Decreased agonist, but not antagonist, binding to the naturally occurring Thr92Lys variant of the h5-HT7(a) receptor.
In the present study on transfected human embryonic kidney (HEK)293 cells, we aimed at establishing whether expression of the naturally occurring Thr92Lys variation of the Gs-coupled h5-HT7(a) receptor leads to changes of ligand binding properties, of agonist-evoked cAMP formation and/or of antagonist-mediated blockade of the latter. Binding of [3H]5-carboxamidotryptamine ([3H]5-CT) to membranes and stimulated [3H]cAMP accumulation in whole cells were determined. Saturation binding experiments in membranes of transiently transfected cells expressing either the wild-type or the variant receptor revealed a single binding site in both cases and no difference in Bmax between both receptor isoforms. In competition binding experiments in membranes of stably transfected cells, the Thr92Lys variant exhibited a 2.8-11 times lower binding affinity of the ligands 5-hydroxytryptamine (5-HT), 5-CT, 5-methoxy-3-(1,2,3,6-tetrahydropyridin-4yl)-1H-indole (RU24969), (+/-)-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT) and sumatriptan compared to the wild-type receptor. However, the variant did not differ from the wild-type with respect to the binding properties of the antagonists (R)-3-(2-(2-(4-methylpiperidin-1-yl)ethyl)-pyrrolodine-1-sulfonyl)phenol hydrochloride (SB-269970), risperidone, mesulergine and clozapine. In agreement with the decreased binding affinity of 5-HT, 5-CT, RU24969 and 8-OH-DPAT for the variant receptor, these agonists were less potent in stimulating [3H]cAMP accumulation in cells stably expressing the Thr92Lys h5-HT7(a) receptor. Sumatriptan did not stimulate cAMP accumulation in spite of its affinity for both receptor isoforms pointing to a putative weak antagonistic property of this drug at the h5-HT7 receptor. SB-269970 and clozapine were equipotent at both the variant and the wild-type receptor in producing a rightward shift of the 5-HT concentration-response curve for its stimulant effect on [3H]cAMP accumulation. In view of, e.g., the purported involvement of the 5-HT7 receptor in the regulation of circadian rhythm, it may be concluded that the decrease in affinity of 5-HT and other 5-HT receptor agonists at the (Thr92Lys) h5-HT7 receptor may be associated with changes of sleep physiology and of actions of new 5-HT7 receptor agonists designed to treat circadian dysregulation.
Brüss
Michael
M
Institute of Pharmacology and Toxicology, University of Bonn, Reuterstrasse 2b, D-53113 Bonn, Germany.
Kiel
Sibylle
S
Bönisch
Heinz
H
Kostanian
Arevat
A
Göthert
Manfred
M
D019943:Amino Acid Substitution / Q000235:genetics; D001665:Binding Sites / Q000187:drug effects / Q000502:physiology; D001667:Binding, Competitive / Q000187:drug effects / Q000502:physiology*; D001921:Brain / Q000187:drug effects / Q000378:metabolism* / Q000503:physiopathology; D001923:Brain Chemistry / Q000187:drug effects / Q000502:physiology*; D002460:Cell Line; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D021081:Chronobiology Disorders / Q000188:drug therapy / Q000378:metabolism; D000242:Cyclic AMP / Q000378:metabolism; D015536:Down-Regulation / Q000187:drug effects / Q000502:physiology; D014644:Genetic Variation; D006801:Humans; D008239:Lysine / Q000378:metabolism; D020033:Protein Isoforms / Q000187:drug effects / Q000378:metabolism; D011869:Radioligand Assay; D011985:Receptors, Serotonin / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D012702:Serotonin Antagonists / Q000378:metabolism* / Q000494:pharmacology; D017366:Serotonin Receptor Agonists / Q000378:metabolism* / Q000494:pharmacology; D013912:Threonine / Q000378:metabolism
D020033:Protein Isoforms; D011985:Receptors, Serotonin; D012702:Serotonin Antagonists; D017366:Serotonin Receptor Agonists; C104425:serotonin 7 receptor; D013912:Threonine; D000242:Cyclic AMP; D008239:Lysine