Coordinated Expression of Dopamine Receptors in Neostriatal Medium Spiny Neurons

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 D 1 and D 2 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 D 1a receptor. A subset of these cells (∼50%) coexpressed D 3 or D 4 receptor mRNA. Neurons expressing detectable levels of ENK but not SP had abundant mRNA for D 2 receptor isoforms (short and long). A subset (10–25%) of these neurons coexpressed D 1a or D 1b mRNAs. Neurons coexpressing ENK and SP mRNAs consistently coexpressed D 1a and D 2 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 D 1a and D 2 receptors is limited, functional D 1 and D 2 class receptors are colocalized in nearly one-half of all medium spiny projection neurons.