Splitting of the circadian rhythm of activity in hamsters: Effects of exposure to constant darkness and subsequent re-exposure to constant light

Summary

The circadian rhythm of wheel running behavior was observed to dissociate into two distinct components (i.e. ‘split’) within 30 to 110 days in 56% of male hamsters exposed to constant light (Figs. 1–2). Splitting was abolished in all 16 animals that were transferred from constant light (LL) to constant darkness (DD) within 1–4 days of DD, and the components of the re-fused activity rhythm assumed a phase relationship that is characteristic of hamsters maintained in DD (Figs. 3–5). Re-fusion of the split activity rhythm was accompanied by a change in period (τ); in 14 animals τ increased while in the other 2 animals τ decreased after transfer to DD.

After 10–30 days in DD, the hamsters were transferred back into LL at various time points throughout the circadian cycle. A few of these animals went through two or three LL to DD to LL transitions. The effect of re-exposure to LL was dependent on the phase relationship between the transition into LL and the activity rhythm. A rapid (i.e. 1–4 days) induction of splitting was observed in 7 of 9 cases when hamsters were transferred into LL 4–5 h after the onset of activity (Fig. 5). In the other 2 animals, the activity pattern was ultradian or aperiodic for 20 to 50 days before eventually coalescing into a split activity pattern. In contrast, transfer of animals (n = 13) from DD to LL at other circadian times did not result in the rapid induction of splitting and the activity rhythm continued to free-run with a single bout of activity (Fig. 5). Importantly, a transfer from DD to LL 4–5 h after the onset of activity did not induce splitting if the hamsters had not shown a split activity rhythm during a previous exposure to LL (n=10; Fig. 6).

These studies indicate that transfer of split hamsters from LL to DD results in the rapid re-establishment of the normal phase relationship between the two circadian oscillators which underlie the two components of activity during splitting. In addition, there appears to be a history-dependent effect of splitting which renders the circadian system susceptible to becoming split again. The rapid re-initiation of the split condition upon transfer from DD to LL at only a specific circadian time is discussed in terms of the phase response curve for this species.