Dopamine receptor circuitry of basal ganglia. Two members of the D₂ receptor family, the D₂ and D₃, are segregated to different circuits within the basal ganglia. D₂ receptors are the primary target of the nigrostriatal DA system originating in the ventral tier of the SNpc and are expressed in higher concentrations at several stations in the motor loop. This includes the sources of afferents (e.g., motor cortex) and the pallidal and thalamic targets of the motor loop (e.g., GPe, VP nucleus of the thalamus). In contrast, D₃ receptors are in higher concentrations within the targets of the mesolimbic DA system originating in the medial and dorsal components of the SN and are also highly concentrated at relay stations in the limbic loop. This includes the sources of afferents (e.g., frontal cortex, nucleus basalis) and pallidal and thalamic targets of the limbic loop (e.g., VP, GPi, MD/AV nuclei of the thalamus). In addition, neurons of the GPe, GPi, RT, and interneurons of the cortex that express the D₄ receptor protein are indicated. Abbreviations: caudate nucleus, CN; globus pallidus external, GPe; globus pallidus internal, GPi; medial dorsal and anteroventral nuclei of the amygdala, MD/AV; nucleus basalis of Meynert, NBM; internal capsule, ic; red nucleus, RN; reticular nucleus of the thalamus, RT; substantia nigra pars compacta, SNpc; ventroposterior nucleus of the amygdala, VP (near thalamus in figure); ventral pallidum, VP.