title: [3H]spiroxatrine: a 5-HT1A radioligand with agonist binding properties.
abstract: Spiroxatrine has been reported to be a 5-HT1A serotonin receptor antagonist. Therefore [3H]spiroxatrine was synthesized and its 5-HT1A receptor binding properties in homogenates of rat hippocampal membranes were characterized with the expectation that it would be the first 5-HT1A antagonist radioligand. [3H]8-Hydroxydipropylaminotetralin [( 3H]8-OH-DPAT), a well-characterized 5-HT1A agonist radioligand, was studied in parallel for comparative purposes. Scatchard analyses of saturation studies of [3H]spiroxatrine and [3H]8-OH-DPAT binding produced KD values of 0.9 nM and 1.8 nM, with Bmax values of 424 and 360 fmol/mg protein, respectively. A highly significant correlation (r = 0.98; p less than 0.001) exists between Ki values obtained for a series of drugs in competing for [3H]-spiroxatrine and [3H]8-OH-DPAT binding. Of special interest was the observation that 5-HT1A agonists such as serotonin, 8-OH-DPAT, and ipsapirone competed with equal high affinities for [3H]spiroxatrine or [3H]8-OH-DPAT-labelled 5-HT1A receptors. [3H]Spiroxatrine and [3H]8-OH-DPAT binding to 5-HT1A receptors was inhibited by guanosine 5'-(beta,gamma-imido)triphosphate (a nonhydrolyzable analog of GTP) in a concentration-dependent manner whereas adenosine 5'-(beta,gamma-imido)triphosphate (a nonhydrolyzable analog of ATP) had no effect. The similarities in the 5-HT1A receptor radiolabelling properties of [3H]spiroxatrine and [3H]8-OH-DPAT, i.e., the high affinities of agonists and the guanyl nucleotide sensitivity, indicate that [3H]spiroxatrine has "agonist-like" binding properties in its interaction with the 5-HT1A receptor.
mesh_terms: D017371:8-Hydroxy-2-(di-n-propylamino)tetralin; D000266:Adenylyl Imidodiphosphate / Q000494:pharmacology; D000818:Animals; D001667:Binding, Competitive; D002462:Cell Membrane / Q000378:metabolism; D004146:Dioxanes / Q000378:metabolism*; D004147:Dioxins / Q000378:metabolism*; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D006624:Hippocampus / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D011985:Receptors, Serotonin / Q000187:drug effects / Q000378:metabolism*; D013141:Spiro Compounds / Q000378:metabolism*; D013764:Tetrahydronaphthalenes / Q000378:metabolism; D014316:Tritium
chemical_list: D004146:Dioxanes; D004147:Dioxins; D011985:Receptors, Serotonin; D013141:Spiro Compounds; D013764:Tetrahydronaphthalenes; D014316:Tritium; D000266:Adenylyl Imidodiphosphate; D006165:Guanylyl Imidodiphosphate; D017371:8-Hydroxy-2-(di-n-propylamino)tetralin; C018387:spiroxatrine