MELATONIN ATTENUATES PHOTIC DISRUPTION OF CIRCADIAN-RHYTHMS IN SIBERIAN HAMSTERS

Body temperature (T-b) was recorded via a biotelemetry system from 28 adult male Siberian hamsters maintained in a light-dark (LD) cycle of 16 h light/day for several months. After T-b was recorded for 3 wk, th e LD cycle was phase delayed by extending the light phase by 5 h for 1 day; animals remained on a 16:8 LD cycle for the remainder of the exp eriment. Hamsters were injected daily with melatonin or vehicle soluti on for several weeks, beginning either 2 mo after (experiment 1) or on the day of (experiment 2) the phase shift; injections occurred within 30 min of dark onset. In Experiment 1, 75% of animals free ran with c ircadian periods >24 h, beginning on the day of the phase shift, and n ever reentrained to the LD cycle; no hamsters unambiguously entrained to daily injections. In contrast, 78% of animals in experiment 2 entra ined to melatonin injections, and 71% of those animals subsequently re entrained to the photocycle when the injection regimen ended. No vehic le-treated animals entrained to the injection schedule. Melatonin had no effect on daily mean T-b and T-b rhythm amplitude in either experim ent; however, melatonin doubled the duration of a hyperthermic respons e that occurred after each injection. Thus melatonin can prevent loss of entrainment induced by a phase shift of the LD cycle but cannot res tore entrainment to free-running animals. Failure to reentrain in the presence of two appropriately coordinated entraining agents also sugge sts that a phase shift of the photocycle can diminish the sensitivity of the circadian system to both photic and nonphotic input.