Daily patterns of clock genes and their timing in the rat brain’s internal clock change with day length

The rhythmic expression of clock genes in the rat suprachiasmatic nucleus is affected by photoperiod, with specific changes observed in mRNA levels.

• Photoperiod influences phase, waveform, and amplitude of rhythmic gene expression in the suprachiasmatic nucleus.
• Long photoperiods (16 h light, 8 h darkness) result in prolonged elevation of Per1 mRNA and shorter elevation of Bmal1 mRNA compared to short photoperiods (8 h light, 16 h darkness).
• Under both photoperiods, the morning rise of Per1 and Cry1 mRNA and the decline of Bmal1 mRNA correlate closely with the onset of morning light.
• The amplitude of Per1, Cry1, and Bmal1 mRNA rhythms is greater during short photoperiods.
• Significant rhythmic expression of Clock mRNA is observed only under short photoperiod conditions.
• These findings suggest that the molecular clock in the rat suprachiasmatic nucleus may vary with seasonal changes in daylength.

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