Regulation of the cerebral microcirculation during neural activity: is nitric oxide the missing link?

The role of nitric oxide and hormone signaling in chronic stress, anxiety, depression and post-traumatic stress disorder

This paper provides a summary of the role of nitric oxide (NO) and hormones in the development of chronic stress, anxiety, depression, and post-traumatic stress disorder (PTSD). These mental health conditions are prevalent globally and involve complex molecular interactions. Although there is a significant amount of research and therapeutic options available, the underlying mechanisms of these disorders are still not fully understood. The primary pathophysiologic processes involved in chronic stress, anxiety, depression, and PTSD include dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis, the intracellular influence of neuronal nitric oxide synthase (nNOS) on transcription factors, an inflammatory response with the formation of nitrergic oxidative species, and reduced serotonergic transmission in the dorsal raphe nucleus. Despite the extensive literature on this topic, there is a great need for further research to clarify the complexities inherent in these pathways, with the primary aim of improving psychiatric care.

Intrinsic organization of the corpus callosum

The corpus callosum-the largest commissural fiber system connecting the two cerebral hemispheres-is considered essential for bilateral sensory integration and higher cognitive functions. Most studies exploring the corpus callosum have examined either the anatomical, physiological, and neurochemical organization of callosal projections or the functional and/or behavioral aspects of the callosal connections after complete/partial callosotomy or callosal lesion. There are no works that address the intrinsic organization of the corpus callosum. We review the existing information on the activities that take place in the commissure in three sections: I) the topographical and neurochemical organization of the intracallosal fibers, II) the role of glia in the corpus callosum, and III) the role of the intracallosal neurons.

Oxidant-induced disruption of vascular K+ channel function: implications for diabetic vasculopathy

Diabetes in humans a chronic metabolic disorder characterised by hyperglycaemia, it is associated with an increased risk of cardiovascular disease, disruptions to metabolism and vascular functions. It is also linked to oxidative stress and its complications. Its role in vascular dysfunctions is generally reported without detailed impact on the molecular mechanisms. Potassium ion channel (K+ channels) are key regulators of vascular tone, and as membrane proteins, are modifiable by oxidant stress associated with diabetes. This review manuscript examined the impact of oxidant stress on vascular K+ channel functions in diabetes, its implication in vascular complications and metabolic and cardiovascular diseases.

Long-range inhibitory neurons mediate cortical neurovascular coupling

To meet the high energy demands of brain function, cerebral blood flow (CBF) parallels changes in neuronal activity by a mechanism known as neurovascular coupling (NVC). However, which neurons play a role in mediating NVC is not well understood. Here, we identify in mice and humans a specific population of cortical GABAergic neurons that co-express neuronal nitric oxide synthase and tachykinin receptor 1 (Tacr1). Through whole-tissue clearing, we demonstrate that Tacr1 neurons extend local and long-range projections across functionally connected cortical areas. We show that whisker stimulation elicited Tacr1 neuron activity in the barrel cortex through feedforward excitatory pathways. Additionally, through optogenetic experiments, we demonstrate that Tacr1 neurons are instrumental in mediating CBF through the relaxation of mural cells in a similar fashion to whisker stimulation. Finally, by electron microscopy, we observe that Tacr1 processes contact astrocytic endfeet. These findings suggest that Tacr1 neurons integrate cortical activity to mediate NVC.

Thermodynamic limitations on brain oxygen metabolism: physiological implications

Recent thermodynamic modelling indicates that maintaining the brain tissue ratio of O2 to CO2 (abbreviated tissue O2 /CO2 ) is critical for preserving the entropy increase available from oxidative metabolism of glucose, with a fall of that available entropy leading to a reduction of the phosphorylation potential and impairment of brain energy metabolism. This provides a novel perspective for understanding physiological responses under different conditions in terms of preserving tissue O2 /CO2 . To enable estimation of tissue O2 /CO2 in the human brain, a detailed mathematical model of O2 and CO2 transport was developed, and applied to reported physiological responses to different challenges, asking: how well is tissue O2 /CO2 preserved? Reported experimental results for increased neural activity, hypercapnia and hypoxia due to high altitude are consistent with preserving tissue O2 /CO2 . The results highlight two physiological mechanisms that control tissue O2 /CO2 : cerebral blood flow, which modulates tissue O2 ; and ventilation rate, which modulates tissue CO2 . The hypoxia modelling focused on humans at high altitude, including acclimatized lowlanders and Tibetan and Andean adapted populations, with a primary finding that decreasing CO2 by increasing ventilation rate is more effective for preserving tissue O2 /CO2 than increasing blood haemoglobin content to maintain O2 delivery to tissue. This work focused on the function served by particular physiological responses, and the underlying mechanisms require further investigation. The modelling provides a new framework and perspective for understanding how blood flow and other physiological factors support energy metabolism in the brain under a wide range of conditions. KEY POINTS: Thermodynamic modelling indicates that preserving the O2 /CO2 ratio in brain tissue is critical for preserving the entropy change available from oxidative metabolism of glucose and the phosphorylation potential underlying energy metabolism. A detailed model of O2 and CO2 transport was developed to allow estimation of the tissue O2 /CO2 ratio in the human brain in different physiological states. Reported experimental results during hypoxia, hypercapnia and increased oxygen metabolic rate in response to increased neural activity are consistent with maintaining brain tissue O2 /CO2 ratio. The hypoxia modelling of high-altitude acclimatization and adaptation in humans demonstrates the critical role of reducing CO2 with increased ventilation for preserving tissue O2 /CO2 . Preservation of tissue O2 /CO2 provides a novel perspective for understanding the function of observed physiological responses under different conditions in terms of preserving brain energy metabolism, although the mechanisms underlying these functions are not well understood.

NO Deficiency Compromises Inter- and Intrahemispheric Blood Flow Adaptation to Unilateral Carotid Artery Occlusion

Carotid artery stenosis (CAS) affects approximately 5-7.5% of older adults and is recognized as a significant risk factor for vascular cognitive impairment (VCI). The impact of CAS on cerebral blood flow (CBF) within the ipsilateral hemisphere relies on the adaptive capabilities of the cerebral microcirculation. In this study, we aimed to test the hypothesis that the impaired availability of nitric oxide (NO) compromises CBF homeostasis after unilateral carotid artery occlusion (CAO). To investigate this, three mouse models exhibiting compromised production of NO were tested: NOS1 knockout, NOS1/3 double knockout, and mice treated with the NO synthesis inhibitor L-NAME. Regional CBF changes following CAO were evaluated using laser-speckle contrast imaging (LSCI). Our findings demonstrated that NOS1 knockout, NOS1/3 double knockout, and L-NAME-treated mice exhibited impaired CBF adaptation to CAO. Furthermore, genetic deficiency of one or two NO synthase isoforms increased the tortuosity of pial collaterals connecting the frontoparietal and temporal regions. In conclusion, our study highlights the significant contribution of NO production to the functional adaptation of cerebrocortical microcirculation to unilateral CAO. We propose that impaired bioavailability of NO contributes to the impaired CBF homeostasis by altering inter- and intrahemispheric blood flow redistribution after unilateral disruption of carotid artery flow.

Universal method for the isolation of microvessels from frozen brain tissue: A proof-of-concept multiomic investigation of the neurovasculature

The neurovascular unit, comprised of vascular cell types that collectively regulate cerebral blood flow to meet the needs of coupled neurons, is paramount for the proper function of the central nervous system. The neurovascular unit gatekeeps blood-brain barrier properties, which experiences impairment in several central nervous system diseases associated with neuroinflammation and contributes to pathogenesis. To better understand function and dysfunction at the neurovascular unit and how it may confer inflammatory processes within the brain, isolation and characterization of the neurovascular unit is needed. Here, we describe a singular, standardized protocol to enrich and isolate microvessels from archived snap-frozen human and frozen mouse cerebral cortex using mechanical homogenization and centrifugation-separation that preserves the structural integrity and multicellular composition of microvessel fragments. For the first time, microvessels are isolated from postmortem ventromedial prefrontal cortex tissue and are comprehensively investigated as a structural unit using both RNA sequencing and Liquid Chromatography with tandem mass spectrometry (LC-MS/MS). Both the transcriptome and proteome are obtained and compared, demonstrating that the isolated brain microvessel is a robust model for the NVU and can be used to generate highly informative datasets in both physiological and disease contexts.

Nitric Oxide/Nitric Oxide Synthase System in the Pathogenesis of Neurodegenerative Disorders-An Overview

Nitric oxide, a ubiquitous molecule found throughout the natural world, is a key molecule implicated in many central and benefic molecular pathways and has a well-established role in the function of the central nervous system, as numerous studies have previously shown. Dysregulation of its metabolism, mainly the upregulation of nitric oxide production, has been proposed as a trigger and/or aggravator for many neurological affections. Increasing evidence supports the implication of this molecule in prevalent neurodegenerative diseases, such as Parkinson's disease, Alzheimer's disease, or amyotrophic lateral sclerosis. The mechanisms proposed for its neurotoxicity mainly center around the increased quantities of nitric oxide that are produced in the brain, their cause, and, most importantly, the pathological metabolic cascades created. These cascades lead to the formation of neuronal toxic substances that impair the neurons' function and structure on multiple levels. The purpose of this review is to present the main causes of increased pathological production, as well as the most important pathophysiological mechanisms triggered by nitric oxide, mechanisms that could help explain a part of the complex picture of neurodegenerative diseases and help develop targeted therapies.

Do Antiepileptic Drugs Change the Levels of Arginine Derivatives in Epileptic Children Treated with Polytherapy? The Results of a Case–Control Study

Previously, a relation between therapy with antiepileptic drugs (AEDs) and the levels of biochemical parameters was observed in adult patients suffering from epilepsy. Among these biochemical factors, arginine derivatives are often analyzed, i.e., asymmetric dimethylarginine (ADMA), symmetric dimethylarginine (SDMA), and homoarginine (hArg) as they may be linked with increased risk for cardiovascular disease (CVD). Since the levels of arginine derivatives may increase during therapy, and the treatment of epilepsy often lasts many years, patients may experience CVD faster. The aim of the present study was to analyze the levels of arginine derivatives in children with epilepsy who were treated with multiple AEDs to answer the question whether pediatric patients may be at increased risk of CVD in the future. We prospectively analyzed 21 children suffering from epilepsy who took ≥2 AEDs for at least 6 months and 22 children without epilepsy (reference group). The levels of the arginine derivatives, e.g., ADMA, SDMA, and hArg, were determined in the blood serum using the HPLC method. No differences in both the mean levels of ADMA and SDMA, as well as in the mean values of the arginine derivative ratios, were observed between the groups. The tendency toward a lower level of hArg was found in epileptic patients more than in the reference group (p = 0.091). Epileptic children receiving three or more AEDs had significantly lower concentrations of hArg and values of the hArg/ADMA ratio than the reference group (p = 0.023 and p = 0.006, respectively). In turn, the mean hArg/ADMA ratio was lower in children receiving three or more AEDs compared to children receiving two AEDs (p = 0.002). There was also a positive correlation between the hArg and ADMA concentrations in children with epilepsy taking two AEDs; the higher the level of hArg, the greater the level of ADMA on average (r = 0.650, p = 0.022). Taking three or more AEDs by epileptic children resulted in lower levels of both hArg and the value of the hArg/ADMA ratio.

Nitric Oxide Pathways in Neurovascular Coupling Under Normal and Stress Conditions in the Brain: Strategies to Rescue Aberrant Coupling and Improve Cerebral Blood Flow

The brain has impressive energy requirements and paradoxically, very limited energy reserves, implying its huge dependency on continuous blood supply. Aditionally, cerebral blood flow must be dynamically regulated to the areas of increased neuronal activity and thus, of increased metabolic demands. The coupling between neuronal activity and cerebral blood flow (CBF) is supported by a mechanism called neurovascular coupling (NVC). Among the several vasoactive molecules released by glutamatergic activation, nitric oxide (^•NO) is recognized to be a key player in the process and essential for the development of the neurovascular response. Classically, ^•NO is produced in neurons upon the activation of the glutamatergic N-methyl-D-aspartate (NMDA) receptor by the neuronal isoform of nitric oxide synthase and promotes vasodilation by activating soluble guanylate cyclase in the smooth muscle cells of the adjacent arterioles. This pathway is part of a more complex network in which other molecular and cellular intervenients, as well as other sources of ^•NO, are involved. The elucidation of these interacting mechanisms is fundamental in understanding how the brain manages its energy requirements and how the failure of this process translates into neuronal dysfunction. Here, we aimed to provide an integrated and updated perspective of the role of ^•NO in the NVC, incorporating the most recent evidence that reinforces its central role in the process from both viewpoints, as a physiological mediator and a pathological stressor. First, we described the glutamate-NMDA receptor-nNOS axis as a central pathway in NVC, then we reviewed the link between the derailment of the NVC and neuronal dysfunction associated with neurodegeneration (with a focus on Alzheimer's disease). We further discussed the role of oxidative stress in the NVC dysfunction, specifically by decreasing the ^•NO bioavailability and diverting its bioactivity toward cytotoxicity. Finally, we highlighted some strategies targeting the rescue or maintenance of ^•NO bioavailability that could be explored to mitigate the NVC dysfunction associated with neurodegenerative conditions. In line with this, the potential modulatory effects of dietary nitrate and polyphenols on ^•NO-dependent NVC, in association with physical exercise, may be used as effective non-pharmacological strategies to promote the ^•NO bioavailability and to manage NVC dysfunction in neuropathological conditions.

Reactive astrocytes: The nexus of pathological and clinical hallmarks of Alzheimer's disease

Astrocyte reactivity is a hallmark of neuroinflammation that arises with Alzheimer’s disease (AD) and nearly every other neurodegenerative condition. While astrocytes certainly contribute to classic inflammatory processes (e.g. cytokine release, waste clearance, and tissue repair), newly emerging technologies for measuring and targeting cell specific activities in the brain have uncovered essential roles for astrocytes in synapse function, brain metabolism, neurovascular coupling, and sleep/wake patterns. In this review, we use a holistic approach to incorporate, and expand upon, classic neuroinflammatory concepts to consider how astrocyte dysfunction/reactivity modulates multiple pathological and clinical hallmarks of AD. Our ever-evolving understanding of astrocyte signaling in neurodegeneration is not only revealing new drug targets and treatments for dementia but is suggesting we reimagine AD pathophysiological mechanisms.

Neurovascular-modulation: A review of primary vascular responses to transcranial electrical stimulation as a mechanism of action

BACKGROUND

The ubiquitous vascular response to transcranial electrical stimulation (tES) has been attributed to the secondary effect of neuronal activity forming the classic neurovascular coupling. However, the current density delivered transcranially concentrates in: A) the cerebrospinal fluid of subarachnoid space where cerebral vasculature resides after reaching the dural and pial surfaces and B) across the blood-brain-barrier after reaching the brain parenchyma. Therefore, it is anticipated that tES has a primary vascular influence.

OBJECTIVES

Focused review of studies that demonstrated the direct vascular response to electrical stimulation and studies demonstrating evidence for tES-induced vascular effect in coupled neurovascular systems.

RESULTS

tES induces both primary and secondary vascular phenomena originating from four cellular elements; the first two mediating a primary vascular phenomenon mainly in the form of an immediate vasodilatory response and the latter two leading to secondary vascular effects and as parts of classic neurovascular coupling: 1) The perivascular nerves of more superficially located dural and pial arteries and medium-sized arterioles with multilayered smooth muscle cells; and 2) The endothelial lining of all vessels including microvasculature of blood-brain barrier; 3) Astrocytes; and 4) Neurons of neurovascular units.

CONCLUSION

A primary vascular effect of tES is highly suggested based on various preclinical and clinical studies. We explain how the nature of vascular response can depend on vessel anatomy (size) and physiology and be controlled by stimulation waveform. Further studies are warranted to investigate the mechanisms underlying the vascular response and its contribution to neural activity in both healthy brain and pathological conditions - recognizing many brain diseases are associated with alteration of cerebral hemodynamics and decoupling of neurovascular units.

Neuronal nitric oxide synthase regulates regional brain perfusion in healthy humans

Aims

Neuronal nitric oxide synthase (nNOS) is highly expressed within the cardiovascular and nervous systems. Studies in genetically modified mice suggest roles in brain blood flow regulation while dysfunctional nNOS signalling is implicated in cerebrovascular ischaemia and migraine. Previous human studies have investigated the effects of non-selective NOS inhibition but there has been no direct investigation of the role of nNOS in human cerebrovascular regulation. We hypothesized that inhibition of the tonic effects of nNOS would result in global or localized changes in cerebral blood flow (CBF), as well as changes in functional brain connectivity.

Methods and results

We investigated the acute effects of a selective nNOS inhibitor, S-methyl-L-thiocitrulline (SMTC), on CBF and brain functional connectivity in healthy human volunteers (n = 19). We performed a randomized, placebo-controlled, crossover study with either intravenous SMTC or placebo, using magnetic resonance imaging protocols with arterial spin labelling and functional resting state neuroimaging. SMTC infusion induced an ∼4% decrease in resting global CBF [−2.3 (−0.3, −4.2) mL/100g/min, mean (95% confidence interval, CI), P = 0.02]. In a whole-brain voxel-wise factorial-design comparison of CBF maps, we identified a localized decrease in regional blood flow in the right hippocampus and parahippocampal gyrus following SMTC vs. placebo (2921 voxels; T = 7.0; x = 36; y = −32; z = −12; P < 0.001). This was accompanied by a decrease in functional connectivity to the left superior parietal lobule vs. placebo (484 voxels; T = 5.02; x = −14; y = −56; z = 74; P = 0.009). These analyses adjusted for the modest changes in mean arterial blood pressure induced by SMTC as compared to placebo [+8.7 mmHg (+1.8, +15.6), mean (95% CI), P = 0.009].

Conclusions

These data suggest a fundamental physiological role of nNOS in regulating regional CBF and functional connectivity in the human hippocampus. Our findings have relevance to the role of nNOS in the regulation of cerebral perfusion in health and disease.

Neurovascular Units and Neural-Glia Networks in Intracerebral Hemorrhage: from Mechanisms to Translation

Intracerebral hemorrhage (ICH), the most lethal type of stroke, often leads to poor outcomes in the clinic. Due to the complex mechanisms and cell-cell crosstalk during ICH, the neurovascular unit (NVU) was proposed to serve as a promising therapeutic target for ICH research. This review aims to summarize the development of pathophysiological shifts in the NVU and neural-glia networks after ICH. In addition, potential targets for ICH therapy are discussed in this review. Beyond cerebral blood flow, the NVU also plays an important role in protecting neurons, maintaining central nervous system (CNS) homeostasis, coordinating neuronal activity among supporting cells, forming and maintaining the blood-brain barrier (BBB), and regulating neuroimmune responses. During ICH, NVU dysfunction is induced, along with neuronal cell death, microglia and astrocyte activation, endothelial cell (EC) and tight junction (TJ) protein damage, and BBB disruption. In addition, it has been shown that certain targets and candidates can improve ICH-induced secondary brain injury based on an NVU and neural-glia framework. Moreover, therapeutic approaches and strategies for ICH are discussed.

Novel Mechanistic Insights and Potential Therapeutic Impact of TRPC6 in Neurovascular Coupling and Ischemic Stroke

Ischemic stroke is one of the most disabling diseases and a leading cause of death globally. Despite advances in medical care, the global burden of stroke continues to grow, as no effective treatments to limit or reverse ischemic injury to the brain are available. However, recent preclinical findings have revealed the potential role of transient receptor potential cation 6 (TRPC6) channels as endogenous protectors of neuronal tissue. Activating TRPC6 in various cerebral ischemia models has been found to prevent neuronal death, whereas blocking TRPC6 enhances sensitivity to ischemia. Evidence has shown that Ca2+ influx through TRPC6 activates the cAMP (adenosine 3',5'-cyclic monophosphate) response element-binding protein (CREB), an important transcription factor linked to neuronal survival. Additionally, TRPC6 activation may counter excitotoxic damage resulting from glutamate release by attenuating the activity of N-methyl-d-aspartate (NMDA) receptors of neurons by posttranslational means. Unresolved though, are the roles of TRPC6 channels in non-neuronal cells, such as astrocytes and endothelial cells. Moreover, TRPC6 channels may have detrimental effects on the blood-brain barrier, although their exact role in neurovascular coupling requires further investigation. This review discusses evidence-based cell-specific aspects of TRPC6 in the brain to assess the potential targets for ischemic stroke management.

Neurovascular coupling and cerebral autoregulation in atrial fibrillation

The risk of cognitive decline and stroke is increased by atrial fibrillation (AF). We sought to determine whether neurovascular coupling and cerebral autoregulation are blunted in people with AF in comparison with age-matched, patients with hypertension and healthy controls. Neurovascular coupling was assessed using five cycles of visual stimulation for 30 s followed by 30 s with both eyes-closed. Cerebral autoregulation was examined using a sit-stand test, and a repeated squat-to-stand (0.1 Hz) manoeuvre with transfer function analysis of mean arterial pressure (MAP; input) and middle cerebral artery mean blood flow velocity (MCA V_m; output) relationships at 0.1 Hz. Visual stimulation increased posterior cerebral artery conductance, but the magnitude of the response was blunted in patients with AF (18 [8] %; mean [SD]) and hypertension (17 [8] %), in comparison with healthy controls (26 [9] %) (P < 0.05). In contrast, transmission of MAP to MCA V_m was greater in AF patients compared to hypertension and healthy controls, indicating diminished cerebral autoregulation. We have shown for the first time that AF patients have impaired neurovascular coupling responses to visual stimulation and diminished cerebral autoregulation. Such deficits in cerebrovascular regulation may contribute to the increased risk of cerebral dysfunction in people with AF.

High-speed optical coherence tomography angiography for the measurement of stimulus-induced retrograde vasodilation of cerebral pial arteries in awake mice

Significance: Having a clear understanding of functional hyperemia is crucial for functional brain imaging and neurological disease research. Vasodilation induced by sensory stimulus propagates from the arterioles to the upstream pial arteries in a retrograde fashion. As retrograde vasodilation occurs briefly in the early stage of functional hyperemia, an imaging technique with a high temporal resolution is required for its measurement. Aim: We aimed to present an imaging method to measure stimulus-induced retrograde vasodilation in awake animals. Approach: An imaging method based on optical coherence tomography angiography, which enables a high-speed and label-free vessel diameter measurement, was developed and applied for the investigation. Results: The propagation speed of retrograde vasodilation of pial artery was measured in awake mice. Other characteristics of functional hyperemia such as temporal profile and amplitude of the vascular response were also investigated. Conclusions: Our results provide detailed information of stimulus-induced hemodynamic response in the brain of awake mice and suggest the potential utility of our imaging method for the study of functional hyperemia in normal and diseased brain.

Quantitative hemodynamic analysis of cerebral blood flow and neurovascular coupling using optical coherence tomography angiography

Functional hyperemia in the rat cortex was investigated using high-speed optical coherence tomography (OCT) angiography and Doppler OCT. OCT angiography (OCTA) was performed to image the hemodynamic stimulus-response over a wide field of view. Temporal changes in vessel diameters in different vessel compartments, which were determined as the diameters of erythrocyte flows in OCT angiograms, were measured in order to monitor localized hemodynamic changes. Our results showed that the dilation of arterioles at the site of activation was accompanied by the dilation of upstream arteries. Relatively negligible dilation was observed in veins. An increase in the OCTA signal was observed during stimulus in multiple capillaries, which may imply that capillary blood flow increases as a result of the expanded arterial blood volume. These results agree with previous observations using two-photon laser scanning microscopy (TPLSM). Doppler OCT was performed to quantitatively measure stimulus-induced blood flow response in pial arteries. The measurement showed small but clear hemodynamic response in upstream arteries with diameters exceeding 100 μm. Our results demonstrate the potential of OCTA and Doppler OCT for the investigation of neurovascular coupling in small animal models.

Effect of chronic nitrate and citrulline supplementation on vascular function and exercise performance in older individuals

Increased nitric oxide (NO) bioavailability may improve exercise performance and vascular function. It remains unclear whether older adults who experience a decreased NO bioavailability may benefit from chronic NO precursor supplementation. This randomised, double-blind, trial aims to assess the effect of chronic NO precursor intake on vascular function and exercise performance in older adults (60-70 years old). Twenty-four healthy older adults (12 females) performed vascular function assessment and both local (knee extensions) and whole-body (incremental cycling) exercise tests to exhaustion before and after one month of daily intake of a placebo (PLA) or a nitrate-rich salad and citrulline (N+C, 520mg nitrate and 6g citrulline) drink. Arterial blood pressure (BP) and stiffness, post-ischemic, hypercapnic and hypoxic vascular responses were evaluated. Prefrontal cortex and quadriceps oxygenation was monitored by near-infrared spectroscopy. N+C supplementation reduced mean BP (-3.3mmHg; p=0.047) without altering other parameters of vascular function and oxygenation kinetics. N+C supplementation reduced heart rate and oxygen consumption during submaximal cycling and increased maximal power output by 5.2% (p<0.05), but had no effect on knee extension exercise performance. These results suggest that chronic NO precursor supplementation in healthy older individuals can reduce resting BP and increase cycling performance by improving cardiorespiratory responses.

Limb linkage rehabilitation training-related changes in cortical activation and effective connectivity after stroke: A functional near-infrared spectroscopy study

Stroke remains the leading cause of long-term disability worldwide. Rehabilitation training is essential for motor function recovery following stroke. Specifically, limb linkage rehabilitation training can stimulate motor function in the upper and lower limbs simultaneously. This study aimed to investigate limb linkage rehabilitation task-related changes in cortical activation and effective connectivity (EC) within a functional brain network after stroke by using functional near-infrared spectroscopy (fNIRS) imaging. Thirteen stroke patients with either left hemiparesis (L-H group, n = 6) and or right hemiparesis (R-H group, n = 7) and 16 healthy individuals (control group) participated in this study. A multichannel fNIRS system was used to measure changes in cerebral oxygenated hemoglobin (delta HbO2) and deoxygenated hemoglobin (delta HHb) in the bilateral prefrontal cortices (PFCs), motor cortices (MCs), and occipital lobes (OLs) during (1) the resting state and (2) a motor rehabilitation task with upper and lower limb linkage (first 10 min [task_S1], last 10 min [task_S2]). The frequency-specific EC among the brain regions was calculated based on coupling functions and dynamic Bayesian inference in frequency intervals: high-frequency I (0.6-2 Hz) and II (0.145-0.6 Hz), low-frequency III (0.052-0.145 Hz), and very-low-frequency IV (0.021-0.052 Hz). The results showed that the stroke patients exhibited an asymmetric (greater activation in the contralesional versus ipsilesional motor region) cortical activation pattern versus healthy controls. Compared with the healthy controls, the stroke patients showed significantly lower EC (p < 0.025) in intervals I and II in the resting and task states. The EC from the MC and OL to the right PFC in interval IV was significantly higher in the R-H group than in the control group during the resting and task states (p < 0.025). Furthermore, the L-H group showed significantly higher EC from the MC and OL to the left PFC in intervals III and IV during the task states compared with the control group (p < 0.025). The significantly increased influence of the MC and OL on the contralesional PFC in low- and very-low-frequency bands suggested that plastic reorganization of cognitive resources severed to compensate for impairment in stroke patients during the motor rehabilitation task. This study can serve as a basis for understanding task-related reorganization of functional brain networks and developing novel assessment techniques for stroke rehabilitation.

Astrocytes drive cortical vasodilatory signaling by activating endothelial NMDA receptors

Astrocytes express neurotransmitter receptors that serve as sensors of synaptic activity and initiate signals leading to activity-dependent local vasodilation and increases in blood flow. We previously showed that arteriolar vasodilation produced by activation of cortical astrocytes is dependent on endothelial nitric oxide synthase (eNOS) and endogenous agonists of N-methyl-D-aspartate (NMDA) receptors. Here, we tested the hypothesis that these effects are mediated by NMDA receptors expressed by brain endothelial cells. Primary endothelial cultures expressed NMDA receptor subunits and produced nitric oxide in response to co-agonists, glutamate and D-serine. In cerebral cortex in situ, immunoelectron microscopy revealed that endothelial cells express the GluN1 NMDA receptor subunit at basolateral membrane surfaces in an orientation suitable for receiving intercellular messengers from brain cells. In cortical slices, activation of astrocytes by two-photon flash photolysis of a caged Ca²⁺ compound or application of a metabotropic glutamate receptor agonist caused endothelial NO generation and local vasodilation. These effects were mitigated by NMDA receptor antagonists and conditional gene silencing of endothelial GluN1, indicating at least partial dependence on endothelial NMDA receptors. Our observations identify a novel astrocyte-endothelial vasodilatory signaling axis that could contribute to endothelium-dependent vasodilation in brain functional hyperemia.

Pressure passivity of cerebral mitochondrial metabolism is associated with poor outcome following perinatal hypoxic ischemic brain injury

Hypoxic ischemic encephalopathy (HIE) leads to significant morbidity and mortality. Impaired autoregulation after hypoxia-ischaemia has been suggested to contribute further to injury. Thalamic lactate/N-Acetylasperate (Lac/NAA) peak area ratio of > 0.3 on proton (¹H) magnetic resonance spectroscopy (MRS) is associated with poor neurodevelopment outcome following HIE. Cytochrome-c-oxidase (CCO) plays a central role in mitochondrial oxidative metabolism and ATP synthesis. Using a novel broadband NIRS system, we investigated the impact of pressure passivity of cerebral metabolism (CCO), oxygenation (haemoglobin difference (HbD)) and cerebral blood volume (total haemoglobin (HbT)) in 23 term infants following HIE during therapeutic hypothermia (HT). Sixty-minute epochs of data from each infant were studied using wavelet analysis at a mean age of 48 h. Wavelet semblance (a measure of phase difference) was calculated to compare reactivity between mean arterial blood pressure (MABP) with oxCCO, HbD and HbT. OxCCO-MABP semblance correlated with thalamic Lac/NAA ( r = 0.48, p = 0.02). OxCCO-MABP semblance also differed between groups of infants with mild to moderate and severe injury measured using brain MRI score ( p = 0.04), thalamic Lac/NAA ( p = 0.04) and neurodevelopmental outcome at one year ( p = 0.04). Pressure passive changes in cerebral metabolism were associated with injury severity indicated by thalamic Lac/NAA, MRI scores and neurodevelopmental assessment at one year of age.

Cerebrovascular reactivity and cerebral perfusion of rats with acute liver failure: role of L-glutamine and asymmetric dimethylarginine in L-arginine-induced response

Cerebral blood flow (CBF) is impaired in acute liver failure (ALF), however, the complexity of the underlying mechanisms has often led to inconclusive interpretations. Regulation of CBF depends at least partially on variations in the local brain L-arginine concentration and/or its metabolic rate. In ALF, other factors, like an increased concentration of asymmetric dimethylarginine (ADMA), an endogenous nitric oxide synthase inhibitor and elevated level of L-glutamine, may contribute to CBF alteration. This study demonstrated strong differences in the reactivity of the middle cerebral arteries and their response to extravascular L-arginine application between vessels isolated from rats with thioacetamide (TAA)-induced ALF and control animals. Our results also showed the decrease in the cerebral perfusion in TAA rats measured by arterial spin labeling perfusion magnetic resonance. Subsequently, we aimed to investigate the importance of balance between the concentration of ADMA and L-arginine in the CBF regulation. In vivo, intraperitoneal L-arginine administration in TAA rats corrected: (i) decrease in cerebral perfusion, (ii) decrease in brain extracellular L-arginine/ADMA ratio and (iii) increase in brain L-glutamine concentration. Our study implicates that impaired vascular tone of cerebral arteries is most likely associated with exposure to high ADMA and L-glutamine levels resulting in limited availability of L-arginine and might be responsible for reduced cerebral perfusion observed in ALF.

A novel high-throughput assay for respiration in isolated brain microvessels reveals impaired mitochondrial function in the aged mice

Cerebral blood flow (CBF) is uniquely regulated by the anatomical design of the cerebral vasculature as well as through neurovascular coupling. The process of directing the CBF to meet the energy demands of neuronal activity is referred to as neurovascular coupling. Microvasculature in the brain constitutes the critical component of the neurovascular coupling. Mitochondria provide the majority of ATP to meet the high-energy demand of the brain. Impairment of mitochondrial function plays a central role in several age-related diseases such as hypertension, ischemic brain injury, Alzheimer's disease, and Parkinson disease. Interestingly, microvessels and small arteries of the brain have been the focus of the studies implicating the vascular mechanisms in several age-related neurological diseases. However, the role of microvascular mitochondrial dysfunction in age-related diseases remains unexplored. To date, high-throughput assay for measuring mitochondrial respiration in microvessels is lacking. The current study presents a novel method to measure mitochondrial respiratory parameters in freshly isolated microvessels from mouse brain ex vivo using Seahorse XFe24 Analyzer. We validated the method by demonstrating impairments of mitochondrial respiration in cerebral microvessels isolated from old mice compared to the young mice. Thus, application of mitochondrial respiration studies in microvessels will help identify novel vascular mechanisms underlying a variety of age-related neurological diseases.

Potential benefits of dietary nitrate ingestion in healthy and clinical populations: A brief review

This article provides an overview of the current literature relating to the efficacy of dietary nitrate (NO₃^-) ingestion in altering aspects of cardiovascular and metabolic health and exercise capacity in healthy and diseased individuals. The consumption of NO₃^--rich vegetables, such as spinach and beetroot, have been variously shown to promote nitric oxide bioavailability, reduce systemic blood pressure, enhance tissue blood flow, modulate muscle O₂ utilisation and improve exercise tolerance both in normoxia and in hypoxia, as is commonly observed in a number of disease states. NO₃^- ingestion may, therefore, act as a natural means for augmenting performance and attenuating complications associated with limited O₂ availability or transport, hypertension and the metabolic syndrome. Recent studies indicate that dietary NO₃^- might also augment intrinsic skeletal muscle contractility and improve the speed and power of muscle contraction. Moreover, several investigations suggest that NO₃^- supplementation may improve aspects of cognitive performance both at rest and during exercise. Collectively, these observations position NO₃^- as more than a putative ergogenic aid and suggest that increasing natural dietary NO₃^- intake may act as a prophylactic in countering the predations of senescence and certain cardiovascular-metabolic diseases.

[Effect of Reading a Book on a Tablet Computer on Cerebral Blood Flow in the Prefrontal Cortex]

OBJECTIVES

By measuring cerebral blood flow in the prefrontal cortex, we aimed to determine how reading a book on a tablet computer affects sleep.

METHODS

Seven students (7 men age range, 21-32 years) participated in this study. In a controlled illuminance environment, the subjects read a novel in printed form or on a tablet computer from any distance. As the subjects were reading, the cerebral blood flow in their prefrontal cortex was measured by near-infrared spectroscopy. The study protocol was as follows. 1) Subjects mentally counted a sequence of numbers for 30 s as a pretest to standardized thinking and then 2) read the novel for 10 min, using the printed book or tablet computer. In step 2), the use of the book or tablet computer was in a random sequence. Subjects rested between the two tasks.

RESULTS

Significantly increased brain activity (increase in regional cerebral blood flow) was observed following reading a novel on a tablet computer compared with that after reading a printed book. Furthermore, the region around Broca's area was more active when reading on a tablet computer than when reading a printed book.

CONCLUSIONS

Considering the results of this study and previous studies on physiological characteristics during nonrapid eye movement sleep, we concluded that reading a book on a tablet computer before the onset of sleep leads to the potential inhibition of sound sleep through mechanisms other than the suppression of melatonin secretion.

The Neurovascular Unit Coming of Age: A Journey through Neurovascular Coupling in Health and Disease

The concept of the neurovascular unit (NVU), formalized at the 2001 Stroke Progress Review Group meeting of the National Institute of Neurological Disorders and Stroke, emphasizes the intimate relationship between the brain and its vessels. Since then, the NVU has attracted the interest of the neuroscience community, resulting in considerable advances in the field. Here the current state of knowledge of the NVU will be assessed, focusing on one of its most vital roles: the coupling between neural activity and blood flow. The evidence supports a conceptual shift in the mechanisms of neurovascular coupling, from a unidimensional process involving neuronal-astrocytic signaling to local blood vessels to a multidimensional one in which mediators released from multiple cells engage distinct signaling pathways and effector systems across the entire cerebrovascular network in a highly orchestrated manner. The recently appreciated NVU dysfunction in neurodegenerative diseases, although still poorly understood, supports emerging concepts that maintaining neurovascular health promotes brain health.

Testing the effects of sensory stimulation as a collateral-based therapeutic for ischemic stroke in C57BL/6J and CD1 mouse strains

Utilizing a rat model of ischemic stroke, we have previously shown that sensory stimulation can completely protect rats from impending ischemic damage of cortex if this treatment is delivered within the first two hours post-permanent middle cerebral artery occlusion (pMCAo). The current study sought to extend our findings in rats to mice, which would allow new avenues of research not available in rats. Thus, young adult C57BL/6J and CD1 mice were tested for protection from ischemic stroke with the same protective sensory stimulation-based treatment. Cortical activity and blood flow were assessed with intrinsic signal optical imaging (ISOI) and laser speckle imaging (LSI), respectively, and histological analysis (TTC) was performed at the completion of the experiments. Standing in stark contrast to the positive results observed in rats, in both strains we found that there were no differences between treated and untreated mice at 24 hours post-pMCAo in terms of infarct volume, negative functional imaging results, and major reduction in retrograde collateral blood flow as compared to pre-pMCAo baseline and surgical controls. Also, no differences were found between the strains in terms of theses variables. Potential reasons for the differences between rats and mice are discussed.

Neurovascular-neuroenergetic coupling axis in the brain: master regulation by nitric oxide and consequences in aging and neurodegeneration

The strict energetic demands of the brain require that nutrient supply and usage be fine-tuned in accordance with the specific temporal and spatial patterns of ever-changing levels of neuronal activity. This is achieved by adjusting local cerebral blood flow (CBF) as a function of activity level - neurovascular coupling - and by changing how energy substrates are metabolized and shuttled amongst astrocytes and neurons - neuroenergetic coupling. Both activity-dependent increase of CBF and O₂ and glucose utilization by active neural cells are inextricably linked, establishing a functional metabolic axis in the brain, the neurovascular-neuroenergetic coupling axis. This axis incorporates and links previously independent processes that need to be coordinated in the normal brain. We here review evidence supporting the role of neuronal-derived nitric oxide (^•NO) as the master regulator of this axis. Nitric oxide is produced in tight association with glutamatergic activation and, diffusing several cell diameters, may interact with different molecular targets within each cell type. Hemeproteins such as soluble guanylate cyclase, cytochrome c oxidase and hemoglobin, with which ^•NO reacts at relatively fast rates, are but a few of the key in determinants of the regulatory role of ^•NO in the neurovascular-neuroenergetic coupling axis. Accordingly, critical literature supporting this concept is discussed. Moreover, in view of the controversy regarding the regulation of catabolism of different neural cells, we further discuss key aspects of the pathways through which ^•NO specifically up-regulates glycolysis in astrocytes, supporting lactate shuttling to neurons for oxidative breakdown. From a biomedical viewpoint, derailment of neurovascular-neuroenergetic axis is precociously linked to aberrant brain aging, cognitive impairment and neurodegeneration. Thus, we summarize current knowledge of how both neurovascular and neuroenergetic coupling are compromised in aging, traumatic brain injury, epilepsy and age-associated neurodegenerative disorders such as Alzheimer's disease and Parkinson's disease, suggesting that a shift in cellular redox balance may contribute to divert ^•NO bioactivity from regulation to dysfunction.

Functional near infra-red spectroscopy (fNIRS) in schizophrenia: A review

The research on the alterations in functional connectivity in schizophrenia has been facilitated by development of an array of functional neuroimaging techniques. Functional Near Infra Red Spectroscopy (fNIRS) is a novel diffuse optical neuromonitring method with its own advantages and limitations. The advantages of fNIRS have made it to be frequently used as a research tool by medical community in different settings. In fNIRS the property of haemoglobin to absorb near infrared light is used to measure brain activity. It provides the indirect measurement of the neuronal activity in the areas of interest. The advantage of fNIRS being less restrictive has made it to be used more commonly in the research of psychiatric disorders in general, schizophrenia in particular. The fNIRS studies on patients with schizophrenia have shown haemodynamic hypo activation primarily in the prefrontal cortex during various cognitive tasks. In this review, initially we have briefly explained the basic principles of fNIRS followed by detailed review of fNIRS findings in patients with schizophrenia.

Gas Diffusion in the CNS

Gases have been long known to have essential physiological functions in the CNS such as respiration or regulation of vascular tone. Since gases have been classically considered to freely diffuse, research in gas biology has so far focused on mechanisms of gas synthesis and gas reactivity, rather than gas diffusion and transport. However, the discovery of gas pores during the last two decades and the characterization of diverse diffusion patterns through different membranes has raised the possibility that modulation of gas diffusion is also a physiologically relevant parameter. Here we review the means of gas movement into and within the brain through "free" diffusion and gas pores, notably aquaporins, discussing the role that gas diffusion may play in the modulation of gas function. We highlight how diffusion is relevant to neuronal signaling, volume transmission, and cerebrovascular control in the case of NO, one of the most extensively studied gases. We point out how facilitated transport can be especially relevant for gases with low permeability in lipid membranes like NH₃ and discuss the possible implications of NH₃ -permeable channels in physiology and hyperammonemic encephalopathy. We identify novel research questions about how modulation of gas diffusion could intervene in CNS pathologies. This emerging area of research can provide novel and interesting insights in the field of gas biology.

Effect of black tea consumption on radial blood pulse spectrum and cognitive health

Black tea consumption has been proven to improve endothelial function and to lower the risk of stroke and cognitive impairment. Several effects of black tea on cardiovascular system had been surveyed. However, the black tea effect on pressure pulse spectrum remains unknown. The study was aimed to investigate the influence of black tea on radial blood pressure and Pulse Spectrum. Fourteen healthy subjects received water and single doses of black tea (0.05g/Kg) in separate weeks. The radial blood pressure and pulse wave were measured and the pressure pulses were evaluated using harmonic analysis. This report confirmed that black tea consumption (dose=0.05g/Kg) significantly increased third, fifth, (P<0.1), sixth, seventh, and eighth harmonics (p<0.05) of radial pressure wave comparing to water control. We proposed that black tea may increase cerebral blood flow (CBF), which was deduced from the results and from the conclusions of previous studies. The results also showed that the harmonic components of pressure pulse could be the vascular kinetic index that assessed the hemodynamic status in each time frame before and after consumption of black tea.

Asymmetric Dimethylarginine and Hepatic Encephalopathy: Cause, Effect or Association?

The methylated derivative of l-arginine, asymmetric dimethylarginine (ADMA) is synthesized in different mammalian tissues including the brain. ADMA acts as an endogenous, nonselective, competitive inhibitor of all three isoforms of nitric oxide synthase (NOS) and may limit l-arginine supply from the plasma to the enzyme via reducing its transport by cationic amino acid transporters. Hepatic encephalopathy (HE) is a relatively frequently diagnosed complex neuropsychiatric syndrome associated with acute or chronic liver failure, characterized by symptoms linked with impaired brain function leading to neurological disabilities. The l-arginine—nitric oxide (NO) pathway is crucially involved in the pathomechanism of HE via modulating important cerebral processes that are thought to contribute to the major HE symptoms. Specifically, activation of this pathway in acute HE leads to an increase in NO production and free radical formation, thus, contributing to astrocytic swelling and cerebral edema. Moreover, the NO-cGMP pathway seems to be involved in cerebral blood flow (CBF) regulation, altered in HE. For this reason, depressed NO-cGMP signaling accompanying chronic HE and ensuing cGMP deficit contributes to the cognitive and motor failure. However, it should be remembered that ADMA, a relatively little known element limiting NO synthesis in HE, may also influence the NO-cGMP pathway regulation. In this review, we will discuss the contribution of ADMA to the regulation of the NO-cGMP pathway in the brain, correlation of ADMA level with CBF and cognitive alterations observed during HE progression in patients and/or animal models of HE.

Fluid and ion transfer across the blood-brain and blood-cerebrospinal fluid barriers; a comparative account of mechanisms and roles

The two major interfaces separating brain and blood have different primary roles. The choroid plexuses secrete cerebrospinal fluid into the ventricles, accounting for most net fluid entry to the brain. Aquaporin, AQP1, allows water transfer across the apical surface of the choroid epithelium; another protein, perhaps GLUT1, is important on the basolateral surface. Fluid secretion is driven by apical Na+-pumps. K+ secretion occurs via net paracellular influx through relatively leaky tight junctions partially offset by transcellular efflux. The blood-brain barrier lining brain microvasculature, allows passage of O2, CO2, and glucose as required for brain cell metabolism. Because of high resistance tight junctions between microvascular endothelial cells transport of most polar solutes is greatly restricted. Because solute permeability is low, hydrostatic pressure differences cannot account for net fluid movement; however, water permeability is sufficient for fluid secretion with water following net solute transport. The endothelial cells have ion transporters that, if appropriately arranged, could support fluid secretion. Evidence favours a rate smaller than, but not much smaller than, that of the choroid plexuses. At the blood-brain barrier Na+ tracer influx into the brain substantially exceeds any possible net flux. The tracer flux may occur primarily by a paracellular route. The blood-brain barrier is the most important interface for maintaining interstitial fluid (ISF) K+ concentration within tight limits. This is most likely because Na+-pumps vary the rate at which K+ is transported out of ISF in response to small changes in K+ concentration. There is also evidence for functional regulation of K+ transporters with chronic changes in plasma concentration. The blood-brain barrier is also important in regulating HCO3- and pH in ISF: the principles of this regulation are reviewed. Whether the rate of blood-brain barrier HCO3- transport is slow or fast is discussed critically: a slow transport rate comparable to those of other ions is favoured. In metabolic acidosis and alkalosis variations in HCO3- concentration and pH are much smaller in ISF than in plasma whereas in respiratory acidosis variations in pHISF and pHplasma are similar. The key similarities and differences of the two interfaces are summarized.

Dietary nitrate supplementation improves sprint and high-intensity intermittent running performance

The influence of dietary nitrate (NO₃^-) supplementation on indices of maximal sprint and intermittent exercise performance is unclear.

PURPOSE

To investigate the effects of NO₃^- supplementation on sprint running performance, and cognitive function and exercise performance during the sport-specific Yo-Yo Intermittent Recovery level 1 test (IR1).

METHODS

In a double-blind, randomized, crossover study, 36 male team-sport players received NO₃^--rich (BR; 70 mL·day^-1; 6.4 mmol of NO₃^-), and NO₃^--depleted (PL; 70 mL·day^-1; 0.04 mmol NO₃^-) beetroot juice for 5 days. On day 5 of supplementation, subjects completed a series of maximal 20-m sprints followed by the Yo-Yo IR1. Cognitive tasks were completed prior to, during and immediately following the Yo-Yo IR1.

RESULTS

BR improved sprint split times relative to PL at 20 m (1.2%; BR 3.98 ± 0.18 vs. PL 4.03 ± 0.19 s; P < 0.05), 10 m (1.6%; BR 2.53 ± 0.12 vs. PL 2.57 ± 0.19 s; P < 0.05) and 5 m (2.3%; BR 1.73 ± 0.09 vs. PL 1.77 ± 0.09 s; P < 0.05). The distance covered in the Yo-Yo IR1 test improved by 3.9% (BR 1422 ± 502 vs. PL 1369 ± 505 m; P < 0.05). The reaction time to the cognitive tasks was shorter in BR (615 ± 98 ms) than PL (645 ± 120 ms; P < 0.05) at rest but not during the Yo-Yo IR1. There was no difference in response accuracy.

CONCLUSIONS

Dietary NO₃^- supplementation enhances maximal sprint and high-intensity intermittent running performance in competitive team sport players. Our findings suggest that NO₃^- supplementation has the potential to improve performance in single-sprint or multiple-sprint (team) sports.

Beneficial effects of L-arginine on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced neuronal degeneration in substantia nigra of Balb/c mice

BACKGROUND

L-arginine has been recently investigated and proposed to reduce neurological damage after various experimental models of neuronal cellular damage. In this study, we aim to evaluate the beneficial effects of L-arginine administration on the numerical density of dark neurons (DNs) in the substantia nigra pars compacta (SNc) of Balb/c mice subjected to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration.

MATERIALS AND METHODS

Male Balb/c mice were randomly divided into 4 groups (n = 7 each): MPTP only; saline only (control); MPTP + L-arginine; and L-arginine only. The animals were infused intranasally with a single intranasal administration of the proneurotoxin MPTP (1 mg/nostril). L-arginine (300 mg/kg) was administrated intraperitoneally once daily for 1-week starting from 3 days after MPTP administration. Cavalieri principle method was used to estimate the numerical density of DNs in the SNc of different studied groups.

RESULTS

Twenty days following MPTP administration, the number of DNs was significantly increased when compared to sham-control and L-arginine-control groups (P < 0.05). Nevertheless, our results showed that L-arginine administration significantly decreased the numerical density of DNs in SNc of mice.

CONCLUSION

This investigation provides new insights in experimental models of Parkinson's disease, indicating that L-arginine represents a potential treatment agent for dopaminergic neuron degeneration in SNc observed in Parkinson's disease patients.

REVIEW ■ : Nitric Oxide: Actions and Pathological Roles

Nitric oxide is a unique biological messenger molecule. It mediates, in part, the immune functions of mac rophages ; it is produced by endothelial cells to mediate blood vessel relaxation; and it also serves as a neurotransmitter in the central and peripheral nervous system. Endothelial nitric oxide synthase and neuronal nitric oxide synthase are thought to be primarily constitutive, with activation induced by calcium entry into cells in the absence of protein synthesis, whereas the macrophage nitric oxide synthase is inducible with large increases in new nitric oxide synthase protein synthesis after immune stimulation. The molecular targets of nitric oxide are expanding, as are its physiological and pathophysiological roles in the nervous system. Nitric oxide may regulate neurotransmitter release, and it may play a key role in nervous system morpho genesis and synaptic plasticity and regulate gene expression. Under conditions of excessive formation, nitric oxide is emerging as an important neurotoxin in a variety of disorders of the nervous system. The Neuro scientist 1:7-18, 1995

Brain imaging of neurovascular dysfunction in Alzheimer's disease

Neurovascular dysfunction, including blood-brain barrier (BBB) breakdown and cerebral blood flow (CBF) dysregulation and reduction, are increasingly recognized to contribute to Alzheimer's disease (AD). The spatial and temporal relationships between different pathophysiological events during preclinical stages of AD, including cerebrovascular dysfunction and pathology, amyloid and tau pathology, and brain structural and functional changes remain, however, still unclear. Recent advances in neuroimaging techniques, i.e., magnetic resonance imaging (MRI) and positron emission tomography (PET), offer new possibilities to understand how the human brain works in health and disease. This includes methods to detect subtle regional changes in the cerebrovascular system integrity. Here, we focus on the neurovascular imaging techniques to evaluate regional BBB permeability (dynamic contrast-enhanced MRI), regional CBF changes (arterial spin labeling- and functional-MRI), vascular pathology (structural MRI), and cerebral metabolism (PET) in the living human brain, and examine how they can inform about neurovascular dysfunction and vascular pathophysiology in dementia and AD. Altogether, these neuroimaging approaches will continue to elucidate the spatio-temporal progression of vascular and neurodegenerative processes in dementia and AD and how they relate to each other.

Astrocyte Ca2+ Signaling Drives Inversion of Neurovascular Coupling after Subarachnoid Hemorrhage

Physiologically, neurovascular coupling (NVC) matches focal increases in neuronal activity with local arteriolar dilation. Astrocytes participate in NVC by sensing increased neurotransmission and releasing vasoactive agents (e.g., K(+)) from perivascular endfeet surrounding parenchymal arterioles. Previously, we demonstrated an increase in the amplitude of spontaneous Ca(2+) events in astrocyte endfeet and inversion of NVC from vasodilation to vasoconstriction in brain slices obtained from subarachnoid hemorrhage (SAH) model rats. However, the role of spontaneous astrocyte Ca(2+) signaling in determining the polarity of the NVC response remains unclear. Here, we used two-photon imaging of Fluo-4-loaded rat brain slices to determine whether altered endfoot Ca(2+) signaling underlies SAH-induced inversion of NVC. We report a time-dependent emergence of endfoot high-amplitude Ca(2+) signals (eHACSs) after SAH that were not observed in endfeet from unoperated animals. Furthermore, the percentage of endfeet with eHACSs varied with time and paralleled the development of inversion of NVC. Endfeet with eHACSs were present only around arterioles exhibiting inversion of NVC. Importantly, depletion of intracellular Ca(2+) stores using cyclopiazonic acid abolished SAH-induced eHACSs and restored arteriolar dilation in SAH brain slices to two mediators of NVC (a rise in endfoot Ca(2+) and elevation of extracellular K(+)). These data indicate a causal link between SAH-induced eHACSs and inversion of NVC. Ultrastructural examination using transmission electron microscopy indicated that a similar proportion of endfeet exhibiting eHACSs also exhibited asymmetrical enlargement. Our results demonstrate that subarachnoid blood causes a delayed increase in the amplitude of spontaneous intracellular Ca(2+) release events leading to inversion of NVC. Significance statement: Aneurysmal subarachnoid hemorrhage (SAH)--strokes involving cerebral aneurysm rupture and release of blood onto the brain surface--are associated with high rates of morbidity and mortality. A common complication observed after SAH is the development of delayed cerebral ischemia at sites often remote from the site of rupture. Here, we provide evidence that SAH-induced changes in astrocyte Ca(2+) signaling lead to a switch in the polarity of the neurovascular coupling response from vasodilation to vasoconstriction. Thus, after SAH, signaling events that normally lead to vasodilation and enhanced delivery of blood to active brain regions cause vasoconstriction that would limit cerebral blood flow. These findings identify astrocytes as a key player in SAH-induced decreased cortical blood flow.

The mechanisms of action of flavonoids in the brain: Direct versus indirect effects

The projected increase in the incidence of dementia in the population highlights the urgent need for a more comprehensive understanding of how different aspects of lifestyle, in particular exercise and diet, may affect neural function and consequent cognitive performance throughout the life course. In this regard, flavonoids, found in a variety of fruits, vegetables and derived beverages, have been identified as a group of promising bioactive compounds capable of influencing different aspects of brain function, including cerebrovascular blood flow and synaptic plasticity, both resulting in improvements in learning and memory in mammalian species. However, the precise mechanisms by which flavonoids exert these actions are yet to be fully established, although accumulating data indicate an ability to interact with neuronal receptors and kinase signaling pathways which are key to neuronal activation and communication and synaptic strengthening. Alternatively or concurrently, there is also compelling evidence derived from human clinical studies suggesting that flavonoids can positively affect peripheral and cerebrovascular blood flow, which may be an indirect effective mechanism by which dietary flavonoids can impact on brain health and cognition. The current review examines the beneficial effects of flavonoids on both human and animal brain function and attempts to address and link direct and indirect actions of flavonoids and their derivatives within the central nervous system (CNS).

Neurovascular and neurometabolic derailment in aging and Alzheimer's disease

The functional and structural integrity of the brain requires local adjustment of blood flow and regulated delivery of metabolic substrates to meet the metabolic demands imposed by neuronal activation. This process—neurovascular coupling—and ensued alterations of glucose and oxygen metabolism—neurometabolic coupling—are accomplished by concerted communication between neural and vascular cells. Evidence suggests that neuronal-derived nitric oxide (^•NO) is a key player in both phenomena. Alterations in the mechanisms underlying the intimate communication between neural cells and vessels ultimately lead to neuronal dysfunction. Both neurovascular and neurometabolic coupling are perturbed during brain aging and in age-related neuropathologies in close association with cognitive decline. However, despite decades of intense investigation, many aspects remain poorly understood, such as the impact of these alterations. In this review, we address neurovascular and neurometabolic derailment in aging and Alzheimer's disease (AD), discussing its significance in connection with ^•NO-related pathways.

Dietary nitrate improves sprint performance and cognitive function during prolonged intermittent exercise

It is possible that dietary nitrate (NO3 (-)) supplementation may improve both physical and cognitive performance via its influence on blood flow and cellular energetics.

PURPOSE

To investigate the effects of dietary NO3 (-) supplementation on exercise performance and cognitive function during a prolonged intermittent sprint test (IST) protocol, which was designed to reflect typical work patterns during team sports.

METHODS

In a double-blind randomised crossover study, 16 male team-sport players received NO3 (-)-rich (BR; 140 mL day(-1); 12.8 mmol of NO3 (-)), and NO3 (-)-depleted (PL; 140 mL day(-1); 0.08 mmol NO3 (-)) beetroot juice for 7 days. On day 7 of supplementation, subjects completed the IST (two 40-min "halves" of repeated 2-min blocks consisting of a 6-s "all-out" sprint, 100-s active recovery and 20 s of rest), on a cycle ergometer during which cognitive tasks were simultaneously performed.

RESULTS

Total work done during the sprints of the IST was greater in BR (123 ± 19 kJ) compared to PL (119 ± 17 kJ; P < 0.05). Reaction time of response to the cognitive tasks in the second half of the IST was improved in BR compared to PL (BR first half: 820 ± 96 vs. second half: 817 ± 86 ms; PL first half: 824 ± 114 vs. second half: 847 ± 118 ms; P < 0.05). There was no difference in response accuracy.

CONCLUSIONS

These findings suggest that dietary NO3 (-) enhances repeated sprint performance and may attenuate the decline in cognitive function (and specifically reaction time) that may occur during prolonged intermittent exercise.

Neurovascular coupling in hippocampus is mediated via diffusion by neuronal-derived nitric oxide

The coupling between neuronal activity and cerebral blood flow (CBF) is essential for normal brain function. The mechanisms behind this neurovascular coupling process remain elusive, mainly because of difficulties in probing dynamically the functional and coordinated interaction between neurons and the vasculature in vivo. Direct and simultaneous measurements of nitric oxide (NO) dynamics and CBF changes in hippocampus in vivo support the notion that during glutamatergic activation nNOS-derived NO induces a time-, space-, and amplitude-coupled increase in the local CBF, later followed by a transient increase in local O2 tension. These events are dependent on the activation of the NMDA-glutamate receptor and nNOS, without a significant contribution of endothelial-derived NO or astrocyte-neuron signaling pathways. Upon diffusion of NO from active neurons, the vascular response encompasses the activation of soluble guanylate cyclase. Hence, in the hippocampus, neurovascular coupling is mediated by nNOS-derived NO via a diffusional connection between active glutamatergic neurons and blood vessels.

Nitric oxide and cerebrovascular regulation

The cerebrovascular regulation involves highly complex mechanisms to assure that the brain is perfused at all times. These mechanisms depend on all components of the neurovascular units: neurons, glia, and vascular cells. All these cell types can produce nitric oxide (NO), a powerful vasodilator through different NO synthases. Many studies underlined the key role of NO in the maintenance of resting cerebral blood flow (CBF) as well as in the mechanisms that control cerebrovascular tone: autoregulation and neurovascular coupling. However, although the role of NO in the control of CBF has been largely investigated, the complexity of the NO system and the lack of specific NO synthase inhibitors led to still unresolved questions such as the origin of NO and the pathways by which it controls the vascular tone. In this chapter, the role of NO in the regulation of CBF is critically reviewed and discussed in the context of the neurovascular unit and the general principles of cerebrovascular regulation.

Astrocyte-induced cortical vasodilation is mediated by D-serine and endothelial nitric oxide synthase

Astrocytes play a critical role in neurovascular coupling by providing a physical linkage from synapses to arterioles and releasing vaso-active gliotransmitters. We identified a gliotransmitter pathway by which astrocytes influence arteriole lumen diameter. Astrocytes synthesize and release NMDA receptor coagonist, D-serine, in response to neurotransmitter input. Mouse cortical slice astrocyte activation by metabotropic glutamate receptors or photolysis of caged Ca(2+) produced dilation of penetrating arterioles in a manner attenuated by scavenging D-serine with D-amino acid oxidase, deleting the enzyme responsible for D-serine synthesis (serine racemase) or blocking NMDA receptor glycine coagonist sites with 5,7-dichlorokynurenic acid. We also found that dilatory responses were dramatically reduced by inhibition or elimination of endothelial nitric oxide synthase and that the vasodilatory effect of endothelial nitric oxide synthase is likely mediated by suppressing levels of the vasoconstrictor arachidonic acid metabolite, 20-hydroxy arachidonic acid. Our results provide evidence that D-serine coactivation of NMDA receptors and endothelial nitric oxide synthase is involved in astrocyte-mediated neurovascular coupling.

Distribution and morphology of nitrergic neurons across functional domains of the rat primary somatosensory cortex

The rat primary somatosensory cortex (S1) is remarkable for its conspicuous vertical compartmentalization in barrels and septal columns, which are additionally stratified in horizontal layers. Whereas excitatory neurons from each of these compartments perform different types of processing, the role of interneurons is much less clear. Among the numerous types of GABAergic interneurons, those producing nitric oxide (NO) are especially puzzling, since this gaseous messenger can modulate neural activity, synaptic plasticity, and neurovascular coupling. We used a quantitative morphological approach to investigate whether nitrergic interneurons, which might therefore be considered both as NO volume diffusers and as elements of local circuitry, display features that could relate to barrel cortex architecture. In fixed brain sections, nitrergic interneurons can be revealed by histochemical processing for NADPH-diaphorase (NADPHd). Here, the dendritic arbors of nitrergic neurons from different compartments of area S1 were 3D reconstructed from serial 200 μm thick sections, using 100x objective and the Neurolucida system. Standard morphological parameters were extracted for all individual arbors and compared across columns and layers. Wedge analysis was used to compute dendritic orientation indices. Supragranular (SG) layers displayed the highest density of nitrergic neurons, whereas layer IV contained nitrergic neurons with largest soma area. The highest nitrergic neuronal density was found in septa, where dendrites were previously characterized as more extense and ramified than in barrels. Dendritic arbors were not confined to the boundaries of the column nor layer of their respective soma, being mostly double-tufted and vertically oriented, except in SG layers. These data strongly suggest that nitrergic interneurons adapt their morphology to the dynamics of processing performed by cortical compartments.

Cerebral blood volume changes during brain activation

Cerebral blood volume (CBV) changes significantly with brain activation, whether measured using positron emission tomography, functional magnetic resonance imaging (fMRI), or optical microscopy. If cerebral vessels are considered to be impermeable, the contents of the skull incompressible, and the skull itself inextensible, task- and hypercapnia-related changes of CBV could produce intolerable changes of intracranial pressure. Because it is becoming clear that CBV may be useful as a well-localized marker of neural activity changes, a resolution of this apparent paradox is needed. We have explored the idea that much of the change in CBV is facilitated by exchange of water between capillaries and surrounding tissue. To this end, we developed a novel hemodynamic boundary-value model and found approximate solutions using a numerical algorithm. We also constructed a macroscopic experimental model of a single capillary to provide biophysical insight. Both experiment and theory model capillary membranes as elastic and permeable. For a realistic change of input pressure, a relative pipe volume change of 21±5% was observed when using the experimental setup, compared with the value of approximately 17±1% when this quantity was calculated from the mathematical model. Volume, axial flow, and pressure changes are in the expected range.