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

Ascending sensory information reaches primary sensory cortical areas via thalamic relay neurons that are organized into modality-specific compartments or nuclei. Although the sensory relay nuclei of the thalamus show consistent modality-specific segregation of afferents, we now show in a wild-type mouse strain that the visual pathway can be surgically "rewired" so as to induce permanent retinal innervation of auditory thalamic cell groups. Applying the same rewiring paradigm to a transgenic mouse lacking the EphA receptor family ligands ephrin-A2 and ephrin-A5 results in more extensive rewiring 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 thalamus. In the absence of this ephrin-A2/A5 border and after rewiring surgery, retinal afferents are better able to invade and innervate the deafferented auditory thalamus. These data suggest that signals that induce retinal axons 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 projections can be surgically manipulated in the mouse and that such plasticity can be controlled by proteins known to regulate topographic mapping.