Ultrastructural features of immunoreactive somatostatin neurons in the rat caudate nucleus

Abstract

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 μm in size) and their emerging processes which were visible for long distances (up to 120 μm) 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 headed processes. At the ultrastructural level, iSS neurons contained deeply intended 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 μm) which were observed in bundles of internal capsule fibers and in small diameter unmyelinated axons (0.1 to 0.2 μm). The latter coursed through the caudate neuropil and gave rise to swellings which made synapses en passant. Somatostatin-containing axon terminals were about 0.5 μm in size, contained numerous clear pleomorphic and few large granular vesicles, and made synaptic contact with dendrites and dendritic spines. The morphologic features of iSS neurons closely correspond to the category of medium size aspiny neurons which have been recognized in Golgi studies and in electron microscopic studies of Golgi-impregnated and intracellular horseradish peroxidase-labeled neurons, and they differ from those of medium size spiny neurons identified by similar Golgi-electron microscopic methods. It is likely that many of the iSS fibers and boutons observed in the caudate originate from intrinsic cells. However, extrinsic sources of somatostatin also may exist.