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Characterization of -Aminobutyric Acid Receptors in the Neurointermediate Lobe of the Amphibian Xenopus Laevis^*

Department of Zoology, Faculty of Science Toernooiueld, 6525 ED Nijmegen, The Netherlands
Faculty of Science (H. V.), University of Haute-Normandie Rouen, France

Address correspondence and requests for reprints to: B. M. L. Verburg-van Kemenade, Department of Zoology, Faculty of Science, Toernooiveld, 6536 ED Nijmegen, The Netherlands.

Abstract

The neurotransmitter -aminobutyric acid (GABA) is involved in the regulation of secretion of MSH from the intermediate lobe of Xenopus laevis. The purpose of this study was to identify the GABA receptor(s) involved by determination of the effect of specific receptor agonists and antagonists on the release of immunoreactive MSH from superfused neurointermediate lobes of Xenopus. Exogenous GABA induces a rapid inhibition of MSH secretion. There was no evidence for a transitory stimulatory effect of GABA as reported for the rat melanotropes. Both the GABA agonists (GABAa) homotaurine and isoguvacine and the GABA agonist (GABAb) baclofen inhibited MSH release in a dose-dependent manner. In vivo, homotaurine and baclofen caused aggregation of pigment in dermal melanophores. The MSH release-inhibiting effect of homotaurine and isoguvacine could be antagonized by the specific GABAa receptor antagonist bicuculline. However, bicuculline and picrotoxin failed to block the effect of exogenous GABA. We conclude that in the neurointermediate lobe of Xenopus laevis both GABAa and GABAb receptors are present, suggesting a dual inhibitory regulation. (Endocrinology 120: 622–628,1987)

Footnotes

^* This work was supported by EEC Grant nr STI-084-5C.

Received July 17, 1986.

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