Molecular mechanisms of the potent and stereospecific nicotinic receptor agonist (+)-anatoxin-a

Anatoxin-a (AnTX) was shown to be a highly potent and stereospecific agonist at nicotinic synapses in frog skeletal muscle and Torpedo electric organs. AnTX binds to the nicotinic-acetyl-choline receptor with a higher affinity than for acetylcholine (ACh) but does not bind to sites in the receptor-gated ionic channel. (+)AnTX caused receptor desensitization, i.e., the loss of agonist-stimulated binding of histrionicotoxin to an allosteric site with time, at a rate significantly slower than that of ACh. Single channel patch clamp recordings indicated that the conductance of channels activated by (+)AnTX (28 pS) and ACh (27 pS) were similar. The (+)AnTX-activated channels contained rapid closing events, the burst times caused by the toxin were shorter than those caused by ACh but had similar voltage dependencies, and the number of short closures per burst was constant at all potentials with both agonists. The bursts of rapid openings and rapid closures (τ = 0.4 msec) appear to result from repetitive opening and closing of the (+)AnTX-bound receptor-ion channel. It is concluded that the semirigid molecule and secondary amine (+)AnTX is a more potent agonist than ACh or carbamylcholine because of a higher affinity for the receptor. At various concentrations the toxin activates the appearance of channels with the same conductances as ACh-induced channels but with a shorter channel lifetime.