The Suprachiasmatic Nucleus

The suprachiasmatic nucleus (SCh) is the major biological clock of the brain and provides an entrance for visual afferent signals to the circadian circuitry of the brain.136, 137; see ref 138 A direct projection from the SCh to both the Hcrt and MCH cells has been described in both rat and human brain;67 see also ref 139. These projections derive from the shell (predominantly containing the neuropeptide, arginine-vasopressin) as well as the core (predominantly containing the neuropeptide, vasoactive intestinal peptide) of the SCh.67 Notably, the diurnal changes in Hcrt expression and peptide content140, 141 disappear following destruction of the SCh.142 The major recipient of SCh innervation in the brain is the subparaventricular zone,139,143 which plays a role opposite to the LHA in arousal, since lesions targeting this region (in its ventral aspects) result in decreased sleep.144 Available data suggest that, at least in part, topographically similar neurons in the SCh project to both the LHA and the subparaventricular zone.13967 It will therefore be of interest to elucidate the synaptology and signalling cascade through which SCh can orchestrate activity in these antagonistic brain regions. Curiously, the retinal ganglion cells, which play a major role in regulating the timing of SCh rhythmicity, also project directly to LHA neurons in a pathway whose functional role is not yet clear.145

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