Daily dark pulses may reset and synchronize the hamster's internal body clock

Daily 1-hour dark pulses can entrain the wheel-running activity rhythm of unsplit hamsters and recouple split components of their circadian rhythm.

• The circadian rhythm in hamsters can split into two components that oscillate antiphase within the suprachiasmatic nuclei (SCN).
• In unsplit hamsters exposed to constant light, a single dark pulse can shift the timing of wheel-running activity and reduce clock gene expression in the SCN.
• Daily 1-hour dark pulses resulted in entrainment of wheel-running activity rhythms in unsplit hamsters.
• In split hamsters, daily dark pulses produced a unimodal coupling of the two distinct locomotor activity components.
• Changes in the expression of proteins such as c-FOS, p-ERK, PERIOD 1, and calbindin in the SCN reflected the behavioral effects of entrainment and recoupling.

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