Light-induced resetting of the circadian pacemaker: Quantitative analysis of transient versus steady-state phase shifts

The suprachiasmatic nuclei of the hypothalamus contain the major circadian pacemaker in mammals, driving circadian rhythms in behavioral and physiolog ical functions. This circadian pacemaker's responsiveness to light allows s ynchronization to the light-dark cycle. Phase shifting by light often invol ves several transient cycles in which the behavioral activity rhythm gradua lly shifts to its steady-state position. In this article, the authors inves tigate in Syrian hamsters, whether a phase-advancing light pulse results in immediate shifts of the PRC at the next circadian cycle. In a first series of experiments, the authors aimed a light pulse at CT 19 to induce a phase advance. It appeared that the steady-state phase advances were highly corr elated with activity onset in the first and second transient cycle. This en abled them to make a reliable estimate of the steady-state phase shift indu ced by a phase-advancing light pulse on the basis of activity onset in the first transient cycle. In the next series of experiments, they presented a light pulse at CT 19, which was followed by a second light pulse aimed at t he delay zone of the PRC on the next circadian cycle. The immediate and ste ady-state phase delays induced by the second light pulse were compared with data from a third experiment in which animals received a phase-delaying li ght pulse only. The authors observed that the waveform of the phase-delay p art of the PRC (CT 12-16) obtained in Experiment 2 was virtually identical to the phase-delay part of the PRC for a single light pulse (obtained in Ex periment 3). This finding allowed for a quantitative assessment of the data . The analysis indicates that the delay part of the PRC-between CT 12 and C T 16-is rapidly reset following a light pulse at CT 19. These findings comp lement earlier findings in the hamster showing that after a light pulse at CT 19, the phase-advancing part of the PRC is immediately shifted. Together , the data indicate that the basis for phase advancing involves rapid reset ting of both advance and delay components of the PRC.