Retinal input to the hypothalamic suprachiasmatic nucleus (SCN)
synchronizes the SCN circadian oscillator to the external day/night
cycle. Retinal ganglion cells that innervate the SCN via the
retinohypothalamic tract are intrinsically light sensitive and express
melanopsin. In this study, we provide data indicating that not all
SCN-projecting retinal ganglion cells express melanopsin. To determine
the proportion of ganglion cells afferent to the SCN that express
melanopsin, ganglion cells were labeled following transsynaptic
retrograde transport of a recombinant of the Bartha strain of
pseudorabies virus (PRV152) constructed to express the enhanced green
fluorescent protein (EGFP). PRV152 injected into the anterior chamber
of the eye retrogradely infects four retinorecipient nuclei in the
brain via autonomic circuits to the eye, resulting in
transneuronally labeled ganglion cells in the contralateral retina 96 h
after intraocular infection. In animals with large bilateral lesions of
the lateral geniculate body/optic tract, ganglion cells labeled
with PRV152 are retrogradely infected from only the SCN. In these
animals, most PRV152-infected ganglion cells were immunoreactive for
melanopsin. However, a significant percentage (10–20%) of
EGFP-labeled ganglion cells did not express melanopsin. These data
suggest that in addition to the intrinsically light-sensitive
melanopsin-expressing ganglion cells, conventional ganglion cells also
innervate the SCN. Thus, it appears that the rod/cone system of
photoreceptors may provide signals to the SCN circadian system
independent of intrinsically light-sensitive melanopsin ganglion
cells.