APPARENT BLOCK OF K+ CURRENTS IN MOUSE MOTOR-NERVE TERMINALS BY TETRODOTOXIN, MU-CONOTOXIN AND REDUCED EXTERNAL SODIUM

1 In mouse triangularis sterni nerve-muscle preparations, reduced extracellular Na+ concentrations and low concentrations of the Na+ channel blocking toxins tetrodotoxin (TTX, 18-36 nm) and mu-conotoxin GIIIB (0.4-2.0-mu-M) selectively decreased the amplitude of the component of perineural waveforms associated with nerve terminal K+ currents, without affecting the main Na+ spike. 2 Intracellular recording of endplate potentials (e.p.ps) and miniature endplate potentials (m.e.p.ps) from triangularis sterni preparations revealed that TTX and mu-conotoxin GIIIB depressed the evoked quantal release of acetylcholine without significant effects on m.e.p.p. amplitude, frequency or time constant of decay. 3 The apparent block of K+ current by low concentrations of TTX and mu-conotoxin is probably not a direct effect on K+ channels but results from a decrease in the passive depolarization of nerve terminals following blockade of a small proportion of axonal Na+ channels.