The Suprachiasmatic Nucleus and the Circadian Timing System - Progress in Molecular Biology and Translational Science - page 19

Retinal ganglion
Cells (melanopsin)

RHT

SCN

POT

SPVZ

IGL

GHT

PTA

RPT

Figure 1.6 Diagram showing retinal projections to the components of the circadian
timing system. IGL, intergeniculate leaflet; GHT, geniculohypothalamic tract; PTA,
pretectal area; POT, primary optic tract; RHT, retinohypothalamic tract; RPT, ret-
inopretectal tract; SCN, suprachiasmatic nucleus; SPVZ, subparaventricular zone.

4.3. Efferent organization
SCN efferents were first described in normal material 115 but were not easily
distinguishable from other systems. Early studies used an autoradiographic
tracing method, 116 but it was difficult in such material to differentiate label-
ing of fibers of passage from terminal plexuses. The most extensive data
come from studies on the rat using anterograde transport using Phaseolus
leucoagglutinin (Refs. 117-119,150 ) . These showed a pattern of efferents
distributing predominantly to adjacent hypothalamus, preoptic area,
anterior hypothalamic area, particularly the subparavenricular zone, retro-
chiasmatic area, tuberal and posterior hypothalamic areas, as well as more
limited projections to basal forebrain, midline thalamus, IGL, and
periaqueductal gray ( Fig. 1.6 ) . With retrograde analysis, it is apparent that
projections to these areas arise differentially from core and shell (Refs.
119,151 ) . In addition to the peptides reviewed earlier, SCN neurons contain
prokineticin 2 (PKC2), a member of a family of secreted proteins with
multiple functions. 120 All SCN neurons appear to produce PKC2, the
protein is found in SCN neurons and projections and appears necessary
for transmission of the circadian signal (Refs. 121,122 ).