Excitatory amino acids (EAA) were originally implicated in neuronal death, and many research efforts were devoted to their role as potential mediators of neuronal death. Besides their role as excitotoxins, it has become apparent that these substances participate in several regulatory homeostatic mechanisms including the endocrine. In collaboration with Dr. Donoso, our laboratory has provided the basis for considering the EAA system a physiological regulatory system acting as a stimulatory input to LHRH neurons and, by that, as a major controller of reproductive physiology. During prior years, we postulated that the actions of EAA on LHRH secretion were mediated primarily via non-NMDA receptors. In contrast, several lines of pharmacological evidence indicated that N- methyl-D-aspartic acid (NMDA), the prototype agonist for NMDA receptors, was more potent in increasing LH secretion when compared to other non- NMDA receptor agonists. During this last year, we have undertaken this apparent contradiction and have evaluated the ability of NMDA and non- NMDA receptor agonists to elicit LHRH secretion using immortalized LHRH neurons (GT1-7 cells) incubated in vitro. In the initial studies, we have used NMDA and alpha-amino-3-hydroxy-5-methyl-4- isoxazole propionic acid (AMPA) as specific agonists for NMDA and non-NMDA receptors, respectively. NMDA, as previously observed by our group, was unable to influence LHRH secretion from GT1-7 cells incubated in vitro at concentrations as high as 20 mM. Conversely, AMPA stimulated LHRH release in a dose-dependent manner with a minimum effective dose as low as 100 mM. Therefore, these observations clearly support the notion that the effects of EAA on LHRH neurons are mediated by non-NMDA receptors acting directly on the LHRH neuronal network. Furthermore, we were able to demonstrate an interaction between inhibitory (IAA) and EAA systems in the regulation of LHRH secretion using hypothamalic explants incubated in vitro. The fact that 1) GT1 cells are responsive to AMPA and 2) express gamma-amino-butyric acid receptor mRNA reinforce the idea that both IAA and EAA establish a dual inhibitory and stimulatory system acting upon the LHRH neuronal network.
