Calories affect zeitgeber properties of the feeding entrained circadian oscillator

Rats with suprachiasmatic (SCN) lesions readily entrain to daily meals by increasing their activity prior to food access. Although previous experiments suggest that entrainment requires a nutritive meal, it is not yet clear what the parameters of the entraining stimulus are. The first experiment investigated the role of caloric content in resetting the feeding entrainable oscillator (FEO). Rats were entrained to 20 g chow/day until anticipatory activity was stable. The food access time was then delayed by 8 h and rats received either 0, 2, 6, or 16 g of chow for two days. Sixteen g of chow produced large delays on the next two cycles, while 0 and 2 g produced no delays. Two of 8 rats receiving 6 g showed delays, indicating that 22 kcal is near the threshold. In a second experiment, the effects of bulk were investigated. After an 8 h phase delay, rats received 0, 6, 10 or 16 g of chow mixed with cellulose for a total of 21 g for all groups. The 10 and 16 g chow groups delayed while the 0 g chow group did not. In the 6 g groups some animals phase shifted while others did not. Thus, the addition of non-nutritive bulk to the phase shifted meal had little or no effect on resetting the FEO and it appears that caloric content rather than gastric distention provides an effective zeitgeber for the FEO.