N-methylanatoxinol isomers: derivatives of the agonist (+)-anatoxin-a block the nicotinic acetylcholine receptor ion channel.
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.
Swanson
K L
KL
Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, Baltimore 21201.
Aracava
Y
Y
Sardina
F J
FJ
Rapoport
H
H
Aronstam
R S
RS
Albuquerque
E X
EX
D000818:Animals; D001427:Bacterial Toxins; D001665:Binding Sites; D001643:Bridged Bicyclo Compounds / Q000494:pharmacology*; D019086:Bridged Bicyclo Compounds, Heterocyclic; D001952:Bridged-Ring Compounds / Q000494:pharmacology*; D000087522:Cyanobacteria Toxins; D004305:Dose-Response Relationship, Drug; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects*; D008387:Marine Toxins / Q000494:pharmacology*; D052998:Microcystins; D009119:Muscle Contraction / Q000187:drug effects; D011894:Rana pipiens; D011978:Receptors, Nicotinic / Q000187:drug effects*; D013237:Stereoisomerism; D013329:Structure-Activity Relationship; D014326:Tropanes
D001427:Bacterial Toxins; D001643:Bridged Bicyclo Compounds; D019086:Bridged Bicyclo Compounds, Heterocyclic; D001952:Bridged-Ring Compounds; D000087522:Cyanobacteria Toxins; D007473:Ion Channels; D008387:Marine Toxins; D052998:Microcystins; D011978:Receptors, Nicotinic; D014326:Tropanes; C058634:N-methylanatoxin; C509783:anatoxin a