Influence of Acute Melatonin Treatment and Light on the Circadian Melatonin Rhythm in the Djungarian Hamster

The present study tested the hypothesis that acute melatonin treatment affects the biological clock mechanism controlling the circadian melatonin rhythm. Adult Djungarian hamsters were raised in LD 16:8 (lights-on from 0300 to 1900 hr). Pineal and serum melatonin concentrations were assessed at specific clock times during the subsequent LD cycle (day 1) and afterwards in hamsters that remained in constant darkness (day 2), to assess the effects of treatment on the circadian melatonin rhythm. In untreated controls (n = 76), melatonin increased in the pineal and in circulation within the first hour after lights-off (day 1) or subjective night (day 2); melatonin concentrations remained elevated for up to 7 hr. A single melatonin injection (5 micrograms/0.2 ml saline, s.c.), administered in the morning (0900 hr, n = 80) or late afternoon (1600 hr, n = 74), transiently increased in circulation but not in the pineal gland. In constant darkness (day 2), the circadian melatonin rhythm in both the pineal gland and circulation was the same as that in untreated controls. No significant differences were found among the three groups in the mean concentration during the day or night; the rise, fall, peak time, peak amplitude, and duration of increased melatonin were also unchanged. The ability of melatonin to override the effect of a light pulse on the circadian melatonin rhythm was also tested. The 5-min light pulse at night (2300-2305 hr of day 1, n = 64) delayed the rise and shortened the duration of increased melatonin in both pineal gland and serum by as much as 3 hr (day 2); these light effects were more pronounced on the pineal than on the serum melatonin rhythm. Injection with melatonin prior to the light pulse (5 micrograms/0.2 ml saline at 2245 hr, n = 64) failed to alter the inhibitory effects of light on the circadian melatonin rhythm. These data suggest that the biological clock mechanism controlling the circadian melatonin rhythm in the Djungarian hamster in long days is responsive to the phase-shifting effects of light, but resistant to the influence of acute melatonin treatment.