Topographic guidance labels in a sensory projection to the forebrain

Visual connections to the mammalian forebrain are known to be patterned by neural activity, but it remains unknown whether the map topography of such higher sensory projections depends on axon guidance labels. Here, we show c omplementary expression and binding for the receptor EphA5 in mouse retina and its ligands ephrin-A2 and ephrin-A5 in multiple retinal targets, includ ing the major forebrain target, the dorsal lateral geniculate nucleus (dLGN ). These ligands can act in vitro as topographically specific repellents fo r mammalian retinal axons and are necessary for normal dLGN mapping in vivo . The results suggest a general and economic modular mechanism for brain ma pping whereby a projecting field is mapped onto multiple targets by repeate d use of the same labels. They also indicate the nature of a coordinate sys tem for the mapping of sensory connections to the forebrain.