Electrophysiological evidence for the existence of GABA A receptors in cultured frog melanotrophs

Abstract

The neurotransmitter GABA exerts a biphasic effect on α-melanocyte-stimulating hormone (α-MSH) secretion from pars intermedia cells: GABA induces a rapid and transient stimulation followed by a sustained inhibition of α-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 GABA A receptors (bicuculline and SR 95531). The depolarizing action of GABA was mimicked by muscimol, an agonist of GABA A receptors. Taken together, our results indicate that the rapid and transient stimulation of α-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 α-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 α-MSH secretion remain to be elucidated.