Neuropeptide Y- and somatostatin-immunoreactive axons in the corpus callosum during postnatal development of the rat.

Corpus callosum (CC) projections in adult mammals were generally thought to be excitatory and to use excitatory amino acids as their transmitters. Little information has been available about the electrical properties and neurochemical status of developing CC connections. The present study investigated the chemical status of rat CC axons during postnatal development by using antibodies to neuropeptide Y (NPY) and to somatostatin (SOM). Both NPY-immunoreactive (ir) and SOM-ir axons were found in the CC of the rat from newborn through adult; however, the number of SOM-ir CC axons is less than that of NPY-ir CC axons at corresponding ages. The density of both NPY-ir and SOM-ir CC axons initially increased, then peaked, and finally decreased to the mature level. In the adult, only a few NPY-ir and SOM-ir CC axons were found in the CC. These results indicate that many NPY-ir and SOM-ir CC axons are transitory during early postnatal development. The results also suggest that the functions of CC connections in adult mammals may be different from that of developing ones. The present results as well as the previous results demonstrate that both developing and mature CC connections are chemically heterogeneous.