EXCITATORY AMINO-ACID REGULATION OF GONADOTROPIN-SECRETION IN PREPUBERTAL HEIFER CALVES

The mechanisms controlling the pulsatile release of gonadotropins in p repubertal heifers are not completely understood. We examined the role of excitatory amino acid neurotransmitters, via activation of N-methy l-D-aspartate (NMDA) receptors, in the control of pulsatile LH and FSH release during prepubertal development in heifers. Hereford heifer ca lves received 4.7 mg/kg of N-methyl-D,L-aspartic acid (NMA), a potent NMDA receptor agonist (n = 5, i.v.), or saline (n = 5, i.v.), as singl e doses, at 4, 8, 12, 24, 36, and 48 wk of age. Blood samples were col lected every 15 min, for 1 h before and 9 h after injection, on the da ys of treatment. Injection of NMA resulted in an acute release of LH ( p < 0.001) in 0, 3, 3, 4, 5, and 5 calves (p < 0.01) and of FSH (p < 0 .001) in 0, 1, 2, 4, 3, and 2 calves at 4, 8, 12, 24, 36, and 48 wk of age, respectively. The peak response of LH and FSH release to NMA was at 15 min posttreatment, and these peak responses were highest at 36 wk of age (p < 0.05). We suggest that neuroexcitatory amino acids, thr ough NMDA receptors, are involved in prepubertal development of LH and FSH secretion in heifer calves.