Target-specific morphology of retinal axon arbors in the adult hamster.

The B fragment of cholera toxin (CT-B) provides a highly sensitive anterograde tracer for labeling retinofugal axons, revealing dense projections to known central retinorecipient nuclei, and sparse but distinct inputs to regions that have not been traditionally recognized as targets of direct retinal projections. In hamsters, we can identify CT-B labeled retinal axons in more than 25 cell groups in the mesencephalon, diencephalon, and basal telencephalon. CT-B labeling additionally delineates the complete arbor morphology, especially in regions that receive a sparse input, offering hitherto unknown views of retinal axon ramifications. We present here the terminal morphology of retinal axons in the lateral geniculate body and superior colliculus, verifying earlier studies, and also document novel findings on the configuration of retinal axon endings in the ventral nucleus of the lateral geniculate body, intergeniculate leaflet, suprachiasmatic nucleus, and in the nuclei of the accessory optic tract. Additionally, the trajectory and terminal morphology of retinal afferents to the hypothalamus, preoptic area, and basal telencephalon are detailed. The results are discussed in the context of possible functional roles for some of these projections.