EFFECT OF A SINGLE INTRAVENOUS-INJECTION OF N-METHYL-D,L-ASPARTIC ACID ON SECRETION OF LUTEINIZING-HORMONE AND GROWTH-HORMONE IN HOLSTEIN BULL CALVES

The role of N-methyl-D-aspartate (NMDA) receptor activation in the cen tral regulation of luteinizing hormone (LH) and growth hormone (GH) wa s tested by administering a bolus intravenous dose of N-methyl-D,L-asp artic acid (NMA), a NMDA receptor agonist, to 24-week-old intact (n=5) , estradiol-treated intact (n=3) and castrated (n=3) Holstein bull cal ves. The calves were bled for 12 h pre- and 100 min post-NMA injection (1.75 mg/kgBW) periods at 10 min intervals. Concentrations of LH and GH in plasma were measured by specific RIA. Prior to administration of NMA, the average concentration of LH, but not GH, differed significan tly among the 3 groups. As expected, administration of estradiol preve nted the normal ontogeny of pulsatile LH secretion, while castration r esulted in an increased frequency of LH discharges. Injection of NMA r esulted in an acute (P<0.001) release of LH in 3 of 5 intact and 3 of 3 estradiol-treated intact calves with the peak response being observe d at 20 min (3.18+/-1.3 and 5.58+/-1.3 ng/ml, respectively) following the challenge. Treatment with NMA did not alter the release of LH in c astrate calves. Concentrations of GH in plasma increased (P<0.001) wit hin 20 to 30 min after administration of NMA in intact, estradiol-trea ted intact and castrate calves with a similar response being observed in each group. Based on these findings, we suggest an involvement of g lutamatergic neurotransmission in the hypothalamic or supra-hypothalam ic control of LH and GH secretion, and that the excitatory effects of NMDA receptor activation on LH release are overtly influenced by gonad al steroids in bull calves.