EFFECT OF NMDA AND NON-NMDA RECEPTOR ANTAGONISTS ON PULSATILE LUTEINIZING-HORMONE SECRETION IN THE ADULT MALE-RAT

To determine the physiological role of excitatory amino acids (EAAs) i n the pulsatile secretion of LH and FSH in the adult male rat, specifi c antagonists for N-methyl-D-aspartic acid (NMDA) receptors [2-amino-5 -phosphono-pentanoic acid (AP-5, 10 mu g)] and quisqualate/kainate (no n-NMDA) receptors [6,7-dinitroquinoxaline-2,3-dione (DNQX; 30 nM)] wer e administered through intracerebroventricular injection to orchiectom ized adult rats. Blood samples were collected from indwelling jugular catheters at 10-min intervals for plasma LH and FSH determinations. Th e results revealed that administration of a single central injection o f AP-5 significantly suppressed LH and FSH mean levels, trough levels and pulse amplitude but not pulse frequency. The non-NMDA receptor ant agonist DNQX had no effect on any parameter of LH or FSH secretion aft er a single central injection. In a second set of experiments, three c entral injections of the EAA antagonists at 0, 20 and 40 min were util ized in order to give a more prolonged block of the EAA receptors and the effect on pulsatile LH secretion was determined. Regardless of the three injections, AP-5 still did not significantly alter LH pulse fre quency. However, the prolonged treatment with DNQX now significantly s uppressed mean and trough LH levels as well as LH pulse amplitude with no effect observed on LH pulse frequency. Taken as a whole, the prese nt studies suggest that EAAs are involved in enhancing gonadotropin pu lsatility in the male rat and that the effect is exerted primarily on pulse amplitude rather than pulse frequency.