Previous immunocytochemical studies have localized immunoreactive (i) somatostatin (SS) to cell bodies and fibers in the rat neostriatum. In order to identify the population of neostriatal neurons that contain iSS, we used the immunoperoxidase technique to study the localization of iSS at both the light and electron microscopic levels. Light microscopic results showed that iSS was present within small to medium size cell bodies (9 to 20 pm in size) and their emerging processes which were visible for long distances (up to 120 pm) away from the somata. Dendrites were of different diameters, branched into secondary and tertiary processes, and exhibited smooth or irregular contours. Dendrites contained prominent swellings, particularly at branch points, and gave rise to fine beaded processes. At the ultrastructural level, iSS neurons contained deeply indented nuclei and a relatively rich cytoplasm, including well developed Golgi apparatus and rough endoplasmic reticulum which was organized into small stacks. Proximal and distal dendrites had an irregular or varicose appearance, rarely exhibited spines, and were contacted by at least two types of unlabeled axon terminals. Immunoreactive SS also was present in myelinated axons (0.2 to 0.5 pm) which were observed in bundles of internal capsule fibers and in small diameter unmyelinated axons (0.1 to 0.2 pm). The latter coursed through the caudate neuropil and gave rise to swellings which made synapses en passant.
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DiFiglia et al. (1982) studied this question.