Circadian variation in photic regulation of immediate-early gene mRNAs in rat suprachiasmatic nucleus cells.

Exposure of rodents to light at daily times at which it can phase-shift circadian rhythms (subjective night) induces an increase in immunoreactivity for the immediate-early gene product Fos in cells of the circadian pacemaker, the suprachiasmatic nuclei (SCN). Light exposure at other phases (subjective day) does not increase Fos immunoreactivity in SCN cells, but it is not known whether this failure reflects the inability of light to induce transcription of appropriate mRNAs, or a post-transcriptional block. We used in situ hybridization studies to examine levels of mRNA in the SCN of rats exposed to light during the subjective day and subjective night. We studied levels of mRNAs for several immediate-early genes: c-fos, NGFI-A, NGFI-B, c-jun, junB and junD, before and after light exposure at these phases. Levels of mRNAs for all of the genes tested were unaffected by light exposure during the subjective day, and all were increased in response to light during the subjective night. With the exception of a weak constitutive label for junD, none of the genes were expressed in the SCN in darkness at either phase. Light-induced increases in the levels of several mRNAs in the SCN occur only during the subjective night; the mechanisms which prevent such responses during the subjective day remain unknown.