title: Melatonin attenuates photic disruption of circadian rhythms in Siberian hamsters.
abstract: Body temperature (Tb) 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 Tb was recorded for 3 wk, the 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 experiment. Hamsters were injected daily with melatonin or vehicle solution 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 circadian periods >24 h, beginning on the day of the phase shift, and never reentrained to the LD cycle; no hamsters unambiguously entrained to daily injections. In contrast, 78% of animals in experiment 2 entrained to melatonin injections, and 71% of those animals subsequently reentrained to the photocycle when the injection regimen ended. No vehicle-treated animals entrained to the injection schedule. Melatonin had no effect on daily mean Tb and Tb rhythm amplitude in either experiment; however, melatonin doubled the duration of a hyperthermic response that occurred after each injection. Thus melatonin can prevent loss of entrainment induced by a phase shift of the LD cycle but cannot restore entrainment to free-running animals. Failure to reentrain in the presence of two appropriately coordinated entraining agents also suggests that a phase shift of the photocycle can diminish the sensitivity of the circadian system to both photic and nonphotic input.
authors.affiliation: Department of Biological Sciences, Stanford University, California 94305, USA.
authors.forename: N F
authors.initials: NF
authors.lastname: Ruby
authors.forename: T
authors.initials: T
authors.lastname: Kang
authors.forename: H C
authors.initials: HC
authors.lastname: Heller
mesh_terms: D000818:Animals; D002940:Circadian Rhythm / Q000187:drug effects* / Q000528:radiation effects*; D006224:Cricetinae; D007279:Injections, Subcutaneous; D008027:Light; D008297:Male; D008550:Melatonin / Q000494:pharmacology*; D017111:Phodopus / Q000502:physiology*; D017440:Photoperiod
chemical_list: D008550:Melatonin