GABA exerts stimulatory and inhibitory influences on gonadotropin II secretion in the Atlantic croaker (Micropogonias undulatus)

Possible involvement of gamma-aminobutyric acid (GABA) in the control of gonadotropin II (GTH II) secretion was investigated during different phases of the seasonal gonadal cycle of the Atlantic croaker (Micropogonias undulatus). Intraperitoneal administration of GABA (100 mu g/g) elicited a significant increase in plasma GTH II levels in croaker with regressed or pre-recrudescence phase gonads, whereas it did not significantly affect GTH II levels during the early- to mid-recrudescence phase of the gonadal cycle in this species. On the other hand, GABA induced a dose-related inhibition of GTH II secretion in fish with fully recrudesced gonads, Similar opposite changes in GTH II levels were observed when croaker with regressed and fully recrudesced gonads were injected with the GABA transaminase inhibitor, gamma-vinyl GABA (10 and 100 mu g/g). This suggests an involvement of endogenous GABA in the control of GTH II secretion. GABA (1-100 mu M) did not significantly alter GTH II release from pituitary fragments from regressed or fully recrudesced fish incubated in vitro, suggesting the lack of a major direct influence at the pituitary level. Musci mol (GABA(A) receptor agonist, 1 mu g/g) treatment mimicked both the stimulatory and inhibitory effects of GABA, whereas injection of baclofen (GABA(B) receptor agonist, 1 mu g/g) had no significant effect on basal or a gonadotropin-releasing hormone (GnRH) analog-induced GTH II secretion. Pretreatment of fish with bicuculline (1 mu g/g), a GABA(A) antagonist, completely blocked the stimulatory influence of GABA on GTH II secretion in croaker with regressed gonads and partially blocked the inhibitory effect in individuals with fully recrudesced gonads. The results demonstrate that GABA exerts opposite influences on GTH II secretion at two different stages of the gonadal cycle of croaker, and acts primarily via GABA(A) type receptors. Thus, GABAergic pathways may be important in mediating seasonal influences on GTH II secretion in this species.