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SUMMARY1. Following motor nerve stimulation there is a period of greatly enhanced quantal release, called the early release period or ERP (Barrett & Stevens, 1972b). Until now, measurements of the probability of quantal releases at different points in the ERP have come from experiments in which quantal output was greatly reduced, so that the time of release of individual quanta could be detected or so that the latency to the release of the first quantum could be measured.2. A method has been developed to estimate the timing of quantal release during the ERP that can be used at much higher levels of quantal output. The assumption is made that each quantal release generates an end-plate current (EPC) that rises instantaneously and then decays exponentially. The peak amplitude of the quantal currents and the time constant for their decay are measured from miniature endplate currents (MEPCs). Then a number of EPCs are averaged, and the times of release of the individual quanta during the ERP estimated by a simple mathematical method for deconvolution derived by Cohen, Van der Kloot & Attwell (1981).3. The deconvolution method was tested using data from preparations in highMg2+ low-Ca2+ solution. One test was to reconstitute the averaged EPCs from the estimated times of quantal release and the quantal currents, by using Fourier convolution. The reconstructions fit well to the originals.4. Reconstructions were also made from averaged MEPCs which do not rise instantaneously and the estimated times of quantal release. In these reconstructions the rise of the EPC is slightly displaced to a later time, but aside from this time displacement the reconstruction is satisfactory.5. Finally the method was tested by working at very low quantal output, comparing direct measurements of the times of appearance of quanta with the times determined by deconvolution. Again the agreement was satisfactory.6. The conclusion is that the method works well with reduced quantal outputs and has the potential for being useful at quantal outputs approaching normal. The method may also be useful at central synapses where the characteristics of individual quanta can be obtained only by noise analysis, if it is first shown that quantal interactions do not occur at the synapse.

Miniature end-plate potentials (MEPPs) are increased in size by pretreatment in a hypertonic Ringer. This paper is about the treatments that increase quantal size and the evidence that it occurs by packing more acetylcholine (ACh) in the quanta. Sartorius muscles were soaked for 2 h in a hypertonic Ringer, containing 200 mM NaCl in place of the usual 120 mM, and then returned to Ringer. In the hypertonic solution the miniature frequency was above 100/s. The treatment increased the size of the miniatures, recorded with an intracellular electrode after the return to usual Ringer, by a factor of approximately 2.5, compared with paired muscles kept in usual Ringer throughout. When the hypertonic solution was made with 200 mM sodium gluconate, after the return to usual Ringer, the miniatures increased in size by a factor of approximately 3.6. The large miniatures produced by the treatment fit a log normal probability distribution function, except for a variable small fraction of the largest events that may represent multiple releases. Miniatures recorded from untreated end plates fit better to a log normal distribution function than to a normal distribution function. The effects of the hypertonic treatments on the acetylcholine receptor (AChR) were accessed by measuring, after the return to usual Ringer, ACh noise, alpha-bungarotoxin binding, and the reversal potential. None of these measurements revealed significant differences between experimental and control preparations. The increases occurred when neostigmine was present in the hypertonic and usual solutions and when it was not, so changes in acetylcholinesterase (AChE) do not appear to be involved. Both the MEPPs and miniature end-plate currents, recorded with the two electrode voltage clamp, increased in size, so the increase is not owing to an rise in the input resistance of the muscle fiber. The increase in quantal size was prevented by including 1-10 microM AH5183 in the hypertonic solution. This drug blocks ACh uptake into synaptic vesicles. Therefore, it seems likely that the treatments act by increasing the quantity of ACh/quantum. Cyclohexamide (10 micrograms/ml), an inhibitor of protein synthesis, did not affect the increase in size caused by hypertonic solutions. Size increases were detected after 15 min in hypertonic solution. Most of the increase was complete after 1 h and there was no further increase after 2 h.(ABSTRACT TRUNCATED AT 400 WORDS)