Light- and electron-microscopic immunocytochemistry of glutamic acid decarboxylase (GAD) in the basal hypothalamus: morphological evidence for neuroendocrine gamma-aminobutyrate (GABA)

Pre- and post-synaptic modulation by GABAB receptors of rat neuroendocrine dopamine neurones

The secretion of prolactin from the pituitary is negatively controlled by tuberoinfundibular dopamine (TIDA) neurones. The electrical properties of TIDA cells have recently been identified as a modulatory target of neurotransmitters and hormones in the lactotrophic axis. The role of the GABA_B receptor in this control has received little attention, yet is of particular interest because it may act as a TIDA neurone autoreceptor. Here, this issue was explored in a spontaneously active rat TIDA in vitro slice preparation using whole-cell recordings. Application of the GABA_B receptor agonist, baclofen, dose-dependently slowed down or abolished the network oscillations typical of this preparation. Pharmacological manipulations identify the underlying mechanism as an outward current mediated by G-protein-coupled inwardly rectifying K⁺ -like channels. In addition to this postsynaptic modulation, we describe a presynaptic modulation where GABA_B receptors restrain the release of glutamate and GABA onto TIDA neurones. Our data identify both pre- and postsynaptic modulation of TIDA neurones by GABA_B receptors that may play a role in the neuronal network control of pituitary prolactin secretion and lactation.

P2X2 Receptor Expression and Function Is Upregulated in the Rat Supraoptic Nucleus Stimulated Through Refeeding After Fasting

Magnocellular neurons in the supraoptic nucleus (SON), which synthesize and release arginine vasopressin (AVP) and oxytocin (OT), express several subtypes of ATP-stimulated purinergic P2X receptors (P2XR) that modulate neuronal activity as well as neurotransmitter and hormone release. However, the physiological impact of this modulation is not well understood. Here, we tested a hypothesis that P2XRs play a role in the sustained release of hormones from SON neurons stimulated through fasting/refeeding. We studied the effect of 2 h of refeeding after 48 h of fasting on P2XR and P2YR mRNA expression and ATP-induced presynaptic and postsynaptic responses in the SON of 30-day-old rats. Quantitative real-time PCR revealed that the expression of P2X2R and AVP mRNA was upregulated, whereas P2X4R, P2X7R, P2Y2R, and OT mRNA levels were not significantly changed and P2Y1R mRNA expression was decreased. Whole-cell patch clamp recordings performed on isolated rat brain slices showed that the amplitude of the ATP-stimulated somatic current and the ATP-induced increases in the frequency of spontaneous GABAergic inhibitory postsynaptic currents were significantly higher in SON neurons from fasted/refed rats than in SON neurons from normally fed rats. No evidence was found for changes in the presynaptic effect of ATP in SON neurons not expressing somatic P2XRs. These results suggest that the increased activity of SON neurons synthesizing AVP is associated with enhanced expression of P2X2Rs on neuronal cell bodies and their GABAergic presynaptic nerve terminals.

Sex-dependent changes in neuroactive steroid concentrations in the rat brain following acute swim stress

Sex differences in hypothalamic-pituitary-adrenal (HPA) axis activity are well established in rodents. In addition to glucocorticoids, stress also stimulates the secretion of progesterone and deoxycorticosterone (DOC) from the adrenal gland. Neuroactive steroid metabolites of these precursors can modulate HPA axis function; however, it is not known whether levels of these steroids differ between male and females following stress. In the present study, we aimed to establish whether neuroactive steroid concentrations in the brain display sex- and/or region-specific differences under basal conditions and following exposure to acute stress. Brains were collected from male and female rats killed under nonstress conditions or following exposure to forced swimming. Liquid chromatography-mass spectrometry was used to quantify eight steroids: corticosterone, DOC, dihydrodeoxycorticosterone (DHDOC), pregnenolone, progesterone, dihydroprogesterone (DHP), allopregnanolone and testosterone in plasma, and in five brain regions (frontal cortex, hypothalamus, hippocampus, amygdala and brainstem). Corticosterone, DOC and progesterone concentrations were significantly greater in the plasma and brain of both sexes following stress; however, the responses in plasma were greater in females compared to males. This sex difference was also observed in the majority of brain regions for DOC and progesterone but not for corticosterone. Despite observing no stress-induced changes in circulating concentrations of pregnenolone, DHDOC or DHP, concentrations were significantly greater in the brain and this effect was more pronounced in females than males. Basal plasma and brain concentrations of allopregnanolone were significantly higher in females; moreover, stress had a greater impact on central allopregnanolone concentrations in females. Stress had no effect on circulating or brain concentrations of testosterone in males. These data indicate the existence of sex and regional differences in the generation of neuroactive steroids in the brain following acute stress, especially for the 5α-reduced steroids, and further suggest a sex-specific expression of steroidogenic enzymes in the brain. Thus, differential neurosteroidogenesis may contribute to sex differences in HPA axis responses to stress.

Generation of neuropeptidergic hypothalamic neurons from human pluripotent stem cells

Hypothalamic neurons orchestrate many essential physiological and behavioral processes via secreted neuropeptides, and are relevant to human diseases such as obesity, narcolepsy and infertility. We report the differentiation of human pluripotent stem cells into many of the major types of neuropeptidergic hypothalamic neurons, including those producing pro-opiolemelanocortin, agouti-related peptide, hypocretin/orexin, melanin-concentrating hormone, oxytocin, arginine vasopressin, corticotropin-releasing hormone (CRH) or thyrotropin-releasing hormone. Hypothalamic neurons can be generated using a 'self-patterning' strategy that yields a broad array of cell types, or via a more reproducible directed differentiation approach. Stem cell-derived human hypothalamic neurons share characteristic morphological properties and gene expression patterns with their counterparts in vivo, and are able to integrate into the mouse brain. These neurons could form the basis of cellular models, chemical screens or cellular therapies to study and treat common human diseases.

Characteristics of GABAergic and cholinergic neurons in perinuclear zone of mouse supraoptic nucleus

The perinuclear zone (PNZ) of the supraoptic nucleus (SON) contains some GABAergic and cholinergic neurons thought to innervate the SON proper. In mice expressing enhanced green fluorescent protein (eGFP) in association with glutamate decarboxylase (GAD)65 we found an abundance of GAD65-eGFP neurons in the PNZ, whereas in mice expressing GAD67-eGFP, there were few labeled PNZ neurons. In mice expressing choline acetyltransferase (ChAT)-eGFP, large, brightly fluorescent and small, dimly fluorescent ChAT-eGFP neurons were present in the PNZ. The small ChAT-eGFP and GAD65-eGFP neurons exhibited a low-threshold depolarizing potential consistent with a low-threshold spike, with little transient outward rectification. Large ChAT-eGFP neurons exhibited strong transient outward rectification and a large hyperpolarizing spike afterpotential, very similar to that of magnocellular vasopressin and oxytocin neurons. Thus the large soma and transient outward rectification of large ChAT-eGFP neurons suggest that these neurons would be difficult to distinguish from magnocellular SON neurons in dissociated preparations by these criteria. Large, but not small, ChAT-eGFP neurons were immunostained with ChAT antibody (AB144p). Reconstructed neurons revealed a few processes encroaching near and passing through the SON from all types but no clear evidence of a terminal axon arbor. Large ChAT-eGFP neurons were usually oriented vertically and had four or five dendrites with multiple branches and an axon with many collaterals and local arborizations. Small ChAT-eGFP neurons had a more restricted dendritic tree compared with parvocellular GAD65 neurons, the latter of which had long thin processes oriented mediolaterally. Thus many of the characteristics found previously in unidentified, small PNZ neurons are also found in identified GABAergic neurons and in a population of smaller ChAT-eGFP neurons.

Distribution of GABA-immunoreactive neurons in the forebrain of the goldfish, Carassius auratus

The distribution of gamma-aminobutyric acid (GABA) immunoreactivity was studied in the forebrain (tel- and diencephalon) of the goldfish by means of immunocytochemistry on Vibratome sections using antibodies against GABA. Positive perikarya were detected in the olfactory bulbs and in all divisions of the telencephalon, the highest density being found along the midline. In the diencephalon, GABA-containing cell bodies were found in the hypothalamus, in particular in the preoptic and tuberal regions. The inferior lobes, the nucleus recessus lateralis, and more laterodorsal regions, such as the nucleus glomerulosus and surrounding structures, also exhibited numerous GABA-positive perikarya. Cell bodies were also noted in the thalamus, in particular in the dorsomedial, dorsolateral and ventromedial nuclei. The relative density of immunoreactive fibers was evaluated for each brain nucleus and classified into five categories. This ubiquitous distribution indicates that, as in higher vertebrates, GABA most probably represents one of the major neurotransmitters in the brain of teleosts.

Alterations in a hypothalamic GABA system in the spontaneously hypertensive rat

The density (Bmax) of muscimol and clonazepam binding to hypothalamic membranes from spontaneously hypertensive rats (SHR) was reduced compared to age-matched Wistar Kyoto (WKY) animals in the period 80 - 120 days. There were no significant differences in dissociation constant (Kd) for either ligand at this time. At 30 - 36 days, prior to development of pronounced hypertension, there were no differences in Kd or Bmax for either ligand in SHR and WKY animals. There were also deficits in endogenous hypothalamic GABA concentrations in SHR at 75 and 120 days as compared to WKY. The hypothesis is advanced, that there may be a dysfunction of a hypothalamic GABA system in the SHR rat as hypertension develops.

Regulation of neurosteroid biosynthesis by neurotransmitters and neuropeptides

The enzymatic pathways leading to the synthesis of bioactive steroids in the brain are now almost completely elucidated in various groups of vertebrates and, during the last decade, the neuronal mechanisms involved in the regulation of neurosteroid production have received increasing attention. This report reviews the current knowledge concerning the effects of neurotransmitters, peptide hormones, and neuropeptides on the biosynthesis of neurosteroids. Anatomical studies have been carried out to visualize the neurotransmitter- or neuropeptide-containing fibers contacting steroid-synthesizing neurons as well as the neurotransmitter, peptide hormones, or neuropeptide receptors expressed in these neurons. Biochemical experiments have been conducted to investigate the effects of neurotransmitters, peptide hormones, or neuropeptides on neurosteroid biosynthesis, and to characterize the type of receptors involved. Thus, it has been found that glutamate, acting through kainate and/or AMPA receptors, rapidly inactivates P450arom, and that melatonin produced by the pineal gland and eye inhibits the biosynthesis of 7α-hydroxypregnenolone (7α-OH-Δ(5)P), while prolactin produced by the adenohypophysis enhances the formation of 7α-OH-Δ(5)P. It has also been demonstrated that the biosynthesis of neurosteroids is inhibited by GABA, acting through GABA(A) receptors, and neuropeptide Y, acting through Y1 receptors. In contrast, it has been shown that the octadecaneuropetide ODN, acting through central-type benzodiazepine receptors, the triakontatetraneuropeptide TTN, acting though peripheral-type benzodiazepine receptors, and vasotocin, acting through V1a-like receptors, stimulate the production of neurosteroids. Since neurosteroids are implicated in the control of various neurophysiological and behavioral processes, these data suggest that some of the neurophysiological effects exerted by neurotransmitters and neuropeptides may be mediated via the regulation of neurosteroid production.

Forebrain ischemia triggers GABAergic system degeneration in substantia nigra at chronic stages in rats

The long-term consequences of forebrain ischemia include delayed Parkinson's syndrome. This study revealed delayed neurodegeneration in the substantia nigra 8 weeks after 12.5 minutes of global ischemia in rat brain. Following neuronal loss of 30-40% in central and dorsolateral striatum at day 3, neuronal damage in the substantia nigra (SN) was assessed at 4-8 weeks using immunohistochemistry for glutamate decarboxylase 67 (GAD67), vesicular GABA transporter (VGAT), and calretinin (CR). At day 56, the optical density of GAD67-, but not VGAT-, immunoreactivity in substantia nigra pars reticulata (SNR)-significantly decreased. CR-neurons concentrated in substantia nigra pars compacta (SNC) were reduced by 27% from day 3 (n = 5) to day 56 (n = 7, ANOVA, p < .01). Movement coordination was impaired at day 56, as evaluated using beam-walking test (time-to-traverse 5.6 ± 1.2 sec versus 11.8 ± 5.4 sec; sham versus ischemia, p < .05, n = 5, and 7, resp.). Our results demonstrate delayed impairment of the GABAergic system components in SN and associated with movement deficits after global ischemia.

Neurosteroid biosynthesis: enzymatic pathways and neuroendocrine regulation by neurotransmitters and neuropeptides

Neuroactive steroids synthesized in neuronal tissue, referred to as neurosteroids, are implicated in proliferation, differentiation, activity and survival of nerve cells. Neurosteroids are also involved in the control of a number of behavioral, neuroendocrine and metabolic processes such as regulation of food intake, locomotor activity, sexual activity, aggressiveness, anxiety, depression, body temperature and blood pressure. In this article, we summarize the current knowledge regarding the existence, neuroanatomical distribution and biological activity of the enzymes responsible for the biosynthesis of neurosteroids in the brain of vertebrates, and we review the neuronal mechanisms that control the activity of these enzymes. The observation that the activity of key steroidogenic enzymes is finely tuned by various neurotransmitters and neuropeptides strongly suggests that some of the central effects of these neuromodulators may be mediated via the regulation of neurosteroid production.

Novel aspects of glutamatergic signalling in the neuroendocrine system

L-glutamate, the main excitatory neurotransmitter, influences virtually all neurones of the neuroendocrine hypothalamus via synaptic mechanisms. Vesicular glutamate transporters (VGLUT1-3), which selectively accumulate L-glutamate into synaptic vesicles, provide markers with which to visualise glutamatergic neurones in histological preparations; excitatory neurones in the endocrine hypothalamus synthesise the VGLUT2 isoform. Results of recent dual-label in situ hybridisation studies indicate that glutamatergic neurones in the preoptic area and the hypothalamic paraventricular, supraoptic and periventricular nuclei include parvocellular and magnocellular neurosecretory neurones which secrete peptide neurohormones into the bloodstream to regulate endocrine functions. Neurosecretory terminals of GnRH, TRH, CRF-, somatostatin-, oxytocin- and vasopressin-secreting neurones contain VGLUT2 immunoreactivity, suggesting the co-release of glutamate with hypophysiotrophic peptides. The presence of VGLUT2 also indicates glutamate secretion from non-neuronal endocrine cells, including gonadotrophs and thyrotrophs of the anterior pituitary. Results of in vitro studies show that ionotropic glutamate receptor analogues can elicit hormone secretion at neuroendocrine/endocrine release sites. Structural constituents of the median eminence, adenohypophysis and neurohypophysis contain elements of glutamatergic transmission, including glutamate receptors and enzymes of the glutamate/glutamine cycle. The synthesis of VGLUT2 exhibits robust up-regulation in response to certain endocrine challenges, indicating that altered glutamatergic signalling may represent an important adaptive mechanism. This review article discusses the newly emerged non-synaptic role of glutamate in neuroendocrine and endocrine communication.

Mechanisms of neuropeptide Y, peptide YY, and pancreatic polypeptide inhibition of identified green fluorescent protein-expressing GABA neurons in the hypothalamic neuroendocrine arcuate nucleus

The fast inhibitory transmitter GABA is robustly expressed in the arcuate nucleus (ARC) and appears to play a major role in hypothalamic regulation of endocrine function and energy homeostasis. Previously, it has not been possible to record selectively from GABA cells, because they have no defining morphological or physiological characteristics. Using transgenic mice that selectively express GFP (green fluorescent protein) in GAD67 (glutamic acid decarboxylase 67)-synthesizing cells, we identified ARC GABA neurons (n > 300) and used whole-cell recording to study their physiological response to neuropeptide Y (NPY), the related peptide YY(3-36) (PYY(3-36)), and pancreatic polypeptide (PP), important modulators of ARC function. In contrast to other identified ARC cells in which NPY receptor agonists were reported to generate excitatory actions, we found that NPY consistently reduced the firing rate and hyperpolarized GABA neurons including neuroendocrine GABA neurons identified by antidromic median eminence stimulation. The inhibitory NPY actions were mediated by postsynaptic activation of G-protein-linked inwardly rectifying potassium (GIRK) and depression of voltage-gated calcium currents via Y1 and Y2 receptor subtypes. Additionally, NPY reduced spontaneous and evoked synaptic glutamate release onto GABA neurons by activation of Y1 and Y5 receptors. The peptide PYY(3-36), a peripheral endocrine signal that can act in the brain, also inhibited GABA neurons, including identified neuroendocrine cells, by activating GIRK conductances and depressing calcium currents. The endogenous Y4 agonist PP depressed the activity of GABA-expressing neurons mainly by presynaptic attenuation of glutamate release. Together, these results show that the family of neuropeptide Y modulators reduces the activity of inhibitory GABA neurons in the ARC by multiple presynaptic and postsynaptic mechanisms.

GABAergic control of neuropeptide gene expression in parvocellular neurons of the hypothalamic paraventricular nucleus

To assess the functional impact of local inhibitory gamma-aminobutyric acid (GABA)ergic interneuron population on the cellular and transcriptional activity of parvocellular neurosecretory neurons in the hypothalamic paraventricular nucleus (PVH), we followed the expression of corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) genes along with the activation marker c-fos in response to the blockade of GABA-A receptors. First, we analysed the effect of the GABA-A receptor antagonist bicuculline methiodide (BMI) in organotypic cultures of hypothalamic slices. These preparations preserve the cytoarchitecture of CRH-synthesizing cell populations and elements of local interneuronal networks, while remote connections originating from limbic- and brainstem areas are missing. In vitro, BMI resulted in a selective induction of c-Fos immunoreactivity that was localized exclusively to the PVH and upregulated both CRH mRNA and AVP hnRNA levels. Local microinjection of BMI into the paraventricular region of freely moving rats increased the adrenocorticotropin secretion and activated PVH neurons ipsilateral to the injection. c-Fos immunoreactivity was distributed within the PVH and in the perinuclear region, where it appeared in GABAergic and also in non-GABAergic profiles. This treatment induced AVP hnRNA expression in the parvocellular compartment without any reliable stimulation of CRH transcription in the parvocellular- and AVP hnRNA levels in the magnocellular neurons. These results reveal an intrinsic GABAergic mechanism in the PVH microenvironment that by itself, without limbic contribution, impinges a tonic inhibitory influence on the parvocellular CRH neurons in vitro. In vivo, remote inputs are superimposed on the local circuit, allowing differential transcriptional regulation of CRH and AVP genes in the hypophysiotropic neurons.

Benzodiazepines and anterior pituitary function

Benzodiazepines (BDZ) are one of the most prescribed classes of drugs because of their marked anxiolytic, anticonvulsant, muscle relaxant and hypnotic effects. The pharmacological actions of BDZ depend on the activation of 2 specific receptors. The central BDZ receptor, present in several areas of the central nervous system (CNS), is a component of the GABA-A receptor, the activation of which increases GABAergic neurotransmission and is followed by remarkable neuroendocrine effects. The peripheral benzodiazepine receptors (PBR), structurally and functionally different from the GABA-A receptor, have been shown in peripheral tissues but also in the CNS, in both neurones and glial cells, and in the pituitary gland. BDZ receptors bind to a family of natural peptides called endozepines, firstly isolated from neurons and glial cells in the brain and then in several peripheral tissues as well. Endozepines modulate several central and peripheral biological activities, including some neuroendocrine functions and synthetic BDZ are likely to mimic them, at least partially. BZD, especially alprazolam (AL), possess a clear inhibitory influence on the activity of the HPA axis in both animals and humans. This effect seems to be mediated at the hypothalamic and/or suprahypothalamic level via suppression of CRH. The strong negative influence of AL on hypothalamicpituitary-adrenal (HPA) axis agrees with its peculiar efficacy in the treatment of panic disorders and depression. BZD have also been shown to increase GH secretion via mechanisms mediated at the hypothalamic or supra-hypothalamic level, though a pituitary action cannot be ruled out. Besides the impact on HPA and somatotrope function, BDZ also significantly affect the secretion of other pituitary hormones, such as gonadotropins and PRL, probably acting through GABAergic mediation in the hypothalamus and/or in the pituitary gland. In all, BDZ are likely to represent a useful tool to investigate GABAergic activity and clarify its role in the neuroendocrine control of anterior pituitary function; their usefulness probably overrides what had been supposed before.

Testosterone modulates the dendritic architecture of arcuate neuroendocrine neurons in adult male rats

Recent studies have demonstrated that gonadectomy of adult male rats induces dendritic growth of neuroendocrine neurons in the arcuate nucleus. We have hypothesized that these changes are secondary to the loss of testosterone negative feedback. In the present study, we examined the effects of testosterone replacement on the dendritic morphology of arcuate neuroendocrine neurons in castrated rats. Rats were orchidectomized and implanted with silastic capsules designed to produce physiological levels of plasma testosterone (n=9) or empty silastic capsules (n=9) for 2 months. Retrograde labeling with systemically injected Fluoro-Gold, followed by intracellular injection of labeled neurons in a fixed slice preparation, were used to visualize arcuate neuroendocrine neurons. Quantitative analysis of dendritic morphology was performed using three-dimensional computer reconstruction. Serum levels of LH (luteinizing hormone) and testosterone were measured by radioimmunoassay. Treatment of castrated rats with physiological levels of testosterone significantly reduced dendritic length, volume and terminal branch number relative to the castrated rats receiving empty silastic capsules. Dendritic spine density was also greater in the testosterone-treated animals, although the total numbers of spines per dendrite was not significantly different between the two groups. In addition, testosterone replacement was effective in reducing serum LH to levels found in intact rats. These studies demonstrate that testosterone replacement suppresses the dendritic outgrowth of arcuate neuroendocrine neurons that occurs in response to castration. The parallel changes in dendritic arbor and serum LH after castration and hormone replacement suggests that the suppressive effects of testosterone are related to steroid negative feedback.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

GABAA receptor mediated elevation of Ca2+ and modulation of gonadotrophin-releasing hormone action in alphaT3-1 gonadotropes

gamma-amino butyric acid (GABA) is the major inhibitory neurotransmitter in the CNS, mediating fast inhibitory synaptic transmission, by activating GABAA receptors. However, these GABA-gated Cl- channels can also be excitatory, causing depolarization, and increasing Ca2+ entry via voltage-operated Ca2+ channels (VOCCs). Evidence exists for excitatory ionotropic GABA receptors in anterior pituitary cells, including gonadotropes, but these have not been directly characterized and their pharmacology remains controversial. Here we have measured the cytosolic Ca2+ concentration ([Ca2+]i) in alphaT3-1 gonadotropes, to test for expression of excitatory GABA receptors. The GABAA agonists, GABA and muscimol, both caused rapid, robust and dose-dependent increases in [Ca2+]i (EC50 values 2.7 and 1 microM), whereas the GABAB agonist, baclofen, did not. The GABAA antagonist, bicuculline, inhibited muscimol's effect, whereas the GABAB antagonist, phaclofen, did not. The neuroactive steroid 5alpha-pregnan-3alpha-ol-11,20-dione (an allosteric activator of GABAA receptors) increased [Ca2+]i, and this effect, like that of muscimol, was inhibited by picrotoxin. The muscimol effect on [Ca2+]i was blocked by the VOCC antagonist, nifedipine, or by Ca2+-free medium. When cells were pretreated with muscimol this increased the spike phase of the [Ca2+]i response to subsequent stimulation with gonadotropin-releasing hormone (GnRH). Similar amplification was seen in muscimol-pretreated cells stimulated with GnRH in Ca2+-free medium, but not when cells were pretreated with muscimol in Ca2+-free medium. The amplification was not, however, GnRH receptor-specific, because the spike response to ionomycin was also increased by muscimol pretreatment. These data provide the first direct evidence for expression of excitatory GABAA receptors, and the first demonstration of acute steroid effects, on GnRH-responsive pituitary cells. They also reveal a novel mechanism by which GABAA activation modulates GnRH action, raising the possibility that this may also influence gonadotrophin secretion from non-immortalized gonadotropes.

Activation of gene expression of the gamma-aminobutyric acid rather than the glutamatergic system in the preoptic area during the preovulatory gonadotropin surge of the rat

There is increasing evidence that in the rat prior to and during the preovulatory LH surge, release rates of GABA in the preoptic area (POA) are decreased while no such changes occurred in the mediobasal hypothalamus (MBH). In addition, GnRH release appears to be facilitated by an increased preoptic excitation of glutamate (GLU). To investigate whether such changes of secretory activity of intrahypothalamic GABA or GLU neurons are associated with altered gene expression of biosynthetic enzymes or transporter proteins characteristic for either neuronal system, we determined mRNA levels of the two forms of the GABA-synthesizing enzyme glutamate decarboxylase (GAD65 and GAD67), the glutamate-synthesizing enzyme glutaminase (GLS), the GABA transporter type 1 (GAT-1) and the glutamate-aspartate transporter type 1 (GLAST). Competitive RT-PCRs using mutant cRNAs as internal standards were conducted with mRNA extracted from microdissected tissue of POA and MBH from diestrous, proestrous, and estrous rats. Proestrous animals were subgrouped according to their endocrine status as follows: 'prior to', on the 'ascending' or on the 'descending' limb of the LH peak, and 'after the LH surge (post)'. During the preovulatory LH surge, mRNA concentrations of GAD67 and GAT-1 in the POA were significantly increased compared to those observed on diestrous (2.8-fold for GAD67 and 2.5-fold for GAT-1, p < 0.01), while in the MBH the amount of both mRNAs remained constant. The expression levels of GAD65, GLS and GLAST were without any changes in the POA as well as in the MBH. These findings support the hypothesis that in rats induction of the preovulatory LH surge is controlled at the level of GnRH perikarya, and suggest that altered activities of intrapreoptic GABA neurons at both transcriptional and secretory levels are pivotal for the preovulatory activation of GnRH neurons.

Visualization of local afferent inputs to magnocellular oxytocin neurons in vitro

We recently showed that oxytocin (OT) neurons in organotypic slice cultures obtained from postnatal rat hypothalamus display complex patterns of electrical activity, similar to those of adult magnocellular OT neurons in vivo. Here we used such cultures to investigate the identity and, in particular, the origin of afferent inputs responsible for this activity. Multiple immunostaining with light and confocal microscopy showed that the somata and dendrites of oxytocinergic neurons were contacted by numerous synapses, visualized by their reaction to the synaptic markers, synaptophysin or synapsin. Many were GABAergic, displaying immunoreactivities for glutamic acid decarboxylase or gamma-aminobutyric acid (GABA); others were enriched in glutamate immunoreactivity. Such afferents presumably arose from GABA- or glutamate-immunoreactive neurons, respectively, with distinct and characteristic morphologies and topographies. A few dopaminergic boutons (tyrosine hydroxylase- or dopamine-immunopositive) impinged on OT neurons; they arose from dopamine-positive neurons located along the third ventricle. No noradrenergic profiles were detected. Despite the presence of choline acetyl-transferase (ChAT)-immunoreactive neurons, there were no cholinergic contacts. Lastly, we found oxytocinergic synapses, identified by immunoreaction for OT-related neurophysin and synapsin, contacting OT somata and dendrites. Our observations thus demonstrate that inhibitory and excitatory inputs to OT neurons derive from local intrahypothalamic GABA and glutamate neurons, in close proximity to the neurons. They also reveal that OT neurons are innervated by hypothalamic dopaminergic neurons. Finally, they confirm the existence of homotypic OT synaptic contacts which derive from local OT neurons.

GABA exerts stimulatory and inhibitory influences on gonadotropin II secretion in the Atlantic croaker (Micropogonias undulatus)

Possible involvement of gamma-aminobutyric acid (GABA) in the control of gonadotropin II (GTH II) secretion was investigated during different phases of the seasonal gonadal cycle of the Atlantic croaker (Micropogonias undulatus). Intraperitoneal administration of GABA (100 microgram/g) elicited a significant increase in plasma GTH II levels in croaker with regressed or pre-recrudescence phase gonads, whereas it did not significantly affect GTH II levels during the early- to mid-recrudescence phase of the gonadal cycle in this species. On the other hand, GABA induced a dose-related inhibition of GTH II secretion in fish with fully recrudesced gonads. Similar opposite changes in GTH II levels were observed when croaker with regressed and fully recrudesced gonads were injected with the GABA transaminase inhibitor, gamma-vinyl GABA (10 and 100 microgram/g). This suggests an involvement of endogenous GABA in the control of GTH II secretion. GABA (1-100 microM) did not significantly alter GTH II release from pituitary fragments from regressed or fully recrudesced fish incubated in vitro, suggesting the lack of a major direct influence at the pituitary level. Muscimol (GABAA receptor agonist, 1 microgram/g) treatment mimicked both the stimulatory and inhibitory effects of GABA, whereas injection of baclofen (GABAB receptor agonist, 1 microgram/g) had no significant effect on basal or a gonadotropin-releasing hormone (GnRH) analog-induced GTH II secretion. Pretreatment of fish with bicuculline (1 microgram/g), a GABAA antagonist, completely blocked the stimulatory influence of GABA on GTH II secretion in croaker with regressed gonads and partially blocked the inhibitory effect in individuals with fully recrudesced gonads. The results demonstrate that GABA exerts opposite influences on GTH II secretion at two different stages of the gonadal cycle of croaker, and acts primarily via GABAA type receptors. Thus, GABAergic pathways may be important in mediating seasonal influences on GTH II secretion in this species.

The effect of excitatory aminoacids on GABA release from mediobasal hypothalamus of female rats

The purpose of the present study was to examine the in vitro effect of L-glutamate and its agonists on basal and potassium-evoked GABA release from incubated mediobasal hypothalamus (MBH) of intact, ovariectomized (OVX) and OVX-estrogenized female rats. L-glutamate (100 microM) decreased evoked GABA release from MBH of intact female rats in diestrus. NMDA and quisqualate (10 and 100 microM) modified neither basal nor evoked hypothalamic GABA release of intact rats. However, kainate (10 and 100 microM) decreased hypothalamic basal and evoked GABA release of intact rats. Kainate induced no changes in basal or in evoked GABA release from hypothalami of OVX rats, but decreased GABA release in chronically estrogenized rats. DNQX (6,7-dinitroquinoxaline-2,3-dione), a non-NMDA receptor antagonist, failed to affect GABA release but blocked the inhibitory effect of kainate. The kainate effect was not Mg2+-sensitive and was not inhibited by D-AP5 (D(-)-2-amino-5-phosphonopentanoic acid), an NMDA-specific receptor antagonist. Kainate induced no changes in nitric oxide synthase activity in MBH of either intact or estrogenized rats. These data indicate that kainate decreases GABA release from MBH of female rats through a non-NMDA receptor subtype, and provide evidence to support the view that kainate-mediated decrease of the hypothalamic GABAergic tone is affected by estrogens.

Age-related development of gamma-aminobutyric acid (GABA)B receptor functions in various brain regions of spontaneously hypertensive rats

The age-related development of GABAB receptors and their coupling to adenylate cyclase were studied in the brains of spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. Compared with WKY rats, the specific [3H]GABA binding to GABAB receptors showed a significant decrease not only in the posterior hypothalamus, midbrain, hippocampus and striatum of eleven-week-old SHR, which maintain a hypertensive state, but also in the posterior hypothalamus of four-week-old normotensive SHR. Similarly, the GABAB receptor agonists (baclofen and DN-2327)-induced suppression of adenylate cyclase activity showed a decrease in the posterior hypothalamus of four-week-old SHR as well as in the posterior hypothalamus and striatum of eleven-week-old SHR. These results suggest that the functions of the GABAB receptor in the brain of SHR may be decreased independently from the occurrence of blood pressure elevation and that such changes may even be involved in the pathogenesis of SHR.

Electrophysiological and morphological characteristics of neurons in perinuclear zone of supraoptic nucleus

Electrophysiological and morphological characteristics of neurons in perinuclear zone of supraoptic nucleus. J. Neurophysiol. 78: 2427-2437, 1997. Neurons in the perinuclear zone (PZ) of the supraoptic nucleus (SON) are thought to serve as interneurons and may mediate changes in neurohypophysial hormone release in response to physiological changes in blood pressure. However, the morphology and electrophysiological characteristics of PZ neurons are unknown. In the present study, PZ neurons from male and female rats were recorded intracellularly to determine some membrane properties, then filled with biocytin or biotinamide for morphological analysis. In general, PZ neurons had faster spikes than magnocellular SON neurons, and the great majority were characterized by a subthreshold depolarizing hump when depolarized from a hyperpolarized (less than -80 mV) membrane potential. In most neurons, this hump was similar to low-threshold spikes described in other CNS regions. Near-threshold, fast action potentials were clustered near the onset of these depolarizations. Conspicuously absent in all PZ neurons was the strong transient and subthreshold outward rectification characteristic of vasopressin and oxytocin neurons of the SON. These results suggest that PZ neurons are electrophysiologically distinct from neurosecretory neurons of the SON. No differences were found between male and female rats in any of the basic properties examined, including input resistance, membrane time constant, spike height, spike width, spike threshold, and the size of the spike afterhyperpolarization. Morphologically, PZ neurons were diverse but were divided into spiny and aspiny groups. Three spiny neurons and one aspiny neuron contributed an axonal projection to the SON characterized by varicosities suggestive of terminals. In the case of the three spiny neurons, the SON projection was clearly a minor collateral projection. The axon arborized in the PZ, but one or more branches were cut at the edge of the explant, indicating a longer projection. In the remaining neurons, no axonal projection to the SON was detected and several had axons leaving the explant. Some portion of the dendritic tree penetrated the SON in several neurons. The morphology of PZ neurons was thus heterogeneous and suggests that, for some cells at least, the projection to the SON may be a minor collateral component of a much wider axonal projection.

Dopamine inhibition: enhancement of GABA activity and potassium channel activation in hypothalamic and arcuate nucleus neurons

Dopamine (DA) decreases activity in many hypothalamic neurons. To determine the mechanisms of DA's inhibitory effect, whole cell voltage- and current-clamp recordings were made from primary cultures of rat hypothalamic and arcuate nucleus neurons (n = 186; 15-39 days in vitro). In normal buffer, DA (usually 10 microM; n = 23) decreased activity in 56% of current-clamped cells and enhanced activity in 22% of the neurons. In neurons tested in the presence of glutamate receptor antagonists D,L-2-amino-5-phosphonovalerate (AP5; 100 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM), DA application (10 microM) revealed heterogeneous effects on electrical activity of cells, either hyperpolarization and decrease in activity (53% of 125) or depolarization and increase in spontaneous activity (22% of 125). The DA-mediated hyperpolarization of membrane potential was associated with a decrease in the input resistance. The reversal potential for the DA-mediated hyperpolarization was -97 mV, and it shifted in a positive direction when the concentration of K+ in the incubating medium was increased, suggesting DA activation of K+ channels. Because DA did not have a significant effect on the amplitude of voltage-dependent K+ currents, activation of voltage-independent K+ currents may account for most of the hyperpolarizing actions of DA. DA-mediated hyperpolarization and depolarization of neurons were found during application of the Na+ channel blocker tetrodotoxin (1 microM). The hyperpolarization was blocked by the application of DA D2 receptor antagonist eticlopride (1-20 microM; n = 7). In the presence of AP5 and CNQX, DA (10 microM) increased (by 250%) the frequency of spontaneous inhibitory postsynaptic currents (IPSCs) in 11 of 19 neurons and evoked IPSCs in 7 of 9 cells that had not previously shown any IPSCs. DA also increased the regularity and the amplitude (by 240%) of spontaneous IPSCs in 9 and 4 of 19 cells, respectively. Spontaneous and DA-evoked IPSCs and inhibitory postsynaptic potentials were blocked by the gamma-aminobutyrate A (GABA(A)) antagonist bicuculline (50 microM), verifying their GABAergic origin. Pertussis toxin pretreatment (200 ng/ml; n = 15) blocked the DA-mediated hyperpolarizations, but did not prevent depolarizations (n = 3 of 15) or increases in IPSCs (n = 6 of 10) elicited by DA. Intracellular neurobiotin injections (n = 21) revealed no morphological differences between cells that showed depolarizing or hyperpolarizing responses to DA. Immunolabeling neurobiotin-filled neurons that responded to DA (n = 13) showed that GABA immunoreactive neurons (n = 4) showed depolarizing responses to DA, whereas nonimmunoreactive neurons (n = 9) showed both hyperpolarizing (n = 6) and depolarizing (n = 3) responses. DA-mediated hyperpolarization, depolarization, and increases in frequency of postsynaptic activity could be detected in embryonic hypothalamic or arcuate nucleus neurons after only 5 days in vitro, suggesting that DA could play a modulatory role in early development. These findings suggest that DA inhibition in hypothalamic and arcuate nucleus neurons is achieved in part through the direct inhibition of excitatory neurons, probably via DA D2 receptors acting through a Gi/Go protein on K+ channels, and in part through the enhancement of GABAergic neurotransmission.

Suprachiasmatic efferents avoid phenestrated capillaries but innervate neuroendocrine cells, including those producing dopamine

The key role of the suprachiasmatic nucleus in the diurnal regulation of anterior pituitary hormone secretions, including PRL, is well established. However, the pathway via suprachiasmatic signals reach the pituitary is ill defined. To determine whether suprachiasmatic efferents innervate neuroendocrine cells, the anterograde tracer, Phaseolus vulgaris leukoagglutinin, was injected iontophoretically into the suprachiasmatic nucleus in parallel with ip administration of fluorogold (20 mg/BW in saline). After visualization of anterogradely labeled processes with a dark blue chromogen. Vibratome sections were immunostained for fluorogold. As fluorogold labeling resulted in dense immunopositive granules without diffuse cytoplasmic labeling, selected sections were further immunostained for cytoplasmic tyrosine hydroxylase (dopamine). Anterogradely labeled suprachiasmatic efferents were observed in the medial preoptic area, periventricular regions, and the lateral aspects of the arcuate and ventromedial nuclei of the hypothalamus, whereas the median eminence and organum vasculosum laminae terminalis lacked labeled suprachiasmatic projections. All of the aforementioned regions contained a high number of cells immunoreactive for fluorogold. However, immunolabeling for fluorogold revealed no retrogradely labeled (ergo neuroendocrine) cells in the suprachiasmatic nucleus. Retrogradely labeled cells in all of these hypothalamic sites, with the exception of the median eminence and organum vasculosum laminae terminalis, were targets of suprachiasmatic nucleus axon terminals. In the preoptic area, anterior hypothalamus, periventricular area, and arcuate nucleus, subpopulations of dopamine cells were retrogradely labeled. In all of these areas, both retrogradely labeled and nonlabeled dopamine cells were frequently found to be in contact with dark blue, anterogradely labeled suprachiasmatic efferents. Electron microscopic examination confirmed the putative connections to be synaptic. This experiment provided evidence that the circadian pacemaker suprachiasmatic nucleus sends efferents onto neuroendocrine cells, but has no contacts with fenestrated capillaries. It was found that a population of median eminence-projective cells targeted by suprachiasmatic axons in the hypothalamus contains dopamine. These observations indicate no direct effect of the circadian pacemaker on the anterior hypophysis, but offer an indirect pathway via circadian signals, mediated by hypothalamic neural systems, that may regulate pituitary hormone secretion, in particular PRL.

GABAergic neurons in rat nuclei of solitary tracts receive inhibitory-type synapses from amygdaloid efferents lacking detectable GABA-immunoreactivity

Gamma-aminobutyric acid (GABA) is a prominent inhibitory transmitter in both the central nucleus of the amygdala (Ce) and the medial nuclei of the solitary tracts (mNTS). These regions are reciprocally connected by anatomical pathways mediating the coordinated visceral responses to emotional stress. To further determine whether GABA is present in the amygdaloid efferents or their targets in the mNTS, we combined peroxidase labeling of Phaseolus vulgaris leucoagglutinin (PHA-L) or biotinylated dextran amine (BDA) anterogradely transported from the Ce with immunogold-silver detection of antibodies against GABA in the rat mNTS. By light microscopy, peroxidase labeling for either PHA-L or BDA was seen in varicose processes, whereas immunogold-silver labeling for GABA was detected in perikarya and processes throughout the rostrocaudal mNTS. The intermediate mNTS at the level of the area postrema, a region receiving mainly cardiorespiratory and gastric visceral afferents, were examined by electron microscopy. In this region, anterograde labeling was observed exclusively in unmyelinated axons and axon terminals. These terminals lacked detectable GABA-immunoreactivity, but formed symmetric synapses that are associated with inhibition. The targets of the anterogradely labeled terminals were medium-sized dendrites both with and without GABA-labeling. These dendrites often also received convergent input from terminals that were intensely GABA-immunoreactive. We conclude that visceral activation accompanying emotional response to stress is likely to involve inhibition of GABAergic neurons in the mNTS by non-GABA-containing amygdaloid efferents. Furthermore, our results indicate that the inhibition of these GABAergic neurons may be further augmented by release of GABA from other converging terminals in the mNTS.

Fos expression in forebrain afferents to the hypothalamic paraventricular nucleus following swim stress

The paraventricular nucleus of the hypothalamus (PVN) serves as the origin of the final common pathway in the secretion of glucocorticoid hormones in response to stress. Various stress-related inputs converge upon the cells of the medial parvocellular division of the PVN. These neurons, which synthesize and release corticotropin-releasing hormone, arginine vasopressin, and other secretagogues, are responsible for a cascade of events which culminates in the adrenocorticotropin-induced release of corticosteroids from the adrenal cortex. Previous data have suggested complex afferent regulation of PVN neurons, although the neuronal pathways by which the effects of stress are mediated remain to be fully disclosed. The present experiment sought to identify forebrain areas potentially involved in afferent regulation of the PVN in response to an acute stressor. Discrete injections of the retrograde tracer Fluoro-gold were delivered to the PVN, and rats were subsequently subjected to an acute swim stress. Brains were processed immunocytochemically for the simultaneous detection of the tracer and Fos, the protein product of the immediate early gene c-fos, utilized as a marker for neuronal activation. The majority of Fluoro-gold/Fos labeled neurons were detected in the parastrial nucleus, the medial preoptic area, the anterior hypothalamic area, the dorsomedial hypothalamic nucleus and adjacent posterior hypothalamic area, and, to a lesser extent, the supramammillary nucleus. These findings are discussed in relation to neural pathways mediating activation and inhibition of the hypothalamic-pituitary-adrenocortical axis.

Bicuculline enhances the corticosterone secretion induced by lipopolysaccharide and interleukin-1 alpha in male rats

Lipopolysaccharide (LPS)-induced inflammatory stress activates the hypothalamus-pituitary-adrenal (HPA) function. Interleukin-I (IL-1) is one of the key factors during this event; however, the mechanisms mediating IL-1 stimulation of HPA axis are still unclear. The present study evaluated the possible involvement of gamma-aminobutyric acid (GABA) in LPS-induced activation of HPA axis in adult male rats. In addition, the possible existence of diurnal changes of LPS-induced HPA axis activity was also investigated. Bicuculline (0.8 mg/kg BW), a GABA-A receptor antagonist and GABA (1 g/kg BW) were intraperitoneally (ip) injected 15 min before LPS (2 mg/kg BW, ip) or recombinant human IL-1 alpha (microgram/rat) administration in intact rats. Control animals received an equivalent volume of 0.9% saline. Rats were sacrificed at 60 min or 90 min after LPS, or IL-1 alpha or saline injection. Plasma corticosterone levels were measured by radioimmunoassay. Results showed that pretreatment with bicuculline enhanced both LPS- and IL-1-induced corticosterone secretion; while pretreatment with GABA significantly reduced the LPS-stimulated corticosterone release (p < 0.05, vs LPS alone). The effect is dependent on the time of sampling and such effect of bicuculline or GABA was not observed when rats were stimulated in the evening. In addition, the maximal changes of plasma corticosterone following LPS administration in the evening were significantly lower than in the morning (p < 0.01). The present study provides evidence that GABA is involved, at least in part, in the neuroendocrine regulation of LPS/interleukin-1a-induced corticosterone secretion via GABA-A receptor in rats. In addition, the response of plasma corticosterone to LPS has a diurnal variation, which corresponds to a diurnal change of GABAergic modulation of the immunoneuroendocrine response.

Chlordiazepoxide attenuates stress-induced activation of neurons, corticotropin-releasing factor (CRF) gene transcription and CRF biosynthesis in the paraventricular nucleus (PVN)

Corticotropin-releasing factor (CRF) plays a role in coordinating endocrine, autonomic, and behavioral responses to stressful stimuli. Benzodiazepines exert many effects which oppose those of CRF, including anxiolysis and suppression of the pituitary-adrenal axis. In the present study, we employed in situ analysis of CRF heteronucleous RNA (hnRNA) and c-fos mRNA to assess stimulus-induced CRF gene transcription rate following stress and its modulation by chlordiazepoxide (CDP). Male albino rats were exposed to restraint stress for 30 min and sacrificed 30 and 120 min after the onset of stress. Either CDP or vehicle was given intraperitoneally 60 min before stress. To determine plasma ACTH levels by immunoradiometric assay, another group of rats was decapitated 10 min after the onset of restraint stress. Restraint stress induced rapidly and significantly c-fos mRNA and CRF hnRNA expression in the PVN at the 30 min time point. Increases in both RNA copies were significantly inhibited by administration of CDP at doses of 5 and 10 mg/kg. CRF mRNA concentrations were increased significantly in the PVN 120 min after stress and again, CDP attenuated significantly these increases in the PVN. The plasma ACTH increase in response to stress was inhibited significantly by CDP administration at every dose tested. CDP did not change CRF mRNA levels in the non-stressed animal.(ABSTRACT TRUNCATED AT 250 WORDS)

Met5-enkephalin is localized within axon terminals in the subfornical organ: vascular contacts and interactions with neurons containing gamma-aminobutyric acid

Met5-enkephalin inhibits sodium and water excretion and antagonizes the central actions of angiotensin II in subfornical organ of rat brain. We examined the ultrastructural basis for enkephalin modulation in this circumventricular region. Additionally, we examined the possibility that there might be cellular sites for functional interactions involving Met5-enkephalin and gamma-aminobutyric acid (GABA), a known inhibitory transmitter throughout the central nervous system. Met5-enkephalin and GABA were identified in single coronal sections through the subfornical organ using immunoperoxidase and silver-enhanced immunogold labeling methods, respectively. Enkephalin-like immunoreactivity was most prominently localized within axon terminals. These were distributed primarily in the central, highly vascular, regions of the subfornical organ. Enkephalin-labeled terminals were apposed to the basement membranes of fenestrated capillaries and also formed symmetric, inhibitory type synapses with neurons. In terminals associated with either blood vessels or neurons, the enkephalin immunoreactivity was enriched in large (80-150 nm) dense core vesicles. The immunoreactive vesicles were usually located within portions of the axon in close proximity to astrocytic processes. In contrast, smaller vesicles in the same terminals were more often aggregated near the basement membrane of the capillaries and the active zone of the synapse. The targets of enkephalin-immunoreactive terminals were either unlabeled or GABA-labeled dendrites of local neurons. Enkephalin was also co-localized with GABA in perikarya and in axon terminals. Terminals containing only GABA were far more abundant than those containing enkephalin or enkephalin and GABA. GABA-immunoreactive terminals formed symmetric synapses on unlabeled dendrites some of which also received convergent input from terminals containing enkephalin. Additionally, the enkephalin-immunoreactive terminals were closely apposed to GABA-labeled and unlabeled terminals. These results suggest sites for nonsynaptic release of Met5-enkephalin from dense core vesicles in contact with astrocytes near blood vessels and synaptic complexes in the rat subfornical organ. Moreover, the observed dual localization and pre- and postsynaptic associations between neurons containing Met5-enkephalin and GABA indicate that inhibitory effects of opioids in the subfornical organ may be mediated or potentiated by GABA.

Kappa-selective agonists decrease postsynaptic potentials and calcium components of action potentials in the supraoptic nucleus of rat hypothalamus in vitro

To investigate the effects of the endogenous kappa-receptor agonists dynorphin and leumorphin on neurons of the supraoptic nucleus in the rat hypothalamus, intracellular recordings were made from 62 supraoptic neurons in slice preparations. Bath application of dynorphin and leumorphin at 10(-7) M to 3 x 10(-6) M decreased the spontaneous firing rate with slight hyperpolarization of the membrane potential (-3.8 +/- 0.5 mV, mean +/- S.E.M.) but did not detectably change input resistance. The inhibitory effects were blocked by the relatively selective kappa-antagonist MR-2266. The synthetic kappa-receptor agonist U-50,488H had similar inhibitory effects on supraoptic neurons. Postsynaptic potentials evoked by electrical stimulation dorsal or dorsolateral to the supraoptic nucleus were suppressed by dynorphin and leumorphin. Morphine and [D-Ala, D-Leu]enkephalin, which are relatively selective to mu- and delta-receptors, respectively, influenced the postsynaptic potentials less. Dynorphin and leumorphin also decreased the duration of action potentials that were prolonged by either bath application of tetraethylammonium chloride at 5-10 mM or intracellular injection of Cs ions from the recording electrodes which were filled with 3 M cesium citrate. The prolongation was blocked by 1 mM MnCl2 and 2 mM CoCl2, which suggested that the components were due to voltage-dependent Ca2+ influx. The results suggest that endogenous kappa-receptor agonists inhibit neurosecretory cells of the supraoptic nucleus to suppress synaptic events and Ca2+ components of action potentials.

The distribution of GABA-immunoreactive neurons in the brain of the silver eel (Anguilla anguilla L.)

The distribution of GABA-immunoreactivity was studied in the brain of the silver eel (Anguilla anguilla) by means of antibodies directed against GABA. Immunoreactive neuronal somata were distributed throughout the brain. Positive perikarya were detected in the internal cellular layer of the olfactory bulb, and in all divisions of the telencephalon, the highest density being observed along the midline. Numerous GABA-reactive cell bodies were found in the diencephalon, particularly in the preoptic and tuberal regions of the hypothalamus, and the dorsolateral, dorsomedial and ventromedial thalamic nuclei. In the optic tectum, the majority of GABA-positive cell bodies were located in the periventricular layer. A number of immunolabeled cell bodies were observed in different tegmental structures, notably the torus semicircularis. In the cerebellum, the Purkinje cells were either very intensely or very weakly immunoreactive. In the rhombencephalon, reactive cell bodies were observed in the eminentia granularis, the valvula cerebellaris, the octavolateral nucleus, the lobus vagus and in the vagal and glossopharyngeal motor nuclei. Intensely immunoreactive axons and terminals were observed in the external granular layer and internal cellular layer of the olfactory bulb. In the telencephalon, the highest density of reactive fibres and boutons was found in the fields of the medial wall. Many immunolabeled fibres were seen in the medial and lateral forebrain bundles. In the diencephalon, intense labelling of fibres and terminals were observed in the nuclei situated close to the midline. In the optic tectum the highest density of reactive fibres was seen in the sfgs, the layer to which the retina projects massively. Finally, in the rhombencephalon the strongest labelling of neurites was observed in the nuclei of the raphé, the nucleus octavocellularis magnocellularis and the nuclei of the IXth and Xth cranial nerves. The GABAergic system of the eel, which is well developed, appears to be generally comparable to that described in tetrapod vertebrates.

Cellular communication in the circadian clock, the suprachiasmatic nucleus

The hypothalamic suprachiasmatic nucleus functions as the circadian clock in the mammalian brain. Communication between the cells of the suprachiasmatic nucleus is likely to be responsible for the generation and accuracy of this biological clock. Communication between many cells of the brain is mediated by action potentials that pass down the axon and cause release of neurotransmitters at the neuronal synaptic junction. Additional mechanisms of cellular communication appear to operate in the suprachiasmatic nucleus. Several lines of evidence point to multiple modes of cellular communication: these include the continuing operation of the clock after Na(+)-mediated action potentials have been blocked, the orchestrated metabolic rhythms of suprachiasmatic nucleus cells prior to synaptogenesis, the entrainment of fetal to maternal rhythms, and the rapid recovery of function after suprachiasmatic nucleus transplants into arrhythmic rodents. Possible alternative means of intercellular communication in the suprachiasmatic nucleus are examined, including calcium spikes in presynaptic dendrites, ephaptic interaction, paracrine communication, glial mediation, and gap junctions. This paper identifies and examines some of the unanswered questions related to intercellular communication of suprachiasmatic nucleus cells.

Chlordiazepoxide attenuates stress-induced accumulation of corticotropin-releasing factor mRNA in the paraventricular nucleus

Corticotropin-releasing factor (CRF) plays a role in coordinating endocrine, autonomic and behavioral responses to stressful stimuli. Benzodiazepines exert many effects which are antithetical to those of CRF, including anxiolysis and suppression of the pituitary-adrenal axis. Although there is evidence that benzodiazepines can modulate several electrophysiological and behavioral responses to exogenous CRF, we questioned whether this class of drug might also affect CRF biosynthesis as well. We have shown previously that footshock stress increases CRF mRNA levels as monitored by in situ hybridization histochemical techniques in the paraventricular nucleus (PVN) and Barrington's nucleus (the pontine micturition center). We report here the effects of the potent benzodiazepine, chlordiazepoxide (CDP), on stress-induced CRF mRNA accumulation in these two regions. Male albino rats were exposed to electrical footshock (1.5 mA, 1-s duration, 60 times/30 min) twice daily for 4 days and sacrificed 24 h after the last shock session. Either CDP (1, 2.5, 5 or 10 mg/kg) or saline was given i.p. 30 min before each stress. Sections were hybridized with an 35S-labeled prepro-CRF cRNA probe. Relative levels of CRF mRNA were quantified by densitometry of the autoradiography with X-ray film. CRF mRNA concentrations were significantly increased in both the PVN and Barrington's nucleus after stress, and CDP attenuated these increases in the PVN. By contrast, CDP did not affect CRF mRNA accumulation in Barrington's nucleus after stress. The results suggest that the benzodiazepine, CDP, suppresses stress-induced pituitary adrenal activation at least in part through inhibition of CRF production in the PVN.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of dopamine-immunoreactive neurons and their relationships to transmitter and hypothalamic hormone-immunoreactive neuronal systems in the rat mediobasal hypothalamus. A morphometric and microdensitometric analysis

A morphometric and microdensitometric characterization of the dopamine neurons of the mediobasal hypothalamus and their relationships with several other chemically identified systems, including putative tyrosine hydroxylase-positive/dopamine-negative neurons, was carried out after visualization of dopamine content by both immunocytochemistry and the Falck-Hillarp technique. Quantitative assessment of co-existence demonstrated that more than 95% of dopamine-immunoreactive neurons also contained tyrosine hydroxylase immunoreactivity and more than 90% of growth hormone-releasing factor-immunoreactive neurons also contained tyrosine hydroxylase immunoreactivity. Morphometric and densitometric analysis of dopamine, tyrosine hydroxylase and growth hormone-releasing factor-immunoreactive neurons in the arcuate nucleus showed that dopamine/tyrosine hydroxylase-containing and growth hormone-releasing factor/tyrosine hydroxylase-containing neuronal populations are two largely segregated cell groups with specific localization in the arcuate region, rostrocaudal extension and tyrosine hydroxylase-immunoreactivity content. Morphometric characteristics of dopamine-immunoreactive neurons were shown to be equivalent to those of catecholamine fluorescent cell bodies in the arcuate region. In addition, a cell group lacking detectable catecholamine fluorescence in normal animals but accumulating L-DOPA after peripheral loading was identified and characterized from a morphometric standpoint in the ventral premammillary nucleus. Quantitative analysis of nerve terminal co-distribution in the median eminence revealed significant correlations between dopamine and other transmitter or neurohormone systems, such as gamma-aminobutyric acid, galanin, luteinizing hormone-releasing hormone, in specific subregions of the palissade zone. These data point to discrete subregions of the median eminence, which have been called 'medianosomes', as main sites of interactions between transmitter-identified nerve terminal systems in the control of hypothalamic hormone release.

Immunocytochemistry of amino-acids in the rodent pituitary using extremely specific, very high titre antisera

The compartmental interrelationships of the metabolically related amino-acids glutamate, GABA and glutamine and the metabolically unrelated amino-acids taurine and glycine in the rodent pituitary, were investigated by light microscopic immunocytochemistry using highly specific antisera. Glutamate-like immunoreactivity was abundant in astrocytes in the posterior pituitary. Glutamine immunoreactivity was present only at low levels in the posterior pituitary, but was abundant in astrocytes within the intermediate lobe. Other glia-like cells in the anterior pituitary were also glutamine-immunoreactive. GABA immunoreactivity was abundant in the intermediate lobe but absent from anterior and posterior lobes. The GABA immunoreactivity mainly took the form of small punctata, the majority of which were in intimate apposition to the glutamine-immunoreactive glia. Strong taurine immunoreactivity was present in astrocytes in the posterior pituitary but only weak labelling was present in intermediate and anterior lobes of the pituitary. Specific glycine immunoreactivity was not detected in the pituitary. These results suggest that glutamate-immunoreactive astrocytes in the posterior pituitary, unlike glia in loci such as the retina, do not convert much, if any, of their glutamate content into glutamine (or if they do, it is rapidly further metabolized to another compound), whereas those astrocytes in the intermediate lobe do contain glutamine. The spatial association of GABAergic fibres with glutamine-positive astrocytes raises the possibility that astrocytes in the intermediate lobe receive a GABAergic innervation. Glutamate, glutamine and taurine (or their metabolites) may have roles as neuroactive substances regulating pituitary secretion.

Ventral subicular interaction with the hypothalamic paraventricular nucleus: evidence for a relay in the bed nucleus of the stria terminalis

The axonal projections of the ventral subiculum to the bed nucleus of the stria terminalis (BST) were examined in the rat with the anterograde neuronal tracer Phaseolus vulgaris-leucoagglutinin (PHA-L). Axons originating in the ventral subiculum coursed to the BST through either the fimbria-fornix, or a pathway involving the stria terminalis via the amygdala. Ventral subicular axons gave rise to dense terminal networks that were preferentially distributed in medial and ventral subregions of the BST. The distribution of subicular fibers and terminals was examined in relation to BST neurons that project to the hypothalamic paraventricular nucleus (PVN). In these cases, discrete iontophoretic injections of the retrograde tracer Fluoro-gold were made in the PVN, with PHA-L delivered to the ipsilateral ventral subiculum. An immunocytochemical double-labeling protocol was then employed for the simultaneous detection of PHA-L and Fluoro-gold, and provided light microscopic evidence for subicular input to PVN-projecting cells located within the BST. In a second series of experiments, the gamma-amino butyric acid (GABA)ergic nature of the BST was examined by in situ hybridization histochemistry for detection of transcripts encoding GAD67 mRNA. The studies revealed that a high proportion of BST neurons express GAD67 transcripts. Also, experiments combining Fluoro-gold tracing with GAD67 in situ hybridization suggested that a proportion of PVN-projecting neurons in the BST are GABAergic. Taken together, the results of these sets of studies suggest that the inhibitory influences of the hippocampus on the PVN might be relayed through specific portions of the BST. These findings may have important implications for our understanding of the neural regulation of the hypothalamic-pituitary-adrenal axis.

Characterization of neurochemical phenotypes in cultured hypothalamic neurons with immunohistochemistry and in situ hybridization

The expression of neurochemical phenotypes was studied in long-term cultures of dissociated embryonic neurons from rat hypothalamus. With time in culture, these neurons establish a complex dendritic and axonal network, as indicated by staining with antibodies against microtubulin-associated protein (MAP2) and neurofilaments (SMI32 and SMI33) as well as GABA and glutamate decarboxylase mRNA immunoreactivity. Neurons expressing neuropeptide Y (NPY) mRNA and NPY peptide and opioid-like peptides as well as vasopressin were observed. Further, weakly acetylcholinesterase- and NADPH diaphorase (nitric-oxide synthase)-labelled neurons were present. In conclusion, the neurochemical phenotypes reported for hypothalamic neurons in vivo can be observed in these cultures. This indicates that the culture conditions allow morphological and molecular differentiation. These findings support the view that long-term hypothalamic cultures provide a valuable model for studying mechanisms of neurosecretion in hypothalamic networks.

Different distributions of GAD65 and GAD67 mRNAs suggest that the two glutamate decarboxylases play distinctive functional roles

Two genes encode two forms of glutamate decarboxylase, GAD65 and GAD67. Because the two GADs differ in subcellular distribution and interactions with the cofactor pyridoxal phosphate, the two enzymes may play different roles in gamma-aminobutyric acid (GABA) production. In this study we have used in situ hybridization to compare the regional and cellular distributions of the two GAD mRNAs in rat brain. Both GAD mRNAs are abundant in olfactory bulb, olfactory tubercle, zona incerta, reticular nucleus of the thalamus, oculomotor nuclei, and pontine tegmental area. GAD65 mRNA is more abundant in several structures of the visual system, including the lateral geniculate nuclei, superior colliculi, and olivary pretectal nucleus, as well as in several hypothalamic and pontine nuclei. In contrast, GAD67 mRNA is more abundant in neocortex, the granular layer of olfactory bulb, lateral and medial septum, globus pallidus, inferior colliculi, and cerebellar cortex. Both GAD mRNAs are present in interneurons as well as in projection neurons, and both are present in neurons with different types of synapses, including dendrodendiritic, axosomatic, and axodendritic synapses. GAD65 mRNA predominates in the visual and the neuroendocrine systems, which are more subject to phasic changes, while GAD67 is present at relatively higher concentrations in many tonically active neurons. GAD65 and GAD67 together may provide more flexibility in the regulation of GABA synthesis than either could alone.

Release of endogenous GABA in the posterior hypothalamus of the conscious rat; effects of drugs and experimentally induced blood pressure changes

Push-pull superfusion was used to investigate the release of endogenous GABA in the posterior hypothalamus of the conscious, freely moving rat at basal conditions and in response to centrally applied drugs or to peripherally induced blood pressure changes. After an initial, exponential decline, the release rate of GABA remained fairly constant for many hours. Fluctuations in the release rate of GABA point to the existence of an ultradian rhythm with an approximate frequency of 1 cycle/65 min. Hypothalamic superfusion with a potassium-rich (50 or 90 mmol/l) artificial cerebrospinal fluid led to a concentration-dependent increase in the GABA release. The release of GABA was also enhanced by veratridine (1 or 10 mumol/l) in a concentration-dependent way. Hypothalamic superfusion with the neutrotoxin tetrodotoxin (1 mumol/l) led to a long-lasting decrease in the GABA release. The rise in blood pressure (45 mmHg) elicited by an intravenous infusion of noradrenaline was associated with an increased release rate of GABA in the hypothalamus. Hypotension produced by nitroprusside (25 mmHg) led to a counteracting decrease in hypothalamic GABA outflow. The findings suggest that approximately 45% of the basal outflow of GABA found in the superfusate are released from GABA-ergic neurons of the posterior hypothalamus. The release rate of GABA fluctuates according to an ultradian rhythm. The modified release of GABA in response to experimentally induced blood pressure changes suggests that, in the posterior hypothalamus of the conscious rat, GABAergic neurons are involved in cardiovascular control and possess a hypotensive function.

Estradiol induces plasticity of gabaergic synapses in the hypothalamus

The number of axosomatic synapses on arcuate neurons of the adult rat hypothalamus fluctuates following the sequence of increasing circulatory estradiol during the ovarian cycle. To determine whether estrogen is affecting GABAergic synaptic contacts we studied the number of GABA-immunoreactive axosomatic synapses in adult ovariectomized rats injected either with 17 beta estradiol (100 micrograms/100 g body weight) or with sesame oil vehicle. The number of immunoreactive axosomatic synapses was significantly reduced in estradiol-treated rats (77 +/- 8 vs 56 +/- 6 synapses per 1000 microns of perikaryal membrane in control and estradiol-treated rats, respectively) while the number of non-immunoreactive synapses was not significantly affected by the hormonal treatment (44 +/- 6 vs 35 +/- 5 synapses per 1000 microns of perikaryal membrane in control and estradiol-treated rats, respectively). Estradiol administration also resulted in a significant decrease in the percentage of perikaryal membrane covered by immunoreactive synapses. These results suggest that physiological levels of estradiol may induce a remodeling of GABAergic inhibitory inputs on arcuate neurons.

Tachykinergic afferents to the rat arcuate nucleus. A combined immunohistochemical and retrograde tracing study

The location of the cells giving rise to the tachykinergic innervation of the rat arcuate nucleus was studied by combining immunohistochemistry and retrograde axonal transport of a protein-gold complex (WGA-ApoHRP-gold). Small volumes (20 nl) of this marker were injected into the arcuate nucleus of the rat. Twenty-four to 30 h later, rats were injected with colchicine. After 24-h survival time, the paraformaldehyde-fixed brains were investigated for silver intensification of the gold particles and for tachykinin immunohistochemistry. Doubly immuno-silver-labeled cells were observed mainly in brainstem structures such as raphe nuclei, central gray pontine, and laterodorsal tegmental nucleus. Intranuclear and intrahypothalamic (ventromedial, dorsomedial, premamillary, and supramamillary) cell bodies were also doubly labeled, principally ipsilateral to the injection site. Minor afferent projections arise from the medial preoptic area. This anatomohistochemical study demonstrates that the arcuate nucleus receives intra- and extrahypothalamic tachykinergic inputs and shows that infundibular neurons undergo convergent tachykinergic influences.

Amino Acid Neurotransmitters and Dopamine in Brain and Pituitary of the Goldfish: Involvement in the Regulation of Gonadotropin Secretion

An isocratic high-performance liquid chromatographic technique was developed to measure levels of gamma-aminobutyric acid (GABA), glutamate, and taurine in the brain and pituitary of goldfish. Accuracy of this procedure for quantification of these compounds was established by evaluating anesthetic and postmortem effects and by selectively manipulating GABA concentrations by intraperitoneal administration of the glutamic acid decarboxylase (GAD) inhibitor 3-mercaptopropionic acid or the GABA transaminase inhibitor gamma-vinyl GABA. The technique provided a simple, rapid, and reliable method for evaluating the concentrations of these amino acids without the use of complex gradient chromatographic systems. To investigate the relationship between neurotransmitter amino acids and the control of pituitary secretion of gonadotropin, the effects of injection of taurine, GABA, or monosodium glutamate on GABA, glutamate, taurine, and, in some instances, monoamine concentrations in the brain and pituitary were evaluated and related to serum gonadotropin levels. Injection of taurine caused an elevation in serum gonadotropin concentrations. In addition, injection of the taurine precursor hypotaurine but not the taurine catabolite isethionic acid elevated serum gonadotropin levels. Intracerebroventricular injection of either GABA or taurine also elevated serum gonadotropin concentrations. Pretreatment of recrudescent fish with alpha-methyl-p-tyrosine reduced pituitary dopamine concentrations and also potentiated the serum gonadotropin response to taurine. Injection of monosodium glutamate caused an increase of glutamate content in the pituitary at 24 h; this was followed by a decrease at 72 h after administration. Pituitary GABA, taurine, and dopamine concentrations underwent a transient depletion after monosodium glutamate administration, and this was associated with an elevation of serum gonadotropin content.(ABSTRACT TRUNCATED AT 250 WORDS)

Ibotenate lesions of the diagonal band of broca attenuate baroreceptor sensitivity of rat supraoptic vasopressin neurons

Previous electrophysiological studies in the rat suggest that neurons in the diagonal band of Broca participate in baroreceptor-induced suppression of the spontaneous activity of vasopressin-secreting neurons in the hypothalamic supraoptic nucleus. In order to test this hypothesis, extracellular recordings were obtained from phasically-active vasopressin neurons in the supraoptic nucleus of anesthetized rats injected at least 3 days previously with ibotenic acid (1.25 μg/250 nl) in the diagonal band of Broca, the medial and lateral septum, or the median preoptic nucleus. In normal rats, brief increases in blood pressure produced by injections of metaraminol (10 μg/10 μl iv) that were sufficient to activate peripheral baroreceptors, suppressed the activity of a majority (21 tested, 19 suppressed) of phasically-active vasopressin-secreting neurons. In rats with ibotenic acid lesions of the diagonal band of Broca, the number of phasically-active neurons that were baroreceptor-sensitive was significantly reduced (21 tested, 8 suppressed) while lesions of the medial and lateral septum (17 tested, 16 suppressed) or the median preoptic nucleus (21 tested, 20 suppressed) had no effect. The results support the hypothesis that diagonal band of Broca neurons participate in a central pathway mediating the inhibitory effects of peripheral baroreceptor stimulation on the activity of vasopressin-secreting neurons in the rat supraoptic nucleus.

Influence of sodium valproate on late follicular phase pulsatile LH secretion in normal women

OBJECTIVE

To study whether modulation of the GABAergic system (with sodium valproate) affects pulsatile LH secretion in the late follicular phase of normal women.

DESIGN

Fifteen normal women volunteers were studied over an 8-hour period in the late follicular phase of two successive menstrual cycles. On each occasion, blood samples were taken every 10 minutes between 1000 and 1800 h. Nine of the volunteers--the short treatment group--were administered 400 mg of sodium valproate every 8 hours on the two days preceding their second session, and a further 400 mg at 0900 h on the day of the session. The other six--the long treatment group--were administered 400 mg of sodium valproate every 8 hours on the seven days preceding their second session and at 0900 and 1400 h on the day of the session.

MEASUREMENTS

LH, oestradiol and progesterone were determined by radioimmunoassay, and sodium valproate by repolarization fluorescence spectrophotometry. Pulse detection was carried out both by the program ULTRA and by a method developed by the authors.

RESULTS

There were no significant differences in LH pulse amplitude or relative pulse amplitude between records taken in the first and second menstrual cycles, i.e. without or with prior sodium valproate treatment. Short treatment did change interpulse interval and mean secretion period, but the changes, though statistically significant, were small (about 10 minutes), so that the values for both post-treatment and control sessions were within the normal range; these parameters were unaffected by long treatment.

CONCLUSIONS

Activation of the GABAergic system with sodium valproate had no biologically significant effect on the late follicular phase pulsatile LH secretion of these normal women.

Localization of GAD-like immunoreactivity in the pancreas and stomach of the rat and mouse

The aim of this study was to localize cells immunoreactive for glutamate decarboxylase (GAD), the enzyme of GABA synthesis, in pyloric and oxyntic regions of the rat stomach as well as in the rat and mouse pancreas. GAD immunocytochemistry was carried out on polyethylene glycol or cryostat sections of alkaline paraformaldehyde fixed tissue, with simultaneous immunolabelling of various gastro-pancreatic hormones for topographical comparison. In the rat stomach, nerve fibers displaying intense GAD-like immunoreactivity were seen in the myenteric plexus, the circular muscular layer, the submucosa and the lamina propria of the mucosa. But, they were absent from the submucous plexus. Colchicine treatment of the rats allowed to detect some labelled perikarya in the myenteric plexus suggesting that the GABAergic innervation is at least partly intrinsic to the stomach. In the oxyntic and pyloric mucosa, endocrine cells appeared immunostained for GAD. However, the nature of their hormones remained unknown since double immunodetections revealed that they were immunoreactive neither for gastrin nor for somatostatin. In the rat and mouse pancreas, GAD-like immunoreactivity was found in islet cells which corresponded only to insulin-secreting cells. Somatostatin-, glucagon- and pancreatic polypeptide-immunopositive cells were devoid of GAD immunolabelling. No GAD-like immunoreactivity was detected in the exocrine tissue and innervation. These results strenghten the hypothesis that GABA is not only a neurotransmitter in the stomach but that it could also be an endocrine or paracrine factor in the stomach and pancreas.