Regulatory role of excitatory amino acids in reproduction.

Glutamate, the major excitatory amino acid (EAA) transmitter in the central nervous system, has been implicated as a critical mediator in brain function. Glutamate and its receptors are found in all key hypothalamic areas critically involved in reproduction. Administration of glutamate and its agonists can bring about LH release in animals with a steroid background. Antagonists of the ionotropic glutamate receptors inhibited LH release and abolished the steroid-induced and the preovulatory LH surge. Both NMDA and non-NMDA receptor antagonists can also inhibit pulsatile LH release in castrated animals. The preoptic area has been implicated as a primary site of action of NMDA, while non-NMDA agonists have been suggested to act primarily at the arcuate/median eminence level. While EAAs may act directly on GnRH neurons to enhance GnRH release, the majority of evidence suggests that an indirect mechanism, involving EAA activation of nitric oxide and/or catecholamines, plays a major role in the GnRH-releasing effects of EAAs. Furthermore, there is also some evidence that the tonic inhibitory effect of opioids on GnRH may also involve, at least in part, a suppression of glutamate. Finally, EAA stimulation of GnRH/LH release is markedly attenuated in middle-aged rats, suggesting that a defect in glutamate neurotransmission may underlie the attenuated LH surge observed in aging.