LIGHT-INDUCED C-FOS EXPRESSION IN THE MOUSE SUPRACHIASMATIC NUCLEUS -IMMUNOELECTRON MICROSCOPY REVEALS COLOCALIZATION IN MULTIPLE CELL-TYPES

Although light is known to regulate the level of c-fos gene expression in the suprachiasmatic nucleus (SCN), the site of an endogenous circa dian clock, little is known about the identities of the photically act ivated cells. We used light-microscopic immunocytochemistry and immuno electron microscopy to detect c-Fos protein in the SCN of Sabra mice e xposed to brief nocturnal light pulses at zeitgeber time 15-16. Stimul ation with light pulses that saturated the phase-shifting response of the circadian locomotor rhythm revealed an upper limit to the number o f photo-inducible c-Fos cells at about one-fifth of the estimated tota l SCN cell population. This functionally defined set was morphological ly and phenotypically heterogeneous. About 24% could be labelled for v asoactive intestinal polypeptide, 13% for vasopressin-neurophysin, and 7% for glial fibrillary acidic protein, The remaining 56% of c-Fos-po sitive cells were largely of unknown phenotype, although many were pre sumptive interneurons, some of which were immunoreactive for nitric ox ide synthase.