@article {Boess515, author = {Frank G. Boess and Frederick J. Monsma, Jr. and Valerie Meyer and Catherine Zwingelstein and Andrew J. Sleight}, title = {Interaction of Tryptamine and Ergoline Compounds with Threonine 196 in the Ligand Binding Site of the 5-Hydroxytryptamine6 Receptor}, volume = {52}, number = {3}, pages = {515--523}, year = {1997}, doi = {10.1124/mol.52.3.515}, publisher = {American Society for Pharmacology and Experimental Therapeutics}, abstract = {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{\textendash}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.}, issn = {0026-895X}, URL = {https://molpharm.aspetjournals.org/content/52/3/515}, eprint = {https://molpharm.aspetjournals.org/content/52/3/515.full.pdf}, journal = {Molecular Pharmacology} }