Abstract
Photic stimulation during the subjective night induces the expression of Fos among a discrete population of cells in the suprachiasmatic nuclei (SCN) region of the Syrian hamster. Light appears to stimulate Fos expression only when administered at circadian times (CTs) at which exposure causes a phase shift. Different populations of SCN cells express Fos in response to light pulses that result in phase advances versus phase delays, raising the possibility that different cell populations in the suprachiasmatic hypothalamus participate in light-induced phase advances and delays of the circadian oscillator. Microinjection of excitatory amino acid (EAA) antagonists into the region of the SCN attenuates light-induced phase advances of the free-running activity rhythm and light-induced Fos expression in the hamster SCN. However, injection of
