A sparse projection from the suprachiasmatic nucleus to the sleep active ventrolateral preoptic area in the rat

The circadian dock of the suprachiasmatic nucleus (SCN) may control the sle ep-wake cycle by modulating the activity of brain regions important in slee p onset and maintenance, such as the ventrolateral preoptic area (VLPO). Th e aim of this study was to determine whether the VLPO receives direct proje ctions from the SCN. The retrograde tracer cholera toxin (beta subunit; CT beta) was injected into the VLPO of male rats and the SCN was examined for the presence of labeled, VLPO-projecting neurons. After injections restrict ed to the VLPO only a few labeled cells were found within the SCN, with mor e labeled cells located around the nucleus. Therefore, the circadian regula tion of the VLPO is likely to be achieved through multisynaptic pathways or via a diffusible signal, rather than by direct axonal outputs from the SCN to the VLPO. NeuroReport 11:93-96 (C) 2000 Lippincott Williams & Wilkins.