title: A role for melatonin as a timekeeping hormone in the ewe.
abstract: This report has focused on the actions of melatonin and how it serves as a timekeeping hormone in the seasonal reproductive process of the ewe. In this short-day breeder, the characteristics of the 24-hour secretory profile of melatonin are highly susceptible to regulation by photoperiod. Changes in the nature of the melatonin pattern, in turn, provoke changes in hypothalamic/pituitary activity and the ability of estradiol to inhibit gonadotropin secretion. These latter changes drive the seasonal occurrence of estrous cycles. Special emphasis was placed on how melatonin codes for daylength in the ewe. Evidence was evaluated for two current theoretical models (phase and duration of the nighttime melatonin rise) and the importance of change in the melatonin pattern was stressed. Clearly, the systems which respond to the melatonin signal and convert it into changes in hypothalamic activity are highly complex. Nevertheless, solid concepts are beginning to emerge and rich questions continue to arise, making this a fertile area for future research.
authors.forename: F J
authors.initials: FJ
authors.lastname: Karsch
mesh_terms: D000818:Animals; D002940:Circadian Rhythm; D003624:Darkness; D004958:Estradiol / Q000378:metabolism; D004971:Estrus; D005246:Feedback; D005260:Female; D008027:Light; D008550:Melatonin / Q000378:metabolism / Q000502:physiology*; D012098:Reproduction; D012621:Seasons; D012756:Sheep / Q000502:physiology*
chemical_list: D004958:Estradiol; D008550:Melatonin