Entrainment of anticipatory activity to various durations of food access.

Rats with lesions of the suprachiasmatic nuclei (SCN) and intact rats were maintained on restricted feeding with the duration of food access ranging from 4 to 12 hr. All rats with SCN lesions displayed at least some anticipatory activity (AA) at all food access durations. The amount of AA diminished when food access was extended to 12 hr and was lowest in a group that was exposed only to a 10-hr access period. The onset of AA (phase angle of entrainment) appeared to be more sensitive to the time of food availability than to its termination. Most intact rats maintained in constant light displayed some AA at food access durations between 4 and 10 hr. In most cases the period of the free running rhythm increased as it crossed food access and the free running rhythm became increasingly disrupted as the experiment progressed in all rats. In some cases the free running rhythm appeared to force AA out of entrainment. These results demonstrate that AA occurs in conditions that impose only minor deficits on the energy balance of rats. Furthermore, they provide additional evidence of interactions between two separate circadian pacemaking systems.