Single cell model for re‐entrainment to a shifted light cycle

Our daily 24‐h rhythm is synchronized to the external light–dark cycle resulting from the Earth's daily rotation. In the mammalian brain, the suprachiasmatic nucleus (SCN) serves as the master clock and receives light‐mediated input via the retinohypothalamic tract. Abrupt changes in the timing of the light–dark cycle (e.g., due to jet lag) cause a phase shift in the circadian rhythms in the SCN. Here, we investigated the effects of a 6‐h delay in the light–dark cycle on PERIOD2::LUCIFERASE expression at the single‐cell level in mouse SCN organotypic explants. The ensemble pattern in phase shift response obtained from individual neurons in the anterior and central SCN revealed a bimodal distribution; specifically, neurons in the ventrolateral SCN responded with a rapid phase shift, while neurons in the dorsal SCN generally did not respond to the shift in the light–dark cycle. We also stimulated the hypothalamic tract in acute SCN slices to simulate light‐mediated input to the SCN; interestingly, we found similarities between the distribution and fraction of rapid shifting neurons (in response to the delay) and neurons that were excited in response to electrical stimulation. These results suggest that a subpopulation of neurons in the ventral SCN that have an excitatory response to light input, shift their clock more readily than dorsal located neurons, and initiate the SCN's entrainment to the new light–dark cycle. Thus, we propose that light‐excited neurons in the anterior and central SCN play an important role in the organism's ability to adjust to changes in the external light–dark cycle.