GAMMA-AMINO-BUTYRIC ACID AND THE CONTROL OF GNRH SECRETION IN SHEEP

The release of GnRH from nerve terminals in the median eminence into t he portal vessels is influenced by factors in the internal and externa l environment of the animal. In the former category are the gonadal st eroid hormones oestrogen and progesterone which alter the characterist ics of GnRH secretion during the oestrous and seasonal cycles. These c annot exert their actions directly on the GnRH neurones as they do not possess hormone receptors. Therefore, some other steroid-sensitive ne uronal system must relay this information to the GnRH neurones. Gamma amino-butyric acid (GABA) neurones are good candidates for this role a s they contain steroid hormone receptors and synapse on GnRH neurones. Recent studies in ewes have sought to identify a role for GABA in med iating the actions of both oestrogen and progesterone on GnRH release. The technique of microdialysis was used to monitor GABA concentration s in areas containing GnRH cell bodies during the oestrogen-induced su rge of GnRH and during progesterone negative feedback. Concentrations of this inhibitory neurotransmitter have been shown to fall in the for mer situation where GnRH release is being stimulated, but to be increa sed when progesterone is depressing GnRH release. GABA may also be imp ortant in mediating the seasonal switch in the negative feedback actio ns of oestradiol. During the anoestrous season, when oestradiol is a p otent inhibitor of GnRH secretion, specific GABA receptor antagonists can stimulate neurohormone release, an action that is not observed in the breeding season when oestrogen is much less potent.