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The medial subdivision of the monkey nucleus accumbens (NAC) is rich in dopamine (DA) and peptides. In the present investigation the mode of DA transmission in the medial subdivision was studied morphologically by light- and electron-microscopic immunocytochemistry using a monoclonal antibody raised against dopamine. The medial subdivision showed extremely dense accumulation of thick DA-immunoreactive varicose fibers. Electron-microscopic observation of single sections revealed that DA afferents had a relatively high incidence (33.2%) of asymmetric junctions in this area. Approximately 50% of the targets were dendritic shafts, 44.2% dendritic spines, and 5.1% somata. Some DA axons showed terminal profiles en passant within the synaptic complex, some of which showed synaptic triads. The unique ultrastructural features of DA terminals in the medial NAC indicate the existence of specific styles of DA transmission in the limbic structure.

In order to further investigate the neurochemical anatomy of the primate nucleus accumbens (NAC), the distributions of the neuropeptides leucine-enkephalin (Leu-ENK), neurotensin (NT), and substance P (SP) and of haloperidol-induced c-fos expression were investigated in the macaque monkey using immunohistochemical methods. To define the boundaries of the NAC, dopamine (DA) and tyrosine hydroxylase (TH) immunohistochemistry was performed. In addition, to formulate the distinction between subdivisions of the nucleus accumbens, immunohistochemistry for calbindin-D28 (CBD) and SP was employed. In general, the medial part of NAC, which consisted of small to medium-sized cells, was low for CBD immunoreactivity and moderate to high for SP immunoreactivities, while the dorsolateral part, which was composed of small cells, showed the opposite pattern of immunostaining for CBD and SP. Many Leu-ENK-immunoreactive perikarya were observed in the dorsal NAC at its middle and caudal levels. There were moderate densities of Leu-ENK-positive fibers throughout the medial part of the NAC. At the dorsolateral margin of the NAC, Leu-ENK-positive fibers formed patches. Most NT-positive perikarya were found in the dorsolateral subdivision. SP-positive perikarya were scarce in the NAC. Dense distribution of NT- and SP-containing fibers or puncta were observed in the mediodorsal part (medial subdivision), where a dense field of DA-immunoreactive fibers was observed. The ventral part (ventral subdivision) contained moderate numbers of NT- and SP-immunoreactive fibers. Haloperidol-induced c-fos expression was very extensive in the medial half of NAC, particularly in the mediodorsal region, which overlapped with the DA- and peptide-rich region. The present study indicates that the NAC of the primate can be subdivided into at least three subterritories, the dorsolateral, medial and ventral subdivision, by neuropeptide histochemistry as well as by the response of its constituent neurons to haloperidol.