Light‐induced c‐Fos Expression in the Mouse Suprachiasmatic Nucleus: Immunoelectron Microscopy Reveals Co‐localization in Multiple Cell Types

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Abstract
Although light is known to regulate the level of c‐fos gene expression in the suprachiasmatic nucleus (SCN), the site of an endogenous circadian clock, little is known about the identities of the photically activated cells. We used light‐microscopic immunocytochemistry and immunoelectron microscopy to detect c‐Fos protein in the SCN of Sabra mice exposed to brief nocturnal tight pulses at zeitgeber time 15–16. Stimulation with light pulses that saturated the phase‐shifting response of the circadian locomotor rhythm revealed an upper limit to the number of photo‐inducible c‐Fos cells at about one‐fifth of the estimated total SCN cell population. This functionally defined set was morphologically and phenotypically heterogeneous. About 24% could be labelled for vasoactive intestinal polypeptide, 13% for vasopressin‐neurophysin, and 7% for glial fibrillary acidic protein. The remaining 56% of c‐Fos‐positive cells were largely of unknown phenotype, although many were presumptive interneurons, some of which were immunoreactive for nitric oxide synthase.

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