Neuronal control of brain microvessel function

Blood-brain barrier on a chip

The blood-brain barrier (BBB) plays a vital role in the maintenance of brain homeostasis. It strictly restricts the passage of molecules from the brain vasculature into the brain via its high transendothelial electrical resistance and low paracellular and transcellular permeability. Specialized brain endothelial cells, astrocytes, pericytes, neurons, and microglia contribute synergistically to the functional properties of the BBB. Because of its complexity and relative inaccessibility, BBB research is fraught with difficulties. Most studies rely on animal or cell culture models, which are not able to fully recapitulate the properties of the human BBB. The recent development of three-dimensional (3D) microfluidic models of the BBB could address this issue. This chapter aims to provide an overview of the recent advances in modeling the BBB on microdevices, and illustrate important considerations for the design of such models. In addition, protocols for the fabrication of a 3D BBB microfluidic chip and BBB assessment experiments, including immunocytochemistry for analyzing cell morphology and protein marker expression, permeability assay, and calcium imaging for studying neuronal function as a measure of BBB integrity, are presented here. It is envisioned that continued advancements in microtechnology can lead to the creation of realistic in vivo-like BBB-on-chip models.

The blood-brain barrier-gatekeeper to neuronal homeostasis: clinical implications in the setting of stroke

The blood-brain barrier is part of the neurovascular unit and serves as a functional and anatomical barrier between the blood and the extracellular space. It controls the flow of solutes in and out of the brain thereby providing an optimal environment for neuronal functioning. Paracellular transport between endothelial cells is restricted by tight junctions and transendothelial transport is reduced and more selective compared to capillaries of other organs. Further, the blood-brain barrier is involved in controlling blood flow and it is the site for signaling damage of the nervous system to the peripheral immune system. As an important player in brain homeostasis, blood-brain barrier dysfunction has been implicated in the pathophysiology of many brain diseases including stroke, traumatic brain injury, brain tumors, epilepsy and neurodegenerative disorders. In this article - highlighting recent advances in basic science - we review the features of the blood-brain barrier and their significance for neuronal homeostasis to discuss clinical implications for neurological complications following cerebral ischemia.

Evidence for a predominant intrinsic sympathetic control of cerebral blood flow alterations in an animal model of cerebral arteriovenous malformation

In terms of neurogenic cerebral blood flow (CBF) control, the activity of the sympathetic nervous system (SNS) has a regulating effect. The impact of a manipulation of both the peripheral (via the perivascular sympathetic net) and central components (via the intracortical noradrenergic terminals originating from the locus coeruleus) on CBF-and especially on hyperperfusion syndromes-is unclear. To test the specific patterns following such alterations, cortical oxygen saturation (rSO2), regional CBF (rCBF), and cortical interstitial norepinephrine (NE) concentrations were measured. Twelve weeks after either the creation of an extracranial AV fistula or sham operation, 80 male Sprague-Dawley rats underwent one of the following procedures: (1) no SNS manipulation, (2) peripheral SNS inhibition via bilateral sympathectomy, (3) central SNS inhibition via the neurotoxin DSP-4, or (4) complete SNS inhibition. Norepinephrine concentrations were lowest after complete inhibition (NE [nmol]: pre, 1.8 ± 1.2; post, 2.4 ± 1.8) and highest following peripheral inhibition (NE [nmol]: pre, 3.6 ± 1.9; post, 6.6 ± 4.4). Following fistula occlusion, rCBF (laser Doppler unit [LDU]) and rSO2 (%SO2) increases were highest after complete inhibition (pre: 204 ± 14 LDU, 34 ± 3%SO2; post: 228 ± 18 LDU, 39 ± 3%SO2) and lowest after peripheral inhibition (pre: 221 ± 18 LDU, 41 ± 2%SO2; post: 226 ± 14 LDU, 47 ± 2%SO2). Thus, a complete inhibition down-regulates SNS activity and provokes a cortical hyperperfusion condition. With this, the hitherto unknown predominant role of the intrinsic component could be demonstrated for the first time in vivo.

Reaction of rat brain capillaries to immobilization stress

The calcium adenosine triphosphate method of Chilingaryan was used to study the morphofunctional state of the capillary component of the microcirculatory bed of the brain in rats at different time points after experimental immobilization stress (fixation of the animal on its back for 2 h). Analysis of morphometric data showed that in comparison with intact animals, stress was immediately followed by constriction of capillaries by 17.2%, with compensatory dilation by 2.5% occurring at two days, and subsequent minor constriction by 5.6%. Morphometric measures in placid and aggressive animals showed that the behavioral stereotype of placid animals produced a more sparing physiological response to stress. It is suggested that the difference in capillary dysfunction is largely dependent on impairment of neuronal systems involved in regulating the microcirculation.

The blood-brain barrier/neurovascular unit in health and disease

The blood-brain barrier (BBB) is the regulated interface between the peripheral circulation and the central nervous system (CNS). Although originally observed by Paul Ehrlich in 1885, the nature of the BBB was debated well into the 20th century. The anatomical substrate of the BBB is the cerebral microvascular endothelium, which, together with astrocytes, pericytes, neurons, and the extracellular matrix, constitute a "neurovascular unit" that is essential for the health and function of the CNS. Tight junctions (TJ) between endothelial cells of the BBB restrict paracellular diffusion of water-soluble substances from blood to brain. The TJ is an intricate complex of transmembrane (junctional adhesion molecule-1, occludin, and claudins) and cytoplasmic (zonula occludens-1 and -2, cingulin, AF-6, and 7H6) proteins linked to the actin cytoskeleton. The expression and subcellular localization of TJ proteins are modulated by several intrinsic signaling pathways, including those involving calcium, phosphorylation, and G-proteins. Disruption of BBB TJ by disease or drugs can lead to impaired BBB function and thus compromise the CNS. Therefore, understanding how BBB TJ might be affected by various factors holds significant promise for the prevention and treatment of neurological diseases.

New aspects of the blood-brain barrier transporters; its physiological roles in the central nervous system

The blood-brain barrier (BBB) segregates the circulating blood from interstitial fluid in the brain, and restricts drug permeability into the brain. Our latest studies have revealed that the BBB transporters play important physiological roles in maintaining the brain milieu. The BBB supplies creatine to the brain for an energy-storing system, and creatine transporter localized at the brain capillary endothelial cells (BCECs) is involved in BBB creatine transport. The BBB is involved in the brain-to-blood efflux transport of the suppressive neurotransmitter, gamma-aminobutyric acid, and GAT2/BGT-1 mediates this transport process. BCECs also express serotonin and norepinephrine transporters. Organic anion transporter 3 (OAT3) and ASCT2 are localized at the abluminal membrane of the BCECs. OAT3 is involved in the brain-to-blood efflux of a dopamine metabolite, a uremic toxin and thiopurine nucleobase analogs. ASCT2 plays a role in L-isomer-selective aspartic acid efflux transport at the BBB. Dehydroepiandrosterone sulfate and small neutral amino acids undergo brain-to-blood efflux transport mediated by organic anion transporting polypeptide 2 and ATA2, respectively. The BBB transporters are regulated by various factors, ATA2 by osmolarity, taurine transporter by TNF-alpha, and L-cystine/L-glutamic acid exchange transporter by oxidative stress. Clarifying the physiological roles of BBB transport systems should give us important information allowing the development of better CNS drugs and improving our understanding of the relationship between CNS disorders and BBB function.

Molecular Mechanisms and Drug Development in Aquaporin Water Channel Diseases: Aquaporins in the Brain

Water homeostasis of the brain is essential for its neuronal activity. Changes in water content in the intra- and extra-cellular space affect ionic concentrations and therefore modify neuronal activity. Aquaporin (AQP) water channels may have a central role in keeping water homeostasis in the brain. Among AQP subtypes cloned in mammalian, only AQP1, AQP4, and AQP9 were identified in the brain. Changes in AQP expression may be correlated with edema formation of the brain. In this review, we describe the physiological function of AQPs and the regulatory mechanism of their expression in the brain.

Species, strain and developmental variations in hippocampal neuronal and endothelial nitric oxide synthase clarify discrepancies in nitric oxide-dependent synaptic plasticity

Nitric oxide (NO) has been implicated in long-term potentiation (LTP) in pyramidal neurons in cellular area 1 (CA1) of the hippocampus. However, considerable confusion exists about the exact role of NO, and the contribution of the endothelial nitric oxide synthase (eNOS) and neuronal nitric oxide synthase (nNOS) isoforms of NO synthase to NO-dependent LTP (NO-LTP), with results often varying, depending on the organism and experimental paradigm used. Using immunohistochemistry and in situ hybridization, we contrast NO synthase expression and activity in rat, mouse, and human hippocampus. nNOS is prominently expressed in all CA1 pyramidal cells of C57B6 mice and humans, while in rats and SV129 mice, its levels are much lower and restricted to the caudal hippocampus. By contrast, eNOS is restricted to endothelial cells. We observe N-methyl-D-aspartate-dependent citrulline production in pyramidal cells of mouse hippocampus, which is absent in nNOS(Delta/Delta) animals. Finally, we observe robust nNOS expression in human CA1 pyramidal cells.The considerable axial, developmental, strain and species-dependent variations in nNOS expression in CA1 pyramidal neurons can explain much of the variation observed in reports of NO-dependent LTP. Moreover, our data suggest that NO produced by eNOS in endothelial cells may play a paracrine role in modulating LTP.

Localization of norepinephrine and serotonin transporter in mouse brain capillary endothelial cells

Monoamines function as a vasoactive modulator in the central nervous system (CNS) and are believed to regulate blood-brain barrier (BBB) function. Although monoamine transport is an essential process for regulating the extracellular monoamine concentration, the transport systems for monoamines at the BBB are poorly understood. mRNA expression of norepinephrine transporter (NET) and serotonin transporter (SERT) has been detected in a conditionally immortalized mouse brain capillary endothelial cell line (TM-BBB4) used as an in vitro model of the BBB, whereas no dopamine transporter (DAT) was detected. Western blot analysis showed the expression of NET and SERT protein in the membrane fraction of mouse brain capillaries and TM-BBB4 cells. Immunohistochemical analysis revealed that NET and SERT are localized at the brain capillaries in the mouse cerebral cortex, and suggests that NET is localized at the abluminal side of brain capillary endothelial cells, and SERT is localized at the luminal and abluminal sides. NET and SERT expressed at the BBB may be involved in the inactivation of monoamines released from neurons around the BBB.

[Regulation of brain microvessel function]

The brain microvessels are formed by a specialized endothelium and regulate the movement of solutes between blood and brain. The endothelial cells are sealed together by tight junctions and play a role as the blood-brain barrier. The brain microvessels express GLUT1 as the major form of glucose transporter, aquaporin-4 as a water channel, and p-glycoprotein as a xenobiotic transporter. Occludin and claudin-5 have been identified as the components of tight junction. Increasing evidence suggests that the activities of the transporters are regulated by adrenergic nerve activity as well as by bioactive peptides such as adrenomedullin. The regulation of the activity as well as expression of these transporters may become a strategy for prophylaxis and treatment of not only cerebral vascular diseases but also neurodegenerative disorders, developmental abnormalities and aging of the brain.

alpha(1)-Adrenergic receptor subtypes in rat cerebral microvessels

To identify alpha(1)-adrenergic receptor subtypes involved in the regulation of cerebral microcirculation, we studied alpha(1)-adrenergic receptor subtypes expressed in the rat cerebral microvessels. The microvessels were prepared from rat cerebral cortex by albumin flotation and glass bead filtration techniques. [(125)I]HEAT binding to the cerebral microvessels was displaced by low concentrations of 5-methylurapidil, a selective antagonist for alpha(1A)-receptors, and modified Scatchard analysis of the data revealed that half of alpha(1)-receptors is alpha(1A)-subtype, and that alpha(1B)- and/or alpha(1D)-receptors are also present. The K(i) value of the high-affinity component for 5-methylurapidil was 3.90+/-1.08 nM, which is comparable with the value obtained in the rat cerebral cortex (2.17+/-0.88 nM). Reverse transcription and polymerase chain reaction experiments showed that mRNAs of alpha(1A)- and alpha(1B)-receptors, but not alpha(1D)-receptors, were expressed in the cerebral microvessels. These results suggest that alpha(1)-receptors involved in the regulation of cerebral microvessel function are alpha(1A)- and alpha(1B)-receptor subtypes.

Pathogenesis of leukoaraiosis: a review

BACKGROUND

Changes in the cerebral hemispheric white matter, detectable with increasing frequency by modern neuroimaging methods, are associated with aging and conceivably may contribute to the development of specific cognitive deficits. The pathogenesis of these cerebral white matter abnormalities (sometimes described as leukoaraiosis) is unknown. This review evaluates the available evidence in support of the hypothesis that the etiology of leukoaraiosis is related to a specific type of cerebral ischemia and highlights mechanisms by which ischemic injury to the brain may induce selected structural alterations limited to the cerebral white matter.

SUMMARY OF REVIEW

The review is based on the critical analysis of over 100 publications (most appearing in the last decade) dealing with the anatomy and physiology of the arterial circulation to the cerebral white matter and with the pathogenesis of leukoaraiosis.

CONCLUSIONS

A significant number of clues support the hypothesis that some types of leukoaraiosis may be the result of ischemic injury to the brain. Structural changes affecting the small intraparenchymal cerebral arteries and arterioles that are associated with aging and with stroke risk factors, altered cerebral blood flow autoregulation, and the conditions created by the unique arterial blood supply of the hemispheric white matter each seem to contribute to the development of leukoaraiosis. To the best of our ability to interpret current information, the type of ischemic injury that is most likely responsible for these white matter changes involves transient repeated events characterized by moderate drops in regional cerebral blood flow that induce an incomplete form of infarction. This hypothesis could be tested in appropriate experimental models.

Systemic cytokine administration can affect blood-brain barrier permeability in the rat

The aim of the present study was to clarify the effect of intracarotid injection of interleukin-1 beta (IL-1 beta), interleukin-2 (IL-2), interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha) on the permeability of the blood-brain barrier (BBB) in the rat. A regional blood-to-brain transfer constant (Ki) for [14C] alpha-aminoisobutyric acid ([14C]AIB) and the cerebral residual blood volume were calculated 10 min following administration of cytokines (CKs; 1000 U/rat). The injection of IL-2 and IL-6 (but not of IL-1 beta) induced a significant enhancement of Ki values for [14C]AIB within several brain areas; conversely, when the rats were given TNF-alpha, a striking decrease in BBB permeability was observed. The cerebral regional blood volumes appeared significantly lower in the rats injected with IL-6 than in the control animals, but markedly increased following TNF-alpha administration. Our findings confirm the ability of some CKs to affect the permeability of the BBB and/or to act, probably indirectly, as vasomodulator agents of the cerebral microvessel endothelium.

Atrial natriuretic peptide-like (ANP-LIR) and ANP prohormone immunoreactive astrocytes and neurons of human cerebral cortex

Atrial natriuretic peptide (ANP) represents a family of related peptides originally isolated from cardiac atria that have potent natriuretic, diuretic, and vasorelaxant properties. ANP has previously been localized in neurons of the rat brain in regions subserving cardiovascular functions and fluid/electrolyte balance and has been localized in astroglia of the canine brain. To determine whether ANP is present in astrocytes of the human brain and to validate the canine model for future studies, human brain tissue was obtained from autopsy cases with no brain damage or neurological or vascular disease. Human brains were obtained less than 3 h postmortem, and anterior cingulate and striate cortices were dissected following perfusion or immersion fixation. Immunohistochemical processing utilized antibodies against the processed form of ANP (ANP IV, ANP104-128) and against rat proANP (amino terminus) and the avidin-biotin-peroxidase technique. Isolated, strongly ANP-immunoreactive protoplasmic astrocytes were observed in all layers of the cingulate and striate cortex gray matter. ANP-positive fibrous astrocytes were observed in the white matter. Additionally, distinctive immunopositive astrocytes were found both within and immediately subjacent to the glia limitans. Antibody against the prohormone stained only protoplasmic astrocytes and sublimitans astrocytes and processes. In addition to the astroglia, ANP was detected in scattered multipolar neurons in the cerebral gray matter. These results provide additional evidence for diversity of peptide localization in astrocytes and suggest roles for ANP in the local regulation of cerebral blood flow, blood-brain barrier permeability, or cerebrospinal fluid volume.

C-type natriuretic peptide increases cyclic GMP in rat cerebral microvessels in primary culture

Effect of CNP on cGMP level in cultured rat cerebral microvessels was investigated. The cerebral microvessels were prepared from rat cerebral cortex by dispase and collagenase digestion and Percoll gradient centrifugation, and cultured. CNP increased cGMP level in a dose-dependent manner suggesting that CNP has a regulatory role in the cerebral microvessel function.

Pituitary adenylate cyclase-activating polypeptides (PACAPs) increase cAMP in rat cerebral microvessels

Effect of PACAP on cAMP level in the rat cerebral microvessels was investigated. The cerebral microvessels were prepared from rat cerebral cortex by albumin flotation and glass beads filtration technique. When the microvessels were incubated with PACAP 27, PACAP 38 and VIP, cAMP in the microvessels was increased rapidly reaching a plateau value within 60 s. PACAP 27, PACAP 38 and VIP increased cAMP level in a dose-dependent manner with EC50 values of 4.7, 7.0 and 34 nM, respectively. These results suggest that PACAPs play a role in the regulation of the cerebral microvessel function.

Effect of sumatriptan, a selective 5-HT1-like receptor agonist, on pial vessel diameter in anaesthetised cats

The action of sumatriptan, a selective 5-HT1-like receptor agonist that is effective for the acute treatment of migraine, was compared on pial vessel diameter following perivascular or intravenous administration to anaesthetised cats. Sumatriptan (0.01-10 microM), when microinjected perivascularly, caused a decrease in pial artery diameter (maximum change of -19 +/- 9%; mean +/- SD) but had no effect on the diameter of pial veins. Sumatriptan (1 microM)-induced pial artery vasoconstriction was unaffected by coadministration of ketanserin (1 microM) or ondansetron (1 microM) but was significantly (p less than 0.01) attenuated by methiothepin (1 microM). Intravenous infusion of a clinically effective dose of sumatriptan (64 micrograms/kg/10 min) caused selective carotid vasoconstriction (22 +/- 6% increase in carotid vascular resistance with little or no change in blood pressure or heart rate) and no change in pial artery diameter, although sumatriptan (1 microM) administered perivascularly in these animals before and after the infusion caused pial artery vasoconstriction. These results demonstrate that perivascularly administered sumatriptan causes pial artery vasoconstriction via activation of 5-HT1-like receptors. However, intravenously administered sumatriptan does not cause pial artery vasoconstriction, which suggests that sumatriptan does not readily penetrate the cerebrovascular intima.

Decreased alpha 1-adrenergic receptors after experimental brain injury

The magnitude of neuronal damage in central nervous system (CNS) injury may be related, in part, to alterations in the balance between excitatory and inhibitory neurotransmitters. Previous studies have implicated a role of central inhibitory noradrenergic mechanisms in the pathophysiologic sequelae of traumatic brain injury. In the present study, we examined alpha 1-adrenergic receptor binding after parasaggital lateral fluid percussion (FP) brain injury of moderate severity (2.3 atm) in the rat. At 30 min following injury, the specific binding of [3H]prazosin to membranes isolated from left cortex (injury site) was reduced by 37% in brain-injured animals when compared to sham-operated noninjured animals (p < 0.05). However, there were no significant differences in [3H]prazosin binding to membranes of either contralateral (right) cortex or left and right hippocampi between brain-injured and sham-operated animals. Conversely, at 24 h posttrauma, specific binding to membranes of left cortex, cortex adjacent to injury site, contralateral (right) cortex, and left hippocampus was reduced by 25%, 16%, 27%, and 24%, respectively (all p < 0.05). Scatchard analysis revealed that a reduction of [3H]prazosin binding to membranes of injured animals resulted from a decrease in alpha 1-receptor binding density (B-max) but not from changes in ligand affinity. Histopathologic assessment of neuronal damage at 24 h postinjury revealed neuronal loss within injury site cortex and left hippocampus but no clearly discernible cell loss in contralateral right cortex, suggesting that the decrease in B-max might be a consequence of early pathophysiology of trauma rather than of neuronal cell loss. We suggest that alterations in alpha 1-adrenergic receptors after brain injury may result in decreased inhibitory neurotransmitter action of norepinephrine and may thus contribute to the pathophysiology of traumatic brain injury.

Evaluation of local cerebral glucose utilization and the permeability of the blood-brain barrier in the genetically epilepsy-prone rat

The genetically epileptic-prone rat (GEPR) is a valuable model for the study of gene-linked abnormalities involved in epilepsy. In comparison with normal Sprague-Dawley controls, we found, in GEPRs, a marked depression in local cerebral glucose utilization, widespread throughout the brain. This depression was accompanied by a significant increase of blood-brain barrier permeability and a reduction in regional blood volume. Finally GEPRs showed lower plasma levels of total triiodothyronine than normal controls. One can speculate that alterations in cerebral metabolism and microvascular regulation and thyroid hormone imbalance may be gene-linked factors involved in seizure susceptibility.

Cerebrovascular permeability and cognition in the aging rat

Regional cerebrovascular permeability-capillary surface area products (rPS) and brain vascular space (BVS) were measured in aging, conscious, unrestrained Sprague-Dawley rats. Three groups of animals were examined: young-mature (6 months), middle-aged (12-14 months), and old (24-26 months) rats. Complex maze learning had been previously characterized in these same animals. Maze learning declined with age. Brain vascular space did not differ significantly with age in any brain region. However, small, but significant age-dependent decreases in rPS (25-33%) were observed. These decreases occurred mainly in the old animals in the basal ganglia and parietal cortex, and in the middle-aged and old rats in the olfactory bulbs. Significant and unexpected positive average correlations between brain permeability-capillary surface area products (PS) and learning errors occurred primarily in young rats and were attributable mainly to changes in 5 of 14 brain regions; hypothalamus, hippocampus, parietal cortex, septal area and superior colliculus. The higher correlations between maze learning errors and PS in young animals may indicate dynamic regulation of this cerebrovascular parameter which is lessened with aging. Average correlations between PS and cerebral blood flow also were determined and found to be generally small and not significant for most brain regions and age groups.

The role of neuroeffector mechanisms in cerebral hyperperfusion syndromes

Cerebral hyperperfusion, a state in which blood flow exceeds the metabolic needs of brain, may complicate a number of neurological and neurosurgical conditions. It may account for the propensity with which hemorrhage, cerebral edema, or seizures follow embolic stroke, carotid endarterectomy, or the excision of large arteriovenous malformations, and for some of the morbidity that accompanies acute severe head injury, prolonged seizures, and acute severe hypertension. Hyperperfusion syndromes have in common acute increases in blood pressure, vasodilatation, breakdown of the blood-brain barrier, and the development of cerebral edema. These common features suggest the possibility that they share the same pathogenic mechanisms. It was believed until recently that reactive hyperemia was caused primarily by the generation of vasoactive metabolites, which induced vasodilatation through relaxation of vascular smooth muscle. However, the authors have recently established that the release of vasoactive neuropeptides from perivascular sensory nerves via axon reflex-like mechanisms has a significant bearing upon a number of hyperperfusion syndromes. In this article, the authors summarize their data and discuss possible therapeutic implications for blockade of these nerves or their constituent neuropeptides.

Angiotensin II, vasopressin and GTP[gamma-S] inhibit inward-rectifying K+ channels in porcine cerebral capillary endothelial cells

Cerebral capillaries from porcine brain were isolated, and endothelial cells were grown in primary culture. The whole-cell tight seal patch-clamp method was applied to freshly isolated single endothelial cells, and cells which were held in culture up to one week. With high K+ solution in the patch pipette and in the bath we observed inward-rectifying K+ currents, showing a time-dependent decay in part of the experiments. Ba2+ (1-10 mM) in the bath blocked this current, whereas outside tetraethylammonium (10 mM) decreased the peak current but increased the steady-state current. Addition of 1 microM of angiotensin II or of arginine-vasopressin to the extracellular side caused a time-dependent inhibition of the inward-rectifying K+ current in part of the experiments. Addition of 100 microM GTP[gamma-S] to the patch pipette blocked the K+ inward rectifier. In cell-attached membrane patches two types of single inward-rectifying K+ channels were observed, with single channel conductances of 7 and 35 pS. Cell-attached patches were also obtained at the antiluminal membrane of intact isolated cerebral capillaries. Only one type of K+ channel with g = 30 pS was recorded. In conclusion, inwardly rectifying K+ channels, which can be inhibited by extracellular angiotensin II and arginine-vasopressin, are present in cerebral capillary endothelial cells. The inhibition of this K+ conductance by GTP[gamma-S] indicates that G-proteins are involved in channel regulation. It is suggested that angiotensin II and vasopressin regulate K+ transport across the blood-brain barrier, mediating their effects via G-proteins.

ANP-like immunoreactivity in neuronal perikarya and processes associated with vessels of the pia and cerebral parenchyma in dog

The existence of neocortical neurons displaying processes which penetrate the glia limitans (GL) and closely approach pial as well as intracerebral microvessels was determined in the dog from immunohistochemical localization of atrial natriuretic peptide (ANP). Scattered ANP-positive pyramidal somata located in cortical layers II and III displayed spinous dendritic arbors and delicate, beaded axon collaterals. Dendritic branches, as well as axon collaterals, traversed the GL near blood vessels entering the parenchyma, or encircled microvessels deep to the GL. These findings suggest that single ANP-like immunoreactive cortical neurons may monitor and control local cerebrovascular flow or permeability of the blood-brain barrier.

Arecoline, but not haloperidol, induces changes in the permeability of the blood-brain barrier in the rat

The aim of the present study was to investigate the existence of alterations of the blood-brain barrier (BBB) permeability in rats injected with centrally acting drugs, by calculating a unidirectional blood-to-brain transfer constant (Ki) for the circulating tracer [14C]-alpha-aminoisobutyric acid. The intraperitoneal (i.p.) injection of the dopaminergic antagonist haloperidol (1 mg kg-1) did not modify the regional BBB permeability. When the cholinomimetic agent arecoline hydrobromide (6.25 mg kg-1) was injected i.p. into methylatropine-pretreated rats, it induced a significant decrease of Ki values within the frontal cortex, parietal cortex, striatum and brain-stem. Our findings emphasize two concepts: (1) centrally acting drugs, such as arecoline, can induce changes in the BBB permeability, through several mechanisms; (2) there is no predictable correlation of drug stimulation of specific brain neuronal pathways and changes in the permeability of the BBB.

Evidence for cholinergic regulation of microvessel hydraulic conductance during tissue hypoxia

Cholinergic regulation of single-vessel hydraulic conductivity (Lp) during normoxia and hypoxia was tested in single mesenteric vessels of pithed frogs (Rana pipiens). Capillaries were cannulated in situ and perfused with frog Ringer's solution containing 10 mg/ml albumin and human erythrocytes while the mesentery was continuously superfused with frog Ringer's solution (15 degrees C). Lp was first measured under normoxic (room air equilibrated) conditions by the modified Landis microocclusion method. Repeated measurements of filtration coefficient under control conditions, for periods up to 80 minutes, demonstrated that Lp did not change with time in normoxic vessels (n = 18). After initial control measurement (Lpo), perfusion with 1 microM acetylcholine increased Lp by 4.6 +/- 1.0-fold (mean +/- SEM, n = 6). The response to acetylcholine was antagonized by the addition of 10 microns atropine to the perfusate (Lp/Lpo = 1.8 +/- 0.4). Perfusion with atropine alone reduced Lp in three of six capillaries Lp/Lpo = 0.56 +/- 0.04); Lp in the remaining three vessels was unaffected. Tissue hypoxia was simulated by exposing the mesentery to deoxygenated superfusate Po2 less than or equal to 10 mm Hg) for 10-15 minutes. Tissue hypoxia had no effect on Lp in atropine-treated vessels (n = 8). Without atropine, tissue hypoxia increased Lp by 2.3 +/- 0.7-fold, whereas the addition of atropine completely antagonized this response (n = 5). In contrast to the inhibitory action of atropine during tissue hypoxia, Lp rose 5.2 +/- 1.6-fold (n = 4) in vessels simultaneously exposed to deoxygenated perfusate.(ABSTRACT TRUNCATED AT 250 WORDS)

Proposals for the mechanism of action of convulsive therapy: a synthesis

The convulsive therapies have been powerful additions to somatic treatment in psychiatry and have enjoyed widespread application for the benefit of many. A number of theories have been advanced in efforts to account for therapeutic efficacy but none has been comprehensive. These theories can be distinguished by whether they posit a central therapeutic role for stimulation, inhibition, psychological effects, or mixed processes induced by treatment. After critically reviewing extant theories, we propose a model for the effects of the convulsive therapies: Convulsive therapy is essentially nonspecific; "nonphysiological" depolarizations are distinctly important for the restoration of aberrant intravesicular transmitter ratios with resultant therapeusis. We present this model as a working hypothesis that may contribute to the guidance of research in the mechanism of action of convulsive treatment and offer several testable hypotheses in this regard.

Cortical microvascular changes in chronological aging, cortical insults and chronic alcohol intoxication in rats. Effects of antihypoxic drug on these phenomena

Chronic alcohol pulmonary exposure in rats produced a cortical hypervascularization from one to four weeks after the onset of the alcoholization procedure. This alcohol-induced cortical hypervascularization, resembling closely the enhanced cortical vascular network observed in chronologically aged rats as well as around a lesion-induced cavity performed on the cortex, was significantly reduced by a concomitant treatment with Sabeluzole, a chemically benzothiazol derivative with antihypoxic and antiischaemic properties. The blood alcohol level of rats treated with the antihypoxic agent remained stable and constant at a mean level of 1 g/l during a whole 2-week-alcoholization duration in contrast to that of untreated rats which was directly related to the increased alcohol concentration of the atmosphere insufflated in the alcoholization chamber. Finally, a free-choice paradigm achieved after the chronic intoxication also revealed that Sabeluzole-treated rats chose to drink less alcohol as compared to untreated rats suggesting Sabeluzole well modulated the alcohol-induced behavioral preference.

Involvement of central histaminergic and cholinergic systems in the morphine-induced increase in blood-brain barrier permeability to sodium fluorescein in mice

Morphine (5 mg/kg, s.c.) caused a submaximal increase in the brain level of sodium fluorescein administered i.v. Histamine H1-antagonists, diphenhydramine and mepyramine, given either i.p. or i.c.v., had no significant influence on the effect of morphine. H2-Antagonists, cimetidine and ranitidine, administered i.c.v., but not i.p., significantly inhibited the morphine effect. alpha-Fluoromethylhistidine, a specific histidine decarboxylase inhibitor (given i.p. and i.c.v.) and antimuscarinic drugs, atropine and biperiden, but not methylatropine (given i.p.) also significantly reduced the morphine effect. Physostigimine (i.p.) significantly enhanced the effects of 0.5 and 1 mg/kg of morphine. Similar effects of histaminergic and cholinergic drugs were also observed on the buprenorphine- and DAGO-induced increase in blood-brain barrier (BBB) permeability to sodium fluorescein. None of the treatments with 6-hydroxydopamine, alpha-methyltyrosine, 5,7-dihydroxytryptamine or p-chlorophenylamine had any significant effect on the morphine-induced increase in BBB permeability. These findings suggest that the activation of brain H2-receptors by neuronal histamine and muscarinic receptors by acetylcholine is involved in the increase in BBB permeability to sodium fluorescein caused by mu opioid receptor agonists.

Absence of N-methyl-D-aspartate receptors on ovine cerebral microvessels

Radioreceptor methods were used to quantitate the N-methyl-D-aspartate (NMDA) receptor-complex of ovine cerebral microvessels and cerebral gray matter. Specific binding of D[3H]2-amino-5-phosphono-pentanoate and [3H]1-[1-(2-thienyl)cyclohexyl]piperidine, ligands for the NMDA primary acceptor site and ionophore, respectively, was found in cerebral gray matter but was not detectable in membranes prepared from brain microvessels enriched in capillaries. Sigma receptors, another locus of action for phencyclidine congeners, were also not present on microvessels but were found in cortical homogenates. On the other hand, cerebral microvessels and gray matter contained significant numbers of beta-adrenoceptors. Our results indicate the NMDA receptors and NMDA antagonists are unlikely to regulate the function of the cerebral microvasculature.

Vasomotor responses of rat intracerebral arterioles to vasoactive intestinal peptide, substance P, neuropeptide Y, and bradykinin

The effect of vasoactive peptides on vascular smooth muscle in the cerebral microcirculation was examined using an isolated intracerebral arteriole preparation. Extraluminally applied vasoactive intestinal peptide (VIP) dilated the spontaneous tone of intracerebral arterioles to 118.9 +/- 3.1% of control diameter at pH 7.30, with an EC50 of 7.27 X 10(-8) M. Similar degrees of dilation to VIP were seen in vessels preconstricted by changing bath solution to pH 7.60. Substance P had no effect on vessel diameter at pH 7.30. However, in vessels precontracted by pH 7.60, significant dose-dependent dilation was observed with an EC50 of 2.55 x 10(-10) M. Neuropeptide Y constricted intracerebral arterioles to 81.22 +/- 2.7% of control diameter, with an EC50 of 6.23 x 10(-10) M. Bradykinin dilated intracerebral arterioles at pH 7.30 and pH 7.60 to 130 +/- 3.0% of control diameter. VIP and bradykinin are potent vasodilators of intracerebral arterioles. Neuropeptide Y is a vasoconstrictor. The effect of substance P appeared to be either pH-dependent or dependent on some degree of precontraction by another agonist, but no effect on vessel diameter was seen at pH 7.30.

Role of alpha-adrenoceptors in the control of the cerebral blood flow response to hypoxia

This study assessed the role of vascular and central alpha-adrenoceptors in the regional cerebral blood flow response to moderate hypoxia. Studies were conducted in 21 rabbits using radioactive microspheres under normoxic and hypoxic (10% O2 in N2) conditions. Animals were divided into three groups and administered either saline, N-methyl chlorpromazine, or phenoxybenzamine. During normoxia, there were regional differences in cerebral blood flow distribution in the saline- and N-methyl chlorpromazine-treated rabbits which were eliminated by phenoxybenzamine. In control, hypoxia significantly increased average cerebral blood flow from 57 +/- 22 to 132 +/- 52 ml/min per 100 g. Flow to the hindbrain increased to a significantly greater extent than to the mid- or forebrain during hypoxia. The increase in average cerebral blood flow during hypoxia was significantly reduced to 97 +/- 34 ml/min per 100 g by phenoxybenzamine. Both alpha-adrenoceptor antagonists prevented the significantly greater increase in hindbrain flow during hypoxia. The greater flow responsiveness of the hindbrain to hypoxia appears to be related at least in part to alpha-adrenoceptors found in the cerebral vasculature.

Adrenergic receptors in bovine retinal microvessels: presence of alpha 2- and beta- but not alpha 1-receptors

In bovine retinal microvessels, alpha 1, alpha 2- and beta-adrenergic receptors were characterized by binding assay, using [3H]prazosin, [3H]para-aminoclonidine and [125I]iodocyanopindolol as radioligands, respectively. The microvessels were purified from bovine eyes by differential centrifugation through a high concentration of bovine serum albumin followed by use of a glass bead filtration technique. In the preparation, specific binding sites for [3H]para-aminoclonidine and [125I]iodocyanopindolol were observed, whereas [3H]prazosin binding was not detected. The [3H]para-aminoclonidine binding sites localized to the microvessels were characterized by high affinity and saturability (KD: 173 +/- 9 pM; Bmax: 394 +/- 11 fmol/mg protein) as well as the [125I]iodocyanopindolol binding sites (KD: 20 +/- 3 pM; Bmax: 43 +/- 4 fmol/mg protein). Furthermore, the specificity of both binding sites was pharmacologically evaluated by measuring the inhibitory effects of various adrenergic reagents on binding. The existence of alpha 2- and beta-adrenergic receptors which were characterized by high affinity, saturability and stereospecificity, leads to the hypothesis that the retinal microcirculation is under neuronal control.

Aging modifies the asymmetry in brain microvascular regulation

Cerebral ischemia induced by unilateral carotid occlusion in rats decreases in an asymmetric manner the number of beta-adrenergic receptors in microvessels prepared from cerebral cortexes ipsilateral and contralateral to the side of the ligature. In particular, the reduction is more pronounced in the left hemisphere in case of both right and left carotid ligature. The greater receptor decrease in the left side of the brain was shown to depend on the integrity of interhemispheric connections. We show that the changes in capillary beta-adrenergic receptors in response to unilateral carotid occlusion are qualitatively modified during aging. In particular, the asymmetry in the response pattern observed in young rats is lost. The mechanisms underlying this phenomenon may be based on an age-related impairment in the transfer of neuronal information between the two sides of the brain.

Characterization of the dihydropyridine binding sites of rat neocortical synaptosomes and microvessels

The dihydropyridine binding sites associated with rat neocortical synaptosomes and microvessels were compared using an in vitro [3H]PN 200-110 [(+)-[methyl-3H]-isopropyl 4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-2,6-dimethyl-5- methoxycarbonylpyridine-3-carboxylate] binding assay. Saturation experiments yielded similar KD values (approximately 70 pM) and Bmax values (approximately 400 fmol/mg of protein) for the two membrane preparations. Interaction experiments with [3H]PN 200-110 and various calcium-modulating substances provided further evidence for the practically identical nature of the synaptosomal and microvascular dihydropyridine binding sites. These findings predict that lipophilic dihydropyridines, simultaneously occupying the two central binding sites, have the dual effect of altering neuronal function and local blood flow.

Protective effect of antihistamines on cerebral oedema induced by experimental pneumothorax in newborn piglets

As a consequence of general hypoxaemia evoked experimentally by bilateral pneumothorax, brain oedema of vasogenic type developed in newborn piglets after 4 h survival. Histamine receptor antagonists, mepyramine (H1-receptor blocker), metiamide, cimetidine and ranitidine (H2-receptor antagonists) were administered either intraperitoneally or intrathecally to check to what extent the formation of brain oedema could be reduced. Mepyramine and ranitidine decreased the accumulation of water, sodium and albumin in the parietal cortex. By measuring the concentration of histamine, the presence of a histamine pool was demonstrated in the cerebral microvessels. The results suggest that histamine, if released upon hypoxic injury from the microvascular store, can take an important part in the development of vasogenic brain oedema.

Specific binding of atrial natriuretic factor in brain microvessels

Cerebral capillaries constitute the blood-brain barrier. Studies of specific receptors (neurotransmitters or hormones) located on this structure can be performed by means of radioligand-binding techniques on isolated brain microvessels. We examined on pure bovine cerebral microvessel preparations the binding of atrial natriuretic factor (ANF), using 125I-labeled ANF. Saturation and competition experiments demonstrated the presence of a single class of ANF-binding sites with high affinity (dissociation constant, approximately 10(-10) M) and with a binding capacity of 58 fmol/mg of protein. The binding of 125I-labeled ANF to brain microvessels is specific, reversible, and time dependent, as is shown by association-dissociation experiments. The demonstration of specific ANF-binding sites on brain microvessels supposes a physiological role of ANF on brain microvasculature. The coexistence of ANF and angiotensin II receptors on this cerebrovascular tissue suggests that the two circulating peptides may act as mutual antagonists in the regulation of brain microcirculation and/or blood-brain barrier function.

Preferential blood flow to brainstem during generalized seizures in the newborn marmoset monkey

The effect of generalized seizures on local cerebral blood flow was studied autoradiographically in 21 immature marmoset monkeys, using either [123I]- or [131I]isopropyliodoamphetamine. Generalized convulsions were induced in ketamine-anesthetized and awake monkeys with bicuculline and continued for 4-59 min. During convulsions in marmosets less than 3 weeks of age, there was a striking rearrangement of blood flow in favor of the brainstem pontomedullary region. The ratios of blood flow in pons-medulla to blood flow in cerebral cortex, putamen, ventroposterior thalamic nuclei, lateral geniculate nuclei, cerebellum and hemispheric white matter increased 1 1/2 to 2 times compared to controls. In seizure animals 4-8 weeks of age, the redistribution of blood flow to brainstem did not occur. Although metabolic acidosis developed after 30 min of bicuculline-induced seizures, mean arterial blood pressure, temperature, arterial pO2 and pCO2 did not differ significantly from controls, indicating that hypoxemia, hypercapnia and hypotension cannot explain the altered cerebral blood flow pattern. The redistribution phenomenon could be explained by more pronounced vasodilatation in brainstem than many other brain regions during generalized seizures in newborn monkeys. Lack of significant vasodilatation in forebrain structures such as cerebral cortex could contribute to neuronal damage by limiting substrate supply at a time of increased metabolic activity.

Blood flow and metabolism in heterotopic cerebellar grafts during hypoglycemia

Hypoglycemia-induced disturbances of brain metabolism and neuronal injury exhibit a distinct predilection for forebrain structures, in particular the caudate-putamen, hippocampus and cerebral cortex, whereas the cerebellum is remarkably resistant. In an attempt to assess the biological basis of this differential regional vulnerability, we have used a neural transplantation technique to compare hemodynamic and metabolic changes in cerebellum during severe hypoglycemia with those in heterotopic cerebellar grafts. To this end, the cerebellar anlage of fetal rat brain (day 15 of gestation) was stereotactically transplanted into the vulnerable caudate-putamen. Following a differentiation period of 8 weeks the grafts had developed into an organotypic population of mature cells with laminar histoarchitecture. Host animals were then subjected to insulin-induced hypoglycemia. After 15 min of isoelectric EEG, blood flow was increased throughout the brain but residual glucose consumption was significantly higher in cerebellum (0.29 mumol/g per min) and cerebellar grafts (0.22 mumol/g per min) as a result of increased glucose extraction. Hypoglycemia caused a depletion of ATP in all brain structures except cerebellum where normal levels were maintained. Correlation of local ATP content and glucose utilization revealed a threshold-like decline of ATP at a glucose utilization rate of 0.27 mumol/g per min. ATP, in consequence, was normal in cerebellum but partially depleted in cerebellar grafts. It is concluded that the resistance of cerebellum to hypoglycemia is due to its capacity for higher glucose extraction at low blood glucose levels, and that this unique intrinsic property is preserved after heterotopic transplantation.