[³H]4-(dimethylamino)-N-(4-(4-(2-methoxyphenyl)piperazin-1-yl)- butyl)benzamide: A selective radioligand for dopamine D₃ receptors. II. Quantitative analysis of dopamine D₃ and D₂ 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 D₃ receptors versus dopamine D₂ 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 [³H]WC-10 to D₃ 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 D₂ selective antagonist [³H]raclopride binds with 11-fold higher affinity to human HEK D_2L (K_d 5 1.6 nM) than HEK D₃ (K_d = 18 nM) receptors; [ ³H]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 [³H]WC-10 and [³H]raclopride were conducted on adult rat and rhesus monkey brain sections. A mathematical model for calculating the absolute densities of dopamine D₂ and D₃ receptors based on the in vitro receptor binding data of [³H]WC-10 and [ ³H]raclopride was developed.