Characterization of [¹²⁵I](R)-trans-7-hydroxy-2-[N-propyl-N-(3'-iodo- 2'-propenyl)amino]tetralin binding to dopamine D3 receptors in rat olfactory tubercle

[¹²⁵I](R,S)-trans-7-Hydroxy-2-[N-propyl-N-(3'-iodo-2'-propenyl)- amino]tetralin {[¹²⁵I](R,S)-trans-7-OH-PIPAT} has been shown to bind with high affinity to dopamine D3 receptors expressed in Spodoptera frugiperda cells. No specific binding was seen in Spodoptera frugiperda cells expressing a high density of D2 receptors. It was therefore, suggested that [¹²⁵I](R,S)-trans-7-OH-PIPAT selectively labels D3 receptors. In the present study, saturation binding of [¹²⁵I](R)-trans-7-OH-PIPAT to membranes from rat olfactory tubercle resulted in markedly curvilinear Scatchard plots, suggesting that the radioligand was binding to more than one receptor class or affinity state. [¹²⁵I](R)-trans-7-OH-PIPAT bound with high affinity to membranes from human embryonic kidney-293 cells expressing transfected D2 or D3 receptors and to membranes from Chinese hamster ovary cells expressing serotonin(1A) receptors. Binding of [¹²⁵I](R)-trans-7- OH-PIPAT to serotonin(1A) and D2 receptors was decreased or eliminated in the presence of NaCl and/or guanylyl-imidodiphosphate [Gpp(NH)p]. In the presence of Gpp(NH)p and NaCl, a linear Scatchard plot with a K(d) value of 0.4 nM and a density of 100 fmol/mg of protein was obtained in membranes from rat olfactory tubercle. Agonists and antagonists inhibited binding of [¹²⁵I](R)-trans-7-OH-PIPAT with a rank order of potency consistent with an interaction at D3 receptors. These results suggest that, in the presence of Gpp(NH)p and NaCl, [¹²⁵I](R)-trans-7-OH-PIPAT specifically labels D3 receptors.