Distribution of NADPH-diaphorase-positive neurons in the opossum neocortex

The distribution of NADPH-diaphorase reactive cells were evaluated both in horizontal sections of a flattened cortex and in transversal sections of the opossum (Didelphis marsupialis) neocortex. The tangential distribution of labeled cells behind the orbitalis fissure was denser in the rostral vs caudal regions and in the lateral vs medial regions. Transversal sections revealed that most of the positive neurons are in the grey matter, although 1/4 of this population is located in the underlying white matter. This pattern of neuronal distribution is similar to that previously described in rodents, but quite different from that observed in higher mammals such as the cat and primates.