The results suggest evidence for a suppressive (gating) effect of light at dawn only during summer which was associated with a phase advance of the onset of melatonin secretion at this time of year. The lack of a major gating effect of environment light on melatonin secretion, and the unchanging duration of secretion through the year in the normally entrained human, highlight differences between the human and those photoperiodic animal species which breed seasonally. Urinary 6-sulphatoxy melatonin proved to be a good indicator of plasma melatonin levels under rigorous examination and is confirmed as a useful clinical measure.

Plasma melatonin was measured at the winter and summer solstices and the autumn and spring equinoxes in four mares held under natural conditions 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 melatonin offset after the time of sunrise. The duration of elevated melatonin was not different from the duration of natural scotophase for each season, with the duration of elevated melatonin longer in winter than the other seasons. Immediately following each 24 hr sampling two mares were resampled in acutely extended darkness to determine the melatonin profile of the endogenous rhythm of the circadian pacemaker, originating from the suprachiasmatic nuclei (SCN). At each season melatonin secretion commenced earlier and decreased later than that measured under the natural photoperiodic condition, suggesting that the expression 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 greater in the spring/summer than the autumn/winter suggesting a possible alternating signal throughout the year. Thus the mare appears to exhibit a similar interaction between endogenous circadian rhythmic activity and the natural photoperiod as the ewe which may underlie the mechanism for timing reproductive activity through the year.

No abstract

Summary. Plasma