Seasonal profiles of melatonin in adult rams

: The objective of this study was to investigate the seasonal changes in melatonin profiles based on frequently collected samples in adult rams maintained under simulated natural photoperiod. In a group of six rams, the seasonal changes of melatonin were characterized in samples collected at 10‐min intervals for an equal period before and after the median of the scotophase during the spring (March) and the autumn (September) equinoxes, and also during the summer (June) and the winter (December) solstices. In an additional two rams, the rapid changes in melatonin concentrations were investigated in samples drawn at 2‐min intervals for a 2‐hr period before and after the median of the scotophase, but only during the summer and the winter solstices. The results show that in adult rams there is a distinct seasonal variation in the nightly rise of melatonin (P < 0.01). Mean concentrations in June and September were higher than in March or December (P < 0.05). There was no difference between the means in June or September. However, the means in March were lower than in December (P < 0.05). Rapid changes in melatonin concentrations occurred in samples collected either at 10‐min or 2‐min intervals. In rams sampled at 2‐min intervals, mean melatonin values in June were also higher than in December (P < 0.01). The results suggest that there are distinct seasonal changes in melatonin concentrations in the ram and that rapid changes in melatonin concentrations reflect pulsatile secretion.