Circadian and Circannual Melatonin Secretion in Finnish Semidomesticated Reindeer
Mammalian species from temperate and subarctic areas display distinct annual fertility cycles and their reproductive physiology responds to cues from the environment. The daily photoperiod is the major environmental factor repeated with great regularity from year to year. There is compelling evidence to suggest that in small laboratory animals the pineal gland transfers the photoperiodic information entering the eyes to the hypothalamo-hypophyseal-gonadal axis via melatonin. The pineal gland is required for photoperiodic regulation of seasonal breeding. The reindeer is a short-day breeder and the association of melatonin with its reproductive cycle was investigated in this study. Ten female reindeer (Rangifer tarandus tarandus) and five calves were studied. The endogenous circadian and circannual rhythms of melatonin were investigated using 4-h samplings for 24h every 3 months in a natural photoperiod. Serum melatonin levels exhibited a pronounced seasonal rhythm (F = 5.88, P < 0.001). In autumn (12L:12D), in winter (0L:24D), and in spring (12L:12D) the circadian rhythm in melatonin secretion was seen in hinds with acrophase at night ( P < 0.001, compared with day). However, in summer (24L:0D) no circadian rhythm occurred. The amplitude in melatonin secretion was at its maximum in autumn. In prepubertal calves, the circadian melatonin rhythm was recorded in autumn and spring, but in autumn the peak levels were lower than those of adults (P<0.05). In winter and in summer the melatonin levels were equal, and no rhythm was recorded in calves.