Subordinate circadian oscillators of the limbic forebrain : synchronization by motivational and metabolic state

The master circadian clock in the suprachiasmatic nucleus (SCN) is responsible for circadian rhythms in physiology and behavior. However, putative clock cells are found in nearly every mammalian tissue, suggesting that circadian rhythms are regulated both globally by the SCN and locally at the tissue level, downstream from the master clock. Immunocytochemistry for the clock gene protein Period2 (PER2), reveals two synchronized, opposite patterns of PER2 expression in the limbic forebrain of Wistar rats: the oval nucleus of the bed nucleus of the stria terminalis (BNST-OV) and central nucleus of the amygdala (CEA) are synchronized and in phase with the SCN; the basolateral amygdala (BLA) and dentate gyrus (DG) of the hippocampus are synchronized but 180{493} out of phase with the SCN. SCN lesions abolish rhythmic expression of PER2 in the limbic forebrain. The BNST-OV and CEA also require adrenal hormones for the rhythmic expression of PER2, as adrenalectomy causes a loss of PER2 rhythm in these two regions, leaving the SCN, BLA, and DG unaffected. In addition, a schedule of restricted feeding (RF) that limits food access to three hours during the day causes behavioral anticipation prior to food availability, and shifts the time of peak PER2 expression. Thus, compared to ad libitum (AL) fed animals, in the RF group the BNST-OV, CEA, BLA and DG all show peak PER2 expression about 12 hours after food access, but the SCN is unaffected. In constant light (LL), AL fed rats show no circadian rhythm of behavior or PER2 expression in the SCN or limbic forebrain. In contrast, RF in LL results in behavioral anticipation of the food access time, and restoration of rhythmic PER2 expression, peaking 12 hours after feeding time, in the BNST-OV, CEA, DG, and surprisingly, the SCN. Limited daily access to nutritive or non-nutritive treats does not produce significant behavioral anticipation or changes in the phase of PER2 expression in the limbic forebrain, whereas sodium deprived animals given limited daily access to saline showed a reduction in PER2 expression in the BLA around the time of saline access, but there was no behavioral effect, suggesting both reward and negative energy balance are necessary to alter the phase of PER2 expression in the limbic forebrain. These results indicate the presence of subordinate circadian oscillators in the limbic forebrain responsible for the integration of emotional, motivational, and behavioral state with circadian rhythm