Immunohistochemical localization of DARPP-32 in striatal projection neurons and striatal interneurons: implications for the localization of D1-like dopamine receptors on different types of striatal neurons.

Immunohistochemical double-label techniques were used to study the localization of DARPP-32, a phosphoprotein that is enriched in neurons possessing members of the D1 subfamily of dopamine receptors, in several different types of striatal neurons in the rat basal ganglia. The vast majority (94.1%) of striatonigral projection neurons (the vast majority of which contain substance P), identified by retrograde labeling with fluorogold, were observed to contain DARPP-32. Similarly, the vast majority of striatopallidal projection neurons (87.7%), identified by immunofluorescence labeling for enkephalin (ENK), were found to label for DARPP-32. In contrast, cholinergic and neuropeptide Y-containing striatal interneurons were never observed to contain DARPP-32. These results suggest that essentially all major types of striatal medium spiny projection neurons may possess members of the D1 subfamily of dopamine receptors, but that striatal local circuit neurons do not possess members of the D1 subfamily of receptors.