Within the dorsal striatum, subcompartments known as the striosome (or patch) and matrix participate in somewhat segregated C-BG-C circuits. These striatal subregions are characterized by different levels of a variety of neuronal proteins (e.g. higher expression of mu opiate receptors in striosome relative to matrix). Projections from cortex and other brain regions also selectively innervate striosomes vs. matrix (Kincaid and Wilson 1996, Crittenden and Graybiel 2011, Smith, Klug et al. 2016). This sub-compartmentalization runs parallel to the circuitry already discussed, at least at the corticostriatal level. However, the functions of patch and matrix subcompartments are still being sorted out, especially in relation to actions of drugs of abuse, and thus we will not have too much to say in this review about differential roles of these regions.
