Abstract
In a previous publication, using human 5-hydroxytryptamine7 (h5-HT7) receptor-expressing human embryonic kidney (HEK) 293 cells, we reported the rapid, potent inactivation of the h5-HT7 receptor stimulation of cAMP production by three antagonists: risperidone, 9-OH-risperidone, and methiothepin (Smith et al., 2006). To better understand the drug-receptor interaction producing the inactivation, we 1) expanded the list of inactivating drugs, 2) determined the inactivating potencies and efficacies by performing concentration-response experiments, and 3) determined the potencies and efficacies of the inactivators as irreversible binding site inhibitors. Three new drugs were found to fully inactivate the h5-HT7 receptor: lisuride, bromocryptine, and metergoline. As inactivators, these drugs displayed potencies of 1, 80, and 321 nM, respectively. Pretreatment of 5-HT7-expressing HEK cells with increasing concentrations of the inactivating drugs risperidone, 9-OH-risperidone, methiothepin, lisuride, bromocriptine, and metergoline potently inhibited radiolabeling of the h5-HT7 receptor, with IC50 values of 9, 5.5, 152, 3, 73, and 10 nM, respectively. We were surprised to find that maximal concentrations of risperidone and 9-OH-risperidone inhibited only 50% of the radiolabeling of h5-HT7 receptors. These results indicate that risperidone and 9-OH risperidone may be producing 5-HT7 receptor inactivation by different mechanisms than lisuride, bromocryptine, metergoline, and methiothepin. These results are not interpretable using the conventional model of G-protein-coupled receptor function. The complex seems capable of assuming a stable inactive conformation as a result of the interaction of certain antagonists. The rapid, potent inactivation of the receptor-G-protein complex by antagonists implies a constitutive, pre-existing complex between the h5-HT7 receptor and a G-protein.
Footnotes
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This work was supported by the United States Public Health Service [Grant MH56650].
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ABBREVIATIONS: 5-HT7, 5-hydroxytryptamine7; HEK, human embryonic kidney; GPCR, G-protein-coupled receptor; LSD, d-lysergic acid diethylamide; SB269970, (R)-3-(2-(2-(4-methylpiperidin-1-yl)ethyl)pyrrolidine-1-sulfonyl)phenol hydrochloride; ICI169369, 2-(2-dimethylamino ethylthio)-3-phenyl quinoline; BIIE0246, N-[(1S)-4-[(aminoiminomethyl)amino]-1-[[[2-(3,5-dioxo-1,2-diphenyl-1,2,4-triazolidin-4-yl)ethyl]amino]carbonyl]butyl]-1-[2-[4-(6,11-dihydro-6-oxo-5H-dibenz[b,e]azepin-11-yl)-1-piperazinyl]-2-oxoethyl]-cyclopentaneacetamide formate.
- Received September 16, 2008.
- Accepted November 7, 2008.
- The American Society for Pharmacology and Experimental Therapeutics
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