Photoperiod regulates clock gene rhythms in the ovine liver

Jun 7, 2005General and comparative endocrinology

Day length affects daily clock gene patterns in sheep liver

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Abstract

Under long days, peak Per2 expression in Soay sheep occurred at the end of the night.

Liver expression of the clock genes Per2 and Bmal1 was rhythmic under all photoperiod treatments.
The timing of Per2 expression differed between long and short photoperiods, with an inverse pattern to Bmal1 under short days.
A significant interaction was observed between photoperiod and time for Per2 expression (P < 0.001).
The pattern of plasma cortisol over 24 hours was aligned with the timing of Per2 expression, indicating a potential role in hormonal regulation.
Clock gene rhythms in the liver were out of sync with those in the central pacemaker regions of the brain, suggesting species differences in circadian regulation.

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To investigate the photoperiodic entrainment of peripheral rhythms in ruminants, we studied the expression of clock genes in the liver in the highly seasonal Soay sheep. Animals were kept under long (LD 16:8) or short photoperiod (LD 8:16). Daily rhythms in locomotor activity were recorded, and blood concentrations of melatonin and cortisol were measured by RIA. Per2, Bmal1, and Cry1 gene expression was determined by Northern blot analyses using ovine RNA probes in liver collected every 4h for 24h. Liver Per2 and Bmal1, but not Cry1, expression was rhythmic in all treatments. Under long days, peak Per2 expression occurred at end of the night with a similar timing to Bmal1, whereas, under short days the Per2 maximum was in the early night with an inverse pattern to Bmal1. There was a photoperiodxtime interaction for only Per2 (P < 0.001). The 24-h pattern in plasma cortisol matched the observed phasing of Per2 expression, suggesting that it may act as an endocrine entraining factor. The clock gene rhythms in the peripheral tissues were different in timing compared with the ovine suprachiasmatic nucleus (SCN, central pacemaker) and pars tuberalis (melatonin target tissue), and the hepatic rhythms were of lower amplitude compared with photoperiodic rodents. Thus, there are likely to be important species differences in the way the central and peripheral clockwork encodes external photoperiod.

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Photoperiod regulates clock gene rhythms in the ovine liver

Abstract

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