A presumptive role for GABA in the stimulatory effects of Des-Gly10, [D-Ala6]-LHRH-ethylamide and pimozide on the gonadotropin release in carp

The role of amino acid neurotransmitters in the regulation of pituitary gonadotropin release in fish

Both glutamate and γ-aminobutyric acid (GABA) are involved in pituitary hormone release in fish. Glutamate serves 2 purposes, both as a neurotransmitter and as a precursor for GABA synthesis. Glutamate can be catabolized to GABA by the actions of 2 distinct but related enzymes, glutamate decarboxylase 65 (GAD65) and GAD67. They derive from 2 different genes that likely arose from an early gene duplication prior to the emergence of teleosts more than 400 million years ago. There is good evidence for the involvement of GABA in luteinizing hormone (LH) release in fish. The mechanism of GABA action to stimulate LH release appears to be a combination of effects on GnRH release, potentiation of gonadotropin hormone-releasing hormone (GnRH) action, and in some cases directly at the LH cell. These actions appear to be dependent on such factors as sex or sex steroid levels, and there may also be species differences. Nevertheless, the stimulatory effects of GABA on LH are present in at least 4 fish species. In contrast, convincing data for the inhibitory effects of GABA on LH release have only been observed in 1 fish species. The sites and mechanisms of action of amino acid neurotransmitters on LH release have yet to be fully characterized. Both N-methyl-D-aspartic acid (NMDA) and S-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type glutamate receptors are likely to have important roles. We suggest that it is a receptor similar to the GABAA type which mediates the effects of GABA on LH release in fish, at least partially acting on the GnRH neuron, but likely directly acting at the gonadotroph as well. GABA may also be involved in regulating the release of other pituitary hormones in fish, namely follicle stimulating hormone (FSH = GTH-I), prolactin, and growth hormone. Based on the findings described in this review, a working model for the involvement of glutamate and GABA in the regulation of LH release in teleost fish is proposed.

Key words: glutamate, GABA, luteinizing hormone, muscimol, patch clamp electrophysiology, reproduction, fish.

Distribution of gamma-aminobutyric acid in catfish (Heteropneustes fossilis) forebrain in relation to season, ovariectomy and E2 replacement, and effects of GABA administration on plasma gonadotropin-II level

In the catfish Heteropneustes fossilis, the hypothalamus and telencephalon showed seasonal variations in gamma-aminobutyric acid (GABA) with high levels in prespawning and spawning phases and low levels in preparatory and postspawning phases. Ovariectomy for 4 and 5 weeks reduced significantly the GABA contents only in the hypothalamus. Replacement with E2 (1 microgram/g BW) restored the levels to that of sham ovariectomized or parallel control group. Treatment with GABA (i.p.; 10 or 50 micrograms/g body weight (BW) alone did not produce any significant effect on plasma gonadotropin-II (GTH-II) level in any of the seasons. Injection of GABA, but not baclofen (a GABAB agonist), stimulated GTH-II secretion in pimozide or GnRH analogue-pimozide pretreated fish at both 0.5 and 2 h in early prespawning phase except at 0.5 h in the pimozide--GABA (10 micrograms) group. This stimulatory effect was not evident in other seasons. The results of the present study suggest that Estradiol-17 beta (E2) seems to stimulate GABA which may account for its high level in the recrudescent phase. GABA seems to have a permissive role in GTH-II secretion when dopamine receptor function is inhibited.

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) 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.