Effects of agonists, partial agonists, and antagonists on the regulation of 5-hydroxytryptamine2 receptors in P11 cells.
Studies of the regulation of 5-hydroxytryptamine (5-HT2) receptors in vivo have generated anomalous and sometimes contradictory results. In particular, administration of antagonists unexpectedly results in a reduction in the density of 5-HT2 receptors. P11 cells, which express a high density of 5-HT2 receptors coupled to phosphoinositide hydrolysis, were used to investigate the regulation of receptors in vitro by agonists, partial agonists, and antagonists. (+/-)-2,5-Dimethoxy-4-iodophenylisopropylamine (DOI) and (+)-lysergic acid diethylamide (LSD) caused marked reductions in the density of 5-HT2 receptors as has been observed in vivo. Down-regulation was prevented by coincubation with ketanserin. The decrease in the density of 5-HT2 receptors after exposure to 5-HT, LSD, or DOI was time dependent and was not a consequence of residual drug in binding assays or irreversibly bound drug. The ability of 5-HT, DOI, and LSD to down-regulate 5-HT2 receptors was not proportional to the ability of these compounds to stimulate phosphoinositide hydrolysis. Ketanserin and mianserin, antagonists which cause paradoxical decreases in the density of 5-HT2 receptors in vivo, did not alter the density of 5-HT2 receptors on P11 cells, even after prolonged incubation with drug. Results of the current studies, which demonstrate agonist- but not antagonist-induced down-regulation of 5-HT2 receptors, lead to the conclusion that the ability of ketanserin and mianserin to down-regulate receptors in vivo is the result of indirect actions of these drugs and is unlikely to be a direct consequence of receptor occupancy by antagonists.
Ferry
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Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia 19104-6084.
Unsworth
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Molinoff
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D000662:Amphetamines / Q000494:pharmacology; D000818:Animals; D015536:Down-Regulation; D006868:Hydrolysis; D007650:Ketanserin / Q000494:pharmacology; D008238:Lysergic Acid Diethylamide / Q000494:pharmacology; D008803:Mianserin / Q000494:pharmacology; D010716:Phosphatidylinositols / Q000378:metabolism; D051381:Rats; D011985:Receptors, Serotonin / Q000187:drug effects*; D012701:Serotonin / Q000494:pharmacology; D012702:Serotonin Antagonists / Q000494:pharmacology*; D017366:Serotonin Receptor Agonists / Q000494:pharmacology*; D014407:Tumor Cells, Cultured
D000662:Amphetamines; D010716:Phosphatidylinositols; D011985:Receptors, Serotonin; D012702:Serotonin Antagonists; D017366:Serotonin Receptor Agonists; D008803:Mianserin; D012701:Serotonin; D008238:Lysergic Acid Diethylamide; D007650:Ketanserin; C015952:4-iodo-2,5-dimethoxyphenylisopropylamine