Striosome-based map of the mouse striatum that is conformable to both cortical afferent topography and uneven distributions of dopamine D1 and D2 receptor-expressing cells

The striatum is critically involved in execution of appropriate behaviors, but its internal structures remain unmapped due to its unique structural organization, leading to ambiguity when interpreting heterogeneous properties of striatal neurons that differ by location. We focused on site-specific diversity of striosomes/matrix compartmentalization to draw the striatum map. Five types of striosomes were discriminated according to diverse immunoreactivities for the µ-opioid receptor, substance P (SP) and enkephalin, and each type occupied a particular domain inside the striatum. Furthermore, there was an additional domain lacking striosomes. This striosome-free space was located at the dorsolateral region and received afferents preferentially from the primary motor and sensory cortices, whereas the striosome-rich part received afferents from associational/limbic cortices, with topography inside both innervations. The proportion of dopamine D1 receptor-expressing, presumptive striatonigral neurons was approximately 70% in SP-positive striosomes, 40% in SP-deficient striosomes, 30% in the striosome-free space, and 50% in the matrix. In contrast, the proportion of D2 receptor-expressing, presumptive striatopallidal neurons was complementary to that of D1 receptor-expressing cells, indicating a close relationship between the map and the direct and indirect parallel circuitry. Finally, the most caudal part of the striatum lacked compartmentalization and consisted of three lamina characterized by intense and mutually exclusive immunoreactivities for SP and enkephalin. This tri-laminar part also received specific afferents from the cortex. The newly obtained map will facilitate broad fields of research in the basal ganglia with higher resolution of the three-dimensional anatomy of the striatum.