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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. Somatostatin-containing axon terminals were about 0.5 pm 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 microcospic studies of Golgiimpregnated 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. Somatostatin (SS), a tetradecapeptide, has been demonstrated by radioimmunoassay (Brownstein et al., 1975; Kobayashi et al., 1977) and immunohistochemistry (Krisch, 1978; Bennett-Clark et al., 1980; Finley et al., ’ This work was supported in part by National Institutes of Health Grants NS 16367 (M. D.) and AM 01126-01 (N. A.), the Hereditary Disease Foundation (M. D.), and University of Massachusetts Medical Center Institutional Biomedical Research Support Grant 2S07RR05712-10 (N. A.). We wish to thank Ms. Jan Carey and Ms. Kathleen Sullivan for their able technical assistance. We appreciate the generosity of Dr. G. Nilaver, Department of Neurology, Columbia University college of Physicians and Surgeons, New York, NY 10032 (National Institutes of Health Grant HD-13147), and Dr. E. Lichtenstein, Department of Medicine, Yale-New Haven Hospital, New Haven, CT 06510, who supplied the somatostatin antiserum and Dr. S. E. Leeman, Department of Physiology, University of Massachusetts Med1981) to have a wide distribution within the central nervous system, including the basal ganglia. Fibers and cell bodies containing immunoreactive (i) SS have been observed in the rat neostriatum at the light microscopic level (Elde et al., 1978; Krisch and Leonhardt, 1980). To date, there are no reports on the ultrastructure of neostriatal iSS neurons or the possible cell type to which ical Center, Worcester, MA 01605 (National Institutes of Health Grant AM 29876-02), and Dr. D. Landis, Department of Neurology, Massachusetts General Hospital, Boston, MA 02114, who allowed us to use their research facilities. We thank Dr. J. B. Martin for his thoughtful

Ultrastructural features of immunoreactive somatostatin neurons in the rat caudate nucleus