Laminar specificity of local circuits in barrel cortex of ephrin-A5 knockout mice

Cortical circuits are characterized by layer-specific axonal arbors. Molecu lar laminar cues are believed to direct the development of this specificity . We have tested the hypothesis that ephrin-A5 is responsible for preventin g layer 2/3 pyramidal cell axons from branching within layer 4 (Castellani et al., 1998) by assessing the laminar specificity of axonal arbors in ephr in-A5 knockout mice. We find that in barrel cortex of knockout mice, layer 2/3 pyramidal neurons form axonal arbors specifically in layers 2/3 and 5, avoiding layer 4. This pattern of arborization is indistinguishable from th at of wild-type littermates. Furthermore, we find that in wild-type mice, l aminar patterns of ephrin-A5 expression differ between cortical areas despi te the similarity of layer-specific local cortical circuits across areas. M ost notably, ephrin-A5 is not expressed preferentially in layer 4 of wild-t ype mouse barrel cortex. We conclude that ephrin-A5 is not responsible for preventing the development of layer 2/3 pyramidal cell axonal arbors in lay er 4 of mouse barrel cortex. These observations also suggest that if ephrin -A5 plays a role in the emergence of layer-specific circuits, that role mus t differ between cortical areas.