Davison L A, McManus C J, Fitzgerald B P
Department of Veterinary Science, The Maxwell Gluck Equine Research Center, University of Kentucky, Lexington, Kentucky 40546-0099, USA.
Biol Reprod. 1998 Nov;59(5):1195-9. doi: 10.1095/biolreprod59.5.1195.
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.
在内分泌的阿片样物质与母马季节性乏情期（AN）促性腺激素分泌的抑制有关。本研究测试了与处于乏情期的母马相比，非繁殖季节（NBS）生殖活动的持续是否反映了阿片样物质张力没有季节性变化。在非繁殖季节，11匹处于乏情期的母马和8匹处于黄体期的母马每隔一天接受0.1、0.05、0.025mg/kg的纳洛酮（NAL）或赋形剂。虽然处于发情周期的母马对所有剂量的NAL都有反应，但处于乏情期的母马仅对较高剂量的促卵泡激素（FSH）有反应，且使用任何剂量的NAL时促黄体生成素（LH）均未增加。在繁殖季节（BS），对12匹处于同步发情周期黄体期的母马重新评估了对这些剂量NAL的反应。虽然处于繁殖季节和非繁殖季节处于发情周期的母马在FSH反应上没有差异，但那些在非繁殖季节有发情周期的母马对0.05mg/kg NAL的LH反应比处于繁殖季节的母马更大。从这些数据中，我们得出结论，繁殖季节的阿片样物质张力低于乏情期，并且这种抑制性张力的变化在非繁殖季节有发情周期的母马中不会发生。因此，阿片样物质张力的降低可能在控制母马发情周期非季节性表现的机制中起作用。
