Cellular colocalization of dopamine D 1 and D 2 receptors in rat medial prefrontal cortex

In a recent study in rat medial prefrontal cortex (mPFC), a fluorescently coupled, high‐affinity ligand for the D 1 receptor subtype was localized to nonpyramidal neurons, while a ligand selective for the D 2 subtype was found on neurons with a size distribution overlapping with both small pyramidal and large nonpyramidal cells. These observations raised the possibility that a subpopulation of cortical neurons with an intermediate size range may coexpress both the D1 and D2 receptor subtypes. In the present study, the D 1 and D 2 receptor subtypes have been simultaneously localized in layer VI of rat mPFC using 20 nM SCH 23390‐Bodipy and 20 nM N‐( p ‐aminophenethyl) spiperone‐Texas red, respectively, in the presence of 100 nM mianserin (5‐HT 2 receptor antagonist). The localization of receptor binding fluorescence was assessed in paired images using fluoresceiN isothiocyanate (FITC) and rhodamine dichroic filters for the D 1 and D 2 subtypes, respectively. Under the conditions employed here, most cell bodies showed either D l ‐like or D 2 ‐like receptor binding fluorescence, while a colocalization of both fluoroprobes was observed on only 25% of the labeled cells. When the size of each single‐labeled cell body was measured using the respective FITC (D 1 ‐probe) and rhodamine (D 2 ‐probe) epifluorescence filters, the distribution of cells showing only D 1 ‐like receptor binding fluorescence was similar to nonpyramidal neurons (68.6 ± 1.8 μm 2 ), while that for cells showing only D 2 ‐like receptor binding fluorescence was similar to that of both large interneurons and small pyramidal cells (106.9 ± 2.4 μm 2 ). Cells showing both D 1 ‐ and D 2 ‐like receptor binding fluorescence were found to overlap in size only with nonpyramidal cells when either fluorescent filter was used. These findings are consistent with the hypothesis that the D 1 and D 2 receptor subtypes are most often found on different populations of neurons in mPFC, although approximately 25% of all such cells appear to be nonpyramidal neurons having both D 1 and D 2 receptor binding activity. These findings suggest that the dopamine projections to rat cortex probably engage in a complex interplay with intrinsic cortical neurons and their respective neurotransmitter systems. © 1995 Wiley‐Liss, Inc.