The inhibitory effects of gamma-aminobutyric acid (GABA) on growth hormonesecretion in the goldfish are modulated by sex steroids

Double-labelling studies at the electron microscopic level demonstrated tha t gamma-aminobutyric acid (GABA)immunoreactive nerve endings are associated with growth-hormone-secreting cells in the proximal pars distalis of the g oldfish pituitary gland, suggesting that GABA may be important for the cont rol of growth hormone release in this species, An ill vitro assay for GABA- transaminase activity demonstrated that the pituitary is a site for the met abolism of GABA to succinic acid. III vitro, GABA or the GABA antagonists b icuculline and saclofen did not affect the rate of growth hormone release f rom dispersed pituitary cells in static incubation. In cent rast, intracere broventricular injection of GABA reduced serum growth hormone levels within 30 min, During the seasonal gonadal cycle, intraperitoneal injection of GA BA was without effect in sexually regressed goldfish, but caused a signific ant decrease in serum growth hormone levels in sexually recrudescent animal s. Intraperitoneal implantation of solid silastic pellets containing oestra diol increased serum GH levels fivefold in sexually regressed acid recrudes cent goldfish; in both groups, GABA suppressed the oestradiol-stimulated in crease in circulating growth hormone levels. The effect of oestradiol on ba sal serum growth hormone levels was specific since progesterone and testost erone were without effect. However, in recrudescent animals treated with pr ogesterone and testosterone, the inhibitory effects of GABA on serum growth hormone levels were absent, indicating a differential role for these stero ids in growth hormone release. Taken together, these results demonstrate th at GABA has an inhibitory effect on growth hormone release in goldfish.