Molecular mechanisms of open-channel blockade in nicotinic acetylcholine receptors of autonomic ganglia neurons.

The results of recent attempts to estimate the dimensions of ionic channels in nicotinic acetylcholine receptors of sympathetic and enteric ganglia neurons are reviewed. The channel dimensions, obtained from comparison of the sizes of the open-channel blocking molecules with their blocking activities, are 6.1 x 8.3 A (1 A = 0.1 nm) in both sympathetic and enteric ganglia. None of the competitive ganglionic blockers fit within this channel size. In addition, a chemical structure that binds the open-channel blockers in ganglionic nicotinic acetylcholine receptors is suggested to be formed by serine and threonine residues, as found by comparing the differences between the structures of the neuronal and muscle nicotinic acetylcholine receptors with the differences in their pharmacology.