Brain metabolism dictates the polarity of astrocyte control over arterioles

Pericyte-mediated regulation of capillary diameter: a component of neurovascular coupling in health and disease

Because regional blood flow increases in association with the increased metabolic demand generated by localized increases in neural activity, functional imaging researchers often assume that changes in blood flow are an accurate read-out of changes in underlying neural activity. An understanding of the mechanisms that link changes in neural activity to changes in blood flow is crucial for assessing the validity of this assumption, and for understanding the processes that can go wrong during disease states such as ischaemic stroke. Many studies have investigated the mechanisms of neurovascular regulation in arterioles but other evidence suggests that blood flow regulation can also occur in capillaries, because of the presence of contractile cells, pericytes, on the capillary wall. Here we review the evidence that pericytes can modulate capillary diameter in response to neuronal activity and assess the likely importance of neurovascular regulation at the capillary level for functional imaging experiments. We also discuss evidence suggesting that pericytes are particularly sensitive to damage during pathological insults such as ischaemia, Alzheimer's disease and diabetic retinopathy, and consider the potential impact that pericyte dysfunction might have on the development of therapeutic interventions and on the interpretation of functional imaging data in these disorders.

Quantification of CMRO(2) without hypercapnia using simultaneous near-infrared spectroscopy and fMRI measurements

Estimation of the cerebral metabolic rate of oxygen (CMRO(2)) and cerebral blood flow (CBF) is important to investigate the neurovascular coupling and physiological components in blood oxygenation level-dependent (BOLD) signals quantitatively. Although there are methods that can determine CMRO(2) changes using functional MRI (fMRI) or near-infrared spectroscopy (NIRS), current approaches require a separate hypercapnia calibration process and have the potential to incur bias in many assumed model parameters. In this paper, a novel method to estimate CMRO(2) without hypercapnia is described using simultaneous measurements of NIRS and fMRI. Specifically, an optimization framework is proposed that minimizes the differences between the two forms of the relative CMRO(2)-CBF coupling ratio from BOLD and NIRS biophysical models, from which hypercapnia calibration and model parameters are readily estimated. Based on the new methods, we found that group average CBF, CMRO(2), cerebral blood volume (CBV), and BOLD changes within activation of the primary motor cortex during a finger tapping task increased by 39.5 +/- 21.4%, 18.4 +/- 8.7%, 12.9 +/- 6.7%, and 0.5 +/- 0.2%, respectively. The group average estimated flow-metabolism coupling ratio was 2.38 +/- 0.65 and the hypercapnia parameter was 7.7 +/- 1.7%. These values are within the range of values reported from other literatures. Furthermore, the activation maps from CBF and CMRO(2) were well localized on the primary motor cortex, which is the main target region of the finger tapping task.

Role of adenosine A2 receptors in regulation of cerebral blood flow during induced hypotension

The mechanisms responsible for vascular autoregulation in the brain during changes in mean arterial blood pressure are ambiguous. Potentially, adenosine, a purine nucleoside and potent vasodilator, may be involved as earlier studies have documented an increase in brain adenosine concentrations with cerebral ischemia and hypotension. Consequently, we tested the hypothesis that adenosine is involved in vasodilatation during hypotension within the autoregulatory range (>50 mm Hg) by exposing adenosine 2a receptor (A2aR) knockout and wild type (WT) mice to short (2 to 5 mins) periods of hypotension. We found that autoregulation was significantly (P<0.05) impaired by 29% in A2a knockout mice as compared with WT animals. Furthermore, the A2R antagonist (A2a>A2b:10-85>1), ZM-241385, in a dose (1, 5, 10 mg/kg, intraperitoneally)-related manner, attenuated autoregulation in WT mice. In knockout mice treated with ZM-2413585 (5 and 10 mg/kg), autoregulation was further impaired indicating that A2b receptors also participated in cerebral vasodilatation. Treatment with dipyridamole (1.0 mg/kg) that increases extracellular concentrations of adenosine improved autoregulation in the A2aR knockout mice. We would conclude that adenosine through both A2a and A2b receptors is involved in physiologic vascular regulation during hypotension even within the autoregulatory range.

Neurovascular coupling in rat brain operates independent of hemoglobin deoxygenation

Recently, a universal, simple, and fail-safe mechanism has been proposed by which cerebral blood flow (CBF) might be coupled to oxygen metabolism during neuronal activation without the need for any tissue-based mechanism. According to this concept, vasodilation occurs by local erythrocytic release of nitric oxide or ATP wherever and whenever hemoglobin is deoxygenated, directly matching oxygen demand and supply in every tissue. For neurovascular coupling in the brain, we present experimental evidence challenging this view by applying an experimental regime operating without deoxy-hemoglobin. Hyperbaric hyperoxygenation (HBO) allowed us to prevent hemoglobin deoxygenation, as the oxygen that was physically dissolved in the tissue was sufficient to support oxidative metabolism. Regional CBF and regional cerebral blood oxygenation were measured using a cranial window preparation in anesthetized rats. Hemodynamic and neuronal responses to electrical forepaw stimulation or cortical spreading depression (CSD) were analyzed under normobaric normoxia and during HBO up to 4 ATA (standard atmospheres absolute). Inconsistent with the proposed mechanism, during HBO, CBF responses to functional activation or CSD were unchanged. Our results show that activation-induced CBF regulation in the brain does not operate through the release of vasoactive mediators on hemoglobin deoxygenation or through a tissue-based oxygen-sensing mechanism.

Imaging extrasynaptic glutamate dynamics in the brain

Glutamate is the major neurotransmitter in the brain, mediating point-to-point transmission across the synaptic cleft in excitatory synapses. Using a glutamate imaging method with fluorescent indicators, we show that synaptic activity generates extrasynaptic glutamate dynamics in the vicinity of active synapses. These glutamate dynamics had magnitudes and durations sufficient to activate extrasynaptic glutamate receptors in brain slices. We also observed crosstalk between synapses--i.e., summation of glutamate released from neighboring synapses. Furthermore, we successfully observed that sensory input from the extremities induced extrasynaptic glutamate dynamics within the appropriate sensory area of the cerebral cortex in vivo. Thus, the present study clarifies the spatiotemporal features of extrasynaptic glutamate dynamics, and opens up an avenue to directly visualizing synaptic activity in live animals.

Astroglia are a possible cellular substrate of angiotensin(1-7) effects in the rostral ventrolateral medulla

AIMS

Angiotensin(1-7) (Ang1-7) acting at the level of the rostral ventrolateral medulla (RVLM) affects arterial pressure. The cellular substrate of Ang1-7 remains unknown. We sought to determine which cell types in RVLM could mediate its actions and whether these are altered in the spontaneously hypertensive rat (SHR).

METHODS AND RESULTS

Astrocytes, catecholaminergic (CA-ergic) and non-CA-ergic neurones were targeted with adenoviral vectors in organotypic slice cultures from Wistar rats and SHR. Astrocytic Ca(2+) signalling was monitored using a genetically engineered Ca(2+) sensor Case12. CA-ergic neurones expressed enhanced green fluorescent protein (EGFP) under control of the PRS x 8 promoter, whereas non-CA-neurones expressed EGFP under control of the synapsin-1 promoter. Neurones were recorded in whole cell mode while [Ca(2+)](i) was monitored using Rhod-2. RVLM astrocytes responded to Ang1-7 (200-1000 nM) with concentration-dependent [Ca(2+)](i) elevation. In SHR, the response to 1000 nM was significantly attenuated. The competitive Ang1-7 receptor antagonist A779, but not the AT(1) receptor blocker (losartan), suppressed Ang1-7-induced [Ca(2+)](i) elevations, which were also antagonized by blocking intracellular Ca(2+) stores. Ang1-7 evoked no consistent changes in [Ca(2+)](i) or membrane excitability in CA-ergic or non-CA-ergic neurones in either rat strain.

CONCLUSION

Astroglia are a plausible cellular target of Ang1-7 in RVLM. Our data suggest that astrocytic responsiveness to Ang1-7 is reduced in SHR. We hypothesise that Ang1-7 modulates astrocytic signalling which in vivo may affect local metabolism and microcirculation, resulting in changes in activity of RVLM pre-sympathetic neurones and hence blood pressure.

A dynamic model of neurovascular coupling: implications for blood vessel dilation and constriction

Neurovascular coupling in response to stimulation of the rat barrel cortex was investigated using concurrent multichannel electrophysiology and laser Doppler flowmetry. The data were used to build a linear dynamic model relating neural activity to blood flow. Local field potential time series were subject to current source density analysis, and the time series of a layer IV sink of the barrel cortex was used as the input to the model. The model output was the time series of the changes in regional cerebral blood flow (CBF). We show that this model can provide excellent fit of the CBF responses for stimulus durations of up to 16 s. The structure of the model consisted of two coupled components representing vascular dilation and constriction. The complex temporal characteristics of the CBF time series were reproduced by the relatively simple balance of these two components. We show that the impulse response obtained under the 16-s duration stimulation condition generalised to provide a good prediction to the data from the shorter duration stimulation conditions. Furthermore, by optimising three out of the total of nine model parameters, the variability in the data can be well accounted for over a wide range of stimulus conditions. By establishing linearity, classic system analysis methods can be used to generate and explore a range of equivalent model structures (e.g., feed-forward or feedback) to guide the experimental investigation of the control of vascular dilation and constriction following stimulation.

Astrocytic endfoot Ca2+ and BK channels determine both arteriolar dilation and constriction

Neuronal activity is thought to communicate to arterioles in the brain through astrocytic calcium (Ca(2+)) signaling to cause local vasodilation. Paradoxically, this communication may cause vasoconstriction in some cases. Here, we show that, regardless of the mechanism by which astrocytic endfoot Ca(2+) was elevated, modest increases in Ca(2+) induced dilation, whereas larger increases switched dilation to constriction. Large-conductance, Ca(2+)-sensitive potassium channels in astrocytic endfeet mediated a majority of the dilation and the entire vasoconstriction, implicating local extracellular K(+) as a vasoactive signal for both dilation and constriction. These results provide evidence for a unifying mechanism that explains the nature and apparent duality of the vascular response, showing that the degree and polarity of neurovascular coupling depends on astrocytic endfoot Ca(2+) and perivascular K(+).

Pathophysiology of stroke-like episodes in MELAS: neuron–astrocyte uncoupling in neuronal hyperexcitability

Mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) is a distinct clinical syndrome characterized by fluctuated encephalopathy, migraineous headache, seizure and stroke-like episodes. The molecular mechanism of MELAS mutations has been elucidated; however, the pathogenesis of stroke-like episodes remains largely unknown. Three main hypotheses include ischemic, metabolic and neuronal hyperexcitability hypotheses. Recently, emerging hypotheses include alterations in nitric oxide homeostasis and over-reduction/oxidative stress mechanisms. Although neuron–astrocyte communication is crucial in various physiological functions, it has not been seriously considered in the pathophysiology of stroke-like episodes. This review summarizes what is known about the molecular mechanisms of gene mutation, clinico-radiological, clinico-physiological and pathological features of stroke-like episodes, as well as its pathogenesis. We finally discuss potential mechanisms involved in the pathogenesis of stroke-like episodes based on currently available clinical data and the current understanding of the mechanisms of neuron–astrocyte communications. We propose that neuron–astrocyte uncoupling is a new target of research in mitochondrial disorders.

Use of NAD(P)H and flavoprotein autofluorescence transients to probe neuron and astrocyte responses to synaptic activation

Synaptic stimulation in brain slices is accompanied by changes in tissue autofluorescence, which are a consequence of changes in tissue metabolism. Autofluorescence excited by ultraviolet light has been most extensively studied, and is due to reduced pyridine nucleotides (NADH and NADPH, collectively termed NAD(P)H). Stimulation generates a characteristic compound NAD(P)H response, comprising an initial fluorescence decrease and then an overshooting increase that slowly recovers to baseline levels. Evoked NAD(P)H transients are relatively easy to record, do not require the addition of exogenous indicators and have good signal-noise ratios. These characteristics make NAD(P)H imaging methods very useful for tracking the spread of neuronal activity in complex brain tissues, however the cellular basis of synaptically-evoked autofluorescence transients has been the subject of recent debate. Of particular importance is the question of whether signals are due primarily to changes in neuronal mitochondrial function, and/or whether astrocyte metabolism triggered by glutamate uptake may be a significant contributor to the overshooting NAD(P)H fluorescence increases. This mini-review addresses the subcellular origins of NAD(P)H autofluorescence and the evidence for mitochondrial and glycolytic contributions to compound transients. It is concluded that there is no direct evidence for a contribution to NAD(P)H signals from glycolysis in astrocytes following synaptic glutamate uptake. In contrast, multiple lines of evidence, including from complimentary flavoprotein autofluorescence signals, imply that mitochondrial NADH dynamics in neurons dominate compound evoked NAD(P)H transients. These signals are thus appropriate for studies of mitochondrial function and dysfunction in brain slices, in addition to providing robust maps of postsynaptic neuronal activation following physiological activation.

Non-junction functions of pannexin-1 channels

Pannexins are large-pore ion channels with broad expression in the central nervous system (CNS). The channels function by releasing large signaling molecules, such ATP and arachidonic acid derivatives, from neurons and possibly astrocytes. They might also contribute to novel forms of non-synaptic communication in the CNS, thereby affecting synaptic function, astrocytic Ca(2+) wave propagation and possibly regulation of vascular tone in the brain. Panx1 activation in various in vitro pathological conditions implicates these channels in ischemic, excitotoxic and ATP-dependent cell death, whereas Panx coupling with purinergic receptors triggers the inflammasome. Novel functions for the pannexin channels are likely to be discovered as current understanding of how they are regulated in physiological and pathological situations improves.

Glutamate regulates Ca2+ signals in smooth muscle cells of newborn piglet brain slice arterioles through astrocyte- and heme oxygenase-dependent mechanisms

Glutamate is the principal cerebral excitatory neurotransmitter and dilates cerebral arterioles to match blood flow to neural activity. Arterial contractility is regulated by local and global Ca(2+) signals that occur in smooth muscle cells, but modulation of these signals by glutamate is poorly understood. Here, using high-speed confocal imaging, we measured the Ca(2+) signals that occur in arteriole smooth muscle cells of newborn piglet tangential brain slices, studied signal regulation by glutamate, and investigated the physiological function of heme oxygenase (HO) and carbon monoxide (CO) in these responses. Glutamate elevated Ca(2+) spark frequency by approximately 188% and reduced global intracellular Ca(2+) concentration ([Ca(2+)](i)) to approximately 76% of control but did not alter Ca(2+) wave frequency in brain arteriole smooth muscle cells. Isolation of cerebral arterioles from brain slices abolished glutamate-induced Ca(2+) signal modulation. In slices treated with l-2-alpha-aminoadipic acid, a glial toxin, glutamate did not alter Ca(2+) sparks or global [Ca(2+)](i) but did activate Ca(2+) waves. This shift in Ca(2+) signal modulation by glutamate did not occur in slices treated with d-2-alpha-aminoadipic acid, an inactive isomer of l-2-alpha-aminoadipic acid. In the presence of chromium mesoporphyrin, a HO blocker, glutamate inhibited Ca(2+) sparks and Ca(2+) waves and did not alter global [Ca(2+)](i). In isolated arterioles, CORM-3 [tricarbonylchloro(glycinato)ruthenium(II)], a CO donor, activated Ca(2+) sparks and reduced global [Ca(2+)](i). These effects were blocked by 1H-(1,2,4)-oxadiazolo-(4,3-a)-quinoxalin-1-one, a soluble guanylyl cyclase inhibitor. Collectively, these data indicate that glutamate can modulate Ca(2+) sparks, Ca(2+) waves, and global [Ca(2+)](i) in arteriole smooth muscle cells via mechanisms that require astrocytes and HO. These data also indicate that soluble guanylyl cyclase is involved in CO activation of Ca(2+) sparks in arteriole smooth muscle cells.

Functional and molecular interactions between aquaporins and Na,K-ATPase

The water channel aquaporin 4 (AQP4) is abundantly expressed in astrocytes and provides a mechanism by which water permeability of the plasma membrane can be regulated. Astrocytes play a key role in the clearance of both potassium (K(+)) and glutamate released during neuronal activity. Emerging evidence suggests that AQP4 facilitates K(+) clearance by astrocytes and contributes to recovery of neuronal excitability. Here we report that AQP4 can assemble with its regulator metabotropic glutamate receptor 5 (mGluR5) and with Na,K-ATPase; the enzyme responsible for active K(+) transport and for establishing the electrochemical gradient across the cell plasma membrane. We have, by use of pull down assays in rat brain tissue, identified the segment in the AQP4 NH(2)-terminus containing the amino acid residues 23-32 as the site for interaction with Na,K-ATPase catalytic subunit and with mGluR5. Mutagenesis studies revealed that the AQP4 amino acids K27 and W30 are of key importance for interaction with both Na,K-ATPase and mGluR5. To confirm that interaction also occurs within intact cells, we have performed fluorescence resonance energy transfer (FRET) studies in primary astrocytes derived from rat striatum. The results indicate close proximity of wild type AQP4 and Na,K-ATPase in the plasma membrane of rat astrocytes. FRET efficiencies observed with the mutants AQP4 K27A and AQP4 W30A were significantly lower, highlighting the importance of these residues for the interaction between AQP4 and Na,K-ATPase. We conclude that AQP4/Na,K-ATPase/mGluR5 can form a macromolecular complex/transporting microdomain in astrocytes. This complex may be of functional importance for the regulation of water and K(+) homeostasis in the brain, as well as for neuron-astrocyte metabolic crosstalk.

Coupling between the blood lactate-to-pyruvate ratio and MCA Vmean at the onset of exercise in humans

Activation-induced increase in cerebral blood flow is coupled to enhanced metabolic activity, maybe with brain tissue redox state and oxygen tension as key modulators. To evaluate this hypothesis at the onset of exercise in humans, blood was sampled at 0.1 to 0.2 Hz from the radial artery and right internal jugular vein, while middle cerebral artery mean flow velocity (MCA Vmean) was recorded. Both the arterial and venous lactate-to-pyruvate ratio increased after 10 s ( P < 0.05), and the arterial ratio remained slightly higher than the venous ( P < 0.05). The calculated average cerebral capillary oxygen tension decreased by 2.7 mmHg after 5 s ( P < 0.05), while MCA Vmean increased only after 30 s. Furthermore, there was an unaccounted cerebral carbohydrate uptake relative to the uptake of oxygen that became significant 50 s after the onset of exercise. These findings support brain tissue redox state and oxygenation as potential modulators of an increase in cerebral blood flow at the onset of exercise.

Glioblastoma cells express functional cell membrane receptors activated by daily used medical drugs

PURPOSE

Calcium ions are highly versatile spacial and temporal intracellular signals of non-excitable cells and have an important impact on nearly every aspect of cellular life controlling cell growth, metabolism, fluid secretion, information processing, transcription, apoptosis, and motility. Neurons and glia respond to stimuli, including neurotransmitters, neuromodulators, and hormones, which increase the intracellular calcium concentration. The function of intracellular calcium in gliomas is unknown. Lots of daily used drugs may act via receptors that can be linked to the intracellular calcium system and therefore could influence glioma biology.

METHODS

Glioma cells were loaded with the calcium ion sensitive dye Fura 2-AM. Subsequently, cells were stimulated with 25 different medical drugs for 30 s. The increase of free intracellular calcium ions was measured and calculated by a microscope-camera-computer-unit.

RESULTS

Except for the buffer solution HEPES that served as negative control and for the cortisol derivative dexamethasone, all other 24 tested drugs induced a rise of intracellular calcium ions. The cellular calcium responses were classified into seven functional groups. The tested substances activated several types of calcium channels and receptors.

CONCLUSIONS

Our study impressively demonstrates that medical drugs are potent inducers of intracellular calcium signals. Totally unexpected, the results show a high amount of functional cellular receptors and channels on glioma cells, which could be responsible for certain biological effects like migration and cell growth. This calcium imaging study proves the usability of the calcium imaging as a screening system for functional receptors on human glioma cells.

Cortical spreading depression-new insights and persistent questions

Since its original extensive description by Leao in 1944, thousands of publications have characterized the phenomenon of cortical spreading depression (CSD). Despite the attention that CSD has received over more than six decades, however, many fundamental questions regarding its initiation, propagation, functional consequences, and relationship to migraine and other human disorders remain unanswered. Advances in genetics and cellular imaging have led to important insights into the basic mechanisms of CSD, with increasing attention focused on specific neuronal ion channels, neurotransmitters and neuromodulators. In addition, there is growing recognition that astrocytes and the vasculature may play an active, rather than simply a passive or reactive role in CSD. Several recent descriptions of CSD in humans in the setting of brain injury provide definitive evidence that this phenomenon can occur and have important functional consequences in the human brain. Although the exact role of CSD in migraine has yet to be conclusively established, there is strong evidence that the investigation of CSD in animal models can provide meaningful information about migraine that can be translated into the clinical setting. This review will briefly address the extensive work that has been done on CSD over more than half a century, but focus primarily on more recent studies with a particular emphasis on relevance to migraine.

Glucose and lactate supply to the synapse

The main source of energy for the mammalian brain is glucose, and the main sink of energy in the mammalian brain is the neuron, so the conventional view of brain energy metabolism is that glucose is consumed preferentially in neurons. But between glucose and the production of energy are several steps that do not necessarily take place in the same cell. An alternative model has been proposed that states that glucose preferentially taken by astrocytes, is degraded to lactate and then exported into neurons to be oxidized. Short of definitive data, opinions about the relative merits of these competing models are divided, making it a very exciting field of research. Furthermore, growing evidence suggests that lactate acts as a signaling molecule, involved in Na(+) sensing, glucosensing, and in coupling neuronal and glial activity to the modulation of vascular tone. In the present review, we discuss possible dynamics of glucose and lactate in excitatory synaptic regions, focusing on the transporters that catalyze the movement of these molecules.

Oxygen transport in brain tissue

Oxygen is essential to maintaining normal brain function. A large body of evidence suggests that the partial pressure of oxygen (pO(2)) in brain tissue is physiologically maintained within a narrow range in accordance with region-specific brain activity. Since the transportation of oxygen in the brain tissue is mainly driven by a diffusion process caused by a concentration gradient of oxygen from blood to cells, the spatial organization of the vascular system, in which the oxygen content is higher than in tissue, is a key factor for maintaining effective transportation. In addition, a local mechanism that controls energy demand and blood flow supply plays a critical role in moment-to-moment adjustment of tissue pO(2) in response to dynamically varying brain activity. In this review, we discuss the spatiotemporal structures of brain tissue oxygen transport in relation to local brain activity based on recent reports of tissue pO(2) measurements with polarographic oxygen microsensors in combination with simultaneous recordings of neural activity and local cerebral blood flow in anesthetized animal models. Although a physiological mechanism of oxygen level sensing and control of oxygen transport remains largely unknown, theoretical models of oxygen transport are a powerful tool for better understanding the short-term and long-term effects of local changes in oxygen demand and supply. Finally, emerging new techniques for three-dimensional imaging of the spatiotemporal dynamics of pO(2) map may enable us to provide a whole picture of how the physiological system controls the balance between demand and supply of oxygen during both normal and pathological brain activity.

Generation of nitric oxide from nitrite by carbonic anhydrase: a possible link between metabolic activity and vasodilation

In catalyzing the reversible hydration of CO2 to bicarbonate and protons, the ubiquitous enzyme carbonic anhydrase (CA) plays a crucial role in CO2 transport, in acid-base balance, and in linking local acidosis to O2 unloading from hemoglobin. Considering the structural similarity between bicarbonate and nitrite, we hypothesized that CA uses nitrite as a substrate to produce the potent vasodilator nitric oxide (NO) to increase local blood flow to metabolically active tissues. Here we show that CA readily reacts with nitrite to generate NO, particularly at low pH, and that the NO produced in the reaction induces vasodilation in aortic rings. This reaction occurs under normoxic and hypoxic conditions and in various tissues at physiological levels of CA and nitrite. Furthermore, two specific inhibitors of the CO2 hydration, dorzolamide and acetazolamide, increase the CA-catalyzed production of vasoactive NO from nitrite. This enhancing effect may explain the known vasodilating effects of these drugs and indicates that CO2 and nitrite bind differently to the enzyme active site. Kinetic analyses show a higher reaction rate at high pH, suggesting that anionic nitrite participates more effectively in catalysis. Taken together, our results reveal a novel nitrous anhydrase enzymatic activity of CA that would function to link the in vivo main end products of energy metabolism (CO2/H+) to the generation of vasoactive NO. The CA-mediated NO production may be important to the correlation between blood flow and metabolic activity in tissues, as occurring for instance in active areas of the brain.

GABA uptake-dependent Ca(2+) signaling in developing olfactory bulb astrocytes

We studied GABAergic signaling in astrocytes of olfactory bulb slices using confocal Ca(2+) imaging and two-photon Na(+) imaging. GABA evoked Ca(2+) transients in astrocytes that persisted in the presence of GABA(A) and GABA(B) receptor antagonists, but were suppressed by inhibition of GABA uptake by SNAP 5114. Withdrawal of external Ca(2+) blocked GABA-induced Ca(2+) transients, and depletion of Ca(2+) stores with cyclopiazonic acid reduced Ca(2+) transients by approximately 90%. This indicates that the Ca(2+) transients depend on external Ca(2+), but are mainly mediated by intracellular Ca(2+) release, conforming with Ca(2+)-induced Ca(2+) release. Inhibition of ryanodine receptors did not affect GABA-induced Ca(2+) transients, whereas the InsP(3) receptor blocker 2-APB inhibited the Ca(2+) transients. GABA also induced Na(+) increases in astrocytes, potentially reducing Na(+)/Ca(2+) exchange. To test whether reduction of Na(+)/Ca(2+) exchange induces Ca(2+) signaling, we inhibited Na(+)/Ca(2+) exchange with KB-R7943, which mimicked GABA-induced Ca(2+) transients. Endogenous GABA release from neurons, activated by stimulation of afferent axons or NMDA application, also triggered Ca(2+) transients in astrocytes. The significance of GABAergic Ca(2+) signaling in astrocytes for control of blood flow is demonstrated by SNAP 5114-sensitive constriction of blood vessels accompanying GABA uptake. The results suggest that GABAergic signaling is composed of GABA uptake-mediated Na(+) rises that reduce Na(+)/Ca(2+) exchange, thereby leading to a Ca(2+) increase sufficient to trigger Ca(2+)-induced Ca(2+) release via InsP(3) receptors. Hence, GABA transporters not only remove GABA from the extracellular space, but may also contribute to intracellular signaling and astrocyte function, such as control of blood flow.

Spontaneous glial calcium waves in the retina develop over early adulthood

Intercellular glial Ca(2+) waves constitute a signaling pathway between glial cells. Artificial stimuli have previously been used to evoke these waves, and their physiological significance has been questioned. We report here that Ca(2+) waves occur spontaneously in rat retinal glial cells, both in the isolated retina and in vivo. These spontaneous waves are propagated by ATP release. In the isolated retina, suramin (P2 receptor antagonist) reduces the frequency of spontaneous wave generation by 53%, and apyrase (ATP-hydrolyzing enzyme) reduces frequency by 95-100%. Luciferin-luciferase chemiluminescence reveals waves of ATP matching the spontaneous Ca(2+) waves, indicating that ATP release occurs as spontaneous Ca(2+) waves are generated. Wave generation also depends on age. Spontaneous wave frequency rises from 0.27 to 1.0 per minute per mm(2), as rats age from 20 to 120 d. The sensitivity of glia to ATP does not increase with age, but the ATP released by evoked waves is 31% greater in 120-d-old than in 20-d-old rats, suggesting that increased ATP release in older animals could account for the higher frequency of wave generation. Simultaneous imaging of glial Ca(2+) and arterioles in the isolated retina demonstrates that spontaneous waves alter vessel diameter, implying that spontaneous waves may have a significant impact on retinal physiology. Spontaneous intercellular glial Ca(2+) waves also occur in the retina in vivo, with frequency, speed, and diameter similar to the isolated retina. Increased spontaneous wave occurrence with age suggests that wave generation may be related to retinal pathology.

The significance of neuronal and glial cell changes in the rat retina during oxygen-induced retinopathy

Retinopathy of prematurity is a devastating vascular disease of premature infants. A number of studies indicate that retinal function is affected in this disease. Using the rat model of oxygen-induced retinopathy, it is possible to explore more fully the complex relationship between neuronal, glial and vascular pathology in this condition. This review examines the structural and functional changes that occur in the rat retina following oxygen-induced retinopathy. We highlight that vascular pathology in rats is characterized by aberrant growth of blood vessels into the vitreous at the expense of blood vessel growth into the body of the retina. Moreover, amino acid neurochemistry, a tool for examining neuronal changes in a spatially complete manner reveals widespread changes in amacrine and bipolar cells. In addition, neurochemical anomalies within inner retinal neurons are highly correlated with the absence of retinal vessels. The key cell types that link blood flow with neuronal function are macroglia. Macroglia cells, which in the retina include astrocytes and Müller cells, are affected by oxygen-induced retinopathy. Astrocyte loss occurs in the peripheral retina, while Müller cells show signs of reactive gliosis that is highly localized to regions that are devoid of intraretinal blood vessels. Finally, we propose that treatments, such as blockade of the renin-angiotensin system, that not only targets pathological angiogenesis, but that also promotes re-vascularization of the retina are likely to prove important in the treatment of those with retinopathy of prematurity.

Persistent increase in oxygen consumption and impaired neurovascular coupling after spreading depression in rat neocortex

Cortical spreading depression (CSD) is associated with a dramatic failure of brain ion homeostasis and increased energy metabolism. There is strong clinical and experimental evidence to suggest that CSD is the mechanism of migraine, and involved in progressive neuronal injury in stroke and head trauma. Here we tested the hypothesis that single episodes of CSD induced acute hypoxia, and prolonged impairment of neurovascular and neurometabolic coupling. Cortical spreading depression was induced in rat frontal cortex, whereas cortical electrical activity and local field potentials (LFPs) were recorded by glass microelectrodes, cerebral blood flow (CBF) by laser-Doppler flowmetry, and tissue oxygen tension (tpO(2)) with polarographic microelectrodes. Cortical spreading depression increased cerebral metabolic rate of oxygen (CMRO(2)) by 71%+/-6.7% and CBF by 238%+/-48.1% for 1 to 2 mins. For the following 2 h, basal tpO(2) and CBF were reduced whereas basal CMRO(2) was persistently elevated by 8.1%+/-2.9%. In addition, within first hour after CSD we found impaired neurovascular coupling (LFP versus CBF), whereas neurometabolic coupling (LFP versus CMRO(2)) remained unaffected. Impaired neurovascular coupling was explained by both reduced vascular reactivity and suppressed function of cortical inhibitory interneurons. The protracted effects of CSD on basal CMRO(2) and neurovascular coupling may contribute to cellular dysfunction in patients with migraine and acutely injured cerebral cortex.

Astrocytes are poised for lactate trafficking and release from activated brain and for supply of glucose to neurons

Brain is a highly-oxidative organ, but during activation, glycolytic flux is preferentially up-regulated even though oxygen supply is adequate. The biochemical and cellular basis of metabolic changes during brain activation and the fate of lactate produced within brain are important, unresolved issues central to understanding brain function, brain images, and spectroscopic data. Because in vivo brain imaging studies reveal rapid efflux of labeled glucose metabolites during activation, lactate trafficking among astrocytes and between astrocytes and neurons was examined after devising specific, real-time, sensitive enzymatic fluorescent assays to measure lactate and glucose levels in single cells in adult rat brain slices. Astrocytes have a 2- to 4-fold faster and higher capacity for lactate uptake from extracellular fluid and for lactate dispersal via the astrocytic syncytium compared to neuronal lactate uptake from extracellular fluid or shuttling of lactate to neurons from neighboring astrocytes. Astrocytes can also supply glucose to neurons as well as glucose can be taken up by neurons from extracellular fluid. Astrocytic networks can provide neuronal fuel and quickly remove lactate from activated glycolytic domains, and the lactate can be dispersed widely throughout the syncytium to endfeet along the vasculature for release to blood or other brain regions via perivascular fluid flow.

Cell adhesion and signaling networks in brain neurovascular units

PURPOSE OF REVIEW

Central nervous system (CNS) neurovascular units are multicellular complexes consisting of neurons and astrocytes, vascular endothelial cells and pericytes, as well as an assortment of growth factors and extracellular matrix (ECM) proteins. Here, I will discuss the current knowledge of signaling networks essential for the development and physiology of CNS neurovascular units, particularly in the brain.

RECENT FINDINGS

Molecular genetic studies have identified various signaling proteins that regulate the formation and function of CNS neurovascular units. These include members of the integrin family of ECM adhesion receptors, ECM proteins such as Wnts and latent transforming growth factor betas, and various transcriptional regulators, including beta-catenin and the inhibitors of DNA binding (Ids).

SUMMARY

Neurovascular units are the cellular and molecular interfaces between the circulatory system and the CNS. Recent molecular genetic analyses in mice and other model organisms have revealed the first mechanisms underlying bidirectional communication between neural and vascular components. In particular, ECM-mediated adhesion and signaling pathways have been identified as essential for neurovascular development and physiology. Understanding how these various gene products normally control neurovascular unit formation and function will lend new insights into the causes and possible treatments of debilitating neurovascular-related diseases such as birth defects, stroke, and age-related dementia.

Deciphering neuron-glia compartmentalization in cortical energy metabolism

Energy demand is an important constraint on neural signaling. Several methods have been proposed to assess the energy budget of the brain based on a bottom-up approach in which the energy demand of individual biophysical processes are first estimated independently and then summed up to compute the brain's total energy budget. Here, we address this question using a novel approach that makes use of published datasets that reported average cerebral glucose and oxygen utilization in humans and rodents during different activation states. Our approach allows us (1) to decipher neuron-glia compartmentalization in energy metabolism and (2) to compute a precise state-dependent energy budget for the brain. Under the assumption that the fraction of energy used for signaling is proportional to the cycling of neurotransmitters, we find that in the activated state, most of the energy (∼80%) is oxidatively produced and consumed by neurons to support neuron-to-neuron signaling. Glial cells, while only contributing for a small fraction to energy production (∼6%), actually take up a significant fraction of glucose (50% or more) from the blood and provide neurons with glucose-derived energy substrates. Our results suggest that glycolysis occurs for a significant part in astrocytes whereas most of the oxygen is utilized in neurons. As a consequence, a transfer of glucose-derived metabolites from glial cells to neurons has to take place. Furthermore, we find that the amplitude of this transfer is correlated to (1) the activity level of the brain; the larger the activity, the more metabolites are shuttled from glia to neurons and (2) the oxidative activity in astrocytes; with higher glial pyruvate metabolism, less metabolites are shuttled from glia to neurons. While some of the details of a bottom-up biophysical approach have to be simplified, our method allows for a straightforward assessment of the brain's energy budget from macroscopic measurements with minimal underlying assumptions.

Modeling premature brain injury and recovery

Premature birth is a growing and significant public health problem because of the large number of infants that survive with neurodevelopmental sequelae from brain injury. Recent advances in neuroimaging have shown that although some neuroanatomical structures are altered, others improve over time. This review outlines recent insights into brain structure and function in these preterm infants at school age and relevant animal models. These animal models have provided scientists with an opportunity to explore in depth the molecular and cellular mechanisms of injury as well as the potential of the brain for recovery. The endogenous potential that the brain has for neurogenesis and gliogenesis, and how environment contributes to recovery, are also outlined. These preclinical models will provide important insights into the genetic and epigenetic mechanisms responsible for variable degrees of injury and recovery, permitting the exploration of targeted therapies to facilitate recovery in the developing preterm brain.

Suppression of GABA input by A1 adenosine receptor activation in rat cerebellar granule cells

Synaptic transmission has been shown to be modulated by purinergic receptors. In the cerebellum, spontaneous inhibitory input to Purkinje neurons is enhanced by ATP via P2 receptors, while evoked excitatory input via the granule cell parallel fibers is reduced by presynaptic P1 (A1) adenosine receptors. We have now studied the modulation of the complex GABAergic input to granule cells by the purinergic receptor agonists ATP and adenosine in acute rat cerebellar tissue slices using the whole-cell patch-clamp technique. Our experiments indicate that ATP and adenosine substantially reduce the bicuculline- and gabazine-sensitive GABAergic input to granule cells. Both phasic and tonic inhibitory components were reduced leading to an increased excitability of granule cells. The effect of ATP and adenosine could be blocked by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), but not by other P1 and P2 receptor antagonists, indicating that it was mediated by activation of A1 adenosine receptors. Our results suggest that, in the cerebellar network, A1 receptor activation, known to decrease the excitatory output of granule cells, also increases their excitability by reducing their complex GABAergic input. These findings extend our knowledge on purinergic receptors, mediating multiple modulations at both inhibitory and excitatory input and output sites in the cerebellar network.

Stimulus-induced changes in blood flow and 2-deoxyglucose uptake dissociate in ipsilateral somatosensory cortex

The present study addresses the relationship between blood flow and glucose consumption in rat primary somatosensory cortex (SI) in vivo. We examined bilateral neuronal and hemodynamic changes and 2-deoxyglucose (2DG) uptake, as measured by autoradiography, in response to unilateral forepaw stimulation. In contrast to the contralateral forepaw area, where neuronal activity, blood oxygenation/flow and 2DG uptake increased in unison, we observed, in the ipsilateral SI, a blood oxygenation/flow decrease and arteriolar vasoconstriction in the presence of increased 2DG uptake. Laminar electrophysiological recordings revealed an increase in ipsilateral spiking consistent with the observed increase in 2DG uptake. The vasoconstriction and the decrease in blood flow in the presence of an increase in 2DG uptake in the ipsilateral SI contradict the prominent metabolic hypothesis regarding the regulation of cerebral blood flow, which postulates that the state of neuroglial energy consumption determines the regional blood flow through the production of vasoactive metabolites. We propose that other factors, such as neuronal (and glial) release of messenger molecules, might play a dominant role in the regulation of blood flow in vivo in response to a physiological stimulus.

Integrin-mediated regulation of neurovascular development, physiology and disease

The mammalian central nervous system (CNS) is comprised of billions of neurons and glia that are intertwined with an elaborate network of blood vessels. These various neural and vascular cell types actively converse with one another to form integrated, multifunctional complexes, termed neurovascular units. Cell-cell communication within neurovascular units promotes normal CNS development and homeostasis, and abnormal regulation of these events leads to a variety of debilitating CNS diseases. This review will summarize (1) cellular and molecular mechanisms that regulate physiological assembly and maintenance of neurovascular units; and (2) signaling events that induce pathological alterations in neurovascular unit formation and function. An emphasis will be placed on neural-vascular cell adhesion events mediated by integrins and their extracellular matrix (ECM) ligands. I will highlight the role of a specific adhesion and signaling axis involving alphavbeta8 integrin, latent transforming growth factor beta's (TGFbeta's), and canonical TGFbeta receptors. Possible functional links between components of this axis and other signal transduction cascades implicated in neurovascular development and disease will be discussed. Comprehensively understanding the pathways that regulate bidirectional neural-vascular cell contact and communication will provide new insights into the mechanisms of neurovascular unit development, physiology and disease.

Increased intravascular flow rate triggers cerebral arteriogenesis

Peripheral arteriogenesis is distinctly enhanced by increased fluid shear stress. Thus, the aim of this study was to investigate in the rat brain whether increased fluid shear stress can also stimulate cerebral arteriogenesis. To increase fluid shear stress in the cerebral circulation, we developed different shear stress models as the ligature of both common carotid arteries (Double-Ligature model), bilateral carotid ligature followed by creation of a unilateral arterio-venous fistula (two-stage protocol, Ligature-Shunt model), and unilateral arterio-venous fistula-creation alone (Solo-Shunt model). Blood flow changes were monitored in vivo by quantitative magnetic resonance imaging-analysis. Cerebral arteriogenesis was analyzed by magnetic resonance imaging and contrast agent-angiography. For proliferation and accumulation of mononuclear cells, immunohistochemistry was performed. During the 14 days-observation period, blood flow increased maximal by 5.5-fold in the A. basilaris and 10.3-fold in the fistula-sided A. cerebri posterior of the Ligature-Shunt model. Considerable vessel growth was found in all shear stress-stimulated arteries. Comparative analysis of vessel length and diameter versus blood flow indicated a correlation between the growth of cerebral collaterals and rising intravascular flow rates (R2=0.90/0.96). Immunohistochemistry showed the typical phases of arteriogenesis and accumulation of mononuclear cells. In conclusion, we provide evidence that fluid shear stress is not only the pivotal trigger of peripheral but also of cerebral arteriogenesis.

Custom astrocyte-mediated vasomotor responses to neuronal energy demand

Brain astrocytes regulate local blood flow and neuronal energy supply by modulating blood vessel tone in response to changes in oxygen levels.

Astrocytes mediate either constriction or dilation of local brain arterioles in response to synaptic activity. Recent work indicates that the directionality of this response may be dictated by ambient oxygen levels.

Astrocytes going live: advances and challenges

Astrocytes are one of the most numerous cell types in the CNS. They have emerged as sophisticated cells participating in a large and diverse variety of functions vital for normal brain development, adult physiology and pathology. Recent in vivo studies have provided exciting new insight into astrocyte physiology in the intact healthy brain. This review will summarize some of their most intriguing findings, discuss some of their implications, and look ahead at some of the challenges we face in studying astrocyte function in vivo.