Ultrastructural localization of cholinergic muscarinic receptors in rat brain cortical capillaries

3D Reconstruction of the Neurovascular Unit Reveals Differential Loss of Cholinergic Innervation in the Cortex and Hippocampus of the Adult Mouse Brain

Increasing evidence supports a role for cerebrovasculature dysfunction in the etiology of Alzheimer’s disease (AD). Blood vessels in the brain are composed of a collection of cells and acellular material that comprise the neurovascular unit (NVU). The NVU in the hippocampus and cortex receives innervation from cholinergic neurons that originate in the basal forebrain. Death of these neurons and their nerve fibers is an early feature of AD. However, the effect of the loss of cholinergic innervation on the NVU is not well characterized. The purpose of this study was to evaluate the effect of the loss of cholinergic innervation of components of the NVU at capillaries, arteries and veins in the hippocampus and cortex. Adult male C57BL/6 mice received an intracerebroventricular injection of the immunotoxin p75NTR mu-saporin to induce the loss of cholinergic neurons. Quadruple labeling immunohistochemistry and 3D reconstruction were carried out to characterize specific points of contact between cholinergic fibers and collagen IV, smooth muscle cells and astrocyte endfeet. Innate differences were observed between vessels of the hippocampus and cortex of control mice, including a greater amount of cholinergic contact with perivascular astrocytes in hippocampal capillaries and a thicker basement membrane in hippocampal veins. Saporin treatment induced a loss of cholinergic innervation at the arterial basement membrane and smooth muscle cells of both the hippocampus and the cortex. In the cortex, there was an additional loss of innervation at the astrocytic endfeet. The current results suggest that cortical arteries are more strongly affected by cholinergic denervation than arteries in the hippocampus. This regional variation may have implications for the etiology of the vascular pathology that develops in AD.

All muscarinic acetylcholine receptors (M1-M5) are expressed in murine brain microvascular endothelium

Clinical and experimental studies indicate that muscarinic acetylcholine receptors are potential pharmacological targets for the treatment of neurological diseases. Although these receptors have been described in human, bovine and rat cerebral microvascular tissue, a subtype functional characterization in mouse brain endothelium is lacking. Here, we show that all muscarinic acetylcholine receptors (M₁-M₅) are expressed in mouse brain microvascular endothelial cells. The mRNA expression of M₂, M₃, and M₅ correlates with their respective protein abundance, but a mismatch exists for M₁ and M₄ mRNA versus protein levels. Acetylcholine activates calcium transients in brain endothelium via muscarinic, but not nicotinic, receptors. Moreover, although M₁ and M₃ are the most abundant receptors, only a small fraction of M₁ is present in the plasma membrane and functions in ACh-induced Ca²⁺ signaling. Bioinformatic analyses performed on eukaryotic muscarinic receptors demonstrate a high degree of conservation of the orthosteric binding site and a great variability of the allosteric site. In line with previous studies, this result indicates muscarinic acetylcholine receptors as potential pharmacological targets in future translational studies. We argue that research on drug development should especially focus on the allosteric binding sites of the M₁ and M₃ receptors.

Review : The Vicious Circle of Stress and Anticholinesterase Responses

After either acute psychological stress or exposure to acetylcholinesterase (AChE) inhibitors, long-lasting deleterious changes of a similar nature occur in the mammalian brain. We explored the molecular and neurophysiological mechanisms preceding these convergent delayed consequences in vivo and in perfused hippocampal brain slices. In stressed mice, we observed a disruption of the blood-brain barrier, which leads to efficient brain penetrance of anti-AChEs. This increase in penetrance of anti-AChEs, and consequently in acetylcholine (ACh) levels, induces a cascade of c-fos-mediated transcriptional responses dependent on intracellular Ca2+ accumulation. Consequently, the capacity for synthesis and vesicle packaging of ACh is suppressed simultaneously with enhanced AChE production that potentiates ACh hydrolysis. This bimodal decrease in ACh bioavailability, which is independent of the hypothalamic-pituitary-adrenal axis, then ter minates the initial neurophysiological excitation. In vivo, this AChE overexpression leads to enzyme accu mulation that is evident for more than 80 hr. The overexpressed enzyme can protect the brain from sustained hyperexcitability and from increased susceptibility to seizure activity and neuronal toxicity. However, exper imental accumulation of AChE in brain neurons through transgenic manipulations leads to a slowly pro gressive deterioration in cognitive and neuromotor faculties. The transcriptional consequences of stress and anti-AChEs may therefore be beneficial in the short term but deleterious in the long term. NEURO SCIENTIST 5:173-183, 1999

Ultrastructural analysis of vascular features in cerebral cavernous malformations

OBJECTIVE

Investigation of the structure of vascular malformations highlights the pathogenic mechanisms underlying their clinical behavior. One of the vascular malformations is called cerebral cavernous malformation (CCM). However, the ultrastructural features of the vascular malformations are not defined in detail.

METHODS

We aimed to investigate the ultrastructural features of CCMs using transmission (TEM), scanning (SEM) electron microscopy, and also immunohistochemistry methods with antibodies against CCM proteins such as CCM2 and CCM3. CCM tissues (n=6) microsurgically excised from patients for conventional indications.

RESULTS

CCM2 and CCM3 were strongly detected in the vascular endothelium. However, there was a very weak immunostaining in stroma. SEM observations revealed that there were ruptures and damages in the luminal endothelium, possibly due to the damage of intercellular junctions. TEM observations also showed a few ruptures and detachments between the endothelium and basal lamina as observed with partially damages and disconnections. The architecture of pericytes showed protrusions and shrinkages. Our results suggest that the thin vessel walls of CCMs were lacking of subendothelial support and intact basal lamina underlying the endothelial cells.

CONCLUSION

This study is so far the first study attempting to show human CCM lesions with SEM. We believe that an understanding of the ultrastructural features of these lesions by light and electron microscopy techniques would help to understand the pathology of these diseases.

The potential role of nutritional components in the management of Alzheimer's Disease

Epidemiological evidence linking nutrition to the incidence and risk of Alzheimer Disease is rapidly increasing. The specific nutritional deficiencies in Alzheimer patients may suggest a relative shortage of specific macro- and micronutrients. These include omega-3 fatty acids, several B-vitamins and antioxidants such as vitamins E and C. Recent mechanistic studies in cell systems and animal models also support the idea that nutritional components are able to counteract specific aspects of the neurodegenerative and pathological processes in the brain. In addition, it has been shown that several nutritional components can also effectively stimulate membrane formation and synapse formation as well as improve behavior and cerebrovascular health. The suggested synergy between nutritional components to improve neuronal plasticity and function is supported by epidemiological studies as well as experimental studies in animal models. The ability of nutritional compositions to stimulate synapse formation and effectively reduce Alzheimer Disease neuropathology in these preclinical models provides a solid basis to predict potential to modify the disease process, especially during the early phases of Alzheimer Disease.

Vascular dementia and the cholinergic pathways

Vascular cognitive impairment/vascular dementia have been the subject of a large number of studies, due to their high prevalence and broad preventive and compensatory therapeutic potential. The knowledge of the cerebral anatomy correlated to the vascular territories of irrigation enables understanding of clinical manifestations, as well as classification into the several types of syndromic presentations. The central cholinergic system exercises important neuromodulatory functions on cerebral circuits related to cognitive and behavioral integration, as well as on vasomotor control related to cerebral blood flow adjustments. The acquisition of data on the anatomy of the cholinergic pathways, including the localization of the nuclei of the basal prosencephalon and the routes of their projections, established an important milestone. The knowledge of the vascular distribution and of the trajectories of the cholinergic pathways allows identification of the strategic points where a vascular lesion can cause interruption. The ensuing denervation leads to cholinergic hypofunction in the involved territories. This information proves important to better evaluate the sites of vascular lesions, emphasizing their strategic localizations in relation to the cholinergic pathways, and offering more robust foundations for treatment aiming at enhancing cholinergic activity.

Cholinergically Mediated Augmentation of Cerebral Perfusion in Alzheimer's Disease and Related Cognitive Disorders: The Cholinergic-Vascular Hypothesis

The treatment of Alzheimer's disease (AD) with cholinesterase inhibitors (ChEIs) is based on the cholinergic hypothesis. This hypothesis fails to account for the global nature of the clinical effects of ChEIs, for the replication of these effects in other dementias, and for the strong and unpredictable intraindividual variation in response to treatment. These findings may be better explained by the premise that ChEIs primarily act by augmenting cerebral perfusion: the cholinergic-vascular hypothesis. This article will review the evidence from preclinical and clinical investigations on the vascular role of the cholinergic neural system. The clinical relevance of this hypothesis is discussed with respect to its interactions with the vascular and amyloid hypotheses of AD. Implications for treatment are indicated. Finally, we propose that the role of the cholinergic system in neurovascular regulation and functional hyperemia elucidates how the cholinergic deficit in AD contributes to the clinical and pathological features of this disease.

Microsphere embolism-induced cortical cholinergic deafferentation and impairments in attentional performance

Ischemic events have been hypothesized to play a critical role on the pathogenesis of dementia and the acceleration of cognitive impairments. This experiment was designed to determine the consequences of microvascular ischemia on the cortical cholinergic input system and associated attention capacities. Injections of microspheres ( approximately 50 microm diameter; approximately 5000 microspheres/100 microL) into the right common carotid artery of rats served as a model of microvascular ischemia and resulted in decreases in the density of cholinergic fibers in the ipsilateral medial prefrontal cortex and frontoparietal areas. Furthermore, dense astrogliosis, indicated by glial fibrillary acidic protein (GFAP) immunohistochemistry, was observed in the globus pallidus, including the areas of origin of cholinergic projections to the cortex. Fluoro-Jade B staining indicated that loss of neurons in the cortex was restricted to areas of microsphere-induced infarcts. Attentional performance was assessed using an operant sustained attention task; performance in this task was previously demonstrated to reflect the integrity and activity of the cortical cholinergic input system. Embolized animals' performance was characterized by a decrease in the animals' ability to detect signals. Their performance in non-signal trials remained unaffected. The residual density of cholinergic axons in prefrontal and frontoparietal areas correlated with the animals' performance. The present data support the hypothesis that microvascular ischemia results in loss of cortical cholinergic inputs and impairs associated attentional performance. Microsphere embolism represents a useful animal model for studying the role of interactions between microvascular disorder and impaired forebrain cholinergic neurotransmission in the manifestation of cognitive impairments.

Retinal vascular image analysis as a potential screening tool for cerebrovascular disease: a rationale based on homology between cerebral and retinal microvasculatures

The retinal and cerebral microvasculatures share many morphological and physiological properties. Assessment of the cerebral microvasculature requires highly specialized and expensive techniques. The potential for using non-invasive clinical assessment of the retinal microvasculature as a marker of the state of the cerebrovasculature offers clear advantages, owing to the ease with which the retinal vasculature can be directly visualized in vivo and photographed due to its essential two-dimensional nature. The use of retinal digital image analysis is becoming increasingly common, and offers new techniques to analyse different aspects of retinal vascular topography, including retinal vascular widths, geometrical attributes at vessel bifurcations and vessel tracking. Being predominantly automated and objective, these techniques offer an exciting opportunity to study the potential to identify retinal microvascular abnormalities as markers of cerebrovascular pathology. In this review, we describe the anatomical and physiological homology between the retinal and cerebral microvasculatures. We review the evidence that retinal microvascular changes occur in cerebrovascular disease and review current retinal image analysis tools that may allow us to use different aspects of the retinal microvasculature as potential markers for the state of the cerebral microvasculature.

Developmental origins of the age-related decline in cortical cholinergic function and associated cognitive abilities

Ontogenetic abnormalities in the regulation of the cortical cholinergic input system are hypothesized to mediate early-life cognitive limitations (ECL) that later escalate, based on reciprocal interactions between a dysregulated cholinergic system and age-related neuronal and vascular processes, to mild cognitive impairment (MCI) and, subsequently, for a majority of subjects, senile dementia. This process is speculated to begin with the disruption of trophic factor support of the basal forebrain ascending cholinergic system early in life, leading to dysregulation of cortical cholinergic transmission during the initial decades of life and associated limitations in cognitive capacities. Results from neurochemical and behavioral experiments support the possibility that aging reveals the vulnerability of an abnormally regulated cortical cholinergic input system. The decline of the cholinergic system is further accelerated as a result of interactions with amyloid precursor protein metabolism and processing, and with cerebral microvascular abnormalities. The determination of the developmental variables that render the cortical cholinergic input system vulnerable to age-related processes represents an important step toward the understanding of the role of this neuronal system in the age-related decline in cognitive functions.

In vivo modulation of a cortical functional sensory representation shortly after topical cholinergic agent application

The aim of the present study was to determine whether cholinergic increase in the size of a functional representation (collective evoked response from a large population of neurons) can be observed shortly (within an hour) after treatment onset and whether nicotinic receptors can participate in this type of modulation. Cholinergic agonist application has been found previously to increase the response of a single cortical neuron to a stimulus. Also, pairing cholinergic basal forebrain stimulation with delivery of a tone has been reported to increase the size of that tone's functional representation. Whereas the increase in a single cortical neuron response can occur within seconds after cholinergic agonist application, to date the increase in the size of a functional representation has only been investigated within one to several weeks after the onset of pairing basal forebrain stimulation with tone delivery. Furthermore, primarily muscarinic receptors have been implicated in these types of changes in cortical activity. By using optical imaging of intrinsic signals in vivo, we found that the size of a whisker's functional representation in the primary somatosensory cortex of the rat increases substantially within 69 or 46 minutes after topical application of either a muscarinic or nicotinic agonist to the exposed cortex, respectively, and decreases within 23 minutes after topical application of a muscarinic antagonist. For each cholinergic agent, we verified that delivery of a cholinergic agent by means of topical application can lead to the agent's successful penetration through the cortical layers in the time allotted to complete an imaging experiment. Furthermore, the time course of penetration for each agent was characterized. Based on the combined imaging/penetration results, we speculate on potential sites of cholinergic action in the cortex. Irrespective of the exact mechanism of action, we demonstrate here that an increase in the size of a functional sensory representation can occur shortly by means of activation of either nicotinic or muscarinic receptors.

Muscarinic receptor subtypes are differentially distributed in the rat cochlea

Five different genes encode the muscarinic acetylcholine receptors. The muscarinic receptor subtypes M1, M3, and M5 are typically coupled to activation of the Galpha(q/11)-phosphatidyl inositol pathway, whereas the M2 and M4 subtypes are typically linked to Galpha(i) and adenylyl cyclase inhibition. In order to localize muscarinic receptors in the rat cochlea, we applied polyclonal antibodies for subtypes M1, M2, M3, and M5, and monoclonal antibody for subtype M4 to paraffin sections. In the organ of Corti, outer hair cells exhibited strong immunoreactivity for M3 and weak immunoreactivity for M1. Deiters' cells were strongly immunoreactive to antibodies for the M1 and M2 subtypes, with weak staining observed for M3, and weaker yet for M5. Inner hair cells showed moderate immunoreactivity for the M1 subtype, weaker staining for the M5 subtype, and slight staining for the M3 subtype. Among the spiral ganglion neurons, weak to moderate immunoreactivity was detected for M3 and M5 subtypes and weak staining was observed for the M1 subtype. The efferent fibers of the intraganglionic spiral bundle were positive for M2 and M5. In the lateral wall, weak to moderate staining was detected for M5 in the stria vascularis corresponding in position to the basolateral extensions of marginal cells. Staining for M3 was observed associated with capillaries. Fibrocytes of the spiral ligament exhibited limited but selective subtype immunoreactivity. No immunoreactivity was detected in the cochlea for the M4 subtype. From the present findings we suggest that M3 is the primary muscarinic receptor subtype in outer hair cells mediating a postsynaptic response to the medial olivocochlear cholinergic efferent input. The muscarinic receptor subtypes M1, M3, and M5 appear to subserve the action of cholinergic lateral olivocochlear efferent stimulation on postsynaptic responses in type I afferents. Whether M1, M3, and M5 protein in inner hair cells indicates constitutive or vestigial expression remaining from development is unknown. M2 and M5 muscarinic receptors expressed presynaptically may modulate the efferent signal. Finally, expression by Deiters' cells of several muscarinic subtypes raises the possibility that cholinergic efferents couple to these non-sensory cells through muscarinic receptors.

Cerebral microvascular pathology in aging and Alzheimer's disease

The aging of the central nervous system and the development of incapacitating neurological diseases like Alzheimer's disease (AD) are generally associated with a wide range of histological and pathophysiological changes eventually leading to a compromised cognitive status. Although the diverse triggers of the neurodegenerative processes and their interactions are still the topic of extensive debate, the possible contribution of cerebrovascular deficiencies has been vigorously promoted in recent years. Various forms of cerebrovascular insufficiency such as reduced blood supply to the brain or disrupted microvascular integrity in cortical regions may occupy an initiating or intermediate position in the chain of events ending with cognitive failure. When, for example, vasoconstriction takes over a dominating role in the cerebral vessels, the perfusion rate of the brain can considerably decrease causing directly or through structural vascular damage a drop in cerebral glucose utilization. Consequently, cerebral metabolism can suffer a setback leading to neuronal damage and a concomitant suboptimal cognitive capacity. The present review focuses on the microvascular aspects of neurodegenerative processes in aging and AD with special attention to cerebral blood flow, neural metabolic changes and the abnormalities in microvascular ultrastructure. In this context, a few of the specific triggers leading to the prominent cerebrovascular pathology, as well as the potential neurological outcome of the compromised cerebral microvascular system are also going to be touched upon to a certain extent, without aiming at total comprehensiveness. Finally, a set of animal models are going to be presented that are frequently used to uncover the functional relationship between cerebrovascular factors and the damage to neural networks.

Presence of aquaporin-4 and muscarinic receptors in astrocytes and ependymal cells in rat brain: a clue to a common function?

Using combined double immunofluorescence and laser confocal microscopy, we studied the common cellular localization of cholinergic muscarinic receptors (mAChRs) and aquaporin-4 water channels (AQP4) in the cortex, the corpus callosum and in ependymal cells of the rat brain. In the cortex, AQP4 staining was restricted to the perivascular end-feet of astrocytes. It was more widely distributed on the astrocytes of the corpus callosum. On astrocytes, mAChRs were often present in regions immunoreactive to AQP4. Ependymal cells bordering the third ventricle were also stained by both antibodies. The double staining of mAChRs with AQP4 on two different cell-types might indicate that further interactions exist which may be important in the regulation of water and electrolyte movements in the brain.

beta-Amyloid excitotoxicity in rat magnocellular nucleus basalis. Effect of cortical deafferentation on cerebral blood flow regulation and implications for Alzheimer's disease

Alzheimer's disease is the most common type of dementia with a still largely unclear etiopathology. One of the factors that may directly contribute to the development and progression of the disorder is the abundant accumulation of beta-amyloid peptides (A beta) in senile plaques. In the present account we review coherent in vivo experimental evidence that A beta infusion into the rat magnocellular nucleus basalis (MBN) induces abrupt and persistent behavioral dysfunctions, perturbations of sensory information processing, storage, and retrieval. These substantial behavioral changes are due to the loss of cholinergic neurons in the MBN and their ascending projections to the frontoparietal cortex. Both neuroanatomical and neurochemical observations pin-point that infusion of A beta into the rat basal forebrain significantly decreases choline-acetyltransferase and acetylcholinesterase activities and the population of--probably--M2 muscarinic acetylcholine receptors in the cerebral cortex. Neuropharmacological data indicate that A beta toxicity is mediated by an excitotoxic cascade involving blockade of astroglial glutamate uptake, sustained activation of N-methyl-D-aspartate receptors and an overt intracellular Ca2+ influx. These changes are associated with increased nitric oxide synthase activity in cortical target areas that may directly lead to the generation of free radicals. Besides, as microvessels of the neocortex receive direct input from the MBN we assume that the loss of cholinergic innervation and hence that of tonic cholinergic vasoregulation ultimately leads to disturbances of vascular (endothelial) function and nutrient supply that may directly enhance neuronal vulnerability during aging and in Alzheimer's disease.

Muscarinic acetylcholine receptors in the hippocampus, neocortex and amygdala: a review of immunocytochemical localization in relation to learning and memory

Immunocytochemical mapping studies employing the extensively used monoclonal anti-muscarinic acetylcholine receptor (mAChR) antibody M35 are reviewed. We focus on three neuronal muscarinic cholinoceptive substrates, which are target regions of the cholinergic basal forebrain system intimately involved in cognitive functions: the hippocampus; neocortex; and amygdala. The distribution and neurochemistry of mAChR-immunoreactive cells as well as behaviorally induced alterations in mAChR-immunoreactivity (ir) are described in detail. M35+ neurons are viewed as cells actively engaged in neuronal functions in which the cholinergic system is typically involved. Phosphorylation and subsequent internalization of muscarinic receptors determine the immunocytochemical outcome, and hence M35 as a tool to visualize muscarinic receptors is less suitable for detection of the entire pool of mAChRs in the central nervous system (CNS). Instead, M35 is sensitive to and capable of detecting alterations in the physiological condition of muscarinic receptors. Therefore, M35 is an excellent tool to localize alterations in cellular cholinoceptivity in the CNS. M35-ir is not only determined by acetylcholine (ACh), but by any substance that changes the phosphorylation/internalization state of the mAChR. An important consequence of this proposition is that other neurotransmitters than ACh (especially glutamate) can regulate M35-ir and the cholinoceptive state of a neuron, and hence the functional properties of a neuron. One of the primary objectives of this review is to provide a synthesis of our data and literature data on mAChR-ir. We propose a hypothesis for the role of muscarinic receptors in learning and memory in terms of modulation between learning and recall states of brain areas at the postsynaptic level as studied by way of immunocytochemistry employing the monoclonal antibody M35.

Functional acetylcholine muscarinic receptor subtypes in human brain microcirculation: identification and cellular localization

Acetylcholine is an important regulator of local cerebral blood flow. There is, however, limited information available on the possible sites of action of this neurotransmitter on brain intraparenchymal microvessels. In this study, a combination of molecular and functional approaches was used to identify which of the five muscarinic acetylcholine receptors (mAChR) are present in human brain microvessels and their intimately associated astroglial cells. Microvessel and capillary fractions isolated from human cerebral cortex were found by reverse transcriptase-polymerase chain reaction to express m2, m3, and, occasionally, m1 and m5 receptor subtypes. To localize these receptors to a specific cellular compartment of the vessel wall, cultures of human brain microvascular endothelial and smooth muscle cells were used, together with cultured human brain astrocytes. Endothelial cells invariably expressed m2 and m5 receptors, and occasionally the m1 receptor; smooth muscle cells exhibited messages for all except the m4 mAChR subtypes, whereas messages for all five muscarinic receptors were identified in astrocytes. In all three cell types studied, acetylcholine induced a pirenzepine-sensitive increase (62% to 176%, P<0.05 to 0.01) in inositol trisphosphate, suggesting functional coupling of m1, m3, or m5 mAChR to a phospholipase C signaling cascade. Similarly, coupling of m2 or m4 mAChR to adenylate cyclase inhibition in endothelial cells and astrocytes, but not in smooth muscle cells, was demonstrated by the ability of carbachol to significantly reduce (44% to 50%, P<0.05 to 0.01) the forskolin-stimulated increase in cAMP levels. This effect was reversed by the mAChR antagonist AFDX 384. The results indicate that microvessels are able to respond to neurally released acetylcholine and that mAChR, distributed in different vascular and astroglial compartments, could regulate cortical perfusion and, possibly, blood-brain barrier permeability, functions that could become jeopardized in neurodegenerative disorders such as Alzheimer's disease.

Na+-Ca2+ exchange and its implications for calcium homeostasis in primary cultured rat brain microvascular endothelial cells

1. The role of Na+-Ca2+ exchange in the regulation of the cytosolic free Ca2+ concentration ([Ca2+]i) was studied in primary cultured rat brain capillary endothelial cells. [Ca2+]i was measured by digital fluorescence imaging in cells loaded with fura-2. 2. ATP (100 microM) applied for a short period of time (6 s) caused a rise in [Ca2+]i from 127 +/- 3 (n = 290) to 797 +/- 25 nM, which then declined to the resting level, with a t time required for [Ca2+]i to decline to half of peak [Ca2+]i) of 5.4 +/- 0.09 s. This effect was independent of external Ca2+ and could be abolished by previously discharging the Ca2+ pool of the endoplasmic reticulum with thapsigargin (1 microM). 3. Application of thapsigargin (1 microM) or cyclopiazonic acid (10 microM) to inhibit the Ca2+-ATPase of the endoplasmic reticulum 6 s prior to ATP application did not influence the peak [Ca2+]i but greatly reduced the rate of decline of [Ca2+]i, with t values of 15 +/- 1.6 and 23 +/- 3 s, respectively. 4. In the absence of external Na+ (Na+ replaced by Li+ or N-methylglucamine) the basal [Ca2+]i was slightly elevated (152 +/- 6 nM) and the restoration of [Ca2+]i after the ATP stimulation was significantly slower (t , 7.3 +/- 0.46 s in Li+ medium, 8.12 +/- 0.4 s in N-methylglucamine medium). 5. The external Na+-dependent component of the [Ca2+]i sequestration was also demonstrated in cells stimulated by ATP subsequent to addition of cyclopiazonic acid; in a Na+-free medium [Ca2+]i remained at the peak level in 88 % of the cells after stimulation with ATP. 6. Addition of monensin (10 microM) in the presence of external Na+ increased the resting [Ca2+]i to 222 +/- 9 nM over approximately 1 min and subsequent removal of extracellular sodium resulted in a further increase in [Ca2+]i to a peak of 328 +/- 11 nM, which was entirely dependent on external Ca2+. 7. These findings indicate that a functional Na+-Ca2+ exchanger is present at the blood-brain barrier, which plays a significant role in shaping the stimulation-evoked [Ca2+]i signal and is able to work in reverse mode under pharmacological conditions.

Distribution of muscarinic receptors on the endothelium of cortical vessels in the rat brain

Functional and pharmacological studies have suggested that there are muscarinic receptors (mAChRs) on the endothelial cells of major cerebral arteries, while recent immunological studies indicate that there are no mAChRs on the endothelium of brain capillaries. This difference may be because the distribution of mAChR on the endothelium varies with the type of vessel. This paper examines the distribution of mAChR on the vascular endothelium along intraparenchymal blood vessels in the rat brain by immunolabelling and laser confocal microscopy. Sections were immunostained by combinations of an anti-mAChR antibody (M35) with antibodies to endothelial (anti-GLUT1), or to smooth muscle markers (anti-actin). Antibody labellings were detected with fluorescent second antibodies. Most of the penetrating vessels bore mAChR immunolabelling which coincided over almost all the vessel surface with endothelial labelling. The mAChR immunolabelling was less widespread over the endothelium on the medium sized vessels (diameter < 50 microm) and only 50% of these vessels had mAChR staining on the endothelium. There was no mAChR immunostaining on the endothelium of the capillaries. In contrast with the basilar artery, there was no mAChR immunolabelling on the smooth muscle layer of the intracortical vessels. These data indicate that the intensity of mAChR immunolabelling decreases along the vascular tree from large conducting vessels to capillaries.