N-methylanatoxinol isomers: derivatives of the agonist (+)-anatoxin-a block the nicotinic acetylcholine receptor ion channel

KL Swanson, Y Aracava, FJ Sardina, H Rapoport, RS Aronstam and EX Albuquerque

Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, Baltimore 21201.

Using biochemical and patch-clamp techniques, we investigated the pharmacology of S- and R-epimers of N-methylanatoxinol, which are analogs of the semi-rigid, stereoselective, nicotinic agonist (+)- anatoxin-a. In contrast to (+)-anatoxin-a, both isomers had poor ability to inhibit the binding of 125I-alpha-bungarotoxin or to open acetylcholine channels, and they were unable to elicit contracture of frog rectus abdominis muscles. However, both isomers were able to demonstrate significant concentration-dependent blockade of the nicotinic acetylcholine receptor ion channel. The R-isomer was approximately 4-fold more potent in causing inhibition of [3H]H12HTX binding than was the S-isomer, in the absence of carbamylcholine. In the presence of carbamylcholine, the affinity of the R-isomer of N- methylanatoxinol for the ion channel sites was further enhanced, so that its affinity became much greater than that of the S-isomer. Refinement of voltage- and concentration-dependent terms for the ion channel blocking and unblocking rates yielded functions that were able to predict the channel open times and short closed times well. The S- isomer bound and dissociated from the ion channel site of the nicotinic acetylcholine receptor more rapidly and with greater voltage sensitivity than the R-isomer. The present characterization of the antagonistic properties of these new analogs of (+)-anatoxin-a introduces a new aspect to the molecular pharmacology of (+)-anatoxin-a analogs; the semi-rigid compounds could be useful in describing the allosteric binding sites of the acetylcholine receptor, as well as in delimiting the agonist binding site.

Volume 35, Issue 2, pp. 223-231, 02/01/1989
Copyright © 1989 by American Society for Pharmacology and Experimental Therapeutics