Reactivity of rat pial arterioles and venules to adenosine and carbon dioxide: with detailed description of the closed cranial window technique in rats

Plastic vasomotion entrainment

The presence of global synchronization of vasomotion induced by oscillating visual stimuli was identified in the mouse brain. Endogenous autofluorescence was used and the vessel 'shadow' was quantified to evaluate the magnitude of the frequency-locked vasomotion. This method allows vasomotion to be easily quantified in non-transgenic wild-type mice using either the wide-field macro-zoom microscopy or the deep-brain fiber photometry methods. Vertical stripes horizontally oscillating at a low temporal frequency (0.25 Hz) were presented to the awake mouse, and oscillatory vasomotion locked to the temporal frequency of the visual stimulation was induced not only in the primary visual cortex but across a wide surface area of the cortex and the cerebellum. The visually induced vasomotion adapted to a wide range of stimulation parameters. Repeated trials of the visual stimulus presentations resulted in the plastic entrainment of vasomotion. Horizontally oscillating visual stimulus is known to induce horizontal optokinetic response (HOKR). The amplitude of the eye movement is known to increase with repeated training sessions, and the flocculus region of the cerebellum is known to be essential for this learning to occur. Here, we show a strong correlation between the average HOKR performance gain and the vasomotion entrainment magnitude in the cerebellar flocculus. Therefore, the plasticity of vasomotion and neuronal circuits appeared to occur in parallel. Efficient energy delivery by the entrained vasomotion may contribute to meeting the energy demand for increased coordinated neuronal activity and the subsequent neuronal circuit reorganization.

Cranial and Spinal Window Preparation for in vivo Optical Neuroimaging in Rodents and Related Experimental Techniques

Optical neuroimaging provides an effective neuroscience tool for multi-scale investigation of the neural structures and functions, ranging from molecular, cellular activities to the inter-regional connectivity assessment. Amongst experimental preparations, the implementation of an artificial window to the central nervous system (CNS) is primarily required for optical visualization of the CNS and associated brain activities through the opaque skin and bone. Either thinning down or removing portions of the skull or spine is necessary for unobstructed long-term in vivo observations, for which types of the cranial and spinal window and applied materials vary depending on the study objectives. As diversely useful, a window can be designed to accommodate other experimental methods such as electrophysiology or optogenetics. Moreover, auxiliary apparatuses would allow the recording in synchrony with behavior of large-scale brain connectivity signals across the CNS, such as olfactory bulb, cerebral cortex, cerebellum, and spinal cord. Such advancements in the cranial and spinal window have resulted in a paradigm shift in neuroscience, enabling in vivo investigation of the brain function and dysfunction at the microscopic, cellular level. This Review addresses the types and classifications of windows used in optical neuroimaging while describing how to perform in vivo studies using rodent models in combination with other experimental modalities during behavioral tests. The cranial and spinal window has enabled longitudinal examination of evolving neural mechanisms via in situ visualization of the brain. We expect transformable and multi-functional cranial and spinal windows to become commonplace in neuroscience laboratories, further facilitating advances in optical neuroimaging systems.

Closed cranial window rodent model for investigating hemodynamic response to elevated intracranial pressure

Background

Elevated intracranial pressure (ICP) occurs in several physiological and pathological conditions, yet long-term sequellae are not common, which implies that blood flow is preserved above ischemic thresholds.

Methods

This pilot study sought to confirm this hypothesis using a closed cranial window model in a rat in which ICP was elevated to 120 mmHg for 12 min, and superficial cortical perfusion was measured by laser Doppler flowmetry and laser speckle flowmetry.

Results

Following a transient increase, cortical blood flow decreased to between 25% and 75% of baseline. These levels correspond to disrupted metabolism and decreased protein synthesis but did not exceed thresholds for electrical signaling or membrane integrity. This may partially explain how some episodes of elevated ICP remain benign.

Conclusion

The closed cranial window model provides a platform for prospective study of physiologic responses to artificially elevated ICP during neurosurgery to promote hemostasis.

The Good, the Bad, and the Deadly: Adenosinergic Mechanisms Underlying Sudden Unexpected Death in Epilepsy

Adenosine is an inhibitory modulator of neuronal excitability. Neuronal activity results in increased adenosine release, thereby constraining excessive excitation. The exceptionally high neuronal activity of a seizure results in a surge in extracellular adenosine to concentrations many-fold higher than would be observed under normal conditions. In this review, we discuss the multifarious effects of adenosine signaling in the context of epilepsy, with emphasis on sudden unexpected death in epilepsy (SUDEP). We describe and categorize the beneficial, detrimental, and potentially deadly aspects of adenosine signaling. The good or beneficial characteristics of adenosine signaling in the context of seizures include: (1) its direct effect on seizure termination and the prevention of status epilepticus; (2) the vasodilatory effect of adenosine, potentially counteracting postictal vasoconstriction; (3) its neuroprotective effects under hypoxic conditions; and (4) its disease modifying antiepileptogenic effect. The bad or detrimental effects of adenosine signaling include: (1) its capacity to suppress breathing and contribute to peri-ictal respiratory dysfunction; (2) its contribution to postictal generalized EEG suppression (PGES); (3) the prolonged increase in extracellular adenosine following spreading depolarization waves may contribute to postictal neuronal dysfunction; (4) the excitatory effects of A2A receptor activation is thought to exacerbate seizures in some instances; and (5) its potential contributions to sleep alterations in epilepsy. Finally, the adverse effects of adenosine signaling may potentiate a deadly outcome in the form of SUDEP by suppressing breathing and arousal in the postictal period. Evidence from animal models suggests that excessive postictal adenosine signaling contributes to the pathophysiology of SUDEP. The goal of this review is to discuss the beneficial, harmful, and potentially deadly roles that adenosine plays in the context of epilepsy and to identify crucial gaps in knowledge where further investigation is necessary. By better understanding adenosine dynamics, we may gain insights into the treatment of epilepsy and the prevention of SUDEP.

Chronic cranial window for photoacoustic imaging: a mini review

Photoacoustic (PA) microscopy is being increasingly used to visualize the microcirculation of the brain cortex at the micron level in living rodents. By combining it with long-term cranial window techniques, vasculature can be monitored over a period of days extending to months through a field of view. To fulfill the requirements of long-term in vivo PA imaging, the cranial window must involve a simple and rapid surgical procedure, biological compatibility, and sufficient optical-acoustic transparency, which are major challenges. Recently, several cranial window techniques have been reported for longitudinal PA imaging. Here, the development of chronic cranial windows for PA imaging is reviewed and its technical details are discussed, including window installation, imaging quality, and longitudinal stability.

Direct wavefront sensing enables functional imaging of infragranular axons and spines

We advance two-photon microscopy for near-diffraction-limited imaging up to 850 µm below the pia in awake mice. Our approach combines direct wavefront sensing of light from a guidestar (formed by descanned fluorescence from Cy5.5-conjugated dextran in brain microvessels) with adaptive optics to compensate for tissue-induced aberrations in the wavefront. We achieve high signal-to-noise ratios in recordings of glutamate release from thalamocortical axons and calcium transients in spines of layer 5b basal dendrites during active tactile sensing.

Optical imaging and modulation of neurovascular responses

The cerebral microvasculature consists of pial vascular networks, parenchymal descending arterioles, ascending venules and parenchymal capillaries. This vascular compartmentalization is vital to precisely deliver blood to balance continuously varying neural demands in multiple brain regions. Optical imaging techniques have facilitated the investigation of dynamic spatial and temporal properties of microvascular functions in real time. Their combination with transgenic animal models encoding specific genetic targets have further strengthened the importance of optical methods for neurovascular research by allowing for the modulation and monitoring of neuro vascular function. Image analysis methods with three-dimensional reconstruction are also helping to understand the complexity of microscopic observations. Here, we review the compartmentalized cerebral microvascular responses to global perturbations as well as regional changes in response to neural activity to highlight the differences in vascular action sites. In addition, microvascular responses elicited by optical modulation of different cell-type targets are summarized with emphasis on variable spatiotemporal dynamics of microvascular responses. Finally, long-term changes in microvascular compartmentalization are discussed to help understand potential relationships between CBF disturbances and the development of neurodegenerative diseases and cognitive decline.

Rat Pial Microvascular Changes During Cerebral Blood Flow Decrease and Recovery: Effects of Cyanidin Administration

The reactive oxygen species (ROS) are known to play a major role in many pathophysiological conditions, such as ischemia and reperfusion injury. The present study was aimed to evaluate the in vivo cyanidin (anthocyanin) effects on damages induced by rat pial microvascular hypoperfusion-reperfusion injury by cerebral blood flow decrease (CBFD) and subsequent cerebral blood flow recovery (CBFR). In particular, the main purpose was to detect changes in ROS production after cyanidin administration. Rat pial microvasculature was investigated using fluorescence microscopy through a cranial window (closed); Strahler's method was utilized to define the geometric features of pial vessels. ROS production was investigated in vivo by 2'-7'-dichlorofluorescein-diacetate assay and neuronal damage was measured on isolated brain sections by 2,3,5-triphenyltetrazolium chloride staining. After 30 min of CBFD, induced by bilateral common carotid artery occlusion, and 60 min of CBFR, rats showed decrease of arteriolar diameter and capillary perfusion; furthermore, increase in microvascular leakage and leukocyte adhesion was observed. Conversely, cyanidin administration induced dose-related arteriolar dilation, reduction in microvascular permeability as well as leukocyte adhesion when compared to animals subjected to restriction of cerebral blood flow; moreover, capillary perfusion was protected. ROS generation increase and marked neuronal damage were detected in animals subjected to CBFD and CBFR. On the other hand, cyanidin was able to reduce ROS generation and neuronal damage. In conclusion, cyanidin treatment showed dose-related protective effects on rat pial microcirculation during CBFD and subsequent CBFR, inducing arteriolar dilation by nitric oxide release and inhibiting ROS formation, consequently preserving the blood brain barrier integrity.

Laser Speckle Imaging of Rat Pial Microvasculature during Hypoperfusion-Reperfusion Damage

The present study was aimed to in vivo assess the blood flow oscillatory patterns in rat pial microvessels during 30 min bilateral common carotid artery occlusion (BCCAO) and 60 min reperfusion by laser speckle imaging (LSI). Pial microcirculation was visualized by fluorescence microscopy. The blood flow oscillations of single microvessels were recorded by LSI; spectral analysis was performed by Wavelet transform. Under baseline conditions, arterioles and venules were characterized by blood flow oscillations in the frequency ranges 0.005-0.0095 Hz, 0.0095-0.021 Hz, 0.021-0.052 Hz, 0.052-0.150 Hz and 0.150-0.500 Hz. Arterioles showed oscillations with the highest spectral density when compared with venules. Moreover, the frequency components in the ranges 0.052-0.150 Hz and 0.150-0.500 were predominant in the arteriolar total power spectrum; while, the frequency component in the range 0.150-0.500 Hz showed the highest spectral density in venules. After 30 min BCCAO, the arteriolar spectral density decreased compared to baseline; moreover, the arteriolar frequency component in the range 0.052-0.150 Hz significantly decreased in percent spectral density, while the frequency component in the range 0.150-0.500 Hz significantly increased in percent spectral density. However, an increase in arteriolar spectral density was detected at 60 min reperfusion compared to BCCAO values; consequently, an increase in percent spectral density of the frequency component in the range 0.052-0.150 Hz was observed, while the percent spectral density of the frequency component in the range 0.150-0.500 Hz significantly decreased. The remaining frequency components did not significantly change during hypoperfusion and reperfusion. The changes in blood flow during hypoperfusion/reperfusion caused tissue damage in the cortex and striatum of all animals. In conclusion, our data demonstrate that the frequency component in the range 0.052-0.150 Hz, related to myogenic activity, was significantly impaired by hypoperfusion and reperfusion, affecting cerebral blood flow distribution and causing tissue damage.

Malvidin's Effects on Rat Pial Microvascular Permeability Changes Due to Hypoperfusion and Reperfusion Injury

The present study was aimed to evaluate the malvidin's protective effects on damage induced by 30 min bilateral common carotid artery occlusion (BCCAO) and 60 min reperfusion (RE) in rat pial microcirculation. Rat pial microcirculation was observed using fluorescence microscopy through a closed cranial window. Western blotting analysis was performed to investigate the endothelial nitric oxide synthase (eNOS), phosphorylated eNOS (p-eNOS) and matrix metalloproteinase 9 (MMP-9) expression. Moreover, MMP-9 activity was evaluated by zymography. Finally, neuronal damage and radical oxygen species (ROS) formation were assessed. In all animals, pial arterioles were classified in five orders of branching according to Strahler's method. In hypoperfused rats, 30 min BCCAO and 60 min RE caused a decrease in arteriolar diameter, an increase in microvascular leakage and leukocyte adhesion, accompanied by decreased capillary perfusion and red blood cell velocity (VRBC). Moreover, marked neuronal damage and evident ROS generation were detected. Conversely, malvidin administration induced arteriolar dilation in dose-related manner, reducing microvascular leakage as well as leukocyte adhesion. Capillary perfusion and VRBC were protected. Nitric oxide (NO) synthase inhibition significantly attenuated malvidin's effects on arteriolar diameter. Western blotting analysis revealed an increase in eNOS and p-eNOS expression, while zymography indicated a decrease in MMP-9 activity after malvidin's administration. Furthermore, malvidin was able to prevent neuronal damage and to decrease ROS generation. In conclusion, malvidin protects rat pial microcirculation against BCCAO/RE injury, preventing blood-brain impairment and neuronal loss. Malvidin's effects appear to be mediated by eNOS activation and scavenger activity.

The Effects of Vaccinium myrtillus Extract on Hamster Pial Microcirculation during Hypoperfusion-Reperfusion Injury

Introduction

The present study was aimed to assess the in vivo hamster pial microvessel alterations due to 30 min transient bilateral common carotid artery occlusion (BCCAO) and reperfusion (60 min); moreover, the neuroprotective effects of Vaccinium myrtillus extract, containing 34.7% of anthocyanins, were investigated.

Materials and Methods

Two groups of male hamsters were used: the first fed with control diet and the other with Vaccinium myrtillus supplemented diet. Hamster pial microcirculation was visualized by fluorescence microscopy through an open cranial window. Pial arterioles were classified according to Strahler’s method.

Results

In age-matched control diet-fed hamsters, BCCAO caused a decrease in diameter of all arterioles. At the end of reperfusion, the reduction of diameter in order 3 arterioles was by 8.4 ± 3.1%, 10.8 ± 2.3% and 12.1 ± 1.1% of baseline in the 2, 4 and 6 month control diet-fed hamsters, respectively. Microvascular permeability and leukocyte adhesion were markedly enhanced, while perfused capillary length (PCL) decreased. The response to acetylcholine and papaverine topical application was impaired; 2’-7’-dichlorofluoresceine-diacetate assay demonstrated a significant ROS production. At the end of BCCAO, in age-matched Vaccinium myrtillussupplemented diet-fed hamsters, the arteriolar diameter did not significantly change compared to baseline. After 60 min reperfusion, order 3 arterioles dilated by 9.3 ± 2.4%, 10.6 ± 3.1% and 11.8 ± 2.7% of baseline in the 2, 4 and 6 month Vaccinium myrtillus supplemented diet-fed hamsters, respectively. Microvascular leakage and leukocyte adhesion were significantly reduced in all groups according to the time-dependent treatment, when compared with the age-matched control diet-fed hamsters. Similarly, the reduction in PCL was progressively prevented. Finally, the response to acetylcholine and papaverine topical application was preserved and there was no significant increase in ROS production in all groups.

Conclusions

In conclusion, Vaccinium myrtillusextract protected pial microcirculation during hypoperfusion-reperfusion, preventing vasoconstriction, microvascular permeability, leukocyte adhesion, reduction in PCL and preserving the endothelium function.

T2* and T1 assessment of abdominal tissue response to graded hypoxia and hypercapnia using a controlled gas mixing circuit for small animals

PURPOSE

To characterize T2* and T1 relaxation time response to a wide spectrum of gas challenges in extracranial tissues of healthy rats.

MATERIALS AND METHODS

A range of graded gas mixtures (hyperoxia, hypercapnia, hypoxia, and hypercapnic hypoxia) were delivered through a controlled gas-mixing circuit to mechanically ventilated and intubated rats. Quantitative magnetic resonance imaging (MRI) was performed on a 3T clinical scanner; T2* and T1 maps were computed to determine tissue response in the liver, kidney cortex, and paraspinal muscles. Heart rate and blood oxygen saturation (SaO2 ) were measured through a rodent oximeter and physiological monitor.

RESULTS

T2* decreases consistent with lowered SaO2 measurements were observed for hypercapnia and hypoxia, but decreases were significant only in liver and kidney cortex (P < 0.05) for >10% CO2 and <15% O2 , with the new gas stimulus, hypercapnic hypoxia, producing the greatest T2* decrease. Hyperoxia-related T2* increases were accompanied by negligible increases in SaO2 . T1 generally increased, if at all, in the liver and decreased in the kidney. Significance was observed (P < 0.05) only in kidney for >90% O2 and >5% CO2 .

CONCLUSION

T2* and T1 provide complementary roles for evaluating extracranial tissue response to a broad range of gas challenges. Based on both measured and known physiological responses, our results are consistent with T2* as a sensitive marker of blood oxygen saturation and T1 as a weak marker of blood volume changes. J. Magn. Reson. Imaging 2016;44:305-316.

Constriction and dysfunction of pial arterioles after regional hemorrhage in the subarachnoid space

Increasing evidence indicates that poor outcomes after brain hemorrhage, especially after subarachnoid hemorrhage (SAH), can be attributed largely to dysfunction of the cerebral microcirculation. However, the cause of this dysfunction remains unclear. Here, we investigated changes in the cerebral microcirculation after regional hemorrhage in the subarachnoid space using the closed cranial window technique in mice. A single pial arteriole on the surface of the brain was punctured to induce a regional hemorrhage in the subarachnoid space. Physiological parameters were monitored during the procedure, and microvessel diameter was measured after hemorrhage. The vasoreactivity of the arterioles in response to hypercapnia as well as to topical application of the vasodilator acetylcholine (ACh) and S-nitroso-N-acetyl-penicillamine (SNAP) were assessed. The constriction of pial arterioles was detected without changes in other physiological parameters. Decreased reactivity of pial arterioles to all of the applied vasodilatory stimuli was observed after hemorrhage. Our results indicate that regional hemorrhage in the subarachnoid space can induce the vasospasm of microvessels and also reduce the vasoreactivity of pial arterioles.

Correlation of oxygenation and perfusion sensitive MRI with invasive micro probe measurements in healthy mice brain

The non-invasive assessment of (patho-)physiological parameters such as, perfusion and oxygenation, is of great importance for the characterization of pathologies e.g., tumors, which may be helpful to better predict treatment response and potential outcome. To better understand the influence of physiological parameters on the investigated oxygenation and perfusion sensitive MRI methods, MRI measurements were correlated with subsequent invasive micro probe measurements during free breathing conditions of air, air+10% CO2 and 100% O2 in healthy mice brain. MRI parameters were the irreversible (R2), reversible (R2') and effective (R2*) transverse relaxation rates, venous blood oxygenation level assessed by quantitative blood oxygenation level dependent (qBOLD) method and cerebral blood flow (CBF) assessed by arterial spin labeling (ASL) using a 7 T small animal MRI scanner. One to two days after MRI, tissue perfusion and pO2 were measured by Laser-Doppler flowmetry and fluorescence quenching micro probes, respectively. The tissue pO2 values were converted to blood oxygen saturation by using the Hill equation. The animals were anesthetized by intra peritoneal injection of ketamine-xylazine-acepromazine (10-2-0.3 mg/ml · kg). Results for normal/hypercapnia/hyperoxia conditions were: R2[s(∧)-1] = 20.7/20.4/20.1, R2*[s(∧)-1] = 31.6/29.6/25.9, R2'[s-(∧)1] = 10.9/9.2/5.7, qBOLD venous blood oxygenation level = 0.43/0.51/0.56, CBF[ml · min(∧)-1 · 100 g(∧)-1] = 70.6/105.5/81.8, Laser-Doppler flowmetry[a.u.] = 89.2/120.2/90.6 and pO2[mmHg] = 6.3/32.3/46.7. All parameters were statistically significantly different with P < 0.001 between all breathing conditions. All MRI and the corresponding micro probe measurements were also statistically significantly (P ≤ 0.03) correlated with each other. However, converting the tissue pO2 to blood oxygen saturation = 0.02/0.34/0.63, showed only very limited agreement with the qBOLD venous blood oxygenation level. We found good correlation between MRI and micro probe measurements. However, direct conversion of tissue pO2 to blood oxygen saturation by using the Hill equation is very limited. Furthermore, adverse effects of anesthesia and trauma due to micro probe insertion are strong confounding factors and need close attention for study planning and conduction of experiments. Investigation of the correlation of perfusion and oxygenation sensitive MRI methods with micro probe measurements in pathologic tissue such as tumors is now of compelling interest.

A chronic scheme of cranial window preparation to study pial vascular reactivity in murine cerebral malaria

OBJECTIVE

The acute implantation of a cranial window for studying cerebroarteriolar reactivity in living animals involves a highly surgically invasive craniotomy procedure at the time of experimentation, which limits its application in severely ill animals such as in the experimental murine model of cerebral malaria (ECM). To overcome this problem, a chronic window implantation scheme was designed and implemented.

METHODS

A partial craniotomy is first performed by creating a skull bone flap in the healthy mice, which are then left to recover for one to two weeks, followed by infection to induce ECM. Uninfected animals are utilized as control. When cranial superfusion is needed, the bone flap is retracted and window implantation completed by assembling a perfusion chamber for compound delivery to the exposed brain surface. The presurgical step is intended to minimize surgical trauma on the day of experimentation.

RESULTS

Chronic preparations in uninfected mice exhibited remarkably improved stability over acute ones by significantly reducing periarteriolar tissue damage and enhancing cerebroarteriolar dilator responses. The chronic scheme was successfully implemented in ECM mice, which unveiled novel preliminary insights into impaired cerebroarteriolar reactivity and eNOS dysfunction.

CONCLUSION

The chronic scheme presents an innovative approach for advancing our mechanistic understanding on cerebrovascular dysfunction in ECM.

Optogenetic micro-electrocorticography for modulating and localizing cerebral cortex activity

OBJECTIVE

Spatial localization of neural activity from within the brain with electrocorticography (ECoG) and electroencephalography remains a challenge in clinical and research settings, and while microfabricated ECoG (micro-ECoG) array technology continues to improve, complementary methods to simultaneously modulate cortical activity while recording are needed.

APPROACH

We developed a neural interface utilizing optogenetics, cranial windowing, and micro-ECoG arrays fabricated on a transparent polymer. This approach enabled us to directly modulate neural activity at known locations around micro-ECoG arrays in mice expressing Channelrhodopsin-2. We applied photostimuli varying in time, space and frequency to the cortical surface, and we targeted multiple depths within the cortex using an optical fiber while recording micro-ECoG signals.

MAIN RESULTS

Negative potentials of up to 1.5 mV were evoked by photostimuli applied to the entire cortical window, while focally applied photostimuli evoked spatially localized micro-ECoG potentials. Two simultaneously applied focal stimuli could be separated, depending on the distance between them. Photostimuli applied within the cortex with an optical fiber evoked more complex micro-ECoG potentials with multiple positive and negative peaks whose relative amplitudes depended on the depth of the fiber.

SIGNIFICANCE

Optogenetic ECoG has potential applications in the study of epilepsy, cortical dynamics, and neuroprostheses.

Functional imaging of cerebral perfusion

The functional imaging of perfusion enables the study of its properties such as the vasoreactivity to circulating gases, the autoregulation and the neurovascular coupling. Downstream from arterial stenosis, this imaging can estimate the vascular reserve and the risk of ischemia in order to adapt the therapeutic strategy. This method reveals the hemodynamic disorders in patients suffering from Alzheimer's disease or with arteriovenous malformations revealed by epilepsy. Functional MRI of the vasoreactivity also helps to better interpret the functional MRI activation in practice and in clinical research.

Two-photon microscopy as a tool to study blood flow and neurovascular coupling in the rodent brain

The cerebral vascular system services the constant demand for energy during neuronal activity in the brain. Attempts to delineate the logic of neurovascular coupling have been greatly aided by the advent of two-photon laser scanning microscopy to image both blood flow and the activity of individual cells below the surface of the brain. Here we provide a technical guide to imaging cerebral blood flow in rodents. We describe in detail the surgical procedures required to generate cranial windows for optical access to the cortex of both rats and mice and the use of two-photon microscopy to accurately measure blood flow in individual cortical vessels concurrent with local cellular activity. We further provide examples on how these techniques can be applied to the study of local blood flow regulation and vascular pathologies such as small-scale stroke.

Rat Pial Microvascular Responses to Transient Bilateral Common Carotid Artery Occlusion and Reperfusion: Quercetin's Mechanism of Action

The aim of the present study was to assess quercetin's mechanism of action in rat pial microvessels during transient bilateral common carotid artery occlusion (BCCAO) and reperfusion. Rat pial microcirculation was visualized using fluorescence microscopy through a closed cranial window. Pial arterioles were classified in five orders of branchings. In ischemic rats, 30 min BCCAO and 60 min reperfusion caused arteriolar diameter decrease, microvascular leakage, leukocyte adhesion in venules, and reduction of capillary perfusion. Quercetin highest dose determined dilation in all arteriolar orders, by 40 ± 4% of baseline in order 2 vessels, and prevented microvascular permeability [0.15 ± 0.02 normalized gray levels (NGL)], leukocyte adhesion, and capillary failure. Protein kinase C (PKC) inhibition exerted by chelerythrine prior to quercetin attenuated quercetin-induced effects: order 2 arterioles dilated by 19.0 ± 2.4% baseline, while there was an increase in permeability (0.40 ± 0.05 NGL) and leukocyte adhesion with a marked decrease in capillary perfusion. Tyrosine kinase (TK) inhibition by tyrphostin 47 prior to quercetin lessened smaller pial arterioles responses, dilating by 20.7 ± 2.5% of baseline, while leakage increased (0.39 ± 0.04 NGL) sustained by slight leukocyte adhesion and ameliorated capillary perfusion. Inhibition of endothelium nitric oxide synthase (eNOS) by N(G)-nitro-L-arginine-methyl ester (L-NAME) prior to PKC or TK reduced the quercetin's effects on pial arteriolar diameter and leakage. eNOS inhibition by L-NAME reduced quercetin effects on pial arteriolar diameter and leakage. Finally, combined inhibition of PKC and TK prior to quercetin abolished quercetin-induced effects, decreasing eNOS expression, while blocking ATP-sensitive potassium (K(ATP)) channels by glibenclamide suppressed arteriolar dilation. In conclusion, the protective effects of quercetin could be due to different mechanisms resulting in NO release throughout PKC and TK intracellular signaling pathway activation.

Protective Effects of Quercetin on Rat Pial Microvascular Changes during Transient Bilateral Common Carotid Artery Occlusion and Reperfusion

The aim of this study was to assess the in vivo effects of quercetin on pial microvascular responses during transient bilateral common carotid artery occlusion (BCCAO) and reperfusion. Rat pial microcirculation was visualized by fluorescence microscopy through a closed cranial window. Pial arterioles were classified in five orders of branchings. Capillaries were assigned order 0, the smallest arterioles order 1, and the largest ones order 5. In ischemic rats, 30 min BCCAO and 60 min reperfusion caused arteriolar diameter decrease (by 14.5 ± 3.3% of baseline in order 2), microvascular leakage [0.47 ± 0.04, normalized gray levels (NGL)], leukocyte adhesion in venules (9 ± 2/100 μm venular length, v.l./30 s), and reduction of capillary perfusion (by 40 ± 7% of baseline). Moreover, at the end of BCCAO and reperfusion there was a significant increase in reactive oxygen species (ROS) formation when compared with baseline. Quercetin highest dose determined dilation in all arteriolar orders (by 40 ± 4% of baseline in order 2) and prevented microvascular permeability (0.15 ± 0.02 NGL), leukocyte adhesion (3 ± 1/100 μm v.l./30 s) as well as ROS formation, while capillary perfusion was protected. Inhibition of endothelial nitric oxide synthase (NOS) prior to quercetin reduced arteriolar dilation (order 2 diameter increase by 10.3 ± 2.5% of baseline) and caused permeability increase (0.29 ± 0.03 NGL); inhibition of neuronal NOS or inducible NOS did not affect quercetin-induced effects. Inhibition of guanylyl cyclase prior to quercetin reversed the quercetin’s effects on pial arteriolar diameter and leakage. In conclusion, quercetin was able to protect pial microcirculation from ischemia–reperfusion damage inducing arteriolar dilation likely by nitric oxide release. Moreover, quercetin scavenger activity blunted ROS formation preserving the blood–brain barrier integrity.

Rat pial microvascular responses to melatonin during bilateral common carotid artery occlusion and reperfusion

The present study assessed the in vivo rat pial microvascular responses induced by melatonin during brain hypoperfusion and reperfusion (RE) injury. Pial microcirculation of male Wistar rats was visualized by fluorescence microscopy through a closed cranial window. Hypoperfusion was induced by bilateral common carotid artery occlusion (BCCAO, 30 min); thereafter, pial microcirculation was observed for 60 min. Arteriolar diameter, permeability increase, leukocyte adhesion to venular walls, perfused capillary length (PCL), and capillary red blood cell velocity (V(RBC) ) were investigated by computerized methods. Melatonin (0.5, 1, 2 mg/kg b.w.) was intravenously administered 10 min before BCCAO and at the beginning of RE. Pial arterioles were classified in five orders according to diameter, length, and branchings. In control group, BCCAO caused decrease in order 2 arteriole diameter (by 17.5 ± 3.0% of baseline) that was reduced by 11.8 ± 1.2% of baseline at the end of RE, accompanied by marked leakage and leukocyte adhesion. PCL and capillary V(RBC) decreased. At the end of BCCAO, melatonin highest dosage caused order 2 arteriole diameter reduction by 4.6 ± 2.0% of baseline. At RE, melatonin at the lower dosages caused different arteriolar responses. The highest dosage caused dilation in order 2 arteriole by 8.0 ± 1.5% of baseline, preventing leakage and leukocyte adhesion, while PCL and V(RBC) increased. Luzindole (4 mg/kg b.w.) prior to melatonin caused order 2 arteriole constriction by 12.0 ± 1.5% of baseline at RE, while leakage, leukocyte adhesion, PCL and V(RBC) were not affected. Prazosin (1 mg/kg b.w.) prior to melatonin did not significantly change melatonin's effects. In conclusion, melatonin caused different responses during hypoperfusion and RE, modulating pial arteriolar tone likely by MT1 and MT2 melatonin receptors while preventing blood-brain barrier changes through its free radical scavenging action.

The brain tumor window model: a combined cranial window and implanted glioma model for evaluating intraoperative contrast agents

OBJECTIVE

Optical contrast agents for brain tumor delineation have been previously evaluated in ex vivo specimens from animals with implanted gliomas and may not reflect the true visual parameters encountered during surgery. This study describes a novel model system designed to evaluate optical contrast agents for tumor delineation in vivo.

METHODS

Biparietal craniectomies were performed on 8-week-old Sprague-Dawley rats. 9L glioma cells were injected intraparenchymally. A cover slip was bonded to the cranial defect with cyanoacrylate glue. When the tumor radius reached 1 mm, Coomassie Blue was administered intravenously while the appearance of the cortical surface was recorded. Computerized image analysis of the red/green/blue color components was used to quantify visible differences between tumor and nonneoplastic tissue and to compare delineation in the brain tumor window (BTW) model with the conventional 9L glioma model.

RESULTS

The tumor margin in the BTW model was poorly defined before contrast administration but readily apparent after contrast administration. Based on red component intensity, tumor delineation improved 4-fold at 50 minutes after contrast administration in the BTW model (P < .002). The conventional 9L glioma model overestimated the degree of delineation compared with the BTW model at the same dose of Coomassie Blue (P < .03).

CONCLUSION

Window placement overlying an implanted glioma is technically possible and well tolerated in the rat. The BTW model is a valid system for evaluating optical contrast agents designed to delineate brain tumor margins. To our knowledge, we have described the first in vivo model system for evaluating optical contrast agents for tumor delineation.

The role of adenosine in hypercarbic hyperemia: in vivo and in vitro studies in adenosine 2(A) receptor knockout and wild-type mice

OBJECT

The authors tested the hypothesis that adenosine, acting through the A(2A) receptor, is not involved in hypercarbic hyperemia by assessing the effects of increased PaCO(2) on cerebral blood flow (CBF) in vivo in wild-type and A(2A) receptor knockout mice. In addition, they evaluated the effect of abluminal pH changes in vitro on the diameter of isolated perfused penetrating arterioles harvested from wild-type and A(2A) receptor knockout mice.

METHODS

The authors evaluated in a blinded fashion the CBF response during transient (60-second) hypercapnic (7% CO(2)) hypercarbia in anesthetized, ventilated C57Bl/6 wild-type and adenosine A(2A) receptor knockout mice. They also evaluated the hypercarbic response in the absence and presence of the nonselective and selective adenosine antagonists.

RESULTS

Cerebral blood flow was measured using laser Doppler flowmetry. There were no differences between the CBF responses to hypercarbia in the wild-type and the knockout mice. Moreover, the hypercarbic hyperemia response was not affected by the adenosine receptor antagonists. The authors also tested the response to alteration in abluminal pH in isolated perfused, pressurized, penetrating arterioles (average diameter 63.3 +/- 3.6 microm) harvested from wild-type (6 mice) and knockout (5 mice) animals. Arteriolar dilation in response to a decrease in abluminal pH, simulating the change in vivo during hypercarbia, was similar in wild-type (15.9 +/- 2.6%) and A(2A) receptor knockout (17.7 +/- 1.3%) mice. With abluminal application of CGS 21680 (10(-6) M), an A(2A) receptor agonist, wild-type arterioles dilated in an expected manner (9.8 +/- 0.7%), whereas A(2A) receptor knockout vessels had minimal response.

CONCLUSIONS

The results of the in vivo and in vitro studies in wild-type and A(2A) receptor knockout mice support the authors' hypothesis that hypercarbic vasodilation does not involve an adenosine A(2A) receptor-related mechanism.

In vivo cerebrovascular effects of cocaine- and amphetamine-regulated transcript (CART) peptide

Cocaine- and amphetamine-regulated transcript (CART) and its associated peptides have been implicated in a number of physiologic processes including modulation of the hypothalamo-pituitary-adrenal (HPA) axis and cardiovascular regulation. Recently, we reported that in isolated cerebral arterioles, CART peptide (CARTp) acts directly to produce endothelium-dependent constriction via the endothelin signaling pathway. We used the rat closed cranial window model to determine the in vivo effects of CARTp on pial arteriolar diameter. Intravenous administration of 30 microg/kg CARTp produced a significant pressor effect and constriction of pial arterioles. The pressor response to systemic CARTp was blocked by the beta-adrenergic receptor antagonist propranolol (2 mg/kg IV). Direct application of 0.1 nM-1 microM CARTp to pial arterioles produced a dose-dependent and long-lasting constriction to approximately 88% of baseline diameter. The constriction response to topically applied 100 nM CARTp was blocked by both the endothelin A (ETA) receptor antagonist BQ-123 (10 microM) and the inhibitor of endothelin-converting enzyme, phosphoramidon (100 nM). These results demonstrate for the first time that CARTp constricts cerebral vessels in vivo, an action mediated by its effects on the endothelin system, specifically via activation of ETA receptors. This supports the notion that CARTp plays a physiologic role in cerebrovascular regulation, particularly during times of HPA axis activation.

Correlation of intrinsic optical signal, cerebral blood flow, and evoked potentials during activation of rat somatosensory cortex

OBJECT

This study was undertaken to test the hypothesis that cerebral blood flow (CBF) and the intrinsic optical signal could be dissociated by altering adenosine receptor activity and to uncover the origin of the optic signal using a cranial window in the anesthetized rat.

METHODS

In anesthetized, ventilated, and temperature-controlled rats with closed cranial windows, the authors evaluated simultaneously the alterations in pial arteriolar diameter, intrinsic optical signals (690 nm), and somatosensory evoked potentials during cortical activation evoked by contralateral sciatic nerve stimulation (SNS). To dissociate the vascular and intrinsic signal, they topically applied the adenosine receptors antagonists theophylline (5 microM), which affects A1 and A2A receptors, and 8-cyclopentyl-1,3-dipropylxanthine (CPX, 1 microM), which blocks the A(1) receptor. The former interacts primarily with the vasculature whereas the latter influences the parenchyma exclusively.

RESULTS

During 20 seconds of contralateral SNS, pial arterioles in the hindlimb somatosensory cortex dilated in a characteristic peak and shoulder pattern. As compared with mock cerebrospinal fluid alone, theophylline significantly (p<0.05) attenuated SNS-induced vasodilation (mean+/-standard deviation 8.1+/-2.5% vs 21.7+/-1.9%; 4 rats in each group). In contrast, CPX potentiated vasodilation significantly (p<0.05) during SNS (54.7+/-15.8% for the CPX group vs 20.1+/-1.9% for the controls; 5 rats in each group). The change in optical signal persisted after cessation of SNS in all the animals. Thus, the pattern of change of the optical signal was distinctly different from the pattern of changes in arteriolar diameter (which returned rapidly to baseline). Moreover, the optical signal during SNS was increased by 50% by theophylline and by almost 5-fold by CPX (p<0.05). The area of change of the intrinsic signal was also increased by the topical application of theophylline and CPX. The somatosensory evoked potential recordings revealed no significant changes after theophylline application, but CPX caused a small diminution of the N1 wave (p<0.01).

CONCLUSIONS

The noncongruent temporal profiles of the changes in pial arteriolar diameter and optical signal, imaged at 690 nm, indicate that the optical signal at 690 nm is not related to CBF. Alteration of adenosine receptor activity independently changed cortical activity, as measured by the optical signal, and CBF, as determined by pial arteriolar diameter. Manipulation of the adenosine receptor activity during increased cortical activity confirmed the temporal dissociation of optical signal and CBF and provided further evidence for the role of adenosine in regulating CBF.

Regulation of cerebral vasculature in normal and ischemic brain

We outline the mechanisms currently thought to be responsible for controlling cerebral blood flow (CBF) in the physiologic state and during ischemia, focusing on the arterial pial and penetrating microcirculation. Initially, we categorize the cerebral circulation and then review the vascular anatomy. We draw attention to a number of unique features of the cerebral vasculature, which are relevant to the microcirculatory response during ischemia: arterial histology, species differences, collateral flow, the venous drainage, the blood-brain barrier, astrocytes and vascular nerves. The physiology of the arterial microcirculation is then assessed. Lastly, we review the changes during ischemia which impact on the microcirculation. Further understanding of the normal cerebrovascular anatomy and physiology as well as the pathophysiology of ischemia will allow the rational development of a pharmacologic therapy for human stroke and brain injury.

Modelling vascular reactivity to investigate the basis of the relationship between cerebral blood volume and flow under CO2 manipulation

Changes in cerebral blood flow (f) and vascular volume (v) are of major interest in mapping cerebral activity and metabolism, but the relation between them currently lacks a sufficient theoretical basis. To address this we considered three models: a uniform reactive tube model (M1); an extension of M1 that includes passive arterial inflow and venous volume (M2); and a more anatomically plausible model (M3) consisting of 19 compartments representing the whole range of vascular sizes and respective CO2 reactivities, derived from literature data. We find that M2 cannot be described as the simple scaling of a tube law, but any divergence from a linear approximation is negligible within the narrow physiological range encountered experimentally. In order to represent correctly the empirically observed slope of the overall v-f relationship, the reactive bed should constitute about half of the total vascular volume, thus including a significant fraction of capillaries and/or veins. Model M3 demonstrates systematic variation of the slope of the v-f relationship between 0.16 and 1.0, depending on the vascular compartment under consideration. This is further complicated when other experimental approaches such as flow velocity are used as substitute measurements. The effect is particularly large in microvascular compartments, but when averaged with larger vessels the variations in slope are contained within 0.25 to 0.55 under conditions typical for imaging methods. We conclude that the v-f relationship is not a fixed function but that both the shape and slope depend on the composition of the reactive volume and the experimental methods used.

Time-dependent alterations in functional and pharmacological arteriolar reactivity after subarachnoid hemorrhage

BACKGROUND AND PURPOSE

Disturbances in cerebral arteriolar function, in addition to large vessel vasospasm, may be responsible for ischemia after subarachnoid hemorrhage. The purpose of this study was to test the hypothesis that subarachnoid hemorrhage alters cerebral microvascular reactivity.

METHODS

An endovascular filament model was used to induce subarachnoid hemorrhage in halothane-anesthetized male Sprague-Dawley rats. We evaluated pial arteriolar responses to sciatic nerve stimulation, topically applied vasoactive agents (adenosine or sodium nitroprusside), and CO(2) inhalation in rats subjected to subarachnoid hemorrhage at 1 to 5 days after insult.

RESULTS

In sham-operated rats, sciatic nerve stimulation evoked a 23.5+/-1.8% increase in arteriolar diameter, which was significantly attenuated to 13.7+/-0.9%, 12.8+/-2.5%, and 18.8+/-2.9% at 24, 48, and 72 hours after subarachnoid hemorrhage, respectively (P<0.05; n> or =7). At 96 and 120 hours after subarachnoid hemorrhage, sciatic nerve stimulation-induced dilation recovered to sham levels. Somatosensory-evoked potentials were unaltered by subarachnoid hemorrhage. Pial vasodilatation to adenosine (10 micromol/L) and sodium nitroprusside (1 micromol/L) were significantly impaired, by 47% and 41%, respectively, at 48 hours after subarachnoid hemorrhage (P<0.05; n=7). In contrast, CO(2) reactivity was unaffected by subarachnoid hemorrhage.

CONCLUSIONS

Pial arteriolar responses to cortical activation may be decreased in the initial 2 to 3 days after experimental subarachnoid hemorrhage.

Role of astrocytes in matching blood flow to neuronal activity

The brain is critically dependent on oxygen and glucose supply for normal function. Various neurovascular control mechanisms assure that the blood supply of the brain is adequate to meet the energy needs of its components. Emerging evidence shows that neuronal activity can control microcirculation using astrocytes as a mediator. Astrocytes can sense neuronal activity and are involved in signal transmission. Synaptic activity triggers an increase in the intracellular calcium concentration [Ca(2+)]i of adjacent astrocytes, stimulating the release of adenosine triphosphate (ATP) and glutamate. The released ATP mediates the propagation of Ca(2+) waves between neighboring astrocytes, thereby recruiting them to mediate adequate cerebrovascular response to neuronal activation. Simultaneously, sodium-dependent glutamate uptake in astrocytes generates Na(+) waves and subsequently increases glucose uptake and metabolism that leads to the formation of lactate, which is then delivered to neurons as an energy substrate. Further, astrocytic Ca(2+) elevations can lead to secretion of vasodilatory substances from perivascular endfeet, such as epoxyeicosatrienoic acid (EETs), adenosine, nitric oxide (NO), and cyclooxygenase-2 (COX-2) metabolites, resulting in increased local blood flow. Thus, astrocytes by releasing vasoactive molecules mediate the neuron-astrocyte-endothelial signaling pathway and play a profound role in coupling blood flow to neuronal activity.

Spatial flow-volume dissociation of the cerebral microcirculatory response to mild hypercapnia

The spatial and temporal response of the cerebral microcirculation to mild hypercapnia was investigated via two-photon laser-scanning microscopy. Cortical vessels, traversing the top 200 microm of somatosensory cortex, were visualized in alpha-chloralose-anesthetized Sprague-Dawley rats equipped with a cranial window. Intraluminal vessel diameters, transit times of fluorescent dextrans and red blood cells (RBC) velocities in individual capillaries were measured under normocapnic (PaCO2= 32.6 +/- 2.6 mm Hg) and slightly hypercapnic (PaCO2= 45 +/- 7 mm Hg) conditions. This gentle increase in PaCO2 was sufficient to produce robust and significant increases in both arterial and venous vessel diameters, concomitant to decreases in transit times of a bolus of dye from artery to venule (14%, P < 0.05) and from artery to vein (27%, P < 0.05). On the whole, capillaries exhibited a significant increase in diameter (16 +/- 33%, P < 0.001, n = 393) and a substantial increase in RBC velocities (75 +/- 114%, P < 0.001, n = 46) with hypercapnia. However, the response of the cerebral microvasculature to modest increases in PaCO2 was spatially heterogeneous. The maximal relative dilatation (range: 5-77%; mean +/- SD: 25 +/- 34%, P < 0.001, n = 271) occurred in the smallest capillaries (1.6 microm-4.0 microm resting diameter), while medium and larger capillaries (4.4 microm-6.8 microm resting diameter) showed no significant changes in diameter (P > 0.08, n = 122). In contrast, on average, RBC velocities increased less in the smaller capillaries (39 +/- 5%, P < 0.002, n = 22) than in the medium and larger capillaries (107 +/- 142%, P < 0.003, n = 24). Thus, the changes in capillary RBC velocities were spatially distinct from the observed volumetric changes and occurred to homogenize cerebral blood flow along capillaries of all diameters.

Tissue oxygenation response to mild hypercapnia during cardiopulmonary bypass with constant pump output

BACKGROUND

Tissue oxygenation is the primary determinant of wound infection risk. Mild hypercapnia markedly improves cutaneous, subcutaneous (s.c.), and muscular tissue oxygenation in volunteers and patients. However, relative contributions of increased cardiac output and peripheral vasodilation to this response remains unknown. We thus tested the hypothesis that increased cardiac output is the dominant mechanism.

METHODS

We recruited 10 ASA III patients, aged 40-65 yr, undergoing cardiopulmonary bypass for this crossover trial. After induction of anaesthesia, a Silastic tonometer was inserted s.c. in the upper arm. S.C. tissue oxygen tension was measured with both polarographic electrode and fluorescence-based systems. Oximeter probes were placed bilaterally on the forehead to monitor cerebral oxygenation. After initiation of cardiopulmonary bypass, in random order patients were exposed to two arterial CO(2) partial pressures for 30 min each: 35 (normocapnia) or 50 mm Hg (hypercapnia). Bypass pump flow was kept constant throughout the measurement periods.

RESULTS

Hypercapnia during bypass had essentially no effect on Pa(CO(2)) , mean arterial pressure, or tissue temperature. Pa(CO(2)) and pH differed significantly. S.C. tissue oxygenation was virtually identical during the two Pa(CO(2)) periods [139 (50-163) vs 145 (38-158), P=0.335] [median (range)]. In contrast, cerebral oxygen saturation (our positive control measurement) was significantly less during normocapnia [57 (28-67)%] than hypercapnia [64 (37-89)%, P=0.025].

CONCLUSIONS

Mild hypercapnia, which normally markedly increases tissue oxygenation, did not do so during cardiopulmonary bypass with fixed pump output. This suggests that hypercapnia normally increases tissue oxygenation by increasing cardiac output rather than direct dilation of peripheral vessels.

Effect of caffeine on cerebral blood flow response to somatosensory stimulation

Despite caffeine's wide consumption and well-documented psychoactive effects, little is known regarding the effects of caffeine on neurovascular coupling. In the present study, we evaluated the effects of caffeine, an adenosine receptor antagonist, on intracerebral arterioles in vitro and subsequently, on the pial circulation in vivo during cortical activation induced by contralateral sciatic nerve stimulation (SNS). In our in vitro studies, we utilized isolated intracerebral arterioles to determine the effects of caffeine (10 or 50 micromol/L) on adenosine-induced vasodilatation. At the lower concentration, caffeine was without effect, but at the higher concentration, caffeine produced significant attenuation. In our in vivo studies, we determined the cerebrospinal fluid (CSF) caffeine concentrations at 15, 30, and 60 mins after intravenous administration of 5, 10 and 40 mg/kg. At the latter two concentrations, CSF levels exceeded 10 micromol/L. We then evaluated the pial arteriolar response during cortical activation caused by contralateral SNS after administering caffeine intravenously (0, 5, 10, 20 30, and 40 mg/kg). The pial circulation was observed through a closed cranial window in chloralose-anesthetized Sprague-Dawley rats. The contralateral sciatic nerve was isolated, positioned on silver electrodes and stimulated for 20 secs (0.20 V, 0.5 ms, and 5 Hz). Arteriolar diameter was quantified using an automated video dimension analyzer. Contralateral SNS resulted in a 23.8% +/-3.9% increase in pial arteriolar diameter in the hindlimb sensory cortex under control conditions. Intravenous administration of caffeine at the lowest dose studied (5 mg/kg) had no effect on either resting arteriolar diameter or SNS-induced vasodilatation. However, at higher doses (10, 20, 30, and 40 mg/kg, intravenously), caffeine significantly (P < 0.05; n = 6) attenuated both resting diameter and cerebral blood flow (CBF) responses to somatosensory stimulation. Intravenous administration of theophylline (10, 20, and 40 mg/kg), another adenosine receptor antagonist, also significantly reduced SNS-induced vasodilatation in a dose-dependent manner. Hypercarbic vasodilatation was unaffected by either caffeine or theophylline. The results of the present study show that caffeine significantly reduces cerebrovascular responses to both adenosine and to somatosensory stimulation and supports a role of adenosine in the regulation of CBF during functional neuronal activity.

Adenosine and the regulation of cerebral blood flow

OBJECTIVES

This review summarizes the 30 year effort of my collaborator and mentor Dr J. W. Phillis to establish the role of adenosine in the regulation of cerebral blood flow.

METHODS

While most of the experiments described utilized the rat cerebral cortex as a model, several different and complementary methodologies were employed. Superfusate samples were collected from the cortical surface and analysed for purines using HPLC. Laser-Doppler flowmetry was utilized to measure blood flow in the pial vasculature, while pial diameters were monitored by videomicroscopy. An additional series of experiments looked at coronary blood flow in a Langendorff preparation.

RESULTS

Adenosine is released from the cortex in response to decreased nutrient supply (hypoxia/ ischemia) and during conditions that mimic alterations in the extracellular environment associated with increased metabolism. The application of pharmacological agents that alter adenosine metabolism resulted in the appropriate alterations in ECF adenosine levels and also in blood flow. Selective blockade of the adenosine A(2A) receptor reduced the pial vasodilation evoked by hypercapnoea. Results from the isolated rat heart, utilizing similar agents, support a role for adenosine in the regulation of coronary blood flow during respiratory and metabolic acidosis.

DISCUSSION

Adenosine is released when there is a mismatch between supply and demand. If the effects of adenosine are blocked with receptor antagonists, the vasodilation is also reduced. However, the effects of adenosine on the hyperemia evoked by hypercapnoea are complicated by the arousal evoked by adenosine receptor antagonists and the effects of upstream regulation.

Direct Observation of the Human Cerebral Microcirculation During Aneurysm Surgery Reveals Increased Arteriolar Contractility

Background and Purpose— The effects of aneurysmal subarachnoid hemorrhage on morphology and function of the cerebral microcirculation are poorly defined, partly due to the lack of suitable techniques to visualize the microvessels in vivo. We used orthogonal polarization spectral (OPS) imaging on the brain cortex during aneurysm surgery to directly observe the small cortical blood vessels and quantify their responses to hypocapnia.

Methods— In 16 patients undergoing aneurysm surgery, the diameter changes of small cortical vessels (15 to 180 μm) were observed using OPS imaging. Ten patients were operated on early (within 48 hours after bleeding) and 6 underwent late surgery. Immediately after dura opening, the response to hyperventilation of arterioles and venules was observed with OPS imaging under sevoflurane anesthesia.

Results— In patients operated on early, layers of subarachnoid blood were clearly visible. In this group, hyperventilation resulted in a 39±15% decrease in arteriolar diameter with a “bead-string” constriction pattern occurring in 60% of patients. In late surgery and in controls, no subarachnoid blood was seen. The arteriolar diameter decrease with hyperventilation was 17±20% in patients undergoing late surgery and 7±7% in controls. Venules were not affected by hyperventilation in any of the groups studied.

Conclusions— OPS imaging allows direct in vivo observation of the cerebral microcirculation enabling us, for the first time, to visually observe and quantify microvascular reactivity in the human brain. The present study demonstrates increased contractile responses of the cerebral arterioles in the presence of subarachnoid blood, suggesting increased microvascular tonus with possibly greater susceptibility to ischemia.

Capillary flow and diameter changes during reperfusion after global cerebral ischemia studied by intravital video microscopy

The reaction of cerebral capillaries to ischemia is unclear. Based on Hossmann's observation of postischemic "delayed hypoperfusion," we hypothesized that capillary flow is decreased during reperfusion because of increased precapillary flow resistance. To test this hypothesis, we measured cerebral capillary erythrocyte velocity and diameter changes by intravital microscopy in gerbils. A cranial window was prepared over the frontoparietal cortex in 26 gerbils anesthetized with halothane. The animals underwent either a sham operation or fifteen minutes of bilateral carotid artery occlusion causing global cerebral ischemia. Capillary flow velocities were measured by frame-to-frame tracking of fluorescein isothiocyanate labeled erythrocytes in 1800 capillaries after 1-hour reperfusion. Capillary flow velocities were decreased compared to control (0.25 +/- 0.27 mm/s vs. 0.76 +/- 0.45 mm/s; P<0.001). Precapillary arteriole diameters in reperfused animals were reduced to 76.3 +/- 6.9% compared to baseline (P<0.05). Capillary diameters in reperfused animals (2.87 +/- 0.97 microm) were reduced (P<0.001) compared to control (4.08 +/- 1.19 microm). Similar reductions of precapillary (24%) and capillary vessel diameters (30%) and absolute capillary flow heterogeneity indicate that delayed (capillary) hypoperfusion occurs as a consequence of increased precapillary arteriole tone during reperfusion.

Nitric oxide synthase inhibition depresses the height of the cerebral blood flow-pressure autoregulation curve during moderate hypotension

Variations in the height of the CBF response to hypotension have been described recently in normal animals. The authors evaluated the effects of nitric oxide synthase (NOS) inhibition on these variations in height using laser Doppler flowmetry in 42 anesthetized (halothane and N2O) male Sprague-Dawley rats prepared with a superfused closed cranial window. In four groups (time control, enantiomer control, NOS inhibition, and reinfusion control) exsanguination to MABPs from 100 to 40 mm Hg was used to produce autoregulatory curves. For each curve the lower limit of autoregulation (the MABP at the first decrease in CBF) was identified; the pattern of autoregulation was classified as "peak" (15% increase in %CBF), "classic" (plateau with a decrease at the lower limit of autoregulation), or "none" (15% decrease in %CBF); and the autoregulatory height as the %CBF at 70 mm Hg (%CBF(70)) was determined. NOS inhibition decreased %CBF(70) in the NOS inhibition group (P = 0.014), in the control (combined time and enantiomer control) group (P = 0.015), and in the reinfusion control group (P = 0.025). NOS inhibition via superfusion depressed the autoregulatory pattern (P = 0.02, McNemar test on changes in autoregulatory pattern) compared with control (P = 0.375). Analysis of covariance showed that changes induced by NOS inhibition in the parameters of autoregulatory height are not related to changes in the lower limit, but are strongly (P < 0.001) related to each other. NOS inhibition depressed the autoregulatory pattern, decreasing the seemingly paradoxical increase in CBF as blood pressure decreases. These results suggest that nitric oxide increases CBF near the lower limit and augments the hypotensive portion of the autoregulatory curve.

Adenosine receptors mediate glutamate-evoked arteriolar dilation in the rat cerebral cortex

We tested the hypothesis that adenosine (Ado) mediates glutamate-induced vasodilation in the cerebral cortex by monitoring pial arteriole diameter in chloralose-anesthetized rats equipped with closed cranial windows. Topical application of 100 microM glutamate and 100 microM N-methyl-d-aspartate (NMDA) dilated pial arterioles (baseline diameter 25 +/- 2 microm) by 17 +/- 1% and 18 +/- 4%, respectively. Coapplication of the nonselective Ado receptor antagonist theophylline (Theo; 10 microM) significantly reduced glutamate- and NMDA-induced vasodilation to 4 +/- 2% (P < 0.01) and 6 +/- 2% (P < 0.05), whereas the Ado A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (0.1 microM) had no effect. Moreover, application of the Ado A(2A) receptor-selective antagonist 4-(2-[7-amino-2-(2-furyl)(1,2,4)triazolo(2,3-a)(1,3,5)triazin-5-ylamino]ethyl)phenol (ZM-241385), either by superfusion (0.1 microM, 1 microM) or intravenously (1 mg/kg), significantly inhibited the pial arteriole dilation response to glutamate. Neither Theo nor ZM-241385 affected vascular reactivity to mild hypercapnia induced by 5% CO(2) inhalation. These results suggest that Ado contributes to the dilation of rat cerebral arterioles induced by exogenous glutamate, and that the Ado A(2A) receptor subtype may be involved in this dilation response.

cGMP-dependent and not cAMP-dependent kinase is required for adenosine-induced dilation of intracerebral arterioles

Adenosine (ADO) is a potent cerebral vasodilator and has been proposed as a metabolic regulator of cerebral blood flow. However, the signal transduction pathway by which ADO causes vasodilation in cerebral microvessels is currently unknown. The current study was designed to investigate the role of cyclic nucleotides and cyclic nucleotide-dependent protein kinases in ADO-induced dilation of resistance-sized rat cerebral arterioles that develop spontaneous tone. Arterioles were cannulated and perfused intraluminally at constant flow (2 microl/min) and pressure (60 mm Hg). ADO (29.7 +/- 2.0%; 1 microM), CGS-21680 (16 +/- 4%, 1 microM), 8-bromo-cyclic guanosine monophosphate (8 Br-cGMP; 29.9 +/- 3.9%; 100 microM), sodium nitroprusside (SNP; 30.6 +/- 3.3%, 1 microM), cyclic guanine monophosphate-dependent protein kinase activator (Sp-8-pCPT-cGMPS, 25.9 +/- 4.2%; 10 microM), forskolin (30.5 +/- 5.9%; 0.1 microM), and pH 6.8 all produced large dilations. The selective cGMP-dependent protein kinase inhibitor, Rp-8-pCPT-cGMPS (10 microM), had no effect on resting diameter or reactivity to acidic pH, but significantly ( < 0.05) attenuated arteriolar dilations to ADO (59%, n = 8), CGS-21680 (60%, n = 4), SNP (62%, n = 3), 8 Br-cGMP (88%, n = 3), and Sp-8-pCPT-cGMPS (98%, n = 3). H8, the less-selective cyclic nucleotide-dependent protein kinase inhibitor, had similar effects as Rp-8-pCPT-cGMPS. Additionally, the inhibitor of the soluble guanylate cyclase, 1H-[1,24]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ), blocked the response to SNP (70% inhibition) and significantly inhibited the ADO response (43% inhibition). In contrast, inhibition of the cyclic ADO monophosphate (cAMP)-dependent protein kinase Rp-8-CPT-cAMPS had no effect on the ADO, SNP, or pH responses, but significantly blocked forskolin-induced vasodilation (53%). It is concluded that ADO-induced vasodilation in cerebral microvessels, at least in part, involves cGMP and cGMP-dependent protein kinase, but not cAMP or cAMP-dependent kinase. Our data therefore provides a new insight into mechanisms by which ADO invokes vasodilation in cerebral microvascular arterioles.

Effects of Ginkgo biloba extract (EGb 761) on cerebral thrombosis and blood pressure in stroke-prone spontaneously hypertensive rats

1. An extract of Ginkgo biloba (EGb 761) has been reported to alleviate cerebrovascular problems. In the present study, we investigated the antithrombotic effects of EGb 761 in cerebral blood vessels of stroke-prone spontaneously hypertensive rats (SHRSP/Izm). 2. In the present study, EGb 761 was administered orally to SHRSP/Izm at 60 and 120 mg/kg each day for 3 weeks from the age of 7 weeks. The age-related increase in blood pressure observed in SHRSP was suppressed significantly by EGb 761 at both doses 3 weeks after treatment. 3. Thrombotic potential was assessed in vivo using a He-Ne laser-induced thrombosis model and was significantly suppressed by EGb 761. 4. The anti-oxidant effects of EGb 761 were determined by measurement of urinary 8-hydroxy-2'-deoxyguanosine (8-OHdG). At 120 mg/kg, EGb 761 decreased 8-OHdG significantly compared with control animals. 5. Urinary nitrite/nitrate, nitric oxide (NO) metabolites, were increased significantly after administration of EGb 761. Expression of endothelial NO synthase (eNOS) mRNA was measured using a real-time quantitative reverse transcription-polymerase chain reaction method. The expression of eNOS mRNA in the EGb 761 group (120 mg/kg) was significantly higher than in the control group. 6. The results indicate that EGb 761 decreases blood pressure and mediates strong antithrombotic and anti-oxidant effects in SHRSP. These pharmacological activities may contribute to the beneficial properties of EGb 761 observed in clinical practice.

Endothelial nitric oxide synthase pathophysiology after nonocclusive common carotid artery thrombosis in rats

Although vascular dysregulation has been documented in patients with extracranial vascular disease, transient ischemic attacks, and stroke, the pathomechanisms are poorly understood. To model thromboembolic stroke in rats, photochemically induced nonocclusive common carotid artery thrombosis (CCAT) was used to generate a platelet thrombus in the carotid artery of anesthetized rats. After CCAT, platelet aggregates break off the thrombus, travel to the distal cerebral vasculature, damage blood vessels, and cause small infarctions. The authors hypothesized that deficits in the endothelial nitric oxide synthase (eNOS) pathway may be responsible for vascular dysfunction after embolic stroke. To examine the functional status of the eNOS system, they measured eNOS-dependent dilation after CCAT by applying acetylcholine through a cranial window over the middle cerebral artery. The authors also measured eNOS mRNA and protein in the middle cerebral artery to determine whether functional changes were caused by alterations in expression. eNOS-dependent dilation was reduced at 6 hours, elevated at 24 hours, and returned to baseline 72 hours after CCAT. Endothelial nitric oxide synthase mRNA increased at 2 hours and was followed by a rise in protein 24 hours after CCAT. Changes in the eNOS system may account for some of the observed vascular deficits in patients with cerebrovascular disease.

Pial arteriole dilation during somatosensory stimulation is not mediated by an increase in CSF metabolites

Pial arterioles supplying the hindlimb somatosensory cortex dilate in response to contralateral sciatic nerve stimulation. The mechanism of this pial vasodilation is not well understood. One possibility is that vasoactive metabolites released during brain activation may diffuse to subarachnoid cerebrospinal fluid (CSF) to dilate pial vessels. To test this hypothesis, we implanted closed cranial windows in rats and measured pial arteriolar dilation to sciatic nerve stimulation during constant rate superfusion of the pial surface with artificial CSF. We reason that flushing the pial surface with CSF should quickly dissipate vasoactive substances and prevent these substances from dilating pial arterioles. CSF flow (1 and 1.5 ml/min) significantly reduced pial arteriole dilation induced by 5% CO2 inhalation, but the same flow rates did not affect dilator responses to sciatic nerve stimulation. We conclude that brain-to-CSF diffusion of vasoactive metabolites does not play a significant role in the dilation of pial arterioles during somatosensory activity.