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-hydroxytr yptamine (5-HT)(2A) (formerly known as the 5-HT2) receptor are related to aliphatic substitution at the N1-position on the indole nucleus. T he 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. Als o, the N1-unsubstituted analogues had higher affinity for the human 5- HT2A receptor, whereas the N1-alkyl analogues had a higher affinity fo r the rat receptor. By mutating the rat 5-HT2A receptor, the importanc e of the Ala/Ser(242) species variation in amino acid sequence was exa mined in relation to this structure-activity relationship. Three mutat ions of the rat 5-HT2A receptor were made, i.e., A242S, A242V, and A24 2T. All three mutations resulted in functional (able to stimulate inos itol phosphate hydrolysis) 5-HT2A receptors with high affinity for [H- 3]ketanserin and 2,5-dimethoxy-4-[(125)l]iodophenyl)isopropylamine. Th e A242S mutation resulted in a pharmacological profile that was almost identical to that of the human 5-HT2A receptor but differed significa ntly from that of the wild-type rat receptor. This strongly suggests t hat the Ala/Ser(242) species variation accounts for the differences in the structure-activity relationship. The A242V and A242T mutations re sulted 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 r eceptors. In addition, arguments are presented that suggest that a hyd rogen-bonding interaction occurs between the human 5-HT2A receptor at Ser(242) and the N1-hydrogen of N1-unsubstituted ergolines and tryptam ines and may serve as an important contact point in the receptor.