Cellular colocalization of Fos and neuropeptide Y in the intergeniculate leaflet after nonphotic phase-shifting events.

Nonphotic and photic stimuli that phase shift circadian rhythms were presented to hamsters, Mesocricetus auratus. The nonphotic stimulus was a 3-h pulse of novelty-induced wheel running starting at circadian time 4-5. The photic stimulus used was a 0.5 h light pulse starting at circadian time 18. Double immunocytochemistry was used to determine the neurochemical phenotype of cells in the intergeniculate leaflet that were activated by these stimuli. Both the nonphotic and the photic phase-shifting stimuli induced the expression of c-fos in the intergeniculate leaflet compared to unstimulated controls. However, after nonphotic stimulation, Fos-like immunoreactivity was common in neurons that also were NPY positive. Such colocalization of Fos and NPY after photic stimuli was rare. These findings suggest that the NPY pathway from the intergeniculate leaflet to the suprachiasmatic nucleus carries information about nonphotic events.