The Suprachiasmatic Nucleus and the Circadian Timing System - Progress in Molecular Biology and Translational Science - page 18

projection is about equally dense to the contralateral and ipsilateral SCN in
all species studied thus far. Because the only photosensitive elements known
in the retina at the time were rods and cones, and many mammals have pre-
dominantly rod retinas, it was assumed that rods were the necessary receptors
for entrainment. This conclusion was rendered untenable by data showing
normal entrainment in animals lacking both rods and cones, leading to an
elegant elucidation of the mechanisms of photoentrainment over the past
15 years, 105 identified and characterized a novel photopigment, melanopsin,
derived from the gene, Opn4 . As noted earlier, studies on mice without rod
and cone photoreceptors had shown a loss of vision but retention of visual
reflexes and light-mediated entrainment of circadian rhythms. 106 This was
followed by the discovery of photosensitive retinal ganglion cells containing
the photopigment, melanopsin, in the inner retina and that this mediated
circadian entrainment and all other nonimage forming visual func-
tions. 107-110,134,135 The full pathway of the RHT can be shown by immu-
nocytochemistry with antisera to melanopsin including the RHT extension
beyond the SCN into the SPVZ and anterior hypothalamic area and nucleus
and caudally into the area of the dorsomedial nucleus and ventral tuberal area
(Ref. 111 , for review of RHT). Proof that the RHT is the entrainment
pathway was shown with selective transection of the tract that led to a selec-
tive loss of entrainment with preservation of other visual functions. 112
4.2.1.2 Lateral geniculate nucleus and the pretectal area
The pattern of RHT innervation of the SCN core is largely mimicked by
a projection from the intergeniculate leaflet (IGL) of the lateral geniculate
complex through the geniculohypothalamic tract (GHT), a projection
from neurons that contain NPY colocalized with GABA ( Ref. 141 ). This
projection is also coextensive with another secondary visual projection from
the pretectal area to the SCN in the rat ( Ref. 138 ) . The function of these
visual projections to the SCN has remained elusive. The components of
the “visual” portions of the CTS are shown in Fig. 1.6 . The other major
input to the SCN core is a serotonin projection from the midbrain raphe
nuclei, particularly the median raphe. 96,113,114 The origin of afferents to
the shell is complex with dense afferents arising from other hypothalamic
nuclei, basal forebrain, limbic cortex, septal area, and brainstem. 96 There also
are afferents from the paraventricular nucleus of the midline thalamus that
distribute to both shell and core ( Ref. 139 ). All afferents to the core termi-
nate over the entire subdivision.