Light and food signals cooperate to entrain the rat pineal circadian system.

Light and food signals are two dominant Zeitgeber for central and peripheral circadian clocks, respectively. However, whether and how changes in light and food stimuli affect circadian rhythms of mammalian pineal clock genes are still largely unknown. For the first time, in the present study, we established a method for analysis of the expression levels of most clock genes (Bmal1, Per1, Per2, Per3, Cry1, Dec1 and Dec2) and AANAT in a single rat pineal gland and used it to investigate individual and cooperative effects of feeding and light cycle reversals on the pineal circadian system. Feeding reversal (RF) alone for 7 days had only weak effect on the rhythmicity of clock gene expression in the pineal gland, whereas light/dark reversal for the same period without any change in the feeding regime could not absolutely reverse the expression phases of all examined genes either. However, reversal of the LD cycle together with the feeding regime completely resynchronized the circadian gene expression in the pineal gland within 7 days. These data suggest that photic regulation is the dominant cue that synchronizes the pineal circadian rhythm more efficiently than the feeding-related cue, but food availability does contribute to reentrainment of the pineal circadian system. In addition, the inability of RF in the pineal circadian system suggests that the pineal gland is more strongly coupled to the master clock than other peripheral organs and alternatively distinguishes it from other peripheral organs in the hierarchically organized mammalian circadian systems.