Pineal and ovarian response to 22- and 24-h days in the ewe.
Melatonin secretion in ewes was entrained by 22-h light-dark cycles whether of long (16L:6D) or short (6L:16D) photoperiod. In photoperiods of 6L:16D, a phase-delay of melatonin secretion was evident, leading to a dark-phase duration shorter than that found in 8L:16D. Early onset of estrus was induced in anestrous ewes kept in 8L:16D, but not 6L:16D, from 22 July compared to controls in natural light. In photoperiods of 16L:6D, the melatonin profile corresponded precisely to the dark phase. Early offset of estrus was induced in estrous ewes kept in both 18L:6D and 16L:6D from 18 December compared to controls in natural light. Thus, when the duration of melatonin secretion was appropriate to the long photoperiod (16L:6D), but with a constantly changing phase position, a long-day reproductive response was found. Activity-rest cycles were not entrained by 16L:6D; thus the synchronization of melatonin and activity-rest cycles does not appear to be essential for the induction of a long-day reproductive response. These results support the hypothesis that the duration, not the circadian-phase position, of melatonin is critical to the induction of photoperiodic effects.
English
J
J
Department of Biochemistry, University of Surrey, Guildford, England.
Arendt
J
J
Symons
A M
AM
Poulton
A L
AL
Tobler
I
I
D000818:Animals; D002940:Circadian Rhythm; D003624:Darkness; D004971:Estrus; D005260:Female; D008027:Light; D008550:Melatonin / Q000097:blood / Q000378:metabolism; D009043:Motor Activity; D010053:Ovary / Q000502:physiology*; D010507:Periodicity; D010870:Pineal Gland / Q000502:physiology*; D011374:Progesterone / Q000097:blood; D012756:Sheep / Q000502:physiology*
D011374:Progesterone; D008550:Melatonin