Nitric oxide as a mechanistic link between metabolic dysfunction and cognitive decline in type 2 diabetes.

Nitric oxide as a mechanistic link between metabolic dysfunction and cognitive decline in type 2 diabetes

Abstract

Type 2 diabetes mellitus (T2DM) is increasingly recognised as a driver of cerebrovascular dysfunction and cognitive decline, yet the molecular mechanisms underlying this association remain incompletely understood. Here, we review and support a key mechanistic role for nitric oxide (•NO), a pleiotropic diffusible messenger that integrates vascular, neuronal, and metabolic functions within the neurovascular unit (NVU). Under physiological conditions, •NO integrates vascular and metabolic responses by coupling blood supply to neuronal activity and fine-tuning cellular energy metabolism. In T2DM, chronic hyperglycemia and insulin resistance converge to reduce •NO bioavailability through multiple mechanisms: mitochondrial and NADPH oxidase-derived superoxide production, endothelial nitric oxide synthases (eNOS) uncoupling via BH4 oxidation, and impaired PI3K-Akt-dependent eNOS activation. The resulting redox imbalance shifts the cerebrovascular environment towards oxidant-mediated damage, compromising the functional and structural integrity of the NVU. This translates into impaired cerebral blood flow regulation and maladaptive remodelling of the cerebrovascular network, ultimately disrupting neurovascular coupling (NVC) and reducing regional cerebral perfusion. Together, impaired perfusion and disrupted NVC result in a sustained mismatch between energy supply and neuronal demand, particularly in metabolically vulnerable regions such as the hippocampus, ultimately leading to progressive cognitive impairment. In sum, the present review integrates current mechanistic evidence positioning •NO dysregulation as a central driver of neurovascular and metabolic dysfunction in T2DM, linking impaired cerebral perfusion, disrupted NVC, and structural vascular remodelling to cognitive decline.

Keywords: Cerebrovascular dysfunction, Neurovascular coupling, Nitric oxide, Type 2 diabetes mellitus

Introduction

Diabetes mellitus, nowadays, represents a major and growing public health challenge. Estimates calculate approximately 588 million individuals living with diabetes worldwide, significantly impacting both individual health and healthcare systems [1]. Among the different forms of the disease, type 2 diabetes mellitus (T2DM) accounts for nearly 90% of cases and is characterised by chronic hyperglycemia and insulin resistance (IR) [1]. Traditionally viewed as peripheral disorders, diabetes mellitus and IR are now recognised as systemic problems, affecting the brain as well [2,3]. Despite its small size, the brain is the most energy-consuming organ in the human body, accounting for ∼20% of the energy produced at rest, yet it has minimal energy reserves [4]. This high metabolic demand emphasises the critical role of cerebral blood flow (CBF) in meeting the brain’s energy requirements, essential for synaptic transmission, neuroplasticity, and overall neuronal performance [5].

The regulation of cerebral blood perfusion is choreographed by the neurovascular unit (NVU), a cooperative structural and functional assembly composed of neurons, astrocytes, endothelial cells (ECs), smooth muscle cells (SMCs), and pericytes [6]. Within this system, neurons initiate activity-dependent signals, astrocytes amplify and pass metabolic information, ECs regulate vascular tone and barrier integrity, and pericytes control capillary diameter and microvascular flow [7,8]. Through these coordinated interactions, neuronal metabolic demand is translated into vascular responses that adjust blood supply to neuronal activity, a process known as neurovascular coupling (NVC) [6]. While NVC represents a key component of cerebral microvascular regulation, specifically representing the rapid increase in regional CBF in response to neuronal activation, in a broader sense cerebral microvascular regulation also includes processes that maintain vascular homeostasis within the brain, including control of basal vascular tone and the maintenance of the blood–brain barrier (BBB) [4]. The NVU’s capacity to integrate neuronal, glial, and vascular signals is essential for maintaining metabolic and vascular stability in brain physiology. Among the molecular mediators taking part in these processes, nitric oxide (•NO) has been identified as a central player [9–11]. Due to its ability to rapidly diffuse across cellular membranes, •NO is in a unique position to coordinate the tight connection between the NVU components [12,13]. Accumulating evidence associates T2DM and IR with changes in CBF and cognitive function [14–16]. While the pathophysiology of diabetic cognitive decline is multifactorial, encompassing contributions from neuroinflammation, impaired glucose metabolism, and synaptic dysfunction, among others, in the present review we focus on •NO as a central mechanistic link through which T2DM and its hallmarks (hyperglycemia and IR) converge to disrupt cerebrovascular function, positioning •NO not as an exclusive cause but as a key integrative node within this complex disease landscape.

Nitric oxide signalling within the neurovascular unit

•NO is a pleiotropic free radical and highly diffusible paracrine messenger that operates through volume signalling to integrate neuronal, vascular, and metabolic processes within the brain [12]. Its unique physicochemical characteristics enable it to diffuse across biological membranes, making it an important regulator of cerebral homeostasis [17]. In the brain, •NO production involves a family of nitric oxide synthases (NOS) [18] that enzymatically convert Larginine into L-citrulline and •NO, requiring oxygen and cofactors, including NADPH and tetrahydrobiopterin (BH4). Among these enzymes, the endothelial nitric oxide synthase (eNOS) and neuronal nitric oxide synthase (nNOS) are constitutively expressed, contrasting with the inducible nitric oxide synthase, which is primarily regulated at the transcriptional level, being expressed in immune and other cell types in response to inflammatory stimuli [19]. nNOS is the most abundant isoform in the central nervous system, mainly expressed in neurons where it is often tethered to the N-methyl-D-aspartate (NMDA) glutamate receptor via the scaffolding protein postsynaptic density protein-95 [20]. This arrangement allows nNOS to translate synaptic activity into rapid •NO production. During synaptic activity, the glutamate released into the synaptic cleft binds to postsynaptic NMDA receptors, eliciting a rapid influx of Ca2+ activates nNOS via calmodulin, leading to •NO production [21]. Endothelial NOS is primarily expressed in ECs within the caveolae, forming a complex with caveolin-1 that inhibits its activity [22]. Its activity is modulated through several mechanisms, including shear stress, involving the enzyme’s dissociation from its interaction with caveolin-1, Ca2+/calmodulin binding or phosphorylation of specific serine residues (e.g. Ser1177) via PI3K-Akt pathway [22]. Recent evidence also indicates that group 1 metabotropic glutamate receptors (mGluR1/5) are expressed on human brain microvascular ECs and can act as sensors on neuronal activity, triggering Ca2+ dependent eNOS activation [23].

A prominent physiological role of •NO in the brain is the regulation of CBF (Figure 1). •NO synthesised by neurons and ECs readily diffuses to reach nearby vascular SMCs in arteriolar walls, where it activates soluble guanylate cyclase (sGC) and leads to the formation of cyclic guanosine monophosphate (cGMP). Elevated levels of cGMP activate protein kinase G, leading to smooth muscle relaxation and vasodilation, thereby regulating changes in vascular tone and cerebral blood perfusion [24]. In addition to regulating basal vascular tone, •NO also critically contributes to activity-dependent adjustments in CBF during neuronal activity [9]. Its production, tightly coupled to neuronal activity, together with its rapid diffusion, enables efficient matching of CBF to the metabolic demands imposed by neuronal activation. Accordingly, nNOS is recognised as a primary driver of the NVC, with its selective inhibition reducing the hemodynamic response by 64%, exceeding the effect of any other individual target [10]. While it has been classically implicated in the initial rapid phase, recent evidence suggests that this contribution is context-dependent and may not be universal across experimental conditions [25]. In turn, endothelial-derived •NO contributes to the regulation of vascular tone and to NVC by facilitating retrograde dilation from microvessels to upstream pial arteries [26]. In contrast, feedback signalling mechanisms contribute to the maintenance of vascular supply during prolonged neuronal activity [27,28].

Beyond its role in vascular regulation, •NO also acts as a metabolic regulator, helping to coordinate energy supply with neuronal activity. In astrocytes, •NO enhances glycolysis by increasing phosphofructokinase-1 activity via fructose-2,6-bisphosphate, promoting lactate production that is subsequently shuttled to neurons to sustain oxidative phosphorylation [29,30]. Simultaneously, •NO reversibly competes with O2 at cytochrome c oxidase, modulating mitochondrial respiration and influencing the balance between oxidative phosphorylation and glycolysis [31]. Through these actions, •NO coordinates neuronal energy demand with vascular supply and substrate utilisation. In addition, •NO can regulate protein function via S-nitrosation of proteins, a reversible reaction in which •NO modifies cysteine thiols to form S-nitrosothiols, thereby modulating the activity of different proteins involved in cellular signalling [17].

Taken together, these mechanisms illustrate the multifaceted role of •NO in the brain, acting simultaneously as a regulator of vascular tone, cellular metabolism and intracellular signalling.

Nitric oxide dysregulation and functional cerebrovascular impairment

In the diabetic brain, metabolic stress arising from chronic hyperglycemia and redox imbalance disrupts •NO homeostasis, shifting it from physiological regulation to a state of chronic distress (Figure 2). This transformation may be driven by a series of mechanistic pathways that reduce •NO bioavailability and generate reactive oxygen species (ROS), ultimately compromising the integrity of the NVU and leading to neuronal deficits. Elevated intracellular glucose in neural and vascular cells enhances mitochondrial superoxide (O2•−) production by increasing electron flux through the electron transport chain, leading to mitochondrial hyperpolarisation and electron leak to molecular oxygen [32]. In addition to mitochondria, the NADPH-oxidase (NOX) family of enzymes is a major source of O2•−, particularly at the vascular level. Persistent hyperglycemia, through increased diacylglycerol formation, and advanced glycation endproducts (AGEs), through RAGE signalling, trigger protein kinase C activation that, in turn, promotes NOX activation by phosphorylating its regulatory subunits, leading to O2•− production [32]. The overproduction of O2•− leads to the rapid inactivation of •NO because these two free radicals rapidly react at near diffusion-controlled rates to form peroxynitrite (ONOO⁻), a highly reactive molecule that redirects •NO bioactivity towards oxidative and nitrative chemistries [33]. Peroxynitrite promotes irreversible tyrosine nitration of proteins, forming 3-nitrotyrosine, a widely used marker of nitroxidative stress that is elevated in the brains of diabetic animals [34]. In parallel, ONOO⁻ and its reactive intermediaries (•OH and •NO2) promote oxidative damage to DNA, lipids, and proteins that compromises cellular integrity. For instance, ONOO⁻-mediated nitration of cytoskeletal proteins such as actin is particularly detrimental in the cerebrovasculature, as they contribute to loss of myogenic tone [35]. •NO bioavailability is further compromised by reduced •NO synthesis due to NOS uncoupling. Excessive ROS impairs eNOS activity by oxidising BH4, an essential cofactor for eNOS coupling. The reduced availability of BH4 shifts eNOS function from •NO production to the generation of O2•−, thereby establishing a feed-forward cycle of oxidative distress that further limits •NO bioavailability and exacerbates nitroxidative damage [36]. This redox-dependent shift in NOS function also helps to explain why global pharmacological manipulation of the NOS/NO pathway may generate outcomes difficult to interpret. For instance, in STZ-induced diabetic rats, both L-NAME and L-arginine impaired active avoidance performance, despite complex effects on nitrite levels, lipid peroxidation, body weight, and glucose [37]. These findings reinforce the concept that therapeutic strategies should not aim simply to increase or inhibit global NOS activity, but rather to restore NOS coupling, redox balance, and compartmentalised •NO bioavailability.

Not only hyperglycemia, but also IR contributes to decreased •NO bioavailability. In an insulin-sensitive state, insulin binds its receptor, activating the PI3K-Akt pathway leading to eNOS phosphorylation and activation. This pathway plays an important role in maintaining endothelial function and vascular homeostasis. However, IR in T2DM down-regulates the PI3K pathway, thereby reducing eNOS phosphorylation and •NO production [38]. In parallel, insulin signalling through the mitogen-activated protein kinase (MAPK) pathway remains preserved, leading to the production of endothelin-1, a potent vasoconstrictor. This imbalance between reduced PI3K-Akt signalling and enhanced MAPK activity tilts the vascular environment from •NO-mediated vasodilation towards increased vasoconstrictive signalling, contributing to compromised cerebral perfusion in IR [36]. A further endogenous mechanism contributing to reduced •NO bioavailability in T2DM is the accumulation of asymmetric dimethylarginine (ADMA), a methylated derivative of L-arginine that competitively inhibits all NOS isoforms. Under physiological conditions, ADMA is primarily catabolised by dimethylarginine dimethylaminohydrolase (DDAH), which exists as two isoforms: DDAH1 is mainly associated with tissues expressing nNOS, including the brain, whereas DDAH2 is enriched in tissues expressing eNOS, including ECs. Importantly, DDAH activity is highly sensitive to oxidative inactivation, as excessive ROS can oxidise the catalytic cysteine residue within its active site, thereby impairing ADMA degradation [39]. Consistently, circulating ADMA levels have been reported to be increased in patients with T2DM [40,41]

Beyond its vascular effects, reduced •NO bioavailability in T2DM also compromises its metabolic support functions within the NVU. Under physiological conditions, eNOS-produced •NO stabilises HIF-1α in astrocytes, boosting glycolytic enzymes and the lactate transporter MCT-4, which promotes lactate production for neuronal use [42]. This regulation occurs at nanomolar levels with rapid, reversible signalling [43]. Therefore, even modest decreases in eNOS activity can impair astrocytic glycolysis and disrupt the lactate shuttle between astrocytes and neurons. At the same time, the nitroxidative environment shifts mitochondrial regulation from reversible •NO effects on cytochrome c oxidase to irreversible damage caused by peroxynitrite. While astrocytes adapt by increasing glycolysis, neurons, which cannot do so, face bioenergetic crises, increasing neuronal vulnerability beyond vascular issues alone [44].

Structural remodelling of vasculature in the diabetic brain

The vascular dysfunction in T2DM extends beyond functional impairment, being further associated with structural alterations. One of the earliest and most prominent features of this structural deterioration is microvascular rarefaction, characterised by reduced vascular density and loss of functional capillaries [15,45]. Advanced 3D imaging demonstrated a significant reduction in the percentage area of perfused vessels across different brain regions [45]. Parallel comparisons in rodent models reveal that T2DM triggers a measurable decrease in vascular density in critical regions, such as the striatum and hippocampus, and, coupled with this reduced density, they showed that there was also reduced CBF [15]. In diabetic animals, hippocampal vascular density has been reported to decrease by approximately 8.73%, reflecting progressive microvascular loss [15]. These microstructural alterations might create localised perfusion dysregulation that contributes to impaired NVC [46,47].

In addition to microvascular rarefaction, diabetes is associated with alterations in vascular architecture, including increased tortuosity and disrupted branching patterns. In the retina, often viewed as a ‘window to the brain’ due to shared anatomical and physiological features [48], numerous studies consistently demonstrate increased vessel tortuosity and altered branching geometry as early features of diabetic microangiopathy [49–51]. Although evidence in the brain is comparatively limited, similar alterations have been described in the cerebral microvasculature. Studies in diabetic models reveal that microvascular hypoperfusion is detectable as early as four weeks of age. In adult diabetic mice, newly formed vessels exhibit dysfunctional loop structures rather than typical terminal points, a maladaptive pattern that contributes to poor tissue formation [52]. This architectural decline is linked to a profound remodelling of the gliovascular unit, characterised by pericyte loss and hyperactive astrogliosis with finer processes wrapping around cerebral vessels [53]. Molecular analysis reveals that these structural shifts are driven by reduced PPARy expression and activation of the NLRP3 inflammasome, which elevates inflammatory cytokines such as IL-1β and TNF-α [52,53]. Furthermore, BBB dysfunction is evidenced by either overt IgG leakage into the brain parenchyma or the specific accumulation of IgG on the luminal side of microvascular walls [53,54]. Importantly, early pharmacological interventions with glycemic control agents such as metformin or pioglitazone can mitigate this dysfunctional remodelling and suppress the underlying chronic neuroinflammation [53,54].

While vascular remodelling is a recognised feature of T2D, the molecular mechanisms driving these alterations are not fully elucidated. However, a prominent role has been attributed to redox-dependent alterations of the extracellular matrix (ECM). Alterations in •NO bioavailability and excessive production of RONS can disrupt the balance between ECM synthesis and degradation, promoting a shift towards pathological ECM accumulation. Hyperglycemia and redox imbalance activate transforming growth factor-β (TGF-β) signalling which leads to SMAD2/3-dependent transcription of ECM components, including collagen IV, laminin, and fibronectin, contributing to basement membrane thickening [55]. In parallel, oxidative modifications and glycation of matrix proteins enhance cross-linking, further increasing matrix stiffness and resistance to degradation [56]. Concomitantly, dysregulation of matrix metalloproteinases (MMPs), particularly MMP-2 and MMP-9, contributes to structural instability of microvasculature by degrading key components of the vascular basement membrane. These enzymes, present in ECs, astrocytes, and microglia, are tightly regulated by redox signalling mechanisms, both at the transcription and post-translational levels. Reflecting its dual role under different redox conditions, •NO thought to regulate MMPs in a biphasic manner: At physiological settings, •NO inhibits MMP-9 expression and activity via cGMPdependent pathways, while under oxidative stress conditions, it contributes to increased MMPs activation via ONOO−-induced oxidative modification of the cysteine residue within the ‘cysteine switch’ of pro-MMPs [57,58]. In addition to enhancing MMP activity, nitroxidative stress also promote the up-regulation of MMPs’ expression via redox-sensitive pathways, including NF-κB and AP-1 activation, and impairs endogenous regulatory mechanisms via the tissue inhibitors of metalloproteinases oxidative-inactivation [58]. The combined effects of increased MMP expression, direct enzymatic activation, and impaired inhibition drive uncontrolled ECM remodelling, leading to microvascular instability. In parallel with TGF-β-mediated matrix deposition, this results in maladaptive remodelling characterised by concurrent matrix accumulation and degradation, ultimately disrupting vascular architecture and neurovascular function.

A further process to be considered is that angiogenesis and neovascularisation are also compromised. While T2DM can increase the risk of cerebral ischemia and hypoperfusion and trigger the up-regulation of vascular endothelial factor to initiate capillary formation, the resulting neovascularisation is mainly pathological and aberrant [59]. The newly formed vessels are poorly formed, structurally fragile, and prone to premature regression, failing to restore perfusion to hypoxic areas. These aberrant vascular patterns may reflect impaired •NO-dependent regulation of angiogenesis and vessel maturation, as •NO is known to modulate endothelial migration, branching, and stabilisation of newly formed vessels [60].

The loss and dysfunction of pericytes is another key driver of the structural failure in diabetic cerebrovasculature. These cells are essential in the NVU, positioned between ECs, astrocytes, and neurons; they coordinate and process signals from their neighbouring cells to generate diverse functional responses, including regulation of the BBB permeability, angiogenesis, and capillary hemodynamic responses [61]. Pericytes are particularly vulnerable to hyperglycemia-induced redox stress, which drives apoptosis via ROS production and AGE-RAGE signalling [62]. This leads to reduced pericyte coverage of cerebral microvessels, while surviving cells exhibit functional impairment [63]. As contractile regulators of capillary diameter, pericytes are essential for maintaining microvascular tone; however, they may induce contractile states under metabolic and nitrosative stress. Persistent capillary constriction limits effective reperfusion and impairs microvascular flow regulation [64]. In addition, emerging evidence indicates that •NO may modulate pericyte-endothelial communication through extracellular vesicle-mediated signalling, thereby influencing vascular remodelling and revascularisation processes [65]. Although demonstrated in ischemic contexts, these findings support a broader role for •NO in coordinating cellular interactions within the NVU.

In sum, structural remodelling of the diabetic brain vasculature can be grouped into key processes: microvascular rarefaction, marked by reduced capillary density and perfusion; architectural and gliovascular alterations, including vessel tortuosity, abnormal branching, inflammation, and BBB disruption; ECM and redox-driven remodelling, where •NO imbalance and oxidative distress promote both matrix accumulation and degradation; impaired angiogenesis, leading to fragile, dysfunctional vessels; and pericyte loss and dysfunction, which compromises vascular stability and flow regulation. Together, these changes drive neurovascular impairment in type 2 diabetes.

From cerebrovascular dysfunction to cognitive decline

The structural and molecular alterations discussed above, and compiled in Figure 2, converge functionally at the level of impaired NVC and cerebral perfusion, which ultimately translates into cognitive decline. In patients with T2DM, the hemodynamic response function becomes significantly slower, characterised by a higher peak latency and lower peak amplitude across all brain regions. Detailed analysis reveals a pronounced initial dip, suggesting that the brain’s initial avidity for oxygen is not compensated for by a timely blood supply, and a less prominent but longer-lasting undershoot [66]. In clinical settings, this failure is often detected by transcranial Doppler as a reduction in the ‘natural frequency’, a parameter that reflects the oscillatory properties and tone of the cerebrovascular system. Reduced natural frequency serves as a sensitive early marker for NVC dysfunction, occurring even in diabetic and hypertensive subjects without symptomatic cerebrovascular disease [67].

Epidemiological studies consistently demonstrated that patients with diabetes have a significantly higher risk of developing mild cognitive impairment and dementia [68]. For instance, individuals with T2DM are 60% more prone to develop Alzheimer’s disease in comparison with non-diabetic subjects [69]. As previously stated, although the mechanisms linking diabetes and neurodegeneration are multifactorial, increasing evidence suggests that cerebrovascular dysfunction emerges as a central factor and implicates compromised •NO signalling as a key driver. When cerebral perfusion becomes impaired, neuronal activations are no longer matched by adequate increases in local perfusion, resulting in a quantitative and temporal neurovascular uncoupling that can lead to chronic energetic stress within neuronal circuits. In T2DM, this stress is compounded by a 31% reduction in the maximum glucose transport rate and impaired glycogen metabolism, which hampers the availability of energy for synaptic plasticity [70]. Certain brain regions appear particularly vulnerable to diabetes-associated cerebrovascular dysfunction, reflecting their high metabolic demand and tight dependence on finely tuned NVC [71,72]. In a rodent model of T2DM, impaired hippocampal NVC has been demonstrated as an early event, occurring in close association with disrupted glutamate-evoked •NO dynamics and deficits in spatial learning and memory performance [73]. In the same animal model, it was also reported marked cerebrovascular remodelling, characterised by increased cerebral vascular density, together with a higher proportion of immature/non-perfused microvessels [74].Consistent with these findings, reduced regional CBF in the hippocampus has been associated with poorer memory performance in humans [75]. Additionally, impaired CBF in the cerebellar network has been linked to both gait alterations and executive processing deficits [76]. Importantly, this vulnerability is not restricted to individual regions but involves large-scale brain networks. Hypoperfusion within the default mode network, particularly in the precuneus and posterior cingulate cortex, is strongly associated with deficits in executive function, attention, and information processing speed [76]. Persistent neurovascular dysfunction ultimately contributes to structural brain damage, manifested as cerebral small vessel disease (CSVD). Imaging markers, such as white matter hyperintensities, lacunes, and enlarged perivascular spaces in the basal ganglia, are significantly more prevalent in patients with T2DM [77]. Research indicates that a moderate or severe total CSVD burden is a risk factor for mild cognitive impairment. These structural abnormalities reflect end-stage parenchymal damage stemming from endothelial dysfunction and increased BBB. This increased permeability allows plasma constituents to leak into the perivascular space, which may directly damage neurons, trigger neuroinflammation, and disrupt the neural connectivity necessary for cognitive stability [77,78]. Ultimately, the progression from cerebrovascular dysfunction to cognitive decline follows the ‘ticking clock hypothesis’, suggesting that microvascular disease processes often begin during the prediabetic stage, long before the clinical onset of diabetes [78]. This cumulative evidence supports a model in which early functional failures gradually progress to manifest structural lesions, ultimately resulting in clear deficits in memory and executive control.

Conclusion and future perspectives

T2DM is firmly recognised as a driver of cerebrovascular dysfunction and a risk factor for the development of cognitive decline. Within this framework, •NO emerges as a key integrator of neuronal signalling, vascular tone, and metabolic processes, required to sustain cerebral blood perfusion and brain homeostasis.

Disruption of •NO signalling constitutes a critical mechanistic link between metabolic and cerebrovascular dysfunction. Persistent hyperglycemia, oxidative stress, and IR converge to limit •NO bioavailability, leading to structural and functional impairment of the cerebral vasculature compromising cerebral perfusion and increasing the vulnerability of metabolically demanding brain regions, such as the hippocampus, thereby contributing to the cognitive deficits frequently observed in T2DM patients.

Although progress has been made, multiple crucial questions remain unanswered. One important challenge is to clarify the temporal dynamics of •NO-mediated dysfunction during the disease progression. Emerging longitudinal evidence suggests that neurovascular impairment develops early in T2DM patients [16] and animal models [73], yet the point at which these alterations become irreversible remains poorly defined. In parallel, increasing evidence indicates that neurovascular impairment may manifest heterogeneously across brain regions [79], suggesting the existence of tissue-specific mechanisms that determine regional vulnerability. Understanding how differences in endothelial signalling, oxidative distress responses, and local metabolic regulation shape the susceptibility of specific vascular networks will be essential for explaining selective neurovascular dysfunction and its contribution to diabetes-associated cognitive decline. Beyond mechanistic understanding, the framework outlined in the present review has direct therapeutic implications. Strategies aimed at restoring •NO bioavailability represent a rational approach to preserving neurovascular function and preventing cognitive decline in T2DM. Among non-pharmacological interventions, inorganic dietary nitrate supplementation has attracted considerable attention, as it can replenish •NO pools through the nitrate-nitrite-NO pathway, even under conditions of NOS dysfunction [80,81]. Phosphodiesterase type 5 (PDE5) inhibitors, such as sildenafil, represent a complementary pharmacological approach that amplifies •NO-mediated signalling downstream of its synthesis by preventing cGMP degradation. Experimental evidence suggests that PDE5 inhibition can attenuate neuroinflammation [82] and cognitive dysfunction [83] and improve cerebrovascular function [84]. Additional promising strategies include restoring eNOS coupling through BH4 supplementation and antioxidant approaches aimed at limiting superoxide-mediated •NO inactivation, particularly NOX inhibition and mitochondria-targeted antioxidants [85–87]. Addressing these unresolved issues may uncover new strategies to preserve •NO signalling and sustain neurovascular homeostasis in metabolic disease.

Perspectives

Cerebrovascular dysfunction is a key driver of cognitive decline in T2DM, shifting the focus towards neurovascular mechanisms. •NO sits at the interface of vascular, neuronal, and metabolic regulation within the neurovascular unit.

T2DM reduces •NO bioavailability and promotes nitro-oxidative stress, impairing NVC, cerebral blood flow, and metabolic support. This leads to a mismatch between energy supply and demand, compounded by vascular remodelling and culminating in cognitive decline.

A major challenge is to determine whether restoring •NO bioavailability can effectively reverse early neurovascular dysfunction in T2DM. This will require targeted and stage-specific therapeutic approaches.

Abbreviations

AGEsadvanced glycation endproducts

ADMAasymmetric dimethylarginine

BBBblood–brain barrier

BH4tetrahydrobiopterin

CBFcerebral blood flow

cGMPcyclic guanosine monophosphate

CSVDcerebral small vessel disease

DDAHdimethylarginine dimethylaminohydrolase

ECsendothelial cells

ECMextracellular matrix

eNOSendothelial nitric oxide synthase

IgGimmunoglobulin G

IRinsulin resistance

L-NAMENω-nitro-L-arginine methyl ester

MAPKmitogen-activated protein kinase

MMPsmatrix metalloproteinases

NF-κBnuclear factor kappa B

NMDAN-methyl-D-aspartate

nNOSneuronal nitric oxide synthase

•NOnitric oxide

NOXNADPH oxidase

NOSnitric oxide synthases

NVCneurovascular coupling

NVUneurovascular unit

PDE5phosphodiesterase type 5

RAGEreceptor for advanced glycation end products

ROSreactive oxygen species

sGCsoluble guanylate cyclase

SMCssmooth muscle cells

TGF-βtransforming growth factor-β

T2DMtype 2 diabetes mellitus

Contributor Information

João Laranjinha, Email: laranjin@ci.uc.pt.

Cátia F. Lourenço, Email: cflmarques@uc.pt.

Competing Interests

The authors declare that there are no competing interests associated with the manuscript.

Funding

This work was also financed by the European Regional Development Fund (ERDF), through the Centro 2020 Regional Operational Programme under project 2022.05454.PTDC (https://doi.org/10.54499/2022.05454.PTDC); through the COMPETE 2030—Operational Programme for Competitiveness and Internationalisation under the project COMPETE2030FEDER-00669500-15619; and through the COMPETE 2020—Operational Programme for Competitiveness and Internationalisation and Portuguese national funds via FCT—Fundação para a Ciência e a Tecnologia, under the projects UID/04539/2025, UID/PRR/04539/2025 and LA/P/0058/2020 (https://doi.org/10.54499/LA/P/0058/2020).

Open Access

This article has been published open access under our Subscribe to Open programme, made possible through the support of our subscribing institutions, learn more here: https://portlandpress.com/pages/open_access_options_and_prices#conditional.

CRediT Author Contribution

Ana L. Marçal: Writing—original draft. João Laranjinha: Conceptualization, Writing & critical review. Cátia F. Lourenço: Conceptualization, Writing & critical review. All authors have read and agreed to the published version of the manuscript.

References

1.(International Diabetes Federation (2025) IDF Diabetes Atlas. 11th edition
Brussels, Belgium:
Available from: https://diabetesatlas.org/

2.de Galan B.E. (2024) Diabetes and brain disorders, a new role for insulin?
Neurosci. Biobehav. Rev.
163, 105775.
10.1016/j.neubiorev.2024.105775

3.Heni M. (2024) The insulin resistant brain: impact on whole-body metabolism and body fat distribution. Diabetologia
67, 1181–1191

10.1007/s00125-024-06104-9

4.Claassen J.A.H.R., Thijssen D.H.J., Panerai R.B. and Faraci F.M. (2021) Regulation of cerebral blood flowin humans: Physiology and clinical implications of autoregulation. Physiol. Rev.
101, 1487–1559

10.1152/physrev.00022.2020

5.Mokhber N., Shariatzadeh A., Avan A., Saber H., Babaei G.S., Chaimowitz G.et al. (2021) Cerebral blood flow changes during aging process and in cognitive disorders: A review. Neuroradiol. J.
34, 300–307

10.1177/19714009211002778

6.Iadecola C. (2017) The neurovascular unit coming of age: a journey through neurovascular coupling in health and disease. Neuron
96, 17–42

10.1016/j.neuron.2017.07.030

7.Gong Y., Wu M., Huang Y., He X., Yuan J. and Dang B. (2025) Research developments in the neurovascular unit and the blood-brain barrier (Review). Biomed. Rep.
22, 1–14

10.3892/br.2025.1966

8.Bonkowski D., Katyshev V., Balabanov R.D., Borisov A. and Dore-Duffy P. (2011) The CNS microvascular pericyte: pericyte-astrocyte crosstalk in the regulation of tissue survival. Fluids Barriers CNS
8, 8.
10.1186/2045-8118-8-8

9.Lourenço C.F., Santos R.M., Barbosa R.M., Cadenas E., Radi R. and Laranjinha J. (2014) Neurovascular coupling in hippocampus is mediated via diffusion by neuronal-derived nitric oxide. Free Radic. Biol. Med.
73, 421–429

10.1016/j.freeradbiomed.2014.05.021

10.Hosford P.S. and Gourine A.V. (2019) What is the key mediator of the neurovascular coupling response?
Neurosci. Biobehav. Rev.
96, 174–181

10.1016/j.neubiorev.2018.11.011

11.O'Gallagher K., Rosentreter R.E., Elaine Soriano J., Roomi A., Saleem S., Lam T.et al. (2022) The effect of a neuronal nitric oxide synthase inhibitor on neurovascular regulation in humans. Circ. Res.
131, 952–961

10.1161/CIRCRESAHA.122.321631

12.Laranjinha J., Nunes C., Ledo A., Lourenço C., Rocha B. and Barbosa R.M. (2021) The peculiar facets of nitric oxide as a cellular messenger: from disease-associated signaling to the regulation of brain bioenergetics and neurovascular coupling. Neurochem. Res.
46, 64–76

10.1007/s11064-020-03015-0

13.Garthwaite J. (2019) NO as a multimodal transmitter in the brain: discovery and current status. Br. J. Pharmacol.
176, 197–211

10.1111/bph.14532

14.Garcia-Serrano A.M. and Duarte J.M.N. (2020) Brain metabolism alterations in type 2 diabetes: what did we learn from diet-induced diabetes models?
Front. Neurosci.
14, 219.
10.3389/fnins.2020.00229

15.Ren X., Zhang G., Zhou B., Gu W., Jiang X., Liao H.et al. (2025) Revealing cerebral microvascular changes in diabetic rodents with ultrasound localization microscopy. Diabetes
74, 1469–1479

10.2337/db25-0007

16.Zhang Y., Zhang X., Ma G., Qin W., Yang J., Lin J.et al. (2021) Neurovascular coupling alterations in type 2 diabetes: a 5-year longitudinal MRI study. BMJ Open Diabetes Res. Care
9, e001433
10.1136/bmjdrc-2020-001433

17.Lundberg J.O. and Weitzberg E. (2022) Nitric oxide signaling in health and disease. Cell
185, 2853–2878
10.1016/j.cell.2022.06.010

18.Chachlaki K. and Prevot V. (2020) Nitric oxide signalling in the brain and its control of bodily functions. Br. J. Pharmacol.
177, 5437–5458

10.1111/bph.14800

19.Daff S. (2010) NO synthase: structures and mechanisms. Nitric Oxide
23, 1–11

10.1016/j.niox.2010.03.001

20.Ghasemi A., Jeddi S. and Kashfi K. (2025) Brain glucose metabolism: Role of nitric oxide. Biochem. Pharmacol.
232, 116728.
10.1016/j.bcp.2024.116728

21.Garthwaite J. (2008) Concepts of neural nitric oxide-mediated transmission. Eur. J. Neurosci.
27, 2783–2802

10.1111/j.1460-9568.2008.06285.x

22.Förstermann U. and Sessa W.C. (2012) Nitric oxide synthases: Regulation and function. Eur. Heart J.
33, 829–37

10.1093/eurheartj/ehr304

23.Negri S., Faris P., Pellavio G., Botta L., Orgiu M., Forcaia G.et al. (2020) Group 1 metabotropic glutamate receptors trigger glutamate-induced intracellular Ca2+ signals and nitric oxide release in human brain microvascular endothelial cells. Cell. Mol. Life Sci.
77, 2235–2253

10.1007/s00018-019-03284-1

24.Lourenço C.F., Ledo A., Barbosa R.M. and Laranjinha J. (2017) Neurovascular-neuroenergetic coupling axis in the brain: master regulation by nitric oxide and consequences in aging and neurodegeneration. Free Radic. Biol. Med.
108, 668–682

10.1016/j.freeradbiomed.2017.04.026

25.Lee L., Boorman L.W., Glendenning E., Shen C., Berwick J. and Howarth C. (2025) Nitric oxide is not responsible for initial sensory-induced neurovascular coupling response in the barrel cortex of lightly anesthetized mice. Neurophotonics
12, S22802
10.1117/1.NPh.12.S2.S22802

26.Chen B.R., Kozberg M.G., Bouchard M.B., Shaik M.A. and Hillman E.M.C. (2014) A critical role for the vascular endothelium in functional neurovascular coupling in the brain. J. Am. Heart Assoc.
3, e000787
10.1161/JAHA.114.000787

27.Hopper R.A. and Garthwaite J. (2006) Tonic and phasic nitric oxide signals in hippocampal long-term potentiation. J. Neurosci.
26, 11513–11521

10.1523/JNEUROSCI.2259-06.2006

28.Vo T.T., Jung W.B., Jin T., Im G.H., Lee S. and Kim S.G. (2025) Somatostatin-expressing interneurons induce early NO-driven and late specific astrocyte-mediated vasodilation. Nat. Commun.
16, 6606.
10.1038/s41467-025-61771-5

29.Tran C.H.T., Peringod G. and Gordon G.R. (2018) Astrocytes integrate behavioral state and vascular signals during functional hyperemia. Neuron
100, 1133.e3–1148.e3

10.1016/j.neuron.2018.09.045

30.Almeida A., Moncada S. and Bolaños J.P. (2004) Nitric oxide switches on glycolysis through the AMP protein kinase and 6-phosphofructo-2-kinase pathway. Nat. Cell Biol.
6, 45–51

10.1038/ncb1080

31.Cooper C.E. and Giulivi C. (2007) Nitric oxide regulation of mitochondrial oxygen consumption II: molecular mechanism and tissue physiology. Am. J. Physiol. Cell Physiol.
292, C1993–C2003

10.1152/ajpcell.00310.2006

32.Rastogi R., Geng X., Li F. and Ding Y. (2017) NOX activation by subunit interaction and underlying mechanisms in disease. Front. Cell Neurosci.
10, 301.
10.3389/fncel.2016.00301

33.Piacenza L., Zeida A., Trujillo M. and Radi R. (2022) The superoxide radical switch in the biology of nitric oxide and peroxynitrite. Physiol. Rev.
102, 1881–1906

10.1152/physrev.00005.2022

34.Aggarwal A., Yadav A., Saini N. and Sandhir R. (2022) S-nitrosoglutathione alleviates hyperglycemia-induced neurobehavioral deficits involving nitro-oxidative stress and aberrant monaminergic system. Nitric Oxide
122-123, 35–44

10.1016/j.niox.2022.03.001

35.Kelly-Cobbs A.I., Prakash R., Coucha M., Knight R.A., Li W., Ogbi S.N.et al. (2012) Cerebral myogenic reactivity and blood flow in type 2 diabetic rats: Role of peroxynitrite in hypoxia-mediated loss of myogenic tone. J. Pharmacol. Exp. Ther.
342, 407–415

10.1124/jpet.111.191296

36.Takeda Y., Matoba K., Sekiguchi K., Nagai Y., Yokota T., Utsunomiya K.et al. (2020) Endothelial dysfunction in diabetes. Biomedicines
8, 182
10.3390/biomedicines8070182

37.Küçükatay V., Hacioğlu G., Ozkaya G., Ağar A. and Yargiçoğlu P. (2009) The effect of diabetes mellitus on active avoidance learning in rats: the role of nitric oxide. Med. Sci. Monit.
15, BR88–BR93

38.Barone E., Di Domenico F., Perluigi M. and Butterfield D.A. (2021) The interplay among oxidative stress, brain insulin resistance and AMPK dysfunction contribute to neurodegeneration in type 2 diabetes and Alzheimer disease. Free Radic. Biol. Med.
176, 16–33

10.1016/j.freeradbiomed.2021.09.006

39.Palm F., Onozato M.L., Luo Z. and Wilcox C.S. (2007) Dimethylarginine dimethylaminohydrolase (DDAH): expression, regulation, and function in the cardiovascular and renal systems. Am. J. Physiol. Heart Circ. Physiol.
293, H3227–H3245

10.1152/ajpheart.00998.2007

40.Guo X., Xing Y. and Jin W. (2023) Role of ADMA in the pathogenesis of microvascular complications in type 2 diabetes mellitus. Front. Endocrinol. (Lausanne)
14, 1183586.
10.3389/fendo.2023.1183586

41.Celik M., Cerrah S., Arabul M. and Akalin A. (2014) Relation of asymmetric dimethylarginine levels to macrovascular disease and inflammation markers in type 2 diabetic patients. J. Diabetes Res.
2014, 139215.
10.1155/2014/139215

42.Katusic Z.S., d'Uscio L.V. and He T. (2023) Emerging roles of endothelial nitric oxide in preservation of cognitive health. Stroke
54, 686–696

10.1161/STROKEAHA.122.041444

43.San Martín A., Arce-Molina R., Galaz A., Pérez-Guerra G. and Barros L.F. (2017) Nanomolar nitric oxide concentrations quickly and reversibly modulate astrocytic energy metabolism. J. Biol. Chem.
292, 9432–9438

10.1074/jbc.M117.777243

44.Bolanos J.P.& Heales S.J.R. (2010) Persistent mitochondrial damage by nitric oxide and its derivatives: neuropathological implications. Front. Neuroenergetics
2, 1.
10.3389/neuro.14.001.2010

45.Özkan E., Çetin-Taş Y., Şekerdağ E., Yiğit B., Shomalizadeh N., Sapancı S.et al. (2023) Hyperglycemia with or without insulin resistance triggers different structural changes in brain microcirculation and perivascular matrix. Metab. Brain Dis.
38, 307–321

10.1007/s11011-022-01100-7

46.Shao J.W., Wang J.D., He Q., Yang Y., Zou Y.Y., Su W.et al. (2021) Three-dimensional-arterial spin labeling perfusion correlation with diabetes-associated cognitive dysfunction and vascular endothelial growth factor in type 2 diabetes mellitus rat. World J. Diabetes
12, 499–513

10.4239/wjd.v12.i4.499

47.Luo W., Wang J., Chen M., Zhou S., Deng D., Liu F.et al. (2022) Alterations of cerebral blood flow and its connectivity in olfactory-related brain regions of type 2 diabetes mellitus patients. Front. Neurosci.
16, 904468.
10.3389/fnins.2022.904468

48.Kashani A.H., Asanad S., Chan J.W., Singer M.B., Zhang J., Sharifi M.et al. (2021) Past, present and future role of retinal imaging in neurodegenerative disease. Prog. Retin. Eye Res.
83, 100938.
10.1016/j.preteyeres.2020.100938

49.Fayed A.E., Abdelbaki A.M., El Zawahry O.M. and Fawzi A.A. (2019) Optical coherence tomography angiography reveals progressive worsening of retinal vascular geometry in diabetic retinopathy and improved geometry after panretinal photocoagulation. PloS One
14, e0226629.
10.1371/journal.pone.0226629

50.Sasongko M.B., Wong T.Y., Nguyen T.T., Cheung C.Y., Shaw J.E. and Wang J.J. (2011) Retinal vascular tortuosity in persons with diabetes and diabetic retinopathy. Diabetologia
54, 2409–2416

10.1007/s00125-011-2200-y

51.Chen Q., Ma Q., Wu C., Tan F., Chen F., Wu Q.et al. (2017) Macular vascular fractal dimension in the deep capillary layer as an early indicator of microvascular loss for retinopathy in type 2 diabetic patients. Invest. Ophthalmol. Vis. Sci.
58, 3785
10.1167/iovs.17-21461

52.Liu Y., Chen D., Smith A., Ye Q., Gao Y. and Zhang W. (2021) Three-dimensional remodeling of functional cerebrovascular architecture and gliovascular unit in leptin receptor-deficient mice. J. Cereb. Blood Flow Metab.
41, 1547–1562

10.1177/0271678X211006596

53.Guo Y., Zuo W., Yin L., Gu T., Wang S., Fang Z.et al. (2022) Pioglitazone attenuates ischaemic stroke aggravation by blocking PPARγ reduction and inhibiting chronic inflammation in diabetic mice. Eur. J. Neurosci.
56, 4948–4961

10.1111/ejn.15789

54.Prakash R., Johnson M., Fagan S.C. and Ergul A. (2013) Cerebral neovascularization and remodeling patterns in two different models of type 2 diabetes. PloS One
8, e56264.
10.1371/journal.pone.0056264

55.Khan M.S., Jeon Y., Lee C.H., Yang J.M. and Kim S.G. (2025) Dysregulation of extracellular matrix in diabetic complications: clinical impacts and new molecular targets. Innov. Acupunct. Med.
18, 3
10.1186/s44424-025-00005-3

56.Zhang L., Zhou J. and Kong W. (2025) Extracellular matrix in vascular homeostasis and disease. Nat. Rev. Cardiol.
22, 333–353

10.1038/s41569-024-01103-0

57.O'Sullivan S., Medina C., Ledwidge M., Radomski M.W. and Gilmer J.F. (2014) Nitric oxide-matrix metaloproteinase-9 interactions: Biological and pharmacological significance. Biochim. Biophys. Acta Mol. Cell Res.
1843, 603–617
10.1016/j.bbamcr.2013.12.006

58.Ridnour L.A., Windhausen A.N., Isenberg J.S., Yeung N., Thomas D.D., Vitek M.P.et al. (2007) Nitric oxide regulates matrix metalloproteinase-9 activity by guanylyl-cyclase-dependent and -independent pathways. Proc. Natl. Acad. Sci. U.S.A.
104, 16898–16903
10.1073/pnas.0702761104

59.Moran E.P., Wang Z., Chen J., Sapieha P., Smith L.E.H. and Ma J.X. (2016) Neurovascular cross talk in diabetic retinopathy: pathophysiological roles and therapeutic implications. Am. J. Physiol. Heart Circ. Physiol.
311, H738–H749

10.1152/ajpheart.00005.2016

60.Duda D.G., Fukumura D. and Jain R.K. (2004) Role of eNOS in neovascularization: NO for endothelial progenitor cells. Trends Mol. Med.
10, 143–145

10.1016/j.molmed.2004.02.001

61.Sweeney M.D., Ayyadurai S. and Zlokovic B.V. (2016) Pericytes of the neurovascular unit: key functions and signaling pathways. Nat. Neurosci.
19, 771–783

10.1038/nn.4288

62.Price T.O., Sheibani N. and Shah G.N. (2017) Regulation of high glucose-induced apoptosis of brain pericytes by mitochondrial CA VA: a specific target for prevention of diabetic cerebrovascular pathology. Biochim. Biophys. Acta Mol. Basis Dis.
1863, 929–935
10.1016/j.bbadis.2017.01.025

63.Hayden M.R. (2019) Type 2 diabetes mellitus increases the risk of late-onset Alzheimer’s disease: ultrastructural remodeling of the neurovascular unit and diabetic gliopathy. Brain Sci.
9, 262
10.3390/brainsci9100262

64.Attwell D., Buchan A.M., Charpak S., Lauritzen M., Macvicar B.A. and Newman E.A. (2010) Glial and neuronal control of brain blood flow. Nature
468, 232–243

10.1038/nature09613

65.Guo L., Yang Q., Wei R., Zhang W., Yin N., Chen Y.et al. (2023) Enhanced pericyte-endothelial interactions through NO-boosted extracellular vesicles drive revascularization in a mouse model of ischemic injury. Nat. Commun.
14, 7334.
10.1038/s41467-023-43153-x

66.Duarte J.V., Guerra C., Moreno C., Gomes L. and Castelo-Branco M. (2023) Changes in hemodynamic response function components reveal specific changes in neurovascular coupling in type 2 diabetes. Front. Physiol.
13, 1101470.
10.3389/fphys.2022.1101470

67.Monteiro A., Castro P., Pereira G., Ferreira C., Sorond F., Milstead A.et al. (2021) Neurovascular coupling is impaired in hypertensive and diabetic subjects without symptomatic cerebrovascular disease. Front. Aging Neurosci.
13, 728007.
10.3389/fnagi.2021.728007

68.You Y., Liu Z., Chen Y., Xu Y., Qin J., Guo S.et al. (2021) The prevalence of mild cognitive impairment in type 2 diabetes mellitus patients: a systematic review and meta-analysis. Acta Diabetol.
58, 671–685

10.1007/s00592-020-01648-9

69.Cao F., Yang F., Li J., Guo W., Zhang C., Gao F.et al. (2024) The relationship between diabetes and the dementia risk: a meta-analysis. Diabetol. Metab. Syndr.
16, 101.
10.1186/s13098-024-01346-4

70.Soares A.F., Nissen J.D., Garcia-Serrano A.M., Nussbaum S.S., Waagepetersen H.S. and Duarte J.M.N. (2019) Glycogen metabolism is impaired in the brain of male type 2 diabetic Goto-Kakizaki rats. J. Neurosci. Res.
97, 1004–1017

10.1002/jnr.24437

71.Li M., Li Y., Tan X., Qin C., Chen Y., Liang Y.et al. (2024) Resting-state neural activity and cerebral blood flow alterations in type 2 diabetes mellitus: Insights from hippocampal subfields. Brain Behav.
14, e3600.
10.1002/brb3.3600

72.Rabiller G., Ip Z., Zarrabian S., Zhang H., Sato Y., Yazdan-Shahmorad A.et al. (2024) Type-2 diabetes alters hippocampal neural oscillations and disrupts synchrony between the hippocampus and cortex. Aging Dis.
15, 2255–2270

10.14336/AD.2023.1106

73.Gonçalves J.S., Seiça R.M., Laranjinha J. and Lourenço C.F. (2022) Impairment of neurovascular coupling in the hippocampus due to decreased nitric oxide bioavailability supports early cognitive dysfunction in type 2 diabetic rats. Free Radic. Biol. Med.
193, 669–675

10.1016/j.freeradbiomed.2022.11.009

74.Prakash R., Somanath P.R., El-Remessy A.B., Kelly-Cobbs A., Stern J.E., Dore-Duffy P.et al. (2012) Enhanced cerebral but not peripheral angiogenesis in the Goto-Kakizaki model of type 2 diabetes involves VEGF and peroxynitrite signaling. Diabetes
61, 1533–1542

10.2337/db11-1528

75.Bangen K.J., Werhane M.L., Weigand A.J., Edmonds E.C., Delano-Wood L., Thomas K.R.et al. (2018) Reduced regional cerebral blood flow relates to poorer cognition in older adults with type 2 diabetes. Front. Aging Neurosci.
10, 270.
10.3389/fnagi.2018.00270

76.Wang Y., Sun L., He G., Gang X., Zhao X., Wang G.et al. (2021) Cerebral perfusion alterations in type 2 diabetes mellitus—a systematic review. Front. Neuroendocrinol.
62, 100916.
10.1016/j.yfrne.2021.100916

77.Teng Z., Feng J., Liu R., Dong Y., Chen H., Xu J.et al. (2022) Cerebral small vessel disease is associated with mild cognitive impairment in type 2 diabetes mellitus. Diabetes Metab. Syndr. Obes.
15, 1985–1994

10.2147/DMSO.S368725

78.van Sloten T.T., Sedaghat S., Carnethon M.R., Launer L.J. and Stehouwer C.D.A. (2020) Cerebral microvascular complications of type 2 diabetes: stroke, cognitive dysfunction, and depression. Lancet Diab. Endocrinol.
8, 325–336

79.Zhang D., Huang Y., Gao J., Lei Y., Ai K., Tang M.et al. (2021) Altered functional topological organization in type-2 diabetes mellitus with and without microvascular complications. Front. Neurosci.
15, 726350
10.3389/fnins.2021.726350

80.Gonçalves J.S., Marçal A., Laranjinha J. and Lourenço C.F. (2026) Benefits of dietary nitrate in cognitive decline associated with type 2 diabetes: pathways and therapeutic potential. Eur. J. Clin. Invest.
56, e70223.
10.1111/eci.70223

81.Gilchrist M., Winyard P.G., Fulford J., Anning C., Shore A.C. and Benjamin N. (2014) Dietary nitrate supplementation improves reaction time in type 2 diabetes: development and application of a novel nitrate-depleted beetroot juice placebo. Nitric Oxide
40, 67–74

10.1016/j.niox.2014.05.003

82.Zhu L., Zhang Z., Hou X.J., Wang Y.F., Yang J.Y. and Wu C.F. (2019) Inhibition of PDE5 attenuates streptozotocin-induced neuroinflammation and tau hyperphosphorylation in a streptozotocin-treated rat model. Brain Res.
1722, 146344.
10.1016/j.brainres.2019.146344

83.Patil C.S., Singh V.P. and Kulkarni S.K. (2006) Modulatory effect of sildenafil in diabetes and electroconvulsive shock-induced cognitive dysfunction in rats. Pharmacol. Rep.
58, 373–380

84.Samidurai A., Xi L., Das A. and Kukreja R.C. (2023) Beyond erectile dysfunction: cGMP-specific phosphodiesterase 5 inhibitors for other clinical disorders. Annu. Rev. Pharmacol. Toxicol.
63, 585–615

10.1146/annurev-pharmtox-040122-034745

85.Tarantini S., Valcarcel-Ares N.M., Yabluchanskiy A., Fulop G.A., Hertelendy P., Gautam T.et al. (2018) Treatment with the mitochondrial-targeted antioxidant peptide SS-31 rescues neurovascular coupling responses and cerebrovascular endothelial function and improves cognition in aged mice. Aging Cell
17, e12731.
10.1111/acel.12731

86.Urner S., Ho F., Jha J.C., Ziegler D. and Jandeleit-Dahm K. (2020) NADPH oxidase inhibition: preclinical and clinical studies in diabetic complications. Antioxid Redox. Signal.
33, 415–434

10.1089/ars.2020.8047

87.Heitzer T., Krohn K., Albers S. and Meinertz T. (2000) Tetrahydrobiopterin improves endothelium-dependent vasodilation by increasing nitric oxide activity in patients with Type II diabetes mellitus. Diabetologia
43, 1435–1438

10.1007/s001250051551