Dynamic cerebral microcirculatory changes in transient forebrain ischemia in rats: involvement of type I nitric oxide synthase.

The diameter of surface microvessels and the erythrocyte velocity and flux through intraparenchymal capillaries in the parietal cortex were measured during transient global cerebral ischemia and reperfusion using laser-scanning confocal fluorescence microscopy in anesthetized rats. The role of nitric oxide (NO) from neurons in the microcirculatory changes was also investigated using 7-nitro-indazole (7-NI, 25 mg/kg, i.p.). Wistar rats (4 per group) equipped with a closed cranial window were given fluorescein isothiocyanate (FITC)-Dextran and FITC-labeled erythrocytes intravenously to respectively visualize the microvessels and the erythrocytes in the capillaries. Experiments were videorecorded on-line. Forebrains were made ischemic for 15 minutes and then reperfused for 120 minutes under the microscope. Ischemia was associated with a flattened EEG, a low persistent blood flow, and a transient leakage of fluorescein across the arteriole wall. Unclamping the carotid arteries led to immediate high blood flow in the arterioles, but it was not until 5 minutes later that the arterioles dilated significantly (181% +/- 27%) and erythrocyte velocity in the capillaries increased significantly (460% +/- 263%). Neither nonperfused capillaries nor erythrocyte capillary recruitment occurred. 7-Nitro-indazole significantly reduced the arteriole dilatation and prevented the increase in erythrocyte velocity and flux through capillaries in early reperfusion. 7-Nitroindazole had no influence on the fluorescein leakage. The current study suggests a partial role for NO released from neurons in the postischemic microcirculatory changes and provides new findings on the timing of arteriole dilatation and blood-brain barrier opening, and on erythrocyte capillary circulation in global ischemia.     

Cortical electrical stimulation alters erythrocyte perfusion pattern in the cerebral capillary network of the rat

The effect of direct cortical electrical stimulation on the pattern of erythrocyte perfusion in the capillary network of the rat cerebral cortex was studied by fluorescence intravital video-microscopy. The movement of fluorescently labeled red blood cells (FRBCs) in individual capillaries 50-70 microm subsurface in the dorsal somatosensory cortex was visualized using a closed cranial window. Cortical stimulation electrodes were placed on opposite sides of the window. FRBC velocity (mm/s) and supply rate (cells/s) were measured in 51 capillaries from six rats before and during electrical stimulation of increasing intensities (15-s trains of 3-Hz, 3-ms, 0.5-5.0-mA, square pulses). FRBC velocity, supply rate, and the instantaneous capillary erythrocyte content (lineal cell density, LCD, cells/mm) increased with the stimulation current and reached maxima of 110, 160 and 33% above control, respectively. Capillaries with low resting velocity showed a greater response than those with high resting velocity. The fraction of capillaries in which FRBC velocity increased was not constant, but increased with the stimulation current, as did the magnitude of the velocity change in these capillaries. A few capillaries showed a negative FRBC velocity response at stimulations <4 mA. These results suggest that a robust rise in the fraction of responding (engaged) capillaries and a smaller rise in the capillary LCD contribute to neuronal activation-induced cortical hyperemia. Thus, capillary engagement and erythrocyte recruitment appear to represent important components of the cortical functional hyperemic response. These results provide insight into some of the specific hemodynamic changes associated with functional hyperemia occurring at the capillary level.     

Role of endothelium-pericyte signaling in capillary blood flow response to neuronal activity

Local blood flow in the brain is tightly coupled to metabolic demands, a phenomenon termed functional hyperemia. Both capillaries and arterioles contribute to the hyperemic response to neuronal activity via different mechanisms and timescales. The nature and specific signaling involved in the hyperemic response of capillaries versus arterioles, and their temporal relationship are not fully defined. We determined the time-dependent changes in capillary flux and diameter versus arteriolar velocity and flow following whisker stimulation using optical microangiography (OMAG) and two-photon microscopy. We further characterized depth-resolved responses of individual capillaries versus capillary networks. We hypothesized that capillaries respond first to neuronal activation, and that they exhibit a coordinated response mediated via endothelial-derived epoxyeicosatrienoates (EETs) acting on pericytes. To visualize peri-capillary pericytes, we used Tie2-GFP/NG2-DsRed mice, and to determine the role of endothelial-derived EETs, we compared cerebrovascular responses to whisker stimulation between wild-type mice and mice with lower endothelial EETs (Tie2-hsEH). We found that capillaries respond immediately to neuronal activation in an orchestrated network-level manner, a response attenuated in Tie2-hsEH and inhibited by blocking EETs action on pericytes. These results demonstrate that capillaries are first responders during functional hyperemia, and that they exhibit a network-level response mediated via endothelial-derived EETs’ action on peri-capillary pericytes.     

Effect of amphetamine on cerebral blood flow and capillary perfusion

The purpose of this study was to determine the cerebral regional microvascular and vascular responses to amphetamine sulfate at a dose (5 mg/kg) known to affect neuronal function. Cerebral blood flow (14C-iodoantipyrine method) and percent of perfused capillaries (fluorescein isothiocyanate-dextran and alkaline phosphatase staining method) were determined during control and after intravenous administration of amphetamine in conscious Long-Evans rats. Amphetamine caused an increase in blood pressure (34%) and heart rate (31%). There was a significant increase in averaged cerebral blood flow from 98 +/- 8 to 166 +/- 9 ml/min/100 g after amphetamine. This flow increase was significant in the cortex, basal ganglia, pons and medulla, however the increase was not significant in the hypothalamus. In control rats, there were approximately 325 +/- 17 capillaries/mm2 of brain tissue and 52 +/- 1% of them were perfused. Amphetamine increased the percent perfused significantly to 72 +/- 1% in all examined regions. There was a similar significant increase in the percent of perfused cerebral capillary volume fraction. There were both vascular and microvascular responses to amphetamine, increasing cerebral blood flow as well as reducing the diffusion distance for oxygen.     

Functional recruitment of red blood cells to rat brain microcirculation accompanying increased neuronal activity in cerebellar cortex

Scanning laser-Doppler flowmetry (SLDF) combines laser-Doppler flowmetry and laser scanning to provide images of cerebral blood flow (CBF) with high spatial and temporal resolution. We investigated the contribution of single vascular elements to the local increase of CBF accompanying increased neuronal activity in halothane-anesthetized rats. CBF was examined in the cerebellar cortex under control conditions and in response to electrical stimulation of parallel and climbing fibers. At rest, arterioles contributed 9%, venules 11-13% and small vessels (< 20 microm) 8-14%, while the background constituted 64-72% of the total SLDF signal. During activation the background signal decreased to 55-60% while the signal from arterioles increased to 11-12%, from venules to 14-15% and from small vessels to 14-19%. The signal increase in small vessels that did not give any laser-Doppler signal at rest was due to functional recruitment of red blood cells to the capillary bed. We conclude that functional recruitment may be an integral part of the hemodynamic response accompanying neuronal activity.     

Long-term in vivo investigation of mouse cerebral microcirculation by fluorescence confocal microscopy in the area of focal ischemia.

This study was designed to assess that mouse pial and cortical microcirculation can be monitored in the long term directly in the area of focal ischemia, using in vivo fluorescence microscopy. A closed cranial window was placed over the left parieto-occipital cortex of C57BL/6J mice. Local microcirculation was recorded in real time through the window using laser-scanning confocal fluorescence microscopy after intravenous injection of fluorescent erythrocytes and dextran. The basal velocity of erythrocytes through intraparenchymal capillaries was 0.53+/-0.30 mm/sec (n=121 capillaries in 10 mice). Two branches of the middle cerebral artery were topically cauterized through the window. Blood flow evaluated by laser-Doppler flowmetry in two distinct areas indicated the occurrence of an ischemic core (15.2%+/-5.9% of baseline for at least 2 h) and a penumbral zone. Magnetic resonance imaging and histology were used to characterize the ischemic area at 24 h after occlusion. The infarct volume was 7.3+/-3.2 mm(3) (n=6). Microcirculation was repeatedly videorecorded using fluorescence confocal microscopy over the next month. After the decrease following arterial occlusion, capillary erythrocyte velocity was significantly higher than baseline 1 week later, and attained 0.74+/-0.51 mm/sec (n=76 capillaries in six mice, P<0.005) after 1 month, while venous and capillary network remodeling was assessed, with a marked decrease in tortuosity. Immunohistochemistry revealed a zone of necrotic tissue into the infarct epicenter, with activated astrocytes at its border. Such long-term investigations in ischemic cortex brings new insight into the microcirculatory changes induced by focal ischemia and show the feasibility of long-term fluorescence studies in the mouse cortex.     

Cortical microcirculation in a new model of focal laser-induced secondary brain damage

To study the causes of spatial and temporal evolution of progressive neuro-injury in focal brain ischemia, models with consistent lesion topography are required. In such models, continuous monitoring of the microcirculation in a penumbral area undergoing progressive damage could be possible. We used a fixed-pulse (1.0 s, 40 W) Nd-YAG laser (NYL) to produced discrete brain lesions in rats and monitored the cerebral blood flow (CBF) with laser-Doppler flowmetry (LDF) in nonirradiated areas directly adjacent to the maturing lesion. We also examined the time evolution of the lesion topography over a 4 day period. The lesion volume determined by histopathological methods increased from 3.1 +/- 0.5 to 4.5 +/- 0.5 mm3 (p less than 0.05) during the first 2 h. Simultaneously, LDF indicated severe hypoperfusion (-60 +/- 21%, p less than 0.01) at a zone (1 mm distance from the laser lesion) where progressive neuronal degeneration and increased tissue water content (80.0 +/- 3.3% versus 76.8 +/- 2.1% in normal tissue, n = 7, p less than 0.05) were also observed. At a 4 mm distance from the lesion, hyperemic CBF responses were observed, but no histopathological signs or edema. Secondary brain damage progressed up to 4 days (lesion volume of 6.0 +/- 0.7 mm3). The NYL-induced brain lesion produced a highly reproducible focal injury and progressive neuronal death in a spatial relationship with microcirculatory failure and edema formation. The model allows prospective study of tissue state at a discrete zone, which is separate from the initial injury, but susceptible to secondary brain damage.     

Morphometric evaluation of post-ischemic capillary perfusion in selectively vulnerable areas of gerbil brain

Summary In gerbils the hemispheric blood flow was interrupted for 5 min by bilateral carotid artery occlusion to produce delayed selective destruction of the CA 1 sector of hippocampus. The influence of hemodynamic factors was studied by evaluating the microcirculation before and at two times after ischemia (3 min and 7 days), using Evans blue as an intravital vascular tracer. The density of perfused capillaries and the fractional volume of circulating blood were determined by quantitative morphometry and the values for the vulnerable CA 1 sector compared with those for the resistent CA 3 sector and cerebral cortex.In control animals the number of perfused capillaries in the CA 1 sector was about 20% lower, and the volume of circulating blood about 30% lower, than in the CA 3 sector or cerebral cortex. This difference was markedly enhanced after 5-min ischemia. During the early recirculation phase, capillary perfusion improved in the cortex, whereas in the CA 1 sector (and to a lesser degree also in the CA 3 sector) it declined. After 7 days, the density of perfused capillaries and the volume of circulating blood had returned to control levels in the cerebral cortex and CA 3 sector of hippocampus. In the CA 1 sector, in contrast, the microcirculation had further deteriorated. The density of perfused capillaries was less than 30%, and the circulating blood volume even less than 50%, of that in the cerebral cortex.The results obtained indicate that the microcirculatory capacity of the selectively vulnerable CA 1 sector of hippocampus is distinctly lower than that of resistant areas of the brain and suggest that hemodynamic factors may contribute to the difference in vulnerability of these regions to short-lasting cerebral ischemia.     

Comparison between pial and intraparenchymal vascular responses to cervical sympathetic stimulation in cats. Part 1. Under normal resting conditions

To investigate the role of sympathetic regulation in both resistance and capacitance vessels in cerebral circulation, the response of pial and intraparenchymal vessels to sympathetic nerve stimulation were simultaneously examined in 14 cats by means of a newly developed video camera photoelectric system. The system consisted of a video camera system for measurement of pial vascular diameters and a photoelectric apparatus for estimating regional cerebral blood volume in the intraparenchymal vessels. The ipsilateral superior cervical ganglion was electrically stimulated for 5 min. Initially, both the pial and intraparenchymal vessels constricted. The large pial arteries (173 +/- 25 micron, mean +/- SEM) remained constricted throughout the stimulation, whereas the intraparenchymal vessels began to dilate after the initial constriction and exceeded the control level at 175 +/- 25 s despite continued stimulation. In conclusion, such sympathetic nerve stimulation is considered to exert a constrictive effect on the intraparenchymal as well as the pial vessels at the early stage. The compensatory dilation of the intraparenchymal vessels was delayed 3 min after initiation of the stimulation.     

Age-related impairment of coupling mechanism between neuronal activation and functional cerebral blood flow response was restored by cholinesterase inhibition: PET study with microdialysis in the awake monkey brain

The effects of three cholinesterase inhibitors (physostigmine, E2020, and Tacrine), all of which are to be cognitive enhancers, on the functional regional cerebral blood flow (rCBF) response were studied in young (5.9+/-1.8 years old) and aged (18.0+/-3.3 years old) monkeys under awake conditions using high-resolution positron emission tomography (PET). Under control condition, vibrotactile stimulation elicited increases in the rCBF response in the contralateral somatosensory cortices of both young and aged monkeys, but the degree of increase in rCBF response was significantly lower in aged (115.8%) than that in young monkeys (139.9%). Regional cerebral metabolic rate of glucose (rCMRglc) response to the stimulation, measured using [18F]-2-fluoro-2-deoxy-Dphysostigmine) were consistent with the data obtained by microdialysis. In contrast, the cognitive enhancers did not alter rCBF response to stimulation in young monkeys. The present results demonstrated that the functional change in rCBF response to the stimulation was induced during the aging process by impairment of the coupling mechanism between the neuronal activation and rCBF response. Furthermore, the observation that cognitive enhancers partly restored the functional rCBF response suggested that the coupling mechanism might be regulated via cholinergic neuronal transmission.     

Initial oligemia with capillary flow stop followed by hyperemia during K+-induced cortical spreading depression in rats.

Local cerebral blood volume (CBV) and capillary flow changes in regions of depolarizing neurons during K(+)-induced cortical spreading depression (CSD) in the cerebral cortex of alpha-chloralose-urethane-anesthetized rats were examined employing a transillumination (550 nm) video system. Capillary flow was calculated as the reciprocal of mean transit times of blood in pixels of 40 microm x 40 microm, each of which contains a few capillaries. Potassium microinjection into the cortex evoked repetitive wave-ring spreads of oligemia at a speed of ca. 2.33 +/- 0.48 mm/min. During the spread of CSD, tracer (either saline or carbon black) was injected into the internal carotid artery. Colocated with the oligemic wave, we detected capillary flow stop as evidenced by disappearance of the hemodilution curves. At any location in the region of interest within the cerebral cortex, we observed cyclic changes of capillary flow stop/hyperperfusion in synchrony with oligemia/hyperemia fluctuations. The initial flow stop and oligemia were ascribed to capillary compression by astroglial cell swelling, presumably at the pericapillary endfeet, since the oligemia occurred before larger vessel changes. We conclude that local depolarizing neurons can decrease adjacent capillary flow directly and immediately, most likely via astroglial cell swelling, and that the flow stop triggers upstream arteriolar dilatation for capillary hyperperfusion.     

High spatiotemporal resolution imaging of the neurovascular response to electrical stimulation of rat peripheral trigeminal nerve as revealed by in vivo temporal laser speckle contrast

Previous studies have implicated the abnormal activation of the trigeminal system to be a factor in the pathogenesis of migraine. The relationship between vascular changes and migraine, however, is under considerable debate. In this study, temporal laser speckle contrast imaging is combined with ridge tracking based vessel detection to obtain high resolution (6.7 microm x 6.7 microm), high contrast images of cerebral vascular structure. For the first time, the vasomotor and blood flow responses to electrical stimulation in rat peripheral trigeminal system were obtained simultaneously. The system is capable of picking up individual vessels with diameters down to 30 microm. The spatial spread of the blood velocity response relative to the point of stimulation was studied. Analysis of branching vessels showed a 50+/-5% vs. 30+/-5% change in mean peak magnitude and a 54% per second vs. 17% per second change in mean rate of increase for vessels proximal vs. distal to the stimulation site. The penetration depth of the laser used was proven to be sufficient to image dural as well as cortical vessels through a thinned skull preparation. Different responses were observed from cortical and dural vessels. While the diameter of cortical vessels did not change in response to the stimulation the blood velocity went up by 65+/-5% per second. Dural vessels enlarged by 40+/-8% and the blood velocity increased by 50+/-5%. The method described here could be very useful in understanding and studying disorders in the neurovascular system.     

Disturbed microcirculation and hyperaemic response in a murine model of systemic inflammation

Systemic inflammation affects cognitive functions and increases the risk of dementia. This phenomenon is thought to be mediated in part by cytokines that promote neuronal survival, but the continuous exposure to which may lead to neurodegeneration. The effects of systemic inflammation on cerebral blood vessels, and their provision of adequate oxygen to support critical brain parenchymal cell functions, remains unclear. Here, we demonstrate that neurovascular coupling is profoundly disturbed in lipopolysaccharide (LPS) induced systemic inflammation in awake mice. In the 24 hours following LPS injection, the hyperaemic response of pial vessels to functional activation was attenuated and delayed. Concurrently, under steady-state conditions, the capillary network displayed a significant increase in the number of capillaries with blocked blood flow, as well as increased duration of ‘capillary stalls’—a phenomenon previously reported in animal models of stroke and Alzheimer’s disease pathology. We speculate that vascular changes and impaired oxygen availability may affect brain functions following acute systemic inflammation and contribute to the long-term risk of neurodegenerative changes associated with chronic, systemic inflammation.     

Air embolism of the brain in rabbits pretreated with mechlorethamine

Infusion of 400 microliters air into the left internal carotid artery of five anesthetized rabbits caused transient pial arteriole air embolism, an immediate 41.9 +/- 0.8% dilatation of the embolized vessels, suppression of the cortical somatosensory evoked response to 29.4 +/- 2.7% of baseline, and a progressive decline in ipsilateral cerebral blood flow (measured by hydrogen clearance) to 46 +/- 4.1% of baseline after 2 hours. These values were significantly different from those at baseline and from the responses of 10 control rabbits given equivalent intracarotid saline infusions. Twelve other rabbits were made leukopenic by treatment with 1.5 mg/kg i.v. mechlorethamine 72 hours prior to study. Mean +/- SEM leukocyte count decreased from 6,320 +/- 73/mm3 to 1,890 +/- 66/mm3 without any change in the leukocyte differential or erythrocyte and platelet counts. Intracarotid infusion of saline into seven of the leukopenic rabbits caused no changes. In the other five leukopenic rabbits, infusion of 400 microliters air caused air embolism but did not produce the anticipated declines in cerebral blood flow or the cortical somatosensory evoked response, both of which remained indistinguishable from baseline values and responses in the seven saline-treated leukopenic controls. Similarly, air-embolized arterioles showed nonsignificant dilatation in leukopenic rabbits. Our data suggest that the decreases in both cerebral blood flow and brain function seen after air embolism require the presence of leukocytes.     

Neuroprotection by 2-h postischemia administration of two free radical scavengers, alpha-phenyl-n-tert-butyl-nitrone (PBN) and N-tert-butyl-(2-sulfophenyl)-nitrone (S-PBN), in rats subjected to focal embolic cerebral ischemia

Oxygen free radical generation may have important secondary damaging effects after the onset of cerebral ischemia. Free radical scavengers have been used successfully in attenuating neuronal damage in the reperfusion period in transient forebrain ischemia. There are limited data on effectiveness in models of focal ischemia. Two free radical scavengers, alpha-phenyl-n-tert-butyl-nitrone (PBN) and N-tert-butyl-(2-sulfophenyl)-nitrone (S-PBN), have been shown to reduce oxidative-stress-induced neuronal injury. Whereas PBN has been demonstrated to reduce infarct volume in focal ischemia, neuroprotection has not been evaluated with S-PBN. The present study was designed to evaluate the neuroprotective effect of PBN and S-PBN compared to vehicle in a focal embolic middle cerebral artery (MCA) cerebral ischemia model in rats. Wistar rats were randomly divided into three groups (n = 10 each group). Animals in the control group received vehicle and those in the treatment groups were treated with PBN or S-PBN (both 100 mg/kg/day x 3 days, intraperitoneally) starting 2 h after the introduction of an autologous thrombus into the right-side MCA. The neurological outcome was observed and compared before and after treatment and between groups. The percentage of cerebral infarct volume was estimated from 2,3, 5-triphenyltetrazolium chloride stained coronal slices 72 h after the ischemic insult. Two-hour postischemia administration of PBN or S-PBN significantly improved neurobehavioral scores at 24 h following MCA embolization (both P < 0.01). The percentage of infarct volume for animals receiving vehicle was 32.8 +/- 9.4%. Two-hour delayed administration of PBN and S-PBN achieved a 35.4% reduction in infarct volume in treatment groups when compared with animals receiving vehicle (PBN vs control, 21.2 +/- 10.9% vs 32.8 +/- 9.4%; P < 0.05; S-PBN vs control, 21.2 +/- 13.1%, (P < 0.05). These data indicate that free radical generation may be involved in brain damage in this model and 2-h delayed postischemia treatment with PBN and S-PBN may have neuroprotective effects in focal cerebral ischemia. As S-PBN does not normally cross the blood-brain barrier, the neuroprotection evident in this study may be explained by entry into the brain via damaged vessels.     

Changes in vascular and metabolic reactivity as indices of ischaemia in the penumbra

The reactivities of cerebral cortical blood flow (hydrogen clearance) and of compensated NADH fluorescence to local cortical electrical stimulation were examined on the marginal gyrus before and after transorbital occlusion of the middle cerebral artery in cats. Prestimulus cerebral blood flow (CBF) was 38.2 +/- 12.9 (SD) ml 100 g-1 min-1 and fell to 19.8 +/- 11.1 following occlusion (p less than 0.02). Peak hydrogen clearance rate (percent increase above prestimulus clearance) was 81.6 +/- 53.6 and fell to 19.9 +/- 29.8 after middle cerebral artery occlusion (p less than 0.01). Steady-state NADH fluorescence rose from 33.5 +/- 10.7 to 40.5 +/- 17.6% full-scale deflection following MCAO (p less than 0.01). Latency from stimulus to maximal fluorescence depression in response to cortical stimulation increased from 12.2 +/- 8.2 to 22.1 +/- 11.9 s (p less than 0.01). Hyperaemic responses at anteromedial sites on the marginal gyrus significantly exceeded those at posterolateral sites. The results are interpreted as indicating early ischaemic metabolic change; however, the presence of residual vasodilator responses to stimulation suggests that flow reduction and early ischaemic change in the territory studied are not simply due to inadequate collateral input, but may also reflect deafferentation or functional suppression. The possible significance of diminished vascular reactivity in the penumbra as a cause of increased vulnerability to extracellular release of excitatory amino acids is discussed.     

Somatosensory pain does not affect total cerebral blood volume

The effect of somatosensory pain on the total cerebral blood volume was investigated in anesthetized rats. Our results show for the first time that total cerebral blood volume remains unaltered in both brain hemispheres during 2.5 min noxious stimulation of the sensory C-fibres of the sciatic nerve. Regional cerebral blood flow was increased by 97% in the thalamus and by 47% in the hypothalamus at the same time. Blockade of the L-arginine-nitric oxide system reduced significantly the steady-state control level of total cerebral blood volume (i.l.: from 5.7+/-1.3 to 4.58+/-1.6 vol%, c.l.: from 5.0+/-0.6 to 4.24+/-0.9 vol%). Nitric oxide synthase blockade, however, did not affect either the stimulation induced increase of regional cerebral blood flow or the steadiness of total cerebral blood volume during the stimulation.     

Evidence of a cerebrovascular postarteriole windkessel with delayed compliance.

A pronounced temporal mismatch was observed between the responses of relative cerebral blood volume (rCBV) measured by magnetic resonance imaging and relative cerebral blood flow measured by laser-Doppler flowmetry in rat somatosensory cortex after electrical forepaw stimulation. The increase of relative cerebral blood flow after stimulus onset and decrease after stimulus cessation were accurately described with a single exponential time constant of 2.4 +/- 0.8 seconds. In contrast, rCBV exhibited two distinct and nearly sequential processes after both onset and cessation of stimulation. A rapid change of rCBV (1.5 +/- 0.8 seconds) occurring immediately after onset and cessation was not statistically different from the time constant for relative cerebral blood flow. However, a slow phase of increase (onset) and decrease (cessation) with an exponential time constant of 14 +/- 13 seconds began approximately 8 seconds after the rapid phase of CBV change. A modified windkessel model was developed to describe the temporal evolution of rCBV as a rapid elastic response of capillaries and veins followed by slow venous relaxation of stress. Venous delayed compliance was suggested as the mechanism for the poststimulus undershoot in blood oxygen-sensitive magnetic resonance imaging signal that has been observed in this animal model and in human data.     

The effect of caffeine on dilated cerebral circulation and on diagnostic CO2 reactivity testing.

Reduction of cerebral blood flow by caffeine has been shown in multiple studies. However, the effect of this substance on pathologically dilated cerebral vessels is not clearly defined. The aim of this study was to investigate the effect of caffeine on an already dilated cerebral circulation and specify if these vessels are still able to constrict as a consequence of caffeine stimulation. A second aim of this study was to compare results of cerebral vasomotor CO(2) reactivity testing with and without caffeine ingestion. Seventeen healthy adult volunteers had vasomotor reactivity tested before and thirty minutes after ingestion of 300 mg of caffeine. Each vasomotor reactivity test consisted of velocity measurements from both middle cerebral arteries using transcranial Doppler ultrasound during normocapnia, hypercapnia, and hypocapnia. Hemodynamic data and end-tidal CO(2) (etCO(2)) concentration were also recorded. The vasomotor reactivity (VMR) and CO(2) reactivity were calculated from a measured data pool. At a level of etCO(2)=40 mmHg the resting velocity in the middle cerebral artery (V(MCA)) dropped from 70.7+/-22.8 cm/sec to 60.7 +/- 15.4 cm/sec 30 minutes after caffeine stimulation (14.1% decrease, p<0.001). During hypercapnia of etCO(2)=50 mmHg there was also a significant decline of V(MCA) from 103.1+/-25.4 to 91.4+/-21.8 cm/sec (11.3%, p<0.001). There was not a statistically significant reduction of V(MCA) during hypocapnia. Calculated VMR and CO(2) reactivity before and after caffeine intake were not statistically significant. The presented data demonstrate a significant decrease in cerebral blood flow velocities after caffeine ingestion both in a normal cerebrovascular bed and under conditions of peripheral cerebrovascular vasodilatation. These findings support the important role of caffeine in regulation of CBF under different pathological conditions. Despite significant reactive vasodilatation in the brain microcirculation, caffeine is still able to act as a competitive antagonist of CO(2) on cerebral microvessels. The fact that caffeine may decrease CBF despite significant pathological vasodilatation offers the possibility of therapeutic manipulation in patients with traumatic vasoparalysis. For routine clinical testing of CO(2) reactivity it is not necessary to insist on pre-test dietary restrictions.     

Diverse functions of pericytes in cerebral blood flow regulation and ischemia

Pericytes are mural cells with contractile properties. Here, we provide evidence that microvascular pericytes modulate cerebral blood flow in response to neuronal activity (‘functional hyperemia''). Besides their role in neurovascular coupling, pericytes are responsive to brain damage. Cerebral ischemia is associated with constrictions and death of capillary pericytes, followed by fibrotic reorganization of the ischemic tissue. The data suggest that precapillary arterioles and capillaries are major sites of hemodynamic regulation in the brain.