Indoleaminergic innervation of rat choroid plexus: a fluorescence histochemical study

Release of insulin produced by the choroid plexis is regulated by serotonergic signaling

The choroid plexus (ChP) is a highly vascularized tissue found in the brain ventricles, with an apical epithelial cell layer surrounding fenestrated capillaries. It is responsible for the production of most of the cerebrospinal fluid (CSF) in the ventricular system, subarachnoid space, and central canal of the spinal cord, while also constituting the blood-CSF barrier (BCSFB). In addition, epithelial cells of the ChP (EChP) synthesize neurotrophic factors and other signaling molecules that are released into the CSF. Here, we show that insulin is produced in EChP of mice and humans, and its expression and release are regulated by serotonin. Insulin mRNA and immune-reactive protein, including C-peptide, are present in EChP, as detected by several experimental approaches, and appear in much higher levels than any other brain region. Moreover, insulin is produced in primary cultured mouse EChP, and its release, albeit Ca2+ sensitive, is not regulated by glucose. Instead, activation of the 5HT2C receptor by serotonin treatment led to activation of IP3-sensitive channels and Ca2+ mobilization from intracellular storage, leading to insulin secretion. In vivo depletion of brain serotonin in the dorsal raphe nucleus negatively affected insulin expression in the ChP, suggesting an endogenous modulation of ChP insulin by serotonin. Here, we show for the first time to our knowledge that insulin is produced by EChP in the brain, and its release is modulated at least by serotonin but not glucose.

The neural milieu of the developing choroid plexus: neural stem cells, neurons and innervation

The choroid plexus produces cerebrospinal fluid and plays an important role in brain homeostasis both pre and postnatally. In vitro studies have suggested that cells from adult choroid plexus have stem/progenitor cell-like properties. Our initial aim was to investigate whether such a cell population is present in vivo during development of the choroid plexus, focusing mainly on the chick choroid plexus. Cells expressing neural markers were indeed present in the choroid plexus of chick and also those of rodent and human embryos, both within their epithelium and mesenchyme. ß3-tubulin-positive cells with neuronal morphology could be detected as early as at E8 in chick choroid plexus and their morphological complexity increased with development. Whole mount immunochemistry demonstrated the presence of neurons throughout choroid plexus development and they appeared to be mainly catecholaminergic, as indicated by tyrosine-hydroxylase reactivity. The presence of cells co-labeling for BrdU and the neuroblast marker, doublecortin, in organotypic choroid plexus cultures supported the hypothesis that neurogenesis can occur from neural precursors within the developing choroid plexus. Furthermore, we found that extrinsic innervation is present in the developing choroid plexus, unlike previously suggested. Altogether, our data are consistent with the presence of neural progenitors within the choroid plexus, suggest that at least some of the choroid plexus neurons are born locally, and show for the first time that choroid plexus innervation occurs prenatally. Hence, we propose the existence of a complex neural regulatory network within the developing choroid plexus that may play a crucial role in modulating its function during development as well as throughout life.

G-protein-coupled receptors in adult neurogenesis

The importance of adult neurogenesis has only recently been accepted, resulting in a completely new field of investigation within stem cell biology. The regulation and functional significance of adult neurogenesis is currently an area of highly active research. G-protein-coupled receptors (GPCRs) have emerged as potential modulators of adult neurogenesis. GPCRs represent a class of proteins with significant clinical importance, because approximately 30% of all modern therapeutic treatments target these receptors. GPCRs bind to a large class of neurotransmitters and neuromodulators such as norepinephrine, dopamine, and serotonin. Besides their typical role in cellular communication, GPCRs are expressed on adult neural stem cells and their progenitors that relay specific signals to regulate the neurogenic process. This review summarizes the field of adult neurogenesis and its methods and specifies the roles of various GPCRs and their signal transduction pathways that are involved in the regulation of adult neural stem cells and their progenitors. Current evidence supporting adult neurogenesis as a model for self-repair in neuropathologic conditions, adult neural stem cell therapeutic strategies, and potential avenues for GPCR-based therapeutics are also discussed.

Serotonin 5-HT2C receptor-mediated phosphoinositide hydrolysis in rat choroid plexus after fluoxetine and citalopram treatments

Selective serotonin reuptake inhibitors (SSRIs) bind directly to various neurotransmitter receptors. The clinical effects of SSRIs appear gradually during weeks of treatment, suggesting a role for adaptive changes in neurotransmitter receptors. Most clinically used antidepressants, e.g. fluoxetine, bind to 5-HT2C receptors. When administered chronically, many antidepressants elicit adaptive regulation of 5-HT2C receptors. The present study was conducted in order to determine the effects of acute and chronic fluoxetine and citalopram treatments on the density and function of 5-HT2C receptors in the rat choroid plexus. Acute and chronic treatments followed by phosphoinositide (PI) hydrolysis assays and quantitative receptor autoradiography were performed. Acute (single-dose) treatment with neither drug significantly affected basal or 5-HT-stimulated PI hydrolysis, but acute citalopram (20 mg/kg) treatment increased both agonist and antagonist binding to 5-HT(2C) receptors. Chronic (14 days) citalopram treatment (20 mg/kg) increased the maximal PI hydrolysis response by 40%, but fluoxetine lacked this effect. The present data suggest that sensitisation of 5-HT2C receptor-mediated intracellular signal transduction may play a role in the effects of citalopram. In contrast, fluoxetine treatment does not functionally sensitise 5-HT2C receptors. Thus, functional 5-HT2C receptor sensitisation is not a common effect of antidepressants, but the differential effects may explain some of the pharmacodynamic differences seen with these drugs, especially upon repeated administration.

Choroid plexus calcification as a possible clue of serotonin implication in schizophrenia

Choroid plexus calcification (CPC) was measured on computed tomography (CT) scans of 87 schizophrenics and 46 controls divided into age subgroups. We studied the relationship between presence and size of CPC and age in both groups, whilst in the schizophrenic group we also investigated the possible correlation between CPC size and age of onset and duration of illness, duration of formal education, psychopathological features of the illness as well as some neuroradiological brain measures. CPC size correlated with age in healthy controls but not in schizophrenics. In the schizophrenic group, left choroid plexus calcification size correlated with the Scale for the Assessment of Positive Symptoms (SAPS) subscales scores of 'formal thought disorder' whilst right choroid plexus calcification size correlated with the ventricular brain ratio at frontal horns (VBRFH). The data are not conclusive, but a possible correlation with a dysgenetic or functional 5-HT alteration can be hypothesized.

Choroid plexus calcification as a possible marker of hallucinations in schizophrenia

Several studies suggest that disturbances of serotonin (5-HT) functions may be involved in the pathophysiology of hallucinations in schizophrenia. It is now well established that the choroid plexus (CP) is innervated by serotonin (5-HT) neurons, which may regulate its activity, and it is possible that decreased 5-HT functions may facilitate the process of its calcification. It is thus conceivable that calcification of the CP may be associated with hallucinations in schizophrenia. I studied in 18 chronic schizophrenic patients the association of CP calcification (CPC) size as ascertained from CT scan, to severity of hallucinations and, for comparison, to four other positive symptoms as well as global psychopathology score. Analysis of variance indicated that CPC size was specifically associated with hallucinations (p < .001) and none of the other psychopathology measures. These findings reveal a relationship between CPC and hallucinations in schizophrenia and suggest that the former may be a neuroradiological marker of hallucinations in the disease.

Nocturnal melatonin secretion in suicidal patients with multiple sclerosis

Multiple sclerosis (MS) is characterised by the occurrence of patchy CNS demyelinating lesions, leading to various degrees of motor, sensory, affective, and cognitive deficits. MS is associated also with an increased risk of suicide accounting for a substantial rate of death among these patients. Post-mortem studies in suicide victims with various psychiatric disorders demonstrate a decreased concentration of serotonin (5-HT) and its metabolites in the brain. Since 5-HT is a precursor in the synthesis of melatonin and as pineal melatonin content was found to be low in suicide victims, we predicted lower melatonin secretion in suicidal versus non-suicidal MS patients during an acute exacerbation of symptoms. To test this hypothesis, we investigated nocturnal plasma melatonin levels in a cohort of 28 relapsing patients who were admitted consecutively to an inpatient Neurology service, 6 of whom had a history of suicide attempts and were having suicidal ideation at the time of admission. While both cohorts of patients were not distinguishable on any of the demographic data including use of psychotrophic drugs on the day of admission to hospital, the mean melatonin level in the suicidal group was significantly lower than in the control group (19.0 pg/ml +/- 11.9 versus 45.5 pg/ml +/- 27.1; p < .05). These findings support the prediction of the study implicating the pineal gland in the pathogenesis of suicidality in MS and reinforce the concept that a biological rather than a reactive etiology underlies the development of psychiatric symptoms in MS.

Neuroradiological facets of cognitive abnormality in schizophrenia

Cognitive abnormality has long been regarded as a core feature of schizophrenia, but its nature and etiopathology have been poorly understood. Predicated on new tests that characterize fundamental cognitive impairments, we investigated their relationship to four neuroradiological markers that have been previously implicated in schizophrenia: choroid plexus calcification, cerebellar atrophy, third ventricle enlargement, and pineal calcification. Twenty-three chronic schizophrenic inpatients meeting RDC and DSM-III diagnostic criteria were stabilized on chlorpromazine and assessed independently on a cognitive battery and on CT scan. The results indicated that all four neuroradiological variables were independent of one another and of demographic, historical, and general intellectual measures. The neuroradiological assessments, however, correlated significantly with different cognitive parameters, implying separate pathophysiological bases for distinct profiles of cognitive abnormality. The findings support a "dual-process model" of cognitive dysfunction that posits developmental and arousal-related components which may, more generally, underlie the positive-negative dimension of schizophrenia.

Effect of serotonin on blood flow to the choroid plexus

The choroid plexus contains a very high density of serotonin receptors and serotonin has been reported to influence the rate of formation of cerebrospinal fluid. The goal of this study was to examine effects of serotonin (5-hydroxytryptamine) on blood flow to the choroid plexus. Blood flow to the choroid plexus was measured in anesthetized cynomolgus monkeys and dogs using radioactive microspheres. Under control conditions, blood flow to choroid plexus was approximately 4 times greater than blood flow to the cerebrum in monkeys and approximately 7 times greater than blood flow to the cerebrum in dogs. Infusion of serotonin (40 micrograms.kg-1.min-1) into the left atrium increased blood flow to choroid plexus by 101 +/- 26% (mean +/- S.E.M.) in monkeys and by 201 +/- 45% in dogs. Serotonin did not affect cerebral blood flow. These findings suggest that serotonin may play an important role in regulation of blood flow to the choroid plexus.

Serotonin receptors in the human brain--III. Autoradiographic mapping of serotonin-1 receptors

The anatomical distribution of serotonin-1 receptors in human postmortem brain tissue was studied by quantitative light microscopic autoradiography. [3H]Serotonin was used to label all the subtypes of serotonin-1 sites (serotonin-1A, serotonin-1B, serotonin-1C). Serotonin-1A receptors were specifically labelled with [3H]8-hydroxy-2-[N,N-di-N-propyl-amino]tetralin, while [3H]mesulergine was used to identify serotonin-1C receptors. Receptor densities were quantified by means of a computer-assisted microdensitometric system. Confirming previous findings, serotonin-1A and serotonin-1C receptors were found in the human brain, while sites with the pharmacological characteristics of serotonin-1B binding sites could not be identified in this tissue. In addition, serotonin-1C receptors appeared to present differences in terms of pharmacology, depending on the brain area analysed. The distribution of both serotonin-1A and serotonin-1C receptor subtypes throughout the human brain was heterogeneous. High or very high densities of serotonin-1A receptors were found over the Ca1 field of the hippocampus, raphé nuclei, layers I and II of the cortex and some nuclei of the thalamus and amygdala. The claustrum, posterior hypothalamus, mesencephalic and pontine central grey matter and substantia gelatinosa of the cervical spinal cord, among others, presented intermediate concentrations of serotonin-1A receptors. In contrast, high densities of serotonin-1C receptors were present in the choroid plexus, substantia nigra, globus pallidus and ventromedial hypothalamus, while low or very low amounts of this receptor subtype were found in many other human brain areas. The anatomical distribution of serotonin-1A and serotonin-1C receptors is discussed taking into account the distribution of serotonergic neurons and fibres, the central functions in which serotonin appears to be involved and the characteristics of the neurological and psychiatric disorders where changes in brain serotonin-1 receptors have been reported.

Central serotonin receptors: effector systems, physiological roles and regulation

Radioligand binding studies have revealed four distinct serotonin (5HT) binding sites in rat brain that are thought to function as 5HT receptors. These include the 5HT-1a, 5HT-1b, 5HT-1c, and 5HT-2 binding sites. Studies have shown that the 5HT-2 binding site mediates a number of effects of 5HT agonists and serves as a 5HT receptor in neuronal and non-neuronal tissues. The 5HT-2 site employs phosphoinositide hydrolysis for signal transduction. The 5HT-1c binding site is also a functional receptor that is linked to phosphoinositide hydrolysis. However, the physiological role of the 5HT-1c receptor is not yet known. Lack of appropriate pharmacological tools for probing the 5HT-1a and 5HT-1b binding sites has made it difficult to definitively determine whether these binding sites are coupled to biochemical effector systems or mediate any of the physiological responses to 5HT agonists. However, there is some evidence that the 5HT-1a site is coupled to adenylate cyclase, and a number of functional roles for the 5HT-1a and 5HT-1b sites have been proposed.

A unique serotonin receptor in choroid plexus is linked to phosphatidylinositol turnover

A novel serotonergic binding site, the 5-HT1C site, has been characterized recently in choroid plexus and several brain regions. The biochemical and physiological roles of this site have not been previously described. In this report we show that serotonin (5-hydroxytryptamine, 5-HT) stimulates phosphatidylinositol turnover in rat choroid plexus. The pharmacology of serotonin-stimulated phosphatidylinositol hydrolysis in choroid plexus was compared to the pharmacology in cerebral cortex, where this response is mediated by the serotonin 5-HT2 receptor. Serotonin increased phosphatidylinositol turnover in choroid plexus by 6-fold and in cerebral cortex by 2.5-fold. Serotonin was greater than 10-fold more potent in choroid plexus (EC50 = 46 nM) than in cerebral cortex (EC50 = 540 nM). The serotonin antagonists ketanserin, mianserin, and spiperone inhibited the response in the two tissues with different potencies. In cerebral cortex all three exhibited nanomolar affinities consistent with their potencies at the 5-HT2 site. In choroid plexus, however, the rank order (mianserin greater than ketanserin much greater than spiperone) and absolute potencies were consistent with binding to the 5-HT1C site. These data suggest that the 5-HT1C site in choroid plexus is a functional receptor that utilizes phosphatidylinositol turnover as its biochemical effector system.

Third ventricle choroid plexus function and its response to acute perturbations in plasma chemistry

The homeostatic role of the third ventricle choroid plexus (3VCP) in the maintenance of CSF electrolytes was investigated by quantifying alterations in CP epithelial ion concentrations induced by chemical perturbations of plasma in adult Sprague-Dawley rats. Significant regional differences (third vs fourth (4VCP) and lateral ventricle CP (LVCP] were found in epithelial content of Na+ and K+, with respect to baseline levels as well as alterations caused by 5-60 min of systemic metabolic acidosis. 3VCP, which comprises ca. 10% of total choroidal tissue, has a water content, extracellular fluid volume and vascularity comparable to 4VCP and LVCP; yet 3VCP is characterized by relatively high and low values for cellular [Na+] (68 mM) and [K+] (118 mM). Compared to time-matched controls, acute metabolic acidosis (i.p. NH4Cl) effected a response, i.e. increases [K+] and decreases [Na+], in 3VCP that was less than in 4VCP, and substantially smaller than in LVCP. The onset and duration of induced electrolyte changes were qualitatively similar among the 3 plexus regions. Although systemic acidosis severely altered CP electrolyte concentrations, it did not compromise CSF homeostasis of [K+] and [Na+]. The function of 3VCP is discussed in terms of secretory capacity, embryological origin, and innervation. Overall, the findings indicate that transport/permeability phenomena which mediate transmembrane distribution of Na+ and K+ in 3VCP differ quantitatively from other regions of the blood-CSF barrier.

Quantitative autoradiographic mapping of serotonin receptors in the rat brain. I. Serotonin-1 receptors

The distribution of serotonin-1 (5-HT1) receptors in the rat brain was studied by light microscopic quantitative autoradiography. Receptors were labeled with [3H]serotonin (5-[3H]HT), 8-hydroxy-2-[N-dipropylamino-3H]tetralin (8-OH- [3H]DPAT), [3H]LSD and [3H]mesulergine, and the densities quantified by microdensitometry with the aid of a computer-assisted image-analysis system. Competition experiments for 5-[3H]HT binding by several serotonin-1 agonists led to the identification of brain areas enriched in each one of the three subtypes of 5-HT1 recognition sites already described (5-HT1A, 5-HT1B, 5-HT1C). The existence of these 'selective' areas allowed a detailed pharmacological characterization of these sites to be made in a more precise manner than has been attained in membrane-binding studies. While 5-[3H]HT labeled with nanomolar affinity all the 5-HT1 subtypes, the other 3H-labeled ligands labeled selectively 5-HT1A (8-OH-[3H]DPAT), 5-HT1C ([3H]mesulergine) and both of them ([3H]LSD). Very high concentrations of 5-HT1 receptors were localized in the choroid plexus, lateroseptal nucleus, globus pallidus and ventral pallidum, dentate gyrus, dorsal subiculum, olivary pretectal nucleus, substantia nigra, reticular and external layer of the entorhinal cortex. The different fields of the hippocampus (CA1-CA4), some nuclei of the amygdaloid complex, the hypothalamic nuclei and the dorsal raphé, among others, also presented high concentrations of sites. Areas containing intermediate densities of 5-HT1 receptors included the claustrum, olfactory tubercle, accumbens, central grey and lateral cerebellar nucleus. The nucleus caudate-putamen and the cortex, at the different levels studied, presented receptor densities ranging from intermediate to low. Finally, in other brain areas--pons, medulla, spinal cord--only low or very low concentrations of 5-HT1 receptors were found. From the areas strongly enriched in 5-HT1 sites, dentate gyrus and septal nucleus contained 5-HT1A sites, while globus pallidus, dorsal subiculum, substantia nigra and olivary pretectal nucleus were enriched in 5-HT1B. The sites in the choroid plexus, which presented the highest density of receptors in the rat brain, were of the 5-HT1C subtype. The distribution of 5-HT1 receptors reported here is discussed in correlation with the distribution of serotoninergic neurons and fibers, the related anatomical pathways and the effects which appear to be mediated by these sites.

Neurochemical studies on the existence, origin and characteristics of the serotonergic innervation of small pial vessels

Substantial concentrations of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA), comparable to those found in brain tissue, were measured in the small pial vessels of the rat, rabbit and cat. Both rat and rabbit pial vessels exhibited a high affinity uptake process with kinetic parameters similar to those identified for the cerebral cortex. Labelled 5-HT, taken up by isolated rabbit pial vessels was released, in a calcium-dependent manner, by potassium-induced depolarization. Various pharmacological manipulations were carried out in the rat. Systemic administration of the 5-HT precursor, 5-hydroxytryptophan and the monoamine oxidase inhibitor, pargyline, significantly increased the concentration of 5-HT in the pial vessels; in contrast, two depleting agents (p-chloroamphetamine and reserpine) and the tryptophan hydroxylase inhibitor, p-chlorophenylalanine, all decreased the perivascular 5-HT levels. A serotonergic antagonist (methysergide) and a 5-HT receptor agonist (MK 212) respectively increased and decreased the concentrations of 5-HIAA in the pial vessels. These pharmacologically induced changes observed in pial vessels were not dissimilar from those noted for cortical tissue. Electrolytic lesions of the nuclei raphes medianus and/or dorsalis markedly decreased the levels of 5-HT and 5-HIAA in these small cerebral arterioles. Electrical stimulation of these nuclei decreased 5-HT although 5-HIAA concentrations tended to increase. A number of conclusions may be drawn from these studies. Thus, there is a serotonergic innervation of the cerebral circulation in several laboratory species which unequivocally originates in the raphé nuclei. Furthermore, these perivascular fibres possess synthetic, storage, release, inactivation and autoregulatory processes for 5-HT which, when further elucidated, may offer some rationale for the treatment of those cerebrovascular diseases in which this neurotransmitter and vasoactive agent is believed to be of pathological importance.

Characterization of [3H]5-hydroxytryptamine uptake within rat cerebrovascular tree

The in vitro uptake of tritiated serotonin ([3H]5HT) was studied in a preparation of rat extracerebral arteries. The uptake of [3H]5HT was time- and temperature-dependent and of high affinity; linear regression analysis gave a Km value of 6.48 X 10(-7) M for the specific uptake. Bilateral superior cervical ganglionectomy was without effect on [3H]5HT uptake while it significantly reduced the uptake of tritiated norepinephrine by the preparation of rat extracerebral arteries. The serotonergic neurotoxin 5,7-dihydroxytryptamine and lesions to both the medial and the dorsal raphe nuclei caused a marked loss of [3H]5HT uptake but did not change the uptake of tritiated norepinephrine. Competition studies with norepinephrine, desimipramine (a noradrenergic uptake blocker), nomifensine (a dopaminergic uptake blocker) and fluoxetine (a 5HT uptake blocker) confirmed the specificity of the [3H]5HT uptake mechanism. Histoautoradiographic studies showed the highest density of silver grains at the level of the adventitial-medial border of the basilar artery. Fluoxetine inhibited the accumulation of silver grains within the adventitial-medial border in the blood vessel studied. The present data further support the view that a neuronal serotonergic system may play a role in the control of blood flow in the cerebrovascular tree.

Functional bases for a central serotonergic involvement in classic migraine: a speculative view

The rôle of serotonin (5-HT) in the cerebrovascular bed is the subject of the following review. Cerebral blood vessels are supplied with 5-HT-containing fibres which originate in the raphé nuclei in the brainstem. The activation of this system may result in a constriction of large arteries and a dilatation of arterioles. Intra-arterial administration of 5-HT causes reduction in cerebral blood flow and metabolism provided it bypasses the blood-brain barrier. The findings, marked changes in plasma levels of 5-HT and in cerebral blood flow during a classic migraine attack, are suggestive of an involvement of the 5-HT system.

Neurogenic control of cerebral circulation

The cerebral vascular neuromuscular apparatus consists of a varicose perivascular nerve plexus at the adventitial-medial border and smooth muscle cells in the medial coat that are functionally connected. In addition to noradrenaline and acetylcholine, a number of putative non-adrenergic, non-cholinergic neurotransmitters have been identified in cerebral perivascular nerves, including serotonin, substance P, vasoactive intestinal polypeptide, gastrin-releasing peptide, cholecystokinin, somatostatin, neurotensin, calcitonin gene-related peptide and neuropeptide Y. The role of adenosine-5'-triphosphate as a cotransmitter with noradrenaline in some perivascular sympathetic nerves, and of endothelial cells in mediating the vasodilatation produced by some neurohumoral agents is discussed. Speculations are made about the relation between vascular neuroeffector mechanisms and migraine, including the possibility of local vasospasm by serotoninergic nerves, reactive hyperaemia involving purine nucleotides and nucleosides, release of substance P from sensory nerve collaterals during antidromic ('axon reflex') impulses and secondary release of local agents such as prostanoids, histamine and bradykinin.

The binding of serotonergic ligands to the porcine choroid plexus: characterization of a new type of serotonin recognition site

The kinetic and pharmacological characteristics of the binding of [3H]5-HT (serotonin), [3H]8-OH-DPAT (8-OH-2-di-n-propylaminotetraline), [3H]LSD, [3H]ketanserin and [3H]mesulergine to membranes from frontal cortex, hippocampus and choroid plexus of pig brain were studied. The binding of these ligands to frontal cortex and hippocampus demonstrated the presence of 5-HT1 and 5-HT2 sites in both tissues, although hippocampus was richer in 5-HT1 (subtype 5-HT1A) sites. [3H]5-HT, [3H]mesulergine and [3H]LSD labeled the pig choroid plexus with high affinity. The pharmacological profiles of [3H]5-HT and [3H]mesulergine binding to this tissue were closely comparable. Ligands reported as selective for 5-HT1A, 5-HT1B or 5-HT2 subtypes did not show high affinity for these binding sites. Therefore, these 5-HT binding sites in pig choroid plexus could be named 5-HT1C. Other drugs with a high affinity for these sites were methysergide and mianserine. In pig frontal cortex, [3H]5-HT labeled the different subtypes of 5-HT1 sites. In contrast, [3H]mesulergine bound in pig frontal cortex to a small population of sites with pharmacological properties similar to those of the choroid plexus 5-HT1C sites. Possible physiological functions in which these sites might be involved are discussed.