MECHANISM OF ACTION OF GAMMA-AMINOBUTYRIC-ACID ON FROG MELANOTROPHS

We have previously demonstrated that gamma-aminobutyric acid (GABA) is a potent regulator of secretory and electrical activity in melanotrophs of the frog pituitary. The aim of the present study was to investigate the intracellular events which mediate the response of melanotrophs to GABA. We first observed that GABA (1-100 mu M) inhibited both basal and forskolin-stimulated cyclic AMP (cAMP) formation. The inhibitory effect of GABA on cAMP levels was mimicked by the GABA(B) receptor agonist baclofen (100 mu M) and totally abolished by a 4-h pretreatment with pertussis toxin (0.1 mu g/ml). In contrast, the specific GABA(A) agonist 3-aminopropane sulphonic acid (3APS) did not affect cAMP production. Both GABA and 3APS (100 mu M each) induced a biphasic effect on alpha-MSH release from perifused frog neurointermediate lobes, i.e. a transient stimulation followed by an inhibition of alpha-MSH secretion. Administration of forskolin (100 mu M) prolonged the stimulatory phase and attenuated the inhibitory phase evoked by GABA and 3APS, indicating that cAMP modulates the response of melanotrophs to GABA(A) agonists. Ejection of 3APS (1 mu M) in the vicinity of cultured melanotrophs caused a massive increase in intracellular calcium concentration ([Ca2+](i)). The stimulatory effect of 3APS on [Ca2+](i) was abolished when the cells were incubated in a chloride-free medium. The formation of inositol trisphosphate was not affected by 3APS, suggesting that the increase in [Ca2+](i) cannot be ascribed to mobilization of intracellular calcium stores. omega-Conotoxin did not alter the secretory response of frog neurointermediate lobes to 3APS, while nifedipine blocked the stimulation of alpha-MSH secretion induced by 3APS. In conclusion, the present data indicate that, in frog pituitary melanotrophs, (i) the stimulatory phase evoked by GABA(A) agonists can be accounted for by an influx of calcium through L-type calcium channels, (ii) the inhibitory effect evoked by GABA(A) agonists can be ascribed to inhibition of adenylate cyclase activity and (iii) cAMP attenuates the inhibitory phase evoked by GABA(A) agonists. Taken together, these data suggest that activation of GABA(B) receptors may modulate GABA(A) receptor function.