STIMULATION OF THE HAMSTER VENTRAL LATERAL GENICULATE-NUCLEUS INDUCES FOS-LIKE IMMUNOREACTIVITY IN SUPRACHIASMATIC NUCLEUS CELLS

The suprachiasmatic nucleus (SCN) functions as a pacemaker for circadian rhythms in rodents. Its activity and, therefore. circadian rhythms, are synchronized by light information from both a direct retinal projection and an indirect projection from the thalamic intergeniculate leaflet (IGL) and ventral lateral geniculate nucleus (vLGN) (geniculohypothalamic tract; GHT). The GHT also appears to be important for synchronizing circadian rhythms to non-photic cues. Light can activate expression of several immediate-early genes in the SCN, including c-fos. We examined whether electrical stimulation of the IGL/vLGN region would induce Fos-like immunoreactivity (lir) in the hamster SCN. Electrical stimulation was given for one hour during the dark period of the lighting cycle. Stimulation at night of the IGL and adjacent LGN regions induced Fos-lir in cells in the SCN. Labeled cells were found in the dorsolateral part of the caudal SCN, and not in rostral or central regions of the SCN, nor in the ventral part of the caudal SCN. These results indicate that activation of cells contributing to the hamster GHT can induce Fos-lir in a restricted region of the caudal SCN.