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

Both glutamate and gamma-aminobutyric acid (GABA) are involved in pituitary hormone release in fish. Glutamate serves 2 purposes, both as a neurotrans mitter and as a precursor for GABA synthesis. Glutamate can be catabolized to GABA by the actions of 2 distinct but related enzymes, glutamate decarbo xylase 65 (GAD65) and GAD67. They derive from 2 different genes that likely arose from an early gene duplication prior to the emergence of teleosts mo re than 400 million years ago. There is good evidence for the involvement o f GABA in luteinizing hormone (LH) release in fish. The mechanism of GABA a ction to stimulate LH release appears to be a combination of effects on GnR H release, potentiation of gonadotropin hormone-releasing hormone (GnRH) ac tion, 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 L H are present in at least 4 fish species. In contract, convincing data for the inhibitory effects of GABA on LH release have only been observed in 1 f ish species. The sites and mechanisms of action of amino acid neurotransmit ters on LH release have yet to be fully characterized. Both 130N-methyl-D-a spartic acid (NMDA) and S-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropion ic acid (AMPA) type glutamate receptors are likely to have important roles. We suggest that it is a receptor similar to the GABA(A) type which mediate s the effects of GABA on LH release in fish, at least partially acting on t he 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 rel ease in teleost fish is proposed.