The nucleus accumbens is the prominent component of the ventral striatum (the others being the olfactory tubercle and ventral parts of the caudate and putamen) (see STRIATUM). It is now accepted that the nucleus accumbens is a heterogeneous structure containing at least three distinct sub-territories. These are: (1) a central ‘core’ which envelops the ANTERIOR COMMISSURE and has a CYTOARCHITECTURE identical with that of the overlying dorsal striatum with which it merges imperceptibly. (2) A medial ‘shell’ which surrounds the core on its medial, ventral and lateral sides. The core and shell are found predominantly in the caudal three-quarters of the nucleus accumbens in the rat brain. (3) A ‘rostral pole’ which is characterized by a blend of histochemical features, afferent and efferent connections that suggest it is comprised of both core and shell components. Within the core, populations of GABAERGICMEDIUM SPINY NEURONS having the phenotype of similar neurons in the caudate and putamen are found, namely those expressing SUBSTANCE P and the D1 dopamine receptors (see D1–D5 DOPAMINE RECEPTORs)and others expressing ENKEPHALIN and the D2 receptor. There is some evidence of a patch and matrix organization within the nucleus accumbens, but this is not as readily visible as it is in the dorsal striatum.
Cortical afferents to the nucleus accumbens arise in the allocortical (see ALLOCORTEX) or ‘limbic cortical’ sites, particularly the basolateral AMYGDALA, ventral and dorsal subiculum components of the HIPPOCAMPAL FORMATION and specific parts of the anterior cortex, such as the PRELIMBIC CORTEX, and also the anterior CINGULATE CORTEX.
These afferents terminate in distinct subterritories of the nucleus accumbens. For example, the basolateral amygdala projects mainly, but not exclusively, to the core region of the nucleus accumbens; the ventral subiculum projects especially to the dorsal parts of the medial shell region. The nucleus accumbens receives a dense dopaminergic innervation arising in the dopamine neurons of the VENTRAL TEGMENTAL AREA (via the MESOLIMBIC DOPAMINE SYSTEM) and also affcrents from the intralaminar and midline nuclei of the THALAMUS. There is a significant serotoninergic input to the nucleus accumbens arising from the midbrain RAPHE NUCLEI and also a noradrenergic innervation arising in the LOCUS COERULEUS. The core and shell compartments of the nucleus accumbens have distinctive efferent connections. Fibres from the core project to the dorsolateral ventral pallidum, the ENTOPEDUNCULAR NUCLEUS (in the rat and which is homologous to the internal pallidal segment in the primate) and SUBSTANTIA NIGRA pars compacta. The shell of the nucleus accumbens, and also the rostral pole, project to ventromedial parts of the ventral pallidum, entopeduncular nucleus, ventral tegmental area and substantia nigra pars compacta. The shell also gives rise to ‘atypical’ (so far as the striatum is concerned) projections that reach the EXTENDED AMYGDALA and lateral PREOPTIC AREA—lateral hypothalamic continuum. The rostral pole and shell also project to the MESOPONTINE portions of the RETICULAR FORMATION and midbrain PERIAQUEDUCTAL GREY.
BARRY J.EVERITT
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