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 nuc lei (SCN) region of the Syrian hamster. Light appears to stimulate Fos expression only when administered at circadian times (CTs) at which e xposure causes a phase shift. Different populations of SCN cells expre ss Fos in response to light pulses that result in phase advances versu s phase delays, raising the possibility that different cell population s in the suprachiasmatic hypothalamus participate in light-induced pha se advances and delays of the circadian oscillator. Microinjection of excitatory amino acid (EAA) antagonists into the region of the SCN att enuates light-induced phase advances of the free-running activity rhyt hm and light-induced Fos expression in the hamster SCN. However, injec tion of N-methyl-D-aspartate (NMDA) at CT 18, which results in a wides pread pattern of Fos expression in the hypothalamus that includes the retinorecipient zone of the SCN, does not produce phase advances of th e circadian oscillator. The results demonstrate that both light-induce d Fos expression and light-induced phase advances are dependent upon E AA 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.