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 fluorescen tly coupled, high-affinity ligand for the D-1 receptor subtype was loc alized to nonpyramidal neurons, while a ligand selective for the D-2 s ubtype 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 a n intermediate size range may coexpress both the D-1 and D-2 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, resp ectively, in the presence of 100 nM mianserin (5-HT2 receptor antagoni st). The localization of receptor binding fluorescence was assessed in paired images using fluoroscein isothiocyanate (FITC) and rhodamine d ichroic filters for the D-1 and D-2 subtypes, respectively. Under the conditions employed here, most cell bodies showed either D-1-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 s ize of each single-labeled cell body was measured using the respective FITC (D-1-probe) and rhodamine (D-2-probe) epifluorescence filters, t he distribution of cells showing only D-1-like receptor binding fluore scence was similar to nonpyramidal neurons (68.6 +/- 1.8 mu m(2)), whi le that for cells showing only D-2-like receptor binding fluorescence was similar to that of both large interneurons and small pyramidal cel ls (106.9 +/- 2.4 mu m(2)). Cells showing both D-1- and D-2-like recep tor binding fluorescence were found to overlap in size only with nonpy ramidal cells when either fluorescent filter was used. These findings are consistent with the hypothesis that the D-1 and D-2 receptor subty pes 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 find ings suggest that the dopamine projections to rat cortex probably enga ge in a complex interplay with intrinsic cortical neurons and their re spective neurotransmitter systems. (C) 1995 Wiley-Liss, Inc.