Individual differences in circadian waveform of Siberian hamsters under multiple lighting conditions

Abstract

Because the circadian clock in the mammalian brain derives from a network of interacting cellular oscillators, characterizing the nature and bases of circadian coupling is fundamental to understanding how the pacemaker operates. Various phenomena involving plasticity in circadian waveform have been theorized to reflect changes in oscillator coupling; however, it remains unclear whether these different behavioral paradigms reference a unitary underlying process. To test whether disparate coupling assays index a common mechanism, we examined whether there is covariation among behavioral responses to various lighting conditions that produce changes in circadian waveform. Siberian hamsters, Phodopus sungorus, were transferred from long to short photoperiods to distinguish short photoperiod responders (SP-R) from nonresponders (SP-NR). Short photoperiod chronotyped hamsters were subsequently transferred, along with unselected controls, to 24-h light:dark:light: dark cycles (LDLD) with dim nighttime illumination, a procedure that induces bifurcated entrainment. Under LDLD, SP-R hamsters were more likely to bifurcate their rhythms than were SP-NR hamsters or unselected controls. After transfer from LDLD to constant dim light, SP-R hamsters were also more likely to become arrhythmic compared to SP-NR hamsters and unselected controls. In contrast, short photoperiod chronotype did not influence more transient changes in circadian waveform. The present data reveal a clear relationship in the plasticity of circadian waveform across 3 distinct lighting conditions, suggesting a common mechanism wherein individual differences reflect variation in circadian coupling.

Figure 1

Representative double-plotted actograms illustrating the lighting conditions to which Siberian hamsters were exposed. Changes in internal shading represent changes in lighting conditions, with dim nighttime illumination present throughout. Wheel-running rhythms (scale 0 to 75 wheel revolutions/6 min) were recorded after hamsters were transferred to LDLD and released into constant DIM illumination (indicated by DIM and arrow on left). After two weeks under constant DIM, hamsters were provided 15-min bright light pulses (indicated by BL, arrow at left, and white circle within actogram). Numbered sections of record indicate intervals used to measure changes in activity duration after release into DIM (1) and after bright light pulses (2). Note that the SP-R hamster, like all other SP-R hamsters, displays a bifurcated rhythm with an activity component in each scotophase starting on Day3 of LDLD; whereas the SP-NR and control hamster, like the majority of hamsters in these two groups, fail to display two activity components for the entire duration of LDLD.

Figure 2

Incidence (percent) and timing (mean ± SEM) of bifurcated entrainment under LDLD. Sample sizes are indicated below each bar. Distinct letters above each bar distinguish groups that differed in post hoc pairwise χ² tests (p < 0.05).

Figure 3

A. Representative wheel-running record of an animal that developed an arrhythmic activity pattern under constant DIM. When constant DIM was extinguished (constant DARK), a rhythmic activity pattern rapidly developed. B. Incidence of constant DIM-induced arrhythmia differed by short photoperiod chronotype (top, conventions as in Figure 2) and entrainment state (bottom). * p < 0.01.