Dark pulse resetting of the suprachiasmatic clock in Syrian hamsters: behavioral phase-shifts and clock gene expression

Four-hour dark pulses applied during the late subjective day resulted in 1.2 ± 0.4 h phase-advances of locomotor activity rhythm in Syrian hamsters.

• Phase-advances were significant compared to control conditions, which showed non-significant shifts of 0.1 ± 0.2 h.
• Dark pulses led to a transient down-regulation of clock gene levels Per1 and Per2 in the suprachiasmatic nuclei by 40% and 20%, respectively.
• Cry1 mRNA levels remained unaffected by the dark pulses.
• In hamsters with asymmetric Per oscillations, dark pulses reduced Per expression in the more active half of the SCN.
• These findings suggest that dark pulses have nonphotic-like effects on the circadian clock at both behavioral and molecular levels.

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