ONTOGENY OF PHOTIC-INDUCED C-FOS MESSENGER-RNA EXPRESSION IN RAT SUPRACHIASMATIC NUCLEI

THE suprachiasmatic nucleus (SCN) of the hypothalamus contains a circa dian pacemaker that controls a variety of physiological and behavioral rhythms. Photic induction of immediate early genes such as c-fos in t he SCN occurs in a circadian-phase dependent manner, suggesting that c -fos may be part of the pathway for entrainment of circadian rhythms. The purpose of this study was to determine the point in development wh en photic stimuli can first activate c-fos mRNA expression in the rat SCN. The results indicate that photic stimulation can induce statistic ally significant c-fos mRNA expression at circadian time 22 (CT22) on postnatal day 1 (P1), although hybridization above background levels c an be detected in the SCN of some rat pups earlier in the subjective n ight. We infer that a multi-step pathway between visual transduction b y the retina and regulation of c-fos transcription in SCN cells must b e mature by CT22 on P1.