DUAL-ACTION OF GABA-A RECEPTORS ON THE SECRETORY PROCESS OF MELANOTROPHS OF XENOPUS-LAEVIS

The effects of GABA(a) receptor activation on the secretion of alpha-m elanocyte-stimulating hormone (alpha-MSH) from the superfused pars int ermedia of the amphibian Xenopus laevis were examined. The GABA(a) rec eptor agonist isoguvacine inhibited secretion of alpha-MSH from the pa rs intermedia, an action completely antagonized by the chloride channe l blocker picrotoxin. Isoguvacine stimulated secretion from picrotoxin -treated tissue. Both effects were blocked by the GABA(A) receptor ant agonist bicuculline, indicating that it is a specific action via the G ABA(A) receptor. We conclude that, besides the inhibitory chloride cha nnel, there is a stimulatory signaling property associated with the GA BA(A) receptor. Isoguvacine stimulated the production of c-AMP, an act ion that was not blocked by picrotoxin. This suggests that the stimula tory mechanism of the GABA(A) receptor involves, directly or indirectl y, the generation of c-AMP.