Dark pulses reduce activity signals in the hamster's internal clock center

A 6-hour dark pulse at circadian time 8 reduced levels of phosphorylated ERK and c-Fos in the Syrian hamster suprachiasmatic nuclei.

• Phosphorylated ERK (P-ERK) levels showed significant circadian variation in the SCN under constant light.
• Dark pulses can down-regulate the expression of P-ERK and c-Fos in the SCN, but not phosphorylated Elk-1.
• After a dark pulse, c-Fos levels increased beyond time-matched control values, while P-ERK levels remained unchanged.
• A dark pulse at the beginning of the subjective night did not phase-shift behavioral rhythms or alter expression of c-Fos, P-ERK, or P-Elk-1.
• In the visual thalamus, c-Fos expression was higher during the subjective night compared to the subjective day, but dark pulses did not significantly affect it.

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