Human 5-HT₇ Receptor-Induced Inactivation of Forskolin-Stimulated Adenylate Cyclase by Risperidone, 9-OH-Risperidone and Other “Inactivating Antagonists”

Abstract

We have previously reported on the unusual human 5-hydroxytryptamine₇ (h5-HT₇) receptor-inactivating properties of risperidone, 9-OH-risperidone, bromocriptine, methiothepin, metergoline, and lisuride. Inactivation was defined as the inability of 10 μM 5-HT to stimulate cAMP accumulation after brief exposure and thorough removal of the drugs from HEK293 cells expressing h5-HT₇ receptors. Herein we report that brief exposure of the h5-HT₇ receptor-expressing cells to inactivating drugs, followed by removal of the drugs, results in potent and efficacious irreversible inhibition of forskolin-stimulated adenylate cyclase activity. Pretreatment, followed by removal of the inactivating drugs inhibited 10 μM forskolin-stimulated adenylate cyclase activity with potencies similar to the drugs' affinities for the h5-HT₇ receptor. The actions of the inactivating drugs were pertussis toxin-insensitive, indicating the lack of G_i in their mechanism(s) of action. Methiothepin and bromocriptine maximally inhibited 10 μM forskolin-stimulated adenylate cyclase, whereas the other drugs produced partial inhibition, indicating the drugs are inducing slightly different inactive conformations of the h5-HT₇ receptor. Maximal effects of these inactivating drugs occurred within 15 to 30 min of exposure of the cells to the drugs. A G_s-mediated inhibition of forskolin-stimulated activity has never been reported. The inactivating antagonists seem to induce a stable conformation of the h5-HT₇ receptor, which induces an altered state of G_s, which, in turn, inhibits forskolin-mediated stimulation of adenylate cyclase. These and previous observations indicate that the inactivating antagonists represent a unique class of drugs and may reveal GPCR regulatory mechanisms previously unknown. These drugs may produce innovative approaches to the development of therapeutic drugs.