Distribution and Morphological Characteristics of Dopamine‐Immunoreactive Neurons in the Midbrain of the Squirrel Monkey (Saimiri sciureus)

The distribution and morphological characteristics of dopamine (DA) neurons in the midbrain of the squirrel monkey (Saimiri sciureus) were investigated by peroxidase‐antiperoxidase (PAP) immunohistochemistry with a highly specific antiserum raised against DA‐glutaraldehyde‐lysyl‐protein conjugate (donated by M. Geffard). Four contiguous areas contained DA‐immunostained nerve cell bodies: (1) the substantia nigra, pars compacta (SNc), (2) the ventral tegmental area (VTA), (3) the retrorubral area (RRA), and (4) the periaqueductal gray (PAG). The SNc composed the vast majority of DA‐immunostained neurons. Most of these neurons were relatively large (mean diameters: 35 × 15 μm) and varied in shape from fusiform to polygonal, but a few smaller (16 × 10.5 μm) globular cells were dispersed among them. The caudal two‐thirds of the SNc was particularly rich in DA somata. Rostrally, these DA cells formed several distinct columns impinging deeply upon the underlying pars reticulata. Large oval sectors mostly devoid of immunoreactivity were delineated by these trabeculae. The long dendritic processes of DA neurons in the SNc were generally oriented in prominent dorsoventral bundles the ventralmost portion of which arborized diffusely along the dorsal surface of the cerebral peduncle. In the VTA, the DA neurons were regrouped in a triangular zone located dorsal to the interpeduncular nucleus, medial to the substantia nigra and ventral to the oculomotor nucleus. These DA cells were of medium size (19 × 10.5 μm), globular or fusiform, and usually showed one or two thick primary dendrites oriented dorsoventrally. The DA cells in the RRA lay in continuity with the most caudal DA‐containing elements of the substantia nigra but could be distinguished by their smaller size (26 × 12 μm), shorter and more profusely branched dendrites, and darker immunostaining. These DA neurons were characteristically scattered among and medial to the fibers of the medial lemniscus, and a few could be observed as far caudally as the pedunculopontine nucleus. In the PAG, DA‐immunostained neurons were seen in the rostral half of the mesencephalic central gray and predominated in its ventral half. These cells were of medium size (22.5 × 10 μm) and some of them were found in proximity to the ventricular lining. At caudal levels, the DA‐positive cells in the PAG did not intermingle with dorsal raphe neurons. Complementary electron microscopic observations of the medial portion of the SNc revealed that (1) the DA‐immunoreactive somata were endowed with spiny dendrites synaptically contacted by numerous immunonegative axon terminals; (2) immunostained axons in this region were myelinated; and (3) axonal varicosities or terminal boutons did not exhibit detectable DA immunoreactivity in the SNc sector examined. These data provide a basis for future hodological and functional studies of the organization of the DA‐ containing neuronal systems in primates.