Involvement of GABA A receptor in Bufo arenarum oocyte maturation

Amphibian oocytes meiotic arrest is released under the stimulus of progesterone; this hormone interacts with the oocyte surface and starts a cascade of events leading to the activation of a cytoplasmic maturation promoting factor (MPF) that induces germinal vesicle breakdown (GVBD), chromosome condensation and extrusion of the first polar body. The aim of this work was to determine whether the activation of a GABA A receptor is able to induce GVBD in fully grown denuded oocytes of Bufo arenarum and to analyse its possible participation in progesterone-induced maturation. We also evaluated the role of purines and phospholipids in the maturation process induced by a GABA A receptor agonist such as muscimol. Our results indicated that the activation of the GABA A receptor by muscimol induces maturation in a dose- and time-dependent manner and that this activation is a genuine maturation that enables oocytes to form pronuclei. Assays with a receptor antagonist, picrotoxine, showed that the maturation induced by muscimol was inhibited. Treatment with picrotoxine, however, shows that the participation of GABA A receptor in progesterone-induced maturation is not significant. In addition, our results indicate that high intracellular levels of purines obtained by the use of db-AMPc and theophylline or the inhibition of the phosphatidylinositol 4,5-bisphosphate (PIP 2 hydrolysis by neomycin and PIP 2 turn over by LiCl, respectively, inhibited the maturation induced by muscimol. Treatment with H-7 indicated, however, that PKC activation is not necessary for GVBD induced by the GABA A receptor agonist. Results suggest that the transduction pathway used by the GABA A receptor to induce maturation is different from those used by progesterone.