[ 3 H]4‐(dimethylamino)‐ N ‐(4‐(4‐(2‐methoxyphenyl)piperazin‐1‐yl) butyl)benzamide: A selective radioligand for dopamine D 3 receptors. II. Quantitative analysis of dopamine D 3 and D 2 receptor density ratio in the caudate‐putamen

4‐(Dimethylamino)‐ N ‐(4‐(4‐(2‐methoxyphenyl)piperazin‐1‐yl)butyl)benzamide (WC‐10), a N ‐phenyl piperazine analog, displays high affinity and moderate selectivity for dopamine D 3 receptors versus dopamine D 2 receptors (Chu et al. [2005] Bioorg Med Chem 13:77–87). In this study, WC‐10 was radiolabeled with tritium (specific activity = 80 Ci/mmol), and quantitative autoradiography studies were conducted using rhesus monkey and Sprague‐Dawley rat brain sections. K d values for the binding of [ 3 H]WC‐10 to D 3 receptors obtained from quantitative autoradiography with rhesus monkey and rat brain sections are in agreement with K d values obtained from cloned human and rat receptors (Xu et al. [2009] Synapse 63:717‐728). The D 2 selective antagonist [ 3 H]raclopride binds with 11‐fold higher affinity to human HEK D 2L ( K d = 1.6 nM) than HEK D 3 (K d = 18 nM) receptors; [ 3 H]raclopride binds to rat Sf9 rD 2L receptors with a K d of 6.79 nM, a value that is 4‐fold lower than binding to human HEK D 2L receptors and 2.5‐fold higher than binding to rat Sf9 rD 3 receptors. In vitro quantitative autoradiography studies with [ 3 H]WC‐10 and [ 3 H]raclopride were conducted on adult rat and rhesus monkey brain sections. A mathematical model for calculating the absolute densities of dopamine D 2 and D 3 receptors based on the in vitro receptor binding data of [ 3 H]WC‐10 and [ 3 H]raclopride was developed. Synapse 64:449–459, 2010. © 2010 Wiley‐Liss, Inc.