Daily patterns of seven internal clock genes in the brain’s main and seasonal timing centers of sheep suggest an internal coincidence timer

The expression of seven clock genes was measured every 4 hours over 24 hours in the brain and pituitary of female Soay sheep under different light conditions.

• All clock genes were expressed in the suprachiasmatic nucleus (SCN) and the pars tuberalis (PT) of the pituitary gland.
• Clock and Bmal1 showed a 24-hour rhythm in the SCN, while Per1 and Per2 exhibited cycles out of phase with them.
• Photoperiod influenced the expression of Per1 and Per2, resulting in prolonged elevated levels during long daylight hours.
• In the PT, the expression of several clock genes, except CK1 epsilon, displayed high-amplitude 24-hour cycles affected by photoperiod.
• The timing of expression peaks varied, with Per1 and Per2 peaking during the day and Cry1 and Cry2 peaking early at night.
• Melatonin was linked to changes in the phase relationship of clock gene expression in the PT, suggesting different models of photoperiod influence.

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