Seasonal variation in the daily pattern of plasma melatonin in a wild mammal: the mountain hare (Lepus timidus).

Seasonal differences in daily patterns of plasma melatonin concentration were investigated in both free-living and captive mountain hares, in relation to reproductive activity. There was a marked increase in plasma melatonin concentrations at night at all times of the year. The period of elevation of plasma melatonin above the daytime values was longer in winter than in summer, correlating with the longer duration of darkness. The magnitude of the nighttime rise in melatonin concentrations did not differ significantly between seasons. There was no change in the plasma melatonin profile under similar photoperiods before the summer solstice when hares are sexually active or after the summer solstice when gonadal regression occurs, indicating that melatonin is not directly pro- or antigonadal. These field and laboratory observations support the view that the daily rhythm of melatonin secretion plays a role in the transduction of photoperiodic information in the mountain hare. Furthermore, they favor the hypotheses that the circadian melatonin rhythm transduces photoperiod information by means of the duration of nocturnal melatonin secretion or by the coincidence of elevated melatonin with a particular sensitive period, rather than by the amplitude of the nocturnal rise.