Is Fos expression necessary and sufficient to mediate light-induced phase advances of the suprachiasmatic circadian oscillator?

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 N-methyl-D-aspartate (NMDA) at CT 18, which results in a widespread pattern of Fos expression in the hypothalamus that includes the retinorecipient zone of the SCN, does not produce phase advances of the circadian oscillator. The results demonstrate that both light-induced Fos expression and light-induced phase advances are dependent upon EAA neurotransmission within the SCN region. However, expression of Fos in the SCN induced by the EAA agonist NMDA is not sufficient to cause phase advances of the SCN oscillator.