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

In vivo analysis of end-plate noise of human extensor digitorum brevis muscle after intramuscularly injected botulinum toxin type A

To evaluate changes occurring in the neuromuscular junction after injection with botulinum neurotoxin type A (BoTx), three healthy volunteers were injected with 10 U BoTx in the right extensor digitorum brevis muscle. In agreement with previous observations, amplitude of compound muscle action potential (CMAP) decreased to approximately 30% of the initial value at approximately day 8 and slowly returned to baseline values around day 250. Values of the acetylcholine receptor (AChR) open time were determined by spectral analysis of end-plate noise and from single exponential fits to the decay phase of individual miniature end-plate potentials (MEPPs). At baseline, the mean channel open times determined by end-plate noise analysis and the exponential fits were 1.1 +/- 0.2 ms and 1.20 +/- 0.04 ms, respectively. After BoTx injection, initially no end-plate noise could be recorded. From day 9 onwards, however, a gradual recurrence of end-plate noise was observed, with mean channel open times of approximately 2-5 ms, being maximal between days 20 to 140. In addition, the shape of many recorded MEPPs was different from the typical fast rising MEPPs observed at baseline. After day 80, end-plate noise gradually returned to normal and mean channel open times decreased slowly to baseline values. Our findings reflect the changed AChR characteristics of newly formed neuromuscular junctions, which are created after BoTx injection and gradually removed after restoration of the original neuromuscular junctions.

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

The consequence of reduced uptake of neurotransmitters into synaptic vesicles on synaptic transmission was examined in rat hippocampal slices and culture using bafilomycin A1 (Baf), a potent and specific blocker of the vacuolar-type (V-type) ATPase, which eliminates the driving force for the uptake of both glutamate and GABA into synaptic vesicles. After incubation with Baf, both the amplitude and frequency of GABAergic miniature inhibitory postsynaptic currents (mIPSCs) were reduced in the slice preparation. Similar effects were seen with glutamatergic miniature excitatory postsynaptic currents (mEPSCs) and GABAergic mIPSCs from cultured neurons. This result indicates that vesicular content is reduced by Baf. The dramatic reduction in the frequency of mPSCs could result either from the exocytosis of empty vesicles or from a mechanism which prevents the exocytosis of depleted vesicles. Vesicle cycling was directly examined using confocal imaging with FM 1-43. In the presence of Baf, vesicles could still be endocytosed and they were released at the same probability as from control untreated synapses. Prolonged high-frequency electrical stimulation of synapses in culture failed to alter the amplitude of mEPSCs, suggesting that the filling of vesicles is rapid compared to the rate of vesicle recycling during repetitive synaptic stimulation. Profound release of glutamate with alpha-latrotoxin did cause a small, but reproducible, reduction in quantal size. These results indicate that decreasing the amount of glutamate and GABA in synaptic vesicles reduces quantal size. Furthermore, the probability of vesicle exocytosis appears to be entirely independent of the state of filling of the vesicle. However, even during high-frequency action potential-evoked release of glutamate, quantal size remained unchanged.

Recycling and refilling of transmitter quanta at the frog neuromuscular junction

1. Fluorescent dyes have been used at the frog neuromuscular junction to label synaptic vesicular membrane. Retrieved membrane is reformed into vesicles, which are released along with pre-existing vesicles. Consequently, if vesicular refilling with acetylcholine (ACh) is depressed by inhibitors, two sizes of quanta should be released: normal and smaller. As recycling continues the fraction of smaller size quanta should increase exponentially. 2. We enhanced the rate of quantal release by elevating the K+ concentration. The principal inhibitors were (-)-vesamicol (VES), hemicholinium-3 (HC3), and NH4+. Quantal size measurements were fitted to one and to two cumulative lognormal probability distribution functions. When two fitted better, the statistical significance assessment took into account the three additional parameters used in calculating the fit. 3. After recycling in the presence of inhibitor, many sets were fitted better by two lognormal functions. As recycling continued, the fraction of the miniature endplate potential voltage-time integrals ( MEPPs) in the larger sub-population decreased exponentially. 4. The size of the releasable pool was estimated by counting the quanta released by carbonyl cyanide m-chlorophenylhydrazone (CCCP). This was compared to pool sizes calculated from the inhibitor experiments. The two estimates of pool size were indistinguishable, with mean values ranging from about 170,000 to 270,000. 5. With all of the treatments tested, the means of the sizes in the smaller sub-population of MEPPs were about 1/3 those of the larger sub-populations. 6. Recycling synaptic vesicles appear to be incorporated into the releasable pool from which they have roughly the same probability of release as the pre-existing vesicles.

Transmitter release differs at snake twitch and tonic endplates during potassium-induced nerve terminal depolarization

Twitch and tonic muscle fibers of snake skeletal muscle differ in their synpatic as well as mechanical properties. These experiments were aimed at detemining the basis of the difference in vesicular release properties of nerve terminals at twitch and tonic endplates. Miniature endplate currents (MEPCs) were recorded from voltage-clamped garter snake muscle fibers depolarized by high K+ in either a control Ca2+ or high-Ca2+ solution. MEPC frequency increased at twitch and tonic endplates and remained elevated for 8 h during depolarization in control Ca2+. At twitch endplates depolarized in the presence of high Ca2+, an increase in MEPC frequency was followed by a progressive decline. In contrast, MEPC frequency remained elevated in high Ca2+ at tonic endplates. The observed decrease in MEPC frequency at depolarized twitch endplates in high Ca2+ was not a function of the level of depolarization or initial MEPC frequency, nor was it due to a reduction in MEPC amplitude and loss of MEPCs in baseline noise. An optical assay of presynaptic function in which the activity-dependent dye FM1-43 was used confirmed that quantal releases differs at twitch and tonic endplates. Most twitch nerve terminals were labeled by FM1-43 during prolonged depolarization with control Ca2+ or after brief depolarization with high Ca2+. In contrast, the number of twitch nerve terminals and the degree to which they were stained was greatly reduced after prolonged exposure to high K+ and high Ca2+, whereas depolarized tonic endplates were well stained by FM1-43 during brief and prolonged exposure to high Ca2+. FM1-43 staining also revealed variable levels of quantal release between individual boutons at twitch endplates after prolonged depolarization in high-Ca2+ solution. The observed reduction in presynaptic function at twitch nerve terminals after prolonged depolarization in high-Ca2+ solution was reversible and therefore not due to irreversible damage to terminal boutons. MEPC frequency increased at both twitch and tonic endplates when either Sr2+ or Ba2+ was substituted for high Ca2+ during K(+)-induced depolarization. Over time, in Sr2+ or Ba2+ solutions, MEPC frequency remained elevated at tonic endplates but declined at twitch endplates with a time course similar to that observed in high Ca2+. MEPC amplitudes at both endplates remained constant. We conclude that the regulation of quantal release differs in nerve terminals innervating twitch and tonic endplates and postulate that differential intraterminal accumulation of Ca2+ may underlie the observed difference in presynaptic function.

A method of estimating the rate of miniature end-plate potential occurrences based on parametric time series modeling

This paper presents a parametric method of estimating the rate of miniature end-plate potential (MEPP) occurrences. We consider the case where the rate of MEPP occurrences is raised by the constant deporalization of presynaptic terminals by using high-concentration potassium solutions. Under such conditions, since MEPP occurrences cannot be identified by eye due to waveform superposition, it is necessary to estimate the rate with the aid of statistical techniques instead of counting the occurrences by eye. In this paper it is assumed according to the literatures that the MEPP data are modeled as a stationary Poisson impulse process filtered by the linear system the impulse response function of which is the sum of two exponentials. Then, the discretized MEPP data are shown to be a second-order autoregressive (AR(2)) process, driven by the sum of 2 first-order moving average (MA(1)) processes (the residual time series). An explicit formula for estimating the rate can be derived by combining the second- and third-order moments of the residual time series. The validity of the proposed estimation method is verified through Monte Carlo simulations in which the rate is varied ranging from 100 to 10,000 s-1. Likewise, the proposed method is applied to estimation of the rate of actual MEPP data, which were observed at the frog's neuromuscular junction under high-concentration potassium solutions.

Methods for estimating release rates during high frequency quantal secretion and for testing such methods

The rate of spontaneous quantal release must be estimated in some investigations of synaptic transmission, even when frequencies are so high that individual quanta cannot be distinguished. An obvious method is to measure the time integrals of the summed MEPPs and then dividing this value by the integral of an average MEPP. The method was tested by recording miniature end-plate potentials (MEPPs) at frog neuromuscular junctions, counting the number of MEPPS, and then adding together records from the same junction to simulate high frequencies. The estimates from the integral method agreed well with the actual counts. The method can readily be used with a microcomputer and does not require stationary data. Methods based on fluctuation analysis were also used to estimate quantal frequencies, but they did not always give good estimates. This was not a thorough test of the fluctuation method, but an example of testing with MEPP data. The integral method might be reasonably reliable, but there are further potential complications, like changes in MEPP size and short-circuiting of the end-plate membrane, which may make it difficult to obtain reliable measurements of high frequency quantal secretion without voltage clamping and protocols that permit measurements of individual MEPCs during the course of the frequency measurements.

Effect of potassium propionate on free and bound acetylcholine in frog muscle

Frog sartorius muscles were homogenized under various conditions which allowed, by means of mass spectrometry, the measurement of total ACh, and different ACh compartments in the tissue: 'bound', 'free-1' and 'free-2' ACh. Bound ACh presumably corresponded to the vesicular compartment, and the free-1 and free-2 fractions to the cytoplasmic compartments of ACh. Stimulation of ACh release by La3+ ions for 60 min caused a decrease of both bound and free-2 ACh, but at 20 min bound ACh was reduced much more than free-2 ACh. Stimulation of ACh release by isotonic potassium propionate (KPr) solution for only 5 min caused a decrease of bound ACh, in contrast to free-1 and free-2 ACh which were not significantly changed. When muscles after 5 min stimulation in KPr were allowed to recover in normal Ringer, free-1 ACh did not change, but free-2 and bound ACh increased; after 180 min in Ringer bound ACh had recovered to control values. When ACh synthesis was prevented by hemicholinium-3 during recovery of the muscles in Ringer, bound ACh increased at the expense of free-2 ACh. In deuterium labeling experiments, in which the Ringer contained choline-d9, much more ACh-d9 was formed in stimulated than in unstimulated muscles. It appeared that almost all newly formed ACh was ACh-d9, since no significant synthesis of unlabeled ACh (ACh-d0) took place. Yet again, the amount of bound ACh-d0 significantly increased, apparently at the expense of preformed free-2 ACh-d0.(ABSTRACT TRUNCATED AT 250 WORDS)