Analysis of quantal acetylcholine noise at end‐plates of frog muscle during rapid transmitter secretion.

Abstract

SUMMARY1. Using the theory of noise analysis an attempt was made to measure frequency and amplitude of miniature end-plate potentials (MEPPs) under conditions of vigorous transmitter release. Frog sartorius muscles were incubated in a depolarizing (32 mM-K+) medium which lacked Ca2+ to prevent transmitter release. Subsequently, when the membrane potential had become stable at about -40 mV, end-plates were superfused with 4 mM-Ca2+-containing medium for 1 min periods with 5 min intervals between the superfusions.2. Most junctions ('fast' type) responded to Ca2+ with a relatively large, noisy depolarization (5-8-14-5 mV) which subsided rapidly during subsequent challenges with Ca2 . Other junctions ('slow' type) responded with only 1-1 6 mV depolarizations which were rather well sustained during the consecutive Ca2+ applications.3. From the variance, E2, and the depolarization, V, caused by Ca2+ the frequency n and amplitude factor q of the MEPPs were calculated. Values of n were 3-4 x 104 and 0_1-1 x 104 S-1 in the fast-and slow-type junctions, respectively. The mean value of q was 0-16 mV; it remained more or less constant in the fast-type junctions, but tended to decline in the slow-type junctions.4. As expected, cholinesterase inhibitors potentiated V and E2 as well as individual MEPPs. However, no advantage could be taken from this finding, since these drugs caused burst-like peaks superimposed on the voltage signal, precluding application of noise analysis.5. The results strongly suggest that, at least in the fast-type junctions, K+ caused an extremely rapid depletion of the store of transmitter quanta, whose mean size did not change appreciably in the course of the experiment. However, in the slow-type junctions during prolonged incubation, it cannot be excluded that the gradual decline of q was due to the release of newly formed, unripe quanta.

Effects of reduced vesicular filling on synaptic transmission in rat hippocampal neurones

“…The literature from the NMJ on the change of quantal size induced by activity is mixed. In some studies, no change was observed even after prolonged stimulation (Molenaar & Oen, 1988; Reid et al 1999), whereas in other studies small reductions were seen after moderate stimulation (Jones & Kwanbunbumpen 1970; Kriebel & Gross, 1974; Doherty et al 1984; Glavinovic, 1987). Since only a small portion of the vesicle pool was turned over by the stimuli, and the newly endocytosed vesicles mix freely with the reserve pool of vesicles (Betz & Bewick, 1992, 1993) it has been suggested that the docked vesicles are filled to a greater extent than the rest of the pool of vesicles (Van der Kloot & Molgó, 1994).…”

“…It has been estimated that the time between the endocytosis of a vesicle and its priming for release is rapid and in cultured neurons can be as fast as 15 s (Ryan & Smith, 1995). However, at many synapses even during intense high‐frequency stimulation the size of quantal events remains relatively constant (Molenaar & Oen, 1988; Reid et al 1999). These results suggest that vesicles are rapidly filled with transmitter to their final equilibrium concentration.…”

Recycling and refilling of transmitter quanta at the frog neuromuscular junction

“…B, a cumulative plot of the data from a similar experiment (green circles), the fit to a single lognormal distribution (orange line), and the fit to two lognormal distributions (black line). (Molenaar & Oen, 1988). Our method was to place preparations in elevated K¤ solution, digitize the MEPPs at an endplate at 2 kHz and display the trace on the computer screen.…”

Cholinergic agonists decrease quantal output at the frog neuromuscular junction by targeting a calcium channel blocked by ω-conotoxin

“…In such solutions the F MEPP is elevated, because the depolarization produced by the K + opens voltage-gated Ca 2+ channels, producing an elevation of the intracellular [Ca 2+ ]. After adding the elevated K + solution the F MEPP rises to a maximum and then slowly declines ( [6,18] reviewed in [27]). The time course of the K + enhancement of F MEPP had to be considered when we designed our experiments.…”