Miniature endplate potential frequency and amplitude determined by an extension of Campbell's theorem

Reconstruction of intermittent time series as a superposition of pulses

Fluctuations in a vast range of physical systems can be described as a superposition of uncorrelated pulses with a fixed shape, a process commonly referred to as a (generalized) shot noise or a filtered Poisson process. In this paper, we present a systematic study of a deconvolution method to estimate the arrival times and amplitudes of the pulses from realizations of such processes. The method shows that a time series can be reconstructed for various pulse amplitude and waiting time distributions. Despite a constraint on positive-definite amplitudes, it is shown that negative amplitudes may also be reconstructed by flipping the sign of the time series. The method performs well under moderate amounts of additive noise, both white noise and colored noise having the same correlation function as the process itself. The estimation of pulse shapes from the power spectrum is accurate except for excessively broad waiting time distributions. Although the method assumes constant pulse durations, it performs well under narrowly distributed pulse durations. The most important constraint on the reconstruction is information loss, which limits the method to intermittent processes. The ratio between the sampling time and the average waiting time between pulses must be about 1/20 or smaller for a well-sampled signal. Finally, given the system forcing, the average pulse function may be recovered. This recovery is only weakly constrained by the intermittency of the process.

Structure and Function of the Mammalian Neuromuscular Junction

The mammalian neuromuscular junction (NMJ) comprises a presynaptic terminal, a postsynaptic receptor region on the muscle fiber (endplate), and the perisynaptic (terminal) Schwann cell. As with any synapse, the purpose of the NMJ is to transmit signals from the nervous system to muscle fibers. This neural control of muscle fibers is organized as motor units, which display distinct structural and functional phenotypes including differences in pre- and postsynaptic elements of NMJs. Motor units vary considerably in the frequency of their activation (both motor neuron discharge rate and duration/duty cycle), force generation, and susceptibility to fatigue. For earlier and more frequently recruited motor units, the structure and function of the activated NMJs must have high fidelity to ensure consistent activation and continued contractile response to sustain vital motor behaviors (e.g., breathing and postural balance). Similarly, for higher force less frequent behaviors (e.g., coughing and jumping), the structure and function of recruited NMJs must ensure short-term reliable activation but not activation sustained for a prolonged period in which fatigue may occur. The NMJ is highly plastic, changing structurally and functionally throughout the life span from embryonic development to old age. The NMJ also changes under pathological conditions including acute and chronic disease. Such neuroplasticity often varies across motor unit types. © 2022 American Physiological Society. Compr Physiol 12:1-36, 2022.

The Structural Basis of Long-Term Potentiation in Hippocampal Synapses, Revealed by Electron Microscopy Imaging of Lanthanum-Induced Synaptic Vesicle Recycling

Hippocampal neurons in dissociated cell cultures were exposed to the trivalent cation lanthanum for short periods (15–30 min) and prepared for electron microscopy (EM), to evaluate the stimulatory effects of this cation on synaptic ultrastructure. Not only were characteristic ultrastructural changes of exaggerated synaptic vesicle turnover seen within the presynapses of these cultures—including synaptic vesicle depletion and proliferation of vesicle-recycling structures—but the overall architecture of a large proportion of the synapses in the cultures was dramatically altered, due to large postsynaptic “bulges” or herniations into the presynapses. Moreover, in most cases, these postsynaptic herniations or protrusions produced by lanthanum were seen by EM to distort or break or “perforate” the so-called postsynaptic densities (PSDs) that harbor receptors and recognition molecules essential for synaptic function. These dramatic EM observations lead us to postulate that such PSD breakages or “perforations” could very possibly create essential substrates or “tags” for synaptic growth, simply by creating fragmented free edges around the PSDs, into which new receptors and recognition molecules could be recruited more easily, and thus, they could represent the physical substrate for the important synaptic growth process known as “long-term potentiation” (LTP). All of this was created simply in hippocampal dissociated cell cultures, and simply by pushing synaptic vesicle recycling way beyond its normal limits with the trivalent cation lanthanum, but we argued in this report that such fundamental changes in synaptic architecture—given that they can occur at all—could also occur at the extremes of normal neuronal activity, which are presumed to lead to learning and memory.

Acute Complexin Knockout Abates Spontaneous and Evoked Transmitter Release

SNARE-mediated synaptic vesicle (SV) fusion is controlled by multiple regulatory proteins that determine neurotransmitter release efficiency. Complexins are essential SNARE regulators whose mode of action is unclear, as available evidence indicates positive SV fusion facilitation and negative "fusion clamp"-like activities, with the latter occurring only in certain contexts. Because these contradictory findings likely originate in part from different experimental perturbation strategies, we attempted to resolve them by examining a conditional complexin-knockout mouse line as the most stringent genetic perturbation model available. We found that acute complexin loss after synaptogenesis in autaptic and mass-cultured hippocampal neurons reduces SV fusion probability and thus abates the rates of spontaneous, synchronous, asynchronous, and delayed transmitter release but does not affect SV priming or cause "unclamping" of spontaneous SV fusion. Thus, complexins act as facilitators of SV fusion but are dispensable for "fusion clamping" in mammalian forebrain neurons.

Level crossings and excess times due to a superposition of uncorrelated exponential pulses

A well-known stochastic model for intermittent fluctuations in physical systems is investigated. The model is given by a superposition of uncorrelated exponential pulses, and the degree of pulse overlap is interpreted as an intermittency parameter. Expressions for excess time statistics, that is, the rate of level crossings above a given threshold and the average time spent above the threshold, are derived from the joint distribution of the process and its derivative. Limits of both high and low intermittency are investigated and compared to previously known results. In the case of a strongly intermittent process, the distribution of times spent above threshold is obtained analytically. This expression is verified numerically, and the distribution of times above threshold is explored for other intermittency regimes. The numerical simulations compare favorably to known results for the distribution of times above the mean threshold for an Ornstein-Uhlenbeck process. This contribution generalizes the excess time statistics for the stochastic model, which find applications in a wide diversity of natural and technological systems.

Making quantal analysis more convenient, fast, and accurate: user-friendly software QUANTAN

Quantal analysis of synaptic transmission is an important tool for understanding the mechanisms of synaptic plasticity and synaptic regulation. Although several custom-made and commercial algorithms have been created for the analysis of spontaneous synaptic activity, software for the analysis of action potential evoked release remains very limited. The present paper describes a user-friendly software package QUANTAN which has been created to analyze electrical recordings of postsynaptic responses. The program package is written using Borland C++ under Windows platform. QUANTAN employs and compares several algorithms to extract the average quantal content of synaptic responses, including direct quantal counts, the analysis of synaptic amplitudes, and the analysis of integrated current traces. The integration of several methods in one user-friendly program package makes quantal analysis of action potential evoked release more reliable and accurate. To evaluate the variability in quantal content, QUANTAN performs deconvolution of the distributions of amplitudes or areas of synaptic responses employing a ridge regression method. Other capabilities of QUANTAN include the analysis of the time-course and stationarity of quantal release. In summary, QUANTAN uses digital records of synaptic responses as an input and computes the distribution of quantal content and synaptic parameters. QUANTAN is freely available to other scholars over the internet.

Adaptation of single photon responses in photoreceptors of the housefly, Musca domestica: a novel spectral analysis

The absorption of a photon by a photoreceptor triggers a small voltage fluctuation termed the 'bump'. Here, in the housefly, I introduce the bispectrum of photoreceptor noise to characterise the bump under dim light. The bispectrum provides explicit phase information and is not contaminated by Gaussian background noise. Over the photon rates examined (<10(4) s(-1)), I show that bumps are minimum-phase, noise spectra are little affected by natural variations in bump shape and bumps adapt such that amplitude is approximately proportional to duration squared. In the dark exists a 'dark event', which I suggest represents spontaneous activation of G-protein.

Transmission between type II hair cells and bouton afferents in the turtle posterior crista

Synaptic activity was recorded with sharp microelectrodes during rest and during 0.3-Hz sinusoidal stimulation from bouton afferents identified by their efferent-mediated inhibitory responses. A glutamate antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) decreased quantal size (qsize) while lowering external Ca(2+) decreased quantal rate (qrate). Miniature excitatory postsynaptic potentials (mEPSPs) had effective durations (qdur) of 3.5-5 ms. Their timing was consistent with Poisson statistics. Mean qsizes ranged in different units from 0.25 to 0.73 mV and mean qrates from 200 to 1,500/s; there was an inverse relation across the afferent population between qrate and qsize. qsize distributions were consistent with the independent release of variable-sized quanta. Channel noise, measured during AMPA-induced depolarizations, was small compared with quantal noise. Excitatory responses were larger than inhibitory responses. Peak qrates, which could approach 3,000/s, led peak excitatory mechanical stimulation by 40 degrees . Quantal parameters varied with stimulation phase with qdur and qsize being maximal during inhibitory stimulation. Voltage modulation (vmod) was in phase with qrate and had a peak depolarization of 1.5-3 mV. On average, 80% of vmod was accounted for by quantal activity; the remaining 20% was a nonquantal component that persisted in the absence of quantal activity. The extracellular accumulation of glutamate and K(+) are potential sources of nonquantal transmission and may provide a basis for the inverse relation between qrate and qsize. Comparison of the phases of synaptic and spike activity suggests that both presynaptic and postsynaptic mechanisms contribute to variations across afferents in the timing of spikes during sinusoidal stimulation.

How Retinal Ganglion Cells Prevent Synaptic Noise From Reaching the Spike Output

Synaptic vesicles are released stochastically, and therefore stimuli that increase a neuron's synaptic input might increase noise at its spike output. Indeed this appears true for neurons in primary visual cortex, where spike output variability increases with stimulus contrast. But in retinal ganglion cells, although intracellular recordings (with spikes blocked) showed that stronger stimuli increase membrane fluctuations, extracellular recordings showed that noise at the spike output is constant. Here we show that these seemingly paradoxical findings occur in the same cell and explain why. We made intracellular recordings from ganglion cells, in vitro, and presented periodic stimuli of various contrasts. For each stimulus cycle, we measured the response at the stimulus frequency (F1) for both membrane potential and spikes as well as the spike rate. The membrane and spike F1 response increased with contrast, but noise (SD) in the F1 responses and the spike rate was constant. We also measured membrane fluctuations (with spikes blocked) during the response depolarization and found that they did increase with contrast. However, increases in fluctuation amplitude were small relative to the depolarization (<10% at high contrast). A model based on estimated synaptic convergence, release rates, and membrane properties accounted for the relative magnitudes of fluctuations and depolarization. Furthermore, a cell's peak spike response preceded the peak depolarization, and therefore fluctuation amplitude peaked as the spike response declined. We conclude that two extremely general properties of a neuron, synaptic convergence and spike generation, combine to minimize the effects of membrane fluctuations on spiking.

Estimation of nonuniform quantal parameters with multiple-probability fluctuation analysis: theory, application and limitations

Synapses are a key determinant of information processing in the central nervous system. Investigation of the mechanisms underlying synaptic transmission at central synapses is complicated by the inaccessibility of synaptic contacts and the fact that their temporal dynamics are governed by multiple parameters. Multiple-probability fluctuation analysis (MPFA) is a recently developed method for estimating quantal parameters from the variance and mean amplitude of evoked steady-state synaptic responses recorded under a range of release probability conditions. This article describes the theoretical basis and the underlying assumptions of MPFA, illustrating how a simplified multinomial model can be used to estimate mean quantal parameters at synapses where quantal size and release probability are nonuniform. Interpretations of the quantal parameter estimates are discussed in relation to uniquantal and multiquantal models of transmission. Practical aspects of this method are illustrated including a new method for estimating quantal size and variability, approaches for optimising data collection, error analysis and a method for identifying multivesicular release. The advantages and limitations of investigating synaptic function with MPFA are explored and contrasted with those for traditional quantal analysis and more recent optical quantal analysis methods.

Combining deconvolution and fluctuation analysis to determine quantal parameters and release rates

Analysis methods are described which integrate information from fluctuation analysis with that from deconvolution. Together the two approaches allow to derive a consistent quantitative description of quantal release (both evoked, spontaneous and asynchronous) under conditions in which quantal parameters may change during a repetitively applied stimulation protocol. Specifically, our methods take into account the effects of accumulating transmitter in the synaptic cleft and postsynaptic receptor desensitization, which may develop during strong stimulation. Several ways to handle non-stationarities are described. Examples are provided for the Calyx of Held, a glutamatergic synapse, in which both the pre- and the postsynaptic compartments can be voltage-clamped.

Estimating transmitter release rates from postsynaptic current fluctuations

A method is presented that allows one to estimate transmitter release rates from fluctuations of postsynaptic current records under conditions of stationary or slowly varying release. For experimental applications, we used the calyx of Held, a glutamatergic synapse, in which "residual current," i.e., current attributable to residual glutamate in the synaptic cleft, is present. For a characterization of synaptic transmission, several postsynaptic parameters, such as the mean amplitude of the miniature postsynaptic current and an apparent single channel conductance, have to be known. These were obtained by evaluating variance and two more higher moments of the current fluctuations. In agreement with Fesce et al. (1986), we found both by simulations and by analyzing experimental records that high-pass filtering of postsynaptic currents renders the estimates remarkably tolerant against nonstationarities. We also found that release rates and postsynaptic parameters can be reliably obtained when release rates are low ( approximately 10 events/msec). Furthermore, during a long-lasting stimulus, the transmitter release at the calyx of Held was found to decay to a low, stationary rate of 10 events/msec after depletion of the "releasable pool" of synaptic vesicles. This stationary release rate is compatible with the expected rate of recruitment of new vesicles to the release-ready pool of vesicles. MiniatureEPSC (mEPSC) size is estimated to be similar to the value of spontaneously occurring mEPSC under this condition.

Estimating transmitter release rates from postsynaptic current fluctuations

A method is presented that allows one to estimate transmitter release rates from fluctuations of postsynaptic current records under conditions of stationary or slowly varying release. For experimental applications, we used the calyx of Held, a glutamatergic synapse, in which "residual current," i.e., current attributable to residual glutamate in the synaptic cleft, is present. For a characterization of synaptic transmission, several postsynaptic parameters, such as the mean amplitude of the miniature postsynaptic current and an apparent single channel conductance, have to be known. These were obtained by evaluating variance and two more higher moments of the current fluctuations. In agreement with Fesce et al. (1986), we found both by simulations and by analyzing experimental records that high-pass filtering of postsynaptic currents renders the estimates remarkably tolerant against nonstationarities. We also found that release rates and postsynaptic parameters can be reliably obtained when release rates are low ( approximately 10 events/msec). Furthermore, during a long-lasting stimulus, the transmitter release at the calyx of Held was found to decay to a low, stationary rate of 10 events/msec after depletion of the "releasable pool" of synaptic vesicles. This stationary release rate is compatible with the expected rate of recruitment of new vesicles to the release-ready pool of vesicles. MiniatureEPSC (mEPSC) size is estimated to be similar to the value of spontaneously occurring mEPSC under this condition.

Analysis of spontaneous EPSCs in retinal horizontal cells of the carp

Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded under Whole-cell voltage clamp from carp type 1 horizontal cells (H1 cells) uncoupled by dopamine in retinal slices. Red light steps, which hyperpolarise cones and reduce glutamate release, induced outward current responses accompanied by a suppression of sEPSCs. sEPSCs decayed exponentially with a mean time constant of 0.71+/-0.07 ms and had a reversal potential near 0 mV. Power spectral analysis of sEPSCs revealed a similar decay time constant. They were suppressed by a non-NMDA receptor antagonist, CNQX at 10 microM, and a relatively specific AMPA receptor antagonist, GYKI52466 at 20 microM. The presence of sEPSCs suggests that the release of glutamate from cone synaptic terminals is vesicular. The reduction in mean sEPSC frequency with red light was not accompanied by a significant change in the mean sEPSC conductance increase (482+/-59 pS), suggesting that a decrease in the vesicular release rate from cones does not alter the vesicular glutamate concentration (quantal contents). The results suggest that the spontaneous events in H1 cells were contributed by non-NMDA (possibly AMPA) type glutamate receptors modulated by the red cone input.

Combining deconvolution and noise analysis for the estimation of transmitter release rates at the calyx of held

The deconvolution method has been used in the past to estimate release rates of synaptic vesicles, but it cannot be applied to synapses where nonlinear interactions of quanta occur. We have extended this method to take into account a nonlinear current component resulting from the delayed clearance of glutamate from the synaptic cleft. We applied it to the calyx of Held and verified the important assumption of constant miniature EPSC (mEPSC) size by combining deconvolution with a variant of nonstationary fluctuation analysis. We found that amplitudes of mEPSCs decreased strongly after extended synaptic activity. Cyclothiazide (CTZ), an inhibitor of glutamate receptor desensitization, eliminated this reduction, suggesting that postsynaptic receptor desensitization occurs during strong synaptic activity at the calyx of Held. Constant mEPSC sizes could be obtained in the presence of CTZ and kynurenic acid (Kyn), a low-affinity blocker of AMPA-receptor channels. CTZ and Kyn prevented postsynaptic receptor desensitization and saturation and also minimized voltage-clamp errors. Therefore, we conclude that in the presence of these drugs, release rates at the calyx of Held can be reliably estimated over a wide range of conditions. Moreover, the method presented should provide a convenient way to study the kinetics of transmitter release at other synapses.

Combining deconvolution and noise analysis for the estimation of transmitter release rates at the calyx of held

The deconvolution method has been used in the past to estimate release rates of synaptic vesicles, but it cannot be applied to synapses where nonlinear interactions of quanta occur. We have extended this method to take into account a nonlinear current component resulting from the delayed clearance of glutamate from the synaptic cleft. We applied it to the calyx of Held and verified the important assumption of constant miniature EPSC (mEPSC) size by combining deconvolution with a variant of nonstationary fluctuation analysis. We found that amplitudes of mEPSCs decreased strongly after extended synaptic activity. Cyclothiazide (CTZ), an inhibitor of glutamate receptor desensitization, eliminated this reduction, suggesting that postsynaptic receptor desensitization occurs during strong synaptic activity at the calyx of Held. Constant mEPSC sizes could be obtained in the presence of CTZ and kynurenic acid (Kyn), a low-affinity blocker of AMPA-receptor channels. CTZ and Kyn prevented postsynaptic receptor desensitization and saturation and also minimized voltage-clamp errors. Therefore, we conclude that in the presence of these drugs, release rates at the calyx of Held can be reliably estimated over a wide range of conditions. Moreover, the method presented should provide a convenient way to study the kinetics of transmitter release at other synapses.

Hypertonic treatment reversibly increases the ratio of giant skew-miniature endplate potentials to bell-miniature endplate potentials

Miniature endplate potentials were recorded from single frog muscle fibers before, during and after treatment with hypertonic saline (200-500 mM NaCl or Na gluconate added to frog saline). Miniature endplate potential amplitude distributions were plotted from small muscle fibers so that the modes and ratios of the skew-miniature endplate potential to bell-miniature endplate potential classes could be defined. Muscle fibers were voltage clamped with two electrodes to determine the input resistance before, during and after treatment. Input resistance increased from two to 100 times during treatment and rapidly fell towards control values (no more than 30% greater) when preparations were returned to normal frog saline. Short duration treatments with 200-300 mM hypertonic salines immediately increased frequencies (100-fold) of both skew-miniature endplate potential and bell-miniature endplate potential classes. Preparations when returned to normal frog saline after a few minutes of treatment showed control miniature endplate potential distributions within minutes. One to two hour treatments left only the skew-miniature endplate potential class and with hour-long recovery periods bell-miniature endplate potentials reappeared and ratios of skew-miniature endplate potential to bell-miniature endplate potential classes returned to control values. Treatment with 500 mM NaCl added to frog saline immediately increased the percentage of skew-miniature endplate potentials (from 2 to 50%) with little or no increase in overall miniature endplate potential frequencies. The mode of the skew-miniature endplate potential class was unchanged after hypertonic treatment, whereas that of the bell-miniature end plate potential class either remained about the same size or decreased depending on the duration of treatment. The number and percentage of giant-miniature endplate potentials belonging to the skew-miniature endplate potential class increased as a function of the duration of 200-300 mM hypertonic saline treatments. Most giant-miniature endplate potentials had a slow rising phase with a foot and/or breaks demonstrating a composite structure. Sequentially recorded giant-miniature endplate potentials had similar initial slopes indicating either repetitive releases from single sites or releases from cooperative sites. After hypertonic treatment the bell-miniature endplate potential size was never more than that expected with the increase (under 30%) in input resistance. The results presented here are completely different from those of Yu and Van der Kloot [(1991) J. Physiol. 433, 677-704] who reported that the bell-miniature endplate potential amplitude was increased two- to four-fold after hypertonic treatment. The wide range of results in the ratio of skew-miniature endplate potential to bell-miniature endplate potential classes is discussed in regards to the quantal hypothesis which is based on a single class of immutable amounts of transmitter; and, a hypothesis based on a dynamical process that meters transmitter in subunit amounts to control miniature endplate potential size and class during release.

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.

Alpha-latrotoxin channels in neuroblastoma cells

The changes in ionic permeability induced by the application of alpha-latrotoxin to NG108-15 neuroblastoma x glioma cells were examined using the nystatin perforated-patch technique for whole-cell recording. Complex single channel activity appeared in the plasmalemmas after delays that ranged from 1-20 min in Krebs' solution. The conductance of a channel fluctuated among at least three broad, approximately equispaced bands, the maximum conductance being about 300 pS, and the reversal potential approximately 0 mV. The channels were permeable to Na+, K+, Ca2+ and Mg2+, poorly permeable to glucosamineH+ and Cl-, and were blocked by La3+. The channels stayed fully open in Ca(2+)-free solutions with 4 mM Mg2+, in solutions with no divalent cations and in solutions with 2 mM Ca2+ and 96 mM Mg2+. They opened infrequently if both internal and external Cl- were replaced by glutamate-. If alpha-latrotoxin opened similar channels in nerve terminals, the flux of ions through them could account for the massive release of neurotransmitter induced by the toxin.

Quantal behaviour of synaptic transmission can be statistically examined using the Fourier line spectrum of the histogram of synaptic potentials

A statistical approach to reveal the quantal behaviour of postsynaptic potentials (PSPs) is described. This includes: (1) obtaining the Fourier line spectrum (decomposition into harmonics) of the PSP histogram; and (2) testing the null hypothesis, 'the spectrum is that of white noise', using an ANOVA. The harmonic that rejects the hypothesis will indicate the regular peaks in the histogram, i.e., the quantal size of PSPs. The method was tested using published results of three experimental studies in central synapses and yielded the quantal sizes close to those derived from other methods. However, using three examples of published simulation studies (where the quantal model of synaptic transmission was known a priori), it was shown that the approach can estimate quantal sizes of PSPs more reliably than other methods.

Synapsin I partially dissociates from synaptic vesicles during exocytosis induced by electrical stimulation

The distribution of the synaptic vesicle-associated phosphoprotein synapsin I after electrical stimulation of the frog neuromuscular junction was investigated by immunogold labeling and compared with the distribution of the integral synaptic vesicle protein synaptophysin. In resting terminals both proteins were localized exclusively on synaptic vesicles. In stimulated terminals they appeared also in the axolemma and its infoldings, which however exhibited a lower synapsin I/synaptophysin ratio with respect to synaptic vesicles at rest. The value of this ratio was intermediate in synaptic vesicles of stimulated terminals, and an increased synapsin I labeling of the cytomatrix was observed. These results indicate that synapsin I undergoes partial dissociation from and reassociation with synaptic vesicles, following physiological stimulation, and are consistent with the proposed modulatory role of the protein in neurotransmitter release.

Modulation of decay kinetics and frequency of GABAA receptor-mediated spontaneous inhibitory postsynaptic currents in hippocampal neurons

Inhibitory postsynaptic currents mediated by spontaneous activation of GABAA receptors were studied using whole-cell voltage-clamp recordings in granule cells of the adult rat (postnatal day 60+) dentate gyrus in 400-microns-thick coronal half-brain slices maintained at 34-35 degrees C. The average amplitude of spontaneous inhibitory postsynaptic currents remained constant during a given recording period (i.e. no rundown was noted). The spontaneous currents had an average conductance between 200-400 pS, were mediated by Cl- flux through GABAA receptor/channels since they reversed at the Cl- equilibrium potential and were blocked by bicuculline or picrotoxin. Their mono-exponential decay time-constants (range: 4.2-7.2 ms) were prolonged by midazolam and pentobarbital in a dose-dependent manner. The effect of midazolam was reversed by the benzodiazepine receptor antagonist flumazenil (RO 15-1788) which, by itself, had no effect on the decay time-constant. The decay time-constant was also dependent on membrane voltage and on temperature. A 132-mV change in membrane potential produced an e-fold prolongation of the decay while the Q10 (between 22-37 degrees C) of the decay rate was 2.1. Within a given neuron, the frequency of spontaneous GABAergic events was remarkably constant over long time-periods, though the mean frequency among different cells showed large variability. Spontaneous miniature inhibitory postsynaptic currents also persisted under experimental conditions such as the presence of extracellular tetrodotoxin (1 microM), Cd2+ (200 microM) or lowered extracellular Ca2+/elevated Mg2+, which effectively abolished all stimulus-evoked GABAergic neurotransmission. The frequency of tetrodotoxin-resistant miniature events was increased by elevating extracellular K+ concentration and was diminished by the GABAB receptor agonist (-)baclofen only at a dose (50 microM) which was an order of magnitude larger than that required to depress stimulus-evoked responses. These findings are consistent with different mechanisms being responsible for the spontaneous and stimulus-evoked release of GABA from interneuron terminals and also identify pre- and postsynaptic modulatory factors of the endogenous, action-potential-independent, GABAergic neurotransmission as being important determinants of the excitability level of mammalian CNS neurons.

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.

Correlation between quantal secretion and vesicle loss at the frog neuromuscular junction

1. We measured the rate of occurrence of miniature endplate potentials (MEPPs) at identified endplates in frog cutaneous pectoris muscles treated with crude black widow spider venom (BWSV) or purified alpha-latrotoxin (alpha-LTX) in calcium-free solutions, and we examined the relationship between the length of the nerve terminal and the total number of quanta secreted, and the relationship between the number of quanta secreted and the number of vesicles remaining at different times. 2. The venom, or toxin, was applied in a modified Ringer solution with tetrodotoxin, 1 mM-EGTA and no divalent cations, and quantal secretion was started by applying Ca2(+)-free solutions with Mg2+. This was done to synchronize the quantal discharge at the various junctions in a muscle. Ringer solution was applied after the MEPP rate had declined to low levels, and then the muscle fibre was injected with Lucifer Yellow, the endplate stained for acetylcholinesterase and the length of the nerve terminal and the length of a sarcomere were measured on the fluorescent fibre. 3. The total number of quanta secreted by a terminal was measured under a wide variety of experimental conditions: the weights of the frogs ranged from 13 to 68 g, the temperature from 9 to 28 degrees C, and the concentration of Mg2+ from 2 to 10 mM. In one series of experiments the Mg2+ was withdrawn after 3-4 min and reapplied 35-40 min later in order to divide the total output of quanta into two approximately equal bouts of secretion that were well separated in time. 4. The total number of MEPPs recorded at a junction was loosely correlated with the length of its nerve terminal, but it was not affected by the temperature, the concentration of Mg2+ or the division of secretion into well-separated bouts of quantal release. The average total secretion per unit length was about 3700 quanta/sarcomere or about 1200 quanta/microns. 5. The average time course of quantal secretion per micrometre of terminal was determined at single junctions in muscles held at 22-23 degrees C or at 9-10 degrees C. Other muscles were fixed at various times during the course of secretion at each temperature and the number of synaptic vesicles remaining in cross-sections of the terminals were counted on electron micrographs. The number of vesicles remaining per micrometre of terminal was determined from the number per cross-section and the section thickness.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of the actions of botulinum neurotoxin type E at the rat neuromuscular junction

Botulinum neurotoxin (BoTx) serotype E blocks spontaneous and evoked quantal release of acetylcholine at the rat neuromuscular junction. Increasing extracellular Ca2+ to 8 mmol l-1 or substituting Ca2+ with La3+ (0.1 and 1.0 mmol l-1) or depolarizing the nerve terminals by 20 mmol l-1 K+ markedly increases miniature end-plate potential frequency in normal muscle, but in BoTx-E poisoned preparations none of these ions, with the exception of 1 mmol l-1 La3+, was able to restore spontaneous quantal transmitter release to levels recorded at unpoisoned junctions. In absolute values the enhancement with La3+ was much less than that reported at normal junctions. Nerve stimulation in the presence of 3,4-diaminopyridine (10-20 mumol l-1) and high calcium (8 mmol l-1) evoked multiquantal end-plate potentials and muscle twitches. We conclude that the neuromuscular block produced by BoTx serotype E is similar to that previously described for BoTx serotype A but differs from that produced by BoTx serotypes B, D and F in not causing desynchronization of nerve impulse-evoked transmitter release. 3,4-Diaminopyridine might be useful in the treatment of poisoning by BoTx serotype E since it markedly enhanced synchronous transmitter release from poisoned motor nerve terminals.

A re-examination of the effects of lanthanum on the frog neuromuscular junction

Lanthanum (La3+, 0.1-2mM) was applied to frog cutaneous pectoris muscles at 20-25 degrees C, or at 3-5 degrees C, and the mean amplitude and rate of occurrence, mean value of r, of the miniature endplate potentials (mepps) were measured as functions of time at single neuromuscular junctions. Some muscles were fixed at 3-5 degrees C and their nerve terminals examined in the electron microscope. When 1 or 2 mM La3+ was applied at room temperature, mean value of r rose to peak values of 0.8-3.4 x 10(3) and then declined to less than 100/s after 30-60 min. When the results are corrected for the dispersion in mepp amplitudes, we estimate that approximately 1.8 x 10(6) mepps occurred in this time. If 0.1 mM La3+ was applied, or if 1 mM La3+ was removed when mean value of r was near its peak, then mean value of r remained high for at least 1 h and approximately 4 x 10(6) mepps occurred. All these mepp counts exceed the 0.7 x 10(6) quanta stored in resting nerve terminals. When 1 or 2 mM La3+ was applied at 3-5 degrees C, mean value of r rose to peak values of 50-700/s and then fell to 20-200/s after 20-30 min. If the La3+ was then removed, mean value of r declined approximately 50% over the next hour; approximately 0.7 x 10(6) mepps occurred at the junctions treated with 1 mM La3+, and their terminals still contained about 69% of their vesicles. Thus, vesicles can be recycled at 3-5 degrees C. Millimolar concentrations of La3+ reduced the mepp amplitude by 70-80% at both temperatures and abolished almost completely the depolarization produced by bath applied acetylcholine or carbachol. These effects were reversible.

Quantal stores of excitatory transmitter in nerve-muscle synapses of crayfish evaluated from high-frequency asynchronous quantal release induced by veratridine or high concentrations of potassium

At single voltage-clamped opener muscle fibres of crayfish claw, 10-100 mumol/l veratridine increased within a few seconds the rate of asynchronous quantal release, ñ, of excitatory transmitter from ñ less than 1 quantum/s to ñ congruent to 10,000 quanta/s. Thereafter ñ declined exponentially either with a single, tau(2) congruent to 50 s, or with two time constants tau(1) congruent to 19 s, tau(2) congruent to 50 s. In total (t----infinity), about 0.3 million quanta were released by veratridine in a single short fibre of about 1 mm length. These values were estimated by means of the noise analysis technique and they agreed with equivalent parameters of release when 100 mmol/l K+ were used as release stimulus. Strong quantal release could be elicited only once in a single muscle by veratridine. Furthermore, the effect of veratridine on quantal release could be completely prevented by pretreatment with tetrodotoxin. In another nerve-muscle preparation of crayfish, the abdominal superficial extensor muscle, up to 3 million excitatory quanta could be released by veratridine in a single fibre. In the latter muscle veratridine-induced asynchronous quantal release was strongly dependent on the extracellular concentration of Ca2+ whereas in the claw opener dependence of quantal release on extracellular Ca2+ was negligible.

Effects of cadmium on quantal transmitter release and ultrastructure of frog motor nerve endings

Exposure of frog cutaneous pectoris nerve-muscle preparations to cadmium (0.1-1 mM) results in an increase in miniature end-plate potential (m.e.p.p.) frequency. The increase is dependent on the concentration, the time of exposure and the co-presence of other divalent cations in the extracellular fluid. The stimulatory effect of cadmium is most marked in a calcium-free medium. Increased levels of calcium (4-10 mM) or of magnesium (10 mM) reduce the stimulatory effect suggesting that those cations interfere with the entry of cadmium into nerve endings. Once the effect of cadmium on m.e.p.p. frequency is attained, washing with a cadmium-free solution fails to abolish its effect. The action of cadmium on m.e.p.p. frequency slowly declines towards zero after about 3 hrs. An ultrastructural study of nerve terminals exposed for one hr to 1 mM cadmium reveals that neither in calcium-containing nor in a nominally calcium-free medium are there any significant changes in the number of synaptic vesicles as compared to controls. However, after 3 hrs of cadmium action in a calcium-free medium there is about 65% depletion of synaptic vesicles, while in calcium-containing media there is only about 25% depletion. The results suggest that cadmium by itself can support transmitter release but not synaptic vesicle recycling which instead might depend upon calcium.

Reappearance of miniature endplate potentials in frog neuromuscular junctions "silenced" by lanthanum ions

Miniature endplate potentials and currents were recorded from frog sartorius muscle fibres. As expected from previous work, La3+ caused a very large increase in miniature endplate potential frequency, followed by a progressive decrease, ending in the virtually complete disappearance of miniature endplate potentials. Soon after its application, La3+ caused an increase in the amplitude of miniature endplate potentials due to a lengthening of the underlying miniature endplate currents. Analysis of membrane current fluctuations produced by acetylcholine showed that La3+ caused an increase in the lifetime of the channels opened by acetylcholine. After miniature endplate potentials had vanished following La3+ treatment, the muscles were placed in organ culture conditions to see if miniature endplate potentials would reappear. After several hours miniature endplate potential activity was resumed in about 25% of the endplates, and disappeared again a few hours later, presumably because of accelerated nerve terminal degeneration. Preliminary electron microscopic observations suggest that the recovery of miniature endplate potential activity was accompanied by re-formation of synaptic vesicles in the motor endings.

Reversible effect of depolarization by K-propionate on sub-miniature endplate potential to bell-miniature endplate potential ratios, on miniature endplate potential frequencies and amplitudes, and on synaptic vesicle diameters and densities in frog neuromuscular junctions

Miniature endplate potentials were recorded from edge muscle fibers of frog sartorius muscles during high frequencies induced with K-propionate and during recovery. The identified neuromuscular junctions were studied with the electron microscope and their ultrastructure was correlated with amplitude and numbers of miniature endplate potentials generated. Miniature endplate potential amplitudes were maintained during the first 10 min of depolarization. They then decreased during the next 2-3 h until the mode was lost to the noise. Miniature endplate potential frequency was greatly increased during the first hour and there was initial depletion of vesicles. Miniature endplate potential frequencies remained high (5 x 10(5)/h) for 3 h but vesicle densities returned to nearly normal values during the second to third hour of treatment. The conspicuous infolding of the presynaptic membrane noted during the first hour of treatment suggests that recycling of vesicles is initially slower than fusion. Calculated recycling time is shorter than 25 min. During recovery after prolonged K-propionate treatment, the sub-miniature endplate potential class reappeared within minutes but about 20 min were required before it returned to control size. Subsequently, the bell-miniature endplate potentials reappeared and slowly increased in amplitude. The ultrastructure returned to a normal state. There was no change in vesicle diameters. No significant difference was found between the diameters of "touching vesicles" (vesicles touching the presynaptic membrane) and the non-touching vesicles. By comparison, lanthanum ions (1 mM) released a smaller number of quanta which did not exceed the number of vesicles present at the start of the experiment. Variations of the subunit hypothesis of the quantum of transmitter release are discussed.

Synaptophysin (p38) at the frog neuromuscular junction: its incorporation into the axolemma and recycling after intense quantal secretion

Recycling of synaptophysin (p38), a synaptic vesicle integral membrane protein, was studied by the use of antisera raised against the protein purified from frog brain. When frog cutaneous pectoris muscles were fixed at rest, a bright, specific immunofluorescent signal was observed in nerve-terminal regions only if their plasma membranes had been previously permeabilized. When muscles were fixed after they had been treated for 1 h with a low dose of alpha-latrotoxin in Ca2+-free medium, an equally intense fluorescence could be observed without previous permeabilization. Under this condition, alpha-latrotoxin depletes nerve terminals of their quantal store of acetylcholine and of synaptic vesicles. These results indicate that fusion of synaptic vesicles leads to the exposure of intravesicular antigenic determinants of synaptophysin on the outer surface of the axolemma, and provide direct support for the vesicle hypothesis of neurotransmitter release. After 1 h treatment with the same dose of alpha-latrotoxin in the presence of 1.8 mM extracellular Ca2+, immunofluorescent images were obtained only after permeabilization with detergents. Under this condition, the vesicle population was maintained by an active process of recycling and more than two times the initial store of quanta were secreted. Thus, despite the active turnover of synaptic vesicles and of quanta of neurotransmitter, no extensive intermixing occurs between components of the vesicle and presynaptic plasma membrane.

Structural and functional correlates of synaptic transmission in the vertebrate neuromuscular junction

Because vertebrate neuromuscular junctions are readily accessible for experimental manipulation, they have provided a superb model in which to examine and test functional correlates of chemical synaptic transmission. In the neuromuscular synapse, acetylcholine receptors have been localized to the crests of the junctional folds and visualized by a variety of ultrastructural techniques. By using ultrarapid freezing techniques with a temporal resolution of less than 1 msec, quantal transmitter release has been correlated with synaptic vesicle exocytosis at discrete sites called "active zones." Mechanisms for synaptic vesicle membrane retrieval and recycling have been identified by using immunological approaches and correlated with endocytosis via coated pits and coated vesicles. In this review, available ultrastructural, physiological, immunological, and biochemical data have been used to construct an ultrastructural model of neuromuscular synaptic transmission that correlates structure and function at the molecular level.

Differential effect of alpha-latrotoxin on exocytosis from small synaptic vesicles and from large dense-core vesicles containing calcitonin gene-related peptide at the frog neuromuscular junction

The regulatory peptide called calcitonin gene-related peptide (CGRP) was detected by immunofluorescence in frog motor neurons and motor nerve terminals. In motor nerve terminals, CGRP-like immunoreactivity was found to be segregated within large dense-core vesicles. To determine whether exocytosis from acetylcholine-containing small synaptic vesicles and from CGRP-containing large dense-core vesicles can be independently stimulated, nerve-muscle preparations were exposed to alpha-latrotoxin. This toxin induced complete depletion of acetylcholine-containing small synaptic vesicles but did not induce a parallel depletion of CGRP-like immunoreactivity and of large dense-core vesicles. These effects were independent of the presence of extracellular Ca2+ and occurred both at room temperature and at low temperature (1-3 degrees C). These findings suggest that exocytosis from the two vesicle populations is mediated by distinct biochemical mechanisms, which might be differentially regulated by physiological stimuli.

Effect of alpha-latrotoxin on the frog neuromuscular junction at low temperature

1. alpha-Latrotoxin (alpha-LTx) was applied to frog cutaneous pectoris muscles bathed at 1-3 degrees C in either Ringer solution, Ca2+-free Ringer solution with 1 mM-EGTA and 4 mM-Mg2+ or Ringer solution plus 4 mM-Mg2+, and its effects on miniature end-plate potential (MEPP) frequency, nerve terminal ultrastructure and uptake of horseradish peroxidase (HRP) were studied. 2. Large concentrations (2 micrograms/ml) of alpha-LTx increased MEPP rates to levels above 100/s at all junctions, but the time course of the increases depended upon the divalent cation content of the bathing solution. However, similar numbers of MEPPs (0.3-0.7 x 10(6] were recorded at all junctions during 2 h of secretion. 3. Nerve terminals exposed to alpha-LTx for 2 h lost 60-75% of their synaptic vesicles and were swollen; their presynaptic membranes were deeply infolded and they often contained many large vesicular structures. Terminals in Ringer solution retained the largest number of synaptic vesicles; terminals in Ringer solution plus Mg2+ swelled the least and contained the largest number of coated vesicles. The average number of synaptic vesicles lost was approximately equal to the average number of MEPPs recorded. 4. Few vesicles became loaded with HRP when this extracellular tracer was present in the bathing solution and the muscles were fixed near the peak of secretion. 5. When the terminals were warmed to 20 degrees C, those in the Ca2+-free solution with Mg2+ secreted additional quanta and lost almost all their residual vesicles; those in Ringer solution without Mg2+ secreted few additional quanta and retained most of their residual vesicles. 6. These results suggest that recycling was blocked at these terminals and that for each quantum secreted a vesicle became permanently incorporated into the axolemma.

The effect of potassium on exocytosis of transmitter at the frog neuromuscular junction

1. Electrophysiology and morphology have been combined to investigate the time course of the exocytosis of quanta of neurotransmitter induced by elevated concentrations of K+ at the frog neuromuscular junction. 2. Replicas of freeze-fractured resting nerve terminals fixed in the presence of 20 mM-K+ showed images of fusion of synaptic vesicles with the presynaptic axolemma which were closely associated with the active zones. After 1 min in 20 nM-K+ fusions appeared also outside the active zones, and by 5 min they became uniformly distributed over the presynaptic membrane. 3. The average total density of fusions was not significantly different at the various times examined since it decreased at the active zones while it increased over the rest of the membrane. 4. Resting terminals fixed in 20 mM-K+ released 33,000-45,000 quanta after the addition of fixative; terminals stimulated by 20 mM-K+ for 1-5 min released 50,000-100,000 quanta during fixation. The fixative potentiated K+-induced transmitter release. 5. Fusions were uniformly distributed in terminals pre-incubated for 5 min in 20 mM-K+ without added Ca2+, stimulated by adding Ca2+ for 30 s, and then fixed. Conversely, after 5 min stimulation in hypertonic Ringer solution fusions remained predominantly located near the active zones. A similar distribution was observed after 15 min stimulation by a lower concentration of K+ (15 mM). 6. At all concentrations of K+ tested (10, 15, 20, 25 mM) miniature end-plate potential (MEPP) rate attained a steady-state value within 10-15 min. Values from a single junction were generally lower at higher concentrations of K+, which indicates partial inactivation of the secretion-recycling process. 7. The data indicate that K+ initially activates exocytosis at the active zones. Subsequently, ectopic exocytosis is activated while sites at the active zones appear to undergo partial inactivation. These phenomena are not related to the intensity or to the amount of previous secretion.

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

1. 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 10(4) and 0.1-1 x 10(4) 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.

Veratridine-induced high-frequency asynchronous release of inhibitory transmitter quanta in crayfish nerve-muscle synapses superfused with normal and low-calcium saline

Crayfish fibres of opener muscles were voltage clamped to E = -80 mV membrane potential (T = 19-22 degrees C), and veratridine (10-100 mumol/l) was added to the superfusate. Within 30-60 s this caused large fluctuations of the clamp current due to vigorous asynchronous quantal release from the inhibitory nerve terminals along the muscle fibre. Excitatory postsynaptic receptors were previously desensitized by application of 5 mmol/l glutamate. Current fluctuations were evaluated by means of the noise analysis technique. Typically, 100 mumol/l veratridine increased instantaneously the quantal release rate n from n less than 1 quantum/s to n congruent to 10,000 quanta/s. Thereafter, n declined exponentially with a time constant of congruent to 70 s. On average, about 500,000 inhibitory quanta could be liberated in this way from the terminals on a single muscle fibre of congruent to 1 mm length. Serotonin (1 mumol/l) facilitated the effect of lower veratridine concentrations (1-10 mumol/l). In opener muscles veratridine-induced asynchronous quantal release showed little dependence on the bath concentration of Ca2+. The opposite was found for fibres of the superficial abdominal extensor muscle. Beside postsynaptic current fluctuations, veratridine elicited slowly changing average postsynaptic DC-currents which could be explained partly by superposition of individual inhibitory quantal currents. These DC-currents suggest that beside inhibitory quantal release another factor activates inhibitory postsynaptic receptors after application of veratridine.

Localization of synapsin I at the frog neuromuscular junction

We report here the results of immunocytochemical and biochemical studies on the localization of synapsin I, a nerve terminal--specific phosphoprotein, at the frog neuromuscular junction. Our results show that in this in situ synapse synapsin I is concentrated in the presynaptic compartment, where it appears to be associated with the synaptic vesicle membrane. Double immunoprecipitated synapsin I from homogenates of frog cutaneous pectoris muscles could be phosphorylated by the catalytic subunit of cyclic adenosine 5'-monophosphate-dependent protein kinase after gel electrophoresis and blotting onto nitrocellulose and could be subsequently identified by an immunoperoxidase technique. Experiments carried out in frog brain preparations indicate that frog synapsin I, like the mammalian protein, can be phosphorylated at different sites by exogenously added catalytic subunit of cyclic adenosine 5'-monophosphate-dependent protein kinase and Ca2+/calmodulin-dependent protein kinase II prepared from mammalian sources. The phosphorylation sites of frog synapsin I, as judged by phosphopeptide mapping, are somewhat different from those of mammalian synapsin I. The study of synapsin I and of the regulation of its state of phosphorylation at the neuromuscular junction may provide important information on its role in synaptic function, since at the present time this is one of the few systems in which a correlation among biochemical, immunocytochemical and electrophysiological results is possible.

Effects of carbonyl cyanide m-chlorophenylhydrazone on quantal transmitter release and ultrastructure of frog motor nerve terminals

The quantal acetylcholine release and the ultrastructural effects of the metabolic inhibitor carbonyl cyanide m-chlorophenylhydrazone have been examined at frog neuromuscular junctions. Carbonyl cyanide m-chlorophenylhydrazone (2 microM) caused a temperature-dependent block of evoked quantal transmitter release accompanied by an increase in the rate of spontaneous quantal release. The carbonyl cyanide m-chlorophenylhydrazone-induced increase in miniature endplate potential frequency was neither antagonized nor prevented by tetrodotoxin. It also occurred in a Ca2+-free medium and after replacement of Ca2+ by Sr2+, indicating that it does not depend upon a Na+ or Ca2+ influx from the external medium but may act by releasing Ca2+ from intraterminal stores. Spontaneous quantal transmitter release was exhausted irreversibly within 4 h of carbonyl cyanide m-chlorophenylhydrazone (2 microM) action, during which time an average of 4.7 x 10(5) acetylcholine quanta were released per junction. The morphologic analysis revealed a significant temperature and time-dependent reduction in the number of synaptic vesicles with swelling and dispersion of mitochondria within the motor nerve terminals. Changes in synaptic vesicle number appear to be directly related to the intensity of transmitter release. The good correlation observed between the number of quanta secreted and the number of vesicles lost by nerve terminals in the absence of vesicle recycling provides an estimate of the initial store of transmitter quanta.

Inhibitory effect of intraterminal lithium on asynchronous release of excitatory quanta induced by veratridine in nerve-muscle synapses of crayfish

Crayfish muscle fibres were voltage-clamped at E = -80 mV membrane potential and superfused for about 10 min with Li+ saline (Na+ replaced by Li+) which contained picrotoxin to block inhibitory post-synaptic currents. Addition of veratridine (100 mumol/l) caused intense fluctuations in the voltage clamp current within 20-60 s due to vigorous asynchronous quantal release of excitatory transmitter from the nerve terminals distributed over the muscle fibre surface. Most likely, this quantal release resulted from loading the nerve terminals with Li+ via voltage-gated Na+ channels activated by veratridine. However, in the presence of Li+ quantal release was only transient; the quantal release rate, ñ, attained a maximum of congruent to 10,000 quanta/s and then declined exponentially with tau congruent to 10 to 20 s. Removal of Li+ and reapplication of normal Na+ increased ñ a second time. The amount of quanta released in the presence of Na+ was about an order of magnitude larger than that released previously in the presence of Li+. In preparations pretreated with Li+ superfusate for t greater than 45 min no marked quantal release could be elicited by veratridine. The experiments suggest an inhibitory effect of intraterminal Li+ on the quantal release process.

Dependence on multivalent cations of quantal release of transmitter induced by black widow spider venom

Application of alpha-latrotoxin (alpha-LT), the active component of black widow spider venom (BWSV), to a vertebrate neuromuscular junction, in the presence of millimolar bath concentrations of Ca2+ or Mg2+, greatly increases the frequency of miniature end-plate potentials (Fmepp). We have further characterized the cation dependence of alpha-LT action at the frog cutaneous pectoris neuromuscular junction. The divalent cations, Ca, Sr, Ba at less than or equal to 50 microM, Zn, Mn, Cd at greater than or equal to 50-100 microM, and Mg at greater than or equal to 1.0 mM, as well as the trivalent cation La at greater than or equal to 15 microM, all increase Fmepp exponentially to greater than or equal to 100-200 s-1 over several minutes time. The exponential rate of rise is graded with extracellular cation concentration and can be reduced by increasing [K+] of the bath from 2 to 25-40 mM. Long-term exposure to alpha-LT in the presence of Sr2+ or Mn2+ results in the exhaustion of the releasable quantal store of transmitter, which in the case of Mn2+ correlates well with depletion of synaptic vesicles. These data support the hypothesis that BWSV promotes an increase in Fmepp by increasing nerve terminal permeability to multivalent cations that enter the nerve terminal down their electrochemical gradients and then may bind to quantal release activating sites or displace Ca2+ from intracellular stores.

Differential effect of intraterminal sodium on spontaneous quantal release of transmitter in two neuromuscular junctions of crayfish

Nerve terminals on the superficial abdominal extensor muscle and the claw opener muscle of small crayfish were loaded with sodium by bath application of 100 mumol/l veratridine in superfusates where normal Ca2+ was removed (low-Ca2+ superfusate). In both preparations this caused an increase in spontaneous quantal release of excitatory and inhibitory transmitter which was evaluated by means of the noise analysis technique. About 2.5 min after application of veratridine, when spontaneous quantal release had largely ceased, the normal Ca2+ concentration was reestablished. This increased transiently the quantal release rate a second time. However, release activated by Ca2+ application was much more vigorous at the superficial abdominal extensor muscle than at the claw opener. At the superficial abdominal extensor muscle on average about 8% of the total number of quanta could be released in low Ca2+ and 92% in normal Ca2+ superfusate, while at the claw opener 75% of the quanta were released in low Ca2+ and 25% in normal Ca2+ superfusate. The experiments suggest that intraterminal sodium has a differential effect in the terminals of the two preparations. Possibly, the intraterminal source from which Na+ may liberate Ca2+ is more restricted in the superficial abdominal extensor muscle than in the opener muscle of the claw.

Coated vesicles and pits during enhanced quantal release of acetylcholine at the neuromuscular junction

Frog neuromuscular junctions were stimulated by different methods to secrete quanta of ACh, and the attendant changes in the ultrastructure of the nerve terminal were assessed by morphometric analysis of electron micrographs. Secretion was stimulated by electrical stimulation at 2 Hz or by application of the secretagogues, lanthanum, ouabain or black widow spider venom, either in the presence or in the absence of extracellular Ca2+. The numbers of synaptic vesicles, coated vesicles and coated pits, and the length of axolemma and area of axoplasm were measured on the micrographs. There was a significant increase (about threefold) in the total number of coated structures (vesicles plus pits) per micron2 of axoplasm, but the fractional increase in the number of coated pits exceeded the fractional increase in the number of coated vesicles. These increases were positively correlated with the increase in the length of axolemma per unit area and negatively correlated with the changes in concentration of synaptic vesicles, suggesting that they were due to the increases in the surface area of the terminal that accompany a loss of vesicles. However, the increase in the concentration of coated structures was not related to the number of quanta secreted or to the estimated number of vesicles recycled. The lack of correspondence between the fractional increases in the coated pits and coated vesicles and the poor correlation between the numbers of these structures and the overall parameters of the secretory process suggest that, in contrast to the situation in other secretory systems, coated pits and coated vesicles may not play a crucial role in maintaining the functional population of synaptic vesicles at rapidly secreting neuromuscular junctions.

Measurement of quantal secretion induced by ouabain and its correlation with depletion of synaptic vesicles

Ouabain (0.1 and 0.05 mM) was applied to frog cutaneous pectoris nerve-muscle preparations bathed in modified Ringer's solution containing either 1.8 mM Ca2+ (and 4 mM Mg2+) or no added Ca2+ (4 mM Mg2+ and 1 mM EGTA). During the intense quantal release of acetylcholine (ACh) induced by ouabain, the parameters of the miniature endplate potentials (mepps) were deduced from the variance, skew, and power spectra of the endplate recordings by applying a recently described modification of classical fluctuation analysis. Often the high frequency of mepps is not stationary; therefore, the signal was high-pass filtered (time constant of the resistance-capacitance filter of 2 ms) to remove the errors introduced by nonstationarity. When ouabain was applied in the presence of Ca2+, mepp frequency started to rise exponentially after a lag of 1.5-2 h, reached an average peak frequency of 1,300/s in approximately 30 min, and then suddenly subsided to low level (10/s). In Ca2+-free solution, after a shorter lag (1-1.5 h), mepp frequency rose to peak rate of 700/s in approximately 20 min and then gradually subsided. In spite of the different time course of secretion in the two experimental conditions, the cumulative quantal release was not significantly different (7.4 +/- 1.3 X 10(5) in Ca2+-containing and 8.8 +/- 2.7 X 10(5) in Ca2+-free solutions). 60 min after the peak secretion, the muscles were fixed for observation in the electron microscope. Morphometric analysis on micrographs of neuromuscular junctions revealed in both cases a profound depletion of synaptic vesicles and deep infoldings of presynaptic membrane. This rapid depletion and the lack of uptake of horseradish peroxidase suggest that ouabain impairs the recycling process that tends to conserve the vesicle population during intense secretion of neurotransmitter. The good correlation observed between the reduction in the store of synaptic vesicles and the total number of quanta of ACh secreted in the absence of a vigorous membrane recycling strongly supports the view that the secretion of a quantum of ACh requires the fusion of a synaptic vesicle with the axolemma.