GLUTAMATE - A MAJOR NEUROENDOCRINE EXCITATORY SIGNAL MEDIATING STEROID EFFECTS ON GONADOTROPIN-SECRETION

The preovulatory gonadotropin surge is induced by progesterone in the cycling female rat or in the ovariectomized estrogen-treated female ra t after adequate estrogen-priming activity is present. The source of p rogesterone under physiological conditions could be the ovary and/or t he adrenal. Since the GnRH neuron does not possess estrogen and proges terone receptors, its function is modulated by other CNS neurotransmit ters and neurosecretory products. Among these, excitatory amino acids (EAAs) have now been shown to play an important role in the regulation of pulsatile gonadotropin release, induction of puberty and preovulat ory and steroid-induced gonadotropin surges. Glutamate, the major endo genous EAA exerts its action through ionotropic and metabotropic recep tors. The ionotropic receptors consist of two major classes, the NMDA (N-methyl-D-aspartate) and non-NMDA: kainate and AMPA (DL-alpha-amino- 3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors. EAA receptor s are found in hypothalamic areas involved with reproduction. While bo th NMDA and non-NMDA receptors are involved in the regulation of LH se cretion, the NMDA receptors appear to be involved with the regulation of puberty and FSH secretion as well. Steroids increase the release ra tes of glutamate and aspartate in the preoptic area during the gonadot ropin surge. Steroids may also regulate the hypothalamic AMPA receptor s.