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 be en identified as containing the pacemaker for circadian rhythms. Photi c stimulation is known to induce the expression of the immediate early gene c-fos in the SCN of rodents during the subjective night. In orde r to determine the exact time course of the light sensitivity in the d ifferent cell subgroups of the SCN, we have investigated the effect of a light pulse every hour of the subjective night in golden hamsters k ept in constant darkness for 3 days. Three neuronal populations inside and outside the SCN have been identified as sensitive to light at dif ferent times of the subjective night. These findings indicate (1) that there are neurons outside the SCN that are activated by light which m ight be part of the pacemaker system, and (2) that the switch from lig ht-induced phase delays to phase advances as illustrated by phase-resp onse curves is linked to the appearance of sensitivity to light in the three cell populations defined here.