Two families of phase-response curves characterize the resetting of the hamster circadian clock.

Phase-response curves (PRCs) have been reported for a wide variety of agents that induce phase shifts in the circadian rhythm of locomotor activity in the golden hamster. Many of these PRCs appear to be quite similar to one another. Because of the important role that the interpretation of PRCs has played in understanding the dynamics of the mammalian circadian pacemaker, a review of PRCs for the golden hamster reported from 1964 to 1991 was conducted to systematically summarize the common characteristics among these PRCs. Plots of phases associated with the peak of the advance portions, or of phases associated with the transitions between the delay and advance portion of the PRCs, revealed bimodal distributions of PRCs 11-13 h apart. Mardia-Watson-Wheeler circular test statistics indicated that the PRCs were distributed as two distinct populations. PRCs were either characteristic of those for light pulses (L-PRCs), or of those for dark pulses (D-PRCs). Taken with anatomical and physiological evidence, these data suggest that either one or two final common pathways may exist to mediate the phase-shifting effects of different stimuli.