Gene expression in the geniculate induced by a nonphotic circadian phase shifting stimulus.

We examined the effect of a nonphotic stimulus (running in a novel wheel) on the induction of Fos-related proteins in the two well-established neural components of the circadian clock: the suprachiasmatic nucleus (SCN) and the intergeniculate leaflet (IGL) of the lateral geniculate complex. There was no induction of Fos immunoreactivity (IR) in the SCN, but the IGL showed distinct Fos IR in animals whose running levels were associated with maximal rhythm shifts. Induction of Fos immunoreactivity was greatest at circadian phases when wheel running induces phase shifts. This temporal and spatial specificity of Fos induction is evidence that the intergeniculate leaflet mediates nonphotic signals to the circadian clock.