Fos expression in the suprachiasmatic nucleus during photic entrainment of circadian rhythms in retinally damaged rats.

The protein product of the immediate-early gene c-fos is expressed rhythmically in the shell region of the suprachiasmatic nucleus (SCN), the mammalian circadian clock. Recently, we found that exposure to an entraining light pulse caused a suppression of Fos expression in the SCN shell in rats. To study the hypothesis that suppression of Fos in the shell is a correlate of photic entrainment, we used rats that were treated with the retinal neurotoxin monosodium glutamate (MSG) during the neonatal period. In spite of retinal degeneration, MSG-treated rats entrained normally and displayed light-induced suppression of Fos within the SCN shell. These results support the view that light-induced suppression of Fos within the SCN shell is a cellular correlate of photic entrainment.