The Involvement of GABAA receptors in the control of GnRH and beta-endorphin release, and catecholaminergic activity in the preoptic area in anestrous ewes.

This study examined role of GABA A receptors in the control of GnRH, beta-endorphin release and catecholaminergic system activity in the preoptic area and LH secretion in anestrous ewes. Stimulation of GABA A receptors in the medial preoptic area (MPOA) by muscimol attenuated GnRH release and dopaminergic system activity and increased extracellular noradrenaline (NE) and MHPG concentration. Muscimol has no evident effect on the extracellular concentration of beta-endorphin-like immunoreactivity (B-END-LI) in the MPOA. The decrease of LH pulse frequency and concentration of this hormone in blood plasma suggests that GABA A receptor agonist applied in the MPOA suppresses GnRH release from the GnRH axon terminals in the ventromedial hypothalamus-nucleus infundibularis region (VEN/NI) into the hypophyseal vascular system. Blockade of GABA A receptors with bicuculline did not change GnRH release, catecholaminergic activity, B-END-LI concentration in the MPOA, and LH release. The presented data indicate that activation of GABA A receptors in the MPOA decreases extracellular concentration of GnRH in this structure and LH level in the blood plasma thus suggesting that GABA may act in the MPOA to inhibit GnRH release in the VEN/NI. These results suggest that suppression of GnRH/LH release during muscimol treatment may result from activation of GABA A receptors on the GnRH perikarya and/or through GABA A receptor mechanism on the dopaminergic and noradrenergic system in the MPOA. Lack of changes in B-END-LI concentration during stimulation or blocking GABA A receptors suggests, that beta-endorphinergic system in the MPOA does not participate in the GABA A receptors mechanism modulating GnRH release.