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

Presynaptic Acetylcholine Receptors Modulate the Time Course of Action Potential-Evoked Acetylcholine Quanta Secretion at Neuromuscular Junctions

For effective transmission of excitation in neuromuscular junctions, the postsynaptic response amplitude must exceed a critical level of depolarization to trigger action potential spreading along the muscle-fiber membrane. At the presynaptic level, the end-plate potential amplitude depends not only on the acetylcholine quanta number released from the nerve terminals in response to the nerve impulse but also on a degree of synchronicity of quanta releases. The time course of stimulus-phasic synchronous quanta secretion is modulated by many extra- and intracellular factors. One of the pathways to regulate the neurosecretion kinetics of acetylcholine quanta is an activation of presynaptic autoreceptors. This review discusses the contribution of acetylcholine presynaptic receptors to the control of the kinetics of evoked acetylcholine release from nerve terminals at the neuromuscular junctions. The timing characteristics of neurotransmitter release is nowadays considered an essential factor determining the plasticity and efficacy of synaptic transmission.

Estimation of the time course of neurotransmitter release at central synapses from the first latency of postsynaptic currents

Measurement of the release time course (RTC) and of the quantal content is important for quantifying synaptic precision and understanding the molecular basis of the release process at central synapses. In theory, the RTC can be determined directly from the histogram of first latencies of quantal events only if a maximum of one vesicle is released per trial, but at most synapses multiple vesicles are released. Traditionally, first latency histograms have been corrected for multiple releases using a simple correction, derived by Barrett and Stevens (BS; 1972b) for quantifying release at the neuromuscular junction. This correction has also been used to quantify release at central synapses. We show, by combining an analytical approach and numerical simulations of stochastic quantal release, that the BS correction gives a biased estimate for RTC and quantal content. The bias increases with release probability, and is therefore particularly problematic for central synapses. We show that this is due to assuming infinite availability of releasable vesicles and we derive a formula for estimating the RTC from first latencies without this assumption. The resulting ‘binomial correction’ requires knowledge of the maximum number of quanta that can be released following an action potential (N), which can be estimated with variance-mean analysis. We show with simulations that estimating RTC and quantal content from first latencies using the binomial correction is robust in the presence of noise and when release probability is non-uniform. We also provide an alternative method for estimating RTC from the first latencies when N cannot be determined.

Characteristics of the time course of evoked secretion of transmitter quanta in different parts of the motor nerve ending in the frog

Experiments were performed on neuromuscular preparations from frogs, in which three extracellular microelectrodes were used to record nerve ending currents and single-quantum endplate currents simultaneously from the proximal, central, and distal parts of single synaptic contacts. The rate of propagation of excitation across terminals was measured. along with the minimum synaptic delay, the intensity. and the degree of synchronicity of the secretion of transmitter quanta in different parts of the nerve ending, and the relationships between these factors and the calcium ion concentration in the medium. These studies showed that along with gradients in the rate of conduction of excitation and the intensity of secretion in different parts of the ending. there were also differences in the kinetics of the release of transmitter quanta. As the distance from the end of the myelinated part of the axon increased, the rate of conduction of the nerve impulse and the duration of the synaptic delay decreased, while the synchronicity of the release of quanta increased. Increases in the calcium concentration in the medium produced greater increases in the synchronicity of transmitter quantum release in the distal parts of the synapse than in the proximal parts. Mathematical modeling of multiple-quantum endplate currents showed that the characteristics of the kinetics of the secretion process observed here in different parts of the nerve ending represent a factor which partially compensates for the decrease in the amplitude and extending of the duration of the leading front of the multiple-quantum endplate current which are associated with the low rate of conduction of excitation across the nerve ending. The contribution of this compensation increases as the intensity of secretion of transmitter quanta increases in the distal parts of the synaptic contact.

Protein kinase A cascade regulates quantal release dispersion at frog muscle endplate

Uniquantal endplate currents (EPCs) were recorded simultaneously at the proximal, central and distal parts of the frog neuromuscular synapse, and their minimal synaptic latencies, latency dispersions and sensitivity to noradrenaline, cAMP and protein kinase A inhibition were measured. The latency dispersion was highest in the proximal part (P90 = 1.25 ms); it decreased to P90 = 0.95 ms in the central part and to P90 = 0.75 ms (60 % of the proximal part) in the distal part. In the proximal parts of the long neuromuscular synapse, stimulation-evoked EPCs with long release latencies were eliminated when the intracellular cAMP was increased by beta1 activation by noradrenaline, by the permeable analogue db-cAMP, by activation of adenylyl cyclase or by inhibition of cAMP hydrolysis. This makes the evoked release more compact, and the amplitude of the reconstructed multiquantal currents increases. Protein kinase A is a target of this regulation, since a specific inhibitor, Rp-cAMP, prevents the action of cAMP in the proximal parts and increases the occurrence of long-latency events in the distal parts of the synapse. Our results show that protein kinase A is involved in the timing of quantal release and can be regulated by presynaptic adrenergic receptors.

Statistics of transmitter release at nerve terminals

This review presents an historical account of the developments of the statistical analysis of quantal transmission over the past half century and of the progress made in using this approach to reveal new properties of nerve terminals. In the early 1950s, Katz and his colleagues showed that evoked transmitter release occurred in quanta at the neuromuscular junction, opening up the study of transmitter release at nerve terminals to statistical analysis. In the subsequent two decades attempts were made to see if evoked quantal release could be described by binomial or compound binomial statistics, as originally suggested by Katz, and to relate the parameters of the statistic to various structures of the nerve terminal. During this period two hypotheses were enunciated, namely the 'vesicle hypothesis', which states that quanta arise as a consequence of the packaging of transmitter in vesicles; and the 'active zone hypothesis', which states that vesicles undergo exocytosis at discrete sites on the nerve terminal. Unsuccessful attempts were made to relate the binomial parameter n to the elements in these hypotheses, that is to the number of active zones possessed by the terminal or the number of vesicles available for release at these zones. This difficulty was part resolved in the late 1970s with the application of non-uniform binomial statistics to transmitter release from nerve terminals, in which n is the number of active zones each with their individual probabilities, p(j). Autocorrelation functions were subsequently introduced to detect if transmitter release is quantised at a particular nerve terminal. Statistical methods which would allow discrimination between different models of transmitter release over the active zones of a terminal were then developed. The introduction of maximum likelihood estimation procedures then allowed estimates to be made of the parameters in the statistical models of quantal release. The application of these procedures to experimental data from a variety of nerve terminals provided evidence for the concept that each synapse, taken as possessing a single active zone, possesses its own individual probability of secretion of a quantum by the exocytosis of a vesicle. In the late 1960s Stevens introduced the first stochastic approach to the analysis of the kinetics of the release of a quantum of transmitter at the neuromuscular junction following an impulse. In the subsequent decades this was developed into an explicit theory for the interaction of proteins involved in regulated exocytosis of a vesicle at an active zone. The parameters were the number of transition steps in the release process (k), each occurring at the same rate (alpha), with the possibility of each of these steps becoming blocked at the same rate (gamma). Maximum likelihood estimation procedures could then be used to obtain these parameter values. The discovery was made in the 1990s of the core proteins of the SNARE complex that govern regulated exocytosis. This offers the possibility in the near future of identifying the kinetic interaction of these proteins with the parameters of the stochastic process of exocytosis which confer a particular probability on individual synapses.

Synchronization of evoked secretion of quanta of mediator as a mechanism facilitating the action of sympathomimetics

Experiments on frog neuromuscular junction preparations with extracellular recording of nerve terminal action potentials and single-quantum end-plate currents (EPC) were used to assess the time course of evoked quantum secretion of mediator by analyzing histograms of the distribution of true synaptic delays. These studies showed that noradrenaline, isoproterenol, and dobutamine change the kinetics of secretion of quanta, leading to synchronization of the process of mediator release; substances blocking beta-adrenoceptors (atenolol, propranolol) blocked this effect. Clonidine and phenylephrine, which activate alpha-receptors, had no effect on the kinetics of secretion, while the alpha-blocker phentolamine had no effect on the synchronizing action of noradrenaline. Reconstruction of multiquantum EPC from changes in the level of synchronization in the release of individual quanta, showed that EPC amplitude increased in response to noradrenaline by 17%, and that this was due only to alterations in the time course of secretion. These data led to the conclusion that there is a special presynaptic mechanism which facilitates the action of sympathomimetics, acting via beta-adrenoceptors.

Noradrenaline synchronizes evoked quantal release at frog neuromuscular junctions

1. Noradrenaline (NA) increases synaptic efficacy at the frog neuromuscular junction. To test the hypothesis that one of the actions of NA is to shorten the period over which evoked quanta are released, we measured the latencies of focally recorded uniquantal endplate currents (EPCs). 2. NA shortened the release period for evoked quantal release. The interval between the time when responses with minimal delay appeared and the point at which 90 % of all latencies had occurred was shortened in the presence of 1 x 10-5 M NA by about 35 % at 20 C and by about 45 % at 8 C. Inhibitor and agonist experiments showed that NA acts on a beta-adrenoreceptor. 3. The better synchronization of release significantly increased the size of reconstructed multi- quantal EPCs. This suggests that NA facilitates synaptic transmission by making the release of quanta more synchronous. 4. The synchronizing action of NA might potentiate neuromuscular transmission during nerve regeneration, transmitter exhaustion and other extreme physiological states where the quantal content is reduced, such as survival in cold and hibernation.

Asynchrony of quantal events in evoked multiquantal responses indicates presynaptic quantal interaction

Asynchrony of quantal events in evoked multiquantal responses indicates presynaptic quantal interaction. We have analyzed the possibility of quantal interactions by inspecting action potential-evoked postsynaptic multiquantal responses recorded extracellularly from the lobster neuromuscular junction. These recorded responses were compared with simulated multiquantal responses constructed from statistically independent quantal events. The simulated multiquantal responses were generated by random superposition of single quantal responses aligned according to the timing of the action potential. The methods of analysis consisted of 1) the comparison of quantal contents obtained from direct counting or by measuring of the size of the responses and 2) the analysis of distributions of quantal latencies. This analysis revealed a large error in the detection of quantal events for responses simulated with no quantal interaction. In contrast, very few errors in quantal detection were made in the analysis of experimental recordings. Latency histograms of recorded responses demonstrate that the proportion of late quantal events (those with latencies of >/=5 ms) increased as a function of quantal content. This shift in latency histograms was not observed for simulated responses. Our interpretation is that quanta interact presynaptically to cause asynchrony of quantal events in evoked responses.

Probabilistic secretion of quanta at somatic motor-nerve terminals: the fusion-pore model, quantal detection and autoinhibition

The probability of detecting first, second, and later quanta secreted at release sites of a motor-nerve terminal during the early release period following a nerve impulse has been addressed. The possibility that early quantal release autoinhibits later quantal release during this period has also been ascertained. In this investigation, a model for the secretion of a quantum at a release site is developed in which, following the influx and diffusion of calcium ions to a release site protein associated with synaptic vesicles, kappa steps of association of the ions with the protein then occur at rate alpha. The release site protein then undergoes a conformational change which may not go on to completion if calcium ions dissociate from the protein at rate gamma. If this process does reach completion then a fusion-pore between the vesicle and the presynaptic membrane is created; this happens at rate delta. Key assumptions of this fusion-pore model are that the quantal secretions from each site are independent of each other, and that there is a large number of vesicles, each with a small probability of secretion, so that the number of secretions is Poisson in nature. These assumptions allow analytical expressions to be obtained for predicting the times at which first, second and later quanta are secreted during the early release period following an impulse. To test the model, experiments were performed in which the times of first, second and later quantal releases were determined at discrete regions along the length of visualized motor-terminal branches in toad (Bufo marinus) muscles. Estimates of model rate constants and of kappa from the times for first quantal secretions failed to give satisfactory predictions of the observed times of later secretions. Therefore, either the model fails, or the procedure used for detecting later quantal events as a consequence of their being masked by earlier quantal events is inadequate. To solve this detection problem, a two-dimensional analysis of the spread of charge following the secretion of a quantum at a random site on the motor-terminal branch has been done. This allows determination of the probability that later quanta will be detected following secretion of earlier quanta. The detection model was then incorporated into the fusion-pore model to predict the times at which second and later quanta occur during the early release period, based on the estimates of the model parameters derived from the analysis of first quantal releases. Good estimates were now obtained for the observed times of second and later quantal releases, indicating that appropriate procedures must be adopted for adequate detection of quantal secretions. Furthermore, the experiments provide support for the fusion-pore model. It has been suggested that the binomial nature of quantal release from the entire motor-nerve terminal may be explained if early quantal release inhibits later quantal release during the early quantal release phase (M. R. Bennett & J. Robinson 1990, Proc. R. Soc. Lond. B 239, 329-358). Although the fusion-pore detection error model gave good predictions of the observed times of first, second and later quantal releases, these may be improved if a model for autoinhibition is included. In this model the first quantum was taken as giving rise to an inhibition of secretion that propagates to surrounding release sites with a constant velocity, v. A combined model incorporating the fusion-pore detection error model and that for autoinhibition was then used to predict second and later quantal latencies, by using the first quantal latencies to determine the estimates for the parameters in the combined model. When this analysis was done on the times for quantal secretion at sites on thirteen different motor-nerve terminals, the value of v was estimated as zero in each case, so that no autoinhibitory effect was observed.

Quantal transmitter release mediated by strontium at the mouse motor nerve terminal

1. In isolated mouse diaphragm, nerve stimulation in the presence of Sr2+ evokes phasic quantal transmitter release (endplate potentials, EPPs) with the same time course as in the presence of Ca2+. 2. Brief tetanic trains of nerve stimuli in the presence of Sr2+ cause an increase in quantal content of EPPs accompanied by an increase in the frequency of miniature EPPs (MEPPs); the latter persists as a 'tail' that subsides within about a second. Pseudo-random stimulation sequences were used to characterize these changes. 3. The fourth root of MEPP frequency during or after stimulation rose and fell in accordance with first order kinetics with the same time constants for rising and falling phases, in agreement with a 'residual ion' model in which (a) each nerve impulse causes the same entry of Sr2+ into the nerve terminal, (b) transmitter release (MEPP frequency) is proportional to the fourth power of [Sr2+] at release sites, and (c) Sr2+ removal is a first order process with a time constant of about 250 ms. 4. After exposure to bis (O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, acetoxymethyl ester form (BAPTA AM), 'Sr2+ tails' of MEPP frequency were reduced in magnitude and prolonged. 5. During stimulation trains, growth of phasic transmitter release rates (and quantal content of EPPs) were related to growth of MEPP frequency in almost exact agreement with a residual ion model, in which 'phasic' release (EPPs) and MEPP frequency are governed by the same equation, with the same parameters, and without any effect of depolarization per se to affect phasic release. 6. Prolonged (1 s) nerve terminal depolarizations in the presence of Sr2+ produce increased MEPP frequency with a time course corresponding to a model in which depolarization per se has little or no effect to increase transmitter release. 7. It was concluded that in the presence of Sr2+ the intense 'phasic' acceleration of quantal release induced by nerve impulse manifest in an EPP can be attributed to a transient rise of intracellular [Sr2+] in the vicinity of release sites, while the modulation of 'phasic' release by antecedent nerve impulses can be attributed to residual Sr2+ which is also manifest in a rise in MEPP frequency.

Estimating the timing of quantal releases during end-plate currents at the frog neuromuscular junction

1. 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 end-plate 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 high-Mg2+ 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.(ABSTRACT TRUNCATED AT 250 WORDS)

The kinetics of quantal releases during end-plate currents at the frog neuromuscular junction

1. The preceding paper (Van der Kloot, 1988) described a method for estimating the timing of quantal releases during an end-plate current. This period of elevated quantal release is called the early release period or ERP (Barrett & Stevens, 1972b). In the present paper, this deconvolution method is used to study the effects of varying quantal output by extracellular ions, stimulus patterns and drugs. 2. The data were obtained by voltage clamping end-plates in low-Ca2+ high-Mg2+ solutions, or in solutions containing tubocurarine (measuring the decay of the miniature end-plate currents (MEPCs) before curarization and assuming a value for MEPC amplitude after curarization). Data were also obtained by extracellular recording in Ca2+-free solution, using a recording pipette filled with CaCl2 and regulating Ca2+ release with a bias current. The three approaches led to similar conclusions. 3. Quantal release rose during the ERP along a sigmoid curve and reached a maximum after about 1.4 ms at 10 degrees C. This is called the time to peak. Quantal release then fell, following an exponential time course with a time constant of about 1.2 ms (10 degrees C). This is called the time constant for decline. 4. The ERP was followed by further, elevated quantal release, at a much lower rate, which declined over a longer time course. This is called late release. The magnitude of late release appears to be almost independent of the magnitude of release during the ERP, although the deconvolution method is a poor one for determining late release. The remainder of the results therefore focus on the ERP. 5. Increasing [Ca2+]o increased quantal output, and the rate of quantal output. It did not change the time to peak or the time constant of decline. Similarly, replacing Ca2+ with Sr2+ did not alter the time course of the ERP. 6. Two-pulse facilitation increased quantal output without changing the time to peak or the time constant of decline. 7. Quantal output was enhanced still more following a brief series of repetitive nerve stimulations. There was a lengthening of the time to peak; there was no change in the decline. The depression produced by longer series of repetitive stimulations did not change the time course of the ERP. 8. 4-Aminopyridine (4-AP) and dimethylsulphoxide (DMSO) increased quantal output and lengthened the time to peak, without altering the time constant for decline. 9. Adenosine decreased quantal output without altering the time course of the ERP.(ABSTRACT TRUNCATED AT 400 WORDS)