Release of glutamate and aspartate from the preoptic area during the progesterone-induced LH surge: In vivo microdialysis studies

To examine the role of excitatory amino acid neurotransmitters in the regulation of LH and FSH secretion in ovariectomized (ovx) adult rats treated with estradiol and progesterone, we measured the release rates of the excitatory amino acid neurotransmitters, glutamate and aspartate, as well as the neurally inactive amino acid serine in microdialysis perfusate samples of the preoptic area (POA) of the hypothalamus collected at 30-rnin intervals during the LH surge induced by progesterone. The concentrations of the amino acids were measured by assay using orthophthalaldehyde prederivatization followed by HPLC coupled with fluorometric detection. The release rates of glutamate in the POA increased at 15:30 and 16:00 h in the progesterone-treated estrogen- primed ovx rat as compared to estrogen controls. Release rates of aspartate in the POA were increased at 12:00, 12:30, 15:00 and 16:00 h in the progesterone-treated estrogen-primed ovx rat as compared to estrogen controls. The increase in POA release rates of glutamate and aspartate in the progesterone- treated rats occured immediately prior to peak serum LH levels induced by progesterone. The specificity of the increase in POA release rates of glutamate and aspartate was evidenced by the finding that the preoptic release of neurally inactive neurotransmitter serine was unchanged by steroid treatment. Estradiol alone had no significant effect on the POA release rates of glutamate, aspartate or serine as compared to ovariectomized controls. In conclusion, the present study demonstrates that the POA release rates of glutamate and aspartate are increased immediately preceding the peak of the progesterone- induced LH surge in the estrogen-primed adult ovx rat. This finding adds further evidence that excitatory amino acid neurotransmission may be an important component in the neurotransmission line mediating gonadotropin surge expression in the female rat.