroxyphenylalanine (L‐DOPA) and subsequently decarboxylated to dopamine by aromatic L‐amino acid decarboxylase. DA can be further converted into norepinephrine (NE) via dopamine‐β‐hydroxylase (DβH), and NE may be methylated by phenylethanolamine N‐methyltransferase to form epinephrine. Dopamine is metabolized by monoamine oxidase (MAO) to 3,4‐dihydroxyphenylacetic acid (DOPAC), which is subsequently converted by catechol‐O‐methyltransferase (COMT) into homovanillic acid (HVA) (Nagatsu 2007).

Within the CNS, DA is synthesized by dopaminergic neurons that are widely distributed throughout the brain and project to multiple target regions, thereby regulating a broad spectrum of physiological functions. Dopaminergic neurons are primarily located in the substantia nigra, projecting to the CPu and forming the nigrostriatal pathway, which is essential for motor control. Additional dopaminergic populations reside in the VTA, projecting to the NAc, AMY, and prefrontal cortex (PFC). This mesolimbic pathway plays a key role in emotional regulation, motivation, reward processing, and memory formation. Dopaminergic neurons are also present in the hypothalamus, where they project to the pituitary gland and modulate endocrine function (Gerfen 1992; Grimm et al. 2004; Kobayashi 2001; Le Moal and Simon 1991).

DA exerts its effects via dopamine receptors, which belong to the GPCR family. These receptors are classified into five subtypes (D1–D5), each encoded by distinct genes and exhibiting specific functional roles and regional distributions (Kebabian and Calne 1979). Based on G protein coupling, dopamine receptors are divided into two major groups. D1‐like receptors (D1 and D5) couple to stimulatory Gs proteins, leading to activation of AC and increased cAMP levels. This activates PKA, which regulates transcription factors and other intracellular targets. In addition, these receptors can activate phospholipase C (PLC), resulting in the production of inositol 1,4,5‐trisphosphate (IP₃) and diacylglycerol (DAG), thereby promoting intracellular calcium release and activation of protein kinase C (PKC). These receptors are widely distributed in the striatum, NAc, olfactory bulb, and substantia nigra, where they contribute to reward processing, motor control, learning, and memory (Bueschbell et al. 2021; Jaber et al. 1996). In contrast, D2‐like receptors (D2, D3, and D4) couple to inhibitory Gi proteins, resulting in suppression of AC activity and modulation of ion channels, including activation of potassium channels. These receptors are involved in cognitive processes, attention, sleep regulation, and neuronal development and are primarily expressed in the striatum, globus pallidus, NAc, HIP, AMY, and Ctx (Jaber et al. 1996; Lacey et al. 1987; Salvi et al. 2016).

3.2.The Glutamatergic System

Glutamate is a nonessential amino acid that plays a central role in amino group transfer and serves as the principal excitatory neurotransmitter in the CNS. It can be synthesized via reversible catalysis by glutamate dehydrogenase (GDH) from α‐ketoglutarate, an intermediate of the Krebs cycle, or through enzymatic conversion from glutamine (Mathioudakis et al. 2019; Reitzer 2004). Following its release into the synaptic cleft, glutamate is taken up by glial cells and converted into glutamine by glutamine synthetase (GS). Glutamine is subsequently transported back to neurons, where it is reconverted into glutamate, thereby completing the glutamate–glutamine cycle (Palmada and Centelles 1998). Alternatively, glutamate may be cleared via excitatory amino acid transporters or bind to its receptors to exert its effects.

Glutamate acts on two major classes of receptors: ionotropic receptors (iGluRs), which function as ligand‐gated ion channels, and metabotropic receptors (mGluRs), which are GPCRs. Both receptor classes are critically involved in numerous physiological and pathophysiological processes within the CNS. iGluRs are tetrameric complexes composed of four subunits forming an ion‐conducting pore. They are classified into four main groups: AMPA receptors (GluA1–GluA4), kainate receptors (GluK1–GluK5), NMDA receptors (GluN1, GluN2A–GluN2D, GluN3A, GluN3B), and δ receptors (GluD1 and GluD2) (Traynelis et al. 2010). These receptors mediate fast excitatory neurotransmission and are essential for processes such as synaptic plasticity, learning, and memory. Their expression is developmentally regulated and region‐specific. AMPA receptors are widely expressed in the Ctx, HIP, and cerebellum (CB) and are often colocalized with NMDA receptors. In contrast, kainate receptors are predominantly expressed on inhibitory interneurons, particularly within hippocampal circuits (Fisahn et al. 2004; Traynelis et al. 2010). mGluRs are subdivided into three groups based on their signaling mechanisms and sequence homology. Group I mGluRs are predominantly neuronal, and their activation stimulates PLC and AC, as well as the MAPK signaling pathways. Group II mGluRs are expressed in both neurons and astrocytes (notably mGluR3) and inhibit AC and calcium channels while activating potassium channels. Group III mGluRs are widely distributed in neurons (e.g., mGluR4 and mGluR7) and in the retina (mGluR6); their activation is associated with inhibition of AC and calcium channels, activation of potassium channels, and modulation of cGMP signaling (Niswender and Conn 2010). The broad distribution of mGluRs underscores their involvement in neural development, synaptic plasticity, and a range of neurological and psychiatric disorders, including Alzheimer's disease, Parkinson's disease, schizophrenia, addiction, anxiety, and depression (Niswender and Conn 2010; Sherman 2014).

3.3.The GABAergic System

Gamma‐aminobutyric acid (GABA) is the principal inhibitory neurotransmitter in the adult CNS (Vargas 2018). It is synthesized from glutamate by glutamate decarboxylase (GAD). Following synthesis, GABA is stored in synaptic vesicles and released into the synaptic cleft in response to neuronal stimulation. After release, GABA is taken up by GABA transporters (GATs) and metabolized to succinate via the sequential actions of GABA transaminase and succinate semialdehyde dehydrogenase or it binds to its receptors to exert inhibitory effects (Schousboe et al. 2004).

Three main classes of GABA receptors have been identified: GABAA, GABAA‐ρ, and GABAB receptors. GABAA and GABAA‐ρ receptors are ligand‐gated chloride ion channels, whereas GABAB receptors are GPCRs that inhibit AC activity and reduce calcium influx, thereby decreasing the release of excitatory neurotransmitters (Bormann 2000; Naffaa et al. 2017; Sigel and Steinmann 2012). GABA receptors are widely distributed throughout the CNS and play essential roles in numerous physiological processes, including sleep regulation, neurodevelopment, and neuroendocrine function (Chebib et al. 2004). In addition, GABAergic signaling influences peripheral systems such as the cardiovascular and reproductive systems (Kah et al. 1992; Vargas 2018; Wang et al. 2008). GABAergic neurons located in the rostromedial tegmental nucleus exert inhibitory control over dopaminergic neurons in the VTA and are involved in nociceptive processing as well as opioid‐mediated analgesia (Taylor et al. 2019). GABAergic neurons contribute to the formation and modulation of neural circuits linking regions such as the Ctx, HIP, limbic system, and pituitary gland. Dysregulation of the GABAergic system has been implicated in various pathological conditions, including anxiety and stress‐related disorders, pain, addiction, and withdrawal syndromes (Smith and Rudolph 2012; Stachowicz et al. 2008).

3.4.The Serotonergic System

The principal neurotransmitter of the serotonergic system is the indoleamine 5‐hydroxytryptamine (5‐HT, serotonin). Its synthesis begins with the essential amino acid L‐tryptophan, which enters the kynurenine metabolic pathway (Höglund et al. 2019). In serotonergic neurons, tryptophan hydroxylase (TPH) catalyzes the conversion of L‐tryptophan to 5‐hydroxy‐L‐tryptophan, which is subsequently decarboxylated to serotonin by aromatic L‐amino acid decarboxylase. Serotonin is primarily metabolized by MAO to 5‐hydroxyindoleacetic acid, a process that may occur directly within the synaptic cleft (Adaway et al. 2016; Höglund et al. 2019).

Serotonergic neurons are primarily located in the raphe nuclei and project extensively to numerous brain regions, including the Ctx, AMY, CB, HIP, thalamus, and hypothalamus. In addition to its central role, serotonin is also synthesized in large quantities in the gastrointestinal tract by enterochromaffin cells. Owing to its widespread distribution, serotonin regulates a diverse range of physiological functions, including mood, cognition, aggression, sleep, thermoregulation, cardiovascular function, and pain perception (Cao et al. 2017; Charnay and Léger 2010).

Serotonin exerts its effects through a diverse family of GPCRs, comprising more than 15 subtypes grouped into seven major classes (5‐HT₁–5‐HT₇) based on structural and pharmacological characteristics (Pithadia and Jain 2009). The 5‐HT₁ receptor family generally inhibits AC and is expressed in regions such as the raphe nuclei, HIP, striatum, and Ctx, as well as in peripheral tissues including smooth muscle and the uterus (Hoyer et al. 2002). The 5‐HT2 receptor family activates PLC and is distributed in the Ctx, CB, hypothalamus, and peripheral tissues such as smooth muscle and platelets (Conn and Sanders‐Bush 1984). The 5‐HT₃ receptor is a ligand‐gated ion channel that modulates the release of other neurotransmitters and is primarily localized in the area postrema and visceral afferent neurons (Alhaider et al. 1991). The 5‐HT₄ receptor activates AC and is found in the HIP and gastrointestinal tract. The 5‐HT₅ receptor is expressed in the olfactory bulb and habenula, although its function remains incompletely understood (Pithadia and Jain 2009). The 5‐HT₆ receptor is predominantly expressed in the NAc, olfactory tubercle, striatum, and HIP, where it regulates glutamatergic and cholinergic neurotransmission and contributes to the modulation of mood and anxiety (Riccioni 2010). The 5‐HT₇ receptor is located in thalamocortical and limbic regions, suggesting a role in sensory processing and affective behavior (Hoyer et al. 2002).

3.5.The Cholinergic System

ACh is a classical neurotransmitter synthesized in cholinergic neurons from choline and acetyl‐coenzyme A by choline acetyltransferase. It is rapidly degraded in the synaptic cleft by acetylcholinesterase (AChE) (Potter 1970). Cholinergic neurons can be broadly categorized into projection neurons, located in the basal forebrain and brainstem, and interneurons, primarily found in the striatum and NAc. Basal forebrain projection neurons innervate the Ctx, HIP, and AMY, where they play essential roles in cognition, learning, and arousal. Brainstem cholinergic neurons project to midbrain and brainstem structures, including the VTA, where they modulate dopaminergic activity (Ananth et al. 2023).

ACh acts on two major receptor classes: nicotinic receptors, which are ligand‐gated ion channels, and muscarinic receptors, which are GPCRs. Nicotinic receptors are subdivided into muscular (N1) and neuronal (N2) types. In the CNS, they are primarily localized presynaptically in regions such as the Ctx, substantia nigra, NAc, CPu, HIP, AMY, and striatum, where they regulate the release of multiple neurotransmitters, including DA, NE, glutamate, and GABA (Papke 2014). Muscarinic receptors are divided into five subtypes (M1–M5). M1, M3, and M5 receptors are coupled to Gq proteins and activate PLC signaling pathways, whereas M2 and M4 receptors are coupled to Gi proteins and inhibit AC activity. Within the CNS, M1, M4, and M5 are the predominant subtypes. M1 receptors are highly expressed in the Ctx, HIP, and striatum, whereas M4 and M5 receptors are mainly localized in the HIP and substantia nigra (Jiang et al. 2014; Lebois et al. 2018).

The cholinergic system plays a pivotal role in modulating both dopaminergic and glutamatergic neurotransmission, particularly within the VTA, NAc, and PFC. In the VTA, activation of both nicotinic and muscarinic receptors enhances dopaminergic and glutamatergic signaling, thereby influencing reward‐related processes. In the NAc, cholinergic interneurons integrate cortical and subcortical inputs, contributing to reward processing and behavioral reinforcement. In the PFC, cholinergic signaling is critically involved in cognitive aspects of addiction, including decision‐making, attention, and impulse control (Jensen et al. 2018; Mohebi et al. 2023).

4.Effects of Opioids on Neurotransmitter Systems

4.1.Opioids and the Dopaminergic System

4.1.1.Effects of Acute Administration

Acute opioid administration exerts profound effects on the dopaminergic system through both direct and indirect mechanisms. Opioids can directly stimulate dopaminergic neurons in key regions of the brain's reward circuitry, including the VTA, striatum, PFC, AMY, HIP, and other limbic structures. This activation reinforces reward processing and increases motivation for continued opioid use (Ahmadi Soleimani et al. 2018; Matthews and German 1984). Indirectly, opioids act via activation of MORs located on GABAergic neurons in the VTA. This reduces GABA release, thereby disinhibiting dopaminergic neurons and enhancing dopaminergic transmission, a mechanism strongly associated with addiction development (Doyle and Mazei‐Robison 2021; Johnson and North 1992; Matsui et al. 2014; Ting‐A‐Kee and van der Kooy 2012). Activation of VTA neurons also increases DA release in the HIP, influencing hippocampal plasticity. Notably, NMDA receptor activation within the VTA–HIP pathway and D1 receptor signaling in the HIP are critical for both rewarding and aversive drug effects, highlighting the role of dopaminergic output in drug‐associated learning and memory (Hu et al. 2014).

D1 receptor signaling is closely linked to DARPP‐32, a key integrator of neurotransmission. Acute morphine administration induces DARPP‐32 phosphorylation, inhibiting protein phosphatase‐1 and preventing dephosphorylation of downstream targets such as glutamate receptors and ion channels. This enhances cAMP signaling cascades (Greengard et al. 1999; Hemmings et al. 1984). Although MOR or KOR agonists do not appear to alter D1 receptor numbers, acute administration has been associated with upregulation of D2 receptors, suggesting complex dopaminergic adaptations (Rooney et al. 1991).

Acute morphine increases extracellular DA levels in reward‐related regions, accompanied by elevated tyrosine levels and increased tyrosine hydroxylase activity, enhancing DA synthesis (García‐Pérez et al. 2013; Pothos et al. 1991). Similarly, MOR agonists (e.g., DAMGO) and DOR agonists (e.g., DPDPE) increase DA and its metabolites (DOPAC, HVA), whereas KOR agonists (e.g., E‐2078) decrease DA levels in the NAc. These findings support opposing roles of MOR and KOR signaling within the reward system (Narita et al. 1993; Spanagel et al. 1990; Xi et al. 1998). Their interaction within the mesolimbic pathway may underlie opioid reinforcement (Xi 2002).

Notably, a single dose of the KOR agonist U‐69593 significantly increased DA uptake in the NAc of male Sprague Dawley rats, indicating enhanced dopamine transporter (DAT) function (Thompson et al. 2000). Otherwise, there are no reports demonstrating effects of acute opioid administration on DAT function or metabolic enzymes. Consistent with this, acute methadone administration did not alter MAO activity in different brain regions (Kaufmann et al. 1983; Simantov 1993).

4.1.2.Effects of Chronic Administration

Chronic opioid exposure enhances dopaminergic activity by increasing neuronal firing and inducing long‐term plasticity within the mesolimbic pathway (Simmons et al. 2019). MOR activation on GABAergic neurons continues to play a central role in shaping reward‐related behavior (Galaj et al. 2020; Galaj and Xi 2021). Elevated DA levels are accompanied by increased concentrations of its metabolites, DOPAC and HVA (Pothos et al. 1991; Spampinato et al. 1985).

Although many MOR agonists similarly elevate DA levels, their effects on metabolic enzymes differ. Morphine, oxycodone, and codeine do not affect MAO activity, whereas methadone, tramadol, and dextromethorphan inhibit MAO, prolonging dopaminergic signaling (Gillman 2005; Mercuri et al. 1997; VanderWende and Spoerlein 1979). These effects are partly mediated through nonopioid mechanisms. In addition to MAO, opioids also influence other enzymes involved in catecholamine metabolism. Genetic variability in COMT has been associated with differential responses to opioids, with reduced COMT activity leading to increased catecholamine levels and heightened pain sensitivity (Duan et al. 2020; Nielsen et al. 2017). DβH activity has also been shown to increase following chronic morphine exposure, further altering catecholamine metabolism (Anderson and Slotkin 1975).

Chronic opioid use additionally induces alterations in receptor expression and intracellular signaling pathways. D1 receptor activity contributes to ΔFosB induction, while reduced mGluR2 sensitivity and altered D2/D3 receptor expression modulate mesolimbic signaling in a maladaptive direction (Maldonado et al. 1997; Muller and Unterwald 2005; Narita et al. 2003; Nestler 2008). D4 receptor signaling has also been implicated in opioid‐induced neuroplasticity (Lüscher et al. 1997; Rivera et al. 2021; Samani et al. 2023). In parallel, chronic opioid exposure impairs dopamine DAT function, with reduced transporter activity observed in both animal models and human opioid users (Liu et al. 2013; Shi et al. 2008; Simantov 1993; Thompson et al. 2000).

4.1.3.Effects of Opioid Withdrawal

Opioid withdrawal is characterized by decreased dopamine and metabolite levels in key reward regions, such as the VTA and NAc, contributing to negative affective states (Acquas et al. 1991; Rada et al. 2004; Rossetti et al. 1992). In addition, experimental studies have shown that morphine withdrawal markedly alters spine density in the NAc, further impairing already reduced dopaminergic transmission (Diana et al. 2006). During withdrawal, a compensatory dopaminergic hypersensitivity emerges, driven primarily by altered D2 receptor responsiveness rather than receptor upregulation (Gianutsos et al. 1974; Strickland et al. 2022).

Recently, epigenetic mechanisms, including histone H3 dopaminylation, have been implicated in long‐lasting transcriptional adaptations and increased vulnerability to relapse (Fulton et al. 2022; Cheng et al. 2023; Stewart et al. 2023). H3 dopaminylation at glutamine 5 (Q5), induced by prolonged heroin exposure, has been identified as an epigenetic mechanism in the VTA that recruits specific transcriptional programs, thereby altering dopaminergic neurotransmission and contributing to drug‐seeking behavior during prolonged abstinence (Fulton et al. 2022; Cheng et al. 2023; Stewart et al. 2023).

The effects of acute and chronic opioid administration, as well as withdrawal, on the dopaminergic system are summarized in Table 1 and illustrated in Figure 1.

TABLE 1Effects of opioids on the dopaminergic system.

Category Acute administration Chronic administration Withdrawal Reference

Neurotransmitter levels ↑ DA, DOPAC, and HVA in the mesolimbic areas (MOR and DOR agonists)

↓ DA levels in the NAc (KOR agonists)

↑ DA, DOPAC, and HVA in

the mesolimbic areas (MOR agonists)

↓ DA, DOPAC, HVA in the

VTA and NAc (MOR agonists)

Spampinato et al. (1985), Spanagel et al. (1990), Acquas et al. (1991), Pothos et al. (1991), Rossetti et al. (1992), Narita et al. (1993), Xi et al. (2004), Simmons et al. (2019)

Receptor expression and function ↑ DR signaling → enhanced cAMP cascade

↑ D2R signaling

↑ D2, D3, D4 R expression (MOR agonists) ↑ Sensitivity of D2R (MOR agonists) Hemmings et al. (1984), Lüscher et al. (1997), Maldonado et al. (1997), Rooney et al. (1991), Greengard et al. (1999), Narita et al. (2003), Spangler et al. (2003), Rivera et al. (2021)

Transporters ↑ DAT function in the NAc (KOR agonists) ↓ DAT function in the VTA and NAc (MOR agonists) Anderson and Slotkin (1975), Simantov (1993), Shi et al. (2008), Thompson et al. (2000), Liu et al. (2013)

Enzymes ↑ TH activity and ↑ DA synthesis (MOR agonists) ↓ MAO activity (MOR agonists, e.g., methadone tramadol)

↑ DBH activity (MOR agonists)

Gauchy et al. (1973), Anderson and Slotkin (1975), VanderWende and Spoerlein (1979), Pothos et al. (1991), Mercuri et al. (1997), Gillman (2005), Pathan and Williams (2012), García‐Pérez et al. (2013)

4.2.Opioids and the Glutamatergic System

4.2.1.Effects of Acute Administration

Opioids significantly modulate the glutamatergic system, which involves glutamate—the brain's primary excitatory neurotransmitter—and plays a central role in addiction‐related processes (Peters and De Vries 2012). Acute activation of MOR or KOR can reduce extracellular glutamate levels in glutamate‐rich regions such as the HIP, Ctx, and NAc, indicating the presence of an opioid–glutamatergic regulatory pathway (Guo et al. 2005; Zhu et al. 1998). At the same time, opioids can activate NMDA receptors, lowering the nociceptive threshold and diminishing opioid analgesic efficacy (Pourzitaki et al. 2018).

The glutamatergic and dopaminergic systems are tightly interconnected. Acute morphine preferentially modulates glutamatergic transmission in cortical‐projecting dopaminergic neurons via AMPA receptors, thereby influencing drug‐seeking and motivational behaviors (Floresco and Magyar 2006; Lane et al. 2008). A single dose of morphine has also been shown to increase mRNA expression of NMDA receptor subunits (NR1, NR2A, NR2B) within hours of administration (Le Grevès et al. 1998). In addition to regulating glutamate release, opioids—including MOR, KOR, and DOR agonists—modulate Ca2+ influx through glutamatergic receptor signaling following K+‐induced depolarization, thereby influencing neurotransmitter release dynamics (Frank 1998; Vlaskovska et al. 1997). DOR agonists, such as SNC80, suppress activity in hippocampal glutamatergic neurons as well as in brainstem serotonergic and dopaminergic neurons. This effect is likely mediated by increased serotonin levels and activation of 5‐HT₁A receptors, contributing to anxiolytic effects (Grinchii et al. 2023; Rawls and Cowan 2006).

Acute opioid exposure also affects glutamate metabolism. For example, tramadol increases the activity of GDH and GS while decreasing glutaminase (GLS) activity across multiple brain regions, including the Ctx, hypothalamus, HIP, and CB (Chetan et al. 2015).

4.2.2.Effects of Chronic Administration

Chronic opioid exposure generally reduces glutamate levels (Verdejo‐García et al. 2013; Yücel et al. 2007), both directly through presynaptic MOR activation on glutamatergic neurons (Ostermeier et al. 2000; Vogt et al. 2001) and indirectly via dopaminergic modulation of VTA glutamatergic neurons (McGovern et al. 2023; Rada, Mark, et al. 1991; Yamamoto and Davy 1992). However, repeated cycles of opioid use and withdrawal can produce sensitization, characterized by progressively enhanced drug responses. In contrast to acute exposure, chronic MOR activation may increase glutamate release in the LC due to enhanced Ca2+ influx during neuronal depolarization (Farahmandfar et al. 2011; Tai et al. 2006; Zhu et al. 1998). This is accompanied by upregulation of cAMP/PKA and PKC signaling pathways, contributing to opioid tolerance (Bie and Pan 2005).

Opioids modulate both presynaptic glutamate release and postsynaptic signaling through AMPA and NMDA receptors in the NAc and VTA. NMDA receptor activation negatively regulates opioid analgesia and contributes to tolerance via receptor upregulation (Chartoff and Connery 2014; Mao 2002; Rodríguez‐Muñoz et al. 2012). Some opioids, such as methadone, may act as NMDA receptor antagonists, thereby enhancing analgesic efficacy (Weschules et al. 2008). Importantly, excessive glutamatergic signaling may lead to excitotoxicity, whereas reduced mGluR2 expression increases sensitivity to opioid reward and exacerbates withdrawal symptoms (Gao et al. 2018; Koob and Volkow 2016).

Chronic opioid use also disrupts the glutamate/GABA–glutamine metabolic axis. Prolonged tramadol exposure increases GDH and GS activity while reducing GLS activity across most brain regions (Caspani et al. 2022; Chetan et al. 2015). Subsequent increases in extracellular glutamate levels may result from downregulation of glutamate transporters such as GLT‐1 and EAAT1 (Farahmandfar et al. 2011; Ozawa et al. 2001; Tai et al. 2006).

4.2.3.Effects of Opioid Withdrawal

Opioid withdrawal markedly enhances glutamatergic activity. Naloxone‐precipitated withdrawal increases extracellular glutamate levels in the NAc and LC, contributing to the development of physical dependence (Aghajanian et al. 1994; Sepulveda et al. 1998; Tokuyama et al. 1998). In parallel, glutamate uptake is increased through upregulation of GLT‐1, particularly in the HIP, likely representing a compensatory response aimed at limiting excitotoxicity (Wang et al. 1998; Xu, Bao, et al. 2003). Increased dynorphin expression during withdrawal activates KORs and further enhances GLT‐1 expression via p38 MAPK signaling, thereby contributing to depressive‐like states (Zan et al. 2021).

Withdrawal also induces pronounced alterations in glutamate receptor expression, including increased levels of AMPA, NMDA, and mGluR subunits. Pharmacological blockade of these receptors has been shown to alleviate withdrawal symptoms (Guo et al. 2009; Salisbury et al. 2021). In the case of MOR agonists such as oxycodone, withdrawal is associated with increased mRNA expression of AMPA receptor subunits GluA1–3, NMDA receptor subunits GluN2A–C, and mGluR subtypes 3, 5, 6, and 8. The involvement of glutamate receptor signaling in withdrawal is particularly pronounced, as antagonists of NMDA receptors and mGluRs can attenuate withdrawal‐related behavioral and physiological symptoms (Guo et al. 2009; Salisbury et al. 2021).

Opioid withdrawal syndrome is also associated with disruptions in neurotransmitter metabolism, including decreased expression of glutamine synthetase and GLS. Notably, these alterations can persist for up to 3 months after morphine withdrawal in rat models, highlighting the long‐lasting impact of opioid exposure on glutamatergic homeostasis (Chen et al. 2007; Ujcikova et al. 2021).

The effects of acute and chronic opioid administration, as well as withdrawal, on the glutamatergic system are summarized in Table 2 and illustrated in Figure 2.

TABLE 2Effects of opioids on the glutamatergic system.

Category Acute administration Chronic administration Withdrawal Reference

Neurotransmitter levels ↓ GLU levels in the HIP, Ctx, and NAc (MOR and KOR) ↓ GLU levels in the NAc and Ctx (MOR agonists) ↑ GLU in the NAc and LC (MOR agonists) Aghajanian et al. (1994), Sepulveda et al. (1998), Tokuyama et al. (1998), Zhu et al. (1998), Guo et al. (2005), Yücel et al. (2007), Verdejo‐García et al. (2013), Pourzitaki et al. (2018)

Receptor expression and function ↑ NMDA receptor subunits (NR1, NR2A, NR2B) mRNA expression (MOR agonists) ↑ NMDAR signaling (MOR agonists)

↓ NMDAR activity (methadone)

↑ GluA1–3 (AMPAR), GluN2a‐c (NMDAR), and mGluR 3, 5, 6, 8 expression (MOR agonists) Le Grevès et al. (1998), Mao (2002), Guo et al. (2009), Rodríguez‐Muñoz et al. (2012), Chartoff and Connery (2014), Salisbury et al. (2021)

Transporters ↓ GLT1 and EAAT 1 function (MOR agonists) ↑ GLT1 expression in the HIP (MOR agonists) Wang et al. (1998), Ozawa et al. (2001), Xu, Bao, et al. (2003), Tai et al. (2006), Farahmandfar et al. (2011)

Enzymes ↑ GDH and GS activity

↓ GLS activity in the Ctx, HIP, and hypothalamus (MOR agonists)

↑ GDH and GS activity in the Ctx, CER, HIP, and hypothalamus

↓ GLS activity in the CER HIP, and hypothalamus

↑ GLS activity in the Ctx (MOR agonists)

↓ GS and GLS expression in the Ctx (MOR agonists) Chen et al. (2007), Chetan et al. (2015), Ujcikova et al. (2021), Caspani et al. (2022)

4.3.Opioids and the GABAergic System

4.3.1.Effects of Acute Administration

Opioids strongly influence the GABAergic system, which plays a central role in regulating dopaminergic activity. Under physiological conditions, GABA exerts inhibitory control over DA release (Kreek et al. 2012). MOR agonists suppress GABAergic interneurons in the VTA, reducing GABA release and thereby disinhibiting dopaminergic neurons, which leads to increased dopamine release in the NAc (Johnson and North 1992; Kelley et al. 1980; Kiyatkin et al. 1993; Xi 2002). At the same time, increased GABA levels have also been reported in the NAc and PFC following opioid exposure, suggesting region‐specific adaptations within the GABAergic system (De Vries and Shippenberg 2002; Schoffelmeer et al. 2001).

MOR‐mediated inhibition of GABA release involves multiple mechanisms, including modulation of potassium conductance and G protein–dependent suppression of vesicular neurotransmitter release (Capogna et al. 1993; Stiller et al. 1996; Vaughan et al. 1997). KOR agonists similarly reduce GABA release, in part by decreasing Ca2+ influx at presynaptic terminals (Hjelmstad and Fields 2003).

Acute opioid exposure can also reduce GABA receptor binding, primarily by decreasing high‐affinity receptor sites in the CB, Ctx, and striatum, which may contribute to opioid‐induced antinociception (Rashvand et al. 2014; Ticku and Huffman 1980). In contrast to chronic administration, acute opioid exposure has not been shown to significantly affect GAD activity, suggesting that enzymatic regulation of GABA synthesis is more relevant in long‐term adaptations rather than acute effects (Tzeng and Ho 1978).

4.3.2.Effects of Chronic Administration

Chronic opioid exposure induces significant adaptations in GABAergic neurotransmission, contributing to tolerance, dependence, and addiction (Gao et al. 2007; Hu et al. 2012; Tsuji et al. 1996; Westerink et al. 1996). A key mechanism underlying these changes is the transition from acute inhibition of AC/cAMP signaling to AC superactivation during chronic exposure and withdrawal. This shift leads to increased cAMP production and enhanced GABA release onto dopaminergic neurons in the VTA (Avidor‐Reiss et al. 1996; Ronström et al. 2023).

Altered GABAergic plasticity disrupts dopaminergic signaling and contributes to opioid reward‐related behaviors (Madhavan et al. 2010; Spyraki et al. 1983). Pharmacological activation of GABA‐B receptors can counteract opioid‐induced sensitization and reduce elevated DA levels (Fu et al. 2012; Shippenberg et al. 1996). Chronic opioid exposure also affects GABA metabolism by reducing GAD activity, thereby impairing GABA synthesis and further contributing to neurotransmitter imbalance (Gao et al. 2007; Shi et al. 2020; Szpręgiel et al. 2021; Tzeng and Ho 1978).

4.3.3.Effects of Opioid Withdrawal

Opioid withdrawal is associated with increased GABA release, driven by cAMP‐dependent mechanisms (Bajo et al. 2014; Bonci and Williams 1997; Collier and Francis 1975; Hack et al. 2003). This enhanced GABAergic activity contributes to withdrawal symptoms and can be attenuated by inhibiting cAMP signaling pathways (Madhavan et al. 2010).

Withdrawal is also accompanied by alterations in GABA receptor function. Modulation of GABA‐A receptors influences withdrawal severity, with agonists exacerbating and antagonists reducing symptoms (Capasso and Sorrentino 1997). In contrast, activation of GABA‐B receptors, for example by baclofen, can alleviate withdrawal symptoms (Capasso 1999). In addition, increased GAT‐1 transporter activity enhances GABAergic excitability in the periaqueductal gray (PAG), directly contributing to the expression of withdrawal symptoms (Bagley et al. 2011; Williams et al. 2001).

The effects of acute and chronic opioid administration, as well as withdrawal, on the GABAergic system are summarized in Table 3 and illustrated in Figure 3.

TABLE 3Effects of opioids on the GABAergic system.

Category Acute administration Chronic administration Withdrawal Reference

Neurotransmitter levels ↓ GABA release in the VTA, PAG, and HIP (MOR agonists) ↑ GABA release in the VTA (MOR agonists) ↑ GABA release in the NAc (MOR agonists) Collier and Francis (1975), Kelley et al. (1980), Johnson and North (1992), Capogna et al. (1993), Kiyatkin et al. (1993), Avidor‐Reiss et al. (1996), Xi (2002), Hjelmstad and Fields (2003), Kreek et al. (2012), Bajo et al. (2014), Ronström et al. (2023)

Receptor expression and function ↓ GABA receptor binding sites in the CER, Ctx, and striatum (MOR agonists) ↑ GABA receptor binding sites in the pons and medulla

↑ GABA receptor affinity in the CER

↓ in the striatum (MOR agonists)

↑ mRNA expression of ε GABAA subunit in the LC (MOR agonists) Ticku and Huffman (1980), Sivam et al. (1982), Shippenberg et al. (1996), Capasso and Sorrentino (1997), Capasso (1999), Heikkilä et al. (2001), Xi (2002), Fu et al. (2012)

Transporters ↑ GAT‐1 activity in the PAG (MOR agonists) Williams et al. (2001), Bagley et al. (2011)

Enzymes ↓ GAD activity (MOR agonists) Tzeng and Ho (1978), Gao et al. (2007), Shi et al. (2020), Szpręgiel et al. (2021)

4.4.Opioids and the Serotonergic System

4.4.1.Effects of Acute Administration

Opioids also modulate serotonergic neurotransmission. Administration of MOR agonists, such as morphine and heroin, increases serotonin synthesis and release, leading to elevated levels of its metabolite 5‐HIAA, likely through enhanced activity of TPH (Boadle‐Biber et al. 1987; Pérez‐Cruet et al. 1975; Sawa and Oka 1976; Spampinato et al. 1985). This effect may result from reduced inhibitory input secondary to opioid‐induced suppression of GABAergic transmission, a mechanism analogous to that observed in dopaminergic neurons within the VTA (Bloom et al. 1972; Bonci and Williams 1997).

Region‐specific effects have been reported. MOR and DOR agonists increase serotonin release in the dorsal raphe nucleus (DRN) but not in the NAc, whereas KOR agonists reduce serotonin release in both regions. These findings suggest that OR subtypes differentially regulate serotonergic transmission across brain circuits (Tao and Auerbach 2002). Similar to their effects on dopamine signaling, KOR agonists appear to act primarily at presynaptic terminals, modulating neurotransmitter release without significantly altering neuronal excitability (Jolas and Aghajanian 1997; Spanagel et al. 1990; Thompson et al. 2000).

Although acute opioid administration does not appear to directly alter serotonergic receptor expression, functional interactions between these systems are well‐established. Activation of 5‐HT2A and 5‐HT2C receptors can counteract morphine‐induced increases in extracellular dopamine and reduce dopaminergic neuronal activity (Willins and Meltzer 1998).

4.4.2.Effects of Chronic Administration

Chronic opioid‐induced modulation of the serotonergic system appears to contribute more to antinociception than to reward or dependence mechanisms (Duman et al. 2004; Sasaki et al. 2021; Zhao et al. 2007). Opioid analgesia is partly mediated through serotonergic activation (Nemmani and Mogil 2003). However, prolonged opioid use leads to tolerance and may induce opioid‐induced hyperalgesia, thereby contributing to physical dependence (Mao 2002; Sasaki et al. 2021; Xu, Colpaert, and Wiesenfeld‐Hallin 2003).

Certain MOR agonists, such as tramadol, methadone, and pethidine, increase extracellular serotonin levels by inhibiting the serotonin transporter (SERT), thereby enhancing their analgesic properties. This effect is not observed in structurally distinct opioids such as morphine or oxycodone (Barann et al. 2015; Codd et al. 1995; Rickli et al. 2018; Sotgiu et al. 2009). However, SERT inhibition also increases the risk of serotonin syndrome, particularly with long‐acting agents such as methadone (Baldo and Rose 2020; Fox et al. 2009; Gillman 2005).

Chronic opioid exposure also alters serotonergic receptor function, including facilitation of 5‐HT₁ receptor signaling (associated with anxiolytic and analgesic effects) and inhibition of 5‐HT₃ receptors (linked to nausea, hyperalgesia, and tolerance) (Tao and Auerbach 2002). Additionally, some MOR agonists can interact with 5‐HT2A receptors, potentially contributing to withdrawal‐related phenomena (Pang et al. 2016; Rickli et al. 2018).

Serotonin can form covalent bonds with cytosolic and nuclear proteins through a process known as serotonylation. This post‐translational modification can alter protein signaling properties and has been implicated in synaptic plasticity, depression, and neurodegeneration (Bader 2019; Fu and Zhang 2019; Jiang et al. 2021; Muma and K. Kapadia 2020). The impact of opioids on serotonylation has not yet been investigated; however, this mechanism may represent a potentially relevant but currently unexplored pathway in the development of opioid addiction.

4.4.3.Effects of Opioid Withdrawal

Opioid withdrawal produces effects opposite to those observed during drug administration. Reduced serotonergic transmission and decreased TPH expression in the DRN have been reported (Harris and Aston‐Jones 2001; Lunden and Kirby 2013). Enhanced 5‐HT₁A receptor function during chronic exposure diminishes after cessation (Lunden and Kirby 2013; Lutz et al. 2011).

These alterations contribute to mood disturbances during withdrawal, including depressive‐like symptoms that intensify with prolonged abstinence. Dysfunction of the serotonergic system is therefore likely involved in both withdrawal symptomatology and relapse vulnerability (Goeldner et al. 2011; Lunden and Kirby 2013; Renoir et al. 2012).

The effects of acute and chronic opioid administration, as well as withdrawal, on the serotonergic system are summarized in Table 4 and illustrated in Figure 4.

TABLE 4Effects of opioids on the serotonergic system.

Category Acute administration Chronic administration Withdrawal Reference

Neurotransmitter levels ↑ 5‐HT and 5‐HIAA levels (MOR and DOR agonists) in the DRN

↓ 5‐HT levels in the NAc and DRN (KOR agonists)

↑ 5‐HT and 5‐HIAA levels in the DRN ↓ 5‐HT levels in the DRN Pérez‐Cruet et al. (1975), Sawa and Oka (1976), Spampinato et al. (1985), Harris, (2021)

Receptor expression and function Activation of 5‐HT2A and 5‐HT2C receptors counteracts opioid‐induced↑ DA levels in the NAc (MOR agonists) ↑ 5‐HT1R activity, ↓ 5‐HT3R activity (MOR agonists)

↑ binding of MOR agonists to 5‐HT2A receptors

↓ 5‐HT1R activity (MOR agonists) Willins and Meltzer (1998), Tao and Auerbach (2002), Lutz et al. (2011), Lunden and Kirby (2013), Pang et al. (2016), Rickli et al. (2018)

Transporters ↓ SERT activity (MOR agonists without oxygen bridge structure, e.g., pethidine, tramadol, and methadone) Codd et al. (1995), Sotgiu et al. (2009), Barann et al. (2015), Rickli et al. (2018)

Enzymes ↑ TPH activity (MOR agonists) ↓ TPH mRNA expression (MOR agonists) Boadle‐Biber et al. (1987), Lunden and Kirby (2013)

4.5.Opioids and the Cholinergic System

4.5.1.Effects of Acute Administration

The effects of acute opioid exposure on the cholinergic system vary depending on brain region and dose. MOR and DOR agonists inhibit striatal cholinergic interneurons, whereas KOR agonists show minimal effects on these neurons (Arenas et al. 1990). Morphine also suppresses glutamate‐stimulated ACh release in the Ctx, thereby impairing cognitive and arousal‐related processes (Osman et al. 2005). Despite these inhibitory actions, morphine has been reported to increase ACh levels in regions such as the striatum and trigeminal nuclei, likely due to reduced release followed by intracellular accumulation and compensatory dynamics (Green et al. 1976; Taguchi et al. 1993; Zhu et al. 2008). These region‐specific effects may involve interactions with other neurotransmitter systems, particularly GABAergic disinhibition (Johnson and North 1992; Zhu et al. 2008).

Opioids also influence AChE activity. Acute morphine reduces AChE activity in the medulla, whereas tramadol inhibits AChE across multiple brain regions, leading to increased ACh availability (Pandanaboina et al. 2012; Wahba et al. 1987).

Although opioids do not directly act on cholinergic receptors, functional interactions between these systems are well‐established. For example, MOR and DOR agonists suppress dopamine release indirectly by inhibiting cholinergic interneurons, an effect that can mimic nicotinic receptor desensitization. In contrast, KOR‐mediated effects appear largely independent of nicotinic receptor function (Britt and McGehee 2008; Das et al. 1986).

4.5.2.Effects of Chronic Administration

Chronic opioid exposure induces significant adaptations in cholinergic signaling, influencing analgesia, reward, and dependence (Jensen et al. 2018). Changes in ACh release are strongly region‐dependent. Chronic morphine increases ACh release in the HIP and laterodorsal tegmental nucleus, thereby enhancing dopaminergic transmission, while simultaneously decreasing cortical ACh release, a change associated with withdrawal‐like states (Bouaziz et al. 1996; Jhamandas et al. 1975; Jhamandas and Sutak 1974).

Chronic opioid use also alters AChE activity. Increased AChE activity has been observed in the Ctx and medulla, which may contribute to tolerance development. In contrast, reduced AChE activity has been reported in the midbrain and medial habenula, suggesting region‐specific and functionally divergent adaptations within the cholinergic system (Giacobini 2004; Hikida et al. 2003; Neugebauer et al. 2013; Slatkin and Rhiner 2004).

Modulation of KOR signaling further influences cholinergic‐dependent cognitive processes by regulating hippocampal ACh release, thereby affecting learning and memory functions (Hiramatsu et al. 1998; Serra et al. 1997; Takahashi et al. 2018).

Muscarinic receptors, particularly the M1 subtype, also interact functionally with opioid systems. Blocking or genetically deleting M1 receptors enhances morphine‐induced analgesia and reduces conditioned place preference, indicating that cholinergic signaling contributes to both pain modulation and reward‐related learning processes (Carrigan and Dykstra 2007; McKendrick and Graziane 2020).

4.5.3.Effects of Opioid Withdrawal

Opioid withdrawal is associated with increased cholinergic activity, particularly elevated ACh levels in the NAc, contributing to withdrawal symptoms (Rada, Pothos, et al. 1991; Rada et al. 1996). This may reflect accumulated ACh during chronic exposure followed by excessive release upon withdrawal. Pharmacological modulation of cholinergic signaling can influence withdrawal severity. Clonidine reduces ACh release and alleviates withdrawal symptoms (Bhargava and Way 1975; Pinsky et al. 1973).

Opioid withdrawal is associated with increased cholinergic activity, particularly elevated ACh levels in the NAc, which contribute to withdrawal symptomatology (Rada, Pothos, et al. 1991; Rada et al. 1996). This increase may reflect intracellular accumulation of ACh during chronic opioid exposure, followed by excessive release upon drug cessation. Pharmacological modulation of cholinergic signaling can significantly influence the severity of withdrawal. For example, clonidine reduces ACh release and alleviates withdrawal symptoms, likely through its inhibitory effects on central noradrenergic and downstream cholinergic activity (Bhargava and Way 1975; Pinsky et al. 1973).

Muscarinic receptor signaling also plays an important modulatory role. Deletion of M5 muscarinic receptors reduces both opioid reinforcement and withdrawal symptoms, suggesting a role in reward–withdrawal circuitry. Conversely, muscarinic agonists such as arecoline may attenuate withdrawal‐related disturbances, further supporting the involvement of muscarinic receptor pathways in opioid dependence and withdrawal regulation (Basile et al. 2002; Buccafusco et al. 2000; Neugebauer et al. 2013).

The effects of acute and chronic opioid administration, as well as withdrawal, on the cholinergic system are summarized in Table 5 and illustrated in Figure 5.

TABLE 5Effects of opioids on the cholinergic system.

Category Acute administration Chronic administration Withdrawal Reference

Neurotransmitter levels ↓ ACh release in the Ctx and striatum

↑ ACh levels in the striatum and trigeminal nuclei (MOR agonists)

↑ ACh release in the HIP and LDTg enhancing DA transmission in the VTA (MOR agonists)

↓ ACh release in the Ctx and striatum (MOR, KOR, and DOR agonists)

↓ ACh release in the Ctx and striatum

↑ ACh levels in the NAc (MOR agonists)

Jhamandas and Sutak (1974), Jhamandas et al. (1975), Green et al. (1976), Mulder et al. (1984), Arenas et al. (1990), Rada et al., (1991), Bouaziz et al. (1996), Imperato et al. (1996), Rada et al. (1996), Schoffelmeer et al., 1997, Osman et al. 2005, Zhu et al. 2008

Receptor expression and function No data available No data available No data available

Transporters No data available No data available No data available

Enzymes ↓ AChE activity in the medulla oblongata, Ctx, and HIP (MOR agonists) ↑ AChE activity in the medulla oblongata and Ctx

↓ AChE activity in the medulla oblongata and medial habenula (MOR agonists)

↑ AChE activity in the medial habenula (MOR agonists) Wahba et al. (1987), Pandanaboina et al. (2012), Neugebauer et al. (2013), Elwy and Tabl (2017)

5.Conclusions, Translational Challenges, and Perpectives

Opioids exert widespread and complex effects across multiple neurotransmitter systems, including dopaminergic, glutamatergic, GABAergic, serotonergic, and cholinergic pathways. These effects vary across acute exposure, chronic use, and withdrawal and involve changes in receptor expression, intracellular signaling, synaptic plasticity, and circuit function. Acute opioid administration enhances dopaminergic signaling through direct receptor‐mediated effects and disinhibition of dopaminergic neurons, thereby reinforcing reward pathways. Chronic exposure induces persistent neuroadaptations that contribute to tolerance, dependence, and addiction, whereas withdrawal is characterized by reduced dopaminergic activity and compensatory hypersensitivity, contributing to negative affective states and relapse vulnerability.

Opioids also substantially alter glutamatergic, GABAergic, serotonergic, and cholinergic neurotransmission. Chronic exposure and withdrawal disrupt glutamatergic plasticity and excitatory signaling, contributing to tolerance, sensitization, maladaptive drug‐related learning, and withdrawal‐associated negative affect. GABAergic adaptations, including changes in cAMP‐dependent signaling, contribute to neuronal hyperexcitability and withdrawal symptoms. Serotonergic signaling contributes to analgesia and mood regulation but is also altered during chronic exposure and withdrawal, potentially exacerbating affective symptoms. Cholinergic neurotransmission is modulated in a region‐specific manner and may influence cognition, reward processing, and withdrawal through changes in ACh release, AChE activity, and muscarinic receptor function. Collectively, these findings highlight the extensive interdependence of opioid effects across neurotransmitter systems rather than isolated dysfunction within individual pathways.

This complexity is particularly relevant to the reward circuitry, where dopaminergic, glutamatergic, GABAergic, serotonergic, and cholinergic networks interact with stress‐ and memory‐related circuits. These interconnected systems therefore represent potential targets for understanding the neurobiological mechanisms underlying OUD and for developing novel therapeutic strategies. However, their substantial overlap and compensatory adaptations create important translational challenges, and the therapeutic value of targeting individual neurotransmitter systems remains limited compared with established treatments.

Dopaminergic signaling has received considerable attention because chronic opioid exposure produces marked changes in dopamine function. During withdrawal, reduced dopaminergic activity contributes to anhedonia, diminished motivation, and negative affect, whereas opioid‐associated cues can evoke strong dopaminergic responses that promote drug‐seeking and relapse. Dopamine D2‐ and D3‐receptor systems, particularly D3 receptors, have been investigated as potential targets for reducing reward‐related effects and cue‐induced craving (de Guglielmo et al. 2017; Nickols et al. 2023). Nevertheless, dopamine‐targeting medications have not demonstrated sufficient clinical efficacy to become evidence‐based first‐line treatments for OUD.

Glutamatergic signaling is similarly altered by chronic opioid exposure and withdrawal. Increased glutamatergic activity, including within the NAc, may contribute to aversive states and maladaptive drug‐related learning. Consequently, glutamate‐modulating strategies, including NMDA receptor antagonism, memantine, and topiramate, have been investigated as approaches to reduce withdrawal symptoms or relapse (Comer and Sullivan 2007; Jain et al. 2011; Pysiewicz et al. 2025). However, clinical evidence for these interventions remains considerably weaker than that supporting established pharmacological treatments for OUD.

As noted above, GABAergic adaptations also contribute to the neurobiological consequences of chronic opioid exposure and withdrawal. GABAergic agents such as benzodiazepines, zolpidem, and related compounds have been investigated for the management of withdrawal‐associated anxiety, pain, and sleep disturbances (Bueschbell et al. 2021; Martin et al. 2023). However, it should be noted that these substances have abuse potential, which limits their clinical utility, and they have not replaced established pharmacological treatments for OUD. Moreover, concomitant use of GABAergic or other CNS depressant medications with opioids requires caution because of the potential for additive respiratory depression.

Serotonergic and cholinergic systems represent additional, but less well‐established, therapeutic targets. Serotonergic dysfunction may contribute to depression, anxiety, and other psychiatric symptoms associated with opioid dependence, while serotonergic agents have also been investigated for the management of specific withdrawal symptoms (Farren and O'Malley 2002; Chu et al. 2017). However, manipulation of serotonin signaling alone has not demonstrated efficacy comparable to established OUD treatments. Potential pharmacological interactions should also be considered, including the uncommon risk of serotonin toxicity when serotonergic medications are combined with methadone or buprenorphine. Cholinergic mechanisms may influence withdrawal, reward‐related learning, and relapse through their effects on ACh release and dopaminergic signaling, but pharmacological modulation of this system remains largely experimental (Jensen et al. 2018).

Overall, although dopaminergic, glutamatergic, GABAergic, serotonergic, and cholinergic systems provide important insights into the neurobiology of OUD and represent potential therapeutic targets, the evidence supporting their pharmacological manipulation remains substantially weaker than that for established treatments such as methadone and buprenorphine. These systems should therefore be considered primarily as complementary or experimental targets rather than replacements for evidence‐based first‐line therapies. A major translational challenge is to determine whether selective modulation of specific receptor subtypes, intracellular signaling pathways, or circuit‐level interactions can provide therapeutic benefits without disrupting physiological neurotransmission or introducing additional adverse effects. Future research should therefore move beyond the study of individual neurotransmitter systems toward an integrated, circuit‐level understanding of opioid‐induced neuroadaptations. Greater attention to the dynamic interactions among reward, stress, and memory circuits may facilitate the development of multimodal, mechanism‐based interventions capable of addressing both the positive and negative motivational states that contribute to opioid dependence and relapse. Such approaches may ultimately lead to more precise and effective strategies for preventing relapse and improving clinical outcomes in OUD.

Author Contributions

Daniela Novakova: writing – original draft, conceptualization. Eliska Pritulova: writing – review and editing, funding acquisition. Jiri Novotny: writing – review and editing, project administration, conceptualization, resources, supervision.

Funding

This work was supported by Charles University GA UK project (no. 148524) and Charles University Institutional Research Fund (SVV‐260790).

Conflicts of Interest

The authors declare no conflicts of interest.

Acknowledgments

The authors gratefully acknowledge Charles University, Faculty of Science, for providing the academic environment and resources that greatly contributed to the completion of this review. Figures were created using BioRender (https://www.biorender.com). The authors used ChatGPT (OpenAI, GPT‐5 series) to assist with grammar checking and to improve the clarity and readability of the text. All suggested changes were subsequently carefully evaluated and reviewed by the authors, who take full responsibility for the final content of the work. Open access publishing facilitated by Univerzita Karlova, as part of the Wiley ‐ CzechELib agreement.

Novakova, D.
, Pritulova E., and Novotny J.. 2026. “Opioid Modulation of Major Neurotransmitter Systems: Acute, Chronic, and Withdrawal Effects.” European Journal of Neuroscience
64, no. 9: e70735. 10.1111/ejn.70735.

Associate Editor: Miriam Melis

Data Availability Statement

Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.

References

Acquas, E.
, Carboni E., and Di Chiara G.. 1991. “Profound Depression of Mesolimbic Dopamine Release After Morphine Withdrawal in Dependent Rats.” European Journal of Pharmacology
193: 133–134.

Adaway, J. E.
, Dobson R., Walsh J., et al. 2016. “Serum and Plasma 5‐Hydroxyindoleacetic Acid as an Alternative to 24‐h Urine 5‐Hydroxyindoleacetic Acid Measurement.” Annals of Clinical Biochemistry
53: 554–560.

Aghajanian, G. K.
, Kogan J. H., and Moghaddam B.. 1994. “Opiate Withdrawal Increases Glutamate and Aspartate Efflux in the Locus Coeruleus: An In Vivo Microdialysis Study.” Brain Research
636: 126–130.

Ahmadi Soleimani, S. M.
, Mohamadi M A H M. H., Raoufy M. R., Azizi H., Nasehi M., and Zarrindast M. R.. 2018. “Acute Morphine Administration Alters the Power of Local Field Potentials in Mesolimbic Pathway of Freely Moving Rats: Involvement of Dopamine Receptors.” Neuroscience Letters
686: 168–174.

Alhaider, A. A.
, Lei S. Z., and Wilcox G. L.. 1991. “Spinal 5‐HT3 Receptor‐Mediated Antinociception: Possible Release of GABA.” Journal of Neuroscience
11: 1881–1888.

Al‐Hasani, R.
, and Bruchas M. R.. 2011. “Molecular Mechanisms of Opioid Receptor‐Dependent Signaling and Behavior.” Anesthesiology
115: 1363–1381.

Ananth, M. R.
, Rajebhosale P., Kim R., Talmage D. A., and Role L. W.. 2023. “Basal Forebrain Cholinergic Signalling: Development, Connectivity and Roles in Cognition.” Nature Reviews Neuroscience
24: 233–251.

Anderson, T. R.
, and Slotkin T. A.. 1975. “Effects of Morphine on the Rat Adrenal Medulla.” Biochemical Pharmacology
24: 671–679.

Arenas, E.
, Alberch J., Sanchez Arroyos R., and Marsal J.. 1990. “Effect of Opioids on Acetylcholine Release Evoked by K+ or Glutamic Acid From Rat Neostriatal Slices.” Brain Research
523: 51–56.

Avidor‐Reiss, T.
, Nevo I., Levy R., Pfeuffer T., and Vogel Z.. 1996. “Chronic Opioid Treatment Induces Adenylyl Cyclase V Superactivation. Involvement of Gbetagamma.” Journal of Biological Chemistry
271: 21309–21315.

Bader, M.

2019. “Serotonylation: Serotonin Signaling and Epigenetics.” Frontiers in Molecular Neuroscience
12: 288.

Bagley, E. E.
, Hacker J., Chefer V. I., et al. 2011. “Drug‐Induced GABA Transporter Currents Enhance GABA Release to Induce Opioid Withdrawal Behaviors.” Nature Neuroscience
14: 1548–1554.

Bajo, M.
, Madamba S. G., Roberto M., and Siggins G. R.. 2014. “Acute Morphine Alters GABAergic Transmission in the Central Amygdala During Naloxone‐Precipitated Morphine Withdrawal: Role of Cyclic AMP.” Frontiers in Integrative Neuroscience
8: 45–45.

Baldo, B. A.
, and Rose M. A.. 2020. “The Anaesthetist, Opioid Analgesic Drugs, and Serotonin Toxicity: A Mechanistic and Clinical Review.” British Journal of Anaesthesia
124: 44–62.

Barann, M.
, Stamer U. M., Lyutenska M., Stüber F., Bönisch H., and Urban B.. 2015. “Effects of Opioids on Human Serotonin Transporters.” Naunyn‐Schmiedeberg's Archives of Pharmacology
388: 43–49.

Basile, A. S.
, Fedorova I., Zapata A., et al. 2002. “Deletion of the M5 Muscarinic Acetylcholine Receptor Attenuates Morphine Reinforcement and Withdrawal but Not Morphine Analgesia.” Proceedings of the National Academy of Sciences of the United States of America
99: 11452–11457.

Beckett, A. H.
, and Casy A. F.. 1954. “Synthetic Analgesics: Stereochemical Considerations.” Journal of Pharmacy and Pharmacology
6: 986–1001.

Bhargava, H. N.
, and Way E. L.. 1975. “Brain Acetylcholine and Choline Following Acute and Chronic Morphine Treatment and During Withdrawal.” Journal of Pharmacology and Experimental Therapeutics
194: 65–73.

Bie, B.
, and Pan Z. Z.. 2005. “Increased Glutamate Synaptic Transmission in the Nucleus Raphe Magnus Neurons From Morphine‐Tolerant Rats.” Molecular Pain
1: 7.

Bloom, F. E.
, Hoffer B. J., Siggins G. R., Barker J. L., and Nicoll R. A.. 1972. “Effects of Serotonin on Central Neurons: Microiontophoretic Administration.” Federation Proceedings
31: 97–106.

Boadle‐Biber, M. C.
, Johannessen J. N., Narasimhachari N., and Phan T. H.. 1987. “Activation of Cortical Tryptophan Hydroxylase by Acute Morphine Treatment: Blockade by 6‐Hydroxydopamine.” European Journal of Pharmacology
139: 193–204.

Bohn, L. M.
, Gainetdinov R. R., Lin F. T., Lefkowitz R. J., and Caron M. G.. 2000. “Mu‐Opioid Receptor Desensitization by Beta‐Arrestin‐2 Determines Morphine Tolerance but Not Dependence.” Nature
408: 720–723.

Bonci, A.
, and Williams J. T.. 1997. “Increased Probability of GABA Release During Withdrawal From Morphine.” Journal of Neuroscience
17: 796–803.

Bormann, J.

2000. “The 'ABC' of GABA Receptors.” Trends in Pharmacological Sciences
21: 16–19.

Bouaziz, H.
, Tong C., Yoon Y., Hood D. D., and Eisenach J. C.. 1996. “Intravenous Opioids Stimulate Norepinephrine and Acetylcholine Release in Spinal Cord Dorsal Horn. Systematic Studies in Sheep and an Observation in a Human.” Anesthesiology
84: 143–154.

Bowery, N. G.
, and Smart T. G.. 2006. “GABA and Glycine as Neurotransmitters: A Brief History.” British Journal of Pharmacology
147, no. Suppl 1: S109–S119.

Britt, J. P.
, and McGehee D. S.. 2008. “Presynaptic Opioid and Nicotinic Receptor Modulation of Dopamine Overflow in the Nucleus Accumbens.” Journal of Neuroscience
28: 1672–1681.

Brook, K.
, Bennett J., and Desai S. P.. 2017. “The Chemical History of Morphine: An 8000‐Year Journey, From Resin to De‐Novo Synthesis.” Journal of Anesthesia History
3: 50–55.

Buccafusco, J. J.
, Zhang L. C., Shuster L. C., Jonnala R. R., and Gattu M.. 2000. “Prevention of Precipitated Withdrawal Symptoms by Activating Central Cholinergic Systems During A Dependence‐Producing Schedule of Morphine in Rats.” Brain Research
852: 76–83.

Bueschbell, B.
, Manga P., Penner E., and Schiedel A. C.. 2021. “Evidence for Protein‐Protein Interaction Between Dopamine Receptors and the G Protein‐Coupled Receptor 143.” International Journal of Molecular Sciences
22: 8328.

Burgraff, N. J.
, Baertsch N. A., and Ramirez J.‐M.. 2023. “A comparative Examination of Morphine and Fentanyl: Unravelling the Differential Impacts on Breathing and Airway Stability.” Journal of Physiology
601: 4625–4642.

Cao, L.
, Hu R., Xu T., Zhang Z. N., Li W., and Lu J.. 2017. “Characterization of Induced Pluripotent Stem Cell‐derived Human Serotonergic Neurons.” Frontiers in Cellular Neuroscience
11: 131–131.

Capasso, A.

1999. “GABAB Receptors Are Involved in the Control of Acute Opiate Withdrawal in Isolated Tissue.” Progress in Neuro‐Psychopharmacology & Biological Psychiatry
23: 289–299.

Capasso, A.
, and Sorrentino L.. 1997. “GABAA Receptor Antagonists Reduce Acute Opiate Withdrawal in Isolated Tissue.” Progress in Neuro‐Psychopharmacology & Biological Psychiatry
21: 315–330.

Capogna, M.
, Gähwiler B. H., and Thompson S. M.. 1993. “Mechanism of Mu‐Opioid Receptor‐Mediated Presynaptic Inhibition in the Rat Hippocampus In Vitro.” Journal of Physiology
470: 539–558.

Carrigan, K. A.
, and Dykstra L. A.. 2007. “Behavioral Effects of Morphine and Cocaine in M1 Muscarinic Acetylcholine Receptor‐Deficient Mice.” Psychopharmacology
191: 985–993.

Caspani, G.
, Sebők V., Sultana N., Swann J. R., and Bailey A.. 2022. “Metabolic Phenotyping of Opioid and Psychostimulant Addiction: A Novel Approach for Biomarker Discovery and Biochemical Understanding of the Disorder.” BritishJournal of Pharmacology
179: 1578–1606.

Charnay, Y.
, and Léger L.. 2010. “Brain Serotonergic Circuitries.” Dialogues in Clinical Neuroscience
12: 471–487.

Chartoff, E. H.
, and Connery H. S.. 2014. “It's MORe Exciting Than Mu: Crosstalk Between Mu Opioid Receptors and Glutamatergic Transmission in the Mesolimbic Dopamine System.” Frontiers in Pharmacology
5: 116.

Chebib, M.
, Hanrahan J. R., Mewett K. N., Duke R. K., and Johnston G. A. R.. 2004. “Ionotropic GABA Receptors as Therapeutic Targets for Memory and Sleep Disorders.” In Ionotropic GABA Receptors as Therapeutic Targets for Memory and Sleep Disorders, 13–23. Elsevier.

Chen, X.‐L.
, Lu G., Gong Y.‐X., et al. 2007. “Expression Changes of Hippocampal Energy Metabolism Enzymes Contribute to Behavioural Abnormalities During Chronic Morphine Treatment.” Cell Research
17: 689–700.

Cheng, J.
, He Z., Chen Q., et al. 2023. “Histone Modifications in Cocaine, Methamphetamine and Opioids.” Heliyon
9: e16407.

Chetan, P. S.
, Sangeetha R., Mohan P. M., and Rajendra W.. 2015. “Alterations in Glutamate Metabolism in Rat Brain by Tramadol Analgesia During Non‐Induction of Pain.” Saudi Journal of Medical and Pharmaceutical Sciences
1: 26–36.

Childers, S. R.
, Creese I., Snowman A. M., and Synder S. H.. 1979. “Opiate Receptor Binding Affected Differentially by Opiates and Opioid Peptides.” European Journal of Pharmacology
55: 11–18.

Chu, L. F.
, Sun J., Clemenson A., et al. 2017. “Ondansetron Does Not Reduce Withdrawal in Patients With Physical Dependence on Chronic Opioid Therapy.” Journal of Addiction Medicine
11: 342–349.

Clarke, S.
, Chen Z., Hsu M. S., Pintar J., Hill R., and Kitchen I.. 2001. “Quantitative Autoradiographic Mapping of the ORL1, Mu‐, Delta‐ and Kappa‐Receptors in the Brains of Knockout Mice Lacking the ORL1 Receptor Gene.” Brain Research
906: 13–24.

Codd, E. E.
, Shank R. P., Schupsky J. J., and Raffa R. B.. 1995. “Serotonin and Norepinephrine Uptake Inhibiting Activity of Centrally Acting Analgesics: Structural Determinants and Role in Antinociception.” Journal of Pharmacology and Experimental Therapeutics
274: 1263–1270.

Collier, H. O.
, and Francis D. L.. 1975. “Morphine Abstinence Is Associated With Increased Brain Cyclic AMP.” Nature
255: 159–162.

Comer, S. D.
, and Sullivan M. A.. 2007. “Memantine Produces Modest Reductions in Heroin‐Induced Subjective Responses in Human Research Volunteers.” Psychopharmacology
193: 235–245.

Conn, P. J.
, and Sanders‐Bush E.. 1984. “Selective 5HT‐2 Antagonists Inhibit Serotonin Stimulated Phosphatidylinositol Metabolism in Cerebral Cortex.” Neuropharmacology
23: 993–996.

Cox, B. M.
, Christie M. J., Devi L., Toll L., and Traynor J. R.. 2015. “Challenges for Opioid Receptor Nomenclature: IUPHAR Review 9.” British Journal of Pharmacology
172: 317–323.

Das, S.
, Matwyshyn G. A., and Bhargava H. N.. 1986. “Effect of Acute and Chronic Morphine Administration on Brain Cholinergic Muscarinic Receptors.” General Pharmacology
17: 173–178.

de Guglielmo, G.
, Kallupi M., Scuppa G., Demopulos G., Gaitanaris G., and Ciccocioppo R.. 2017. “Pioglitazone Attenuates the Opioid Withdrawal and Vulnerability to Relapse to Heroin Seeking in Rodents.” Psychopharmacology
234: 223–234.

De Vries, T. J.
, and Shippenberg T. S.. 2002. “Neural Systems Underlying Opiate Addiction.” Journal of Neuroscience: The Official Journal of the Society for Neuroscience
22: 3321–3325.

Dhawan, B. N.
, Cesselin F., Raghubir R., et al. 1996. “International Union of Pharmacology. XII. Classification of Opioid Receptors.” Pharmacological Reviews
48: 567–592.

Diana, M.
, Spiga S., and E. Acquas E.. 2006. Persistent and Reversible Morphine Withdrawal‐Induced Morphological Changes in the Nucleus Accumbens, 446–457. Cellular and Molecular Mechanisms of Drugs of Abuse and Neurotoxicity: Cocaine, Ghb, and Substituted Amphetamines.

Doyle, M. A.
, and Mazei‐Robison M. S.. 2021. “Opioid‐Induced Molecular and Cellular Plasticity of Ventral Tegmental Area Dopamine Neurons.” Cold Spring Harbor Perspectives in Medicine
11: a039362.

Drastichova, Z.
, Hejnova L., Moravcova R., and J. Novotny J.. 2021. “Proteomic Analysis Unveils Expressional Changes in Cytoskeleton‐ and Synaptic Plasticity‐Associated Proteins in Rat Brain Six Months After Withdrawal From Morphine.” Life (Basel)
11: 683.

Drewes, A. M.
, Jensen R. D., Nielsen L. M., et al. 2013. “Differences Between Opioids: Pharmacological, Experimental, Clinical and Economical Perspectives.” British Journal of Clinical Pharmacology
75: 60–78.

Duan, L.
, Li X., Yan J., et al. 2020. “Association of COMT Gene Polymorphisms With Response to Methadone Maintenance Treatment Among Chinese Opioid‐Dependent Patients.” Genetic Testing and Molecular Biomarkers
24: 364–369.

Duman, E. N.
, Kesim M., Kadioglu M., Yaris E., Kalyoncu N. I., and Erciyes N.. 2004. “Possible Involvement of Opioidergic and Serotonergic Mechanisms in Antinociceptive Effect of Paroxetine in Acute Pain.” Journal of Pharmacological Sciences
94: 161–165.

Elwy, A. E.
, and Tabl G.. 2017. “Impact of Tramadol and Morphine Abuse on the Activities of Acetylcholine Esterase, Na+/K+‐ATPase and Related Parameters in Cerebral Cortices of Male Adult Rats.” Electronic Physician
9: 4027–4034.

Farahmandfar, M.
, Karimian S. M., Zarrindast M.‐R., Kadivar M., Afrouzi H., and Naghdi N.. 2011. “Morphine Sensitization Increases the Extracellular Level of Glutamate in CA1 of Rat Hippocampus via Μ‐Opioid Receptor.” Neuroscience Letters
494: 130–134.

Farren, C. K.
, and O'Malley S.. 2002. “A Pilot Double Blind Placebo Controlled Trial of Sertraline With Naltrexone in the Treatment of Opiate Dependence.” American Journal on Addictions
11: 228–234.

Fisahn, A.
, Contractor A., Traub R. D., Buhl E. H., Heinemann S. F., and McBain C. J.. 2004. “Distinct Roles for the Kainate Receptor Subunits GluR5 and GluR6 in Kainate‐Induced Hippocampal Gamma Oscillations.” Journal of Neuroscience
24: 9658–9668.

Fleming, L. M.
, Ponjee G., and Childers S. R.. 1992. “Inhibition of Protein Phosphorylation by Opioid‐Inhibited Adenylyl Cyclase in Rat Brain Membranes.” Journal of Pharmacology and Experimental Therapeutics
260: 1416–1424.

Floresco, S. B.
, and Magyar O.. 2006. “Mesocortical Dopamine Modulation of Executive Functions: Beyond Working Memory.” Psychopharmacology
188: 567–585.

Fox, M. A.
, Jensen C. L., and Murphy D. L.. 2009. “Tramadol and Another Atypical Opioid Meperidine Have Exaggerated Serotonin Syndrome Behavioural Effects, but Decreased Analgesic Effects, in Genetically Deficient Serotonin Transporter (SERT) Mice.” International Journal of Neuropsychopharmacology
12: 1055–1065.

Frank, C.

1998. “Effects of Opioids on Glutamate Induced Calcium Intracellular Increase in Individually Perfused Rat Hippocampal Neurons In Vitro.” Progress in Neuro‐Psychopharmacology and Biological Psychiatry
22: 1331–1339.

Fu, L.
, and Zhang L.. 2019. “Serotonylation: A Novel Histone H3 Marker.” Signal Transduction and Targeted Therapy
4: 15–15.

Fu, Z.
, Yang H., Xiao Y., Zhao G., and H. Yang H.. 2012. “The γ‐Aminobutyric Acid Type B (GABAB) Receptor Agonist Baclofen Inhibits Morphine Sensitization by Decreasing the Dopamine Level in Rat Nucleus Accumbens.” Behavioral and Brain Functions
8: 20.

Fulton, S. L.
, Mitra S., Lepack A. E., et al. 2022. “Histone H3 Dopaminylation in Ventral Tegmental Area Underlies Heroin‐Induced Transcriptional and Behavioral Plasticity in Male Rats.” Neuropsychopharmacology
47: 1776–1783.

Galaj, E.
, Han X., Shen H., et al. 2020. “Dissecting the Role of GABA Neurons in the VTA versus SNr in Opioid Reward.” Journal of Neuroscience
40: 8853–8869.

Galaj, E.
, and Xi Z.‐X.. 2021. “Progress in Opioid Reward Research: From a Canonical Two‐Neuron Hypothesis to Two Neural Circuits.” Pharmacology, Biochemistry, and Behavior
200: 173072–173072.

Gao, H.
, Xiang Y., Sun N., et al. 2007. “Metabolic Changes in Rat Prefrontal Cortex and Hippocampus Induced by Chronic Morphine Treatment Studied Ex Vivo by High Resolution 1H NMR Spectroscopy.” Neurochemistry International
50: 386–394.

Gao, J.‐T.
, Jordan C. J., Bi G.‐H., et al. 2018. “Deletion of the Type 2 Metabotropic Glutamate Receptor Increases Heroin Abuse Vulnerability in Transgenic Rats.” Neuropsychopharmacology
43: 2615–2626.

García‐Pérez, D.
, Sáez‐Belmonte F., Laorden M. L., Núñez C., and Milanés M. V.. 2013. “Morphine Administration Modulates Expression of Argonaute 2 and Dopamine‐Related Transcription Factors Involved in Midbrain Dopaminergic Neurons Function.” British Journal of Pharmacology
168: 1889–1901.

Gauchy, C.
, Agid Y., Glowinski J., and Cheramy A.. 1973. “Acute Effects of Morphine on Dopamine Synthesis and Release and Tyrosine Metabolism in the Rat Striatum.” European Journal of Pharmacology
22: 311–319.

George, S. R.
, Zastawny R. L., Briones‐Urbina R., et al. 1994. “Distinct Distributions of Mu, Delta and Kappa Opioid Receptor mRNA in Rat Brain.” Biochemical and Biophysical Research Communications
205: 1438–1444.

Gerfen, C. R.

1992. “The Neostriatal Mosaic: Multiple Levels of Compartmental Organization.” Trends in Neurosciences
15: 133–139.

Ghasemi, F.
, Moradi A., Izadpanah E., Moloudi M. R., Hassanzadeh K., and Rahimmi A.. 2015. “Simvastatin Prevents Morphine Antinociceptive Tolerance and Withdrawal Symptoms in Rats.” Journal of the Formosan Medical Association = Taiwan Yi Zhi
114: 399–406.

Giacobini, E.

2004. “Cholinesterase Inhibitors: New Roles and Therapeutic Alternatives.” Pharmacological Research
50: 433–440.

Gianutsos, G.
, Hynes M. D., Puri S. K., Drawbaugh R. B., and Lal H.. 1974. “Effect of Apomorphine and Nigrostriatal Lesions on Aggression and Striatal Dopamine Turnover During Morphine Withdrawal: Evidence for Dopaminergic Supersensitivity in Protracted Abstinence.” Psychopharmacologia
34: 37–44.

Gillman, P. K.

2005. “Monoamine Oxidase Inhibitors, Opioid Analgesics and Serotonin Toxicity.” British Journal of Anaesthesia
95: 434–441.

Goeldner, C.
, Lutz P.‐E., Darcq E., et al. 2011. “Impaired Emotional‐Like Behavior and Serotonergic Function During Protracted Abstinence From Chronic Morphine.” Biological Psychiatry
69: 236–244.

Goody, R. J.
, Oakley S. M., Filliol D., Kieffer B. L., and Kitchen I.. 2002. “Quantitative Autoradiographic Mapping of Opioid Receptors in the Brain of Delta‐Opioid Receptor Gene Knockout Mice.” Brain Research
945: 9–19.

Green, J. P.
, Glick S. D., Crane A. M., and Szilagyi P. I.. 1976. “Acute Effects of Morphine on Regional Brain Levels of Acetylcholine in Mice and Rats.” European Journal of Pharmacology
39: 91–99.

Greengard, P.
, Allen P. B., and Nairn A. C.. 1999. “Beyond the Dopamine Receptor: The DARPP‐32/Protein Phosphatase‐1 Cascade.” Neuron
23: 435–447.

Grimm, J.
, Mueller A., Hefti F., and Rosenthal A.. 2004. “Molecular Basis for Catecholaminergic Neuron Diversity.” Proceedings of the National Academy of Sciences of the United States of America
101: 13891–13896.

Grinchii, D.
, Lacinova L., and Dremencov E.. 2023. “Effects of the Acute Administration of Delta‐Opioid Receptor Ligands on the Excitability of Rat Hippocampal Glutamate and Brainstem Monoamine Neurons In Vivo.” General Physiology and Biophysics
42: 273–283.

Guo, M.
, Xu N. H., Li Y. T., et al. 2005. “Morphine Modulates Glutamate Release in the Hippocampal CA1 Area in Mice.” Neuroscience Letters
381: 12–15.

Guo, Y.
, Wang H.‐L., Xiang X.‐H., and Zhao Y.. 2009. “The Role of Glutamate and Its Receptors in Mesocorticolimbic Dopaminergic Regions in Opioid Addiction.” Neuroscience and Biobehavioral Reviews
33: 864–873.

Hack, S. P.
, Vaughan C. W., and Christie M. J.. 2003. “Modulation of GABA Release During Morphine Withdrawal in Midbrain Neurons In Vitro.” Neuropharmacology
45: 575–584.

Harris, G. C.
, and Aston‐Jones G.. 2001. “Augmented Accumbal Serotonin Levels Decrease the Preference for a Morphine Associated Environment During Withdrawal.” Neuropsychopharmacology
24: 75–85.

Heikkilä, A. T.
, Echenko O., Uusi‐Oukari M., Sinkkonen S. T., and Korpi E. R.. 2001. “Morphine Withdrawal Increases Expression of GABA(A) Receptor Epsilon Subunit mRNA in Locus Coeruleus Neurons.” Neuroreport
12: 2981–2985.

Hejnova, L.
, Hronova A., Drastichova Z., and Novotny J.. 2024. “Long‐Term Administration of Morphine Specifically Alters the Level of Protein Expression in Different Brain Regions and Affects the Redox State.” Open Life Sciences
19: 20220858.

Hemmings, H. C.
, Greengard P., Tung H. Y. L., and Cohen P.. 1984. “DARPP‐32, a Dopamine‐Regulated Neuronal Phosphoprotein, Is a Potent Inhibitor of Protein Phosphatase‐1.” Nature
310: 503–505.

Higginbotham, J. A.
, Markovic T., Massaly N., and Morón J. A.. 2022. “Endogenous Opioid Systems Alterations in Pain and Opioid Use Disorder.” Frontiers in Systems Neuroscience
16: 1014768.

Hikida, T.
, Kitabatake Y., Pastan I., and Nakanishi S.. 2003. “Acetylcholine Enhancement in the Nucleus Accumbens Prevents Addictive Behaviors of Cocaine and Morphine.” Proceedings of the National Academy of Sciences of the United States of America
100: 6169–6173.

Hiramatsu, M.
, Murasawa H., Mori H., and Kameyama T.. 1998. “Reversion of Muscarinic Autoreceptor Agonist‐Induced Acetylcholine Decrease and Learning Impairment by Dynorphin A (1‐13), an Endogenous Kappa‐Opioid Receptor Agonist.” British Journal of Pharmacology
123: 920–926.

Hjelmstad, G. O.
, and Fields H. L.. 2003. “Kappa Opioid Receptor Activation in the Nucleus Accumbens Inhibits Glutamate and GABA Release Through Different Mechanisms.” Journal of Neurophysiology
89: 2389–2395.

Ho, J.‐H.
, Stahl E. L., Schmid C. L., Scarry S. M., Aubé J., and Bohn L. M.. 2018. “G Protein Signaling‐Biased Agonism at the κ‐Opioid Receptor Is Maintained in Striatal Neurons.” Science Signaling
11: 542.

Höglund, E.
, Øverli Ø., and S. Winberg S.. 2019. “Tryptophan Metabolic Pathways and Brain Serotonergic Activity: A Comparative Review.” Frontiers in Endocrinology
10: 158.

Hoyer, D.
, Hannon J. P., and Martin G. R.. 2002. “Molecular, Pharmacological and Functional Diversity of 5‐HT Receptors.” Pharmacology Biochemistry and Behavior
71, no. 4: 533–554.

Hu, L.
, Jing X.‐H., Cui C.‐L., Xing G.‐G., and Zhu B.. 2014. “NMDA Receptors in the Midbrain Play a Critical Role in Dopamine‐Mediated Hippocampal Synaptic Potentiation Caused by Morphine.” Addiction Biology
19: 380–391.

Hu, Z.
, Deng Y., Hu C., et al. 2012. “1H NMR‐Based Metabonomic Analysis of Brain in Rats of Morphine Dependence and Withdrawal Intervention.” Behavioural Brain Research
231: 11–19.

Imperato, A.
, Obinu M. C., Casu M. A., Mascia M. S., Carta G., and Gessa G. L.. 1996. “Chronic Morphine Increases Hippocampal Acetylcholine Release: Possible Relevance in Drug Dependence.” European Journal of Pharmacology
302: 21–26.

Jaber, M.
, Robinson S. W., Missale C., and Caron M. G.. 1996. “Dopamine Receptors and Brain Function.” Neuropharmacology
35: 1503–1519.

Jain, K.
, Jain R., and Dhawan A.. 2011. “A Double‐Blind, Double‐Dummy, Randomized Controlled Study of Memantine Versus Buprenorphine in Naloxone‐Precipitated Acute Withdrawal in Heroin Addicts.” Journal of Opioid Management
7: 11–20.

Jensen, K. P.
, DeVito E. E., Yip S., Carroll K. M., and Sofuoglu M.. 2018. “The Cholinergic System as a Treatment Target for Opioid Use Disorder.” CNS Drugs
32: 981–996.

Jhamandas, K.
, Hron V., and Sutak M.. 1975. “Comparative Effects of Opiate Agonists Methadone, Levorphanol, and Their Isomers on the Release of Cortical ACh In Vivo and In Vitro.” Canadian Journal of Physiology and Pharmacology
53: 540–548.

Jhamandas, K.
, and Sutak M.. 1974. “Modification of Brain Acetylcholine Release by Morphine and Its Antagonists in Normal and Morphine‐Dependent Rats.” British Journal of Pharmacology
50: 57–62.

Jiang, S. T.
, Li Y. F., Zhang C. L., et al. 2014. “M1 Muscarinic Acetylcholine Receptor in Alzheimer's Disease.” Neuroscience Bulletin
30: 295–307.

Jiang, S.‐H.
, Wang Y.‐H., Hu L.‐P., et al. 2021. “The Physiology, Pathology and Potential Therapeutic Application of Serotonylation.” Journal of Cell Science
134: jcs257337.

Johnson, S. W.
, and North R. A.. 1992. “Opioids Excite Dopamine Neurons by Hyperpolarization of Local Interneurons.” Journal of Neuroscience
12: 483–488.

Jolas, T.
, and Aghajanian G. K.. 1997. “Opioids Suppress Spontaneous and NMDA‐Induced Inhibitory Postsynaptic Currents in the Dorsal Raphe Nucleus of the Rat In Vitro.” Brain Research
755: 229–245.

Jomary, C.
, Gairin J. E., Cros J., and Meunier J. C.. 1988. “Autoradiographic Localization of Supraspinal Kappa‐Opioid Receptors With [125I‐Tyr1, D‐Pro10]dynorphin A‐(1‐11).” Proceedings of the National Academy of Sciences of the United States of America
85: 627–631.

Kah, O.
, Trudeau V. L., Sloley B. D., et al. 1992. “Influence of GABA on Gonadotrophin Release in the Goldfish.” Neuroendocrinology
55: 396–404.

Kaufmann, C. A.
, Kreek M. J., Raghunath J., and Arns P.. 1983. “Methadone, Monoamine Oxidase, and Depression: Opioid Distribution and Acute Effects on Enzyme Activity.” Biological Psychiatry
18: 1007–1021.

Kawai, H.
, Lazar R., and Metherate R.. 2007. “Nicotinic Control of Axon Excitability Regulates Thalamocortical Transmission.” Nature Neuroscience
10: 1168–1175.

Kebabian, J. W.
, and Calne D. B.. 1979. “Multiple Receptors for Dopamine.” Nature
277: 93–96.

Kelley, A. E.
, Stinus L., and Iversen S. D.. 1980. “Interactions Between D‐Ala‐Met‐Enkephalin, A10 Dopaminergic Neurones, and Spontaneous Behaviour in the Rat.” Behavioural Brain Research
1: 3–24.

Kitchen, I.
, Slowe S. J., Matthes H. W., and B. Kieffer B.. 1997. “Quantitative Autoradiographic Mapping of Mu‐, Delta‐ and Kappa‐Opioid Receptors in Knockout Mice Lacking the Mu‐Opioid Receptor Gene.” Brain Research
778: 73–88.

Kiyatkin, E. A.
, Wise R. A., and A. Gratton A.. 1993. “Drug‐ and Behavior‐Associated Changes in Dopamine‐Related Electrochemical Signals During Intravenous Heroin Self‐Administration in Rats.” Synapse (New York, N.Y.)
14: 60–72.

Ko, J. H.
, and Strafella A. P.. 2012. “Dopaminergic Neurotransmission in the Human Brain: New Lessons From Perturbation and Imaging.” Neuroscientist
18: 149–168.

Kobayashi, K.

2001. “Role of Catecholamine Signaling in Brain and Nervous System Functions: New Insights From Mouse Molecular Genetic Study.” Journal of Investigative Dermatology. Symposium Proceedings
6: 115–121.

Koob, G. F.
, and Volkow N. D.. 2016. “Neurobiology of Addiction: A Neurocircuitry Analysis.” Lancet. Psychiatry
3: 760–773.

Kosten, T. R.
, and Baxter L. E.. 2019. “Review Article: Effective Management of Opioid Withdrawal Symptoms: A Gateway to Opioid Dependence Treatment.” American Journal on Addictions
28: 55–62.

Kreek, M. J.
, Levran O., Reed B., Schlussman S. D., Zhou Y., and Butelman E. R.. 2012. “Opiate Addiction and Cocaine Addiction: Underlying Molecular Neurobiology and Genetics.” Journal of Clinical Investigation
122: 3387–3393.

Lacey, M. G.
, Mercuri N. B., and North R. A.. 1987. “Dopamine Acts on D2 Receptors to Increase Potassium Conductance in Neurones of the Rat Substantia Nigra Zona Compacta.” Journal of Physiology
392: 397–416.

Lane, D. A.
, Lessard A. A., Chan J., et al. 2008. “Region‐Specific Changes in the Subcellular Distribution of AMPA Receptor GluR1 Subunit in the Rat Ventral Tegmental Area After Acute or Chronic Morphine Administration.” Journal of Neuroscience
28: 9670–9681.

Le Grevès, P.
, Huang W., Zhou Q., Thörnwall M., and Nyberg F.. 1998. “Acute Effects of Morphine on the Expression of mRNAs for NMDA Receptor Subunits in the Rat Hippocampus, Hypothalamus and Spinal Cord.” European Journal of Pharmacology
341: 161–164.

Le Moal, M.
, and Simon H.. 1991. “Mesocorticolimbic Dopaminergic Network: Functional and Regulatory Roles.” Physiological Reviews
71: 155–234.

Lebois, E. P.
, Thorn C., Edgerton J. R., Popiolek M., and Xi S.. 2018. “Muscarinic Receptor Subtype Distribution in the Central Nervous System and Relevance to Aging and Alzheimer's Disease.” Neuropharmacology
136: 362–373.

Listos, J.
, Baranowska‐Bosiacka I., Talarek S., et al. 2013. “The Effect of Perinatal Lead Exposure on Dopamine Receptor D2 Expression in Morphine Dependent Rats.” Toxicology
310: 73–83.

Liu, Y.
, Han M., Liu X., et al. 2013. “Dopamine Transporter Availability in Heroin‐Dependent Subjects and Controls: Longitudinal Changes During Abstinence and the Effects of Jitai Tablets Treatment.” Psychopharmacology
230: 235–244.

Lunden, J. W.
, and Kirby L. G.. 2013. “Opiate Exposure and Withdrawal Dynamically Regulate mRNA Expression in the Serotonergic Dorsal Raphe Nucleus.” Neuroscience
254: 160–172.

Lüscher, C.
, Jan L. Y., Stoffel M., Malenka R. C., and Nicoll R. A.. 1997. “G Protein‐Coupled Inwardly Rectifying K+ Channels (GIRKs) Mediate Postsynaptic but Not Presynaptic Transmitter Actions in Hippocampal Neurons.” Neuron
19: 687–695.

Lutz, P.‐E.
, Pradhan A. A., Goeldner C., and Kieffer B. L.. 2011. “Sequential and Opposing Alterations of 5‐HT(1A) Receptor Function During Withdrawal From Chronic Morphine.” European Neuropsychopharmacology
21: 835–840.

Madhavan, A.
, Bonci A., and Whistler J. L.. 2010. “Opioid‐Induced GABA Potentiation After Chronic Morphine Attenuates the Rewarding Effects of Opioids in the Ventral Tegmental Area.” Journal of Neuroscience
30: 14029–14035.

Mafi, A.
, Kim S.‐K., and Goddard W. A.. 2021. “The G Protein‐First Activation Mechanism of Opioid Receptors by Gi Protein and Agonists.” QRB Discovery
2: e9.

Maldonado, R.
, Saiardi A., Valverde O., Samad T. A., Roques B. P., and Borrelli E.. 1997. “Absence of Opiate Rewarding Effects in mice Lacking Dopamine D2 Receptors.” Nature
388: 586–589.

Mansour, A.
, Fox C. A., Akil H., and Watson S. J.. 1995. “Opioid‐Receptor mRNA Expression in the Rat CNS: Anatomical and Functional Implications.” Trends in Neurosciences
18: 22–29.

Mansour, A.
, Khachaturian H., Lewis M. E., Akil H., and Watson S. J.. 1987. “Autoradiographic Differentiation of Mu, Delta, and Kappa Opioid Receptors in the Rat Forebrain and Midbrain.” Journal of Neuroscience
7: 2445–2464.

Mao, J.

2002. “Opioid‐Induced Abnormal Pain Sensitivity: Implications in Clinical Opioid Therapy.” Pain
100: 213–217.

Martin, C. E.
, Patel H., Dzierewski J. M., et al. 2023. “Benzodiazepine, Z‐Drug, and Sleep Medication Prescriptions in Male and Female People With Opioid Use Disorder on Buprenorphine and Comorbid Insomnia: An Analysis of Multistate Insurance Claims.” Sleep
46: zsad083.

Mathioudakis, L.
, Bourbouli M., Daklada E., Kargatzi S., Michaelidou K., and Zaganas I.. 2019. “Localization of Human Glutamate Dehydrogenases Provides Insights into Their Metabolic Role and Their Involvement in Disease Processes.” Neurochemical Research
44: 170–187.

Matsui, A.
, Jarvie B. C., Robinson B. G., Hentges S. T., and Williams J. T.. 2014. “Separate GABA Afferents to Dopamine Neurons Mediate Acute Action of Opioids, Development of Tolerance, and Expression of Withdrawal.” Neuron
82: 1346–1356.

Matthews, R. T.
, and German D. C.. 1984. “Electrophysiological Evidence for Excitation of Rat Ventral Tegmental Area Dopamine Neurons by Morphine.” Neuroscience
11: 617–625.

McGovern, D. J.
, Polter A. M., Prévost E. D., et al. 2023. “Ventral Tegmental Area Glutamate Neurons Establish a Mu‐Opioid Receptor Gated Circuit to Mesolimbic Dopamine Neurons and Regulate Opioid‐Seeking Behavior.” Neuropsychopharmacology
48: 1889–1900.

McKendrick, G.
, and Graziane N. M.. 2020. “Drug‐Induced Conditioned Place Preference and Its Practical Use in Substance Use Disorder Research.” Frontiers in Behavioral Neuroscience
14: 582147–582147.

Mercuri, N. B.
, Scarponi M., Bonci A., Siniscalchi A., and Bernardi G.. 1997. “Monoamine Oxidase Inhibition Causes a Long‐Term Prolongation of the Dopamine‐Induced Responses in Rat Midbrain Dopaminergic Cells.” Journal of Neuroscience
17: 2267–2272.

Mohebi, A.
, Collins V. L., and Berke J. D.. 2023. “Accumbens Cholinergic Interneurons Dynamically Promote Dopamine Release and Enable Motivation.” eLife
12: e85011.

Montandon, G.
, Ren J., Victoria N. C., et al. 2016. “G‐Protein‐Gated Inwardly Rectifying Potassium Channels Modulate Respiratory Depression by Opioids.” Anesthesiology
124: 641–650.

Mulder, A. H.
, Wardeh G., Hogenboom F., and Frankhuyzen A. L.. 1984. “Kappa‐ and Delta‐Opioid Receptor Agonists Differentially Inhibit Striatal Dopamine and Acetylcholine Release.” Nature
308: 278–280.

Muller, D. L.
, and Unterwald E. M.. 2005. “D1 Dopamine Receptors Modulate deltaFosB Induction in Rat Striatum After Intermittent Morphine Administration.” Journal of Pharmacology and Experimental Therapeutics
314: 148–154.

Muma, N.
, and K. Kapadia K.. 2020. Serotonylation and Neuronal Function, 257–265. Elsevier.

Naffaa, M. M.
, Hung S., Chebib M., Johnston G. A. R., and Hanrahan J. R.. 2017. “GABA‐ Receptors: Distinctive Functions and Molecular Pharmacology.” British Journal of Pharmacology
174: 1881–1894.

Nagatsu, T.

2007. “The Catecholamine System in Health and Disease ‐ Relation to Tyrosine 3‐Monooxygenase and Other Catecholamine‐Synthesizing Enzymes.” Proceedings of the Japan Academy. Series B, Physical and Biological Sciences
82: 388–415.

Narita, M.
, Mizuo K., Mizoguchi H., et al. 2003. “Molecular Evidence for the Functional Role of Dopamine D3 Receptor in the Morphine‐Induced Rewarding Effect and Hyperlocomotion.” Journal of Neuroscience
23: 1006–1012.

Narita, M.
, Suzuki T., Funada M., Misawa M., and Nagase H.. 1993. “Blockade of the Morphine‐Induced Increase in Turnover of Dopamine on the Mesolimbic Dopaminergic System by κ ‐Opioid Receptor Activation in Mice.” Life Sciences
52: 397–404.

Nemmani, K. V. S.
, and Mogil J. S.. 2003. “Serotonin‐GABA Interactions in the Modulation of Mu‐ and Kappa‐Opioid Analgesia.” Neuropharmacology
44: 304–310.

Nestler, E. J.

2008. “Review. Transcriptional Mechanisms of Addiction: Role of DeltaFosB.” Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences
363: 3245–3255.

Neugebauer, N. M.
, Einstein E. B., Lopez M. B., McClure‐Begley T. D., Mineur Y. S., and Picciotto M. R.. 2013. “Morphine Dependence and Withdrawal Induced Changes in Cholinergic Signaling.” Pharmacology, Biochemistry, and Behavior
109: 77–83.

Nickols, J. E. R.
, Dursun S. M., and Taylor A. M. W.. 2023. “Preclinical Evidence for the Use of the Atypical Antipsychotic, Brexpiprazole, for Opioid Use Disorder.” Neuropharmacology
233: 109546.

Nielsen, L. M.
, Christrup L. L., Sato H., Drewes A. M., and Olesen A. E.. 2017. “Genetic Influences of OPRM1, OPRD1 and COMT on Morphine Analgesia in a Multi‐Modal, Multi‐Tissue Human Experimental Pain Model.” Basic & Clinical Pharmacology & Toxicology
121: 6–12.

Niswender, C. M.
, and Conn P. J.. 2010. “Metabotropic Glutamate Receptors: Physiology, Pharmacology, and Disease.” Annual Review of Pharmacology and Toxicology
50: 295–322.

Osman, N. I.
, Baghdoyan H. A., and Lydic R.. 2005. “Morphine Inhibits Acetylcholine Release in Rat Prefrontal Cortex When Delivered Systemically or by Microdialysis to Basal Forebrain.” Anesthesiology
103: 779–787.

Ostermeier, A. M.
, Schlösser B., Schwender D., and Sutor B.. 2000. “Activation of Mu‐ and Delta‐Opioid Receptors Causes Presynaptic Inhibition of Glutamatergic Excitation in Neocortical Neurons.” Anesthesiology
93: 1053–1063.

Ozawa, T.
, Nakagawa T., Shige K., Minami M., and Satoh M.. 2001. “Changes in the Expression of Glial Glutamate Transporters in the Rat Brain Accompanied With Morphine Dependence and Naloxone‐Precipitated Withdrawal.” Brain Research
905: 254–258.

Özcete, O. D.
, Banerjee A., and Kaeser P. S.. 2024. “Mechanisms of Neuromodulatory Volume Transmission.” Molecular Psychiatry
29: 3680–3693.

Palmada, M.
, and Centelles J. J.. 1998. “Excitatory Amino Acid Neurotransmission. Pathways for Metabolism, Storage and Reuptake of Glutamate in Brain.” Frontiers in Bioscience
3: d701–d718.

Pandanaboina, S. C.
, Rajbanshi S. L. A., Kunala P. N. V., et al. 2012. “Tramadol Safety‐Cholinergic System of Rat Brain Without Nociception.” Acta Poloniae Pharmaceutica
69: 833–841.

Pang, G.
, Wu X., Tao X., et al. 2016. “Blockade of Serotonin 5‐HT2A Receptors Suppresses Behavioral Sensitization and Naloxone‐Precipitated Withdrawal Symptoms in Morphine‐Treated Mice.” Frontiers in Pharmacology
7: 514.

Papke, R. L.

2014. “Merging Old and New Perspectives on Nicotinic Acetylcholine Receptors.” Biochemical Pharmacology
89: 1–11.

Pathan, H.
, and Williams J.. 2012. “Basic Opioid Pharmacology: An Update.” British Journal of Pain
6: 11–16.

Pérez‐Cruet, J.
, Thoa N. B., and Ng L. K. Y.. 1975. “Acute Effects of Heroin and Morphine on Newly Synthesized Serotonin in Rat Brain.” Life Sciences
17: 349–362.

Pert, C. B.
, and Snyder S. H.. 1973. “Opiate Receptor: Demonstration in Nervous Tissue.” Science (New York, N.Y.)
179: 1011–1014.

Peters, J.
, and De Vries T. J.. 2012. “Glutamate Mechanisms Underlying Opiate Memories.” Cold Spring Harbor Perspectives in Medicine
2: a012088.

Pinsky, C.
, Frederickson R. C. A., and Vazquez A. J.. 1973. “Morphine Withdrawal Syndrome Responses to Cholinergic Antagonists and to a Partial Cholinergic Agonist.” Nature
242: 59–60.

Pithadia, A. B.
, and Jain S. M.. 2009. “5‐Hydroxytryptamine Receptor Subtypes and Their Modulators With Therapeutic Potentials.” Journal of Clinical Medicine Research
1: 72–80.

Pothos, E.
, Rada P., Mark G. P., and Hoebel B. G.. 1991. “Dopamine Microdialysis in the Nucleus Accumbens During Acute and Chronic Morphine, Naloxone‐Precipitated Withdrawal and Clonidine Treatment.” Brain Research
566: 348–350.

Potter, L. T.

1970. “Synthesis, Storage and Release of [14C]Acetylcholine in Isolated Rat Diaphragm Muscles.” Journal of Physiology
206: 145–166.

Pourzitaki, C.
, Tsaousi G., Papazisis G., et al. 2018. “Fentanyl and Naloxone Effects on Glutamate and GABA Release Rates From Anterior Hypothalamus in Freely Moving Rats.” European Journal of Pharmacology
834: 169–175.

Pysiewicz, A.
, Mazur A., Kotlińska J., et al. 2025. “The Influence of Topiramate on Morphine Dependence in Mice.” Biomolecules
15: 730.

Rada, P.
, Johnson D. F., Lewis M. J., and Hoebel B. G.. 2004. “In Alcohol‐Treated Rats, Naloxone Decreases Extracellular Dopamine and Increases Acetylcholine in the Nucleus Accumbens: Evidence of Opioid Withdrawal.” Pharmacology, Biochemistry and Behavior
79: 599–605.

Rada, P.
, Mark G. P., Pothos E., and Hoebel B. G.. 1991. “Systemic Morphine Simultaneously Decreases Extracellular Acetylcholine and Increases Dopamine in the Nucleus Accumbens of Freely Moving Rats.” Neuropharmacology
30: 1133–1136.

Rada, P.
, Pothos E., Mark G. P., and Hoebel B. G.. 1991. “Microdialysis Evidence That Acetylcholine in the Nucleus Accumbens is Involved in Morphine Withdrawal and Its Treatment With Clonidine.” Brain Research
561: 354–356.

Rada, P. V.
, Mark G. P., Taylor K. M., and Hoebel B. G.. 1996. “Morphine and Naloxone, i.p. or Locally, Affect Extracellular Acetylcholine in the Accumbens and Prefrontal Cortex.” Pharmacology, Biochemistry, and Behavior
53: 809–816.

Ranjbar‐Slamloo, Y.
, and Z. Fazlali Z.. 2020. “Dopamine and Noradrenaline in the Brain; Overlapping or Dissociate Functions?” Frontiers in Molecular Neuroscience
12: 12:334.

Rapport, M. M.
, Green A. A., and Page I. H.. 1948. “Serum Vasoconstrictor, Serotonin; Chemical Inactivation.” Journal of Biological Chemistry
176: 1237–1241.

Rashvand, M.
, Khajavai A., Parviz M., Hasanein P., and Keshavarz M.. 2014. “GABAA Receptors Are Involved in the Analgesic Effects of Morphine Microinjected Into the Central Nucleus of the Amygdala.” Clinical and Experimental Pharmacology & Physiology
41: 338–344.

Rawls, S. M.
, and Cowan A.. 2006. “Modulation of Delta Opioid‐Evoked Hypothermia in Rats by WAY 100635 and Fluoxetine.” Neuroscience Letters
398: 319–324.

Raynor, K.
, Kong H., Chen Y., et al. 1994. “Pharmacological Characterization of the Cloned Kappa‐, Delta‐, and Mu‐Opioid Receptors.” Molecular Pharmacology
45: 330–334.

Reeves, K. C.
, Shah N. K., Munoz B., and Atwood B. K.. 2022. “Opioid Receptor‐Mediated Regulation of Neurotransmission in the Brain.” Frontiers in Molecular Neuroscience
15: 919773.

Reitzer, L.

2004. “Biosynthesis of Glutamate, Aspartate, Asparagine, L‐Alanine, and D‐Alanine.” EcoSal Plus
1: 1.

Renoir, T.
, Pang T. Y., and Lanfumey L.. 2012. “Drug Withdrawal‐Induced Depression: Serotonergic and Plasticity Changes in Animal Models.” Neuroscience and Biobehavioral Reviews
36: 696–726.

Riccioni, T.

2010. “5‐Ht6 Receptor Characterization.” Pharmacology of 5‐Ht6 Receptors, Pt 1
94: 67–88.

Rickli, A.
, Liakoni E., Hoener M. C., and Liechti M. E.. 2018. “Opioid‐Induced Inhibition of the Human 5‐HT and Noradrenaline Transporters in Vitro: Link to Clinical Reports of Serotonin Syndrome.” British Journal of Pharmacology
175: 532–543.

Rivera, A.
, Suárez‐Boomgaard D., Miguelez C., et al. 2021. “Dopamine D4 Receptor Is a Regulator of Morphine‐Induced Plasticity in the Rat Dorsal Striatum.” Cells
11: 31.

Rizo, J.

2018. “Mechanism of Neurotransmitter Release Coming Into Focus.” Protein Science
27: 1364–1391.

Rodríguez‐Muñoz, M.
, Sánchez‐Blázquez P., Vicente‐Sánchez A., Berrocoso E., and Garzón J.. 2012. “The Mu‐Opioid Receptor and the NMDA Receptor Associate in PAG Neurons: Implications in Pain Control.” Neuropsychopharmacology
37: 338–349.

Ronström, J. W.
, Johnson N. L., Jones S. T., et al. 2023. “Opioid‐Induced Reductions in Amygdala Lateral Paracapsular GABA Neuron Circuit Activity.” International Journal of Molecular Sciences
24: 1929.

Rooney, K. F.
, Armstrong R. A., and Sewell R. D.. 1991. “Increased Dopamine Receptor Sensitivity in the Rat Following Acute Administration of Sufentanil, U50,488H and D‐Ala2‐D‐Leu5‐Enkephalin.” Naunyn‐Schmiedeberg's Archives of Pharmacology
343: 458–462.

Rossetti, Z. L.
, Hmaidan Y., and Gessa G. L.. 1992. “Marked Inhibition of Mesolimbic Dopamine Release: A Common Feature of Ethanol, Morphine, Cocaine and Amphetamine Abstinence in Rats.” European Journal of Pharmacology
221: 227–234.

Salisbury, A. J.
, Blackwood C. A., and Cadet J. L.. 2021. “Prolonged Withdrawal From Escalated Oxycodone Is Associated With Increased Expression of Glutamate Receptors in the Rat Hippocampus.” Frontiers in Neuroscience
14: 617973.

Salvi, R.
, Steigleder T., Schlachetzki J. C. M., et al. 2016. “Distinct Effects of Chronic Dopaminergic Stimulation on Hippocampal Neurogenesis and Striatal Doublecortin Expression in Adult Mice.” Frontiers in Neuroscience
10: 77.

Samani, S. K.
, Khalifeh S., Vahiddastjerdi M., et al. 2023. “The Effect of Memantine on the Expression of Dopamine Receptors in the Striatum of Male Rats With Morphine Addiction.” Qom University of Medical Sciences Journal
17: 11.

Sasaki, M.
, Kamiya Y., Bamba K., et al. 2021. “Serotonin Plays a Key Role in the Development of Opioid‐Induced Hyperalgesia in Mice.” Journal of Pain
22: 715–729.

Sawa, A.
, and Oka T.. 1976. “Effects of Narcotic Analgesics on Serotonin Metabolism in Brain of Rats and Mice.” Japanese Journal of Pharmacology
26: 599–605.

Schoffelmeer, A. N.
, Hogenboom F., and Mulder A. H.. 1997. “Kappa1‐ and kappa2‐opioid receptors mediating presynaptic inhibition of dopamine and acetylcholine release in rat neostriatum.” British Journal of Pharmacology
122: 520–524.

Schoffelmeer, A. N.
, Wardeh G., and Vanderschuren L. J.. 2001. “Morphine Acutely and Persistently Attenuates Nonvesicular GABA Release in Rat Nucleus Accumbens.” Synapse
42: 87–94.

Schousboe, A.
, Sarup A., and Larsson O. M.. 2004. “GABA Transporters and GABA‐Transaminase: Roles in GABA Homeostasis and as Drug Targets.” Journal of Neurochemistry
90: 31–31.

Sepulveda, M. J.
, Hernandez L., Rada P., Tucci S., and Contreras E.. 1998. “Effect of Precipitated Withdrawal on Extracellular Glutamate and Aspartate in the Nucleus Accumbens of Chronically Morphine‐Treated Rats: An In Vivo Microdialysis Study.” Pharmacology, Biochemistry, and Behavior
60: 255–262.

Serra, M.
, Dazzi L., Cagetti E., et al. 1997. “Effect of Pentylenetetrazole‐Induced Kindling on Acetylcholine Release in the Hippocampus of Freely Moving Rats.” Journal of Neurochemistry
68: 313–318.

Sherman, S. M.

2014. “The Function of Metabotropic Glutamate Receptors in Thalamus and Cortex.” Neuroscientist
20: 136–149.

Shi, J.
, Zhao L.‐Y., Copersino M. L., et al. 2008. “PET Imaging of Dopamine Transporter and Drug Craving During Methadone Maintenance Treatment and After Prolonged Abstinence in Heroin Users.” European Journal of Pharmacology
579: 160–166.

Shi, Y.
, Li Y., Zhang J., Xiao Y., Yan P., and Zhu Y.. 2020. “GAD1 but Not GAD2 Polymorphisms Are Associated With Heroin Addiction Phenotypes.” Neuroscience Letters
717: 134704–134704.

Shippenberg, T. S.
, Heidbreder C., and Lefevour A.. 1996. “Sensitization to the Conditioned Rewarding Effects of Morphine: Pharmacology and Temporal Characteristics.” European Journal of Pharmacology
299: 33–39.

Sigel, E.
, and Steinmann M. E.. 2012. “Structure, Function, and Modulation of GABAA Receptors.” Journal of Biological Chemistry
287: 40224–40231.

Simantov, R.

1993. “Chronic Morphine Alters Dopamine Transporter Density in the Rat Brain: Possible Role in the Mechanism of Drug Addiction.” Neuroscience Letters
163: 121–124.

Simmons, S. C.
, Wheeler K., and Mazei‐Robison M. S.. 2019. “Determination of Circuit‐Specific Morphological Adaptations in Ventral Tegmental Area Dopamine Neurons by Chronic Morphine.” Molecular Brain
12: 10–10.

Sivam, S. P.
, Nabeshima T., and Ho I. K.. 1982. “An Analysis of GABA Receptor Changes in the Discrete Regions of Mouse Brain After Acute and Chronic Treatments With Morphine.” Journal of Neurochemistry
39: 933–939.

Slatkin, N.
, and Rhiner M.. 2004. “Treatment of Opioid‐Induced Delirium With Acetylcholinesterase Inhibitors: A Case Report.” Journal of Pain and Symptom Management
27: 268–273.

Smith, K. S.
, and Rudolph U.. 2012. “Anxiety and Depression: Mouse Genetics and Pharmacological Approaches to the Role of GABAA Receptor Subtypes.” Neuropharmacology
62: 54–62.

Snyder, S. H.
, and Childers S. R.. 1979. “Opiate Receptors and Opioid Peptides.” Annual Review of Neuroscience
2: 35–64.

Sotgiu, M. L.
, Valente M., Storchi R., Caramenti G., and Biella G. E. M.. 2009. “Cooperative N‐Methyl‐d‐aspartate (NMDA) Receptor Antagonism and μ‐Opioid Receptor Agonism Mediate the Methadone Inhibition of the Spinal Neuron Pain‐Related Hyperactivity in a Rat Model of Neuropathic Pain.” Pharmacological Research
60: 284–290.

Spampinato, U.
, Esposito E., Romandini S., and Samanin R.. 1985. “Changes of Serotonin and Dopamine Metabolism in Various Forebrain Areas of Rats Injected With Morphine Either Systemically or in the Raphe Nuclei Dorsalis and Medianus.” Brain Research
328: 89–95.

Spanagel, R.
, Herz A., and Shippenberg T. S.. 1990. “The Effects of Opioid Peptides on Dopamine Release in the Nucleus Accumbens: An In Vivo Microdialysis Study.” Journal of Neurochemistry
55: 1734–1740.

Spangler, R.
, Goddard N. L., Avena N. M., Hoebel B. G., and Leibowitz S. F.. 2003. “Elevated D3 Dopamine Receptor mRNA in Dopaminergic and Dopaminoceptive Regions of the rat Brain in Response to Morphine.” Molecular Brain Research
111: 74–83.

Spyraki, C.
, Fibiger H. C., and Phillips A. G.. 1983. “Attenuation of Heroin Reward in Rats by Disruption of the Mesolimbic Dopamine System.” Psychopharmacology
79: 278–283.

Stachowicz, K.
, Brañski P., Kłak K., et al. 2008. “Selective Activation of Metabotropic G‐Protein‐Coupled Glutamate 7 Receptor Elicits Anxiolytic‐Like Effects in Mice by Modulating GABAergic Neurotransmission.” Behavioural Pharmacology
19: 597–603.

Stevens, C. W.

2009. “The Evolution of Vertebrate Opioid Receptors.” Frontiers in bioscience (Landmark edition)
14: 1247–1269.

Stewart, A. F.
, Lepack A. E., Fulton S. L., Safovich P., and Maze I.. 2023. “Histone H3 Dopaminylation in Nucleus Accumbens, but Not Medial Prefrontal Cortex, Contributes to Cocaine‐Seeking Following Prolonged Abstinence.” Molecular and Cellular Neuroscience
125: 103824–103824.

Stiller, C.‐O.
, Bergquist J., Beck O., Ekman R., and Brodin E.. 1996. “Local Administration of Morphine Decreases the Extracellular Level of GABA in the Periaqueductal Gray Matter of Freely Moving Rats.” Neuroscience Letters
209: 165–168.

Strickland, J. C.
, Gipson C. D., and Dunn K. E.. 2022. “Dopamine Supersensitivity: A Novel Hypothesis of Opioid‐Induced Neurobiological Mechanisms Underlying Opioid‐Stimulant Co‐Use and Opioid Relapse.” Frontiers in Psychiatry
13: 835816.

Svoboda, P.
, Teisinger J., Novotný J., et al. 2004. “Biochemistry of Transmembrane Signaling Mediated by Trimeric G Proteins.” Physiological Research
53, no. Suppl 1: S141–S152.

Szpręgiel, I.
, Wrońska D., Kmiecik M., Pałka S., and Kania B. F.. 2021. “Glutamic Acid Decarboxylase Concentration Changes in Response to Stress and Altered Availability of Glutamic Acid in Rabbit (Oryctolagus cuniculus) Brain Limbic Structures.” Animals: An Open Access Journal From MDPI
11: 455.

Taguchi, K.
, Hagiwara Y., Suzuki Y., and Kubo T.. 1993. “Effects of Morphine on Release of Acetylcholine in the Rat Striatum: An In Vivo Microdialysis Study.” Naunyn‐Schmiedeberg's Archives of Pharmacology
347: 9–13.

Tai, Y.‐H.
, Wang Y.‐H., Wang J.‐J., Tao P. L., Tung C. S., and Wong C. S.. 2006. “Amitriptyline Suppresses Neuroinflammation and Up‐Regulates Glutamate Transporters in Morphine‐Tolerant Rats.” Pain
124: 77–86.

Takahashi, K.
, Nakagawasai O., Sugawara M., et al. 2018. “Kappa Opioid Receptor Agonist Administration in Olfactory Bulbectomized Mice Restores Cognitive Impairment Through Cholinergic Neuron Activation.” Biological and Pharmaceutical Bulletin
41: 957–960.

Tao, R.
, and Auerbach S. B.. 2002. “Opioid Receptor Subtypes Differentially Modulate Serotonin Efflux in the Rat Central Nervous System.” Journal of Pharmacology and Experimental Therapeutics
303: 549–556.

Taylor, N. E.
, Long H., Pei J. Z., et al. 2019. “The Rostromedial Tegmental Nucleus: A Key Modulator of Pain and Opioid Analgesia.” Pain
160: 2524–2534.

Thompson, A. C.
, Zapata A., Justice J. B., Vaughan R. A., Sharpe L. G., and Shippenberg T. S.. 2000. “κ‐Opioid Receptor Activation Modifies Dopamine Uptake in the Nucleus Accumbens and Opposes the Effects of Cocaine.” Journal of Neuroscience
20: 9333–9340.

Ticku, M. K.
, and Huffman R. D.. 1980. “The Effects of Acute and Chronic Morphine Administration on GABA Receptor Binding.” European Journal of Pharmacology
68: 97–106.

Ting‐A‐Kee, R.
, and van der Kooy D.. 2012. “The Neurobiology of Opiate Motivation.” Cold Spring Harbor Perspectives in Medicine
2: a012096.

Tokuyama, S.
, Zhu H., Wakabayashi H., Feng Y. Z., and Ho I. K.. 1998. “The Role of Glutamate in the Locus Coeruleus During Opioid Withdrawal and Effects of H‐7, a Protein Kinase Inhibitor, on the Action of Glutamate in Rats.” Journal of Biomedical Science
5: 45–53.

Traynelis, S. F.
, Wollmuth L. P., McBain C. J., et al. 2010. “Glutamate Receptor Ion Channels: Structure, Regulation, and Function.” Pharmacological Reviews
62: 405–496.

Tsuji, M.
, Nakagawa Y., Ishibashi Y., et al. 1996. “Activation of Ventral Tegmental GABAB Receptors Inhibits Morphine‐Induced Place Preference in Rats.” European Journal of Pharmacology
313: 169–173.

Tzeng, S. F.
, and Ho I. K.. 1978. “Acute and Continuous Morphine Administration on the γ‐Aminobutyric Acid System In the Mouse.” Progress in Neuro‐Psychopharmacology
2: 55–64.

Ujcikova, H.
, Hejnova L., Eckhardt A., Roubalova L., Novotny J., and Svoboda P.. 2021. “Impact of Three‐Month Morphine Withdrawal on Rat Brain Cortex, Hippocampus, Striatum and Cerebellum: Proteomic and Phosphoproteomic Studies.” Neurochemistry International
144: 104975.

VanderWende, C.
, and Spoerlein M. T.. 1979. “Effect of Narcotics on Monoamine Oxidase Activity of Mouse Brain.” Research Communications in Chemical Pathology and Pharmacology
24: 103–113.

Vargas, R.

2018. “The GABAergic System: An Overview of Physiology, Physiopathology and Therapeutics.” International Journal of Clinical Pharmacology and Pharmacotherapy
3: 142.

Vaughan, C. W.
, Ingram S. L., Connor M. A., and Christie M. J.. 1997. “How Opioids Inhibit GABA‐Mediated Neurotransmission.” Nature
390: 611–614.

Verdejo‐García, A.
, Lubman D. I., Roffel K., et al. 2013. “Cingulate Biochemistry in Heroin Users on Substitution Pharmacotherapy.” Australian & New Zealand Journal of Psychiatry
47: 244–249.

Vlaskovska, M.
, Schramm M., Nylander I., et al. 1997. “Opioid Effects on 45 Ca 2+ Uptake and Glutamate Release in Rat Cerebral Cortex in Primary Culture.” Journal of Neurochemistry
68: 517–524.

Vogt, L. J.
, Sim‐Selley L. J., Childers S. R., Wiley R. G., and Vogt B. A.. 2001. “Colocalization of Mu‐Opioid Receptors and Activated G‐Proteins in Rat Cingulate Cortex.” Journal of Pharmacology and Experimental Therapeutics
299: 840–848.

Volkow, N.
, Benveniste H., and McLellan A. T.. 2018. “Use and Misuse of Opioids in Chronic Pain.” Annual Review of Medicine
69: 451–465.

Wahba, Z. Z.
, Oriaku E. T., and Soliman K. F. A.. 1987. “Brain Cholinergic Involvement During the Rapid Development of Tolerance to Morphine.” Pharmacology
34: 66–73.

Wang, D. D.
, Kriegstein A. R., and Ben‐Ari Y.. 2008. “GABA Regulates Stem Cell Proliferation Before Nervous System Formation.” Epilepsy Currents
8: 137–139.

Wang, G. J.
, Chung H. J., Schnuer J., et al. 1998. “Dihydrokainate‐Sensitive Neuronal Glutamate Transport Is Required for Protection of Rat Cortical Neurons in Culture Against Synaptically Released Glutamate.” European Journal of Neuroscience
10: 2523–2531.

Weschules, D. J.
, Bain K. T., and Richeimer S.. 2008. “Actual and Potential Drug Interactions Associated With Methadone.” Pain Medicine
9: 315–344.

Westerink, B. H.
, Kwint H. F., and de Vries J. B.. 1996. “The Pharmacology of Mesolimbic Dopamine Neurons: A Dual‐Probe Microdialysis Study in the Ventral Tegmental Area and Nucleus Accumbens of the Rat Brain.” Journal of Neuroscience: The Official Journal of the Society for Neuroscience
16: 2605–2611.

Whistler, J. L.
, Chuang H.‐h., Chu P., Jan L. Y., and von Zastrow M.. 1999. “Functional Dissociation of μ Opioid Receptor Signaling and Endocytosis.” Neuron
23: 737–746.

Williams, J. T.
, Christie M. J., and Manzoni O.. 2001. “Cellular and Synaptic Adaptations Mediating Opioid Dependence.” Physiological Reviews
81: 299–343.

Willins, D. L.
, and Meltzer H. Y.. 1998. “Serotonin 5‐HT2C Agonists Selectively Inhibit Morphine‐Induced Dopamine Efflux in the Nucleus Accumbens.” Brain Research
781: 291–299.

Wittert, G.
, Hope P., and Pyle D.. 1996. “Tissue Distribution of Opioid Receptor Gene Expression in the Rat.” Biochemical and Biophysical Research Communications
218: 877–881.

Xi, Z. X.

2002. “GABAergic Mechanisms of Opiate Reinforcement.” Alcohol and Alcoholism
37: 485–494.

Xi, Z. X.
, Fuller S. A., and Stein E. A.. 1998. “Dopamine Release in the Nucleus Accumbens During Heroin Self‐Administration Is Modulated by Kappa Opioid Receptors: An In Vivo Fast‐Cyclic Voltammetry Study.” Journal of Pharmacology and Experimental Therapeutics
284: 151–161.

Xie, Z.
, Li Z., Guo L., et al. 2007. “Regulator of G Protein Signaling Proteins Differentially Modulate Signaling of Mu and Delta Opioid Receptors.” European Journal of Pharmacology
565: 45–53.

Xu, N.‐J.
, Bao L., Fan H.‐P., et al. 2003. “Morphine Withdrawal Increases Glutamate Uptake and Surface Expression of Glutamate Transporter GLT1 at Hippocampal Synapses.” Journal of Neuroscience
23: 4775–4784.

Xu, X.‐J.
, Colpaert F., and Wiesenfeld‐Hallin Z.. 2003. “Opioid Hyperalgesia and Tolerance Versus 5‐HT1A Receptor‐Mediated Inverse Tolerance.” Trends in Pharmacological Sciences
24: 634–639.

Yamamoto, B. K.
, and Davy S.. 1992. “Dopaminergic Modulation of Glutamate Release in Striatum as Measured by Microdialysis.” Journal of Neurochemistry
58: 1736–1742.

Yücel, M.
, Lubman D. I., Harrison B. J., et al. 2007. “A Combined Spectroscopic and Functional MRI Investigation of the Dorsal Anterior Cingulate Region in Opiate Addiction.” Molecular Psychiatry
12: 691–702.

Zan, G.‐Y.
, Wang Y.‐J., Li X.‐P., et al. 2021. “Amygdalar κ‐Opioid Receptor‐Dependent Upregulating Glutamate Transporter 1 Mediates Depressive‐Like Behaviors of Opioid Abstinence.” Cell Reports
37: 109913.

Zhang, J.
, Ferguson S. S., Barak L. S., et al. 1998. “Role for G Protein‐Coupled Receptor Kinase in Agonist‐Specific Regulation of Mu‐Opioid Receptor Responsiveness.” Proceedings of the National Academy of Sciences of the United States of America
95: 7157–7162.

Zhao, Z.‐Q.
, Gao Y.‐J., Sun Y.‐G., Zhao C. S., Gereau R. W. IV, and Chen Z. F.. 2007. “Central Serotonergic Neurons Are Differentially Required for Opioid Analgesia but Not for Morphine Tolerance or Morphine Reward.” Proceedings of the National Academy of Sciences of the United States of America
104: 14519–14524.

Zhu, H.
, Rockhold R. W., and Ho I. K.. 1998. “The Role of Glutamate in Physical Dependence on Opioids.” Japanese Journal of Pharmacology
76: 1–14.

Zhu, Z.
, Bowman H. R., Baghdoyan H. A., and Lydic R.. 2008. “Morphine Increases Acetylcholine Release in the Trigeminal Nuclear Complex.” Sleep
31: 1629–1637.

Associated Data

Data Availability Statement

Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.