In order to delineate the striosomes and matrix striatal compartments, 2 transverse sections were taken at the pre-commissural and post-commissural striatal levels and immunostained for μ-opioid receptor (MOR). These sections were used to assess differences in the density of D1/D2 MSNs between the 2 striatal compartments. Briefly, sections were incubated with a primary antibody against MOR (1/200, overnight at 4 oC, catalog no. AB5509; Chemicon, Darmstadt, Germany) followed by incubation into a donkey anti-guinea pig secondary antibody coupled to Cy5 (1/200, 2 h at RT, catalog no. 706-175-148; Jackson ImmunoResearch, West Grove, PA, USA).
