Gonadotropin Response to Naloxone in the Mare: Effect of Time of Year and Reproductive Status¹

^a Department of Veterinary Science, The Maxwell Gluck Equine Research Center, University of Kentucky, Lexington, Kentucky 40546–0099

In the mare, endogenous opioids have been implicated in the suppression 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 (NAL) or vehicle on alternate days. Whereas cycling mares responded to all dosages of NAL, AN mares responded only to the higher dosages for FSH, and LH failed to increase at any dosage employed. During the breeding season (BS), the response to these dosages of NAL was reevaluated in 12 mares in the luteal phase of a synchronized estrous cycle. Although there was no difference between cycling mares during the breeding and nonbreeding seasons in FSH response, those mares that had cycled during 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 does not occur in mares that cycle during the NBS. Thus, reduced opioid tone may play a role in the mechanisms controlling the nonseasonal exhibition of estrous cycles in the mare.

¹ This work was supported by funds from the Kentucky Equine Research Foundation. An abstract containing some of these data was presented at the 29th annual meeting of the Society for the Study of Reproduction, London, Ontario, Canada, 1996. The research reported in this article (No. 97–14–162) is published in connection with a project of the Kentucky Agricultural Experiment Station and is published with approval of its director.

² Correspondence. FAX: 606 257 8542; ldavison{at}pop.uky.edu

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