Characterization of binding sites for [125I]R(+)trans-7-OH-PIPAT in rat brain

Binding characteristics of a novel radioiodinated ligand, ¹²⁵I]R(+)trans-7-hydroxy-2-(N-n-propylN-3-iodo-2-propenyl) aminotetralin (¹²⁵I]R(+)trans-7-OH-PIPAT), were evaluated using homogenate binding and autoradiographic techniques in rat brain. ¹²⁵I]R(+)trans-7-OH-PIPAT bound to sites (dopamine receptors) in homogenates of rat basal forebrain (including caudate putamen, nucleus accumbens and olfactory tubercle) with a high affinity (K_d = 0.42 nM). A majority (70%) of the sites labeled by ¹²⁵I]R(+)trans-7-OH-PIPAT in basal forebrain were GTP-sensitive. In rat hippocampal homogenates, specific and saturable binding of ¹²⁵I]R(+)trans-7-OH-PIPAT to 5-HT_1A receptors, with a K_d value of 1.4 nM and a B_max value of 210 fmol/mg protein, was observed. Binding of ¹²⁵I]R(+)trans-7-OH-PIPAT to sigma sites was also demonstrated in rat cerebellar homogenates. In the presence of GTP (to inhibit binding to D 2 and 5-HT_1A receptors) and DTG (to inhibit binding to sigma sites), dopamine D3 receptors could be selectively labeled with ¹²⁵I]R(+)trans-7-OH-PIPAT. ¹²⁵I]R(+)trans-7-OH-PIPAT offers several unique advantages, including high specific activity and high affinity binding, which make it an excellent probe for the investigation and characterization of the distribution of dopamine D 3 receptors.Abbreviations 7-OH-DPAT 7-hydroxy-2-N,N-(di-n-propyl)aminotetralin - trans-7-OH-PIPAT traps-7-hydroxy-2-(N-n-propyl-N-3-iodo-2propenyl)aminotetralin - 8-OH-DPAT 8-hydroxy-2-N,N-(di-n-propyl)aminotetralin - 8-OH-PIPAT trans-8-hydroxy-2-(N-n-propyl-N-3-iodo-2-propenyl)aminotetralin - NCQ298 S(–)-3-iodo-N-(1-ethyl-2-pyrrolidinyl)]methyl-2-hydroxy-5,6-dimethoxybenzamide - DTG 1,3-diortho-tolyl-guanidine - Sf9 cells Spodoptera frugiperda insect cells - CHO cells Chinese hamster ovary cells - GTP guanosine 5-triphosphate - WB4101 2-(2,6-dimeethoxyphenoxyethyl)aminomethyl-1,4benzodioxane - R(+)3PPP R(+)-3-(3-hydroxyphenyl)-N-propylpiperidine - (+)MK-801 (+)5-methyl-10,11-dihydro-5H-dibenzoa,d]cyclohepten-5,10-imine