CALBINDIN-D-28K CELLS IN THE HAMSTER SCN EXPRESS LIGHT-INDUCED FOS

ALTHOUGH the suprachiasmatic nuclei (SCN) have been intensively analyz ed, they contain a population of cells that has not yet been character ized. In this study, we examined the distribution of cells immunoreact ive (ir) for calbindin-D-28K (CaBP), calretinin (CR), parvalbumin, vas opressin-associated neurophysin (NP), substance P (SP), vasoactive int estinal peptide (VIP), and light-induced Fos-like protein. Previously unidentified cells in the core of the hamster SCN contained CaBP. Phot ic stimulation during the night induced Fos expression in about 75% of the CaBP-positive SCN cells, and about 50% of the Fos-positive cells in the core region expressed CaBP. These findings provide new informat ion in the search for the cellular localization of pacemaker cells in the SCN, as photic input entrains the circadian system, and cells that receive photic input must be either part of the clock itself, or an u pstream component of the clock.