Astroglial and vascular interactions of noradrenaline terminals in the rat cerebral cortex

Calbindin-D28k in cerebrovascular extrinsic innervation system of the rat

Calbindin-D28k, one of the calcium-binding proteins, belongs to the EF hand family and is commonly found in neurons. It serves as a representative neuronal marker for neuroanatomical investigations. The authors' knowledge of its precise function, however, is yet very limited. In this study, we examined the existence of nerve fibers with calbindin-D28k immunoreactivity in the cerebral blood vessels and ganglia that innervate the cerebral blood vessels in the rat. Numerous nerve fibers with calbindin-D28k immunoreactivity were observed on the walls of the major extracerebral arteries forming the circle of Willis and its branches. Calbindin-D28k immunoreactivity was seen in many neurons of the trigeminal, dorsal root and jugular ganglia. A small number of neurons showed calbindin-D28k immunoreactivity in the otic and superior cervical ganglia. Calbindin-D28k immunoreactivity was not detected in the sphenopalatine or internal carotid ganglia. Pericellular basket-like formations of nerve terminals with calbindin-D28k immunoreactivity were observed in the sphenopalatine, otic, internal carotid and superior cervical ganglia. The present study demonstrated calbindin-D28k immunoreactivity in the cerebrovascular nerve fibers as well as in their origins--the cranial ganglia. These findings are significant in understanding the calcium-mediated mechanism of the neural control of the cerebral blood vessels.

Basal forebrain nitric oxide synthase (NOS)-containing neurons project to microvessels and NOS neurons in the rat neocortex: cellular basis for cortical blood flow regulation

Stimulation of basal forebrain neurons results in local increases in cortical cerebral blood flow that are dependent upon cholinergic and nitrergic mechanisms. In the present study, we investigated the possibility that basal forebrain nitric oxide synthase (NOS)-containing neurons project to microvessels and NOS interneurons in the rat cerebral cortex. We performed quisqualic (QUIS) acid lesions of the basal forebrain and evaluated their effects on cortical NOS immunostained nerve terminals, with emphasis on those associated with microvessels and NOS interneurons, both at the light and/or electron microscopic levels. The results show that basal forebrain NOS neurons provide about one third of the overall cortical NOS innervation. Further, the data indicate that basalocortical NOS fibres establish privileged associations with microvessels and NOS neurons, as respective denervations of 60 and 45% were observed following lesion. At the electron microscopic level, most perivascular NOS neuronal elements corresponded to nerve terminals and a majority ( approximately 25%) of these were located in the immediate vicinity of the blood vessels, similar to the perivascular distribution reported previously for classic neurotransmitters/neuromediators. NOS terminals abutting on cortical NOS neurons were primarily nonjunctional. Altogether, these results raise the possibility that not only cholinergic but also nitrergic basal forebrain neurons are involved in the flow response observed following stimulation of the basal forebrain. Further, they suggest interactions between basalocortical and intracortical NOS neurons. We conclude that these interactions are involved in the spatial and temporal regulation of cortical perfusion following basal forebrain activation, and that they may become dysfunctional in pathologies such as Alzheimer's disease which affects both the basal forebrain and the cortical NOS neurons.

Neurons and ECM regulate occludin localization in brain endothelial cells

We report that extracellular matrix and neurons modulate the expression of occludin, one of the main components of tight junctions, by rat brain endothelial cells (RBE4.B). Of the three extracellular matrix proteins which we tested (collagen I, collagen IV, and laminin), collagen IV stimulated at the best the expression of occludin mRNA. The corresponding protein, however, was not synthesized. Significant amounts of occludin accumulated only when RBE4.B cells were cultured on collagen IV-coated inserts, in the presence of cortical neurons, plated on laminin-coated companion wells. Finally, occludin segregated at the cell periphery, only when endothelial cells were co-cultured with neurons for > or = 1 week.

Cyclic adenosine monophosphate regulates the expression of the intercellular adhesion molecule and the inducible nitric oxide synthase in brain endothelial cells

The authors studied whether cyclic AMP (cAMP), a widespread regulator of inflammation, modulates the cytokine-mediated expression of the intercellular adhesion molecule, intercellular adhesion molecule-1 (ICAM-1), and the inflammatory nitric oxide synthase 2 (NOS-2), in primary and immortalized brain endothelial cell cultures (GP8.3 cell line). When measured by enzyme-linked immunosorbent assay (ELISA), ICAM-1 was constitutively expressed and was up-regulated twofold by interleukin-1beta, with no effect of interferon-gamma. The NOS-2 activity, assessed by nitrite accumulation, was absent from untreated cultures but was induced by interleukin-1beta and interferon-gamma acting synergistically. Stimulation of cAMP-dependent pathways with forskolin or dibutyryl cAMP decreased ICAM-1 protein expression, whereas it increased NOS-2 protein expression. For both ICAM-1 and NOS-2, mRNA expression correlated with protein expression. Blockade of NOS activity with L-N-monomethylargiuine (L-NMMA) did not alter ICAM-1 expression, indicating that the nitric oxide released by NOS-2 did not cause the down-regulation of ICAM-1. Analysis of NFKB activation indicated that cAMP acted through a mechanism other than inhibition of nuclear translocation of NFKB. The authors conclude that cAMP modulates the expression of proinflammatory molecules in brain endothelium. This suggests that inflammatory processes at the blood-brain barrier in vivo may be regulated by perivascular neurotransmitters via cAMP.

Regulation of the expression of the inflammatory nitric oxide synthase (NOS2) by cyclic AMP

The enzyme nitric oxide synthase 2 (NOS2), often called inducible NOS, plays a central role in the inflammatory reactions that follow infection or tissue damage. NOS2 has been detected in virtually every cell type, and the NO it produces can perform both beneficial and detrimental actions. It is thus conceivable that regulatory mechanisms exist which control the timing and intensity of NO production by NOS2 in order to outweigh protective effects against detrimental ones. Since cyclic AMP inhibits numerous immunological reactions, studies have been carried out to determine whether cAMP-dependent pathways could inhibit NOS2 expression as well. Pharmacological studies in cultured cells show that, depending on the cell type examined, increased cAMP can exert opposite effects on the endotoxin- or cytokine-induced expression of NOS2, being either stimulatory or inhibitory in macrophages, stimulatory in adipocytes, smooth muscle, skeletal muscle, and brain endothelial cells, and inhibitory in pancreatic, liver, and brain glial cells. Regulation of NOS2 gene transcription appears to be the primary mechanism of action of cAMP, and whether it is stimulatory or inhibitory hinges on the cell-specific regulation of transcription factors including CREB, NF-kappaB, and C/EBP. Cyclic AMP must therefore be considered a modulator rather than a suppressor of NOS2 expression. This review summarizes evidence derived from in vitro studies, considers regulation of NOS2 by cAMP in vivo, and discusses possible therapeutic applications of cAMP treatment.-Galea, E., Feinstein, D. L. Regulation of the expression of the inflammatory nitric oxide synthase (NOS2) by cyclic AMP.

The cell biology of the blood-brain barrier

The blood-brain barrier (BBB) is formed by brain capillary endothelial cells (ECs). In the late embryonic and early postnatal period, these cells respond to inducing factors found in the brain environment by adopting a set of defined characteristics, including high-electrical-resistance tight junctions. Although the factors have not been identified definitively, a great deal of information about brain ECs has been obtained, especially recently. This review concentrates on a cell biological analysis of the BBB, with an emphasis on regulation of the specialized intercellular junctions. The development of these junctions seems to depend on two primary processes: the appearance of high levels of the tight junction protein occludin and intracellular signaling processes that control the state of phosphorylation of junctional proteins. Recent studies have revealed that the BBB can be modulated in an ongoing way to respond to environmental stimuli.

Effects of moderate caloric restriction on cortical microvascular density and local cerebral blood flow in aged rats

The present study was designed to assess the impact of moderate caloric restriction (60% of ad libitum fed animals) on cerebral vascular density and local cerebral blood flow. Vascular density was assessed in male Brown-Norway rats from 7-35 months of age using a cranial window technique. Arteriolar density, arteriole-arteriole anastomoses, and venular density decreased with age and these effects were attenuated by moderate caloric restriction. Analysis of local cerebral blood using [14C]iodoantipyrine indicated that basal blood flow decreased with age in CA1, CA3 and dentate gyrus of hippocampus; similar trends were evident in cingulate, retrosplenal, and motor cortex. Basal blood flow was increased in all brain regions of moderate caloric restricted old animals (compared to old ad libitum fed animals) and no differences were observed between ad libitum fed young and caloric restricted older animals. In response to a CO2 challenge to maximally dilate vessels, blood flow increased in young and old ad libitum fed animals, but a similar increase was not observed in caloric restricted old animals. We conclude that a decrease in cerebral vasculature is an important contributing factor in the reduction in blood flow with age. Nevertheless, vessels from young and old animals have the capacity to dilate in response to a CO2 challenge and, after CO2, no differences are observed between the two age-groups. These results are consistent with the hypothesis that aged animals fail to adequately regulate local cerebral blood flow in response to physiological stimuli. Moderate caloric restriction increases microvascular density and cerebral blood flow in aged animals but tissues exhibit little or no increase in blood flow in response to CO2 challenge. The cause of this deficient response may indicate that vessels are maximally dilated in aged calorically restricted animals or that they fail to exhibit normal regulatory control.

Dopaminergic regulation of cerebral cortical microcirculation

Functional variations in cerebral cortical activity are accompanied by local changes in blood flow, but the mechanisms underlying this physiological coupling are not well understood. Here we report that dopamine, a neurotransmitter normally associated with neuromodulatory actions, may directly affect local cortical blood flow. Using light and electron-microscopic immunocytochemistry, we show that dopaminergic axons innervate the intraparenchymal microvessels. We also provide evidence in an in vitro slice preparation that dopamine produces vasomotor responses in the cortical vasculature. These anatomical and physiological observations reveal a previously unknown source of regulation of the microvasculature by dopamine. The findings may be relevant to the mechanisms underlying changes in blood flow observed in circulatory and neuropsychiatric disorders.

Ectopic pancreas cyst in the mesocolon

Neural activity increases local blood flow in the central nervous system (CNS), which is the basis of BOLD (blood oxygen level dependent) and PET (positron emission tomography) functional imaging techniques. Blood flow is assumed to be regulated by precapillary arterioles, because capillaries lack smooth muscle. However, most (65%) noradrenergic innervation of CNS blood vessels terminates near capillaries rather than arterioles, and in muscle and brain a dilatory signal propagates from vessels near metabolically active cells to precapillary arterioles, suggesting that blood flow control is initiated in capillaries. Pericytes, which are apposed to CNS capillaries and contain contractile proteins, could initiate such signalling. Here we show that pericytes can control capillary diameter in whole retina and cerebellar slices. Electrical stimulation of retinal pericytes evoked a localized capillary constriction, which propagated at approximately 2 microm s(-1) to constrict distant pericytes. Superfused ATP in retina or noradrenaline in cerebellum resulted in constriction of capillaries by pericytes, and glutamate reversed the constriction produced by noradrenaline. Electrical stimulation or puffing GABA (gamma-amino butyric acid) receptor blockers in the inner retina also evoked pericyte constriction. In simulated ischaemia, some pericytes constricted capillaries. Pericytes are probably modulators of blood flow in response to changes in neural activity, which may contribute to functional imaging signals and to CNS vascular disease.