A Methyl Scan of the Pyrrolidinium Ring of Nicotine Reveals Significant Differences in Its Interactions with α7 and α4β2 Nicotinic Acetylcholine Receptors

Abstract

The two major nicotinic acetylcholine receptors (nAChRs) in the brain are the α4β2 and α7 subtypes. A "methyl scan" of the pyrrolidinium ring was used to detect differences in nicotine's interactions with these two receptors. Each methylnicotine was investigated using voltage-clamp and radioligand binding techniques. Methylation at each ring carbon elicited unique changes in nicotine's receptor interactions. Replacing the 1'-N-methyl with an ethyl group or adding a second 1'-N-methyl group significantly reduced interaction with α4β2 but not α7 receptors. The 2'-methylation uniquely enhanced binding and agonist potency at α7 receptors. Although 3'- and 5'-trans-methylations were much better tolerated by α7 receptors than α4β2 receptors, 4'-methylation decreased potency and efficacy at α7 receptors much more than at α4β2 receptors. Whereas cis-5'-methylnicotine lacked agonist activity and displayed a low affinity at both receptors, trans-5'-methylnicotine retained considerable α7 receptor activity. Differences between the two 5'-methylated analogs of the potent pyridyl oxymethylene-bridged nicotine analog A84543 were consistent with what was found for the 5'-methylnicotines. Computer docking of the methylnicotines to the Lymnaea acetylcholine binding protein crystal structure containing two persistent waters predicted most of the changes in receptor affinity that were observed with methylation, particularly the lower affinities of the cis-methylnicotines. The much smaller effects of 1'-, 3'-, and 5'-methylations and the greater effects of 2'- and 4'-methylations on nicotine α7 nAChR interaction might be exploited for the design of new drugs based on the nicotine scaffold. SIGNIFICANCE STATEMENT: Using a comprehensive "methyl scan" approach, we show that the orthosteric binding sites for acetylcholine and nicotine in the two major brain nicotinic acetylcholine receptors interact differently with the pyrrolidinium ring of nicotine, and we suggest reasons for the higher affinity of nicotine for the heteromeric receptor. Potential sites for nicotine structure modification were identified that may be useful in the design of new drugs targeting these receptors.