Axonal Transport and Central Visual Projections of Ganglion Cells in Congenitally Blind Chickens

The rd (retinal degenerate) strain of chicken is an example of a recessively inherited mutation characterized by blindness at the time of hatching, as defined by behavioral and electrophysiological tests. Paradoxically, blind mutants have normal retinal morphology, even at the ultrastructural level. Eventually, however, the entire retina degenerates in this strain, perhaps as a result of disuse atrophy. Results of preliminary studies imply that a defect in the visual transduction cascade in photoreceptor cells is responsible for the lack of vision. As well as being an important animal model for studies on photochemistry and transduction, the rd chicken may afford a paradigm for studies on inner retinal physiology and pathology, as electrical input to this inner neuronal system appears to be absent. In the current study we examined axonal transport (both retrograde and anterograde) in rd retinal ganglion cells and connectivity of ganglion cells to visual centers in the brain and compared these to normally sighted chicks. All visuorecipient nuclei were present in rd animals and appeared normal at the light microscopic level. Whenn 3H-proline was injected into one eye of a blind chicken on the day of hatching, labeled polypeptides or proteins were transported via a fast transport mechanism to the same visual centers in roughly the same quantities as in normally sighted chicks. When horseradish peroxidase (HRP) was injected in the optic tectum of blind and normal 1 day old chicks, this label was transported retrogradely to the soma of retinal ganglion cells. It is therefore concluded that central visual centers develop normally in rd chicks, tht ganglion cell axons differentiate and connect to their genetically determined central destinations, and that both anterograde and retrograde axonal transport mechanisms are not initially compromised by lack of electrical input to ganglion cells in this model of hereditary blindness.