Effects of a one-hour light pulse on the timing of the circadian rhythm in melatonin secretion in rams.

The effects of a 1-hr light pulse on the timing of the circadian rhythm in the blood plasma concentration of melatonin were documented in Soay rams. Groups of 5 to 6 animals were transferred from short days (LD 8:16) to constant dim red light (DD) for 6 days, and were exposed to a 1-hr light pulse at one of 16 different times throughout 24 hr on day 3. Blood samples were collected hourly for 30 hr before (day 2-3) and after the light pulse (day 5-6), and the plasma concentrations of melatonin were measured by radioimmunoassay. The animals were allocated to experimental groups based on the circadian time (CT) when the light pulse was given using two hourly blocks through the circadian day; the onset of enhanced melatonin secretion (melatonin peak) was designated as CT 12. Under DD there was a clearly defined plasma melatonin rhythm in all animals. The mean duration of the melatonin peak was 13.24 +/- 0.16 hr (n = 91) and the mean period between the onset of successive melatonin peaks was 23.55 +/- 0.10 hr (n = 21). The effect of the 1-hr light pulse on the time of onset of the melatonin peak varied significantly with the circadian time when the light pulse was given (ANOVA, P = 0.031).(ABSTRACT TRUNCATED AT 250 WORDS)