Postnatal development of somatostatin- and neuropeptide Y-immunoreactive neurons in rat cerebral cortex: a double-labeling immunohistochemical study.

The postnatal development of somatostatin (SOM)- and neuropeptide Y (NPY)-immunoreactive (ir) neurons was examined in rat cerebral cortex, while considering their coexistence in cortical neurons. Using double immunohistochemical staining for SOM and NPY with diaminobenzidine and benzidine dihydrochloride as chromogens, we subdivided immunoreactive cells into double-labeled SOM/NPY-, SOM only-, and NPY only-ir neurons. SOM/NPY- and SOM only-ir neurons were detectable even at the day of birth, in contrast on NPY only-ir cells which first appeared in most cortices from week two. The morphological features of double-labeled SOM/NPY neurons differed with those of SOM only- and NPY only-ir neurons. No apparent changes in the shape and size of single-labeled neurons occurred with age; throughout their postnatal life they were round and ovoid, had a thin rim of perinuclear cytoplasm, and short processes. However, the features of SOM/NPY-ir neurons were not consistent according to postnatal age; by day P7, these neurons showed immature features and they began to show more advanced neuronal characteristics by week P2, when they had a larger and more intensely-stain cytoplasm. In addition, their processes were longer, thicker and more complex than at earlier ages. At this age, SOM/NPY-ir somata were close to their near maximum size. From week P4, they became smaller and were lightly labeled. SOM/NPY-ir somata were larger than SOM only- and NYP only-ir somata at and after two weeks of age. The present results, showing different postnatal maturation patterns such as time of appearance and morphological features, raise the possibilities that double-labeled SOM/NPY and single-labeled immunoreactive neurons may be different populations regulated by different mechanisms in their development, and with different functional properties during development.