COMPARISON OF THE EFFECTS OF N-METHYL-DL-ASPARTIC ACID ON GONADOTROPIN AND PROLACTIN SECRETION IN ANESTROUS MARES AND MARES EXHIBITING ESTROUS CYCLES DURING ANESTRUS

This study investigated the hypothesis that for a subpopulation of hor se mares continuation of estrous cycles during the nonbreeding season may be attributed to continued stimulatory glutamatergic activity on G nRH-secreting neurons. The gonadotropin response to the glutamatergic agonist N-methyl-DL-aspartic acid (NMA) was compared in cycling and an estrous mares during the nonbreeding season. It was anticipated that t he gonadotropin response to NMA in cycling mares would be attenuated, compared with that of anestrous mares. The experiment used 16 anestrou s mares and 15 mares that cycled during the nonbreeding season. The ef fect of NMA on prolactin secretion was also evaluated. In addition, th e seasonal rhythm of prolactin secretion was compared in anestrous and cycling mares during October-April. In cycling mares, the response to NMA was dependent on the stage of the cycle, and a significantly (p < 0.05) larger proportion responded during the luteal phase (6 of 8), c ompared with the follicular phase (1 of 7 mares). The proportion of an estrous mares that responded to NMA was similar to that of cycling mar es during the luteal phase, but larger than during the follicular phas e. In anestrous and cycling mares, NMA suppressed prolactin secretion, and in both groups prolactin secretion decreased during the nonbreedi ng season. Thus, we conclude that differences in reproductive activity in mares during the nonbreeding season are unlikely to reflect a chan ge in glutamatergic activity.