Petitclerc D, Chapin L T, Tucker H A
Department of Animal Science, Michigan State University, East Lansing 48824.
Proc Soc Exp Biol Med. 1989 May;191(1):37-42. doi: 10.3181/00379727-191-42886.
Early temporal changes in concentrations of prolactin (PRL) in serum after a sudden change in photoperiod and daily responsiveness to PRL-releasing and inhibiting factors were investigated in prepubertal Holstein bull calves exposed to different photoperiods. In calves switched from 8-hr light: 16-hr dark to 16-hr light:8-hr dark, there was no observable change in the daily pattern of serum concentrations of PRL after 1, 2, or 4 days. On the other hand, in animals switched from 16-hr light:8-hr dark to 8-hr light:16-hr dark, there was a consistent increase in serum PRL from 33.4 ng/ml on Day 0 to maximum values of 57.3, 62.7, and 78.9 ng/ml between 14 and 18 hr after onset of light on Days 1, 2, and 4, respectively. Thus, absence of light allowed expression of a daily rhythm in serum concentrations of PRL that persisted for at least 4 days after the photoperiod switch. There were no differences in L-dopa inhibition of PRL release in animals exposed to 16-hr light:8-hr dark at 3 or 15 hr after onset of light. However, thyrotropin-releasing hormone-induced release of PRL was greater 3 hr after onset of light (11 hr after onset of dark) compared with release at 9, 15, and 21 hr after onset of light in animals exposed to 16-hr light:8-hr dark, but not in bulls exposed to 8-hr light:16-hr dark. The results provide evidence that the cue for the putative photosensitive period of PRL secretion in cattle may be more closely associated with onset of dark, not onset of light.
研究了处于不同光照周期的青春期前荷斯坦公牛犊在光照周期突然改变后血清中催乳素（PRL）浓度的早期时间变化以及对PRL释放和抑制因子的每日反应性。在从8小时光照：16小时黑暗转换为16小时光照：8小时黑暗的犊牛中，1天、2天或4天后血清PRL浓度的每日模式没有可观察到的变化。另一方面，在从16小时光照：8小时黑暗转换为8小时光照：16小时黑暗的动物中，血清PRL从第0天的33.4 ng/ml持续增加，在第1天、第2天和第4天光照开始后14至18小时分别达到最大值57.3、62.7和78.9 ng/ml。因此，无光条件下血清PRL浓度的每日节律得以表达，且在光照周期转换后至少持续4天。在光照开始后3小时或15小时处于16小时光照：8小时黑暗条件下的动物中，左旋多巴对PRL释放的抑制作用没有差异。然而，在处于16小时光照：8小时黑暗条件下的动物中，促甲状腺激素释放激素诱导的PRL释放量在光照开始后3小时（黑暗开始后11小时）大于光照开始后9小时、15小时和21小时，但在处于8小时光照：16小时黑暗条件下的公牛中并非如此。结果提供了证据，表明牛PRL分泌假定的光敏期的线索可能与黑暗开始更密切相关，而非光照开始。
