Hormones and neurotransmitters control cyclic AMP metabolism in choroid plexus epithelial cells

Basal Sodium-Dependent Vitamin C Transporter 2 polarization in choroid plexus explant cells in normal or scorbutic conditions

Vitamin C is incorporated into the cerebrospinal fluid (CSF) through choroid plexus cells. While the transfer of vitamin C from the blood to the brain has been studied functionally, the vitamin C transporter, SVCT2, has not been detected in the basolateral membrane of choroid plexus cells. Furthermore, it is unknown how its expression is induced in the developing brain and modulated in scurvy conditions. We concluded that SVCT2 is intensely expressed in the second half of embryonic brain development and postnatal stages. In postnatal and adult brain, SVCT2 is highly expressed in all choroidal plexus epithelial cells, shown by colocalization with GLUT1 in the basolateral membranes and without MCT1 colocalization, which is expressed in the apical membrane. We confirmed that choroid plexus explant cells (in vitro) form a sealed epithelial structure, which polarized basolaterally, endogenous or overexpressed SVCT2. These results are reproduced in vivo by injecting hSVCT2wt-EYFP lentivirus into the CSF. Overexpressed SVCT2 incorporates AA (intraperitoneally injected) from the blood to the CSF. Finally, we observed in Guinea pig brain under scorbutic condition, that normal distribution of SVCT2 in choroid plexus may be regulated by peripheral concentrations of vitamin C. Additionally, we observed that SVCT2 polarization also depends on the metabolic stage of the choroid plexus cells.

Day-night differences in neural activation in histaminergic and serotonergic areas with putative projections to the cerebrospinal fluid in a diurnal brain

In nocturnal rodents, brain areas that promote wakefulness have a circadian pattern of neural activation that mirrors the sleep/wake cycle, with more neural activation during the active phase than during the rest phase. To investigate whether differences in temporal patterns of neural activity in wake-promoting regions contribute to differences in daily patterns of wakefulness between nocturnal and diurnal species, we assessed Fos expression patterns in the tuberomammillary (TMM), supramammillary (SUM), and raphe nuclei of male grass rats maintained in a 12:12 h light-dark cycle. Day-night profiles of Fos expression were observed in the ventral and dorsal TMM, in the SUM, and in specific subpopulations of the raphe, including serotonergic cells, with higher Fos expression during the day than during the night. Next, to explore whether the cerebrospinal fluid is an avenue used by the TMM and raphe in the regulation of target areas, we injected the retrograde tracer cholera toxin subunit beta (CTB) into the ventricular system of male grass rats. While CTB labeling was scarce in the TMM and other hypothalamic areas including the suprachiasmatic nucleus, which contains the main circadian pacemaker, a dense cluster of CTB-positive neurons was evident in the caudal dorsal raphe, and the majority of these neurons appeared to be serotonergic. Since these findings are in agreement with reports for nocturnal rodents, our results suggest that the evolution of diurnality did not involve a change in the overall distribution of neuronal connections between systems that support wakefulness and their target areas, but produced a complete temporal reversal in the functioning of those systems.

Water and solute secretion by the choroid plexus

The cerebrospinal fluid (CSF) provides mechanical and chemical protection of the brain and spinal cord. This review focusses on the contribution of the choroid plexus epithelium to the water and salt homeostasis of the CSF, i.e. the secretory processes involved in CSF formation. The choroid plexus epithelium is situated in the ventricular system and is believed to be the major site of CSF production. Numerous studies have identified transport processes involved in this secretion, and recently, the underlying molecular background for some of the mechanisms have emerged. The nascent CSF consists mainly of NaCl and NaHCO(3), and the production rate is strictly coupled to the rate of Na(+) secretion. In contrast to other secreting epithelia, Na(+) is actively pumped across the luminal surface by the Na(+),K(+)-ATPase with possible contributions by other Na(+) transporters, e.g. the luminal Na(+),K(+),2Cl(-) cotransporter. The Cl(-) and HCO(3) (-) ions are likely transported by a luminal cAMP activated inward rectified anion conductance, although the responsible proteins have not been identified. Whereas Cl(-) most likely enters the cells through anion exchange, the functional as well as the molecular basis for the basolateral Na(+) entry are not yet well-defined. Water molecules follow across the epithelium mainly through the water channel, AQP1, driven by the created ionic gradient. In this article, the implications of the recent findings for the current model of CSF secretion are discussed. Finally, the clinical implications and the prospects of future advances in understanding CSF production are briefly outlined.

Serotonin, bradykinin and endothelin signalling in a sheep choroid plexus cell line

The secretion of cerebrospinal fluid by the epithelial cells of choroid plexus is regulated by membrane receptors coupled to adenylyl cyclases or to phospholipase C. These intracellular signalling pathways as their interactions were investigated in a sheep choroid plexus cell line. Endothelin-1, bradykinin and serotonin induced a transient dose-dependent increase in intracellular calcium. EC 50 were 10(-8) M for endothelin-1, 10(-8) M for bradykinin and 10(-6) M for serotonin. Maximal increase in intracellular calcium was comparable for bradykinin and serotonin, but was 3 to 5 fold larger for endothelin-1. Successive stimulations with endothelin-1, serotonin or bradykinin elicited calcium increases similar to single stimulations reflecting absence of heterologous desensitization between these receptors. Forskolin-induced cAMP accumulation was potentiated by bradykinin, but not by serotonin and endothelin-1. This potentiation resulted from an increase in cAMP production rather than to an inhibition of cAMP hydrolysis. These data suggest that serotonin, endothelin-1 and bradykinin each use specific signalling pathways in the sheep choroid plexus cells.

Ependymal and choroidal cells in culture: characterization and functional differentiation

During the past 10 years, our teams developed long-term primary cultures of ependymal cells derived from ventricular walls of telencephalon and hypothalamus or choroidal cells (modified ependymal cells) derived from plexuses dissected out of fetal or newborn mouse or rat brains. Cultures were established in serum-supplemented or chemically defined media after seeding on serum-, fibronectin-, or collagen-laminin-coated plastic dishes or semipermeable inserts. To identify and characterize cell types growing in our cultures, we used morphological features provided by phase contrast, scanning, and transmission electron microscopy. We used antibodies against intermediate filament proteins (vimentin, glial fibrillary acidic protein, cytokeratin, desmin, neurofilament proteins), actin, myosin, ciliary rootlets, laminin, and fibronectin in single or double immunostaining, and monoclonal antibodies against epitopes of ependymal or endothelial cells, to recognize ventricular wall cell types with immunological criteria. Ciliated or nonciliated ependymal cells in telencephalic cultures, tanycytes and ciliated and nonciliated ependymal cells in hypothalamic cultures always exceeded 75% of the cultured cells under the conditions used. These cells were characterized by their cell shape and epithelial organization, by their apical differentiations observed by scanning and transmission electron microscopy, and by specific markers (e.g., glial fibrillary acidic protein, ciliary rootlet proteins, DARPP 32) detected by immunofluorescence. All these cultured ependymal cell types remarkably resembled in vivo ependymocytes in terms of molecular markers and ultrastructural features. Choroidal cells were also maintained for several weeks in culture, and abundantly expressed markers were detected in both choroidal tissue and culture (Na+-K+-dependent ATPase, DARPP 32, G proteins, ANP receptors). In this review, the culture models we developed (defined in terms of biological material, media, substrates, duration, and subculturing) are also compared with those developed by other investigators during the last 10 years. Focusing on morphological and functional approaches, we have shown that these culture models were suitable to investigate and provide new insights on (1) the gap junctional communication of ependymal, choroidal, and astroglial cells in long-term primary cultures by freeze-fracture or dye transfer of Lucifer Yellow CH after intracellular microinjection; (2) some ionic channels; (3) the hormone receptors to tri-iodothyronine or atrial natriuretic peptides; (4) the regulatory effect of tri-iodothyronine on glutamine synthetase expression; (5) the endocytosis and transcytosis of proteins; and (6) the morphogenetic effects of galactosyl-ceramide. We also discuss new insights provided by recent results reported on in vitro ependymal and choroidal expressions of neuropeptide-processing enzymes and neurosecretory proteins or choroidal expression of transferrin regulated through serotoninergic activation.

The normal and pathological physiology of brain water

The physicochemical properties of water enable it to act as a solvent for electrolytes, and to influence the molecular configuration and hence the function--enzymatic in particular--of polypeptide chains in biological systems. The association of water with electrolytes determines the osmotic regulation of cell volume and allows the establishment of the transmembrane ion concentration gradients that underlie nerve excitation and impulse conduction. Fluid in the central nervous system is distributed in the intracellular and extracellular spaces (ICS, ECS) of the brain parenchyma, the cerebrospinal fluid, and the vascular compartment--the brain capillaries and small arteries and veins. Regulated exchange of fluid between these various compartments occurs at the blood-brain barrier (BBB), and at the ventricular ependyma and choroid plexus, and, on the brain surface, at the pia mater. The normal BBB is relatively permeable to water, but considerably less so to ions, including the principal electrolytes Brain fluid regulation takes place within the context of systemic fluid volume control, which depends on the mutual interaction of osmo-, volume-, and pressure-receptors in the hypothalamus, heart and kidney, hormones such as vasopressin, renin-angiotensin, aldosterone, atriopeptins, and digitalis-like immunoreactive substance, and their respective sites of action. Evidence for specific transport capabilities of the cerebral capillary endothelium, for example high Na+K(+)-ATPase activity and the presence at the abluminal surface of a Na(+)--H+ antiporter, suggests that cerebral microvessels play a more active part in brain volume regulation and ion homoeostasis than do capillaries in other vascular beds. The normal brain ECS amounts to 12-19% of brain volume, and is markedly reduced in anoxia, ischaemia, metabolic poisoning, spreading depression, and conventional procedures for histological fixation. The asymmetrical distributions of Na+ K+ and Ca2+ between ICS and ECS underlie the roles of these cations in nerve excitation and conduction, and in signal transduction. The relatively large volume of the CSF, and extensive diffusional exchange of many substances between brain ECS and CSF, augment the ion-homeostasing capacity of the ECS. The choroid plexus, in addition to secreting CSF principally by biochemical mechanisms (there is an additional small component from the extracellular fluid), actively transports some substances from the blood (e.g. nucleotides and ascorbic acid), and actively removes others from the CSF. In contrast with CSF secretion, CSF reabsorption is principally a biomechanical process, passively dependent on the CSF-dural sinus pressure gradient. Pathological increases in intracranial water content imply development of an intracranial mass lesion. The additional water may be distributed diffusely within the brain parenchyma as brain oedema, as a cyst, or as increase in ventricular volume due to hydrocephalus. Brain oedema is classified on the basis of pathophysiology into four categories, vasogenic, cytotoxic, osmotic and hydrostatic. The clinical conditions in which brain oedema presents the greatest problems are tumour, ischaemia, and head injury. Peritumoural oedema is predominantly vasogenic and related to BBB dysfunction. Ischaemic oedema is initially cytotoxic, with a shift of Na+ and CI- ions from ECS to ICS, followed by osmotically obliged water, this shift can be detected by diffusion-weighted MRI. Later in the evolution of an ischaemic lesion the oedema becomes vasogenic, with disruption of the BBB. Recent imaging studies in patients with head injury suggest that the development of traumatic brain oedema may follow a biphasic time course similar to that of ischaemic oedema. Hydrocephalus is associated in the great majority of cases with an obstruction to the circulation or drainage of CSF, or, occasionally, with overproduction of CSF by a choroid plexus papilloma. In either case, the consequence is a ris

Cyclic GMP inhibits phosphoinositide turnover in choroid plexus: evidence for interactions between second messengers concurrently triggered by 5-HT2C receptors

The present study examined the effects of the nitric oxide generator sodium nitroprusside (SNP), a membrane-permeable cGMP analog (dibutyryl-cGMP) and low calcium buffer incubation on choroid plexus serotonin 5-HT2C receptor-mediated inositol monophosphate (IP) production. SNP (100 microM) substantially inhibited 10(-6)M serotonin-stimulated IP production (-46%, P < 0.02). Serotonin-stimulated IP production was increased in low calcium buffer (+280%, P < 0.01) in which serotonin-stimulated cGMP formation is attenuated. Addition of dibutyryl-cGMP (500 microM) inhibited IP formation in low calcium buffer. The present data are suggestive of an inhibitory effect of cGMP on IP formation in choroid plexus, and raise the intriguing possibility of interactions between second messenger systems concurrently activated by 5-HT2C receptors.

High affinity vasoactive intestinal peptide receptors on fetal human nonpigmented ciliary epithelial cells

The effect of vasoactive intestinal peptide (VIP) on stimulation of adenylyl cyclase in fetal human nonpigmented ciliary epithelial (NPE) and pigmented ciliary epithelial (CPE) cells was studied. 1 microM VIP elicited a 5-10 fold increase in intracellular cAMP in NPE cells from three fetal donors, but caused little or no response in CPE from two fetal donors and other ocular cell types employed as controls. Appearance of cAMP in the extracellular medium was stimulated in NPE but not in CPE in response to VIP. Both NPE and CPE gave similar cAMP responses (8-13 fold) to the beta-adrenergic agonist, isoproterenol. Binding studies of [125I]VIP to intact NPE and CPE revealed that VIP bound to NPE cells at a high affinity site (KD = .33 nM and a low affinity site (KD = 16 nM), whereas VIP bound to CPE cells only at the low affinity site (KD = 18 nM). In NPE cells, VIP stimulated cAMP formation with an EC50 of approximately 0.6-1 nM, similar to the high affinity binding site KD, with maximal stimulation at 10 nM. Four peptides with various degrees of sequence homology to VIP were also studied. Of these, PHM and PHI stimulated cAMP with EC50s of 50 and 300 nM, respectively, while secretin and glucagon stimulated only at concentrations above 0.1 microM. These results suggest that in fetal human ciliary epithelium, as in rabbit ciliary epithelium (Mittag et al., J Pharm Exp Ther 241: 230, [1987]), VIP stimulation of adenylyl cyclase is a characteristic of NPE but not CPE cells.

Cl- current activation in choroid plexus epithelial cells involves a G protein and protein kinase A

The involvement of GTP-binding proteins (G proteins) in the regulation of the Cl- conductance in rat choroid plexus epithelial cells was investigated, using the whole cell patch-clamp technique. Intracellular application of a nonhydrolyzable GTP analogue, guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S; 0.1-0.2 mM), evoked a transient increase in the Cl- conductance. The activated Cl- current exhibited inward rectification and was independent of time at hyperpolarizing or depolarizing voltage pulses. The effect of GTP gamma S was inhibited by a nonhydrolyzable GDP analogue, guanosine 5'-O-(2-thiodiphosphate) (2 mM), and by an inhibitor of protein kinase A, H-89, but was not affected by chelation of cytosolic Ca2+ with 5 mM 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. GTP gamma S failed to activate the current when ATP was omitted from the pipette solution. Intracellular application of adenosine 3',5'-cyclic monophosphate (cAMP; 0.25 mM) or the catalytic subunit of protein kinase A activated a similar Cl- current. These results suggest that G proteins activate Cl- channels via a cAMP-dependent pathway in rat choroid plexus.

Prostaglandins modulate alterations of microvascular permeability, blood flow, edema and serotonin levels following spinal cord injury: an experimental study in the rat

The possibility that prostaglandins influence edema formation, microvascular permeability increase and reduction of blood flow following spinal cord trauma was examined in a rat model. In addition, the influence of prostaglandins on serotonin metabolism of the traumatized spinal cord was evaluated. Trauma to spinal cord (2-mm-deep and 5-mm-long incision in the right dorsal horn of T10-11 segments) resulted in a profound increase of the water content 5 h after injury. At this time, the microvascular permeability to Evans Blue and [131I]sodium was increased by 457 and 394%, respectively. The blood flow was reduced by 30%. The serotonin (5-hydroxytryptamine) content of the spinal cord increased by 205%. The plasma serotonin level rose by 152% in the injured group of rats. Pretreatment with indomethacin (10 mg/kg, i.p.) 30 min before trauma significantly reduced the edema and microvascular permeability increase. The local spinal cord blood flow of traumatized animals was partially restored. The increases of serotonin levels of the spinal cord and plasma were significantly attenuated. These beneficial effects of indomethacin were not present in rats given a lower dose (5 mg/kg). Indomethacin in either dose did not influence these parameters of control rats without trauma to the cord. Since indomethacin is a potential inhibitor of prostaglandins synthesis our observations indicate: (i) that prostaglandins participate in many microvascular responses (permeability changes, edema, blood flow) occurring after a trauma to the spinal cord; (ii) that these effects of the drug seem to be dose dependent, and (iii) that the prostaglandins may influence the serotonin metabolism following trauma to the spinal cord.

Neuroendocrine regulatory mechanisms in the choroid plexus-cerebrospinal fluid system

The CSF is often regarded as merely a mechanical support for the brain, as well as an unspecific sink for waste products from the CNS. New methodology in receptor autoradiography, immunohistochemistry and molecular biology has revealed the presence of many different neuroendocrine substances or their corresponding receptors in the main CSF-forming structure, the choroid plexus. Both older research on the sympathetic nerves and recent studies of peptide neurotransmitters in the choroid plexus support a neurogenic regulation of choroid plexus CSF production and other transport functions. Among the endocrine substances present in blood and CSF, 5-HT, ANP, vasopressin and the IGFs have high receptor concentrations in the choroid plexus and have been shown to influence choroid plexus function. Finally, the choroid plexus produces the growth factor IGF-II and a number of transport proteins, most importantly transthyretin, that might regulate hormone transport from blood to brain. These studies suggest that the choroid plexus-CSF system could constitute an important pathway for neuroendocrine signalling in the brain, although clearcut evidence for such a role is still largely lacking.

Effect of Histamine H2Receptor Antagonists on the Secretion of Cerebrospinal Fluid in the Cat

Following a recent report that epithelial cells of the choroid plexus possess histamine H2 receptors, the effect of cimetidine and ranitidine, histamine H2 receptor antagonists, on the secretion and electrolyte content of CSF was examined. Fifty cats were divided into one control (n = 6) and six experimental groups. CSF was collected by puncture of the cisterna magna following pentobarbital anesthesia, and its volume, concentrations of Na+, K+, Cl-, and pH were determined. Cimetidine or ranitidine (50, 20, or 10 mg/kg) was injected intravenously 2 h after the start of the test, and their concentrations were measured in hourly blood samples and in 30-min aliquots of CSF in the 50 mg/kg experimental groups. Whereas the secretion of CSF did not change over 6 h in the control group, it decreased significantly by 30-60 min after injection of cimetidine or ranitidine and remained low for the following 6 1/2 h in all experimental groups except the 10-mg ranitidine group. Peak cimetidine and ranitidine concentrations in CSF in the 50-mg experimental groups were noted 60 and 90 min, respectively, after intravenous injection. CSF electrolyte concentrations and pH did not change during the test in any group. We conclude that intravenous cimetidine or ranitidine can significantly reduce CSF secretion in the cat, possibly by competitive inhibition of the histamine effect on H2 receptors located on the choroid plexus epithelial cell, or by a direct effect on the capillaries of the choroid plexus.

Changes in blood-brain barrier and cerebral blood flow following elevation of circulating serotonin level in anesthetized rats

Plasma serotonin (5-HT) was elevated by an intravenous infusion of this amine into urethane-anaesthetized rats and the concentration approximated that present in various neurological diseases and mental abnormalities. An infusion of 10 micrograms per kg body weight for 10 min significantly increased blood-brain barrier (BBB) permeability to Evans blue and 131I-sodium measured in whole brain. Regional BBB determinations with labelled 131I-sodium showed that the permeability to this compound was increased in the cerebral cortex, hippocampus, caudate nucleus, hypothalamus, colliculus and the cerebellum but not in the pons and the medulla oblongata. Regional blood flow was reduced in the same parts which showed BBB abnormality tested with 125I-labeled microspheres. Pretreatment with cyproheptadine, a 5-HT2 receptor antagonist, prevented the BBB increase and the regional blood flow was near normal values. Similar effects were obtained with indomethacin, a prostaglandin synthesis inhibitor. Vinblastine, known to influence vesicular transport, eliminated extravasation of the tracers but the regional blood flow remained depressed. A hypothesis is put forward that serotonin after binding to its receptor in the cerebral vessels stimulates prostaglandin which either directly or by means of cyclic adenosine monophosphate causes an increased vesicular transport across the endothelial cells and thus an extravasation of tracer substances in the brain. Obviously, this form of exudation can be influenced by pharmacological means.

EEG changes following increased blood-brain barrier permeability under long-term immobilization stress in young rats

A continuous 8 h of immobilization stress in conscious young rats increased the blood-brain barrier (BBB) permeability to 131I-sodium in 12 out of 14 brain regions studied. A flattening of electroencephalographic (EEG) activity was noted during this time period. The mean cerebral blood flow (CBF) was reduced by 17% (during this time period) but the regional flow reduction was not related to the regional increase in BBB permeability. On the other hand, a correlation was observed between increased plasma and brain 5-HT levels and increased BBB permeability. p-Chloro-phenylalanine (p-CPA) pretreatment has prevented the occurrence of increased BBB permeability, and the flattening of EEG activity as well as 5-HT levels in plasma and brain. These results suggest that the long-term immobilization stress induces causally related sequential events in rats: enhancement of circulating 5-HT, impairment of BBB, free access of 5-HT into the brain, and eventually flattening of EEG.

Effect of 5-hydroxytryptamine on the rate of cerebrospinal fluid production in rabbit

Infusion of 5-hydroxytryptamine in mock cerebrospinal fluid of rabbits lowered cerebrospinal fluid production dose-dependently by nearly 30%. The effect was amplified in the presence of the monoamine oxidase inhibitor, nialamide. The 5-hydroxytryptamine-induced inhibition was partly counteracted by ketanserin, and further addition of practolol completely blocked the reduction in cerebrospinal fluid formation, suggesting that both 5-hydroxytryptamine receptors and beta 1-adrenergic receptors were involved in the response mediated by both the secretory epithelium and the vascular bed of the plexus.

Influence of long-term acute heat exposure on regional blood-brain barrier permeability, cerebral blood flow and 5-HT level in conscious normotensive young rats

Exposure of conscious young rats to 4 h heat stress at 38 degrees C in B.O.D. incubator was associated with increased blood-brain barrier (BBB) permeability in 14 brain regions studied. In the same regions cerebral flow (CBF) diminished by 4-65%, but the magnitude of flow reduction was not correlated with the degree of increased BBB permeability. On the other hand, a correlation was observed with increased plasma and brain 5-hydroxytryptamine (5-HT) levels. p-Chlorophenylalanine (p-CPA), indomethacin and diazepam pretreatment prevented both the increased BBB permeability and 5-HT levels following heat exposure. Whereas cyproheptadine and vinblastine pretreatment prevented the increased BBB permeability alone. The probable mechanism(s) underlying the BBB permeability is discussed.

Serotonin decreases population spike amplitude in hippocampal cells through a pertussis toxin substrate

Activation of the serotonin1A receptor decreases CA1 population spike amplitude and inhibits forskolin-stimulated adenylate cyclase in rat hippocampus. Pretreatment of rats with pertussis toxin blocked both responses. Because the electrophysiological and biochemical responses to serotonin were correlated after pertussis toxin treatment, we conclude that both responses are mediated by a common regulatory protein, presumably Gi.

Agonist-induced phosphoinositide hydrolysis in choroid plexus

5-Hydroxytryptamine (5-HT, serotonin) stimulates phosphoinositide hydrolysis in choroid plexus by interacting with the 5-HTlc site. In the present study, the effects of 5-HT were compared with those of other agonists. 5-HT stimulates a rapid release of all three inositol sugars in a mianserin-sensitive manner. Inositol bisphosphate and inositol trisphosphate levels increase about twofold within 2.5 min, whereas inositol monophosphate levels are not appreciably elevated until 5 min. In contrast, glutamate, carbachol, histamine, substance P, and vasopressin, agents that increase phosphoinositide hydrolysis in other tissues, do not stimulate this response in choroid plexus. High concentrations of norepinephrine increase inositol phosphate release in choroid plexus, but this effect is apparently mediated by activation of the 5-HTlc site. The depolarizing agents KCl and veratrine also fail to stimulate phosphoinositide hydrolysis in choroid plexus. These results, combined with the finding that the phosphoinositide response to 5-HT is insensitive to tetrodotoxin, suggest that the effects of 5-HT are not secondary to neurotransmitter release. Furthermore, an indirect effect mediated via arachidonic acid metabolism is unlikely, since inhibitors of cyclooxygenase and lipoxygenase do not reduce the 5-HT response. We conclude, therefore, that phosphoinositide hydrolysis is the transducing mechanism of the 5-HT 5-HTlc receptor and that the choroid plexus will serve as a useful model system for studies of this receptor.

Vasoactive intestinal peptide stimulates cyclic AMP metabolism in choroid plexus epithelial cells

Some peptides of the glucagon-secretin family were found to stimulate intracellular cyclic AMP accumulation in cultured bovine choroid plexus epithelial cells. Vasointestinal peptide and porcine intestinal peptide at concentrations of 30 and 300 nM, respectively, evoked 50-fold elevations of cyclic AMP; half-maximal responses were obtained with concentrations of 15 and 102 nM for the two peptides, respectively. Secretin and glucagon each produced 25- to 50-fold elevations of cyclic AMP at 330 microM, but showed no effect below 3 microM. Gastric inhibitory peptide and prealbumin had little or no response at any concentration tested. Experiments measuring the cellular cyclic AMP accumulation in response to pairs of peptides suggested that vasointestinal peptide, porcine intestinal peptide and secretin act through a common receptor. Studies with antagonists to isoproterenol and histamine indicated that this receptor is distinct from the beta-adrenergic and H2-histamine receptors known to exist on choroidal cells.

Serotonin-1C sites in the choroid plexus are not linked in a stimulatory or inhibitory way to adenylate cyclase

The association of the serotonin recognition sites in the pig choroid plexus (5-HT-1C sites) with an adenylate cyclase was examined. The interaction of serotonin and mianserin with [3H]mesulergine binding was not affected by the stable GTP analogue GppNHp. The binding of [3H]serotonin to choroid plexus membranes was also unaffected by GppNHp while a dose-dependent decrease was observed in pig cortical and hippocampal membranes. The porcine choroid plexus contained a forskolin- and histamine-sensitive adenylate cyclase. Serotonin, however, was ineffective in this preparation. While forskolin-stimulated adenylate cyclase in the rat hippocampus was inhibited by serotonin forskolin-stimulated adenylate cyclase in the choroid plexus was insensitive to serotonin. These results indicate that the serotonin recognition sites in the choroid plexus are not linked in a stimulatory or inhibitory way to an adenylate cyclase, in contrast with other 5-HT-1 receptor subtypes.

Neurochemical coupled actions of transmitters in the microvasculature of the brain

The discovery that monoamine nerves end on the central microvessels of the choroid plexus, pia-arachnoid and parenchyma has prompted an intense investigation as to their physiological and neuropathological roles. The source of the monoamine fibers to the pial vessels and choroid plexus was shown to be the superior cervical ganglion. Ganglionic stimulation causes vasoconstriction or vasodilation of pial vessels, an event depending upon the functional ratio of alpha to beta adrenergic receptors. Moreover, stimulation of the superior cervical ganglion evokes an inhibition of cerebrospinal fluid formation in choroid plexus. The locus coeruleus is the site of adrenergic nerve supply to the parenchymal capillaries and stimulation of this nucleus increases capillary permeability to small molecules and water. Neurotransmitter receptors (adrenergic, histamine, adenosine, dopamine, prostacyclin, prostaglandins and specific amino acids or neuropeptides) have been identified on microvessels and in many instances these transmitter actions are coupled to cyclic AMP synthesis. Moreover, cyclic AMP has been shown to increase the rate of capillary endothelial pinocytosis and produce brain edema. In small vessels containing smooth muscle cells cyclic AMP production improves cerebral blood flow via an initiation of vasodilatory processes. The presence of receptors for serotonin and acetylcholine have likewise been demonstrated to occur on cerebral microvessels. Limited information is available as to the receptor coupled actions of these two transmitters, but cholinergic mechanisms may act to restrict catecholamine-induced formation of cyclic AMP. Altered sensitivity of microvessels to neurotransmitters has been demonstrated following conditions of stroke, hypertension, aging, diabetes and X-irradiation.

H2 histamine receptors on the epithelial cells of choroid plexus

A major site of cerebrospinal fluid production in vertebrates is the choroid plexus. The epithelial cells of the choroid plexus accumulate intracellular cyclic AMP in response to several effectors, including histamine. Since histamine is known to regulate fluid secretion in the stomach via H2 histamine receptors, we asked whether H2 receptors might also be present on epithelial cells of bovine choroid plexus. Using agonists and antagonists of histamine, we show that an agonist and antagonist pair specific for the H2 subtype were clearly more effective than an H1 agonist and antagonist pair in mimicking or inhibiting histamine stimulation of cellular cyclic AMP. Analysis by Schild plot allowed assignment of an apparent dissociation constant to the H2 antagonist metiamide which was 34-fold lower than that of its H1 counterpart, diphenhydramine. These results indicate that epithelial cells of the choroid plexus possess H2 histamine receptors.

Influence of long-term immobilization stress on regional blood-brain barrier permeability, cerebral blood flow and 5-HT level in conscious normotensive young rats

Eight hours immobilization stress in young rats has increased the blood-brain barrier (BBB) permeability in 12 out of 14 brain regions studied. In the same regions cerebral blood flow (CBF) diminished by 2-37%, but the magnitude of flow reduction was not correlated with the degree of increased BBB permeability. On the other hand, a correlation was observed with increased plasma and brain 5-HT levels. The increased BBB permeability and increased 5-HT levels were prevented by pretreatment with p-CPA, indomethacin and diazepam. Cyproheptadine and vinblastine pretreatment prevented the occurrence of increased BBB permeability alone. The probable mechanism(s) underlying the breakdown of BBB permeability is discussed.