Effects of irradiance and stimulus duration on early gene expression (Fos) in the suprachiasmatic nucleus: temporal summation and reciprocity.

The daily behavioral, physiological, and hormonal rhythms in mammals are regulated by an endogenous circadian clock located in the suprachiasmatic nucleus (SCN) and are synchronized by the natural 24 hr light/dark cycle. We studied the response properties (threshold, saturation, and linearity) of the photic system to irradiance by assaying light induction of Fos, the protein product of the immediate early gene c-fos. Fos expression was quantified by image analysis in the SCN and in the retina. Fos expression in the SCN and retina are unrelated because the response differs in terms of threshold, saturation, and range. In the SCN, Fos expression increases proportionately to increases in both irradiance and duration of light exposure. The photic system shows a linear temporal integration of photons for durations ranging from 3 sec to 47.5 min. The principal result of this study shows that in the SCN, Fos expression is directly proportional to the total number of photons rather than to irradiance or duration alone (reciprocity), and that integration occurs over a range of 5 log units of photon number. This report provides the first demonstration that the mechanism of photon integration by the circadian system is expressed at a cellular level in the SCN.