Immunocytochemical localization of enkephalin in the neostriatum of rat brain: a light and electron microscopic study.

The peroxidase-antiperoxidase (PAP) immunocytochemical technique was used to determine the light and electron microscopic localization of antisera directed against either methionine [Met5]-or leucine [Leu5]-enkephalin in the neostriatum of brains from untreated rats. By light microscopy, neuronal perikarya and processes showing enkephalin-like immunoreactivity (ELI) were unevenly distributed throughout the neostriatum. The greatest accumulation of neuronal structures showing ELI was in the ventro- and caudo- lateral portions of the nucleus. The labeled perikarya measured 10--15 micrometer in diameter and constituted about 15--20% of the total neurons in the neostriatum. By electron microscopy, examination of three areas from horizontal and coronal sections revealed no regional differences in types of neurons showing ELI or in their synaptic organization. All labeled neurons showed a relatively low intensity reaction product which was diffusely distributed throughout the perikarya and dendrites. The cytoplasm contained relatively few organelles, which included mitochondria, endoplasmic reticulum and numerous "alveolate" vesicles. The dendrites had many spiny processes which formed asymmetric synapses with unlabeled axon terminals containing all small clear vesicles. In contrast to the perikarya and dendrites a dense accumulation of reaction product was present in a few myelinated and numerous unmyelinated axons and axonal varicosities. Approximately 75% of the labeled varicosities did not form a specialized synaptic junction in a single plane of section. The remaining 25% of the labeled terminals formed asymmetric junctions primarily with unlabeled dendrites and rarely with unlabeled perikarya or axons. The morphology and synaptic relations of the neurons showing ELI suggest that they may belong to the general group of medium-sized spiny cells characterized in Golgi studies by Kemp and Powell ('71a). At least some of the peptide-containing neurons may also have a myelinated efferent axons.