PT  - JOURNAL ARTICLE
AU  - Boess, Frank G.
AU  - Monsma, Frederick J.
AU  - Meyer, Valerie
AU  - Zwingelstein, Catherine
AU  - Sleight, Andrew J.
TI  - Interaction of Tryptamine and Ergoline Compounds with Threonine 196 in the Ligand Binding Site of the  5-Hydroxytryptamine<sub>6</sub> Receptor
AID  - 10.1124/mol.52.3.515
DP  - 1997 Sep 01
TA  - Molecular Pharmacology
PG  - 515--523
VI  - 52
IP  - 3
4099  - http://molpharm.aspetjournals.org/content/52/3/515.short
4100  - http://molpharm.aspetjournals.org/content/52/3/515.full
SO  - Mol Pharmacol1997 Sep 01; 52
AB  - 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 transmembrane region V are important for ligand binding in several bioamine receptors. In the 5-HT6receptor, 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 mianserin, 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.