differences (Fig. 3), supporting the homogeneous regional distribution of the D1, D2 and D1/D2 MSNs throughout the dorsal striatum. Occasionally, some D1/D2 MSNs can be seen to form small clusters of 2–3 cells at different striatal levels. Assessment of the distribution of D1/D2 MSNs in striosomes and matrix striatal compartments, as delineated on immunostained sections for μ-opioid receptor, reveals no significant difference, neither at the pre-commissural level (P = 0.4857), nor at the post-commissural level (P = 0.8286, Fig. S1). Our quantitative analyses reveal that the D1/D2 MSNs are more heterogeneously distributed in the Acb than in the dorsal striatum. As shown in Fig. 4d, a dense region located in the medial part of the shell of the Acb was almost entirely composed of the D1/D2 type of MSNs. A closer examination of the lateral striatal stripe indicates no significant difference regarding the density of the D1/D2 MSNs compared to other regions of the lateral area of the shell of the Acb. The density of the D1/D2 MSNs is overall significantly higher in the Acb compared to the striatum. This difference becomes statistically significant in the shell compartment of the Acb with 45.0 ± 10.9 × 103 cells/mm3 compared to
