Is the FEO Food-Entrainable? Reexamining the Classic Fred Stephan Experiment Using Canonical-Clock-Less Period 1/2/3...

When nocturnal rodents are subjected to daytime restricted feeding, in which food is only available for a few hours per day, they typically become active a few hours before the onset of the scheduled mealtime. This so-called food-anticipatory activity (FAA) is controlled by an autonomous circadian pacemaker, which is independent from the central circadian pacemaker in the suprachiasmatic nucleus (SCN). Fred Stephan named this pacemaker the food-entrainable oscillator (FEO) because FAA re-entrains to a shifted feeding schedule. We recently developed a method to measure food-seeking nose-poking behavior by an operant feeding device and found that anticipatory food-seeking nose-poking for scheduled daily food availability shifts in parallel with phase-shifted environmental light-dark cycles, raising the possibility that anticipatory food-seeking behavior is controlled by an oscillator entrained to the environmental light-dark cycle. With this possible light-entrainability of the FEO, we revisited Stephan's historical experiment-testing whether the FEO entrains to feeding cycle in the absence of a light-dark cycle without functional SCN-using Period 1/2/3 triple knockout (KO) mice, in which the canonical circadian oscillators in the SCN and peripheral tissues are disabled. KO mice were subjected to restricted feeding under constant darkness. The food-seeking nose-poking activity of a subset of the KO mice indeed occasionally entrained to the feeding cycle and re-entrained to a shifted feeding cycle. Despite our previous study showing that anticipatory food-seeking behavior shifted with the environmental light-dark cycle, these data demonstrate that it can also entrain to the feeding cycle in the absence of an environmental light-dark cycle, supporting Stephan's observation that the FEO is indeed food-entrainable.