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

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.