GABA-ergic control of alpha-melanocyte-stimulating hormone (alpha-MSH) release by frog neurointermediate lobe in vitro

Analysis of the melanotrope cell neuroendocrine interface in two amphibian species, Rana ridibunda and Xenopus laevis: a celebration of 35 years of collaborative research

This review gives an overview of the functioning of the hypothalamo-hypophyseal neuroendocrine interface in the pituitary neurointermediate lobe, as it relates to melanotrope cell function in two amphibian species, Rana ridibunda and Xenopus laevis. It primarily but not exclusively concerns the work of two collaborating laboratories, the Laboratory for Molecular and Cellular Neuroendocrinology (University of Rouen, France) and the Department of Cellular Animal Physiology (Radboud University Nijmegen, The Netherlands). In the course of this review it will become apparent that Rana and Xenopus have, for the most part, developed the same or similar strategies to regulate the release of α-melanophore-stimulating hormone (α-MSH). The review concludes by highlighting the molecular and cellular mechanisms utilized by thyrotropin-releasing hormone (TRH) to activate Rana melanotrope cells and the function of autocrine brain-derived neurotrophic factor (BDNF) in the regulation of Xenopus melanotrope cell function.

Plasticity in the melanotrope neuroendocrine interface of Xenopus laevis

Melanotrope cells of the amphibian pituitary pars intermedia produce alpha-melanophore-stimulating hormone (alpha-MSH), a peptide which causes skin darkening during adaptation to a dark background. The secretory activity of the melanotrope of the South African clawed toad Xenopus laevis is regulated by multiple factors, both classical neurotransmitters and neuropeptides from the brain. This review concerns the plasticity displayed in this intermediate lobe neuroendocrine interface during physiological adaptation to the environment. The plasticity includes dramatic morphological plasticity in both pre- and post-synaptic elements of the interface. Inhibitory neurons in the suprachiasmatic nucleus, designated suprachiasmatic melanotrope-inhibiting neurons (SMINs), possess more and larger synapses on the melanotrope cells in white than in black-background adapted animals; in the latter animals the melanotropes are larger and produce more proopiomelanocortin (POMC), the precursor of alpha-MSH. On a white background, pre-synaptic SMIN plasticity is reflected by a higher expression of inhibitory neuropeptide Y (NPY) and is closely associated with postsynaptic melanotrope plasticity, namely a higher expression of the NPY Y1 receptor. Interestingly, melanotrope cells in such animals also display higher expression of the receptors for thyrotropin-releasing hormone (TRH) and urocortin 1, two neuropeptides that stimulate alpha-MSH secretion. Possibly, in white-adapted animals melanotropes are sensitized to neuropeptide stimulation so that, when the toad moves to a black background, they can immediately initiate alpha-MSH secretion to achieve rapid adaptation to the new background condition. The melanotrope cell also produces brain-derived neurotrophic factor (BDNF), which is co-sequestered with alpha-MSH in secretory granules within the cells. The neurotrophin seems to control melanotrope cell plasticity in an autocrine way and we speculate that it may also control presynaptic SMIN plasticity.

The regulation of alpha-MSH release by GABA is mediated by a chloride-dependent [Ca2+]c increase in frog melanotrope cells

In frog melanotrope cells, gamma-aminobutyric acid (GABA) induces a biphasic effect, i.e. a transient stimulation followed by a more sustained inhibition of alpha-MSH release, and both phases of the GABA effect are mediated by GABAA receptors. We have previously shown that the stimulatory phase evoked by GABAA receptor agonists can be accounted for by calcium entry. In the present study, we have investigated the involvement of the chloride flux on GABA-induced [Ca2+]c increase and alpha-MSH release. We show that GABA evokes a concentration-dependent [Ca2+]c rise through specific activation of the GABAA receptor. The GABA-induced [Ca2+]c increase results from opening of voltage-activated L- and N-type calcium channels, and sodium channels. Variations of the extracellular Cl- concentration revealed that GABA-induced [Ca2+]c rise and alpha-MSH release both depend on the Cl- flux direction and driving force. These observations suggest for the first time that GABA-gated Cl- efflux provokes an increase in [Ca2+]c increase that is responsible for hormone secretion.

Characterization of the GABA(A) receptor in the brain of the adult male bullfrog, Rana catesbeiana

Little is known about the properties of GABA receptors in the amphibian brain. The GABA(A) receptor is widespread in the mammalian brain, and can be specifically labeled with the receptor agonist [3H]muscimol. The binding of [3H]muscimol to membrane preparations from the brain of the bullfrog, Rana catesbeiana, was investigated in kinetic, saturation, and inhibition experiments to determine whether this species possessed a GABA(A)-like receptor. Binding of 20 nM [3H]muscimol to membranes was specific and could be displaced by 1 mM GABA. Association binding curves showed that steady state occurred rapidly, within 2 min, and dissociation occurred within 5 min. The receptor was saturable with a single, high-affinity binding site (K(D)=19.2 nM; B(max)=1.8 pmol/mg protein). Binding of [3H]muscimol was inhibited in a dose-dependent fashion by muscimol, GABA, bicuculline methiodide, and bicuculline (in order of potency). Baclofen (at doses from 10(-9) to 10(-3) M) failed to displace [3H]muscimol. The binding characteristics and ligand specificity of [3H]muscimol binding sites in the bullfrog brain support the hypothesis that this amphibian possesses a GABA(A)-like receptor protein similar to the GABA(A) receptor characterized in mammals.

Amphibian melanotrope subpopulations respond differentially to hypothalamic secreto-inhibitors

The melanotrope population of the frog intermediate lobe consists of two subtypes of cells, referred to as high-(HD) and low-density (LD) melanotrope cells, which differ markedly in their basal morphofunctional features as well as their in vitro response to hypothalamic factors, such as the stimulator thyrotropin-releasing hormone (TRH) and the inhibitor dopamine. In this study, we have investigated whether other major hypothalamic regulators of the release of alpha-melanocyte-stimulating hormone (alpha-MSH), such as gamma-aminobutyric acid (GABA) and neuropeptide Y (NPY), also differentially regulate frog melanotrope subpopulations. Our results show that in LD cells, both factors markedly inhibited proopiomelanocortin (POMC) mRNA accumulation and alpha-MSH secretion. In contrast, the secretory and biosynthetic activity of HD cells was not modified by GABA. NPY inhibited POMC transcript accumulation and tended to reduce alpha-MSH secretion in HD cells, yet these effects were less pronounced than those evoked in LD cells. In addition, GABA and NPY inhibited the KCl-induced rise in cytosolic free calcium levels in both subpopulations. Taken together, these results further indicate that frog melanotrope subpopulations are differentially regulated by the hypothalamus and strongly suggest that the intensity of such regulation is directly related to the activity of the cell subset. Thus, the LD subpopulation represents a highly responsive cell subset which is regulated by multiple neuroendocrine factors (TRH, dopamine, GABA and NPY), whereas the hormone storage HD subpopulation shows a moderate response to single stimulatory (TRH) and inhibitory (NPY) inputs.

Distribution of GAD-immunoreactive neurons in the diencephalon of the african lungfish Protopterus annectens: colocalization of GAD and NPY in the preoptic area

The distribution of GABAergic neurons was investigated in the diencephalon of the African lungfish, Protopterus annectens, by using specific antibodies directed against glutamic acid decarboxylase (GAD). A dense population of immunoreactive perikarya was observed in the periventricular preoptic nucleus, whereas the caudal hypothalamus and the dorsal thalamus contained only scattered positive cell bodies. Clusters of GAD-positive cells were found in the intermediate lobe of the pituitary. The diencephalon was richly innervated by GAD-immunoreactive fibers that were particularly abundant in the hypothalamus. In the periventricular nucleus, GAD-positive fibers exhibited a radial orientation, and a few neurons extended processes toward the third ventricle. More caudally, a dense bundle of GAD-immunoreactive fibers coursing along the ventral wall of the hypothalamus terminated into the median eminence and the neural lobe of the pituitary. Double-labeling immunocytochemistry revealed that GAD and neuropeptide tyrosine (NPY)-like immunoreactivity was colocalized in a subpopulation of perikarya in the periventricular preoptic nucleus. The proportion of neurons that coexpressed GAD and NPY was higher in the caudal region of the preoptic nucleus. The distribution of GAD-immunoreactive elements in the diencephalon and pituitary of the African lungfish indicates that GABA may act as a hypophysiotropic neurohormone in Dipnoans. The coexistence of GAD and NPY in a subset of neurons of the periventricular preoptic nucleus suggests that GABA and NPY may interact at the synaptic level.

Regulation of GABAA receptor by protein tyrosine kinases in frog pituitary melanotrophs

The effects of protein tyrosine kinase (PTK) and PTK inhibitors on the GABAA receptor function were studied in cultured frog pituitary melanotrophs by using the patch-clamp technique. Extracellular application of the PTK inhibitors genistein (10-9 to 10-5 M) or lavendustin A (10-12 to 10-7 M) provoked a bell-shaped potentiation of the whole-cell current induced by GABA (3x10-6 M). In contrast, at high concentrations, genistein (10-4 M) and lavendustin A (10-5 M) reversibly reduced the GABA-evoked current. Daidzein and lavendustin B, the inactive analogs of genistein and lavendustin A, respectively, did not modify the current induced by GABA. In the inside-out configuration, bath application of the recombinant PTK pp60c-src (75 U/ml) inhibited the GABA-activated chloride current, and the inhibitory effect of pp60c-src was prevented by genistein (10-7 M). Immunoblotting revealed that genistein, at doses of 10-7 M or 10-4 M, markedly inhibited tyrosine phosphorylation of the beta2/beta3 subunits of the GABAA receptor. Extracellular application of the PKA activator Bt2cAMP (10-3 M), the PKA/PKC inhibitor H7 (10-5 M) and the Cam KII inhibitor W7 (10-5 M) reversibly diminished the whole-cell GABA-induced current. Internal application of H7 and W7 (10-4 M) did not modify the dose-dependent effects of genistein. Internal application of sodium orthovanadate (10-4 M), a protein tyrosine phosphatase inhibitor, decreased the GABA-evoked current and markedly reduced the potentiating effect of genistein. The present study provides the first evidence that, in frog pituitary melanotrophs, the GABAA receptor is phosphorylated at least on its beta2/beta3 subunits by an endogenous PTK. Our data also demonstrate that tyrosine phosphorylation exerts an inhibitory effect on GABAA receptor function.

Gramicidin-perforated patch revealed depolarizing effect of GABA in cultured frog melanotrophs

1. In frog pituitary melanotrophs, GABA induces a transient stimulation followed by prolonged inhibition of hormone secretion. This biphasic effect is inconsistent with the elevation of cytosolic calcium and the inhibition of electrical activity also provoked by GABA in single melanotrophs. In the present study, standard patch-clamp configurations and gramicidin-perforated patches were used to investigate the physiological GABAA receptor-mediated response and intracellular chloride concentration ([Cl-]i) in cultured frog melanotrophs. 2. In the gramicidin-perforated patch configuration, 1 microM GABA caused a depolarization associated with an action potential discharge and a slight fall of membrane resistance. In contrast, at a higher concentration (10 microM) GABA elicited a depolarization accompanied by a transient volley of action potentials, followed by a sustained inhibitory plateau and a marked fall of membrane resistance. Isoguvacine mimicked the GABA-evoked responses, indicating a mediation by GABAA receptors. 3. In gramicidin-perforated cells, the depolarizing excitatory effect of 1 microM GABA was converted into a depolarizing inhibitory action when 0.4 microM allopregnanolone was added to the bath solution. 4. After gaining the whole-cell configuration, the amplitude and/or direction of the GABA-evoked current (IGABA) rapidly changed before stabilizing. After stabilization, the reversal potential of IGABA followed the values predicted by the Nernst equation for chloride ions when [Cl-]i was varied. 5. In gramicidin-perforated cells, the steady-state I-V relationships of 10 microM GABA- or isoguvacine-evoked currents yielded reversal potentials of -37.5 +/- 1.6 (n = 17) and -38.6 +/- 2.0 mV (n = 8), respectively. These values were close to those obtained by using a voltage-ramp protocol in the presence of Na+, K+ and Ca2+ channel blockers. The current evoked by 1 microM GABA also reversed at these potentials. 6. We conclude that, in frog pituitary melanotrophs, chloride is the exclusive charge carrier of IGABA. In intact cells, the reversal potential of IGABA is positive to the resting potential because of a relatively high [Cl-]i (26.5 mM). Under these conditions, GABA induces a chloride efflux responsible for a depolarization triggering action potentials. However, GABA at a high concentration or in the presence of the potentiating steroid allopregnanolone exerts a concomitant shunting effect leading to a rapid inhibition of the spontaneous firing.

Multiple modulatory effects of the neuroactive steroid pregnanolone on GABAA receptor in frog pituitary melanotrophs

1. The effects of the neuroactive steroid pregnanolone (5 beta-pregnan-3 alpha-ol-20-one) on the electrical response to GABA were investigated in cultured frog pituitary melanotrophs using the patch-clamp technique. 2. Low concentrations of pregnanolone (0.01-1 microM) in the extracellular solution enhanced the current evoked by submaximal concentrations of GABAA receptor agonists and prolonged the GABA-induced inhibition of the spontaneous action potentials in a dose-dependent manner. 3. Pregnanolone augmented the opening probability of the single GABA-activated channels but did not modify the conductance levels. 4. Pregnanolone (1 microM) shifted the GABA dose-response curve towards the low GABA concentrations, reducing the EC50 from 4.2 to 1.8 microM. 5. Internal cell dialysis with pregnanolone (1 or 10 microM) did not alter the GABA-evoked current. 6. Pregnanolone accelerated the desensitization of both the current and conductance increases caused by GABA. 7. High concentrations of pregnanolone (30 microM) markedly and reversibly diminished the current evoked by 10 microM GABA. 8. At high concentrations (10-30 microM), pregnanolone induced an outward current which reversed at the chloride equilibrium potential. 9. It is concluded that, in frog pituitary melanotrophs, pregnanolone exerts a dual inverse modulation and a direct activation of the GABAA receptor-channel depending on the concentrations of both GABA and steroid. Pregnanolone acts on an extracellular site on the GABAA receptor inducing conformational changes of the receptor-channel complex, resulting in a desensitized less-conducting state.

Electrophysiological effects of various neuroactive steroids on the GABA(A) receptor in pituitary melanotrope cells

The action of steroids on the bioelectrical response to gamma-aminobutyric acid (GABA) has never been studied in pituitary cells. In the present study, we have thus investigated the effects of a series of neuroactive steroids on the GABA-activated current in frog melanotrope cells in primary culture, using the patch-clamp technique in the whole-cell configuration. Bath perfusion of 3alpha-isomers of pregnanolone or tetrahydrodeoxycorticosterone (1 microM) significantly enhanced the current evoked by short pulses of GABA (3 microM) and accelerated its desensitization. In contrast, the 3beta-isomers (30 microM) had no effect on the GABA-activated current. Addition to the bath solution of dehydroepiandrosterone or dehydroepiandrosterone sulfate (10 microM) inhibited the GABA-activated current without modifying its kinetics while pregnenolone sulfate (10 microM) both inhibited the GABA-activated current and accelerated its decay rate. The effects of pregnane steroids were not impaired by the central-type benzodiazepine receptor antagonist flumazenil (10 microM). In conclusion, the present study reveals that neuroactive steroids may exert multiple modulatory activities on the GABA(A) receptor borne by melanotrope cells. The effect of steroids on the current evoked by GABA is rapid, reversible, stereospecific and not mediated through the benzodiazepine binding site of the GABA(A) receptor.

Multiple sources of the pituitary pars intermedia innervation in amphibians: a DiI retrograde tract-tracing study

Afferent projections to the pituitary pars intermedia were studied using the DiI tract-tracing technique in two amphibian species, the urodelan Triturus carnifex, and the anuran Rana esculenta. After DiI crystal application into the pituitary intermediate lobe, in both species cells were retrogradely labeled in the preoptic nucleus, in the supra- and retro-chiasmatic hypothalamus and in the brainstem (especially in the area indicated as locus coeruleus). The findings are discussed in relation to data on the neurochemical nature of the innervation of the pars intermedia in amphibians.

Characterization of the GABA-induced current in frog pituitary melanotrophs

The molecular mechanisms regulating GABAA receptor activity in cultured frog melanotrophs were studied using the patch-clamp technique. In the whole-cell configuration, application of GABA evoked a dose-related increase of inward chloride currents. The ED50 value, estimated from the sigmoidal dose-response curve was 2 x 10(-6) M and the Hill coefficient was 1.55. The amplitude of the GABA-induced current decayed with time. Kinetics analysis of the desensitization revealed that the time-course of the current decrement was fitted by one exponential. Graded doses of GABA or association of GABA with the benzodiazepine receptor agonist flunitrazepam accelerated the desensitization process. In contrast, the time-course of the current did not significantly vary at different holding potentials. In the outside-out configuration, GABA was found to activate channels which displayed three unitary conductance levels (8, 15 and 30 pS). The channel openings of the more frequent conductance level (30 pS) exhibited short and long lasting open states (1.2 and 28.3 ms at -60 mV). Altogether these data reveal that frog melanotrophs possess a single population of GABAA receptors which interconvert into a higher affinity state in the presence of benzodiazepine receptor agonists. Two GABA molecules must bind to the receptor to trigger long lasting channel openings. In addition, the activity of the GABAA receptor appears to be independent of the accumulation of intracellular chloride ions.

GABA-ergic control of prolactin release in rainbow trout (Oncorhynchus mykiss) pituitaries in vitro

The involvement of γ-aminobutyric acid (GABA) in the control of prolactin (PRL) release was investigated in rainbow trout using both perifused pituitary fragments and pituitary cells in primary culture. In our perifusion system, infusion of GABA (10(-6) to 10(-4) M) caused an inhibition of PRL release (between 20 and 40%). Administration on perifused pituitary fragments of 3APS, a GABAa agonist, mimicked this inhibitory effect. Moreover, bicuculline, a specific antagonist of GABAa receptors, totally abolished GABA effect. When tested on cultured pituitary cells during 40h exposure, GABA (10(-5) M) caused a significant decrease in PRL release (24.5%). Baclofen, a specific agonist for GABAb receptor tested at 10(-6) and 10(-5) M, also inhibited PRL released from cultured pituitary cells. These results demonstrate that GABA inhibits PRL release by acting directly on pituitary cells and that probably both types of GABA receptor (a and b) are involved in this regulation.

Differential effects of coexisting dopamine, GABA and NPY on alpha-MSH secretion from melanotrope cells of Xenopus laevis

The secretion of alpha-MSH from the intermediate lobe of the pituitary gland of the amphibian Xenopus laevis is under complex neural control. Three neurotransmitters, dopamine, GABA and NPY, coexist in nerve terminals that contact the melanotrope cells. All three neurotransmitters inhibit alpha-MSH release. We have investigated the significance of this neurotransmitter coexistence for the regulation of alpha-MSH release, using an in vitro superfusion system. From experiments where lobes were treated with various combinations of receptor agonists we conclude that the transmitters act in an additive way but have clear, differential actions. Inhibition of secretion by either dopamine, isoguvacine (GABAA receptor agonist) or baclofen (GABAB receptor agonist) occurs rapidly and alpha-MSH secretion rapidly returns when treatment is terminated (recovery from baclofen being relatively fast, that from dopamine relatively slow); in contrast, inhibition by NPY and recovery from NPY-induced inhibition occurs only very slowly. Differential effects of the transmitters were also seen in experiments with 8-bromo-cyclic AMP, which strongly stimulates alpha-MSH secretion from isoguvacine- or baclofen-treated lobes, but is relatively ineffective in stimulating secretion from lobes treated with dopamine or NPY. NPY, furthermore, enables a short phasic stimulation of secretion by isoguvacine and attenuates the inhibitory action of dopamine and baclofen. Altogether it is concluded that the coexisting factors differentially affect the secretory process of the melanotrope cells of Xenopus laevis. NPY has a slow, sustained action whereas dopamine and GABA act fast.

Adenosine inhibits alpha-melanocyte-stimulating hormone release from frog pituitary melanotrophs. Evidence for the involvement of a(1) adenosine receptors negatively coupled to adenylate cyclase

Adenosine is recognized as an important modulator of cell activity. In particular, adenosine regulates the secretion of adrenocorticotropin from anterior pituitary cells. However, the possible role of adenosine on the pars intermedia has never been investigated. In the present study, we have examined the effect of adenosine on α-melanotropin (α-MSH) secretion from the intermediate lobe of the pituitary of the frog (Rana ridibunda), using the perifusion technique. When whole neurointermediate lobes were exposed to graded doses of adenosine (10(-9) to 10(-4) M), a dose-dependent inhibition of a-MSH release was observed. Repeated pulses of adenosine (5 ± 10(-5) M) induced a reproducible inhibition of α-MSH secretion without any desensitization phenomenon. The effect of adenosine was mimicked by the non-selective agonist 5'-N-ethylcarboxamide-adenosine and the highly specific adenosine A, receptor agonist N(6) -[R-phenylisopropyl]-adenosine (R-PIA). In contrast the selective adenosine A(2) receptor agonist, CGS 21680, induced a slight stimulation of α-MSH release. Adenosine-induced inhibition of α-MSH secretion was blocked by the non-selective adenosine antagonist, 8-(p-sulfophenyl)-theophyline. Adenosine and R-PIA also inhibited α-MSH secretion from acutely dispersed pars intermedia cells. Adenosine did not block thyrotropin-releasing hormone-induced α-MSH release from perifused neurointermediate lobes. In contrast, adenosine inhibited both acetylcholine-evoked and muscarine-evoked α-MSH secretion. Finally, R-PIA induced a significant inhibition of basal and forskolin-stimulated cyclic AMP levels in whole neurointermediate lobes. The present results demonstrate that adenosine exerts a direct inhibitory effect on α-MSH release from melanotrope cells through activation of the A(1) receptor subtype, negatively coupled to adenylate cyclase. These data suggest that adenosine may play a physiological role in the regulation of hormone release from the intermediate lobe of the pituitary.

Lack of effect of TRH on alpha-MSH release from the neurointermediate lobe of the lizard Lacerta vivipara

Thyrotropin-releasing hormone (TRH) is a potent stimulator of melanotropin (alpha-MSH) release from pituitary melanotrophs in pig, frog, and fish. Concurrently, it has recently been shown that injection of TRH induces skin darkening in the lizard Anolis carolinensis (Licht and Denver, 1988). In the present study, we have thus investigated in vitro the possible effect of TRH on alpha-MSH release from the lizard (Lacerta vivipara) neurointermediate lobe, by means of the perifusion technique. Using our radioimmunoassay procedure, we found that serial dilutions of L. vivipara NIL extracts and synthetic alpha-MSH gave parallel binding curves. Administration of graded doses of TRH (10(-8)-10(-6) M) did not cause any modification of alpha-MSH release. In contrast, infusion of a depolarizing concentration of K+ induced a robust stimulation of alpha-MSH secretion. These results indicate that, in the lizard L. vivipara, the neuropeptide TRH does not stimulate pituitary melanotrophs.

Co-localization of tyrosine hydroxylase, GABA and neuropeptide Y within axon terminals innervating the intermediate lobe of the frog Rana ridibunda

Possible co-existence of gamma-aminobutyric acid (GABA), catecholamines, and neuropeptide Y (NPY) in the same nerve terminals of the frog intermediate lobe was investigated by immunocytochemistry at the electron microscopic level. Co-localization of GABA and tyrosine hydroxylase (TH) was studied by using a double immunogold labeling procedure. Co-localization of glutamate decarboxylase (GAD) and NPY was studied by combining, respectively, the peroxidase-antiperoxidase method and a radioimmunocytochemical labeling procedure. Catecholamines and GABA were systematically co-localized in nerve endings of the pars intermedia. Most of the NPY-immunoreactive fibers also contained GAD-like immunoreactivity. However, a few NPY-positive nerve terminals were not immunoreactive for GAD. These data provide evidence for co-existence of a regulatory peptide (NPY) and several neurotransmitters (i.e., GABA and catecholamines) within the same axon terminals in the intermediate lobe. Since GABA, dopamine, and NPY have all been shown to inhibit the activity of frog melanotrope cells, the present findings suggest that these neuroendocrine factors may interact either at the pre-synaptic or post-synaptic level.

Central-type benzodiazepines modulate GABAA receptor chloride channels in cultured pituitary melanotrophs

The effects of gamma-aminobutyric acid (GABA) and benzodiazepines on the electrical activity of cultured frog melanotrophs were studied using the patch-clamp technique. In the cell-attached configuration, the exposure to GABA caused a blockage of the spontaneous firing. In the whole-cell configuration, with physiological chloride concentrations, GABA evoked a hyperpolarization associated with a decrease of membrane resistance, generating an inward chloride current. Clonazepam, a central-type benzodiazepine agonist, potentiated the GABA-induced current and the resulting hyperpolarization. In addition, the benzodiazepine inverse agonist Ro 19-4603 totally abolished GABA-induced hyperpolarizing chloride current. Since the pars intermedia of the frog pituitary is composed of a 'pure' population of endocrine cells enriched with GABAA receptors, our results indicate that these cells represent a valuable model in which to investigate the electrophysiological effects of ligands for the GABAA benzodiazepine receptor complex.

[The intermediate lobe of the pituitary, model of neuroendocrine communication]

The intermediate lobe of the pituitary is composed of a homogeneous population of endocrine cells, the melanotrophs, which secrete several bioactive peptides including alpha-melanocyte-stimulating hormone (alpha-MSH) and beta-endorphin. In contrast to most endocrine glands which are richly vascularized, the intermediate lobe of the pituitary contains very few blood vessels; in some species, the pars intermedia is virtually totally avascular. In contrast, pituitary melanotrophs are richly supplied by nerve fibers originating from the hypothalamus. The pars intermedia thus appears as a pure model of neuroendocrine communication, i.e. it is an archetype of the mode of transducing interface between the central nervous system and endocrine effectors. In mammalian species, different types of nerve terminals containing dopamine, norepinephrine, gamma-aminobutyric acid (GABA) and serotonin have been identified. In lower vertebrates, particularly in fish and amphibians, the pars intermedia is also innervated by peptidergic fibers which are though to take part in regulation of the secretory activity of the melanotroph. In these animals, the pars intermedia is regarded as a major center of neuroendocrine integration and an exceptional model to investigate the process of communication between the brain and the endocrine glands. The purpose of the present review is to summarize our current knowledge on the synthesis, processing and release of peptide hormones from pars intermedia cells and to survey the multiple regulatory mechanisms which are involved in the control of the activity of pituitary melanotrophs. Proopiomelanocortin, a multifunctional precursor. Pituitary melanotrophs synthetise a major precursor protein called proopiomelanocortin (POMC) which generates through proteolytic cleavage several biologically active peptides including adrenocorticotropic hormone (ACTH), endorphins and MSHs. In lower vertebrates, alpha-MSH is generally considered as the major hormone secreted by melanotrophs, in that it is involved in the process of skin colour adaptation. The post-translational processing of POMC, which yields to the mature hormones released by melanotrophs, includes a number of steps: glycosylation, phosphorylation, tissue-specific proteolytic cleavage, amidation and acetylation. Some of these posttranslational modifications can be regulated by neuroendocrine factors. For instance, in frogs, it has been shown that dopamine inhibits acetylation of alpha-MSH and thus reduces the secretion of the biologically active form of the peptide. The intermediate lobe of the pituitary: a model of neuroendocrine integration. In most vertebrate species, the intermediate lobe of the pituitary is innervated by catecholamine-containing fibers. In particular, the presence of dopaminergic nerve fibers has been observed in the pars intermedia of mammals and poikilotherms.(ABSTRACT TRUNCATED AT 400 WORDS)

Studies on pituitary melanotrophs reveal the novel GABAB antagonist CGP 35-348 to be the first such compound effective on endocrine cells

One obstacle to understanding the action and physiological significance of the responsiveness of various endocrine cells to gamma-aminobutyric acid (GABA) has been that previously available substances, all active as GABAB antagonists in the nervous system, are ineffective on endocrine cells. The introduction of a potent new member of this class, CGP 35-348, of very different chemical structure, encouraged us to examine its effect on endocrine cells. For this purpose, we studied melanotroph secretion from pituitary neurointermediate lobes. We found that CGP 35-348, in contrast to previously available members of this class, suppressed completely, in rat and toad, secretory responses to baclofen, the classic GABAB agonist. Analysis, in toad, showed CGP 35-348 did not affect responses to GABAA agonists (muscimol; isoguvacine), dopamine, or neuropeptide Y. When tested against GABA, the physiological ligand present in the innervation of melanotrophs (along with dopamine and neuropeptide Y), CGP 35-348 completely suppressed the secretory response, which, in toad, is purely inhibitory and unaffected by bicuculline, the specific GABAA antagonist. In addition, CGP 35-348 unmasked a stimulant effect that bicuculline blocked. In CGP 35-348, we thus have a new tool with which to analyse responses to GABA and their physiological involvement in endocrine cells.

Effect of acetylcholine on the electrical and secretory activities of frog pituitary melanotrophs

The activity of melanotroph cells of the amphibian pars intermedia is regulated by multiple factors including classical neurotransmitters and neuropeptides. In this study, we have examined the possible involvement of acetylcholine (ACh) in the regulation of electrical and secretory activities of frog pituitary melanotrophs. Electrophysiological recordings were conducted on cultured cells by using the patch-clamp technique in the whole-cell configuration. In parallel, alpha-MSH release from acutely dispersed pars intermedia cells was studied by means of the perifusion technique. In all cells tested in the current-clamp mode, superfusion with ACh (10(-6) M) gave rise to a depolarization associated with an enhanced frequency of action potentials. Administration of ACh (10(-6) M) to perifused cells also induced stimulation of alpha-MSH release. These results indicate that the neurotransmitter ACh exerts a direct stimulatory effect on pituitary melanotrophs. The action of ACh on electrical and secretory activities was mimicked by muscarine (10(-5) M), while ACh-induced alpha-MSH secretion was completely abolished by the muscarinic antagonist atropine (10(-6) M). The depolarizing effect of muscarine was suppressed by the specific M1 muscarinic antagonist pirenzepine (10(-5) M), indicating the existence of a M1 subtype muscarinic receptor in frog pars intermedia cells. In addition, using a monoclonal antibody against calf muscarinic receptors, we have visualized, by the immunofluorescence technique, the presence of muscarinic receptor-like immunoreactivity in cultured intermediate lobe cells. Electrophysiological recordings showed that nicotine (10(-5) M) induces membrane depolarization associated with an increase of the frequency of action potentials.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological evidence for the existence of GABAA receptors in cultured frog melanotrophs

The neurotransmitter GABA exerts a biphasic effect on alpha-melanocyte-stimulating hormone (alpha-MSH) secretion from pars intermedia cells: GABA induces a rapid and transient stimulation followed by a sustained inhibition of alpha-MSH release. In the present study, we have investigated the effect of GABA on the electrophysiological properties of frog melanotrophs in primary culture using the patch-clamp technique in the whole cell configuration. In all cells tested, GABA stimulated an inward current and induced depolarization. A transient period of intense firing was consistently observed at the onset of GABA administration. During the depolarization phase, the membrane potential reached a plateau corresponding to the Cl- equilibrium potential. When repeated hyperpolarizing pulses were applied, an increase of membrane conductance was observed throughout the response evoked by GABA. The effect of GABA was abolished by the chloride channel blocker picrotoxin, and by antagonists of GABAA receptors (bicuculline and SR 95531). The depolarizing action of GABA was mimicked by muscimol, an agonist of GABAA receptors. Taken together, our results indicate that the rapid and transient stimulation of alpha-MSH release induced by GABA can be accounted for by activation of a chloride conductance which causes membrane depolarization. These data support the notion that the transient stimulation of alpha-MSH secretion induced by GABA can be accounted for by membrane depolarization which provokes activation of voltage-operated calcium channels. Since no evidence was found for GABA-induced hyperpolarization, the intracellular mechanisms leading to the strong inhibitory effect of GABA on alpha-MSH secretion remain to be elucidated.

Neuropeptide Y: localization in the central nervous system and neuroendocrine functions

Neuropeptide Y (NPY) is a 36-amino acid peptide first isolated and characterized from porcine brain extracts. A number of immunocytochemical investigations have been conducted to determine the localization of NPY-containing neurons in various animal species including both vertebrates and invertebrates. These studies have established the widespread distribution of NPY in the brain and in sympathetic neurons. In the rat brain, a high density of immunoreactive cell bodies and fibers is observed in the cortex, caudate putamen and hippocampus. In the diencephalon, NPY-containing perikarya are mainly located in the arcuate nucleus of the hypothalamus; numerous fibers innervate the paraventricular and suprachiasmatic nuclei of the hypothalamus, as well as the paraventricular nucleus of the thalamus and the periaqueductal gray. At the electron microscope level, using the pre- and post-embedding immunoperoxidase techniques, NPY-like immunoreactivity has been observed in neuronal cell body dendrites and axonal processes. In nerve terminals of the hypothalamus, the product of the immunoreaction is associated with large dense core vesicles. In lower vertebrates, including amphibians and fish, neurons originating from the diencephalic (or telencephalic) region innervate the intermediate lobe of the pituitary where a dense network of immunoreactive fibers has been detected. At the ultrastructural level, positive endings have been observed in direct contact with pituitary melanotrophs of frog and dogfish. These anatomical data suggest that NPY can act both as a neurotransmitter (or neuromodulator) and as a hypophysiotropic neurohormone. In the rat a few NPY-containing fibers are found in the internal zone of the median eminence and high concentrations of NPY-like immunoreactivity are detected in the hypothalamo-hypophyseal portal blood, suggesting that NPY may affect anterior pituitary hormone secretion. Intrajugular injection of NPY causes a marked inhibition of LH release but does not significantly affect other pituitary hormones. Passive immunoneutralization of endogenous NPY by specific NPY antibodies induces stimulation of LH release in female rats, suggesting that NPY could affect LH secretion at the pituitary level. However, NPY has no effect on LH release from cultured pituitary cells or hemipituitaries. In addition, autoradiographic studies show that sites for 125I-labeled Bolton-Hunter NPY or 125I-labeled PYY (2 specific ligands of NPY receptors) are not present in the adenohypophysis, while moderate concentrations of these binding sites are found in the neural lobe of the pituitary. It thus appears that the inhibitory effect of NPY on LH secretion must be mediated at the hypothalamic level.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuropeptide Y inhibits thyrotropin-releasing hormone-induced stimulation of melanotropin release from the intermediate lobe of the frog pituitary

Previous studies have shown that the release of melanotropin from frog neurointermediate lobes is under the control of two neuropeptides: thyrotropin-releasing hormone (TRH) stimulates, while neuropeptide Y (NPY) inhibits alpha-melanocyte-stimulating hormone (alpha-MSH) secretion from intact neurointermediate lobes in vitro. The aim of the present study was to investigate possible interactions between the two regulatory peptides at the pituitary level. Whole neurointermediate lobes or acutely dispersed pars intermedia cells from Rana ridibunda were perifused in vitro for 2 to 7.5 hr and the concentrations of alpha-MSH released into the effluent perifusate were monitored by radioimmunoassay. Administration of TRH (10(-7) M) or NPY (10(-7) M) to dispersed cells induced, respectively, marked stimulation or inhibition of alpha-MSH release. The effects of the two neuropeptides were similar to those observed using intact neurointermediate lobes, suggesting that TRH and NPY act directly on melanotropic cells. Perifused whole neurointermediate lobes were exposed to NPY (10(-8) to 3 x 10(-7) M) for 120 min and a single dose of TRH (10(-8) M) was administered during the prolonged infusion of NPY. Using this procedure, we observed a dose-dependent inhibition of TRH-evoked alpha-MSH release. These data support the concept that TRH and NPY act through a common intracellular pathway to regulate alpha-MSH release.

gamma-Aminobutyric acid inhibits the release of alpha-melanocyte-stimulating hormone from rat hypothalamic slices

The effect of gamma-aminobutyric acid (GABA) on release of alpha-melanocyte-stimulating hormone (alpha-MSH) from hypothalamic neurons was investigated in vitro using the perifusion technique. Rat hypothalamic slices were continuously superfused with Krebs-Ringer medium and the release of alpha-MSH in the effluent perifusate was monitored by means of a sensitive and specific radioimmunoassay method. Infusion of 50 mM K+ for 15 min induced a transient increase of alpha-MSH release (5- to 8-fold above the spontaneous level). Infusion of the same dose of K+ for 75 min caused a brief discharge of alpha-MSH during the first 30 min followed by sustained release of the neuropeptide. The effect of GABA was investigated 27 min after the onset of KCl infusion. Application of GABA (5 x 10(-5) M) resulted in a significant and reversible inhibition of K+-induced alpha-MSH release. The GABAA agonist, muscimol (10(-4) M), produced a prolonged inhibition of K+-evoked alpha-MSH release, while the GABAB agonist, baclofen (10(-4) M), was devoid of effect on hypothalamic alpha-MSH release. Bicuculline (10(-4) M), a specific GABAA antagonist, had no effect when added alone to the medium but totally reversed the inhibitory effect of GABA on K+-induced alpha-MSH release. Taken together, these data suggest that exogenous GABA exerts an inhibitory control on alpha-MSH neurons. Our data also show that the effect of GABA on alpha-MSH release by hypothalamic neurons is mediated through GABAA-type receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Three distinct thyrotropin-releasing hormone-immunoreactive axonal systems project in the median eminence-pituitary complex of the frog Rana ridibunda. Immunocytochemical evidence for co-localization of thyrotropin-releasing hormone and mesotocin in fibers innervating pars intermedia cells

The localization of thyrotropin-releasing hormone-immunoreactive structures was investigated in the hypothalamo-hypophyseal complex of the frog, Rana ridibunda, by light and electron microscopy using the conventional indirect immunoperoxidase technique and the immuno-gold technique, respectively. The localization of mesotocin-, vasotocin- and neurophysin-immunoreactive elements was compared to that of thyrotropin-releasing hormone either by comparing homologous fields on serial sections or by staining the same section with two different antibodies. Thyrotropin-releasing hormone-immunoreactive perikarya occurred mainly in the anterobasal periventricular area and dorsal extension of the preoptic nucleus, and in the lateral zone of the infundibular nucleus. In the anterobasal preoptic nucleus, the distribution of thyrotropin-releasing hormone-immunoreactive perikarya partially overlapped that of vasotocin- and mesotocin-containing neurons; however, co-localization of thyrotropin-releasing hormone with either nonapeptide could not be detected there. In contrast, in the caudal extension of the preoptic nucleus, thyrotropin-releasing hormone- and mesotocin-like immunoreactivities were frequently co-localized in the same neurons. In the external zone of the median eminence, abundant networks of thyrotropin-releasing hormone- and vasotocin-immunoreactive nerve fibers were found in the vicinity of portal capillaries, while mesotocin-immunoreactive axons were only found in the internal zone. Using the immuno-gold technique at the electron microscopic level, three distinct thyrotropin-releasing hormone-immunoreactive systems were identified in the median eminence-neurointermediate lobe complex. (1) In the external zone of the median eminence, a conspicuous population of pericapillary endings contained 100-nm dense core vesicles immunoreactive solely for thyrotropin-releasing hormone. (2) In the neural lobe of the pituitary, thyrotropin-releasing hormone immunoreactivity occurred on secretory vesicles in a subpopulation of the mesotocinergic axons containing 160-nm secretory granules; co-localization with vasotocin was never seen. (3) In the intermediate lobe, thyrotropin-releasing hormone- and mesotocin (or neurophysin I)-immunoreactivities were systematically found in the same 120-nm dense core vesicles; these thyrotropin-releasing hormone-/mesotocin-immunoreactive axon terminals frequently made synaptic contacts with melanotropic cells. The possible modulatory effect of mesotocin on thyrotropin-releasing hormone-induced alpha-melanocyte-stimulating hormone secretion was investigated using perifused frog neurointermediate lobes. Administration of graded doses of mesotocin (from 10(-10) to 10(-5) M) did not affect the spontaneous release of alpha-melanocyte-stimulating hormone. In addition, mesotocin (10(-7) and 10(-6) M) did not modify thyrotropin-releasing hormone-evoked alpha-melanocyte-stimulating hormone release.(ABSTRACT TRUNCATED AT 400 WORDS)

Central-type benzodiazepines and the octadecaneuropeptide modulate the effects of GABA on the release of alpha-melanocyte-stimulating hormone from frog neurointermediate lobe in vitro

The involvement of the GABA-benzodiazepine receptor complex in the regulation of melanotropin secretion has been investigated using perfused frog neurointermediate lobes. The GABAA agonist 3-amino-1 propane sulfonic acid mimicked the biphasic effect of GABA on alpha-melanocyte-stimulating hormone secretion: a brief stimulation followed by an inhibition of melanotropin secretion. The GABAA antagonist SR 95531 (10(-4) M) inhibited both stimulation and inhibition of alpha-melanocyte-stimulating hormone release induced by GABA (10(-4) M). Since the inhibitory effect of baclofen (10(-4) M) was partially antagonized by SR 95531 (10(-4) M), it appears that the GABAergic control of alpha-melanocyte-stimulating hormone release is mainly achieved through activation of GABAA receptors. GABA-induced stimulation of alpha-melanocyte-stimulating hormone release was inhibited by tetrodotoxin (10(-5) M), an Na+ -channel blocker, or nifedipine (10(-5) M), a voltage-dependent Ca2+ -channel blocker, suggesting that Na+ and Ca2+ ions are involved in the stimulatory phase of GABA action. Only central-type benzodiazepine binding site agonists such as clonazepam (10(-4) M) modified alpha-melanocyte-stimulating hormone release. In fact, clonazepam (10(-7) to 10(-5) M) led to a dose-dependent potentiation of both GABA-induced stimulation and inhibition of alpha-melanocyte-stimulating hormone release. This potentiating effect was antagonized by the GABAA antagonist SR 95531 (10(-4) M) or by the central-type benzodiazepine binding site antagonist flumazenil (10(-4) M), whereas picrotoxin (10(-4) M) abolished only the stimulatory phase.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of MSH release from the neurointermediate lobe of Xenopus laevis by CRF-like peptides

Immunocytochemical studies showed the presence of a fiber system containing a CRF-like peptide in the median eminence and in the neural lobe of the pituitary gland of Xenopus laevis. During in vitro superfusion of neurointermediate lobe tissue, CRF, sauvagine and urotensin I induced a rapid and dose-dependent stimulation of secretion of MSH and endorphin. Tissue of white-background adapted animals displayed a remarkably higher sensitivity to CRF and sauvagine than tissue from animals that were adapted to a black background. During superfusion of isolated melanotrope cells in suspension, it was shown that CRF and sauvagine exerted their effect directly on the melanotrope cell. We therefore conclude that there is morphological and biochemical evidence to consider a CRF-like peptide as a physiological MSH-releasing factor.

The benzodiazepine agonist clonazepam potentiates the effects of gamma-aminobutyric acid on alpha-MSH release from neurointermediate lobes in vitro

The action of the central-type benzodiazepine-receptor agonist clonazepam on alpha-MSH release has been studied in vitro using perifused frog neurointermediate lobes. High concentrations of clonazepam (3.16 X 10(-5) and 10(-4) M) caused an inhibition of alpha-MSH release and this effect was reversed by the central-type benzodiazepine-receptor antagonist Ro 15-1788. High doses of GABA (10(-5) and 10(-4) M) induced a biphasic effect on pars intermedia cells: a brief stimulation followed by a sustained inhibition of alpha-MSH secretion. Administration of clonazepam (10(-5) M) in the presence of various concentrations of GABA (10(-6) to 10(-4) M) led to a potentiation of both stimulatory and inhibitory phases of alpha-MSH secretion induced by GABA. Ro 15-1788 completely abolished the potentiating effect of clonazepam. Thus our results indicate that endogenous benzodiazepine receptors may modulate the effects of GABA on alpha-MSH secretion.