The B fragment of cholera toxin (CTb) is a highly sensitive
anterograde tracer for the labelling of retinal axons. It can reveal
dense retinofugal projections to well-known retinorecipient nuclei
along with sparse but distinct input to target areas that are not
commonly recognized. Following a unilateral injection of CTb into the
vitreous chamber of seven adult cats, we localized the toxin
immunohistochemically in order to identify direct retinal projections
in these animals. Consistent with previous findings, the strongest
projections were observed in the superficial layers of the superior
colliculus, the dorsal and ventral lateral geniculate nuclei, the
pretectal nuclei, the accessory optic nuclei, and the suprachiasmatic
nucleus of the hypothalamus. However, we also found labelled terminals
in several other brain areas, including the zona incerta, the medial
geniculate nucleus, the lateral posterior-pulvinar complex, the lateral
habenular nucleus, and the anterior and lateral hypothalamic regions.
The morphological characteristics of the retinal axon terminals in most
of the identified novel target sites are described.