Enhanced plasticity of retinothalamic projections in an ephrin-A2/A5 double mutant

Ascending sensory information reaches primary sensory cortical areas via th alamic relay neurons that are organized into modality-specific compartments or nuclei. Although the sensory relay nuclei of the thalamus show consiste nt modality-specific segregation of afferents, we now show in a wild-type m ouse,strain that the visual pathway can be surgically "rewired" so as to in duce permanent retinal innervation of auditory thalamic cell groups. Applyi ng the same rewiring paradigm to a transgenic mouse lacking the EphA recept or family ligands ephrin-A2 and ephrin-A5 results In more extensive rewirin g than in the wild-type strain. We also show for the first time that ephrin -A2 and ephrin-A5 define a distinct border between visual and auditory thal amus. In the absence of this ephrin-A2/A5 border and after rewiring surgery , retinal afferents are better able to invade and innervate the deafferente d auditory thalamus. These data suggest that signals that induce retinal ax ons to innervate the denervated auditory thalamus may compete with barriers , such as the ephrins, that serve to contain them within the normal target. The present findings thus show that the targeting of retinothalamic projec tions can be surgically manipulated in the mouse and that such plasticity c an be controlled by proteins known to regulate topographic mapping.