Development of the ultrastructural features of neuropeptide Y-immunoreactive neurons in the rat visual cortex

Immunohistochemical studies have localized neuropeptide Y into a small population of non-pyramidal neurons in the mammalian cerebral cortex. In the rat, these cells are distributed in layers II-VI and are characterized at the ultrastructural level by an abundance of cytoplasm containing a plethora or organelles, most conspicuous of which are cisternae of granular endoplasmic reticulum stacked in parallel arrays. In the present study, we used electron microscopic immunocytochemistry to examine the ultrastructural development of neuropeptide Y-labelled neurons in the rat visual cortex from birth, when they first appear in this cortical area, until postnatal day 32. At birth and in the subsequent few days, neuropeptide Y neurons, found exclusively in layers V and VI, often show a deeply infolded nucleus and little cytoplasm containing few organelles. At the end of the first postnatal week, labelled cells are still restricted to layers V and VI and display immature features. However, at this stage, cells often show irregularly enlarged proximal dendrites filled with organelles. During the second postnatal week, neuropeptide Y-immunoreactive cell bodies appear for the first time in layers II and III, and at the end of this week they have a distribution similar to that observed in the adult. Labelled cells are overall more differentiated than at earlier ages showing some of the ultrastructural features which distinguish them in the adult. No differences in maturation are evident between immunoreactive neurons located in the superficial layers and those in the deep layers, suggesting that the neuropeptide Y neurons in the more superficial layers express the peptide after having completed their migration and have acquired their characteristic ultrastructural features. Maturation proceeds during the third postnatal week. At the end of this stage, neuropeptide Y-containing cells acquired their mature nuclear and cytoplasmic features and an adult complement of synapses.