TEMPORAL REGULATION OF LIGHT-INDUCED FOS AND FOS-LIKE PROTEIN EXPRESSION IN THE VENTROLATERAL SUBDIVISION OF THE RAT SUPRACHIASMATIC NUCLEUS

We measured c-fos messenger RNA levels and Fos protein immunoreactivit y in the suprachiasmatic nucleus of rats as a function of light and ti me of day. Immunohistochemistry demonstrated a daily rhythm of immunor eactive Fos in the ventrolateral subdivision of the suprachiasmatic nu cleus of animals entrained to a 12 h/12 h light-dark cycle; expression was low during the dark phase, peaked about 2 h after light onset at dawn, and remained elevated at an intermediate level for the remainder of the light phase. Immunoblots of nuclear extracts showed a 54,000 m ol. wt band that increased in density from the dark phase to the early light phase and decreased again during the late light phase. In situ hybridization using a radiolabeled cDNA probe revealed a c-fos messeng er RNA signal that was detected as early as 15 min after dawn, promine nt at 30 min, and absent by 2 h. The expression of c-fos messenger RNA and Fos immunoreactivity in the suprachiasmatic nucleus depended on t he presence of ambient light. In rats entrained to two daily 1-h light pulses corresponding to dawn and dusk (''skeleton'' photoperiod) inst ead of the complete light-dark cycle, immunoreactive Fos was elicited by the dawn pulse alone and was less persistent than during the comple te photoperiod. In rats free-running in constant darkness, c-fos messe nger RNA and Fos immunoreactivity were stimulated by 2-h light pulses administered only during the subjective night and early subjective day , but not by light pulses during the middle or late subjective day or in the absence of light pulses.