Time course of neuronal sensitivity to light in the circadian timing system of the golden hamster.

The mammalian suprachiasmatic nuclei of the hypothalamus (SCN) have been identified as containing the pacemaker for circadian rhythms. Photic stimulation is known to induce the expression of the immediate early gene c-fos in the SCN of rodents during the subjective night. In order to determine the exact time course of the light sensitivity in the different cell subgroups of the SCN, we have investigated the effect of a light pulse every hour of the subjective night in golden hamsters kept in constant darkness for 3 days. Three neuronal populations inside and outside the SCN have been identified as sensitive to light at different times of the subjective night. These findings indicate (1) that there are neurons outside the SCN that are activated by light which might be part of the pacemaker system, and (2) that the switch from light-induced phase delays to phase advances as illustrated by phase-response curves is linked to the appearance of sensitivity to light in the three cell populations defined here.