Involvement of γ-Aminobutyric Acid in the Control of GTH-1 and GTH-2 Secretion in Male and Female Rainbow Trout

Abstract

The potential role of the neurotransmitter γ-aminobutyric acid (GABA) in the control of the secretion of the two pituitary fish gonadotropins (GTH-1 and GTH-2) was investigated in male and female rainbow trout (Oncorhynchus mykiss). The presence of glutamate decarboxylase-positive fibers in the neurohypophyseal digitations adjacent to the gonadotropic cells was demonstrated by means of double immunohistochemistry, providing a morphofunctional support for potential GABA-gonadotropin interactions in both sexes. In spermiating males, in vivo treatment with GABA did not affect basal gonadotropin release, but stimulated GTH-1 release when coadministered with a gonadotropin-releasing hormone analogue (GnRHa), and potentiated GnRHa-stimulated GTH-2 release. In vitro, using dispersed pituitary cells, GABA stimulated basal 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 vivo 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 recrudescent females, slightly increasing plasma GTH-2 levels in both saline- and GnRHa-treated fish (GnRHa alone has no effect at this stage). Immature fish were unresponsive to GABA/GnRHa treatments but, after steroid implantation [testosterone (T) or estradiol] for 13 days, injection of GABA stimulated GTH-2 release in vivo (also GTH-1 slightly in T-implanted fish). In conclusion, GABA has an overall stimulatory action on GTH-1 and GTH-2 secretion 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 GTH-1 and GTH-2 secretion from dispersed pituitary cells.

Distribution of dopamine D2 receptor mRNAs in the brain and the pituitary of female rainbow trout: An in situ hybridization study

“…This finding suggests that, as in mammals, DA could modulate the activity of GABAergic neurons through D 2 receptors. In fish, GABA stimulates LH release Kah et al, 1992;Mañ anos et al, 1999), and, in goldfish, there is indication that DA inhibits GABA-stimulated GTH-II release, evidencing functional DA-GABA interactions (Trudeau et al, 1993).…”

The Spatial Relationship of γ-Aminobutyric Acid (GABA) Neurons and Gonadotropin-Releasing Hormone (GnRH) Neurons in Larval and Adult Sea Lamprey, <i>Petromyzon marinus</i>

“…Some teleosts in which GABA has been studied include the adult goldfish, rainbow trout, silver eel, red seabream and deep sea armed grenadier [Martinoli et al, 1990;Trudeau et al, 1993aTrudeau et al, , b, 2000aMedina et al, 1994;Mananos et al, 1999;Senthilkumaran et al, 2001]. In these studies, GABA, and agents that alter GABAergic function, significantly influenced reproductive processes by their actions on the hypothalamus and pituitary.…”

Sexually Dimorphic Expression of Glutamate Decarboxylase mRNA in the Hypothalamus of the Deep Sea Armed Grenadier, <i>Coryphaenoides (Nematonurus) armatus</i>

“…For example, at depths shallower than 4,000 m, the ratio of females to males varies between 1 and 5, whereas below 4,000 m, there are approximately 10 to 15 females per male caught in standard trawls. Female C. (N.) armatus are larger than males, especially at depths greater than 4,000 m. Given that GABA is implicated in the neuroendocrine regulation of both growth [Trudeau et al, 2000a] and reproduction [Trudeau, 1997;Khan and Thomas, 1999;Mañanos et al, 1999] in fish, we reasoned that differences in GAD mRNA levels might be apparent in animals collected at a depth where sexual dimorphism in body size has been documented [Merrett, 1992]. Moreover, Smith et al [1997] and Jones et al [1998] present evidence for a seasonal migration by the armed grenadier C. (N.) armatus between the slope regions and abyssal plains of the NE Atlantic Ocean.…”