GABA stimulation of gonadotropin-II release in goldfish: involvement of GABAA receptors, dopamine, and sex steroids.
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)
Department of Zoology, University of Alberta, Edmonton, Canada.
V L
VL
Trudeau
B D
BD
Sloley
R E
RE
Peter
D000614:Aminocaproates / Q000494:pharmacology; D000818:Animals; D001921:Brain / Q000378:metabolism; D004298:Dopamine / Q000502:physiology*; D058787:GABA-A Receptor Antagonists; D006054:Goldfish / Q000378:metabolism*; D012739:Gonadal Steroid Hormones / Q000502:physiology*; D006062:Gonadotropins / Q000097:blood / Q000378:metabolism*; D010902:Pituitary Gland / Q000378:metabolism; D011963:Receptors, GABA-A / Q000502:physiology*; D020888:Vigabatrin; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology*
D000614:Aminocaproates; D058787:GABA-A Receptor Antagonists; D012739:Gonadal Steroid Hormones; D006062:Gonadotropins; D011963:Receptors, GABA-A; D005680:gamma-Aminobutyric Acid; D020888:Vigabatrin; D004298:Dopamine