In recent years, the distribution of dopamine receptor subtypes among the principal neurons of the neostriatum has been the subject of debate. Conventional anatomical and physiological approaches have yielded starkly different estimates of the extent to which D1 and D2 class dopamine receptors are colocalized. One plausible explanation for the discrepancy is that some dopamine receptors are present in physiologically significant numbers, but the mRNA for these receptors is not detectable with conventional techniques. To test this hypothesis, we examined the expression of DA receptors in individual neostriatal neurons by patch-clamp and RT-PCR techniques. Because of the strong correlation between peptide expression and projection site, medium spiny neurons were divided into three groups on the basis of expression of mRNA for enkephalin (ENK) and substance P (SP). Neurons expressing detectable levels of SP but not ENK had abundant mRNA for the D1a receptor. A subset of these cells (approximately 50%) coexpressed D3 or D4 receptor mRNA. Neurons expressing detectable levels of ENK but not SP had abundant mRNA for D2 receptor isoforms (short and long). A subset (10-25%) of these neurons coexpressed D1a or D1b mRNAs. Neurons coexpressing ENK and SP mRNAs consistently coexpressed D1a and D2 mRNAs in relatively high abundance. Functional analysis of neurons expressing lower abundance mRNAs revealed clear physiological consequences that could be attributed to these receptors. These results suggest that, although colocalization of D1a and D2 receptors is limited, functional D1 and D2 class receptors are colocalized in nearly one-half of all medium spiny projection neurons.