PLASMA MELATONIN IN THE HORSE - MEASUREMENTS IN NATURAL PHOTOPERIOD AND IN ACUTELY EXTENDED DARKNESS THROUGHOUT THE YEAR

Plasma melatonin was measured at the winter and summer solstices and t he autumn and spring equinoxes in four mares held under natural condit ions at 35 degrees S. At all seasons the onset of the nightly elevated melatonin was coincident with or after the time of sunset and the mel atonin offset after the time of sunrise, The duration of elevated mela tonin was not different from the duration of natural scotophase for ea ch season, with the duration of elevated melatonin longer in winter th an the other seasons, Immediately following each 24 hr sampling two ma res were resampled in acutely extended darkness to determine the melat onin profile of the endogenous rhythm of the circadian pacemaker, orig inating from the suprachiasmatic nuclei (SCN). At each season melatoni n secretion commenced earlier and decreased later than that measured u nder the natural photoperiodic condition, suggesting that the expressi on of the melatonin rhythm is normally gated by natural environmental light both at dusk and dawn, The interval from the onset of melatonin measured under acutely extended darkness to the time of sunset was gre ater in the spring/summer than the autumn/winter suggesting a possible alternating signal throughout the year. Thus the mare appears to exhi bit a similar interaction between endogenous circadian rhythmic activi ty and the natural photoperiod as the ewe which may underlie the mecha nism for timing reproductive activity through the year.