AJP - Regulatory, Integrative and Comparative Physiology, Vol 265, Issue 2 348-R355, Copyright © 1993 by American Physiological Society
GABA stimulation of gonadotropin-II release in goldfish: involvement of GABAA receptors, dopamine, and sex steroids
V. L. Trudeau, B. D. Sloley and R. E. Peter
Department of Zoology, University of Alberta, Edmonton, Canada.
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)
