Neuropil reactivity, distribution and morphology of NADPH diaphorase type I neurons in the barrel cortex of the adult mouse

The mouse, like a few other rodent and marsupial species, displays a striking modular architecture in its primary somatosensory cortex (SI). These modules, known as barrels, are mostly defined by the peculiar arrangement of granule cells and thalamic axons in layer IV. In the present work, we studied both the distribution and morphology of neurons stained for NADPH diaphorase (NADPH-d) and neuropil reactivity in the posteromedial barrel subfield (PMBSF), which represents the mystacial whiskers. We then compared our results with previous descriptions of NADPH-d distribution in both neonatal and young mice. We found two types of neurons in the PMBSF: type I neurons, which have large cell bodies and are heavily stained by the NADPH-d reaction; and type II neurons, characterized by relatively small and poorly stained cell bodies. The distribution of type I cells in the PMBSF was not homogenous, with cells tending to concentrate in septa between barrels. Moreover, the cells found in septal region possess both a larger and more complex dendritic arborization than cells located inside barrels. Our findings are at variance with results from other groups that reported both an absence of type II cells and a homogeneous distribution of type I cells in the PMBSF of young animals. In addition, our results show a distribution of type I cells which is very similar to that previously described for the rat's barrel field.