Interaction of tryptamine and ergoline compounds with threonine 196 in the ligand binding site of the 5-hydroxytryptamine6 receptor.
We examined the ligand-binding site of the 5-hydroxytryptamine6 (5-HT6) receptor using site-directed mutagenesis. Interactions with residues in two characteristic positions of trans-membrane region V are important for ligand binding in several bioamine receptors. In the 5-HT6 receptor, one of these residues is a threonine (Thr196), whereas in most other mammalian 5-HT receptors, the corresponding residue is alanine. After transient expression in human embryonic kidney 293 cells, we determined the effects of the mutation T196A on [3H]d-lysergic acid diethylamide (LSD) binding and adenylyl cyclase stimulation. This mutation produced a receptor with a 10-fold reduced affinity for [3H]LSD and a 6-fold reduced affinity for 5-HT. The potency of both LSD and 5-HT for stimulation of adenylyl cyclase was also reduced by 18- and 7-fold, respectively. The affinity of other N1-unsubstituted ergolines (e.g., ergotamine, lisuride) was reduced 10-30 fold, whereas the affinity of N1-methylated ergolines (e.g., metergoline, methysergide, mesulergine) and other ligands, such as methiothepine, clozapine, ritanserin, amitriptyline, and mainserin, changed very little or increased. This indicates that in wild-type 5-HT6 receptor, Thr196 interacts with the N1 of N1-unsubstituted ergolines and tryptamines, probably forming a hydrogen bond. Based on molecular modeling, a serine residue in transmembrane region IV of the 5-HT2A receptor has previously been proposed to interact with the N1-position of 5-HT. When the corresponding residue of the 5-HT6 receptor (Ala154) was converted to serine, no change in the affinity of twelve 5-HT6 receptor ligands or in the potency of 5-HT and LSD could be detected, suggesting that this position does not contribute to the ligand binding site of the 5-HT6 receptor.
Boess
F G
FG
Pharma Division, Preclinical Research, F. Hoffman-La Roche Ltd., 4070 Basel, Switzerland. frank.boess@roche.com
Monsma
F J
FJ
Meyer
V
V
Zwingelstein
C
C
Sleight
A J
AJ
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002478:Cells, Cultured; D003001:Cloning, Molecular; D006224:Cricetinae; D004285:Dogs; D004347:Drug Interactions; D004873:Ergolines / Q000378:metabolism*; D006168:Guinea Pigs; D006801:Humans; D007668:Kidney / Q000378:metabolism / Q000648:ultrastructure; D008024:Ligands; D008238:Lysergic Acid Diethylamide / Q000378:metabolism; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D051381:Rats; D011985:Receptors, Serotonin / Q000235:genetics / Q000378:metabolism* / Q000502:physiology; D017386:Sequence Homology, Amino Acid; D012701:Serotonin / Q000378:metabolism; D013912:Threonine / Q000235:genetics / Q000378:metabolism*; D014363:Tryptamines / Q000378:metabolism*
D004873:Ergolines; D008024:Ligands; D011985:Receptors, Serotonin; D014363:Tryptamines; C097216:serotonin 6 receptor; D013912:Threonine; D012701:Serotonin; D008238:Lysergic Acid Diethylamide