Resetting of a circadian clock by food pulses.

Rats with lesions of the suprachiasmatic nuclei were exposed to daily feeding until anticipatory activity (AA) developed. Meals were then phase advanced or delayed and presented for 1-4 consecutive days. The phase of the circadian pacemaker was assessed during probes of total food deprivation before or after 8 days of intervening ad lib feeding. One or two food pulses caused phase delays for one cycle but were insufficient to reset the feeding entrainable pacemaker. Complete or partial resetting to 6- or 9-h advances or delays was observed in some rats after three food pulses and in all rats after four food pulses. In some rats, phase shifts of meals appeared to induce both advancing and delaying transients, and two bouts of AA appeared during food-deprivation probes. This suggests that the feeding entrainable pacemaker consists of two or more oscillators which became uncoupled after phase shifts. The persistence of AA at the preshift phase observed after initial phase delays, concomitantly with AA to the phase shifted meals, also suggests the presence of a second oscillator.