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1. Extensor digitorum longus muscles of rats were paralysed with local, non-lethal doses of botulinum toxin Type A (BoTx). At 2 and 7 days after toxin injection, the nerve-muscle preparations were excised and end-plate currents analysed at 23 degrees C by dual-micro-electrode voltage clamp. 2. At 2 days after BoTx injection, the growth time of miniature end-plate currents (m.e.p.c.s) increased from a rather narrow range with a mean of 0.59 to a mean of 1.35 ms with a large variability between m.e.p.c.s. End-plate currents (e.p.c.s) were reduced compared to unpoisoned muscle. The decay phase of m.e.p.c.s and e.p.c.s, the growth phase of e.p.c.s and the voltage sensitivity of m.e.p.c.s were unchanged. 3. At 7 days after BoTx injection, the findings were similar to 2 days except that the time constant of the decay phase of m.e.p.c.s. and e.p.c.s. was about twice a long as normal and that the voltage sensitivity of m.e.p.c.s was increased. 4. The acetylcholine null potential (about 0 mV) was unchanged after treatment with BoTx. 5. The increase in the growth time of m.e.p.c.s compared to e.p.c.s following the injection of BoTx suggests that the poisoning, besides blocking quantal release, affects the time course of spontaneous but not that of evoked release. After BoTx poisoning the trans-synaptic diffusion of a majority of spontaneously released transmitter quanta seems to occur more slowly or from areas more distant from the highest concentration of the post-synaptic receptor than that of evoked release. 6. The increase in the decay phase of m.e.p.c.s and e.p.c.s and its increased voltage sensitivity observed in muscles poisoned for 7 days with BoTx suggest that appearance at the end-plate of a population of new receptors with a prolonged ion channel opening time similar to that previously described for extrajunctional receptors after denervation and for junctional receptors during development.

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