Djungarian Hamsters: A Species with a Labile Circadian Pacemaker? Arrhythmicity under a Light-Dark Cycle Induced by Short Light Pulses

Abstract

In most cases, phase-shifting effects of light pulses are studied in animals kept in constant darkness (DD) or in animals released into DD following the stimulus. In this study, the authors exposed Djungarian hamsters (Phodopus sungorus) to short light pulses during the dark phase of a 16:8 light-dark (LD) cycle and thus obtained a type VI phase response curve. Light pulses early in the night caused phase delays of the activity onset as well as phase advances of the activity offset, whereas light pulses later in the night resulted in phase advances of the activity offset only. A combination of two 15-min light pulses-the first one given late in the scotophase and the second given early in the dark phase of the following night-led to a strong compression of the activity phase alpha. In 75% of all animals, daily rhythms were no longer visible after complete alpha compression, and long-term arrhythmicity (up to 145 days) persisted despite continued exposure to an LD cycle. Because three independent output rhythms of the clock (i.e., activity, body temperature, and melatonin rhythms) were equally affected, the authors conclude that overt arrhythmicity was due not merely to disrupted output pathways but to an altered state of the central pacemaker. The authors suggest a qualitative two-oscillator model to explain this phenomenon. Their hypothesis assumes that, due to loose coupling, the pacemaker of Djungarian hamsters can be driven to a state of zero phase difference between the two oscillators, with zero amplitude of their outputs.

Trans‐pineal microdialysis in the Djungarian hamster (Phodopus sungorus): a tool to study seasonal changes of circadian clock activities

“…In accordance with our study, Lerchl [39] reported significant effects on melatonin amplitude and phase one night after a light pulse at midnight in the same species. However our results show that those alterations are still visible 2 days after the photic stimulus, fitting with results from Steinlechner et al [40] who showed that already short light pulses can have a long-lasting or even destructive effect on circadian rhythms. It is known as well that during resetting of the mammalian circadian clock, not only the clock's phase is shifted, but amplitude of overt rhythms driven by the clock may be temporarily reduced or even abolished [41].…”

Variation in reproduction of the smallest‐bodied primate radiation, the mouse lemurs (Microcebus spp.): A synopsis

“…When temperatures are low and food is scarce, they may rely on heterothermy (prolonged torpor or hibernation) to save energy. Fat deposition also favors energy investment in later reproduction and thus enhances reproductive success in both sexes (e.g., Bronson, ; Geiser & Ruf, ; Steinlechner, Stieglitz, & Ruf, ). Over evolutionary time, small‐bodied mammals have often adjusted reproduction to annual day length (or photoperiod) fluctuations, predicting seasonal changes in temperature and food abundance.…”

Daily torpor affects the molecular machinery of the circadian clock in Djungarian hamsters (Phodopus sungorus)

“…PER1) feedback loops of clock genes that are expressed rhythmically and generate output signals such as vasopressin (AVP) release from the SCN or melatonin secretion from the pineal gland. This provides the organism with photoperiodic information and thereby determines torpor season (Pevet, 1988; Bartness & Goldman, 1989; Steinlechner et al ., 2002; Sumova et al ., 2003; Tournier et al ., 2003; Johnston et al . ; 2005).…”