DIURNAL EXPRESSION OF C-FOS IN THE MOUSE RETINA

The diurnal levels of c-fos mRNA were studied in the mouse retinas by means of RNA blot analysis. Mice were kept on a 12/12 h dark/light cycle and gene expression was studied at various time points during the day and night periods. The highest levels of c-fos mRNA were measured during the first half of the night period. The high levels persisted for about 4-5 h. The c-fos mRNA levels declined during the second half of the night period and remained low during the day period. Continuous illumination of mice during the first hours of the night period prevented the increase in c-fos mRNA. If mice were kept in the dark during the day period, they failed to show an increase in c-fos mRNA levels in the subsequent night period. Hence, following activation of c-fos during the night period, a refractory period exists at which illumination is required before c-fos can be induced again by dark. Although light appears to suppress the activation of c-fos in the night period, a short lived burst in c-fos expression of 30-60 min in duration was observed when dark-adapted animals were illuminated, either at the transition between the dark and light period or during the dark period. Thus, it appears that c-fos is activated in at least two different cell types in the retina which respond differently to light and dark stimuli. The role of c-fos products in regulation of gene expression might be of importance in mediating the effect of light, via neurotransmitters or neuromodulators, on cellular activities in the retina during the day and night periods.