Studies on the mode of action of botulinum toxin type A at the frog neuromuscular junction

In frogs poisoned with botulinum toxin type A the quantal content of endplate potentials is greatly reduced. Lowering the temperature of the preparation increases quantum content; between 14 and 4 °C the mean Q 10 for this effect is 6.3. Facilitation of synaptic transmission is marked with pairs of stimuli and cooling further enhances facilitation. The time constant of decay of facilitation is 34 ms at 20 °C and 116 ms at 4 °C. The increase in facilitation and in its time constant of decay at low temperature are presumably not a result of a prolongation of the duration of the nerve terminal action potential since such changes are not seen in the presence of K +-channel blockade by 3,4-diaminopyridine. Electrotonic depolarization of nerve terminals in the presence of tetrodotoxin and 3,4-diaminopyridine induces all-or-none endplate currents. Such endplate currents, at a holding potential of −50 mV, show that the amount of charge entry is about1/3 of that in unpoisoned junctions but still corresponds to5−10 ×10 3 transmitter quanta. Transmitter release at this level is maintained during repetitive stimulation even in the presence of 82 mM Ca 2+ in the extracellular solution. We speculate that the blockade of transmitter release in BoTx-poisoned muscles results from a stimulatory effect of the toxin on metabolic systems of Ca 2+ disposal in the nerve terminal.