Investigations of the significance of the crepuscular LH peak in the ovulatory cycle of the hen (Gallus domesticus)

Abstract

ABSTRACT The crepuscular (occurring at dusk) peak of LH was characterized and its functional significance in the ovulatory cycle of the hen, Gallus domesticus, examined. Serial blood sampling on the night before the first ovulation (C1) of the sequence was followed by normal ovulations in six out of eight hens whereas blood sampling before a second or third ovulation (C2 or C3) of the sequence resulted in blocked ovulations in six out of seven hens. The difference in incidence of blocked ovulations was significant (P<0·02). Associated with normal ovulations was a significant (P<0·05) crepuscular peak in plasma LH whereas in those hens in which ovulation was blocked, no rise in LH at onset of darkness was detected. Serial blood sampling of hens on a night when 'lights-off' was delayed revealed a significant (P<0·05) crepuscular peak of LH at the time when the lights usually went off, indicating that the rise in LH had the property of a daily rhythm. When a 'lights-off' signal was given 4 h early before a C1 ovulation, a crepuscular peak of LH was induced (P<0·05) and preovulatory rises of LH and progesterone were significantly advanced (P<0·005 and P<0·02 respectively) compared with control hens, although not by the entire 4-h interval. This indicated that the crepuscular LH peak could be induced by the lights-off signal. Injections of chicken LH (cLH) which simulated a premature crepuscular LH peak were not effective in advancing the preovulatory surges of LH or progesterone or the resulting ovulations. The preovulatory surge of LH was potentiated (P<0·05), however, after the cLH injections, compared with the control values, although the treatment had no effect on preovulatory progesterone values. From these results we conclude that the crepuscular peak of LH may be necessary to initiate preovulatory events, but is not sufficient to do so. J. Endocr. (1984) 100, 307–313