Neuroendocrine γ-Aminobutyric Acid (GABA): Functional Differences in GABAAVersusGABABReceptor Inhibition of the Melanotrope Cell ofXenopus laevis1

Buzzi, Marco; Bemelmans, Frank F. J.; Roubos, Eric W.; Jenks, Bruce G.

doi:10.1210/endo.138.1.4886

Cited by 15 publications

(8 citation statements)

References 45 publications

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“…In the gramicidinperforated patch recording, the currents evoked by isoguvacine at −20 and −50 mV were opposite, whilst in the whole-cell configuration they both remained inwardly directed. Buzzi et al 1997). These observations are inconsistent with electrophysiological data revealing that GABA induces an immediate abolition of the spontaneous action currents (Taraskevich et al 1982;Louiset et al 1992).…”

Section: Discussionmentioning

confidence: 99%

“…In frog melanotrophs, calcium measurements have shown that GABAA receptor activation increased [Ca¥]é through nifedipinesensitive voltage-gated calcium channels (Desrues et al 1995). In Xenopus laevis melanotrophs that exhibit [Ca¥]é oscillations, GABAA receptor agonists induced an initial peak and a subsequent arrest of [Ca¥]é pulsing (Shibuya & Douglas, 1993;Buzzi et al 1997;Shibuya et al 1997).…”

Section: Responses Mediated By Gabaa Receptors In Frog Melanotrophsmentioning

confidence: 99%

“…In these cells, the frequency and amplitude of [Ca¥]é transients are supposed to be dependent on the membrane potential. Action potential-driven calcium delivery is thought to be directly responsible for the basal and stimulated hormone secretions (Louiset et al 1988;Shibuya & Douglas, 1993;Desrues et al 1995;Shibuya et al 1997;Buzzi et al 1997;Valentijn & Valentijn, 1997). Thus, the early transient stimulation of á-MSH release is probably due to the GABA-induced depolarization leading to the activation of voltage-operated sodium and calcium channels.…”

Section: Responses Mediated By Gabaa Receptors In Frog Melanotrophsmentioning

confidence: 99%

“…In the African clawed toad Xenopus laevis and frog Rana ridibunda, GABA induces a biphasic effect. Both phases are mimicked by GABAA receptor agonists and are blocked by bicuculline (Adjeroud et al 1986;Desrues, Vaudry, Lamacz & Tonon, 1995;Buzzi, Bemelmans, Roubos & Jenks, 1997). In the amphibians, the stimulatory and inhibitory actions of GABA can thus be ascribed to the activation of GABAA receptors.…”

mentioning

confidence: 99%

“…In the amphibians, the stimulatory and inhibitory actions of GABA can thus be ascribed to the activation of GABAA receptors. The secretion of á-MSH requires calcium entry through voltage-gated calcium channels (Shibuya & Douglas, 1993;Desrues et al 1995;Buzzi et al 1997;Shibuya, Kongsamut & Douglas, 1997). It has recently been shown that the phasic elevations of cytosolic calcium in Xenopus laevis melanotrophs are triggered by action potentials (Valentijn & Valentijn, 1997).…”

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

Foll

Castel

Soriani

et al. 1998

The Journal of Physiology

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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. In the gramicidin‐perforated patch configuration, 1 μM GABA caused a depolarization associated with an action potential discharge and a slight fall of membrane resistance. In contrast, at a higher concentration (10 μm) 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. In gramicidin‐perforated cells, the depolarizing excitatory effect of 1 μm GABA was converted into a depolarizing inhibitory action when 0.4 μm allopregnanolone was added to the bath solution. 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. In gramicidin‐perforated cells, the steady‐state I–V relationships of 10 μm 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 μm GABA also reversed at these potentials. 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.

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Section: Discussionmentioning

confidence: 99%

Section: Responses Mediated By Gabaa Receptors In Frog Melanotrophsmentioning

confidence: 99%

Section: Responses Mediated By Gabaa Receptors In Frog Melanotrophsmentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

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

Foll

Castel

Soriani

et al. 1998

The Journal of Physiology

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Dynamics and plasticity of peptidergic control centres in the retino‐brain‐pituitary system of Xenopus laevis

Krämer

Kolk

Berghs

et al. 2001

Microscopy Res & Technique

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This review deals particularly with the recent literature on the structural and functional aspects of the retino-brain-pituitary system that controls the physiological process of background adaptation in the aquatic toad Xenopus laevis. Taking together the large amount of multidisciplinary data, a consistent picture emerges of a highly plastic system that efficiently responds to changes in the environmental light condition by releasing POMC-derived peptides, such as the peptide alpha-melanophore-stimulating hormone (alpha-MSH), into the circulation. This plasticity is exhibited by both the central nervous system and the pituitary pars intermedia, at the level of molecules, subcellular structures, synapses, and cells. Signal transduction in the pars intermedia of the pituitary gland of Xenopus laevis appears to be a complex event, involving various environmental factors (e.g., light and temperature) that act via distinct brain centres and neuronal messengers converging on the melanotrope cells. In the melanotropes, these messages are translated by specific receptors and second messenger systems, in particular via Ca(2+) oscillations, controlling main secretory events such as gene transcription, POMC-precursor translation and processing, posttranslational peptide modifications, and release of a bouquet of POMC-derived peptides. In conclusion, the Xenopus hypothalamo-hypophyseal system involved in background adaptation reveals how neuronal plasticity at the molecular, cellular and organismal levels, enable an organism to respond adequately to the continuously changing environmental factors demanding physiological adaptation.

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Intracellular calcium buffering shapes calcium oscillations in Xenopus melanotropes

Koopman

Scheenen

Schoolderman

et al. 2001

Pflügers Arch - Eur J Physiol

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The pituitary melanotrope cell of Xenopus laevis displays cytosolic Ca2+ oscillations that arise for the interplay between the burst-like openings of voltage-operated Ca2+ channels and Ca2+-extrusion mechanisms. We have previously shown that Ca2+-extrusion rates increase with increases in [Ca2+]i, suggesting that Ca2+ itself plays a role in shaping the Ca2+ oscillations. The purpose of the present study was to test this hypothesis by manipulating the intracellular Ca2+ buffering capacity of the cell and determining the consequences of such manipulations for the shape of the Ca2+ oscillations. We manipulated the cytosolic buffering capacity by loading the fast Ca2+ chelator BAPTA into cells. During loading the [Ca2+]i was dynamically imaged with confocal laser scanning microscopy. The basal [Ca2+]i was reduced with BAPTA loading and this reduction was associated with lower Ca2+-extrusion rates, a broadening of the Ca2+ oscillations and declined oscillation frequencies. Short loading periods of the buffer led to new, stable patterns of Ca2+ signaling and to reduced but stable levels of peptide secretion. We propose that the cytosolic Ca2+ buffer capacity, and thus by inference the profile of intracellular Ca2+ buffering proteins, is an important factor in setting the frequency and shape of Ca2+ oscillations.

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Neuroendocrine γ-Aminobutyric Acid (GABA): Functional Differences in GABA_AVersusGABA_BReceptor Inhibition of the Melanotrope Cell ofXenopus laevis¹

Cited by 15 publications

References 45 publications

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

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

Dynamics and plasticity of peptidergic control centres in the retino‐brain‐pituitary system of Xenopus laevis

Intracellular calcium buffering shapes calcium oscillations in Xenopus melanotropes

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