ABSTRACT
<p><b>OBJECTIVE</b>To observe the distribution of neuronal nitric oxide synthase (nNOS)-immunopositive neurons in rat corpus striatum and their ultrastructural features.</p><p><b>METHODS</b>Brain tissue specimens were obtained from normal SD rats, in which nNOS-immunopositive neurons were visualized by ABC immunocytochemistry and observed under immunoelectron microscope with pre-embedding staining.</p><p><b>RESULTS</b>Under light microscope, nNOS-immunopositive neurons appeared brown with distinct profiles of the cell body and processes. These neurons, mostly medium-sized and small cells, were located mainly in the lateral region of the corpus striatum. Only a few immunopositive neurons were detected in the medial region of the corpus striatum. Immunohistochemistry and transmission electron microscopy identified the nNOS-immunopositive neurons as interneurons possessing large nuclei with small amount of cytoplasma. The immunopositive granules were visualized as black plaques, and the larger ones distributed mainly in the cell bodies, some with monolayer membrane encapsulation. The small granules did not have the encapsulation, scattering in perinuclear regions and under the cell membrane, but not in the cell body. The immunopositive granules were also found in the axons and dendrites, but not in the vesicles of the synapses. In addition, many immunopositive terminals were found close to the blood vessels.</p><p><b>CONCLUSIONS</b>nNOS-immunopositive neurons in rat corpus striatum are mainly medium-sized and small cells as is typical of the interneurons. The immunopositive granules locate in the cytoplasma, axons and dendrites, and larger granules have membrane coating while small ones do not, possibly in relation to their functions.</p>