Differential effects of meal size and food energy density on feeding entrainment in goldfish

The synchronizing stimulus, its transduction site, and the afferent pathway s responsible for feeding entrainment remain unknown. In fish, the role of the diet in the development of feeding anticipatory activity (FAA) is not w ell understood and fundamental questions on the mechanisms of feeding entra inment, such as the meal characteristics required to develop FAA, remain un explored. To test the entraining properties of daily meals with different s izes and energy densities, activity rhythms were studied after a 12-h shift of the feeding cycle in individual goldfish under constant light. In the I st experiment, the energy content of a control diet (16.7 kJ/g) was diluted by replacing 50% (8.3 kJ/g) or 90% (1.7 kJ/g) of the diet with cellulose. However, the number of days required to stabilize FAA after the shift did n ot differ statistically between diets. In the 2nd experiment, meal size was modified by reducing the daily feeding ration to 0.5% and 0.1% b.wt.d(-1). In this case, differences in the entraining properties of the two feeding rations appeared because goldfish fed at 0.1% b.wt.d(-1) resynchronized fas ter than those fed at 0.5% b.wt.d(-1). These results revealed that the dilu tion of the dietary energy up to 1.7 kJ/g had no significant effect on the entraining properties of the feeding-entrainable oscillator (FEO), whereas the reduction of the meal size to 0.1% b.wt.d(-1) provoked a faster resynch ronization after shifting the daily meal cycle. Taken together, these resul ts suggest that gut distension may be involved in feeding entrainment, as a reduction in meal size but not in the amount of dietary energy supplied si gnificantly shortened the time required for resynchronization and highlight ed the different synchronizing properties of meal size and energy density a s zeitgebers for the FEO.