GPR155 is an integral membrane protein containing DEP domain, which is highly expressed in the mouse striatum. Expression of GPR155 is confined to the medium spiny neurons of the lateral striatum, as demonstrated by immunohistochemistry and in situ hybridization in the mouse. Changes in the expression levels of GPR155 have been noted in pathologic conditions like autism-spectrum disorders and Huntington's disease. Judging from the above data it might be concluded that GPR155 is contributing to striatal machinery for motor control. To further characterize GPR155 immunopositive neurons in the mouse striatum, we studied its co-expression with D1 and D2 dopamine receptors, DARPP-32, GAD1, MAP2, drebrin and calbindin D28K by means of double immunohistochemistry. Neurons stained for GPR155 were also immunopositive for GAD1. Many neurons double stained for GPR155 and calbindin D28K were scattered in the central and medial regions of the striatum. In the lateral striatum, neurons double stained for GPR155 and D1 receptors were frequently encountered. D1 receptor expressing neurons were also immunopositive for DARPP-32. Cells double immunopositive for D2 receptor and GPR155 were very sparse. Double labeling for GPR155 and MAP2 or drebrin was frequently detected, which suggested that GPR155 exists not only in the membrane of the cell body but also in neuronal process. We have also demonstrated that the mouse substantia nigra, pars reticulata and internal segment of the globus pallidus contained fibers heavily labeled for GPR155. These results suggest that GPR155 is closely associated with the medium-spiny projection neurons of the lateral striatum co-expressing D1 receptors and contributes to the basal ganglia motor circuits via the D1 receptor striatal neuronal outflow system.