Characteristics of Schwann-cell miniature end-plate currents in denervated frog muscle

End-plates in denervated frog sartorius muscles were investigated, under voltage clamp, to determine the characteristics of miniature endplate currents (mepc) caused by the release of acetylcholine (ACh)-quanta from Schwanncells. Staining for acetylcholinesterase ascertained that in most cases Schwann-cell mepc had been recorded focally from the end-plate. The mean amplitude of Schwann-cell mepc (about 1 nA) was smaller than the amplitude (3–4 nA) of normal nerve evoked mepc. The amplitude distribution of Schwann-cell mepc was skewed, as were also the distributions of decay time constants and of rise times. Some Schwann-cell mepc appeared to be of composite nature, as shown by an inflexion on either the rising or falling phase. The decay time constant τ of both fast and slowly decaying Schwann-cell mepc increased with membrane hyperpolarization suggesting an exponential correlation. The potential dependence was 118±37 mV (means±SD from 6 experiments) for ane-fold change of τ, which does not differ significantly from that for neural mepc (98 mV). The cholinesterase inhibitor neostigmine increased the amplitude and prolonged the time course of Schwann-cell mepc to a similar degree after short and long term denervation, which indicates that cholinesterase was still functional after 3–4 months of denervation. The results show that the characteristics of Schwann-cell mepc resemble in many respects those of neural mepc but are more variable in shape and amplitude.