Involvement of gamma-aminobutyric acid in the control of GTH-1 and GTH-2 secretion in male and female rainbow trout

The potential role of the neurotransmitter gamma-aminobutyric acid (GABA) i n the control of the secretion of the two pituitary fish gonadotropins (GTH -1 and GTH-2) was investigated in male and female rainbow trout (Oncorhynch us mykiss). The presence of glutamate decarboxylase-positive fibers in the neurohypophyseal digitations adjacent to the gonadotropic cells was demonst rated by means of double immunohistochemistry, providing a morphofunctional support for potential GABA-gonadotropin interactions in both sexes. In spe rmiating males, in vivo treatment with GABA did not affect basal gonadotrop in release, but stimulated GTH-1 release when coadministered with a gonadot ropin-releasing hormone analogue (GnRHa), and potentiated GnRHa-stimulated GTH-2 release. In vitro, using dispersed pituitary cells, GABA stimulated b asal GTH-1 and GTH-2 secretion, in a dose-dependent manner, and potentiated salmon GnRH effect on both hormones. In mature females, GABA induced in vi vo a strong elevation of plasma GTH-2 levels after 2-6 h of injection, but had no effect in vitro. GABA treatment in vivo was also stimulatory in recr udescent females, slightly increasing plasma GTH-2 levels in both saline- a nd GnRHa-treated fish (GnRHa alone has no effect at this stage). Immature f ish were un responsive to GABA/GnRHa treatments but, after steroid implanta tion [testosterone (T) or estradiol] for 13 days, injection of GABA stimula ted GTH-2 release in vivo (also GTH-1 slightly in T-implanted fish). In con clusion, GABA has an overall stimulatory action on GTH-1 and GTH-2 secretio n in rainbow trout, which depends on the sex and the reproductive stage of the fish. The stimulatory action of GABA might be exerted, at least in part , directly onto the gonadotropes, as it stimulates basal and GnRH-induced G TH-1 and GTH-2 secretion from dispersed pituitary cells.