amesergide and 4-iodo-2-5-dimethoxyphenylisopropylamine

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.

Topics: Amphetamines; Animals; Binding, Competitive; Cloning, Molecular; Ergolines; Humans; Hydrogen Bonding; Hydrolysis; Inositol Phosphates; Ketanserin; Mutation; Rats; Receptors, Serotonin; Species Specificity; Structure-Activity Relationship; Tryptamines

Previous studies indicated that selected ergolines and tryptamines showed species differences for affinity to the antagonist-labeled 5-HT2 receptor. The present study examined these same compounds for affinity at the agonist-labeled 5-HT2 receptor in rat and squirrel monkey cortical homogenates using [125I]DOI ([125I]1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane). As seen with the antagonist-labeled 5-HT2 receptor, N(1) alkyl substitution of either the ergolines or tryptamines resulted in a slight increase or no effect on their affinity for the agonist-labeled rat 5-HT2 receptor. In contrast, these same N(1) substitutions resulted in significant decreases in affinity for the agonist-labeled monkey 5-HT2 receptor. It was also noted that N(1)-unsubstituted ergolines and tryptamines (such as ergonovine, LY86057, LY193525 and 5-methoxytryptamine) tended to have higher affinity for the monkey versus the rat agonist-labeled receptor. However, the N(1) alkyl-substituted ergolines and tryptamines (such as mesulergine, LY53857, amesergide, N(1)-isopropyltryptamine and N(1)-isopropyl-5-methoxytryptamine) showed significantly lower affinity for the monkey versus the rat 5-HT2 receptor. These data suggest that, at least in relation to the N(1) position, ergolines and tryptamines bind in a similar orientation. These results are also discussed in terms of what amino acid differences between species may account for this structure-activity relationship.

Topics: Amphetamines; Animals; Cerebral Cortex; Ergolines; Ergonovine; Rats; Rats, Sprague-Dawley; Receptors, Serotonin; Saimiri; Serotonin Antagonists; Serotonin Receptor Agonists; Species Specificity; Structure-Activity Relationship; Tryptamines