Species variations in transmembrane region V of the 5-hydroxytryptamine type 2A receptor alter the structure-activity relationship of certain ergolines and tryptamines.

Previous work has suggested that species differences in the structure-activity relationship for ergolines and tryptamines at the 5-hydroxytryptamine (5-HT)2A (formerly known as the 5-HT2) receptor are related to aliphatic substitution at the N1-position on the indole nucleus. The present work has confirmed these findings by examining the rat and human cloned 5-HT2A receptors. As previously found, N1-substitution of ergolines or tryptamines had no effect or increased affinity for the rat 5-HT2A receptor but decreased affinity for the human receptor. Also, the N1-unsubstituted analogues had higher affinity for the human 5-HT2A receptor, whereas the N1-alkyl analogues had a higher affinity for the rat receptor. By mutating the rat 5-HT2A receptor, the importance of the Ala/Ser242 species variation in amino acid sequence was examined in relation to this structure-activity relationship. Three mutations of the rat 5-HT2A receptor were made, i.e., A242S, A242V, and A242T. All three mutations resulted in functional (able to stimulate inositol phosphate hydrolysis) 5-HT2A receptors with high affinity for [3H]ketanserin and 1-(2,5-dimethoxy-4-[125I]iodophenyl)isopropylamine. The A242S mutation resulted in a pharmacological profile that was almost identical to that of the human 5-HT2A receptor but differed significantly from that of the wild-type rat receptor. This strongly suggests that the Ala/Ser242 species variation accounts for the differences in the structure-activity relationship. The A242V and A242T mutations resulted in differing but profound effects on affinity for the different ergolines and tryptamines. The results are discussed in terms of the importance of position 242 in the binding of these ligands to 5-HT2A receptors. In addition, arguments are presented that suggest that a hydrogen-bonding interaction occurs between the human 5-HT2A receptor at Ser242 and the N1-hydrogen of N1-unsubstituted ergolines and tryptamines and may serve as an important contact point in the receptor.