GABA stimulation of gonadotropin-II release in goldfish: involvement of GABAA receptors, dopamine, and sex steroids

Abstract

The involvement of gamma-aminobutyric acid (GABA) in regulation of pituitary gonadotropin-II (GTH-II) release was studied in the goldfish. Intraperitoneal injection of GABA (300 micrograms/g) stimulated an increase in serum GTH-II levels at 30 min postinjection. The GABAA receptor agonist muscimol (0.1-10 micrograms/g) stimulated GTH-II in a dose-dependent manner. Baclofen, a GABAB receptor agonist, had a small but significant stimulatory effect at 1 and 10 micrograms/g; the amount of GTH-II released in response to baclofen was significantly less (P < 0.05) than that released by muscimol. Pretreatment of goldfish with bicuculline, a GABAA receptor antagonist, but not saclofen, a GABAB receptor antagonist, blocked the stimulatory effect of GABA on serum GTH-II. Elevation of brain and pituitary GABA levels with the GABA transaminase inhibitor, gamma-vinyl-GABA (GVG), decreased hypothalamic and pituitary dopamine (DA) turnover rates, indicating that GABA may stimulate GTH-II release in the goldfish by decreasing dopaminergic inhibition of GTH-II release. The release of GTH-II stimulated by muscimol and GVG was potentiated by pharmacological agents that decrease inhibitory dopaminergic tone, indicating that DA may also inhibit GABA-stimulated GTH-II release. Based on the linear 24-h accumulation of GABA in brain and pituitary after GVG injection, implantation of testosterone, estradiol, or progesterone, previously shown to regulate the serum GTH-II release response to gonadotropin-releasing hormone and GABA, was also found to modulate GABA synthesis in the brain and pituitary.(ABSTRACT TRUNCATED AT 250 WORDS)

The GABAergic system: a possible component of estrogenic feedback on gonadotropin secretion in rainbow trout (Oncorhynchus mykiss)

“…However, a rôle for GABA on GTH2 sécrétion was clearly demonstrated in goldfish. Indeed, intraperitoneal or intracerebroventricular injections of GABA or GABA-transaminase inhibitors stimulate GTH2 release (KAH et al, 1992 ;TRUDEAU et al, 1993a). Since a direct action on GTH cell has not been demonstrated in vitro, it has been postulated that the stimulatory actions of GABA on GTH2 release are mediated via GnRH and/or dopaminergic Systems.…”

“…Since a direct action on GTH cell has not been demonstrated in vitro, it has been postulated that the stimulatory actions of GABA on GTH2 release are mediated via GnRH and/or dopaminergic Systems. The stimulatory effects of GABA on GTH2 release in goldfish resuit from both increased GnRH release and decreased dopaminergic activity (TRUDEAU et al, 1993a). In addition, several in vivo experiments have shown that GABA stimulâtes GTH2 sécrétion in fish at early stages of gonadal development but was ineffective in mature or sexually regressed goldfish, suggesting that the stimulating effects of GABA on GTH2 levels dépend on the steroid environment (KAH et al, 1992).…”

γ-Aminobutyric Acid Up-Regulates the Expression of a Novel Secretogranin-II Messenger Ribonucleic Acid in the Goldfish Pituitary**The work was supported by the Welcome Trust (UK), Balaguer-Gonell Foundation (Spain), and Natural Sciences and Engineering Research Council (Canada).

“…However, a significant stimulatory role for GABA in the control of hypothalamic GnRH (GnRH) (6) and pituitary gonadotropic hormone (7,8) release is now apparent. Our work using the adult goldfish model (reviewed in Ref.…”

“…In vivo, pharmacological evidence indicates that GABA action to enhance GTH-II release is mediated by dual stimulatory effects on GABA A and GABA B receptors (8), which may explain why GABA has a dominant stimulatory effect in the goldfish model. GABA acts to stimulate GTH-II release by enhancement of GnRH release from the nerve terminal within the goldfish pituitary and also by inhibition of the inhibitory preoptic-hypothalamic dopaminergic system.…”

“…Sex steroids in turn modulate GABA responses; testosterone increases, whereas estradiol decreases GABA-induced GTH-II release in vivo. Both steroids also modulate GABA synthesis in the goldfish preoptic-hypothalamic region and pituitary (8). The role of GABA in stimulating GnRH and GTH-II release, and the exquisite sensitivity of the GABA system to sex steroid feedback, underscores the significance of this neurotransmitter in the central regulation of reproduction.…”