Anomalous functional organization of barrel cortex in GAP-43 deficient mice

Growth associated protein 43 (GAP-43), found only in the nervous system, regulates the response of neurons to axon guidance signals. It is also critical for establishing normal somatotopy. Mice lacking GAP-43 (KO) show aberrant pathfinding by thalamocortical afferents, and do not form cortical whisker/barrels. GAP-43 heterozygous (HZ) mice show more subtle deficits--delayed barrel segregation and enlarged barrels at postnatal day 7. Here, we used cortical intrinsic signal imaging to characterize adult somatotopy in wildtype (WT), GAP-43 KO, and HZ mice. We found clear foci of activation in GAP-43 KO cortex in response to single-whisker stimulation. However, the KO spatial activation patterns showed severe anomalies, indicating a loss of functional somatotopy. In some cases, multiple foci were activated by single whiskers, while in other cases, the same cortical zone was activated by several whiskers. The results are consistent with our previous findings of aberrant pathfinding and clustering by thalamocortical afferent axons, and absence of barrel patterning. Our findings indicate that cortex acts to cluster afferents from a given whisker, even in the absence of normal topography. By contrast, single-whisker stimulation revealed normal adult topographic organization in WT and HZ mice. However, we found that functional representations of adult HZ barrels are larger than those found in WT mice. Since histological HZ barrels recover normal dimensions by postnatal day 26, the altered circuit function in GAP-43 HZ cortex could be a secondary consequence of the rescue of barrel dimensions.