The present study provides the first detailed description of the morphological characteristics, density and regional distribution of D1/D2 MSNs of the dorsal striatum and Acb in normal mice, as well as the first characterization of changes induced in this striatal subpopulation by striatal DA denervation. Our data gathered in normal animals reveal that the D1/D2 MSNs are morphologically distinct from the D1 and D2 MSNs: they have a smaller cell body, a less profusely arborized dendritic tree with branches that bear fewer spines than those of the D1 and D2 MSNs. They are uniformly scattered throughout the striatum, where they represent approximately 2% of the total number of MSNs, but heterogeneously distributed and more abundant in the Acb, where their proportion ranged from 7 to 15% of all MSNs. In 6-OHDA-lesioned mice, the density and regional distribution of all 3 types of MSNs is essentially unaltered. In contrast to the D1 and the D2 MSNs, the D1/D2 neurons do not show any significant reduction in the length of their dendritic arborization after intoxication with 6-OHDA. However, a reduction in dendritic
