Anatomical and physiological evidence for D-1 and D-2 dopamine receptor colocalization in neostriatal neurons

Despite the importance of dopamine signaling, it remains unknown if the two major subclasses of dopamine receptors exist on the same or distinct popul ations of neurons. Here we used confocal microscopy to demonstrate that vir tually all striatal neurons, both in vitro and in vivo, contained dopamine receptors of both classes. We also provide functional evidence for such col ocalization: in essentially all neurons examined, fenoldopam, an agonist of the D-1 subclass of receptors, inhibited both the Na+/K+ pump and tetrodot oxin (TTX)-sensitive sodium channels, and quinpirole, an agonist of the Dr subclass of receptors, activated TTX-sensitive sodium channels. Thus D-1 an d D-2 classes of ligands may functionally interact in virtually all dopamin e-responsive neurons within the basal ganglia.