CIRCANNUAL ALTERATIONS IN THE CIRCADIAN-RHYTHM OF MELATONIN SECRETION

To determine if a circadian rhythm known to be functionally related to the reproductive axis varies on a circannual basis, we monitored the circadian secretion of melatonin at monthly intervals for 2 years in f our ovariectomized, estradiol-implanted ewes held in a constant short- day photoperiod. Prior to the study, ewes had been housed in a short-d ay (8L:16D) photoperiod for 4 years and were exhibiting circannual rep roductive rhythms as assessed by serum luteinizing hormone (LH) levels . Three of the four sheep showed unambiguous deviations from the expec ted nocturnal melatonin secretion at two different times approximately 1 year apart. Nocturnal rises in melatonin, which usually last the du ration of the dark phase, were delayed by 3-14 h or were missing. Alto gether, five of the seven melatonin alterations observed in these thre e ewes occurred during the nadir of the circannual LH cycle. In the re maining ewe, we did not observe an altered melatonin secretory pattern during this period, and this ewe also failed to show a high amplitude circannual cycle of LH. The results provide evidence for a circannual change in the circadian rhythm of melatonin secretion. This alteratio n in melatonin secretion may serve as a ''functional'' change in dayle ngth, and thereby may influence the expression of the circannual repro ductive rhythm of sheep held in a fixed photoperiod for an extended ti me.