Circadian Clocks for all Meal Times: Central and Peripheral Correlates of Anticipation to Multiple Daily Meals in Rats

Rodents anticipate a scheduled daily meal using food-entrained circadian oscillator(s) (FEOs). These oscillators are separate from the light-entrained master circadian pacemaker in the suprachiasmatic nucleus, but their location in the brain or body remains uncertain, despite studies employing lesion, clock gene mapping, and clock gene knockout techniques. Rats and mice can also anticipate two daily meals. Behavioural studies suggest that each bout of anticipation is controlled by a separate FEO, but a single FEO model has not been ruled out. A two-oscillator model predicts that rhythmicity at the tissue level will exhibit bimodality. To determine if anticipation of two daily meals is associated with unimodal or bimodal rhythmicity, we mapped clock gene expression in both the brain (in situ hybridization for bmal1) and periphery (RTPCR for Bmal-1, Rev-erbα, Per1 and Per2) from rats anticipating one or two daily meals provided in the light period (AM) and/or the dark period (PM). AM feeding, relative to PM feeding, inverted the phase of clock genes rhythms in the adrenal gland and stomach, while the 2-meal schedule was associated with unimodal rhythms with an intermediate phase. Similar results were obtained for plasma levels of the adrenal hormone corticosterone and the gastric hormone ghrelin. Within the brain the results were less clear. AM feeding, compared to PM feeding, shifted Bmal-1 rhythms in the olfactory bulb and nucleus accumbens but not in other areas previously reported to exhibited shifting of clock gene rhythms by AM restricted feeding. The 2-meal schedule was again associated with intermediate phases in olfactory bulb and accumbens. In conclusion, assessment of clock gene rhythms in the brain and periphery has provided no definitive evidence for bimodality in rats anticipating two daily meals. Intermediate phases in the two meal group could represent a unique phase of entrainment of one FEO (or population of FEOs), or averaging of two populations of FEOs that are spatially intermingled within the tissues and brain regions examined. Further investigation using a technique that can localize clock gene expression within individual cells will be needed to resolve this issue.