INFLUENCE OF GABA ON GONADOTROPIN-RELEASE IN THE GOLDFISH

The influence of GABA on pituitary gonadotrophin (GTH) release in the goldfish was studied by means of in vivo and in vitro techniques. It w as found that GABA injected intraperitoneally caused an increase of se rum GTH levels in regressed or early maturing fish, but not in late ma turing animals. Moreover, injection of a GABA transminase inhibitor ca used a significant increase of GABA within the hypothalamus and pituit ary, and a dose-dependent increase in serum GTH levels. To determine i f this effect could be exterted directly at the level of the pituitary , dispersed pituitary cells in static incubation or in perifusion were exposed to increasing concentrations of GABA or its agonists muscimol and baclofen. None of these drugs was able to modify the spontaneous or GnRH-induced secretion of GTH, indicating that the in vivo effect o f GABA was most likely mediated via another hypothalamic factor. Using in vitro incubation of pituitary slices, it was found that GABA cause d a dose-related stimulation of GnRH release at the level of the pitui tary, providing a possible explanation for the observed in vivo stimul atory effect of GABA on GTH release. Since the seasonal effect of GABA in vivo indicated a possible interaction of GABA with sexual steroids , GABA was given intraperitoneally to female goldfish implanted with e ither testosterone or estradiol. We found that the stimulatory effect of GABA on GTH release was abolished in estradiol-treated females but was still observed in testosterone-implanted fish. Moreover, estradiol but not testosterone caused a decrease of the GABA concentration with in the telencephalon. Taken together, these data indicate that GABA is involved in the regulation of GTH secretion in the goldfish, possibly by stimulating the release of GnRH from the pituitary, an effect that appeared to be modulated by estrogens. The inhibitory effects of estr ogens on GABA actions may be part of the mechanism of estrogen negativ e feedback on the brain-pituitary axis.