Neuroendocrine gamma-aminobutyric acid (GABA): Functional differences in GABA(A) versus GABA(B) receptor inhibition of the melanotrope cell of Xenopus laevis

The melanotrope cell of Xenopus laevis is innervated by nerve terminals that contain, among other transmitter substances, the neurotransmitter gamma-aminobutyric acid (GABA). Postsynaptically the melanotrope cell possess both GABA(A) and GABA(B) receptors. Activation of either receptor type leads to an inhibition of alpha MSH release from the cell. The present study concerns the functional significance of the existence of two types of GABA receptors on the melanotrope regarding two questions: 1) do the different receptor types have different effects on the melanotrope? and 2) can the endogenous ligand GABA differentially activate these receptors? Concerning the first question, we have tested the hypothesis that the GABA(A) receptor(a chloride ion channel) and the GABA(B) receptor (a G protein-coupled receptor neg negatively linked to adenylyl cyclase) may have differential effects on the sensitivity of the cell to stimulation by cAMP-dependent mechanisms. We show that treatments with either isoguvacine (GABA(A) agonist) or baclofen (GABA(B) agonist) inhibit intracellular Ca2+ oscillations and peptide secretion from melanotrope cells. Treatments known to increase intracellular cAMP in the melanotrope (e.g. use of the peptide sauvagine or the cAMP analog 8-bromo-cAMP) completely overcame the inhibition induced by baclofen, but not that caused by isoguvacine. We conclude that the GABA(A) and GABA(B) receptors have different effects on the Xenopus melanotrope cell by differentially affecting the sensitivity the cell shows to stimulation by cAMP-dependent mechanisms. Concerning possible differential activation of the receptor types, we found that we could use a membrane potential probe (from the bis-oxonol family) to differentiate between GABA(A) and GABA(B) receptor activation. Using this probe we showed that low GABA concentrations (< 10(-7) M) give a response indicative of the GABA(B) receptor, whereas at high GABA concentrations (> 10(-7) hr), the GABA(A) receptor response predominates. We, therefore, conclude that GABA can differentially activate the two types of GABA receptors on the Xenopus melanotrope cell.