SEASONAL ONSET AND DISAPPEARANCE OF DIURNAL RHYTHMICITY IN MELATONIN SECRETION IN FEMALE REINDEER

The reindeer (Rangifer tarandus tarandus) is a semidomesticated holarc tic ruminant subject to economic activity. Our research was carried ou t in Northern Finland, at latitude 69 degrees 10'N, where the daily li ght-dark rhythm has a polar pattern: constant darkness starts on Novem ber 26th and lasts about 10 weeks, while the polar day of about 10 wee ks starts on May 15th. We had previously found distinct daily rhythms in reindeer melatonin secretion in autumn, winter and spring but not a t all in summer. Here we determine how the daily melatonin pattern dev elops after the period of polar day and how it disappears at the begin ning of that period. Melatonin showed increased concentrations very so on after the first sunset. Thereafter the daily pattern developed grad ually and was fully developed around the autumn equinox. Its shape was asymmetric and the maximum amplitude occurred near the end of the dar k period. The disappearance of this daily pattern in spring was also g radual. The maximal concentrations depended on ambient illumination, e specially the duration of the dark period. In spring the reindeer appe ared to be more sensitive to light than in autumn. Additionally illumi nation of the same magnitude suppressed serum melatonin levels more ef fectively before midnight than afterwards, which is in accordance with the asymmetric secretion pattern. Melatonin secretion was always supp ressed when the ambient illumination exceeded 1,000 Ix. The pattern of melatonin secretion in reindeer is different from those described ear lier and hence the reindeer may serve as a model for melatonin secreti on at high latitudes. We suggest that, in addition to the duration of the melatonin secretion, changes in amplitude are also of importance i n the reindeer.