GONADOTROPIN-RESPONSE TO NALOXONE IN THE MARE - EFFECT OF TIME OF YEAR AND REPRODUCTIVE STATUS

In the mare, endogenous opioids have been implicated in the suppressio n of gonadotropin secretion during seasonal anestrus (AN). The present study tested whether continuation of reproductive activity during the nonbreeding season (NBS) reflects the absence of a seasonal shift in opioid tone compared to what occurs in AN mares. During the NBS, 11 AN and 8 luteal-phase mares received 0.1, 0.05, 0.025 mg/kg naloxone (NA L) or vehicle on alternate days. Whereas cycling mares responded to al l dosages of NAL, AN mares responded only to the higher dosages for FS H, and LH failed to increase at any dosage employed. During the breedi ng season (BS), the response to these dosages of NAL was reevaluated i n 12 mares in the luteal phase of a synchronized estrous cycle. Althou gh there was no difference between cycling mares during the breeding a nd nonbreeding seasons in FSH response, those mares that had cycled du ring the NBS showed a greater LH response to 0.05 mg/kg NAL than mares during the BS. From these data, we conclude that opioid tone is lower during the BS than during AN and that this shift in inhibitory tone d oes not occur in mares that cycle during the NBS. Thus, reduced opioid tone may play a role in the mechanisms controlling the nonseasonal ex hibition of estrous cycles in the mare.