Estimating the time course of evoked quantal release at the frog neuromuscular junction using end‐plate current latencies.

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Abstract
1. The use of end-plate current (e.p.c.) latency mesurements to estimate the time course of the stochastic probabilistic process governing evoked release was investigated in the sciatic nerve-sartorius muscle preparation of the frog, Rana pipiens. We also examined the possibility that the release of a quantum depresses or enhances the subsequent release of additional quanta. Muscle end-plates were voltage clamped at 3-4.degree. C. Quantal release was restricted to a short, or localized, region of the nerve terminal using Ca2+-free, EGTA Ringer solution and a Ca2+-filled micropipette. 2. The number of e.p.c.s containing 0, 1, 2, etc. quanta were totalled and compared to numbers predicted using Poisson''s theorem. The differences between the actual and predicted numbers of events were not significant at the nineteen junctions studied (P < 0.05). 3. The latency of the first quantum observed in several hundred e.p.c.s was measured and used to calculate an estimate, .alpha.1(t), of the time-dependent, probabilistic process, .alpha.(t), governing all evoked quantal release (Barett and Stevens, 1972b). In three experiments, all quantal latencies were measured to obtain the actual .alpha.(t). The .alpha.1(t) function gave an excellent approximation of .alpha.(t) (P > 0.2), in real and simulated latency data. 4. The latency of the second quantum in the e.p.c.s. was measured and used to provide another estimate, .alpha.2(t), of .alpha.(t). The .alpha.2(t) function was lower (depressed) during the first few milliseconds of the evoked release period, relative to .alpha.1(t). The difference was significant (P > 0.01) in all experiments. Our measurement procedures were tested using computer-generated ''e.p.c.s'' containing randomly occurring ''quanta''. These tests showed that the early depression was due to inadequate detection of the second quantum in the e.p.c.s. 5. The effect of Sr2+ on evoked release was examined using double-barrelled pipettes containing 1 M-SrCl2 and CaCl2 solutions. The major result was that the durations of .alpha.1(t) and .alpha.2(t) were equally lengthened in Sr2, relative to Ca2+.