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The Journal of Neuroscience, October 15, 2000, 20(20):7790-7797
Effects of Irradiance and Stimulus Duration on Early Gene
Expression (Fos) in the Suprachiasmatic Nucleus: Temporal Summation and
Reciprocity
O.
Dkhissi-Benyahya1,
B.
Sicard2, and
H. M.
Cooper1
1 Institut National de la Santé et de la
Recherche Médicale Unité 371, Cerveau et Vision, 69675 Bron, France, and 2 Laboratoire de Mammalogie, Institut de
Recherche pour le Developpement, BP 2528, Bamako, Mali
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.
Key words:
suprachiasmatic nucleus; immediate early genes; circadian
system; photoreception; retina; endogenous rhythms
Copyright © 2000 Society for Neuroscience 0270-6474/00/20207790-08$05.00/0
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