Output

The efferent projections of the SCN are extensive. Dr. Robert Moore has described four primary projections. These include the following: (1) projections to the anterior paraventricular thalamus, ventral lateral septum, and bed nucleus of the stria terminalis; (2) a periventricular fiber system innervating a large portion of the medial hypothalamus from the preoptic region to the premammillary region; (3) a lateral thalamic projection to the IGL; and (4) projections to the posterior paraventricular thalamus, precommisural nucleus, and olivary pretectal nucleus. The majority of these projections exhibit vasopressin or VIP immu-noreactivity. In general, this restricted subset of projections receives a particularly dense innervation from the SCN and therefore is thought to serve as relays for the distribution of SCN information to other neural structures. The subparaventricular zone (SPVZ), a region ventral to the paraventricular nucleus of the hypothalamus, is another structure receiving dense efferent SCN projections. The SPVZ has parallel projections to most of the same sites as the direct efferents of the SCN. With the exception of the SCN commissural connections, the efferents of each nucleus are ipsilateral.

The efferent projection to the preoptic hypothalamus is of particular importance because this area is involved in the regulation of sleep-wake, reproduction, fluid homeostasis, and thermoregulation. The projection to the PVN is part of a multisynaptic pathway regulating melatonin synthesis. Evidence suggests that the SCN, via the PVN, may influence the intermediate nucleus of the solitary tract (NTS), a structure that integrates gustatory, respiratory, and cardiovascular information. How efferent projections from the SCN convey time of day information to other neural networks is largely unclear.

The SCN also has a humoral output, which is known to include a daily rhythm in levels of the neuropeptide arginine vasopressin (AVP). Cerebrospinal fluid levels of AVP are high during the subjective day and low during the subjective night in both nocturnal and diurnal animals. These rhythms also persist in vitro. Rhythms of other neuroactive peptides synthesized in the SCN have been measured in the cerebral spinal fluid (CSF) and in vitro as rhythms in both mRNA and protein production.

The circadian and noncircadian visual systems may, in fact, interact. Except for the sharing of retinal photoreception, the nature of this interaction is not well-understood. Interconnections of these pathways occur between the superior colliculus and ventrolateral geniculate leaflet and the IGL.

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