The circadian clock in the suprachiasmatic nucleus (SCN) receives direct
retinal input via the retinohypothalamic tract (RHT), and the retinal ganglion cells contributing to this projection may be specialized with respect to direct regulation of the circadian clock. However, some
ganglion cells forming the RHT bifurcate, sending axon collaterals to the intergeniculate leaflet (IGL) through which light has secondary access to the circadian clock. The present studies provide a more extensive examination of
ganglion cell bifurcation and evaluate whether
ganglion cells projecting to several subcortical visual nuclei contain
melanopsin, a putative
ganglion cell photopigment. The results showed that retinal ganglion cells projecting to the SCN send collaterals to the IGL, olivary pretectal nucleus, and superior colliculus, among other places.
Melanopsin-immunoreactive (IR)
ganglion cells are present in the hamster retina, and some of these cells project to the SCN, IGL, olivary pretectal nucleus, or superior colliculus. Triple-label analysis showed that
melanopsin-IR cells bifurcate and project bilaterally to each SCN, but not to the other visual nuclei evaluated. The
melanopsin-IR cells have photoreceptive characteristics optimal for circadian rhythm regulation. However, the presence of moderately widespread bifurcation among
ganglion cells projecting to the SCN, and projection by
melanopsin-IR cells to locations distinct from the SCN and without known rhythm function, suggest that this
ganglion cell type is generalized, rather than specialized, with respect to the conveyance of photic information to the brain.