Central GABAergic neuroendocrine regulations: pharmacological and morphological evidence

A novel GABA-mediated corticotropin-releasing hormone secretory mechanism in the median eminence

Corticotropin-releasing hormone (CRH), which is synthesized in the paraventricular nucleus (PVN) of the hypothalamus, plays an important role in the endocrine stress response. The excitability of CRH neurons is regulated by γ-aminobutyric acid (GABA)-containing neurons projecting to the PVN. We investigated the role of GABA in the regulation of CRH release. The release of CRH was impaired, accumulating in the cell bodies of CRH neurons in heterozygous GAD67-GFP (green fluorescent protein) knock-in mice (GAD67(+/GFP)), which exhibited decreased GABA content. The GABAA receptor (GABAAR) and the Na(+)-K(+)-2Cl(-) cotransporter (NKCC1), but not the K(+)-Cl(-) cotransporter (KCC2), were expressed in the terminals of the CRH neurons at the median eminence (ME). In contrast, CRH neuronal somata were enriched with KCC2 but not with NKCC1. Thus, intracellular Cl(-) concentrations ([Cl(-)]i) may be increased at the terminals of CRH neurons compared with concentrations in the cell body. Moreover, GABAergic terminals projecting from the arcuate nucleus were present in close proximity to CRH-positive nerve terminals. Furthermore, a GABAAR agonist increased the intracellular calcium (Ca(2+)) levels in the CRH neuron terminals but decreased the Ca(2+) levels in their somata. In addition, the increases in Ca(2+) concentrations were prevented by an NKCC1 inhibitor. We propose a novel mechanism by which the excitatory action of GABA maintains a steady-state CRH release from axon terminals in the ME.

Enhanced excitatory input to melanin concentrating hormone neurons during developmental period of high food intake is mediated by GABA

In contrast to the local axons of GABA neurons of the cortex and hippocampus, lateral hypothalamic neurons containing melanin concentrating hormone (MCH) and GABA send long axons throughout the brain and play key roles in energy homeostasis and mental status. In adults, MCH neurons maintain a hyperpolarized membrane potential and most of the synaptic input is inhibitory. In contrast, we found that developing MCH neurons received substantially more excitatory synaptic input. Based on gramicidin-perforated patch recordings in hypothalamic slices from MCH-green fluorescent protein transgenic mice, we found that GABA was the primary excitatory synaptic transmitter in embryonic and neonatal ages up to postnatal day 10. Surprisingly, glutamate assumed only a minor excitatory role, if any. GABA plays a complex role in developing MCH neurons, with its actions conditionally dependent on a number of factors. GABA depolarization could lead to an increase in spikes either independently or in summation with other depolarizing stimuli, or alternately, depending on the relative timing of other depolarizing events, could lead to shunting inhibition. The developmental shift from depolarizing to hyperpolarizing occurred later in the dendrites than in the cell body. Early GABA depolarization was based on a Cl(-)-dependent inward current. An interesting secondary depolarization in mature neurons that followed an initial hyperpolarization was based on a bicarbonate mechanism. Thus during the early developmental period when food consumption is high, MCH neurons are more depolarized than in the adult, and an increased level of excitatory synaptic input to these orexigenic cells is mediated by GABA.

GABAergic mediation of stress-induced secretion of corticosterone and oxytocin, but not prolactin, by the hypothalamic paraventricular nucleus

The hypothalamus-pituitary-adrenal axis (HPA) participates in mediating the response to stressful stimuli. Within the HPA, neurons in the medial parvocellular region of paraventricular nucleus (PVN) of the hypothalamus integrate excitatory and inhibitory signals triggering secretion of corticotropin-releasing hormone (CRH), the main secretagogue of adrenocorticotropic hormone (ACTH). Stressful situations alter CRH secretion as well as other hormones, including prolactin and oxytocin. Most inputs to the PVN are of local origin, half of which are GABAergic neurons, and both GABA-A and GABA-B receptors are present in the PVN. The objective of the present study was to investigate the role of GABA-A and GABA-B receptors in the PVN's control of stress-induced corticosterone, oxytocin and prolactin secretion. Rats were microinjected with saline or different doses (0.5, 5 and 50 pmol) of GABA-A (bicuculine) or GABA-B (phaclofen) antagonists in the PVN. Ten minutes later, they were subjected to a stressor (ether inhalation) and blood samples were collected 30 min before and 10, 30, 60, 90 and 120 min after the stressful stimulus to measure hormone levels by radioimmunoassay. Our results indicate that GABA acts in the PVN to inhibit stress-induced corticosterone secretion via both its receptor subtypes, especially GABA-B. In contrast, GABA in the PVN stimulates oxytocin secretion through GABA-B receptors and does not alter prolactin secretion.

Heterogeneous chloride homeostasis and GABA responses in the median preoptic nucleus of the rat

The median preoptic nucleus (MnPO) is an integrative structure of the hypothalamus receiving periphery-derived information pertinent to hydromineral and cardiovascular homeostasis. In this context, excitability of MnPO neurones is controlled by fast GABAergic, glutamatergic and angiotensinergic projection from the subfornical organ (SFO). Taking advantage of a brain slice preparation preserving synaptic connection between the SFO and the MnPO, and appropriate bicarbonate-free artificial cerebrospinal fluid (CSF), we investigated a possible implication of an active outward Cl- transport in regulating efficacy of the GABA(A) receptor-mediated inhibitory response at the SFO-MnPO synapse. When somata of the MnPO neurones was loaded with 18 mm chloride, stimulation of the SFO evoked outward inhibitory postsynaptic currents (IPSCs) in 81% of the MnPO neurones held at -60 mV. Accordingly, E(IPSC) was found 25 mV hyperpolarized from the theoretical value calculated from the Nernst equation, indicating that IPSC polarity and amplitude were driven by an active Cl- extrusion system in these neurones. E(IPSC) estimated with gramicidin-based perforated-patch recordings amounted -89.2 +/- 4.3 mV. Furosemide (100 microm), a pharmacological compound known to block the activity of the neurone-specific K(+)-Cl- cotransporter, KCC2, reversed IPSC polarity and shifted E(IPSC) towards its theoretical value. Presence of the KCC2 protein in the MnPO was further detected with immunohistochemistry, revealing a dense network of KCC2-positive intermingled fibres. In the presence of a GABA(B) receptor antagonist, high-frequency stimulation (5 Hz) of the SFO evoked a train of IPSCs or inhibitory postsynaptic potentials (IPSPs), whose amplitude was maintained throughout the sustained stimulation. Contrastingly, similar 5 Hz stimulation carried out in the presence of furosemide (50 microm) evoked IPSCs/IPSPs, whose amplitude collapsed during the high-frequency stimulation. Similar reduction in inhibitory neurotransmission was also observed in MnPO neurones lacking the functional Cl- extrusion mechanism. We conclude that a majority of MnPO neurones were characterized by a functional Cl- transporter that ensured an efficient activity-dependent Cl- transport rate, allowing sustained synaptic inhibition of these neurones. Pharmacological and anatomical data strongly suggested the involvement of KCC2, as an essential postsynaptic determinant of the inhibitory neurotransmission afferent to the MnPO, a key-structure in the physiology of the hydromineral and cardiovascular homeostasis.

GABAergic innervation of corticotropin-releasing hormone (CRH)-secreting parvocellular neurons and its plasticity as demonstrated by quantitative immunoelectron microscopy

GABA has been identified as an important neurotransmitter in stress-related circuitry mediating inhibitory effects on neurosecretory neurons that comprise the central limb of the hypothalamo-pituitary-adrenocortical axis. Using combinations of pre-embedding immunostaining and postembedding immunogold methods at the ultrastructural level, direct synaptic contacts were revealed between GABA-containing terminals and neurosecretory cells that were immunoreactive for corticotropin-releasing hormone (CRH) in the hypothalamic paraventricular nucleus (PVN). The vast majority of axo-dendritic GABA synapses was symmetric (inhibitory) type, and 46% of all synaptic boutons in the medial parvocellular subdivision of the PVN were immunoreactive to GABA. Using the disector method, an unbiased stereological method on serial ultrathin sections, the total calculated number of synaptic contacts within the medial parvocellular subdivision of the PVN was 55.4 x 10(6)/mm(3). On CRH-positive profiles 20.1 x 10(6) GABAergic synaptic boutons were detected per mm(3) in control, colchicine-treated rats. In the medial parvocellular subdivision, 79% of GABAergic boutons terminated on CRH neurons. Following adrenalectomy, which increases the synthetic and secretory activities of CRH neurons, the number of GABAergic synapses that terminate on CRH-positive profiles was increased by 55%. GABA-containing boutons appeared to be swollen, while the contact surfaces of cellular membranes between GABAergic boutons and CRH-positive profiles were shorter in adrenalectomized animals than in controls. Our data provide ultrastructural evidence for direct inhibitory GABAergic control of stress-related CRH neurons and suggest a pivotal role of GABA-containing inputs in the functional plasticity of parvocellular neurosecretory neurons seen in response to adrenalectomy.

GABA(A) receptor subunit expression within hypophysiotropic CRH neurons: a dual hybridization histochemical study

Dual hybridization histochemical studies were conducted to investigate the extent of colocalization of mRNA transcripts encoding the alpha1-2 and beta1-3 subunits of the gamma aminobutyric acid (GABA)(A) receptor with those for corticotropin-releasing hormone (CRH) within the rat hypothalamic paraventricular nucleus (PVN). A vast majority of CRH neurons (>94.5%) were found to express transcripts specific for the the alpha2, beta1 and beta3 subunits; mRNAs for the alpha1 and beta2 subunits of the GABA(A) receptor were detected within 53.3% and 65.7% of PVN CRH neurons, respectively. The results may have important implications for studies aimed at understanding GABAergic influences upon the hypothalamic-pituitary-adrenocortical (HPA) axis. Hypophysiotropic CRH neurons serve as the origin of the final common pathway for glucocorticoid secretion in response to stressful stimuli, and GABAergic afferents have been implicated in afferent control of these neurons. The subunit composition of GABA(A) receptors at this key regulatory locus may affect the efficacy of a major inhibitory input, and thus the magnitude and/or duration of stress-induced glucocorticoid secretion. The present findings reveal basal expression patterns of transcripts encoding several subunits of the GABA(A) receptor within stress-integrative CRH neurons, data which may be used to guide regulatory studies of GABAergic influences on the HPA axis under a variety of conditions.

Kainate acts at presynaptic receptors to increase GABA release from hypothalamic neurons

Recent reports suggest that kainate acting at presynaptic receptors reduces the release of the inhibitory transmitter GABA from hippocampal neurons. In contrast, in the hypothalamus in the presence of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor antagonists [1-(4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466) and D,L-2-amino-5-phosphonopentanoic acid (AP5)], kainate increased GABA release. In the presence of tetrodotoxin, the frequency, but not the amplitude, of GABA-mediated miniature inhibitory postsynaptic currents (IPSCs) was enhanced by kainate, consistent with a presynaptic site of action. Postsynaptic activation of kainate receptors on cell bodies/dendrites was also found. In contrast to the hippocampus where kainate increases excitability by reducing GABA release, in the hypothalamus where a much higher number of GABAergic cells exist, kainate-mediated activation of transmitter release from inhibitory neurons may reduce the level of neuronal activity in the postsynaptic cell.

GABAB receptor-mediated inhibition of GABAA receptor calcium elevations in developing hypothalamic neurons

In the CNS, gamma-aminobutyric acid (GABA) affects neuronal activity through both the ligand-gated GABAA receptor channel and the G protein-coupled GABAB receptor. In the mature nervous system, both receptor subtypes decrease neural excitability, whereas in most neurons during development, the GABAA receptor increases neural excitability and raises cytosolic Ca2+ levels. We used Ca2+ digital imaging to test the hypothesis that GABAA receptor-mediated Ca2+ rises were regulated by GABAB receptor activation. In young, embryonic day 18, hypothalamic neurons cultured for 5 +/- 2 days in vitro, we found that cytosolic Ca2+ rises triggered by synaptically activated GABAA receptors were dramatically depressed (>80%) in a dose-dependent manner by application of the GABAB receptor agonist baclofen (100 nM-100 microM). Coadministration of the GABAB receptor antagonist 2-hydroxy-saclofen or CGP 35348 reduced the inhibitory action of baclofen. Administration of the GABAB antagonist alone elicited a reproducible Ca2+ rise in >25% of all synaptically active neurons, suggesting that synaptic GABA release exerts a tonic inhibitory tone on GABAA receptor-mediated Ca2+ rises via GABAB receptor activation. In the presence of tetrodotoxin the GABAA receptor agonist muscimol elicited robust postsynaptic Ca2+ rises that were depressed by baclofen coadministration. Baclofen-mediated depression of muscimol-evoked Ca2+ rises were observed in both the cell bodies and neurites of hypothalamic neurons taken at embryonic day 15 and cultured for three days, suggesting that GABAB receptors are functionally active at an early stage of neuronal development. Ca2+ rises elicited by electrically induced synaptic release of GABA were largely inhibited (>86%) by baclofen. These results indicate that GABAB receptor activation depresses GABAA receptor-mediated Ca2+ rises by both reducing the synaptic release of GABA and decreasing the postsynaptic Ca2+ responsiveness. Collectively, these data suggest that GABAB receptors play an important inhibitory role regulating Ca2+ rises elicited by GABAA receptor activation. Changes in cytosolic Ca2+ during early neural development would, in turn, profoundly affect a wide array of physiological processes, such as gene expression, neurite outgrowth, transmitter release, and synaptogenesis.

GABA immunoreactivity in hypothalamic neurons and growth cones in early development in vitro before synapse formation

GABA (gamma-aminobutyrate) is the most prevalent inhibitory transmitter in the mature hypothalamus. In contrast, in the developing hypothalamus, GABA may exert depolarizing actions leading to neuronal excitation. To determine whether GABA is present in hypothalamic neurons early in development, and whether there is a preferential expression in axonal growth cones, immunogold and peroxidase studies were used with light and whole mount transmission electron microscopy. At embryonic day 15, a stage of development at the beginning of hypothalamic neurogenesis, histological sections showed GABA immunoreactivity in fibers and weakly stained perikarya. Hypothalamic neurons (13%) cultured at embryonic day 15 were immunoreactive after 1 day in vitro. The percentage of neurons stained, and the intensity of staining increased during the next few days to 39% at 4 days in vitro. Neuritic growth cones, including lamellipodia and long filopodia, showed strong immunoreactivity before synaptogenesis. By using neuronal whole mounts studied with transmission electron microscopy and GABA silver-enhanced immunogold staining, a quantitative comparison of growth cones after a day and a half in culture revealed that the growth cone of the longest process, the putative axon, had a greater level of immunogold labeling than that of the shorter processes, the putative dendrites. This finding is one of the earliest biochemical differences between putative axons and dendrites. Astrocytes in the same cultures showed no immunolabeling. These results indicate that GABA is present very early in the development of hypothalamic neurons and is in a position to be released.

Excitatory actions of GABA after neuronal trauma

GABA is the dominant inhibitory neurotransmitter in the CNS. By opening Cl- channels, GABA generally hyperpolarizes the membrane potential, decreases neuronal activity, and reduces intracellular Ca2+ of mature neurons. In the present experiment, we show that after neuronal trauma, GABA, both synaptically released and exogenously applied, exerted a novel and opposite effect, depolarizing neurons and increasing intracellular Ca2+. Different types of trauma that were effective included neurite transection, replating, osmotic imbalance, and excess heat. The depolarizing actions of GABA after trauma increased Ca2+ levels up to fourfold in some neurons, occurred in more than half of the severely injured neurons, and was long lasting (>1 week). The mechanism for the reversed action of GABA appears to be a depolarized Cl- reversal potential that results in outward rather than inward movement of Cl-, as revealed by gramicidin-perforated whole-cell patch-clamp recording. The consequent depolarization and resultant activation of the nimodipine sensitive L- and conotoxin-sensitive N-type voltage-activated Ca2+ channel allows extracellular Ca2+ to enter the neuron. The long-lasting capacity to raise Ca2+ may give GABA a greater role during recovery from trauma in modulating gene expression, and directing and enhancing outgrowth of regenerating neurites. On the negative side, by its depolarizing actions, GABA could increase neuronal damage by raising cytosolic Ca2+ levels in injured cells. Furthermore, the excitatory actions of GABA after neuronal injury may contribute to maladaptive signal transmission in affected GABAergic brain circuits.

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.

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.

Local origins of some GABAergic projections to the paraventricular and supraoptic nuclei of the hypothalamus in the rat

Axonal transport and immunohistochemical methods were used to characterize the organization of glutamic acid decarboxylase-immunoreactive (GAD-ir) projections to the paraventricular (PVH) and supraoptic (SO) nuclei of the hypothalamus in the rat. In line with prior reports, GAD-ir varicosities were found to be densely and quite uniformly distributed throughout the hypothalamus, including the PVH and the SO. Nonetheless, the periventricular part of the PVH was consistently found to contain a disproportionately high density of GAD-ir elements. Small crystalline implants of the retrograde tracer, true blue, into the PVH labeled GAD-ir cells in the anterior perifornical region, portions of the anterior hypothalamic area immediately ventral to the PVH, a region just dorsal to the rostral SO and extending caudomedially over the optic chiasm and tract, and within the anterior one-third of the PVH itself. Because possible uptake of retrograde tracer by local dendritic processes might have yielded false positive filling of nearby GAD-ir cells, anterograde transport, Phaseolus vulgaris-leucoagglutinin, and combined anterograde transport-immunohistochemical methods were used to attempt to confirm these four putative local sources of GAD-ir inputs. Tracer injections in each of the above mentioned regions labeled sparse to moderate axonal projections to the PVH, which ramified preferentially in the parvicellular division of the nucleus. Projections to the magnocellular division of the PVH and the SO were generally sparse and inconsistently observed in this material. A variable, and generally small, proportion of anterogradely labeled axons and terminals in the PVH also displayed GAD-ir. These results suggest that GABAergic projections to visceromotor cell types in the PVH and SO arise, at least in part, from several diffusely distributed local sources. The fact that these afferents were found to terminate preferentially in the parvicellular division of the PVH makes it likely that additional sources of GABAergic projections to the magnocellular neurosecretory system remain to be identified. Peri- and intranuclear GABAergic neurons could provide an intermediary by which documented (and generally inhibitory) limbic system influences on neuroendocrine function are exerted.

Testosterone enhances GABA and taurine but not N-methyl-D,L-aspartate stimulation of gonadotropin secretion in the goldfish: possible sex steroid feedback mechanisms

The effects of gonadal steroids on GABA-, taurine (TAU)- and N-methyl-D,L-aspartate (NMA)-induced gonadotropin-II (GTH-II) release were investigated in male and female goldfish in vivo. In sexually regressed goldfish (both sexes mixed), intraperitoneal implantation for 5 to 10 days with solid Silastic pellets containing testosterone (100 micrograms/g), oestradiol (100 micrograms/g) or progesterone (100 micrograms/g) was previously shown to elevate serum sex steroid levels to values comparable to those in sexually mature animals, and to potentiate gonadotropin-releasing hormone-stimulated GTH-II release. In the present study, testosterone but not oestradiol or progesterone enhanced the stimulatory effects of exogenous GABA (100 micrograms/g) on GTH-II release in vivo. TAU (1 mg/g) stimulated GTH-II release in sexually regressed mixed sex and sexually recrudescent male goldfish, and both testosterone and oestradiol implantation enhanced GTH-II release induced by TAU. The glutamate agonist NMA (25 to 50 micrograms/g) was also found to stimulate GTH-II release; however it was relatively less effective in elevating serum GTH-II levels than GABA and TAU, and its effects were not modulated by sex steroid treatments. Pretreatment of goldfish with alpha-methyl-p-tyrosine to deplete brain and pituitary catecholamines did not affect NMA action on GTH-II release. Our results indicate that GABA, TAU and NMA are involved in the neuroendocrine regulation of GTH-II release in goldfish, and support the idea that testosterone participates in the positive feedback regulation of pituitary gonadotropin release in a non-mammalian vertebrate by enhancing GABA- and TAU-stimulated GTH release in vivo.

Converging GABA- and glutamate-immunoreactive axons make synaptic contact with identified hypothalamic neurosecretory neurons

To study the neurochemical identity of axons in synaptic contact with identified hypothalamic neurosecretory neurons in rats, we combined retrograde axonal transport of a marker molecule with postembedding immunogold staining for amino acid neurotransmitters. After intravenous injections of horseradish peroxidase, neurosecretory neurons with axons in the median eminence or neurohypophysis transported the peroxidase retrogradely back to the cell body of origin. Serial ultrathin sections from the paraventricular and arcuate nuclei were immunostained with glutamate or GABA antisera. Peroxidase-labeled neurons and their dendrites received synaptic contact from colloidal gold-labeled axons immunoreactive for GABA or for glutamate. Axons which were highly immunoreactive for GABA and other axons immunoreactive for glutamate but not for GABA consistently made converging synaptic contact with the same peroxidase-labeled cell. Some of the peroxidase-labeled neurons from the arcuate nucleus which were postsynaptic to both GABA and glutamate axons were themselves identified as being GABA immunoreactive. Serial ultrathin sections revealed that multiple presynaptic axons immunoreactive for glutamate or GABA made repeated contacts with single neurons. These results suggest a widespread convergence of the major inhibitory and excitatory amino acid transmitter on the neurons which control both the anterior and posterior pituitary hormones.

GABAergic neurons and circuits in the pretectal nuclei and the accessory optic system of mammals

Two classes of GABAergic cell bodies have been described. They probably can be divided into GABAergic local interneurons and GABAergic projection neurons. GABAergic cell bodies receive few terminals which is in contrast to non-GABAergic somata, which receive many synaptic contacts. GABAergic dendrites that originate from GABAergic cell bodies, however, receive numerous terminals, both GABAergic and nonGABAergic. It can therefore be concluded that somatic inhibition is not present on GABAergic neurons, but does occur on nonGABAergic neurons. Furthermore, dendrites traverse large parts of the NOT/DTN forming a complex network that enables sampling and integration from a wide area. The projection to the IO is not GABAergic itself, but cells projecting to the IO receive a substantial GABAergic input, that probably originates in part from the MTN. Further investigation on the distribution of this input over a completely identified neuron would provide the quantitative data that are required to verify the above mentioned hypothesis. A GABAergic projection that originates in the pretectal nuclei is directed towards the superficial layers of the SC in the cat (Appell and Behan, 1990) and rat (Van der Want et al., 1991). A second GABAergic projection derives from the pretectum and reaches the LGN (Cucchiaro et al., 1991). Whether this projection originates from the same GABAergic cell bodies that project to the SC and the LGN or is derived from different populations remains to be determined. The ultrastructural studies of the NOT/DTN complex have shown that GABAergic terminals with different morphological characteristics are present and that the GABA positive F and P terminals are widely distributed over somata and the adjacent neuropil. The P terminals probably originate from dendrites of GABAergic interneurons while the F types originate from GABAergic projection and interneurons (Van der Want and Nunes Cardozo, 1988). One of these sources is located in the MTN differ from the intrinsic GABAergic terminals with respect to their relation to R terminals. GABAergic MTN terminals were never observed to receive R terminal input. This is in contrast with other GABAergic terminals which frequently do receive direct contact from R terminals. Within glomeruli triadic arrangements, formed by a single retinal terminal, a dendritic profile and second axonal profile dendritic profile and second axonal profile synapsing with the dendrite, were frequently encountered in the OPN (Campbell and Lieberman, 1985), but only occasionally in the NOT/DTN (Nunes Cardozo and Van der Want, 1987).(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Retinohypothalamic tract in the female albino rat: a study using horseradish peroxidase conjugated to cholera toxin

There are several anatomically and functionally distinct retinofugal pathways, one of which is the retinohypothalamic tract (RHT). In this study, horseradish peroxidase conjugated to cholera toxin (CT-HRP), a sensitive neural tracer, was employed to describe the RHT in the female albino rat. Following uniocular injection of CT-HRP, both medial and lateral components of the RHT were evident. The medial component swept caudally into and through the suprachiasmatic nucleus (SCN) and dorsally to the subparaventricular zone. Terminal label was seen in the medial preoptic region, peri-SCN area, retrochiasmatic area, periventricular nucleus, anterior and central parts of the anterior hypothalamic area, and the subparaventricular zone. In contrast to the more focused and symmetrical medial component, the lateral component was diffuse with light terminal label in the lateral preoptic region, olfactory tubercle, lateral hypothalamus, supraoptic nucleus, and medial and posteroventral medial amygdaloid nuclei. The striking exception to this diffuse pattern of the lateral component was an extremely dense columnar terminal field over the dorsal border of the supraoptic nucleus. Whereas the intensity of label in terminal fields of the medial component was often similar on the sides ipsilateral and contralateral to the injection, the lateral component was consistently asymmetrical with greater labeling on the side contralateral to the injection. In addition, a light projection arrived at several thalamic nuclei by returning toward the thalamus from the tectal or pretectal areas via stria medullaris, and thus was not a part of the RHT. Implications for circadian as well as noncircadian photobiologic effects are discussed.

GABA: a dominant neurotransmitter in the hypothalamus

To study the organization and distribution of the inhibitory amino acid neurotransmitter GABA in the medial hypothalamus, we used a postembedding immunocytochemical approach with colloidal gold. Quantitative analysis showed that half (49%) of all synapsing boutons studied were immunoreactive for GABA, based on immunogold staining of the suprachiasmatic, arcuate, supraoptic, and paraventricular nuclei. This was corroborated with pre-embedding peroxidase immunostaining with antisera against glutamate decarboxylase, the GABA synthetic enzyme. These data suggest that GABA is the numerically dominant neurotransmitter in the hypothalamus, and emphasize the importance of inhibitory circuits in the hypothalamus. Serial ultrathin sections were used to reconstruct GABA immunoreactive boutons and axons in three dimensions. With this type of analysis we found less morphological heterogeneity between GABA immunoreactive boutons than with single ultrathin sections. Single sections sometimes showed boutons containing only small clear vesicles, and other with both clear vesicles and small dense core vesicles. However, with serial sections through individual boutons, dense core vesicles were consistently found at the periphery of the pre-synaptic GABA immunoreactive boutons, suggesting probable co-localization of GABA with unidentified peptides in most if not all boutons throughout the hypothalamus. A positive correlation was found between the density of small clear vesicles and the intensity of immunostaining with colloidal gold particles. GABA immunoreactive axons generally made symmetrical type synaptic specializations, although a small percentage made strongly asymmetrical synaptic specializations. Vesicles in GABA immunoreactive boutons were slightly smaller than those in non-reactive boutons. Synaptic efficacy is related to the position of the synapse on the post-synaptic neuron. While the majority of GABA immunoreactive axons made synaptic contact with dendrites, the distribution of GABA immunoreactive synapses on somata and dendrites was the same as would be expected from a random distribution of all boutons. No preferential innervation of cell bodies by GABA immunoreactive terminals was found. Serial ultrathin sections showed that a GABA immunoreactive axon would sometimes make repeated synaptic contacts with a single postsynaptic neuron, indicating a high degree of direct control by the presynaptic GABAergic cell. Other immunoreactive axons made synaptic contact with a number of adjacent dendrites and cells, suggesting a role for GABA in synchronizing the activity of hypothalamic neurons. Based on the density of immunogold particles per unit area, varying concentrations of immunoreactive GABA were found in different presynaptic boutons in the hypothalamus.

Evidence of an endogenous forebrain GABAergic system capable of inhibiting baroreceptor-mediated vasopressin release

In conscious rats, intracerebroventricular (i.c.v.) injections of gamma-aminobutyric acid (GABA), a GABA-uptake inhibitor (nipecotic acid), and artificial CSF (aCSF) were restricted to forebrain regions and their effect on baroreceptor-mediated arginine-vasopressin (AVP) release was studied. AVP release was stimulated by the hypotension resulting from combined treatment with a converting enzyme inhibitor (CEI) and chlorisondamine (CHLOR), a ganglionic blocking agent. CEI + CHLOR reduced mean arterial pressure (MAP) from 118 +/- 2 to 63 +/- 2 mm Hg, but pressure then rose to a compensated level of 78 +/- 1 mm Hg. The compensation in MAP was shown to be AVP-dependent at the end of the experiment since the vascular AVP antagonist, d(CH2)5Tyr(Me)AVP, reduced MAP from 78 +/- 1 to 63 +/- 1 mm Hg. While AVP was contributing to MAP maintenance, GABA (15, 50 and 150 micrograms) caused dose-related reductions in MAP (5 +/- 1.7 +/- 1 and 11 +/- 2 mm Hg, respectively). Nipecotic acid (3-350 micrograms) also caused dose-related reductions in MAP (from 3 +/- 1 to 15 +/- 2 mm Hg), while aCSF had no effect on MAP. Pretreatment with d(CH2)5Tyr(Me)AVP, antagonized completely the depressor effects of GABA and nipecotic acid. In other rats, blood samples were taken to measure the changes in plasma AVP concentrations (pAVP) induced by CEI + CHLOR and subsequent treatment with aCSF or nipecotic acid (175 micrograms). Hypotension induced by CEI + CHLOR caused a significant increase in pAVP. Forebrain-restricted nipecotic acid significantly suppressed pAVP (61 +/- 8% reduction; P less than 0.05 vs aCSF). These data provide evidence of an endogenous forebrain GABAergic system which, when activated, can inhibit baroreceptor-mediated AVP release.

Neuroendocrine control of blood tonicity and volume

The numerous studies cited in this chapter clearly show that very complex mechanisms are involved in the cardiovascular and osmotic regulation of AVP release. Information from peripheral receptors within the cardiovascular system and from central osmoreceptors is carried by multisynaptic neural inputs to the SON and PVN. This information is co-ordinated at one or more sites within the hypothalamus, the brain stem, or even the neurohypophysis itself, to ensure release of the appropriate amount of AVP. In many cases the origin of the pathway and the nature of the neurotransmitter is known, but further studies are needed to establish their physiological role in AVP release.

Ultrastructural relationships of serotonin and GABA terminals in the rat suprachiasmatic nucleus. Evidence for a close interconnection between the two afferent systems

Serotonin (5-HT) and gamma-aminobutyric acid (GABA) nerve endings were identified in the rat suprachiasmatic nucleus (SCN) by combined [3H]5-HT uptake radioautography and glutamate decarboxylase (GAD) immunocytochemistry at the electron microscope level. In areas of good overlap between radioautographic and immunocytochemical labellings, there were no axonal varicosities exhibiting both labellings, indicating that 5-HT and GABA are not co-localized in the SCN. The systematic survey in these areas of all profiles that had accumulated [3H]5-HT and of all GAD-immunoreactive varicosities allowed the analysis of 247 of the former and 896, i.e. an almost four-fold greater number, of the latter. This seems concordant with the view that GABA endings would be the most numerous of all classes of nerve terminals so far identified in the SCN. More than 22% of the [3H]5-HT labelled profiles showed the membrane specialization typically associated with synaptic junctions. Thereby, it was possible to evaluate that about 45% of the 5-HT terminals actually form a synapse in the SCN. Some 37% of the GAD-positive varicose profiles which could be formally interpreted also showed well differentiated synaptic contacts, suggesting that the GABAergic innervation of the SCN could be entirely junctional. Whereas 5-HT terminals usually innervated only one dendritic or somatic element, a convergence of several GABAergic terminals onto the same postsynaptic target also receiving a 5-HT input was frequently observed.(ABSTRACT TRUNCATED AT 250 WORDS)

GABA immuno-electron microscopic study of the nucleus of the optic tract in the rabbit

The pretectal nucleus of the optic tract (NOT) was investigated immunocytochemically with an antiserum against gamma aminobutyric acid (GABA) employing the pre-embedding peroxidase antiperoxidase technique at the light microscopic level and the postembedding colloidal gold technique at the electron microscopic level. GABA immunoreactivity was observed in cell bodies of different sizes and as punctate structures in the neuropil. In the electron microscope, besides immunoreactive dendrites, four different types of terminals were found to be GABA-immunopositive; three types of terminals with clustered and flattened vesicles (F-profile) and a fourth type with pleomorphic vesicles, presumably of dendritic origin (P-profile). Both P- and F-profiles formed symmetrical synapses with dendritic profiles arranged in clusters ensheathed by glial elements. GABA-immunopositive terminals were observed in synaptic contact with somata and retinal terminals (R-profiles) that were always GABA-immunonegative. Some GABA-immunopositive somata showed presynaptic contacts with dendrites. The presence of GABA in numerous distinct elements in the NOT and the diversity in labeled somata and terminals demonstrate the importance of the inhibitor neurotransmitter in the NOT and suggest that its function is not limited to interneurons.

Central inhibition by gamma-aminobutyric acid of the release of vasopressin by carbachol in the rat

1. gamma-Aminobutyric acid (GABA) inhibited the antidiuretic response and the increased urinary excretion of vasopressin produced by carbachol when both drugs were injected into a lateral cerebral ventricle (i.c.v.) in the water-loaded rat under ethanol anaesthesia. 2. The inhibitory effect of GABA was mimicked by muscimol and 3-amino-1-propane sulphonic acid (3-APS) and blocked by bicuculline. 3. GABA injected i.v. or into the cisterna magna (i.cist.) did not inhibit the release of vasopressin by carbachol injected i.c.v. 4. The results suggest a role for GABA as a putative inhibitory transmitter in the hypothalamo-neurohypophysial system, acting directly on the supraoptic or paraventricular nuclei in the anterior hypothalamus.

Ultrastructural localization of GABA in the supraoptic nucleus and neural lobe

Antibodies directed against the neurotransmitter gamma-aminobutyric acid (GABA) enabled the ultrastructural localization of GABA in conventional glutaraldehyde fixed and osmium postfixed material of the rat supraoptic nucleus and neural lobe. GABA was visualized using immunogold postembedding staining in axonal profiles that terminate on dendrites, axons or cell bodies throughout the supraoptic nucleus. The optimum ultrastructural preservation made possible the visualization of GABA terminals, also in the neural lobe. Here GABA axons were found to terminate synaptically on pituicytes and axonal profiles containing large dense core vesicles. These results emphasize, from an anatomical point of view, the potency of GABA to influence, as a transmitter, the release of vasopressin and oxytocin, both at the level of the cell body and of the neural lobe.

Immunocytochemical study of the GABAergic innervation of the mouse pituitary by use of antibodies against gamma-aminobutyric acid (GABA)

The GABAergic innervation of the mouse pituitary, including the median eminence, was studied at light-microscopic and ultrastructural levels by use of a pre-embedding immunocytochemical technique with antibodies directed against GABA. In the median eminence, a high density of GABA-immunoreactive fibers was found in the external layer where the GABAergic varicosities were frequently observed surrounding the blood vessels of the primary capillary plexus. In the internal and subependymal layers, only few fibers were immunoreactive. The intense labeling of the external layer was observed in the entire rostro-caudal extent of the median eminence. In the pituitary proper, a dense network of GABA-immunoreactive fibers was revealed throughout the neural and intermediate lobes, entering via the hypophyseal stalk. The anterior and tuberal lobes were devoid of any immunoreactivity. The GABA-immunoreactive terminals were characterized in the median eminence, and in the intermediate and posterior lobes at the electron-microscopic level. They contained small clear vesicles, occasionally associated with dense-core vesicles or neurosecretory granules. In the intermediate lobe they were seen to be in contact with the glandular cells. In the posterior lobe and in the median eminence, GABA-immunoreactive terminals were frequently located in the vicinity of blood vessels. These results further support the concept of a role of GABA in the regulation of hypophyseal functions, via the portal blood for the anterior lobe, directly on the cells in the intermediate lobe, and via axo-axonic mechanisms in the median eminence and posterior lobe.

Immunohistochemical evidence of plasticity of gamma-aminobutyric acid neurons in the red nucleus and adjacent reticular formation after contralateral cerebellectomy in the adult cat

A comparative mapping of gamma-aminobutyric acid neurons identified by means of glutamic acid decarboxylase (GAD) immunohistochemistry was performed in the red nucleus (RN) in both intact and hemicerebellectomized adult cats (21 days postoperative). In the deafferented RN (contralateral to the lesion) as well as in the adjacent dorsolateral reticular formation, a marked increase in the number of GAD-positive perikarya was observed. In this mesencephalic area, some neurons may therefore increase their endogenous levels of immunodetectable GAD, as a response to cerebellar deafferentation. This can be viewed as one of the events contributing to functional recovery.

Effect of GABAergic treatment on resting and stress-induced adrenocortical activities in the thalamic pigeon

Muscimol was injected (0.5 mg kg-1) intravenously to either intact or thalamic-lesioned animals through a chronic catheter. Plasma samples were obtained at 7-min intervals and exhibited a moderate and transient increase in corticosterone levels in both groups, together with some symptoms of light discomfort (e.g., ptiloerection or panting). All these signs disappeared within 45 to 60 min. Intraventricular injection of bicuculline (3.5 micrograms) provoked a corticosterone profile quite similar to a stress-induced polyphasic pattern. Acute stress (electrical footshocks for 30 sec) was applied 2 hr after drug treatment. In controls, the stress-induced adrenocortical response appeared to be markedly modified after muscimol injection. The usual polyphasic rebounding profile was replaced by a monophasic one. This effect of muscimol did not occur when bicuculline had been injected in the 3rd ventricle. Bicuculline alone did not alter the stress-induced profile. GABAergic treatment had no effect on the monophasic response of thalamic-lesioned birds.

GABAergic and catecholaminergic synaptic interactions in the macaque hypothalamus: double label immunostaining with peroxidase-antiperoxidase and colloidal gold

Immunogold staining (IGS) for glutamic acid decarboxylase (GAD) was combined with the peroxidase-antiperoxidase (PAP) technique for tyrosine hydroxylase (TH) to analyze gamma-aminobutyric acid-catecholaminergic neuronal interactions in the rhesus hypothalamus. At the light-microscopic level, TH-immunoreactive (-IR) perikarya and their fibers (brown) were observed in the anterior ventral periventricular area (AVPV), the arcuate nucleus (ARC) and the adjacent periventricular zone (ARC-PVZ). GAD-IR processes (light red) were also present throughout the hypothalamus and appeared to contact some TH-IR neurons. At the electron-microscopic level, PAP was present in perikarya, dendrites, axons and axon terminals of TH-IR neurons. Colloidal gold particles (15 nm) were found only in dendrites and axon terminals of GAD-IR neurons. Labeled GAD terminals typically contained small, clear synaptic vesicles, while TH terminals contained these and sometimes one or two dense-core vesicles. In the ARC and ARC-PVZ, asymmetrical (Gray I) axodendritic synapses occurred between GAD and TH-IR profiles, with TH/GAD directionality more prevalent. Symmetrical (Gray II) synapses were less common, with either TH or GAD presynaptic in axodendritic and dendrodendritic contacts. GAD/GAD interactions were not observed, but TH/TH contacts appeared to be mostly dendrodendritic. In the AVPV, only symmetrical synapses were encountered, and their directionality was difficult to determine. GAD- and TH-IR dendrites frequently established dendrodendritic synapses, but GAD/TH dendrosomatic synapses were seldom seen. These results illustrate the complex interactions of GAD- and TH-containing elements in the neuroendocrine hypothalamus.

Lordosis behavior and GABAergic neurotransmission

gamma-Aminobutyric acid (GABA) (1.0 microgram/cannula) or muscimol (50 ng/cannula) was injected into the ventromedial hypothalamus or the lateral septi nuclei of ovariectomized rats brought to sexual receptivity by combined treatment of estrogen and progesterone. No inhibitory effects of GABA or muscimol were observed on the lordosis behavior. Furthermore, systemic (1.0 mg/kg) or intrahypothalamic (50 ng/cannula) picrotoxin administration was followed by a statistically significant increase in lordosis behavior in ovariectomized, estrogen-primed rats. No such effect was observed in ovariectomized-adrenalectomized animals, indicating its dependence on adrenal secretions. Present results do not support the hypothesis of a GABAergic mechanism in the hormonal control of lordosis behavior.

Involvement of endogenous gabaergic system in the modulation of gonadotropin secretion in normal cycling women

To investigate whether endogenous GABA participates in the control of gonadotropin secretion during the menstrual cycle, placebo or sodium valproate (DPA), an anticonvulsant drug which enhances endogenous GABA content by blocking GABA degradation, were administered to regularly cycling women both during early follicular and midluteal phase. In a first set of experiments, the effect of DPA administration (400 mg, orally) on basal gonadotropin secretion was evaluated in 13 subjects. During early follicular phase (n = 6), no significant changes in plasma gonadotropin levels were observed after DPA or placebo administration. Conversely, during midluteal phase (n = 7), DPA administration resulted in a significant fall (p less than 0.01) in plasma LH concentrations, with a maximal percent decrease of 41.8 +/- 6.7% after 120 min. No changes in plasma FSH levels were observed. In a second set of experiments, the effect of DPA pretreatment (400 mg, orally) on gonadotropin release stimulated by a pulse of exogenous GnRH (10 micrograms, iv bolus) was studied in 11 subjects. During both follicular (n = 4) and luteal phase (n = 7), DPA did not modify gonadotropin response to GnRH injected 1h after pretreatment. Finally, 8 subjects were submitted to iv injection with 10 micrograms GnRH 2h after pretreatment with DPA (400 mg, orally) or placebo. During both follicular (n = 4) and luteal phase (n = 4), no statistical differences in gonadotropin response to GnRH were found between DPA and placebo pretreatment. These findings demonstrated that during the estrogen-progesterone (midluteal) phase of menstrual cycle, endogenous GABA is involved in the inhibitory regulation of LH secretion at a central level.

Glutamate decarboxylase-immunoreactive boutons in synaptic contacts with hypothalamic dopaminergic cells: a light and electron microscopy study combining immunocytochemistry and radioautography

Double post-embedding immunolabeling of both tyrosine hydroxylase and glutamate decarboxylase on 1-micron semi-thin sections allowed the visualization of numerous endings that use gamma-aminobutyrate as a transmitter apposed to dopaminergic cell bodies in the periventricular-arcuate hypothalamic complex. Up to fifteen glutamate decarboxylase-positive contacts per tyrosine hydroxylase-positive cell profile could be observed. In some favourable planes of section glutamate decarboxylase-positive endings were also seen in close apposition to proximal dopaminergic dendrites. About 250 tyrosine hydroxylase-positive cell profiles, whose diameter approached the maximum diameter of the dopaminergic cells, were surveyed. An average of 7.4 glutamate decarboxylase-positive contacts were counted on these profiles. From these figures it was estimated that a dopaminergic cell body was contacted on average by 75-175 terminals that use gamma-aminobutyrate as a transmitter. At the electron-microscopic level, the nature of these contacts was investigated by a method combining radioautographic detection of cell bodies having taken up tritiated dopamine and pre-embedding immunostaining of glutamate decarboxylase containing endings. Glutamate decarboxylase-positive axon terminals were seen apposed to somatic and dendritic elements. On some favorable planes of section, they were found to be engaged in morphologically defined synaptic complexes of the symmetrical or asymmetrical type. A number of the postsynaptic perikarya were labelled by tritiated dopamine and, in agreement with the light microscopic observations, they were frequently seen in contact with more than one immunopositive ending. The present findings provide a morphological substratum for a direct gamma-aminobutyrate control of the tuberoinfundibular dopaminergic neurons. Such a control could account more particularly for the central, stimulatory effects of gamma-aminobutyrate on prolactin secretion.

Silver-intensified gold and peroxidase as dual ultrastructural immunolabels for pre- and postsynaptic neurotransmitters

An ultrastructural immunostaining method that uses silver-intensified gold was combined with another procedure that uses biotin peroxidase conjugates to allow simultaneous identification of two neurotransmitter-related antigens in the central nervous system. Tyrosine hydroxylase-immunoreactive neurons labeled with silver-intensified gold could be differentiated at both light and electron microscopic levels from glutamate decarboxylase-immunoreactive neurons labeled with peroxidase. Cross reactivity of the second group of immunoreagents with the first group was reduced by the heavy metal silver shell formed around the colloidal gold immunoglobulin complex. With this dual pre-embedding method, peroxidase-stained axons containing the inhibitory neurotransmitter gamma-aminobutyric acid were found to synapse directly on silver-stained dopamine neurons in the rat dorsomedial hypothalamus. This approach can be used in combination with a post-embedding immunocytochemical colloidal gold procedure, allowing ultrastructural identification of three neurotransmitter-related antigens in the same tissue section.

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

GABAergic cells and axon terminals were localized in the basal hypothalamus of differnet species (rat, mouse and cat), by means of an immunocytochemical approach using a specific and well-characterized antiserum to the GABA biosynthetic enzyme, glutamate decarboxylase. Light-microscopic visualization was performed with an indirect immunofluorescence method and electron-microscopic observations were made on material with pre-embedding staining and use of the peroxidase-antiperoxidase procedure. At the light-microscopic level, a dense immunofluorescent plexus was observed over both the medial and lateral parts of the external layer of the median eminence. The labelling extended from the rostal part of the median eminence up to the pituitary stalk. Over the subependymal and internal layers only a few immunoreactive dots were visible, except around the blood vessels where they appeared more concentrated. Immunoreactive varicosities could be found following the outlines of the capillary loops and lining tanycyte processes, especially in the median eminance midportion. At the electron-microscopic level, the immunolabelling was exclusively found over neuronal profiles in the median eminence. The latter represented a small fraction of the total number of varicosities visible on the same section. Labelled profiles typically contained numerous small clear synaptic vesicles and only a few or no dense-core vesicles. In the subependymal and internal layers, rare labelled endings were found close to ependymal cells or among transversally cut fibers, respectively. In the palisadic zone, elongated positive boutons were visible intermingled with bundles of unlabelled axons and glial or ependymal processes. In the neurohemal contact zone, immunoreactive endings were observed among unlabelled neurosecretory endings in close vicinity to fenestrated capillary perivascular space. Small moderately intense immunofluorescent varicosities were observed all over the hypothalamus. The density of the glutamate decarboxylase-positive network was higher than in most diencephalic regions. Intraventricular or topical injection of colchicine allowed the visualization of small lightly immunoreactive cells in the diffusion area of colchicine. In the arcuate nucleus labelled axonal endings containing small pleomorphic synaptic vesicles and sometimes a few dense-core vesicles were observed at the electron-microscopic level. Typical synaptic junctions were commonly found between positive endings and unlabelled perikarya, or more frequently, unlabelled dendrites. These findings show that glutamate decarboxylase-containing endings are localized ed in several strategic sites for potential GABAergic neuroendocrine regulations. The GABAergic endings found among neurosecretory endings in the neurohemal contact zone may provide the morphological support for the release of gamma-aminobutyrate into the portal blood flow as an hypothalamic hypophysiotropic hormone.

Two-color immunohistochemistry for dopamine and GABA neurons in rat substantia nigra and zona incerta

Dopaminergic and GABAergic neurons were visualized in the same section of rat substantia nigra (SN) and zona incerta (ZI) by a two-color double immunoperoxidase procedure or by double immunofluorescence. Rabbit antiserum to tyrosine hydroxylase (TH) and sheep antiserum to glutamic acid decarboxylase (GAD), markers for catecholaminergic and GABAergic neurons, respectively, were used as primary antisera. These techniques rely on species difference of primary antisera and non-crossreactivity of linking antisera. In normal and colchicine pretreated rats, SN pars compacta (SNC), SN pars lateralis (SNL), and ZI pars medialis (area A13) contained high densities of TH-positive neurons. Relatively few TH-positive cells were scattered in SN pars reticulata (SNR) and ZI pars lateralis (ZIL). In normal rats, GAD-positive boutons were more numerous throughout SNR and ZIL than in SNC, SNL, and area A13. In colchicine pretreated rats, the majority of neurons in SNR and ZIL and few neurons in SNC, SNL, and area A13 were GAD-positive and TH-negative. This study suggests a dichotomy of both SN and ZI into a predominantly dopaminergic and a predominantly GABAergic part.