Axonal regeneration and synapse formation in the superior colliculus by retinal ganglion cells in the adult rat

In adult rats, one optic nerve was transected and replaced by a 4 cm segment of autologous peripheral nerve (PN) that linked one eye and the superior colliculus (SC) along a predominantly extracranial course. Retrograde and orthograde studies with the tracers HRP or rhodamine-B-isothiocyanate (RITC), as well as immunocytochemical neuronal labels, indicated the following: (1) Regenerating axons from the axotomized retinal ganglion cells extended along the entire PN grafts, covering a distance nearly twice that of the normal retinotectal projection of intact rats. (2) Some of these axons penetrated the SC and formed terminal arborizations up to 500 microns from the end of the graft. (3) By electron microscopy, the arborizations of these regenerated axons in the SC were seen as small HRP-labeled axonal profiles that contacted neuronal processes in the SC; some of these contacts showed pre- and postsynaptic membrane specializations. These findings indicate that injured retinal ganglion cells in the adult rat are not only able to regrow lengthy axons, but may also form synapses in the SC.