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

The circadian clock of the suprachiasmatic nucleus (SCN) may control the sleep-wake cycle by modulating the activity of brain regions important in sleep onset and maintenance, such as the ventrolateral preoptic area (VLPO). The aim of this study was to determine whether the VLPO receives direct projections 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 restricted to the VLPO only a few labeled cells were found within the SCN, with more labeled cells located around the nucleus. Therefore, the circadian regulation 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.