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) in the pulsatile secretion of LH and FSH in the adult male rat, specific antagonists for N-methyl-D-aspartic acid (NMDA) receptors [2-amino-5-phosphono-pen-tanoic acid (AP-5, 10 µg)] and quisqualate/kainate (non-NMDA) receptors [6, 7-dinitroquinoxaline-2, 3-dione (DNQX; 30 nM)] were administered through intracerebroventricular injection to orchiectomized adult rats. Blood samples were collected from indwelling jugular catheters at 10-min intervals for plasma LH and FSH determinations. The results revealed that administration of a single central injection of AP-5 significantly suppressed LH and FSH mean levels, trough levels and pulse amplitude but not pulse frequency. The non-NMDA receptor antagonist DNQX had no effect on any parameter of LH or FSH secretion after a single central injection. In a second set of experiments, three central injections of the EAA antagonists at 0, 20 and 40 min were utilized 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 frequency. However, the prolonged treatment with DNQX now significantly suppressed mean and trough LH levels as well as LH pulse amplitude with no effect observed on LH pulse frequency. Taken as a whole, the present studies suggest that EAAs are involved in enhancing gonadotropin pulsatility in the male rat and that the effect is exerted primarily on pulse amplitude rather than pulse frequency.