Matthews C D, Guerin M V, Napier A J
Department of Obstetrics and Gynaecology, University of Adelaide, Queen Elizabeth Hospital, South Australia.
J Biol Rhythms. 1995 Dec;10(4):308-18. doi: 10.1177/074873049501000404.
Two experiments, using Romney Marsh ewes, tested for the existence and role of a critical interval of the circadian pacemaker located near dusk that may be integrally involved in the precise timing of the breeding season. Groups of Romney Marsh ewes (n = 6) were provided with exogenous melatonin by injection at dusk (Experiment 1) or by infusion at dawn or subjected to extended darkness at dawn (Experiment 2) from the winter to the summer solstice before being exposed to natural photoperiod at latitude 35 degrees S. Other than the experimental protocols, all animals were held in natural photoperiod. The onset of the breeding season (defined as cyclic ovarian activity as indicated by plasma progesterone monitoring) was normal in those animals treated with morning melatonin but was delayed in those animals treated with melatonin at dusk or extended darkness at dawn compared to controls in natural photoperiod (p < .01). Exogenous melatonin at dusk was associated with a phase advance of the onset of the circadian pacemaker (as measured by endogenous melatonin in acutely extended darkness); additional darkness at dawn was associated with a phase delay of both the onset and the offset of the circadian pacemaker. Exogenous morning melatonin did not change the phase of the circadian pacemaker relative to the controls. The results are consistent with an external coincidence model of seasonal breeding in which a critical interval of the circadian pacemaker requires exposure to light during spring/summer to time estrus correctly. The proposed critical interval appears to be located near dusk in this model and is phase locked to the circadian pacemaker. The effect of the exogenous melatonin on the timing of the breeding season is similar to darkness when administered at dusk but is not equivalent to darkness at dawn. The timing of anestrus was not affected by any of the experimental treatments and may reflect a common response to an environmental influence.
两项以罗姆尼沼泽母羊为对象的实验，测试了位于黄昏附近的昼夜节律起搏器关键间隔的存在及其作用，该间隔可能与繁殖季节的精确时间安排密切相关。从冬季到夏至，给罗姆尼沼泽母羊组（n = 6）在黄昏时注射外源性褪黑素（实验1），或在黎明时输注外源性褪黑素，或在黎明时使其处于延长黑暗环境（实验2），之后让它们暴露于南纬35度的自然光照周期下。除实验方案外，所有动物都处于自然光照周期中。用清晨褪黑素处理的动物，其繁殖季节的开始（通过监测血浆孕酮来定义为周期性卵巢活动）正常，但与处于自然光照周期的对照组相比，在黄昏时用褪黑素处理或在黎明时处于延长黑暗环境的动物，其繁殖季节开始延迟（p < 0.01）。黄昏时的外源性褪黑素与昼夜节律起搏器开始时间的相位提前相关（通过在急性延长黑暗中内源性褪黑素测量）；黎明时额外的黑暗与昼夜节律起搏器开始和结束时间的相位延迟相关。外源性清晨褪黑素相对于对照组并未改变昼夜节律起搏器的相位。这些结果与季节性繁殖的外部巧合模型一致，在该模型中，昼夜节律起搏器的关键间隔需要在春/夏期间暴露于光照下，才能正确安排发情时间。在这个模型中，所提出的关键间隔似乎位于黄昏附近，并与昼夜节律起搏器锁相。外源性褪黑素对繁殖季节时间的影响在黄昏给药时类似于黑暗，但与黎明时的黑暗不等同。任何实验处理都未影响乏情期的时间，这可能反映了对环境影响的共同反应。
