Agonist, antagonist, and inverse agonist properties of antipsychotics at human recombinant 5-HT_1A receptors expressed in HeLa cells

Agonist and antagonist properties of antipsychotics at human (h) recombinant 5-hydroxytryptamine_1A (h5-HT_1A) receptor have been examined previously in transfected Chinese hamster ovary (CHO) cells using 5′-O-(3-[³⁵S]thio)-triphosphate ([³⁵S] GTPγS) binding. Na⁺-dependent [³⁵S] GTPγS binding to membranes from human epithelioid carcinoma (HeLa) cells, expressing 500 fmol/mg protein of h5-HT_1A receptor (HA7 cells), appears suitable to characterize not only agonist and antagonist properties of 5-HT_1A receptor ligands, but also inverse agonist properties. We therefore examined agonist, antagonist, and inverse agonist activity of antipsychotics at h5-HT_1A receptor in HA7 cells. Some antipsychotics had agonist activity and stimulated [³⁵S] GTPγS binding with the following order of efficacy: nemonapride>ziprasidone>clozapine>ocaperidone. Tiospirone and trans-5-chloro-2-methyl-2,3,3a,12b-tetrahydro-1H-dibenz[2,3:6,7,5]-oxepino- [4,5c]pyrrole (ORG 5222), were more potent h5-HT_1A receptor antagonists than raclopride, olanzapine, and risperidone. Haloperidol, chlorpromazine, thioridazine, pimozide, and sertindole showed Na⁺-dependent inverse agonist activity at h5-HT_1A receptor that could be antagonized by (s)-N-tert-butyl-3-(4-(2-methoxyphenyl)piperazine-1-yl)-2-phenylpropanamide [(s)-WAY 100135]. These results are further evidence that interactions with h5-HT_1A receptors could play a role in the pharmacological profile of certain antipsychotics, and that Na⁺ affects the ability to detect inverse agonist activity at h5-HT_1A receptors, likely by influencing receptor precoupling. Also, the manner in which compounds interact with 5-HT_1A receptors appears to be related to their K_b/K_i ratio.