IS THERE A DIRECT RETINA-RAPHE-SUPRACHIASMATIC NUCLEUS PATHWAY IN THERAT

Possible pathways from the retina to the suprachiasmatic nucleus (SCN) relaying in the dorsal raphe nucleus (DRN) were investigated in rats using combined anterograde and retrograde tracing with immunohistochem istry. After injection of wheat germ agglutinin-conjugated horseradish peroxidase-colloidal gold complex into the SCN, many neurons were ret rogradely labeled in the middle levels of the DRN. Approximately one h alf of these neurons contained serotonin. After injection of cholera t oxin B subunit into the eyes, a few anterogradely labeled afferent fib ers were detected in the rostral DRN, however, not in contact with ret rogradely labeled neurons. Our findings provide direct evidence that s erotonergic projections to the rat SCN stem from the DRN nuclei. They also suggest that retina-raphe-SCN projections, a presumed third visua l input to the mammalian circadian pacemaker, may include further neur onal connections or brain sites.