Dopamine (DA) is a key regulator of action selection and associative learning. The striatum
has long been thought to be a major locus of DA action in this process. Although all striatal
cell types express G protein-coupled receptors for DA, the effects of DA on principal medium
spiny neurons (MSNs) understandably have received the most attention. In the two principal
classes of MSN, DA receptor expression diverges, with striatonigral MSNs robustly
expressing D 1 receptors and striatopallidal MSNs expressing D 2 receptors. In the last …