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Anatoxin-a: a novel, potent agonist at the nicotinic receptor.

Anatoxin-a, a semirigid, bicyclic amine, caused a depolarizing blockade of the indirectly elicited twitch in frog sartorius muscle. Concentration-response studies of contracture in the rectus abdominis and depolarization in the sartorius muscles of the frog showed that it is the most potent of the nicotinic agonists. It produced desensitization, and the kinetic and steady-state parameters found from chronically denervated rat soleus muscle were similar for anatoxin-a and acetylcholine. When endplate regions of frog sartorius fibers were voltage clamped, anatoxin-a induced endplate currents and concomitant increases in endplate current noise. Fourier analysis of this noise revealed that the average single channel lifetime was indistinguishable from that induced by acetylcholine; the single channel conductance was about 25% less for the toxin compared to acetylcholine. Analysis of nerve evoked endplate currents and spontaneous, miniature endplate currents indicated that the toxin did not alter the time constants for decay or the linearity of the current-voltage relationship. We conclude from this evidence that anatoxin-a does not block the open ion channel and does not produce a voltage-dependent blockade of the closed ion channel. The activity of this unusual agonist may be related to the coulombic and hydrogen bond free energies of binding to the receptor.