Testosterone Enhances GABA and Taurine but not N‐Methyl‐D,L‐Aspartate Stimulation of Gonadotropin Secretion in the Goldfish: Possible Sex Steroid Feedback Mechanisms

The effects of gonadal steroids on GABA‐, taurine (TAU)‐ and N‐methyl‐D, L‐aspartate (NMA)‐induced gonadotropin‐II (GTH‐II) release were investigated in male and female goldfish in vivo. In sexually regressed goldfish (both sexes mixed), intraperitoneal implantation for 5 to 10 days with solid Silastic pellets containing testosterone (100 μg/g), oestradiol (100 μg/g) or progesterone (100 μg/g) was previously shown to elevate serum sex steroid levels to values comparable to those in sexually mature animals, and to potentiate gonadotropin‐releasing hormone‐stimulated GTH‐II release. In the present study, testosterone but not oestradiol or progesterone enhanced the stimulatory effects of exogenous GABA (100 μg/g) on GTH‐II release in vivo. TAU (1 mg/g) stimulated GTH‐II release in sexually regressed mixed sex and sexually recrudescent male goldfish, and both testosterone and oestradiol implantation enhanced GTH‐II release induced by TAU. The glutamate agonist NMA (25 to 50 μg/g) was also found to stimulate GTH‐II release; however it was relatively less effective in elevating serum GTH‐II levels than GABA and TAU, and its effects were not modulated by sex steroid treatments. Pretreatment of goldfish with α‐methyl‐p‐tyrosine to deplete brain and pituitary catecholam‐ines did not affect NMA action on GTH‐II release. Our results indicate that GABA, TAU and NMA are involved in the neuroendocrine regulation of GTH‐II release in goldfish, and support the idea that testosterone participates in the positive feedback regulation of pituitary gonadotropin release in a non‐mammalian vertebrate by enhancing GABA‐ and TAU‐stimulated GTH release in vivo.