Interaction of nitric oxide, 20-HETE, and EETs during functional hyperemia in whisker barrel cortex

The Unexpected Role of the Endothelial Nitric Oxide Synthase at the Neurovascular Unit: Beyond the Regulation of Cerebral Blood Flow

Nitric oxide (NO) is a highly versatile gasotransmitter that has first been shown to regulate cardiovascular function and then to exert tight control over a much broader range of processes, including neurotransmitter release, neuronal excitability, and synaptic plasticity. Endothelial NO synthase (eNOS) is usually far from the mind of synaptic neurophysiologists, who have focused most of their attention on neuronal NO synthase (nNOS) as the primary source of NO at the neurovascular unit (NVU). Nevertheless, the available evidence suggests that eNOS could also contribute to generating the burst of NO that, serving as volume intercellular messenger, is produced in response to neuronal activity in the brain parenchyma. Herein, we review the role of eNOS in both the regulation of cerebral blood flow and of synaptic plasticity and discuss the mechanisms by which cerebrovascular endothelial cells may transduce synaptic inputs into a NO signal. We further suggest that eNOS could play a critical role in vascular-to-neuronal communication by integrating signals converging onto cerebrovascular endothelial cells from both the streaming blood and active neurons.

Calcium transients in nNOS neurons underlie distinct phases of the neurovascular response to barrel cortex activation in awake mice

Neuronal nitric oxide (NO) synthase (nNOS), a Ca2+ dependent enzyme, is expressed by distinct populations of neocortical neurons. Although neuronal NO is well known to contribute to the blood flow increase evoked by neural activity, the relationships between nNOS neurons activity and vascular responses in the awake state remain unclear. We imaged the barrel cortex in awake, head-fixed mice through a chronically implanted cranial window. The Ca2+ indicator GCaMP7f was expressed selectively in nNOS neurons using adenoviral gene transfer in nNOScre mice. Air-puffs directed at the contralateral whiskers or spontaneous motion induced Ca2+ transients in 30.2 ± 2.2% or 51.6 ± 3.3% of nNOS neurons, respectively, and evoked local arteriolar dilation. The greatest dilatation (14.8 ± 1.1%) occurred when whisking and motion occurred simultaneously. Ca2+ transients in individual nNOS neurons and local arteriolar dilation showed various degrees of correlation, which was strongest when the activity of whole nNOS neuron ensemble was examined. We also found that some nNOS neurons became active immediately prior to arteriolar dilation, while others were activated gradually after arteriolar dilatation. Discrete nNOS neuron subsets may contribute either to the initiation or to the maintenance of the vascular response, suggesting a previously unappreciated temporal specificity to the role of NO in neurovascular coupling.

Neurotoxic and cytoprotective mechanisms in the ischemic neocortex

Neuronal damage in ischemic stroke occurs due to permanent imbalance between the metabolic needs of the brain and the ability of the blood-vascular system to maintain glucose delivery and adequate gas exchange. Oxidative stress and excitotoxicity trigger complex processes of neuroinflammation, necrosis, and apoptosis of both neurons and glial cells. This review summarizes data on the structural and chemical changes in the neocortex and main cytoprotective effects induced by focal ischemic stroke. We focus on the expression of neurotrophins (NT) and molecular and cellular changes in neurovascular units in ischemic brain. We also discuss how these factors affect the apoptosis of cortical cells. Ischemic damage involves close interaction of a wide range of signaling molecules, each acting as an efficient marker of cell state in both the ischemic core and penumbra. NTs play the main regulatory role in brain tissue recovery after ischemic injury. Heterogeneous distribution of the BDNF, NT-3, and GDNF immunoreactivity is concordant with the selective response of different types of cortical neurons and glia to ischemic injury and allows mapping the position of viable neurons. Astrocytes are the central link in neurovascular coupling in ischemic brain by providing other cells with a wide range of vasotropic factors. The NT expression coincides with the distribution of reactive astrocytes, marking the boundaries of the penumbra. The development of ischemic stroke is accompanied by a dramatic change in the distribution of GDNF reactivity. In early ischemic period, it is mainly observed in cortical neurons, while in late one, the bulk of GDNF-positive cells are various types of glia, in particular, astrocytes. The proportion of GDNF-positive astrocytes increases gradually throughout the ischemic period. Some factors that exert cytoprotective effects in early ischemic period may display neurotoxic and pro-apoptotic effects later on. The number of apoptotic cells in the ischemic brain tissue correlates with the BDNF levels, corroborating its protective effects. Cytoprotection and neuroplasticity are two lines of brain protection and recovery after ischemic stroke. NTs can be considered an important link in these processes. To develop efficient pharmacological therapy for ischemic brain injury, we have to deepen our understanding of neurochemical adaptation of brain tissue to acute stroke.

Astrocytes amplify neurovascular coupling to sustained activation of neocortex in awake mice

Functional hyperemia occurs when enhanced neuronal activity signals to increase local cerebral blood flow (CBF) to satisfy regional energy demand. Ca2+ elevation in astrocytes can drive arteriole dilation to increase CBF, yet affirmative evidence for the necessity of astrocytes in functional hyperemia in vivo is lacking. In awake mice, we discovered that functional hyperemia is bimodal with a distinct early and late component whereby arteriole dilation progresses as sensory stimulation is sustained. Clamping astrocyte Ca2+ signaling in vivo by expressing a plasma membrane Ca2+ ATPase (CalEx) reduces sustained but not brief sensory-evoked arteriole dilation. Elevating astrocyte free Ca2+ using chemogenetics selectively augments sustained hyperemia. Antagonizing NMDA-receptors or epoxyeicosatrienoic acid production reduces only the late component of functional hyperemia, leaving brief increases in CBF to sensory stimulation intact. We propose that a fundamental role of astrocyte Ca2+ is to amplify functional hyperemia when neuronal activation is prolonged.

The Effect of a Neuronal Nitric Oxide Synthase Inhibitor on Neurovascular Regulation in Humans

BACKGROUND

Neurovascular coupling (NVC) is a key process in cerebral blood flow regulation. NVC ensures adequate brain perfusion to changes in local metabolic demands. Neuronal nitric oxide synthase (nNOS) is suspected to be involved in NVC; however, this has not been tested in humans. Our objective was to investigate the effects of nNOS inhibition on NVC in humans.

METHODS

We performed a 3-visit partially randomized, double-blinded, placebo-controlled, crossover study in 12 healthy subjects. On each visit, subjects received an intravenous infusion of either S-methyl-L-thiocitrulline (a selective nNOS-inhibitor), 0.9% saline (placebo control), or phenylephrine (pressor control). The NVC assessment involved eliciting posterior circulation hyperemia through visual stimulation while measuring posterior and middle cerebral arteries blood velocity.

RESULTS

nNOS inhibition blunted the rapidity of the NVC response versus pressor control, evidenced by a reduced initial rise in mean posterior cerebral artery velocity (-3.3% [-6.5, -0.01], P=0.049), and a reduced rate of increase (ie, acceleration) in posterior cerebral artery velocity (slope reduced -4.3% [-8.5, -0.1], P=0.045). The overall magnitude of posterior cerebral artery response relative to placebo control or pressor control was not affected. Changes in BP parameters were well-matched between the S-methyl-L-thiocitrulline and pressor control arms.

CONCLUSIONS

Neuronal NOS plays a role in dynamic cerebral blood flow control in healthy adults, particularly the rapidity of the NVC response to visual stimulation. This work opens the way to further investigation of the role of nNOS in conditions of impaired NVC, potentially revealing a therapeutic target.

Endothelial Cells and the Cerebral Circulation

Endothelial cells form the innermost layer of all blood vessels and are the only vascular component that remains throughout all vascular segments. The cerebral vasculature has several unique properties not found in the peripheral circulation; this requires that the cerebral endothelium be considered as a unique entity. Cerebral endothelial cells perform several functions vital for brain health. The cerebral vasculature is responsible for protecting the brain from external threats carried in the blood. The endothelial cells are central to this requirement as they form the basis of the blood-brain barrier. The endothelium also regulates fibrinolysis, thrombosis, platelet activation, vascular permeability, metabolism, catabolism, inflammation, and white cell trafficking. Endothelial cells regulate the changes in vascular structure caused by angiogenesis and artery remodeling. Further, the endothelium contributes to vascular tone, allowing proper perfusion of the brain which has high energy demands and no energy stores. In this article, we discuss the basic anatomy and physiology of the cerebral endothelium. Where appropriate, we discuss the detrimental effects of high blood pressure on the cerebral endothelium and the contribution of cerebrovascular disease endothelial dysfunction and dementia. © 2022 American Physiological Society. Compr Physiol 12:3449-3508, 2022.

Gradual Not Sudden Change: Multiple Sites of Functional Transition Across the Microvascular Bed

In understanding the role of the neurovascular unit as both a biomarker and target for disease interventions, it is vital to appreciate how the function of different components of this unit change along the vascular tree. The cells of the neurovascular unit together perform an array of vital functions, protecting the brain from circulating toxins and infection, while providing nutrients and clearing away waste products. To do so, the brain's microvasculature dilates to direct energy substrates to active neurons, regulates access to circulating immune cells, and promotes angiogenesis in response to decreased blood supply, as well as pulsating to help clear waste products and maintain the oxygen supply. Different parts of the cerebrovascular tree contribute differently to various aspects of these functions, and previously, it has been assumed that there are discrete types of vessel along the vascular network that mediate different functions. Another option, however, is that the multiple transitions in function that occur across the vascular network do so at many locations, such that vascular function changes gradually, rather than in sharp steps between clearly distinct vessel types. Here, by reference to new data as well as by reviewing historical and recent literature, we argue that this latter scenario is likely the case and that vascular function gradually changes across the network without clear transition points between arteriole, precapillary arteriole and capillary. This is because classically localized functions are in fact performed by wide swathes of the vasculature, and different functional markers start and stop being expressed at different points along the vascular tree. Furthermore, vascular branch points show alterations in their mural cell morphology that suggest functional specializations irrespective of their position within the network. Together this work emphasizes the need for studies to consider where transitions of different functions occur, and the importance of defining these locations, in order to better understand the vascular network and how to target it to treat disease.

Luseogliflozin, a sodium-glucose cotransporter-2 inhibitor, reverses cerebrovascular dysfunction and cognitive impairments in 18-mo-old diabetic animals

Diabetes mellitus (DM) is a leading risk factor for age-related dementia, but the mechanisms involved are not well understood. We previously discovered that hyperglycemia induced impaired myogenic response (MR) and cerebral blood flow (CBF) autoregulation in 18-mo-old DM rats associated with blood-brain barrier (BBB) leakage, impaired neurovascular coupling, and cognitive impairment. In the present study, we examined whether reducing plasma glucose with a sodium-glucose cotransporter-2 inhibitor (SGLT2i) luseogliflozin can ameliorate cerebral vascular and cognitive function in diabetic rats. Plasma glucose and HbA1c levels of 18-mo-old DM rats were reduced, and blood pressure was not altered after treatment with luseogliflozin. SGLT2i treatment restored the impaired MR of middle cerebral arteries (MCAs) and parenchymal arterioles and surface and deep cortical CBF autoregulation in DM rats. Luseogliflozin treatment also rescued neurovascular uncoupling, reduced BBB leakage and cognitive deficits in DM rats. However, SGLT2i did not have direct constrictive effects on vascular smooth muscle cells and MCAs isolated from normal rats, although it decreased reactive oxygen species production in cerebral vessels of DM rats. These results provide evidence that normalization of hyperglycemia with an SGLT2i can reverse cerebrovascular dysfunction and cognitive impairments in rats with long-standing hyperglycemia, possibly by ameliorating oxidative stress-caused vascular damage.NEW & NOTEWORTHY This study demonstrates that luseogliflozin, a sodium-glucose cotransporter-2 inhibitor, improved CBF autoregulation in association with reduced vascular oxidative stress and AGEs production in the cerebrovasculature of 18-mo-old DM rats. SGLT2i also prevented BBB leakage, impaired functional hyperemia, neurodegeneration, and cognitive impairment seen in DM rats. Luseogliflozin did not have direct constrictive effects on VSMCs and MCAs isolated from normal rats. These results provide evidence that normalization of hyperglycemia with an SGLT2i can reverse cerebrovascular dysfunction and cognitive impairments in rats with long-standing hyperglycemia, possibly by ameliorating oxidative stress-caused vascular damage.

Regulation of the Cerebral Circulation During Development

The cerebral microcirculation undergoes dynamic changes in parallel with the development of neurons, glia, and their energy metabolism throughout gestation and postnatally. Cerebral blood flow (CBF), oxygen consumption, and glucose consumption are as low as 20% of adult levels in humans born prematurely but eventually exceed adult levels at ages 3 to 11 years, which coincide with the period of continued brain growth, synapse formation, synapse pruning, and myelination. Neurovascular coupling to sensory activation is present but attenuated at birth. By 2 postnatal months, the increase in CBF often is disproportionately smaller than the increase in oxygen consumption, in contrast to the relative hyperemia seen in adults. Vascular smooth muscle myogenic tone increases in parallel with developmental increases in arterial pressure. CBF autoregulatory response to increased arterial pressure is intact at birth but has a more limited range with arterial hypotension. Hypoxia-induced vasodilation in preterm fetal sheep with low oxygen consumption does not sustain cerebral oxygen transport, but the response becomes better developed for sustaining oxygen transport by term. Nitric oxide tonically inhibits vasomotor tone, and glutamate receptor activation can evoke its release in lambs and piglets. In piglets, astrocyte-derived carbon monoxide plays a central role in vasodilation evoked by glutamate, ADP, and seizures, and prostanoids play a large role in endothelial-dependent and hypercapnic vasodilation. Overall, homeostatic mechanisms of CBF regulation in response to arterial pressure, neuronal activity, carbon dioxide, and oxygenation are present at birth but continue to develop postnatally as neurovascular signaling pathways are dynamically altered and integrated. © 2021 American Physiological Society. Compr Physiol 11:1-62, 2021.

Soluble epoxide hydrolase inhibition improves cognitive function and parenchymal artery dilation in a hypertensive model of chronic cerebral hypoperfusion

OBJECTIVE

Parenchymal arterioles (PAs) regulate perfusion of the cerebral microcirculation, and impaired PA endothelium-dependent dilation occurs in dementia models mimicking chronic cerebral hypoperfusion (CCH). Epoxyeicosatrienoic acids (EETs) are vasodilators; their actions are potentiated by soluble epoxide hydrolase (sEH) inhibition. We hypothesized that chronic sEH inhibition with trifluoromethoxyphenyl-3 (1-propionylpiperidin-4-yl) urea (TPPU) would prevent cognitive dysfunction and improve PA dilation in a hypertensive CCH model.

METHODS

Bilateral carotid artery stenosis (BCAS) was used to induce CCH in twenty-week-old male stroke-prone spontaneously hypertensive rats (SHSRP) that were treated with vehicle or TPPU for 8 weeks. Cognitive function was assessed by novel object recognition. PA dilation and structure were assessed by pressure myography, and mRNA expression in brain tissue was assessed by qRT-PCR.

RESULTS

TPPU did not enhance resting cerebral perfusion, but prevented CCH-induced memory deficits. TPPU improved PA endothelium-dependent dilation but reduced the sensitivity of PAs to a nitric oxide donor. TPPU treatment had no effect on PA structure or biomechanical properties. TPPU treatment increased brain mRNA expression of brain derived neurotrophic factor, doublecortin, tumor necrosis factor-alpha, sEH, and superoxide dismutase 3, CONCLUSIONS: These data suggest that sEH inhibitors may be viable treatments for cognitive impairments associated with hypertension and CCH.

Aging exacerbates impairments of cerebral blood flow autoregulation and cognition in diabetic rats

Diabetes mellitus (DM) is a leading risk factor for aging-related dementia; however, the underlying mechanisms are not well understood. The present study, utilizing a non-obese T2DN diabetic model, demonstrates that the myogenic response of the middle cerebral artery (MCA) and parenchymal arteriole (PA) and autoregulation of cerebral blood flow (CBF) in the surface and deep cortex were impaired at both young and old ages. The impaired CBF autoregulation was more severe in old than young DM rats, and in the deep than the surface cortex. The myogenic tone of the MCA was enhanced at perfusion pressure in the range of 40-100 mmHg in young DM rats but was reduced at 140-180 mmHg in old DM rats. No change of the myogenic tone of the PA was observed in young DM rats, whereas it was significantly reduced at 30-60 mmHg in old DM rats. Old DM rats had enhanced blood-brain barrier (BBB) leakage and neurodegeneration, reduced vascular density, tight junction, and pericyte coverage on cerebral capillaries in the CA3 region in the hippocampus. Additionally, DM rats displayed impaired functional hyperemia and spatial learning and short- and long-term memory at both young and old ages. Old DM rats had impaired non-spatial short-term memory. These results revealed that impaired CBF autoregulation and enhanced BBB leakage plays an essential role in the pathogenesis of age- and diabetes-related dementia. These findings will lay the foundations for the discovery of anti-diabetic therapies targeting restoring CBF autoregulation to prevent the onset and progression of dementia in elderly DM.

Group 1 metabotropic glutamate receptors trigger glutamate-induced intracellular Ca2+ signals and nitric oxide release in human brain microvascular endothelial cells

Neurovascular coupling (NVC) is the mechanism whereby an increase in neuronal activity causes an increase in local cerebral blood flow (CBF) to ensure local supply of oxygen and nutrients to the activated areas. The excitatory neurotransmitter glutamate gates post-synaptic N-methyl-D-aspartate receptors to mediate extracellular Ca2+ entry and stimulate neuronal nitric oxide (NO) synthase to release NO, thereby triggering NVC. Recent work suggested that endothelial Ca2+ signals could underpin NVC by recruiting the endothelial NO synthase. For instance, acetylcholine induced intracellular Ca2+ signals followed by NO release by activating muscarinic 5 receptors in hCMEC/D3 cells, a widely employed model of human brain microvascular endothelial cells. Herein, we sought to assess whether also glutamate elicits metabotropic Ca2+ signals and NO release in hCMEC/D3 cells. Glutamate induced a dose-dependent increase in intracellular Ca2+ concentration ([Ca2+]i) that was blocked by α-methyl-4-carboxyphenylglycine and phenocopied by trans-1-amino-1,3-cyclopentanedicarboxylic acid, which, respectively, block and activate group 1 metabotropic glutamate receptors (mGluRs). Accordingly, hCMEC/D3 expressed both mGluR1 and mGluR5 and the Ca2+ response to glutamate was inhibited by their pharmacological blockade with, respectively, CPCCOEt and MTEP hydrochloride. The Ca2+ response to glutamate was initiated by endogenous Ca2+ release from the endoplasmic reticulum and endolysosomal Ca2+ store through inositol-1,4,5-trisphosphate receptors and two-pore channels, respectively, and sustained by store-operated Ca2+ entry. In addition, glutamate induced robust NO release that was suppressed by pharmacological blockade of the accompanying increase in [Ca2+]i. These data demonstrate for the first time that glutamate may induce metabotropic Ca2+ signals in human brain microvascular endothelial cells. The Ca2+ response to glutamate is likely to support NVC during neuronal activity, thereby reinforcing the emerging role of brain microvascular endothelial cells in the regulation of CBF.

Sex differences in the structure and function of rat middle cerebral arteries

Epidemiological studies demonstrate that there are sex differences in the incidence, prevalence, and outcomes of cerebrovascular disease (CVD). The present study compared the structure and composition of the middle cerebral artery (MCA), neurovascular coupling, and cerebrovascular function and cognition in young Sprague-Dawley (SD) rats. Wall thickness and the inner diameter of the MCA were smaller in females than males. Female MCA exhibited less vascular smooth muscle cells (VSMCs), diminished contractile capability, and more collagen in the media, and a thicker internal elastic lamina with fewer fenestrae compared with males. Female MCA had elevated myogenic tone, lower distensibility, and higher wall stress. The stress/strain curves shifted to the left in female vessels compared with males. The MCA of females failed to constrict compared with a decrease of 15.5 ± 1.9% in males when perfusion pressure was increased from 40 to 180 mmHg. Cerebral blood flow (CBF) rose by 57.4 ± 4.4 and 30.1 ± 3.1% in females and males, respectively, when perfusion pressure increased from 100 to 180 mmHg. The removal of endothelia did not alter the myogenic response in both sexes. Functional hyperemia responses to whisker-barrel stimulation and cognition examined with an eight-arm water maze were similar in both sexes. These results demonstrate that there are intrinsic structural differences in the MCA between sexes, which are associated with diminished myogenic response and CBF autoregulation in females. The structural differences do not alter neurovascular coupling and cognition at a young age; however, they might play a role in the development of CVD after menopause.

NEW & NOTEWORTHY Using perfusion fixation of the middle cerebral artery (MCA) in calcium-free solution at physiological pressure and systematically randomly sampling the sections prepared from the same M2 segments of MCA, we found that there are structural differences that are associated with altered cerebral blood flow (CBF) autoregulation but not neurovascular coupling and cognition in young, healthy Sprague-Dawley (SD) rats. Understanding the intrinsic differences in cerebrovascular structure and function in males and females is essential to develop new pharmaceutical treatments for cerebrovascular disease (CVD).

Administration of a 20-Hydroxyeicosatetraenoic Acid Synthesis Inhibitor Improves Outcome in a Rat Model of Pediatric Traumatic Brain Injury

The arachidonic acid pathway metabolite 20-hydroxyeicosatetraenoic acid (20-HETE) contributes to ischemia/reperfusion brain injury. Inhibition of 20-HETE formation can protect the developing brain from global ischemia. Here, we examined whether treatment with the 20-HETE synthesis inhibitor N-hydroxy-N-4-butyl-2-methylphenylformamidine (HET0016) can protect the immature brain from traumatic brain injury (TBI). Male rats at postnatal day 9-10 underwent controlled cortical impact followed by intraperitoneal injection with vehicle or HET0016 (1 mg/kg, 5 min and 3 h post-injury). HET0016 decreased the lesion volume by over 50% at 3 days of recovery, and this effect persisted at 30 days as the brain matured. HET0016 decreased peri-lesion gene expression of proinflammatory cytokines (tumor necrosis factor-α [TNF-α], interleukin-1β [IL-1β]) at 1 day and increased reparative cytokine (IL-4, IL-10) expression at 3 days. It also partially preserved microglial ramified processes, consistent with less activation. HET0016 decreased contralateral hindlimb foot faults and improved outcome on the novel object recognition memory task 30 days after TBI. In cultured BV2 microglia, HET0016 attenuated the lipopolysaccharide-evoked increase in release of TNF-α. Our data show that HET0016 improves acute and long-term histologic and functional outcomes, in association with an attenuated neuroinflammatory response after contusion of an immature rat brain.

In vivo imaging for neurovascular disease research

Connections between various cell types in the brain enable cognitive function. The neurovascular unit is a structure composed of different cell types that regulate neurovascular coupling, blood-brain barrier permeability, and other interactions with peripheral systems. The relationship among the components of the neurovascular unit is complex and difficult to study without the use of in vivo neurovascular disease imaging. In this review, we introduce principles and examples of various in vivo optical imaging techniques including laser Doppler flowmetry, laser speckle contrast imaging, intrinsic optical signal imaging, optical coherence tomography, and two-photon microscopy. Furthermore, we introduce recent advances of in vivo imaging and future directions for promoting neurovascular disease research.

Cytochrome P450 eicosanoids in cerebrovascular function and disease

Cytochrome P450 eicosanoids play important roles in brain function and disease through their complementary actions on cell-cell communications within the neurovascular unit (NVU) and mechanisms of brain injury. Epoxy- and hydroxyeicosanoids, respectively formed by cytochrome P450 epoxygenases and ω-hydroxylases, play opposing roles in cerebrovascular function and in pathological processes underlying neural injury, including ischemia, neuroinflammation and oxidative injury. P450 eicosanoids also contribute to cerebrovascular disease risk factors, including hypertension and diabetes. We summarize studies investigating the roles P450 eicosanoids in cerebrovascular physiology and disease to highlight the existing balance between these important lipid signaling molecules, as well as their roles in maintaining neurovascular homeostasis and in acute and chronic neurovascular and neurodegenerative disorders.

Global assessment of oxidized free fatty acids in brain reveals an enzymatic predominance to oxidative signaling after trauma

Traumatic brain injury (TBI) is a major health problem associated with significant morbidity and mortality. The pathophysiology of TBI is complex involving signaling through multiple cascades, including lipid peroxidation. Oxidized free fatty acids, a prominent product of lipid peroxidation, are potent cellular mediators involved in induction and resolution of inflammation and modulation of vasomotor tone. While previous studies have assessed lipid peroxidation after TBI, to our knowledge no studies have used a systematic approach to quantify the global oxidative changes in free fatty acids. In this study, we identified and quantified 244 free fatty acid oxidation products using a newly developed global liquid chromatography tandem-mass spectrometry (LC-MS/MS) method. This methodology was used to follow the time course of these lipid species in the contusional cortex of our pediatric rat model of TBI. We show that oxidation peaked at 1h after controlled cortical impact and was progressively attenuated at 4 and 24h time points. While enzymatic and non-enzymatic pathways were activated at 1h post-TBI, enzymatic lipid peroxidation was the predominant mechanism with 15-lipoxygenase (LOX) contributing to the majority of total oxidized fatty acid content. Pro-inflammatory lipid mediators were significantly increased at 1 and 4h after TBI with return to basal levels by 24h. Anti-inflammatory lipid mediators remained significantly increased across all three time points, indicating an elevated and sustained anti-inflammatory response following TBI.

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.

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.

Clinical Implications of 20-Hydroxyeicosatetraenoic Acid in the Kidney, Liver, Lung and Brain: An Emerging Therapeutic Target

Cytochrome P450-mediated metabolism of arachidonic acid (AA) is an important pathway for the formation of eicosanoids. The ω-hydroxylation of AA generates significant levels of 20-hydroxyeicosatetraenoic acid (20-HETE) in various tissues. In the current review, we discussed the role of 20-HETE in the kidney, liver, lung, and brain during physiological and pathophysiological states. Moreover, we discussed the role of 20-HETE in tumor formation, metabolic syndrome and diabetes. In the kidney, 20-HETE is involved in modulation of preglomerular vascular tone and tubular ion transport. Furthermore, 20-HETE is involved in renal ischemia/reperfusion (I/R) injury and polycystic kidney diseases. The role of 20-HETE in the liver is not clearly understood although it represents 50%-75% of liver CYP-dependent AA metabolism, and it is associated with liver cirrhotic ascites. In the respiratory system, 20-HETE plays a role in pulmonary cell survival, pulmonary vascular tone and tone of the airways. As for the brain, 20-HETE is involved in cerebral I/R injury. Moreover, 20-HETE has angiogenic and mitogenic properties and thus helps in tumor promotion. Several inhibitors and inducers of the synthesis of 20-HETE as well as 20-HETE analogues and antagonists are recently available and could be promising therapeutic options for the treatment of many disease states in the future.

Functional vascular contributions to cognitive impairment and dementia: mechanisms and consequences of cerebral autoregulatory dysfunction, endothelial impairment, and neurovascular uncoupling in aging

Increasing evidence from epidemiological, clinical and experimental studies indicate that age-related cerebromicrovascular dysfunction and microcirculatory damage play critical roles in the pathogenesis of many types of dementia in the elderly, including Alzheimer's disease. Understanding and targeting the age-related pathophysiological mechanisms that underlie vascular contributions to cognitive impairment and dementia (VCID) are expected to have a major role in preserving brain health in older individuals. Maintenance of cerebral perfusion, protecting the microcirculation from high pressure-induced damage and moment-to-moment adjustment of regional oxygen and nutrient supply to changes in demand are prerequisites for the prevention of cerebral ischemia and neuronal dysfunction. This overview discusses age-related alterations in three main regulatory paradigms involved in the regulation of cerebral blood flow (CBF): cerebral autoregulation/myogenic constriction, endothelium-dependent vasomotor function, and neurovascular coupling responses responsible for functional hyperemia. The pathophysiological consequences of cerebral microvascular dysregulation in aging are explored, including blood-brain barrier disruption, neuroinflammation, exacerbation of neurodegeneration, development of cerebral microhemorrhages, microvascular rarefaction, and ischemic neuronal dysfunction and damage. Due to the widespread attention that VCID has captured in recent years, the evidence for the causal role of cerebral microvascular dysregulation in cognitive decline is critically examined.

IGF-1 deficiency impairs neurovascular coupling in mice: implications for cerebromicrovascular aging

Aging is associated with marked deficiency in circulating IGF‐1, which has been shown to contribute to age‐related cognitive decline. Impairment of moment‐to‐moment adjustment of cerebral blood flow (CBF) via neurovascular coupling is thought to play a critical role in the genesis of age‐related cognitive impairment. To establish the link between IGF‐1 deficiency and cerebromicrovascular impairment, neurovascular coupling mechanisms were studied in a novel mouse model of IGF‐1 deficiency (Igf1^f/f‐TBG‐Cre‐AAV8) and accelerated vascular aging. We found that IGF‐1‐deficient mice exhibit neurovascular uncoupling and show a deficit in hippocampal‐dependent spatial memory test, mimicking the aging phenotype. IGF‐1 deficiency significantly impaired cerebromicrovascular endothelial function decreasing NO mediation of neurovascular coupling. IGF‐1 deficiency also impaired glutamate‐mediated CBF responses, likely due to dysregulation of astrocytic expression of metabotropic glutamate receptors and impairing mediation of CBF responses by eicosanoid gliotransmitters. Collectively, we demonstrate that IGF‐1 deficiency promotes cerebromicrovascular dysfunction and neurovascular uncoupling mimicking the aging phenotype, which are likely to contribute to cognitive impairment.

Purinergic glio-endothelial coupling during neuronal activity: role of P2Y1 receptors and eNOS in functional hyperemia in the mouse somatosensory cortex

Impairment of moment-to-moment adjustment of cerebral blood flow (CBF) via neurovascular coupling is thought to play a critical role in the genesis of cognitive impairment associated with aging and pathological conditions associated with accelerated cerebromicrovascular aging (e.g., hypertension, obesity). Although previous studies demonstrate that endothelial dysfunction plays a critical role in neurovascular uncoupling in these conditions, the role of endothelial NO mediation in neurovascular coupling responses is not well understood. To establish the link between endothelial function and functional hyperemia, neurovascular coupling responses were studied in mutant mice overexpressing or deficient in endothelial NO synthase (eNOS), and the role of P2Y1 receptors in purinergic glioendothelial coupling was assessed. We found that genetic depletion of eNOS (eNOS(-/-)) and pharmacological inhibition of NO synthesis significantly decreased the CBF responses in the somatosensory cortex evoked by whisker stimulation and by administration of ATP. Overexpression of eNOS enhanced NO mediation of functional hyperemia. In control mice, the selective and potent P2Y1 receptor antagonist MRS2179 attenuated both whisker stimulation-induced and ATP-mediated CBF responses, whereas, in eNOS(-/-) mice, the inhibitory effects of MRS2179 were blunted. Collectively, our findings provide additional evidence for purinergic glio-endothelial coupling during neuronal activity, highlighting the role of ATP-mediated activation of eNOS via P2Y1 receptors in functional hyperemia.

Contribution of epoxyeicosatrienoic acids to the cerebral blood flow response to hypoxemia

Adenosine A2A receptors and ATP-activated K(+) (KATP) channels contribute to part of the cerebral vasodilatory response to systemic hypoxia, but other mediators are likely involved. Epoxyeicosatrienoic acids (EETs) are cerebral vasodilators and are released from astrocytes exposed to hypoxia. Moreover, stimulation of metabotropic glutamate receptors (mGluR) produces vasodilation by an EET-dependent mechanism. Here, we tested the hypothesis that EET signaling and mGluR activation contribute to hypoxic vasodilation. Laser-Doppler flow was measured over cerebral cortex of anesthetized rats subjected to stepwise reductions in arterial oxygen saturation to 50-70%. Hypoxic reactivity was calculated as the slope of the change in laser-Doppler flow vs. the reciprocal of arterial oxygen content. Hypoxic reactivity significantly decreased from 9.2 ± 1.9 (±95% confidence interval) in controls with vehicle treatment to 2.6 ± 1.4 with the EET antagonist 14,15-epoxyeicosa-5(Z)-enoic acid, to 3.0 ± 1.5 with the EET synthesis inhibitor MS-PPOH, to 1.9 ± 2.3 with the combined mGluR subtype 1 and 5 antagonists 2-methyl-6-(phenylethynyl)pyridine and LY367385, to 5.6 ± 1.2 with the KATP channel inhibitor glibenclamide, and to 5.8 ± 2.3 with the A2A receptor antagonist SCH58261. However, reactivity was not significantly altered by the A2B receptor antagonist MRS1754 (6.7 ± 1.8; P = 0.28 Dunnett's test) or by the 20-hydroxyeicosatetraenoic acid synthesis inhibitor HET0016 (7.5 ± 2.3; P = 0.6). These data indicate that, in addition to the known contributions of A2A receptors and KATP channels to the increase in cerebral blood flow during hypoxia, EETs and mGluRs make a major contribution, possibly by mGluR stimulation and hypoxia-induced release of EETs. In contrast, A2B receptors do not make a major contribution, and 20-hydroxyeicosatetraenoic acid does not significantly limit hypoxic vasodilation.

Additive Neuroprotection of a 20-HETE Inhibitor with Delayed Therapeutic Hypothermia after Hypoxia-Ischemia in Neonatal Piglets

The severity of perinatal hypoxia-ischemia and the delay in initiating therapeutic hypothermia limit the efficacy of hypothermia. After hypoxia-ischemia in neonatal piglets, the arachidonic acid metabolite 20-hydroxyeicosatetraenoic acid (20-HETE) has been found to contribute to oxidative stress at 3 h of reoxygenation and to eventual neurodegeneration. We tested whether early administration of a 20-HETE synthesis inhibitor after reoxygenation augments neuroprotection with 3-hour delayed hypothermia. In two hypothermic groups, whole body cooling from 38.5 to 34°C was initiated 3 h after hypoxia-ischemia. Rewarming occurred from 20 to 24 h; then anesthesia was discontinued. One hypothermic group received a 20-HETE inhibitor at 5 min after reoxygenation. A sham-operated group and another hypoxia-ischemia group remained normothermic. At 10 days of recovery, resuscitated piglets with delayed hypothermia alone had significantly greater viable neuronal density in the putamen, caudate nucleus, sensorimotor cortex, CA3 hippocampus, and thalamus than did piglets with normothermic recovery, but the values remained less than those in the sham-operated group. In piglets administered the 20-HETE inhibitor before hypothermia, the density of viable neurons in the putamen, cortex and thalamus was significantly greater than in the group with hypothermia alone. Cytochrome P450 4A, which can synthesize 20-HETE, was expressed in piglet neurons in these regions. We conclude that early treatment with a 20-HETE inhibitor enhances the therapeutic benefit of delayed hypothermia in protecting neurons in brain regions known to be particularly vulnerable to hypoxia-ischemia in term newborns.

Rhythmic expression of cytochrome P450 epoxygenases CYP4x1 and CYP2c11 in the rat brain and vasculature

Mammals have circadian variation in blood pressure, heart rate, vascular tone, thrombotic tendency, and cerebral blood flow (CBF). These changes may be in part orchestrated by circadian variation in clock gene expression within cells comprising the vasculature that modulate blood flow (e.g., fibroblasts, cerebral vascular smooth muscle cells, astrocytes, and endothelial cells). However, the downstream mechanisms that underlie circadian changes in blood flow are unknown. Cytochrome P450 epoxygenases (Cyp4x1 and Cyp2c11) are expressed in the brain and vasculature and metabolize arachidonic acid (AA) to form epoxyeicosatrienoic acids (EETs). EETs are released from astrocytes, neurons, and vascular endothelial cells and act as potent vasodilators, increasing blood flow. EETs released in response to increases in neural activity evoke a corresponding increase in blood flow known as the functional hyperemic response. We examine the hypothesis that Cyp2c11 and Cyp4x1 expression and EETs production vary in a circadian manner in the rat brain and cerebral vasculature. RT-PCR revealed circadian/diurnal expression of clock and clock-controlled genes as well as Cyp4x1 and Cyp2c11, within the rat hippocampus, middle cerebral artery, inferior vena cava, hippocampal astrocytes and rat brain microvascular endothelial cells. Astrocyte and endothelial cell culture experiments revealed rhythmic variation in Cyp4x1 and Cyp2c11 gene and protein expression with a 12-h period and parallel rhythmic production of EETs. Our data suggest there is circadian regulation of Cyp4x1 and Cyp2c11 gene expression. Such rhythmic EETs production may contribute to circadian changes in blood flow and alter risk of adverse cardiovascular events throughout the day.

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.

Ryanodine receptors, calcium signaling, and regulation of vascular tone in the cerebral parenchymal microcirculation

The cerebral blood supply is delivered by a surface network of pial arteries and arterioles from which arise (parenchymal) arterioles that penetrate into the cortex and terminate in a rich capillary bed. The critical regulation of CBF, locally and globally, requires precise vasomotor regulation of the intracerebral microvasculature. This vascular region is anatomically unique as illustrated by the presence of astrocytic processes that envelope almost the entire basolateral surface of PAs. There are, moreover, notable functional differences between pial arteries and PAs. For example, in pial VSMCs, local calcium release events ("calcium sparks") through ryanodine receptor (RyR) channels in SR membrane activate large conductance, calcium-sensitive potassium channels to modulate vascular diameter. In contrast, VSMCs in PAs express functional RyR and BK channels, but under physiological conditions, these channels do not oppose pressure-induced vasoconstriction. Here, we summarize the roles of ryanodine receptors in the parenchymal microvasculature under physiologic and pathologic conditions, and discuss their importance in the control of CBF.

Locus coeruleus stimulation recruits a broad cortical neuronal network and increases cortical perfusion

The locus coeruleus (LC), the main source of brain noradrenalin (NA), modulates cortical activity, cerebral blood flow (CBF), glucose metabolism, and blood-brain barrier permeability. However, the role of the LC-NA system in the regulation of cortical CBF has remained elusive. This rat study shows that similar proportions (∼20%) of cortical pyramidal cells and GABA interneurons are contacted by LC-NA afferents on their cell soma or proximal dendrites. LC stimulation induced ipsilateral activation (c-Fos upregulation) of pyramidal cells and of a larger proportion (>36%) of interneurons that colocalize parvalbumin, somatostatin, or nitric oxide synthase compared with pyramidal cells expressing cyclooxygenase-2 (22%, p < 0.05) or vasoactive intestinal polypeptide-containing interneurons (16%, p < 0.01). Concurrently, LC stimulation elicited larger ipsilateral compared with contralateral increases in cortical CBF (52 vs 31%, p < 0.01). These CBF responses were almost abolished (-70%, p < 0.001) by cortical NA denervation with DSP-4 [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride] and were significantly reduced by α- and β-adrenoceptor antagonists (-40%, p < 0.001 and -30%, p < 0.05, respectively). Blockade of glutamatergic or GABAergic neurotransmission with NMDA or GABA(A) receptor antagonists potently reduced the LC-induced hyperemic response (-56%, p < 0.001 or -47%, p < 0.05). Moreover, inhibition of astroglial metabolism (-35%, p < 0.01), vasoactive epoxyeicosatrienoic acids (EETs; -60%, p < 0.001) synthesis, large-conductance, calcium-operated (BK, -52%, p < 0.05), and inward-rectifier (Kir, -40%, p < 0.05) K+ channels primarily impaired the hyperemic response. The data demonstrate that LC stimulation recruits a broad network of cortical excitatory and inhibitory neurons resulting in increased cortical activity and that K+ fluxes and EET signaling mediate a large part of the hemodynamic response.

The effects of hypertension on the cerebral circulation

Maintenance of brain function depends on a constant blood supply. Deficits in cerebral blood flow are linked to cognitive decline, and they have detrimental effects on the outcome of ischemia. Hypertension causes alterations in cerebral artery structure and function that can impair blood flow, particularly during an ischemic insult or during periods of low arterial pressure. This review will focus on the historical discoveries, novel developments, and knowledge gaps in 1) hypertensive cerebral artery remodeling, 2) vascular function with emphasis on myogenic reactivity and endothelium-dependent dilation, and 3) blood-brain barrier function. Hypertensive artery remodeling results in reduction in the lumen diameter and an increase in the wall-to-lumen ratio in most cerebral arteries; this is linked to reduced blood flow postischemia and increased ischemic damage. Many factors that are increased in hypertension stimulate remodeling; these include the renin-angiotensin-aldosterone system and reactive oxygen species levels. Endothelial function, vital for endothelium-mediated dilation and regulation of myogenic reactivity, is impaired in hypertension. This is a consequence of alterations in vasodilator mechanisms involving nitric oxide, epoxyeicosatrienoic acids, and ion channels, including calcium-activated potassium channels and transient receptor potential vanilloid channel 4. Hypertension causes blood-brain barrier breakdown by mechanisms involving inflammation, oxidative stress, and vasoactive circulating molecules. This exposes neurons to cytotoxic molecules, leading to neuronal loss, cognitive decline, and impaired recovery from ischemia. As the population ages and the incidence of hypertension, stroke, and dementia increases, it is imperative that we gain a better understanding of the control of cerebral artery function in health and disease.

Multifunctional role of astrocytes as gatekeepers of neuronal energy supply

Dynamic adjustments to neuronal energy supply in response to synaptic activity are critical for neuronal function. Glial cells known as astrocytes have processes that ensheath most central synapses and express G-protein-coupled neurotransmitter receptors and transporters that respond to neuronal activity. Astrocytes also release substrates for neuronal oxidative phosphorylation and have processes that terminate on the surface of brain arterioles and can influence vascular smooth muscle tone and local blood flow. Membrane receptor or transporter-mediated effects of glutamate represent a convergence point of astrocyte influence on neuronal bioenergetics. Astrocytic glutamate uptake drives glycolysis and subsequent shuttling of lactate from astrocytes to neurons for oxidative metabolism. Astrocytes also convert synaptically reclaimed glutamate to glutamine, which is returned to neurons for glutamate salvage or oxidation. Finally, astrocytes store brain energy currency in the form of glycogen, which can be mobilized to produce lactate for neuronal oxidative phosphorylation in response to glutamatergic neurotransmission. These mechanisms couple synaptically driven astrocytic responses to glutamate with release of energy substrates back to neurons to match demand with supply. In addition, astrocytes directly influence the tone of penetrating brain arterioles in response to glutamatergic neurotransmission, coordinating dynamic regulation of local blood flow. We will describe the role of astrocytes in neurometabolic and neurovascular coupling in detail and discuss, in turn, how astrocyte dysfunction may contribute to neuronal bioenergetic deficit and neurodegeneration. Understanding the role of astrocytes as a hub for neurometabolic and neurovascular coupling mechanisms is a critical underpinning for therapeutic development in a broad range of neurodegenerative disorders characterized by chronic generalized brain ischemia and brain microvascular dysfunction.

Increased 20-HETE synthesis explains reduced cerebral blood flow but not impaired neurovascular coupling after cortical spreading depression in rat cerebral cortex

Cortical spreading depression (CSD) is associated with release of arachidonic acid, impaired neurovascular coupling, and reduced cerebral blood flow (CBF), caused by cortical vasoconstriction. We tested the hypothesis that the released arachidonic acid is metabolized by the cytochrome P450 enzyme to produce the vasoconstrictor 20-hydroxyeicosatetraenoic acid (20-HETE), and that this mechanism explains cortical vasoconstriction and vascular dysfunction after CSD. CSD was induced in the frontal cortex of rats and the cortical electrical activity and local field potentials recorded by glass microelectrodes, CBF by laser Doppler flowmetry, and tissue oxygen tension (tpO(2)) using polarographic microelectrodes. 20-HETE synthesis was measured in parallel experiments in cortical brain slices exposed to CSD. We used the specific inhibitor HET0016 (N-hydroxy-N'-(4-n-butyl-2-methylphenyl)formamidine) to block 20-HETE synthesis. CSD increased 20-HETE synthesis in brain slices for 120 min, and the time course of the increase in 20-HETE paralleled the reduction in CBF after CSD in vivo. HET0016 blocked the CSD-induced increase in 20-HETE synthesis and ameliorated the persistent reduction in CBF, but not the impaired neurovascular coupling after CSD. These findings suggest that CSD-induced increments in 20-HETE cause the reduction in CBF after CSD and that the attenuation of stimulation-induced CBF responses after CSD has a different mechanism. We suggest that blockade of 20-HETE synthesis may be clinically relevant to ameliorate reduced CBF in patients with migraine and acute brain cortex injuries.

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.

Effects of angiotensin II on the cerebral circulation: role of oxidative stress

Oxidative stress has emerged as a key component of many diseases that affect the vasculature. Oxidative stress is characterized as a cellular environment where the generation of oxidant molecules overwhelms endogenous anti-oxidant defense mechanisms. NADPH oxidases are a family of enzymes whose primary purpose is generation of reactive oxygen species (oxidant molecules) and therefore are likely to be key contributors to oxidative stress. Hypertension is associated with oxidative stress in the vasculature and is a major risk factor for stroke and cognitive abnormalities. Angiotensin II (Ang II) is the main effector peptide of the renin-angiotensin system (RAS) and plays a critical role in promoting oxidative stress in the vasculature. In the cerebral circulation, Ang II has been implicated in reactive oxygen species generation, alterations to vasomotor function, impaired neurovascular coupling, inflammation, and vascular remodeling. Furthermore, studies in humans have shown that cerebral blood flow is altered during hypertension and therapeutically targeting the RAS improves cerebral blood flow. Importantly, many of the aforementioned effects have been shown to be dependent on NADPH oxidases. Thus, Ang II, NADPH oxidases and oxidative stress are likely to play key roles in the pathogenesis of hypertension and associated cerebrovascular disease. This review will focus on our current understanding of the contribution of Ang II and NADPH oxidases to oxidative stress in the cerebral circulation.

The locus coeruleus-norepinephrine network optimizes coupling of cerebral blood volume with oxygen demand

Given the brain's uniquely high cell density and tissue oxygen levels bordering on hypoxia, the ability to rapidly and precisely match blood flow to constantly changing patterns in neural activity is an essential feature of cerebrovascular regulation. Locus coeruleus-norepinephrine (LC-NE) projections innervate the cerebral vasculature and can mediate vasoconstriction. However, function of the LC-mediated constriction in blood-flow regulation has never been addressed. Here, using intrinsic optical imaging coupled with an anesthesia regimen that only minimally interferes with LC activity, we show that NE enhances spatial and temporal aspects of functional hyperemia in the mouse somatosensory cortex. Increasing NE levels in the cortex using an α(2)-adrenergic receptor antagonist paradoxically reduces the extent of functional hyperemia while enhancing the surround blood-flow reduction. However, the NE-mediated vasoconstriction optimizes spatial and temporal focusing of the hyperemic response resulting in a sixfold decrease in the disparity between blood volume and oxygen demand. In addition, NE-mediated vasoconstriction accelerated redistribution to subsequently active regions, enhancing temporal synchronization of blood delivery. These observations show an important role for NE in optimizing neurovascular coupling. As LC neuron loss is prominent in Alzheimer and Parkinson diseases, the diminished ability to couple blood volume to oxygen demand may contribute to their pathogenesis.

Activation of cortical 5-HT(3) receptor-expressing interneurons induces NO mediated vasodilatations and NPY mediated vasoconstrictions

GABAergic interneurons are local integrators of cortical activity that have been reported to be involved in the control of cerebral blood flow (CBF) through their ability to produce vasoactive molecules and their rich innervation of neighboring blood vessels. They form a highly diverse population among which the serotonin 5-hydroxytryptamine 3A receptor (5-HT_3A)-expressing interneurons share a common developmental origin, in addition to the responsiveness to serotonergic ascending pathway. We have recently shown that these neurons regroup two distinct subpopulations within the somatosensory cortex: Neuropeptide Y (NPY)-expressing interneurons, displaying morphological properties similar to those of neurogliaform cells and Vasoactive Intestinal Peptide (VIP)-expressing bipolar/bitufted interneurons. The aim of the present study was to determine the role of these neuronal populations in the control of vascular tone by monitoring blood vessels diameter changes, using infrared videomicroscopy in mouse neocortical slices. Bath applications of 1-(3-Chlorophenyl)biguanide hydrochloride (mCPBG), a 5-HT₃R agonist, induced both constrictions (30%) and dilations (70%) of penetrating arterioles within supragranular layers. All vasoconstrictions were abolished in the presence of the NPY receptor antagonist (BIBP 3226), suggesting that they were elicited by NPY release. Vasodilations persisted in the presence of the VIP receptor antagonist VPAC1 (PG-97-269), whereas they were blocked in the presence of the neuronal Nitric Oxide (NO) Synthase (nNOS) inhibitor, L-NNA. Altogether, these results strongly suggest that activation of neocortical 5-HT_3A-expressing interneurons by serotoninergic input could induces NO mediated vasodilatations and NPY mediated vasoconstrictions.

The complex contribution of NOS interneurons in the physiology of cerebrovascular regulation

Following the discovery of the vasorelaxant properties of nitric oxide (NO) by Furchgott and Ignarro, the finding by Bredt and coll. of a constitutively expressed NO synthase in neurons (nNOS) led to the presumption that neuronal NO may control cerebrovascular functions. Consequently, numerous studies have sought to determine whether neuraly-derived NO is involved in the regulation of cerebral blood flow (CBF). Anatomically, axons, dendrites, or somata of NO neurons have been found to contact the basement membrane of blood vessels or perivascular astrocytes in all segments of the cortical microcirculation. Functionally, various experimental approaches support a role of neuronal NO in the maintenance of resting CBF as well as in the vascular response to neuronal activity. Since decades, it has been assumed that neuronal NO simply diffuses to the local blood vessels and produce vasodilation through a cGMP-PKG dependent mechanism. However, NO is not the sole mediator of vasodilation in the cerebral microcirculation and is known to interact with a myriad of signaling pathways also involved in vascular control. In addition, cerebrovascular regulation is the result of a complex orchestration between all components of the neurovascular unit (i.e., neuronal, glial, and vascular cells) also known to produce NO. In this review article, the role of NO interneuron in the regulation of cortical microcirculation will be discussed in the context of the neurovascular unit.

Astrocyte inositol triphosphate receptor type 2 and cytosolic phospholipase A2 alpha regulate arteriole responses in mouse neocortical brain slices

Functional hyperemia of the cerebral vascular system matches regional blood flow to the metabolic demands of the brain. One current model of neurovascular control holds that glutamate released by neurons activates group I metabotropic glutamate receptors (mGluRs) on astrocytes, resulting in the production of diffusible messengers that act to regulate smooth muscle cells surrounding cerebral arterioles. The acute mouse brain slice is an experimental system in which changes in arteriole diameter can precisely measured with light microscopy. Stimulation of the brain slice triggers specific cellular responses that can be correlated to changes in arteriole diameter. Here we used inositol trisphosphate receptor type 2 (IP(3)R2) and cytosolic phospholipase A(2) alpha (cPLA(2)α) deficient mice to determine if astrocyte mGluR activation coupled to IP(3)R2-mediated Ca(2+) release and subsequent cPLA(2)α activation is required for arteriole regulation. We measured changes in astrocyte cytosolic free Ca(2+) and arteriole diameters in response to mGluR agonist or electrical field stimulation in acute neocortical mouse brain slices maintained in 95% or 20% O(2). Astrocyte Ca(2+) and arteriole responses to mGluR activation were absent in IP(3)R2(-/-) slices. Astrocyte Ca(2+) responses to mGluR activation were unchanged by deletion of cPLA(2)α but arteriole responses to either mGluR agonist or electrical stimulation were ablated. The valence of changes in arteriole diameter (dilation/constriction) was dependent upon both stimulus and O(2) concentration. Neuron-derived NO and activation of the group I mGluRs are required for responses to electrical stimulation. These findings indicate that an mGluR/IP(3)R2/cPLA(2)α signaling cascade in astrocytes is required to transduce neuronal glutamate release into arteriole responses.

Cell-to-cell communication and vascular dementia

OBJECTIVE

VaD is the second-most common form of dementia, second only to that caused by AD. As the name indicates, VaD is predominantly considered a disease caused by vascular phenomena.

METHODS

In this invited review, we introduce the reader to recent developments in defining VaD as a unique form of dementia by reviewing the current pertinent literature. We discuss the clinical and experimental evidence that supports the notion that the microcirculation, specifically cell-to-cell communication, likely contributes to the development of VaD. Through exploration of the concept of the NVU, we elucidate the extensive cerebrovascular communication that exists and highlight models that may help test the contribution(s) of cell-to-cell communication at the microvascular level to the development and progression of VaD. Lastly, we explore the possibility that some dementia, generally considered to be purely neurodegenerative, may actually have a vascular component at the neurovascular level.

CONCLUSION

This latter recognition potentially broadens the critical involvement of microvascular events that contribute to the numerous dementias affecting an increasingly larger sector of the adult population.

Epoxides and soluble epoxide hydrolase in cardiovascular physiology

Epoxyeicosatrienoic acids (EETs) are arachidonic acid metabolites that importantly contribute to vascular and cardiac physiology. The contribution of EETs to vascular and cardiac function is further influenced by soluble epoxide hydrolase (sEH) that degrades EETs to diols. Vascular actions of EETs include dilation and angiogenesis. EETs also decrease inflammation and platelet aggregation and in general act to maintain vascular homeostasis. Myocyte contraction and increased coronary blood flow are the two primary EET actions in the heart. EET cell signaling mechanisms are tissue and organ specific and provide significant evidence for the existence of EET receptors. Additionally, pharmacological and genetic manipulations of EETs and sEH have demonstrated a contribution for this metabolic pathway to cardiovascular diseases. Given the impact of EETs to cardiovascular physiology, there is emerging evidence that development of EET-based therapeutics will be beneficial for cardiovascular diseases.

Calcium signaling in cerebral vasoregulation

The tight coupling of regional neurometabolic activity with synaptic activity and regional cerebral blood perfusion constitutes a single functional unit, described generally as a neurovascular unit. This is central to any discussion of haemodynamic response linked to any neuronal activation. In normal as well as in pathologic conditions, neurons, astrocytes and endothelial cells of the vasculature interact to generate the complex activity-induced cerebral haemodynamic responses, with astrocytes not only partaking in the signaling but actually controlling it in many cases. Neurons and astrocytes have highly integrated signaling mechanisms, yet they form two separate networks. Bidirectional neuron-astrocyte interactions are crucial for the function and survival of the central nervous system. The primary purpose of such regulation is the homeostasis of the brain's microenvironment. In the maintenance of such homeostasis, astrocytic calcium response is a crucial variable in determining neurovascular control. Future work will be directed towards resolving the nature and extent of astrocytic calcium-mediated mechanisms for gene transcription, in modelling neurovascular control, and in determining calcium sensitive imaging assays that can capture disease variables.

Relative contribution of cyclooxygenases, epoxyeicosatrienoic acids, and pH to the cerebral blood flow response to vibrissal stimulation

The increase in cerebral blood flow (CBF) during neuronal activation can be only partially attenuated by individual inhibitors of epoxyeicosatrienoic acids (EETs), cyclooxgenase-2, group I metabotropic glutamate receptors (mGluR), neuronal nitric oxide synthase (nNOS), N-methyl-D-aspartate receptors, or adenosine receptors. Some studies that used a high concentration (500 μM) of the cyclooxygenase-1 inhibitor SC-560 have implicated cyclooxygenase-1 in gliovascular coupling in certain model systems in the mouse. Here, we found that increasing the concentration of SC-560 from 25 μM to 500 μM over whisker barrel cortex in anesthetized rats attenuated the CBF response to whisker stimulation. However, exogenous prostaglandin E(2) restored the response in the presence of 500 μM SC-560 but not in the presence of a cyclooxygenase-2 inhibitor, thereby suggesting a limited permissive role for cyclooxygenase-1. Furthermore, inhibition of the CBF response to whisker stimulation by an EET antagonist persisted in the presence of SC-560 or a cyclooxygenase-2 inhibitor, thereby indicating that the EET-dependent component of vasodilation did not require cyclooxygenase-1 or -2 activity. With combined inhibition of cyclooxygenase-1 and -2, mGluR, nNOS, EETs, N-methyl-D-aspartate receptors, and adenosine 2B receptors, the CBF response was reduced by 60%. We postulated that the inability to completely block the CBF response was due to tissue acidosis resulting from impaired clearance of metabolically produced CO2. We tested this idea by increasing the concentration of superfused bicarbonate from 25 to 60 mM and found a markedly reduced CBF response to hypercapnia. However, increasing bicarbonate had no effect on the initial or steady-state CBF response to whisker stimulation with or without combined inhibition. We conclude that the residual response after inhibition of several known vasodilatory mechanisms is not due to acidosis arising from impaired CO2 clearance when the CBF response is reduced. An unidentified mechanism apparently is responsible for the rapid, residual cortical vasodilation during vibrissal stimulation.

Role of the adenosine(2A) receptor-epoxyeicosatrienoic acid pathway in the development of salt-sensitive hypertension

Activation of rat adenosine(2A) receptors (A(2A) R) dilates preglomerular microvessels, an effect mediated by epoxyeicosatrienoic acids (EETs). High salt (HS) intake increases epoxygenase activity and adenosine levels. A greater vasodilator response to a stable adenosine analog, 2-chloroadenosine (2-CA), was seen in kidneys obtained from HS-fed rats which was mediated by increased EET release. Because this pathway is antipressor, we examined the role of the A(2A) R-EET pathway in a genetic model of salt-sensitive hypertension, the Dahl salt-sensitive (SS) rats. Dahl salt resistant (SR) rats fed a HS diet demonstrated a greater renal vasodilator response to 2-CA. In contrast, Dahl SS rats did not exhibit a difference in the vasodilator response to 2-CA whether fed normal salt (NS) or HS diet. In Dahl SR but not Dahl SS rats, HS intake significantly increased purine flux, augmented the protein expression of A(2A) R and cytochrome P450 2C23 and 2C11 epoxygenases, and elevated the renal efflux of EETs. Thus the Dahl SR rat is able to respond to HS intake by recruiting EET formation, whereas the Dahl SS rat appears to have exhausted its ability to increase EET synthesis above the levels observed on NS intake. In vivo inhibition of the A(2A) R-EET pathway in Dahl SR rats fed a HS diet results in reduced renal EETs levels, diminished natriuretic capacity and hypertension, thus supporting a role for the A(2A) R-EET pathway in the adaptive natriuretic response to modulate blood pressure during salt loading. An inability of Dahl SS rats to upregulate the A(2A) R-EET pathway in response to salt loading may contribute to the development of salt-sensitive hypertension.

The neurovascular unit in brain function and disease

Neurovascular coupling, or functional hyperaemia, refers to complex mechanisms of communication between neurons, astrocytes and cerebral vessels which form the neurovascular unit that spatially and temporally adjusts blood supply to the needs in energy and oxygen of activated neurons. Neurovascular coupling is so precise that it underlies neuroimaging techniques to map changes in neuronal activity. Therefore, understanding its basis is indispensable for the proper interpretation of imaging signals from functional magnetic resonance imaging and positron emission tomography, routinely used in humans. Although neurovascular coupling mechanisms are not yet fully understood, considerable progress has been made over the last decade. In this review, we present recent knowledge from in vivo studies on the cortical cellular network involved in neurovascular coupling responses and the mediators implicated in these haemodynamic changes. Recent findings have emphasized the intricate interplay between both excitatory and inhibitory neurons in neurovascular coupling, together with an intermediary role of astrocytes, which are ideally positioned between neurons and microvessels. Finally, we describe latest findings on the alterations of neurovascular function encountered in neurodegenerative conditions such as Alzheimer's disease.

Experience-dependent plasticity acts via GluR1 and a novel neuronal nitric oxide synthase-dependent synaptic mechanism in adult cortex

Synaptic plasticity directs development of the nervous system and is thought to underlie memory storage in adult animals. A great deal of our current understanding of the role of AMPA receptors in synaptic plasticity comes from studies on developing cortex and cell cultures. In the present study, we instead focus on plasticity in mature neurons in the neocortex of adult animals. We find that the glutamate receptor 1 (GluR1) subunit of the AMPA receptor is involved in experience-dependent plasticity in adult cortex in vivo and that it acts in addition to neuronal nitric oxide synthase (αNOS1), an enzyme that produces the rapid synaptic signaling molecule nitric oxide (NO). Potentiation of the spared whisker response, following single whisker experience, is ∼33% less in GluR1-null mutants than in wild types. We found that the remaining plasticity depended on αNOS1. Potentiation was reduced by >42% in the single αNOS1-null mutants and completely abolished in GluR1/αNOS1 double-knock-out mice. However, potentiation in GluR1/NOS3 double knock-outs occurred at similar levels to that seen in GluR1 single knock-outs. Synaptic plasticity in the layer IV to II/III pathway in vitro mirrored the results in vivo, in that LTP was present in GluR1/NOS3 double-knock-out mice but not in the GluR1/αNOS1 animals. While basal levels of NO in cortical slices depended on both αNOS1 and NOS3, NMDA receptor-dependent NO release only depended on αNOS1 and not on NOS3. These findings demonstrate that αNOS1 acts in concert with GluR1 to produce experience-dependent plasticity in the neocortex.

Epoxyeicosatrienoic acid-dependent cerebral vasodilation evoked by metabotropic glutamate receptor activation in vivo

Group I metabotropic glutamate receptors (mGluR) on astrocytes have been shown to participate in cerebral vasodilation to neuronal activation in brain slices. Pharmacological stimulation of mGluR in brain slices can produce arteriolar constriction or dilation depending on the initial degree of vascular tone. Here, we examined whether pharmacological stimulation of mGluR in vivo increases cerebral blood flow. A 1-mM solution of the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) superfused at 5 μl/min over the cortical surface of anesthetized rats produced a 30 ± 2% (±SE) increase in blood flow measured by laser-Doppler flowmetry after 15-20 min. The response was completely blocked by superfusion of group I mGluR antagonists and attenuated by superfusion of an epoxyeicosatrienoic acid (EET) antagonist (5 ± 4%), an EET synthesis inhibitor (11 ± 3%), and a cyclooxygenase-2 inhibitor (15 ± 3%). The peak blood flow response was not significantly affected by administration of inhibitors of cyclooxygenase-1, neuronal nitric oxide synthase, heme oxygenase, adenosine A(2B) receptors, or an inhibitor of the synthesis of 20-hydroxyeicosatetraenoic acid (20-HETE). The blood flow response gradually waned following 30-60 min of DHPG superfusion. This loss of the flow response was attenuated by a 20-HETE synthesis inhibitor and was prevented by superfusion of an inhibitor of epoxide hydrolase, which hydrolyzes EETs. These results indicate that pharmacological stimulation of mGluR in vivo increases cerebral blood flow and that the response depends on the release of EETs and a metabolite of cyclooxygenase-2. Epoxide hydrolase activity and 20-HETE synthesis limit the duration of the response to prolonged mGluR activation.