Postnatal development of somatostatin‐ andneuropeptide y‐immunoreactive neurons in rat cerebralcortex : 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 theircoexistence in cortical neurons. Using double immunohistochemical staining for SOM and NPYwith diaminobenzidine and benzidine dihydrochloride as chromogens, we subdividedimmunoreactive 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 toNPY only‐ir cells which first appeared in most cortices from week two. The morphologicalfeatures of double‐labeled SOM/NPY neurons differed with those of SOM only‐ and NPYonly‐ir neurons. No apparent changes in the shape and size of single‐labeled neurons occurredwith age ; throughout their postnatal life they were round and ovoid, had a thin rim ofperinuclear cytoplasm, and short processes. However, the features of SOM/NPY‐ir neurons werenot consistent according to postnatal age ; by day P7, these neurons showed immature featuresand they began to show more advanced neuronal characteristics by week P2, when they had alarger and more intensely‐stained cytoplasm. In addition, their processes were longer, thicker andmore complex than at earlier ages. At this age, SOM/NPY‐ir somata were close to their nearmaximum size. From week P4, they became smaller and were lightly labeled. SOM/NPY‐irsomata were larger than SOM only‐ and NPY only‐ir somata at and after two weeks of age. Thepresent results, showing different postnatal maturation patterns such as time of appearance andmorphological features, raise the possibilities that double‐labeled SOM/NPY and single‐labeledimmunoreactive neurons may be different populations regulated by different mechanisms in theirdevelopment, and with different functional properties during development.