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.