Phase angle changes of photically entrained circadian rhythms following a single nonphotic stimulus.

ABSTRACT

Syrian hamsters entrained to a light-dark (LD) cycle of 1410 h were given the opportunity to run in novel wheels for 3 h in the middle of the light phase. This manipulation transiently altered the phase angle of entrainment to the LD cycle activity onset was significantly advanced (by about 0.5 h) on the day after the pulse and gradually drifted back toward its prepulse time. When animals were held in LD 11.512.5 h, a photoperiod in which onset time occurs later relative to the time of lights-off, they again advanced about 0.5 h in response to the pulse of wheel running, but many animals retained an advanced phase angle for at least 7 days, and some for more than 21 days. Individual changes in phase angle were highly correlated with the prepulse phase angle the more negative the phase angle, the greater the advance subsequent to the novel wheel pulse. These results show that a single, short-duration, nonphotic manipulation can produce long-lasting alterations in the phase angle of entrainment to a LD cycle.