The effects of in vitro application of purified botulinum neurotoxin at mouse motor nerve terminals.

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Abstract
1. Purified botulinum neurotoxin type A (10 nM) was applied in vitro to mouse diaphragm muscles. Intracellular micro‐electrode recordings were made continuously in single fibres. 2. This treatment reduced end‐plate potential (e.p.p.) amplitudes with a time to half‐maximal effect of about 75 min at 22‐25 degrees C. E.p.p. rise‐times remained fast and unaffected by the toxin. 3. Miniature end‐plate potential (m.e.p.p.) frequency was reduced by the toxin to less than 5% of control frequency, and followed a similar time course to the block of e.p.p. amplitudes. The m.e.p.p. rise‐time and coefficient of variation (c.v.) of m.e.p.p. amplitude distributions both increased, but the time course of these increases lagged significantly behind the change in frequency. 4. A population of slow rise‐time m.e.p.p.s was present in controls at low frequency. This population was found to be unaffected by the toxin. 5. The above‐detailed in vitro changes could be explained by the toxin acting by a single common mechanism to inhibit the release process underlying both fast rise‐time m.e.p.p.s and e.p.p.s. A distinct release process, which leads to slow rise‐time m.e.p.p.s, was unaffected by the toxin.