Photoperiod regulates clock gene rhythms in the ovine liver

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, Bmall, and Cry1 gene expression was determined by Northern blot analyses using ovine RNA probes in liver collected every 4 h for 24 h. Liver Per2 and Bmall, 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 Bmall, whereas, under short days the Per2 maximum was in the early night with an inverse pattern to Bmall. There was a photoperiod x time 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 supracbiasmatic 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. (c) 2005 Elsevier Inc. All rights reserved.