Topological specificity in reinnervation of the superior colliculus by regenerated retinal ganglion cell axons in adult hamsters

In normal rodents there is a precise topology of the retinocollicular proje ction, the nasotemporal and ventrodorsal axes of the retina being respectiv ely projected onto the caudorostral and mediolateral axes of the contralate ral superior colliculus (SC). We evaluated the distribution of regenerated retinal ganglion cell (RGC) axon terminals in the SC of adult hamsters in w hich an unbranched peripheral nerve graft was directed from the retina to t he contralateral SC. Responses to visual stimulation of individual RGCs wer e recorded from terminal arbors of their regenerated axons in the reinnerva ted SC. Retinal positions of these RGCs were inferred from the locations of their visual receptive fields. At some sites in the reinnervated SC, axon terminal arbors converged from widely separated RGCs. Conversely, axon term inal arbors at widely separated sites in the SC could emanate from contiguo us RGCs. To assess whether any tendency for order was superimposed on the a pparent disorganization of the regenerated projection, we evaluated the rel ative positions of pairs of RGC terminals in the SC in relation to the rela tive retinal locations of the corresponding pairs of RGCs. Among the 983 pa irs of RGCs able to be evaluated from nine animals studied 30-60 weeks afte r grafting, there was a statistically significant 3/2 tendency for the more nasally situated of two RGCs to project its terminal more caudally in the SC than that of the more temporally situated RGC. A similar tendency toward appropriate organization was not found with respect to the ventrodorsal ax is of the retina and the mediolateral axis of the SC.