Identification of D₃ and σ Receptors in the Rat Striatum and Nucleus Accumbens Using (±)‐7‐Hydroxy‐N,N‐Di‐n‐[³H]Propyl‐2‐Aminotetralin and Carbetapentane

Abstract: Cross‐reactions between dopamine D₃ and σ receptor ligands were investigated using (±)‐7‐hydroxy‐N,N‐di‐n‐[³H]propyl‐2‐aminotetralin [(±)‐7‐OH‐[³H]DPAT], a putative D₃‐selective radioligand, in conjunction with the unlabeled σ ligands 1,3‐di(2‐tolyl)guanidine (DTG), carbetapentane, and R(−)‐N‐(3‐phenyl‐1‐propyl)‐1‐phenyl‐2‐aminopropane [R(−)‐PPAP]. In transfected CCL1.3 mouse fibroblasts expressing the human D₃ receptor, neither DTG nor carbetapentane (0.1 µM) displaced (±)‐7‐OH‐[³H]DPAT binding. R(−)‐PPAP (0.1 µM) displaced 39.6 ± 1.0% of total (±)‐7‐OH‐[³H]DPAT binding. In striatal and nucleus accumbens homogenates, (±)‐7‐OH‐[³H]DPAT labeled a single site (15–20 fmol/mg of protein) with high (1 nM) affinity. Competition analysis with carbetapentane defined both high‐ and low‐affinity sites in striatal (35 and 65%, respectively) and nucleus accumbens (59 and 41%, respectively) tissue, yet R(−)‐PPAP identified two sites in equal proportion. Carbetapentane and R(−)‐PPAP (0.1 µM) displaced ∼20–50% of total (±)‐7‐OH‐[³H]DPAT binding in striatum, nucleus accumbens, and olfactory tubercle in autoradiographic studies, with the nucleus accumbens shell subregion exhibiting the greatest displacement. To determine directly (+)‐7‐OH‐[³H]DPAT binding to σ receptors, saturation analysis was performed in the cerebellum while masking D₃ receptors with 1 µM dopamine. Under these conditions (+)‐7‐OH‐[³H]DPAT labeled σ receptors with an affinity of 24 nM. These results suggest that (a) (±)‐7‐OH‐[³H]DPAT binds D₃ receptors with high affinity in rat brain and (b) a significant proportion of (±)‐7‐OH‐[³H]DPAT binding consists of σ₁ sites and the percentages of these sites differ among the subregions of the striatum and nucleus accumbens.