Siberian hamsters free run or become arrhythmic after a phase delay of the photocycle

Abstract

Body temperature (Tb) and locomotor activity were recorded telemetrically from male Siberian hamsters (Phodopus sungorus sungorus) that were 3 or 12 mo of age and maintained in a light-dark (LD) cycle of 16 h light/day for 2-4 mo. After 3 wk of Tb recording, the LD cycle was phase delayed by extending the light phase by 5 h for 1 day; animals remained on a 16:8-h LD cycle for the remainder of the experiment. Tb and activity rhythms of all animals were stably entrained to the LD cycle before the phase shift. After the phase shift, > or = 80% of the animals in each age group failed to reentrain and expressed free-running Tb rhythms with stable periods that ranged from 24.33 to 26.33 h; one hamster in each age group reentrained within several days. Tb became arrhythmic in 10% of all animals immediately after, and in 28% of free running animals several weeks after, the phase shift. Changes in tau and phase of activity rhythms closely paralleled Tb rhythms in individual hamsters. Daily mean Tb was unchanged, but Tb rhythm amplitude decreased by 25-50% in individual animals after the phase shift. We believe this to be the first report of neurologically intact animals failing to reentrain to a phase shift of the LD cycle. These phenomena are not readily explained by current knowledge of circadian systems and suggest that the entrainment process in Siberian hamsters differs markedly from that in other rodent species.

Homeostatic regulation of sleep in arrhythmic Siberian hamsters

“…Hamsters housed in short day lengths also undergo dramatic changes in their physiology that sharply differentiate them from long-day hamsters. Thus sleep in arrhythmic hamsters in the present study cannot be directly compared with sleep in hamsters maintained in short day lengths that are partially arrhythmic because the hamsters in the present study are arrhythmic in all physiological measures determined thus far, i.e., sleep, body temperature, melatonin, and locomotor activity (41,45), and therefore provide a good alternative to SCNx animal model.…”

“…At 3 mo of age, hamsters were housed individually to allow for recording of spontaneous locomotor activity using passive infrared-motion detectors (model 007.4RTE-A, Visonic, Bloomfield, CT; Ref. 41). Activity data were analyzed in 10-min intervals (software by Dataquest, Minneapolis, MN) and analyzed by 2 periodogram analysis and visual inspection of the actograms (ClockLab, Actimetrics, Evanston, IL) to confirm presence or absence of circadian organization.…”

“…Entrainment of locomotor activity was verified during the 7 days preceding the phase shift. On the next day, the LD cycle was phase delayed by 5 h via an extension of the light phase, and activity rhythms were monitored for 2 mo after the phase shift (41). Animals were considered arrhythmic if no significant rhythms were detected by the periodogram analysis and if no LD masking of activity was observed.…”

“…Siberian hamsters (Phodopus sungorus) can be made arrhythmic by a phase shift of the LD cycle (41) or in response to two consecutive light pulses at night (45), despite continued maintenance in an LD cycle. When hamsters are exposed to a 5-h phase shift of the LD cycle, some retain circadian rhythms and reentrain to the new LD cycle, some maintain circadian rhythmicity but fail to reentrain and free run through the new LD cycle, and some become arrhythmic (41). The latter condition is reversible because rhythmicity is sometimes restored spontaneously or by subjecting hamsters to a period of constant darkness (40,41).…”

“…When hamsters are exposed to a 5-h phase shift of the LD cycle, some retain circadian rhythms and reentrain to the new LD cycle, some maintain circadian rhythmicity but fail to reentrain and free run through the new LD cycle, and some become arrhythmic (41). The latter condition is reversible because rhythmicity is sometimes restored spontaneously or by subjecting hamsters to a period of constant darkness (40,41). Arrhythmic Siberian hamsters, therefore, provide a unique model system to study homeostatic regulation of sleep in intact animals that lack overt circadian rhythms in physiological measures and behavior (45), especially because both sleep (7)(8)(9)(10)12) and circadian rhythms (23, 24, 38 -42) have been intensively studied in this species.…”