The effects of calcium ions on the binomial parameters that control acetylcholine release during trains of nerve impulses at amphibian neuromuscular synapses

Seasonal factors influence quantal transmitter release and calcium dependence at amphibian neuromuscular junctions

Amphibian neuromuscular junctions (NMJs) are composed of hundreds of neurotransmitter release sites that exhibit nonuniform transmitter release probabilities and demonstrated seasonal modulation. We examined whether recruitment of release sites is variable when the extracellular calcium concentration ([Ca²⁺]_o) is increased in the wet and dry seasons. The amount of transmitter released from the entire nerve terminal increases by approximately the fourth power as [Ca²⁺]_o is increased. Toad (Bufo marinus) NMJs were visualized using 3,3'-diethyloxardicarbocyanine iodide [DiOC₂(5)] fluorescence, and focal loose patch extracellular recordings were used to record the end-plate currents (EPCs) from small groups of release sites. Quantal content (m̄_e ), average probability of quantal release (p_e ), and the number of active release sites (n_e ) were determined for different [Ca²⁺]_o Our results indicated that the recruitment of quantal release sites with increasing [Ca²⁺]_o differs spatially (between different groups of release sites) and also temporally (in different seasons). These differences were reflected by the nonuniform alterations in p_e and n_e Most release site groups demonstrated an increase in both p_e and n_e when [Ca²⁺]_o increased. In ~30% of release site groups examined, p_e decreased while n_e increased only during the active period (wet season). Although the dry season induced parallel right shift in the quantal release versus extracellular calcium concentration when compared with the wet season, the dependence of quantal content on [Ca²⁺]_o was not changed. These results demonstrate the flexibility, reserve, and adaptive capacity of neuromuscular junctions in maintaining appropriate levels of neurotransmission.

Mechanisms of calcium sequestration during facilitation at active zones of an amphibian neuromuscular junction

The calcium transients (Delta[Ca(2+)](i)) at active zones of amphibian (Bufo marinus) motor-nerve terminals that accompany impulses, visualized using a low-affinity calcium indicator injected into the terminal, are described and the pathways of subsequent sequestration of the residual calcium determined, allowing development of a quantitative model of the sequestering processes. Blocking the endoplasmic reticulum calcium pump with thapsigargin did not affect Delta[Ca(2+)](i) for a single impulse but increased its amplitude during short trains. Blocking the uptake of calcium by mitochondria with CCCP had little effect on Delta[Ca(2+)](i) of a single impulse but greatly increased its amplitude during short trains. This present compartmental model is compatible with our previous Monte Carlo diffusion model of Ca(2+) sequestration during facilitation [Bennett, M.R., Farnell, L., Gibson, W.G., 2004. The facilitated probability of quantal secretion within an array of calcium channels of an active zone at the amphibian neuromuscular junction. Biophys. J. 86(5), 2674-2690], with the single plasmalemma pump in that model now replaced by separate pumps for the plasmalemma and endoplasmic reticulum, as well as the introduction of a mitochondrial uniporter.

The facilitated probability of quantal secretion within an array of calcium channels of an active zone at the amphibian neuromuscular junction

A Monte Carlo analysis has been made of the phenomenon of facilitation, whereby a conditioning impulse leaves nerve terminals in a state of heightened release of quanta by a subsequent test impulse, this state persisting for periods of hundreds of milliseconds. It is shown that a quantitative account of facilitation at the amphibian neuromuscular junction can be given if the exocytosis is triggered by the combined action of a low-affinity calcium-binding molecule at the site of exocytosis and a high-affinity calcium-binding molecule some distance away. The kinetic properties and spatial distribution of these molecules at the amphibian neuromuscular junction are arrived at by considering the appropriate values that the relevant parameters must take to successfully account for the experimentally observed amplitude and time course of decline of F1 and F2 facilitation after a conditioning impulse, as well as the growth of facilitation during short trains of impulses. This model of facilitation correctly predicts the effects on facilitation of exogenous buffers such as BAPTA during short trains of impulses. In addition, it accounts for the relative invariance of the kinetics of quantal release due to test-conditioning sequences of impulses as well as due to change in the extent of calcium influx during an impulse.

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.

Calcium dependence and recovery kinetics of presynaptic depression at the climbing fiber to Purkinje cell synapse

Short-term depression is a widespread form of use-dependent plasticity found in the peripheral and central nervous systems of invertebrates and vertebrates. The mechanism behind this transient decrease in synaptic strength is thought to be primarily the result of presynaptic "depletion" of a readily releasable neurotransmitter pool, which typically recovers with a time constant of a few seconds. We studied the mechanism and dynamics of recovery from depression at the climbing fiber to Purkinje cell synapse, where marked presynaptic depression has been described previously. Climbing fibers are well suited to studies of recovery from depression because they display little, if any, facilitation (even under conditions of low-release probability), which can obscure rapid recovery from depression for hundreds of milliseconds after release. We found that recovery from depression occurred in three kinetic phases. The fast and intermediate components could be approximated by exponentials with time constants of 100 msec and 3 sec at 24 degrees C. A much slower recovery phase was also present, but it was only prominent during prolonged stimulus trains. The fast component was enhanced by raising extracellular calcium and was eliminated by lowering presynaptic calcium, suggesting that, on short time scales, recovery from depression is driven by residual calcium. During regular and Poisson stimulus trains, recovery from depression was dramatically accelerated by accumulation of presynaptic residual calcium, maintaining synaptic efficacy under conditions that would otherwise deplete the available transmitter pool. This represents a novel form of presynaptic plasticity in that high levels of activity modulate the rate of recovery as well as the magnitude of depression.

Calcium dependence and recovery kinetics of presynaptic depression at the climbing fiber to Purkinje cell synapse

Short-term depression is a widespread form of use-dependent plasticity found in the peripheral and central nervous systems of invertebrates and vertebrates. The mechanism behind this transient decrease in synaptic strength is thought to be primarily the result of presynaptic "depletion" of a readily releasable neurotransmitter pool, which typically recovers with a time constant of a few seconds. We studied the mechanism and dynamics of recovery from depression at the climbing fiber to Purkinje cell synapse, where marked presynaptic depression has been described previously. Climbing fibers are well suited to studies of recovery from depression because they display little, if any, facilitation (even under conditions of low-release probability), which can obscure rapid recovery from depression for hundreds of milliseconds after release. We found that recovery from depression occurred in three kinetic phases. The fast and intermediate components could be approximated by exponentials with time constants of 100 msec and 3 sec at 24 degrees C. A much slower recovery phase was also present, but it was only prominent during prolonged stimulus trains. The fast component was enhanced by raising extracellular calcium and was eliminated by lowering presynaptic calcium, suggesting that, on short time scales, recovery from depression is driven by residual calcium. During regular and Poisson stimulus trains, recovery from depression was dramatically accelerated by accumulation of presynaptic residual calcium, maintaining synaptic efficacy under conditions that would otherwise deplete the available transmitter pool. This represents a novel form of presynaptic plasticity in that high levels of activity modulate the rate of recovery as well as the magnitude of depression.

Probabilistic secretion of quanta and the synaptosecretosome hypothesis: evoked release at active zones of varicosities, boutons, and endplates

A quantum of transmitter may be released upon the arrival of a nerve impulse if the influx of calcium ions through a nearby voltage-dependent calcium channel is sufficient to activate the vesicle-associated calcium sensor protein that triggers exocytosis. A synaptic vesicle, together with its calcium sensor protein, is often found complexed with the calcium channel in active zones to form what will be called a "synaptosecretosome." In the present work, a stochastic analysis is given of the conditions under which a quantum is released from the synaptosecretosome by a nerve impulse. The theoretical treatment considers the rise of calcium at the synaptosecretosome after the stochastic opening of a calcium channel at some time during the impulse, followed by the stochastic binding of calcium to the vesicle-associated protein and the probability of this leading to exocytosis. This allows determination of the probabilities that an impulse will release 0, 1, 2,... quanta from an active zone, whether this is in a varicosity, a bouton, or a motor endplate. A number of experimental observations of the release of transmitter at the active zones of sympathetic varicosities and boutons as well as somatic motor endplates are described by this analysis. These include the likelihood of the secretion of only one quantum at an active zone of endplates and of more than one quantum at an active zone of a sympathetic varicosity. The fourth-power relationship between the probability of transmitter release at the active zones of sympathetic varicosities and motor endplates and the external calcium concentration is also explained by this approach. So, too, is the fact that the time course of the increased rate of quantal secretion from a somatic active zone after an impulse is invariant with changes in the amount of calcium that enters through its calcium channel, whether due to changes consequent on the actions of autoreceptor agents such as adenosine or to facilitation. The increased probability of quantal release that occurs during F1 facilitation at the active zones of motor endplates and sympathetic boutons is predicted by the residual binding of calcium to a high-affinity site on the vesicle-associated protein. The concept of the stochastic operation of a synaptosecretosome can accommodate most phenomena involving the release of transmitter quanta at these synapses.

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.

Calcium dependence of quantal secretion from visualized sympathetic nerve varicosities on the mouse vas deferens

1. The effects of calcium on the secretion of quanta recorded from single varicosities on the surface of the mouse vas deferens has been determined. 2. If recordings were made from two adjacent varicosities and the [Ca2+]o increased from a low value (1 mM), then the increase in the mean quantal content of the endplate potential (m(e)) was almost entirely due to an increase in the binomial probability for secretion (pe). At higher [Ca2+]o there was an increasing tendency for the binomial parameter (ne) to increase from 1 to 2. When ne increased there was very little change in pe, indicating that the new release site recruited from the other varicosity has a relatively low probability for secretion. 3. If recordings were restricted to single varicosities and the [Ca2+]o increased in the range from 1 to 4 mM then the increases in m(e) were almost always due to an increase in pe. The gradient relating the log of m(e) to the log of [Ca2+]o for [Ca2+]o of 1.0-1.5 mM was in the range 3.2-5.4 (mean 4.2). 4. Test impulses gave a similar proportional increase in m(e) following a conditioning impulse at all varicosities from which recordings were made. 5. Facilitation of m(e) declined exponentially with an increase in the test-conditioning interval from 0.5 to 4 s. The time constant for this decline was about 6 s. 6. The results indicate that single release sites show a similar fourth power dependency on [Ca2+]o and facilitate to about the same degree following a conditioning impulse.

Probabilistic secretion of quanta from nerve terminals in toad (Bufo marinus) muscle modulated by adenosine

1. A study has been made of the effect of blocking endogenous adenosine on the statistics of quantal secretion at nerve terminals in toad (Bufo marinus) muscle during summer and winter. 2. Exogenous adenosine (10-50 microM) reduces the mean quantal content of the endplate potential (EPP) recorded with an intracellular microelectrode (m) by 36 +/- 6% (mean +/- S.E.M.), independent of the control value of m in both summer and winter. The variance of the EPP (S2) was reduced by adenosine in proportion to m, so that the probability of quantal secretion (p) remained relatively constant. 3. Exogenous adenosine reduces the mean quantal content of secretion recorded with an extracellular electrode (m(e)) to a similar extent (47 +/- 6%) at different relatively high secreting sites along nerve terminal branches in both summer and winter. 4. Both theophylline (20-100 microM) and adenosine deaminase (2.5 i.u./ml) increase the amplitude of the EPP in summer in a [Ca2+]o of 0.5 mM or greater; a maximum increase of about 40% is reached at a [Ca2+]o of 1.2 mM. The amplitude of the EPP in winter is usually reduced by theophylline in a [Ca2+]o of 0.5 mM, but is always increased in a [Ca2+]o greater than 0.9 mM to reach a maximum increase of about 40% at high [Ca2+]o. 5. The variance of the EPP (S2) was always increased by theophylline to a greater extent than m in summer, so that p decreased and Poisson rather than binomial statistics could be used to describe the distribution of EPP amplitudes. In winter, theophylline generally increased m and S2 to about the same extent, so that p did not change much. 6. An autocorrelation analysis of the amplitude of successive EPPs in a long train at 0.5 Hz in high [Ca2+]o showed that these are likely to be independently distributed. Adenosine secreted by one impulse is then unlikely to affect secretion by a subsequent impulse in the train. 7. These observations are discussed in terms of the hypothesis that endogenously secreted adenosine at a release site inhibits secretion by nearby release sites in summer; this has the effect of reducing m and to a greater extent S2 so that binomial rather than Poisson statistics can describe the frequency distribution of EPP amplitudes.

Probabilistic secretion of quanta from nerve terminals in avian ciliary ganglia modulated by adenosine

1. The effects of adenosine on the probability of secretion of acetylcholine quanta and on presynaptic and postsynaptic action potentials was examined in the post-hatched avian ciliary ganglion. 2. Adenosine (20 microM) reduced the average size of the excitatory postsynaptic potential (EPSP) by 33%. This was due to a decrease in quantal content of the EPSP (m). The effect was blocked by theophylline (50 microM). 3. Adenosine deaminase (2.5 i.u./ml) increased the size of the EPSP by 70%, suggesting that endogenous adenosine modulates synaptic transmission in the ciliary ganglion. However, theophylline (20-100 microM) did not affect the EPSP in a low [Ca2+]o of 1 mM and high [Mg2+]o of 6 mM. 4. Plateau-type action potentials with a large calcium component were generated in the ciliary neurones by bathing the ganglion in tetraethylammonium ions (TEA, 10 mM). Adenosine (20 microM) reduced the duration of these action potentials on short exposures (less than 20 min) but increased the duration on longer exposure (greater than 30 min). Adenosine did not affect the normal action potential recorded in the absence of TEA. 5. Adenosine (20 microM) hyperpolarized the nerve terminal and as a consequence increased the size of the presynaptic action potential and reduced its after-hyperpolarization. 6. Plateau-type action potentials with a large calcium component were generated in the nerve terminals using TEA (10 mM). The duration of these action potentials was significantly reduced by adenosine (20 microM). 7. Adenosines action on nerve terminals, to hyperpolarize the membrane and reduce calcium influx, may contribute to its effect in reducing m of the EPSP.

Probabilistic secretion of quanta from nerve terminals at synaptic sites on muscle cells: non-uniformity, autoinhibition and the binomial hypothesis

A model of the secretion of a quantum at a release site is proposed in which, following the influx of calcium ions, synaptic vesicles are made available for release by the activation of kappa phosphorylation steps with rate alpha. At any time during this process the vesicles may become unavailable for secretion at rate gamma. On completion of the kappa phosphorylation steps the vesicles participate in the formation of a fusion pore with the terminal membrane to give exocytosis at rate delta. Changes in alpha, delta and kappa are shown to produce characteristic changes in the number and timecourse of quantal secretions following a nerve impulse, which are similar to those observed following drug treatments that are thought to act selectively on each of these processes. The number of quanta secreted from nerve terminals that consist of many release sites does not fluctuate much during a low frequency train of impulses: the variance is small compared with the mean level, so secretion follows binomial rather than Poisson statistics. A theory is derived that shows that variations in the probability of secretion amongst these release sites of any particular kind fails to reduce the variance of the total secretion from the terminal; Poisson rather than binomial statistics then still apply. The theory shows that an interaction between release sites is required to reduce this variance and such an effect is provided if secretion at a site inhibits secretion at nearby sites. Simulations show that incorporating this process of autoinhibition into the model reproduces the experimental observations on the effects of calcium ions on the binomial parameters p and n as well as on the relative constancy of p during facilitation and depression of quantal secretion. Methods for estimating the timecourse of changes in the probability of secretion at release sites following an impulse, by using either the time of occurrence of first, second, third or later quantal latencies, are given. These procedures show that current methods for estimating the time-dependent probability changes are inadequate for detecting interaction between release sites, such as autoinhibition, unless this is relatively large. Therefore, estimates from third quantal latencies are used.

Effects of (+)-tubocurarine on [3H]acetylcholine release from the rat phrenic nerve at different stimulation frequencies and train lengths

The effect of (+)-tubocurarine (TC) on the release of [3H]acetylcholine from the rat phrenic nerve-hemidiaphragm preincubated with [3H]choline was investigated at different stimulation frequencies and train lengths. At 0.5 Hz (100 pulses) TC failed to modulate the evoked acetylcholine release. A slight (30%) inhibition was observed at 1 Hz (100 pulses). Release of acetylcholine evoked at 5, 25 and 50 Hz (100 pulses) or 100 Hz (200 pulses) was markedly reduced by TC. The degree of inhibition (60%) was similar between 5 Hz and 100 Hz. A concentration of 1 mumol/l TC was a maximal effective concentration at 5 Hz whilst at all higher stimulation frequencies a 10-fold higher concentration was necessary for the maximal effect. When 300 pulses were continuously applied at 5 Hz or 50 Hz TC caused only a slight inhibition (20%). Additionally, the phrenic nerve was stimulated intermittently. Trains of 15 pulses were repeated 10 times with an interval of 3 s between each train. Under this latter stimulation condition TC failed to reduce acetylcholine release. It is concluded that nicotinic autofacilitation of acetylcholine release from the motor nerve operates at frequencies and stimulation conditions similar to the pattern of nerve activity under in vivo conditions. At least more than 15 pulses are required before the nicotinic autofacilitation becomes apparent. It appears unlikely that the TC induced fading of end-organ responses can only be attributed to a blockade of the presynaptic nicotine receptors.

Changes in the dimensions of release sites along terminal branches at amphibian neuromuscular synapses

The probability of transmitter secretion from release sites declines along the length of most long terminal branches (greater than 78 micron) at toad (Bufo marinus) neuromuscular junctions; in contrast, few short terminal branches (less than 78 micron) show such a decline. The present study was carried out to see if any of the dimensions of release sites change along the length of terminal branches in a way that can be correlated with the decrease in secretion probability. The size of presynaptic release site structures was determined by examining serial transverse sections through entire terminal branches with the transmission electron microscope; the size of postsynaptic release site structures was determined by examining terminal gutters with the scanning electron microscope after the removal of terminal branches. Long terminal branches showed a significant decrease in the length of their synaptic contact and cross-sectional area (terminal size) with distance from the origin of the branch. In contrast, there was no significant difference in the length of close apposition (less than 0.2 micron) between the nerve terminal and postsynaptic muscle membrane; furthermore, neither the length of postsynaptic folds nor the frequency of the folds along the length of the terminal gutter changed. Short terminal branches showed no significant differences in the dimensions of either presynaptic or postsynaptic release site structures. The decline in the length of synaptic contacts whilst the length of close apposition remains relatively constant is due to the progressive encroachment of Schwann cell processes between presynaptic and postsynaptic membranes along the length of long terminal branches.

The probability of quantal secretion along visualized terminal branches at amphibian (Bufo marinus) neuromuscular synapses

The number of quanta secreted from selected sites along terminal branches at toad (Bufo marinus) neuromuscular junctions was determined. Terminal branches were visualized by prior staining with the fluorescent dye, 3-3 Diethyloxadicarbocyanine iodide (DiOC2(5)); neither impulse conduction nor quantal release were affected by DiOC2(5) at concentrations less than 10 microM. The evoked quantal release recorded with an extracellular micro-electrode (me) at different sites along the length of terminal branches was determined in an external calcium concentration, [Ca]o, of 0.35-0.45 mM. For short branches (40-80 microns), me remained approximately constant for over 60% of the branches; for the rest, me declined approximately exponentially with an average length constant of 17 +/- 2 microns (mean +/- S.E. of mean). For both medium (81-120 microns) and long branches (121-160 microns), me declined in nearly all cases approximately exponentially with length constants of 39 +/- 5 and 54 +/- 8 microns respectively. These changes in me were observed at synapses having a wide range of terminal branching patterns. Some DiOC2(5)-stained branches possessed discontinuous cholinesterase staining. In general, me declined along these branches in the same way as along DiOC2(5)-stained branches with continuous cholinesterase staining. It is suggested that because of the decline in me along most medium and long terminal branches, many release sites have a very low probability for secretion in low [Ca]o. Release sites near the point of nerve entry, which have a relatively high probability, therefore make the main contribution to secretion recorded with an intracellular micro-electrode. As a consequence, transmitter secretion from the whole terminal does not fluctuate from impulse to impulse as much as expected if there were a large number of release sites, each with a low probability of secretion. Transmitter secretion then follows binomial rather than Poisson statistics.

Transmitter secretion varies between visualized release sites at amphibian neuromuscular junctions

Living amphibian (Bufo marinus) motor-nerve terminals have been stained and electrophysiological techniques have been used to determine the amount of transmitter secreted at selected release sites of the terminals. High secretion occurs at sites laid down near the origin of the terminals; very low or zero secretion occurs at more distal sites. The results indicate that some release sites may fail to secrete transmitter on arrival of a nerve impulse. It is suggested that functionally silent release sites may exist at synapses in the nervous system.

The dependence of excitatory junction potential amplitude on the external calcium concentration in mouse vas deferens during narcotic withdrawal

The dependence of neurotransmitter release on calcium was evaluated in adrenergic terminals from mice that were acutely withdrawn from chronic morphine treatment (CMT). A two fold increase in the number of writhes in response to an i.p. injection of acetylcholine was induced in mice by CMT and subsequent withdrawal. A shift to the left in the relationship between the excitatory junction potential (e.j.p.) amplitude and extracellular calcium concentration ([Ca]o) was induced in vasa deferentia from CMT-withdrawn mice. A reduction in the degree of facilitation of transmitter release during a short low-frequency train of impulses and an increase in the amount of transmitter release during a high-frequency train of impulses was induced in vasa deferentia from CMT-withdrawn mice. The adaptive mechanism of the terminals to the sustained presence of morphine may involve an increase in the probability that the release sites will release transmitter either via increase in calcium influx or an increase in the affinity of calcium to the hypothetical X-receptor.

The dependence of excitatory junction potential amplitude on the external calcium concentration in narcotic tolerant mouse vas deferens

The dependence of neurotransmitter secretion on external calcium ions during development of opiate tolerance in the mouse vas deferens was studied. The writhing response of mice to an i.p. injection of acetylcholine was inhibited by morphine. Reversal of this antinociceptive effect of morphine during chronic treatment signalled the development of tolerance. Tolerance to morphine at the neuromuscular junction was shown as a reversal of the initial shift of the size of the excitatory junction potential (e.j.p.) vs extracellular calcium concentration relationship back towards the control without any change in the power of 2.4. Facilitation in the amplitude of the e.j.p. occurs with low frequency (2 Hz) stimulation. The initial increase in facilitation induced by morphine was reversed by chronic morphine treatment without any change in the plateau e.j.p. amplitude achieved after a long low frequency train of impulses. At high frequencies (10 Hz) the initial increase in e.j.p. amplitude was followed by a depression. Acute morphine administration decreased the size of the e.j.p., this was followed by an increase in facilitation and a decrease in depression. These effects were reversed after chronic morphine treatment. Tolerance to morphine involves a counteradaptive process which restores the normal entry of calcium ions or its actions within the release sites in promoting transmitter release.

Age-associated changes in neuromuscular transmission in the rat

The physiological changes in neuromuscular transmission associated with age were examined in rats between the ages of 1 mo (28 days) and 1 yr (364 days). Intracellular recording techniques were used to monitor end-plate potentials and miniature end-plate potentials at the rat diaphragm neuromuscular junction. Muscle action potentials were blocked by cutting the muscle fibers. Neuromuscular transmission was significantly different in 28-day-old rats compared with the older rats (42-364 days). The 28-day-old rats released fewer quanta. The statistical store in the immature rats was also significantly smaller than in the older age groups. The statistical probability of release, however, was not significantly different from the older animals. Examination of the presynaptic parameters in rats between the ages of 42 and 364 days revealed no significant increases in quantal release. It is concluded that the neuromuscular junction matures physiologically by 6 wk of age in rats and remains stable through the 1st yr of life.

Is the MEPP due to the release of one vesicle or to the simultaneous release of several vesicles at one active zone?

Miniature end-plate potentials (MEPPs) recorded at the neuromuscular junction were initially reported to be normally distributed and have been attributed to quantal ACh release. This quantum was later correlated with the release of the content of one clear vesicle. This is the 'classical vesicular hypothesis'. Recent observations of subminiature end-plate potentials (s-MEPPs) and of multimodal distribution of the MEPP amplitudes have led to the formulation of a new 'multivesicular hypothesis'. It attributes the s-MEPP to the release of one vesicle and the MEPP to the simultaneous release of several vesicles at one active zone. The distribution of MEPP intervals, the evaluation of the ACh content of a vesicle and of the ACh necessary to produce a MEPP, estimates of the number of vesicles missing following repeated stimulation, and the freeze fracture studies of the active zone do not permit a definitive rejection of either hypotheses.

Variation in quantal secretion at different release sites along developing and mature motor terminal branches

The secretion of a quantum by groups of release sites (me) along the length of nerve terminal branches has been studied with extracellular electrodes at mature and juvenile toad neuromuscular junctions. The position of the recording sites with respect to the terminal branches was determined following zinc-iodide staining of terminals. me declined with distance from the last myelin segment along individual terminal branches: in a [Ca]o of 0.35 mM, me declined by 80% over a distance of 60 micron along mature terminal branches. Simultaneous recording of secretion from adjacent groups of release sites with two extracellular electrodes confirmed that secretion of a quantum at release sites declines with the distance of the sites from the last myelin segment. The secretion of a quantum by groups of release sites along juvenile terminal branches also declines with distance from the last myelin segment: me is always largest for the first release sites to be laid down. The facilitated secretion of quanta at sites with low me is larger than those with high me; as a consequence, all sites secrete similar numbers of quanta at the end of a short high-frequency train.

Quantal analysis of a decremental response at hair cell-afferent fibre synapses in the goldfish sacculus

1. On application of a step decrement in the sound intensity, the amplitude of excitatory post-synaptic potentials (e.p.s.p.s) recorded intracellularly from large afferent auditory fibres in goldfish's sacculus showed a decremental response in which the amplitude of the e.p.s.p.s was temporarily reduced to a very low level, but soon returned to a new steady level appropriate to the decreased sound intensity. This response seems to underlie the temporary reduction in the rate of afferent discharge observed in the mammalian cochlea upon cessation of sound or upon reduction of its intensity 2. A statistical analysis revealed that reduction in the size of the mean quantal content (m) during the decremental response was associated with reduction in the size of binomial population (n), but not that of the probability (p) of any one of the available quanta actually being used. 3. The temporary reduction in the size of n during the decremental responses can be explained if it is assumed that replenishment to vacant release sites was channelled to high threshold sites and that the low threshold sites were bypassed. The mechanism underlying this special type of replenishment is discussed in relation to the ultrastructural features of presynaptic sites in the hair cell.

An electrophysiological analysis of the effects of morphine on the calcium dependence of neuromuscular transmission in the mouse vas deferens

1 The effects of morphine on the Ca-dependence of the synaptic potential amplitude in the mouse vas deferens have been determined. 2 The synaptic potential increased with a power factor of 2.4 for [Ca]o between 0.7 mM and 1.8 mM. Morphine (40 nM) decreased the synaptic potential, without altering the second power relationship between the synaptic potential and [Ca]o. 3 Morphine reversed the depression in the synaptic potential which develops during a short high-frequency (10 Hz) train of impulses to facilitation. Consequently the synaptic potential beyond the tenth impulse was unaffected by morphine. 4 Morphine did not alter the facilitation of the synaptic potential which develops during a short low-frequency (less than or equal to 2 Hz) train of impulses in normal [Ca]o. Consequently morphine decreased the synaptic potential for each impulse by about the same percentage amount. 5 Morphine increased the small facilitation in the synaptic potential which occurs during a short low-frequency (less than or equal to 2 Hz) train of impulses in high [Ca]o. This facilitation approximated the predictions based on the assumption that each impulse leaves residual Ca ions bound to receptors involved in transmitter release from the nerve terminal.

The effect of calcium ions on the secretion of quanta evoked by an impulse at nerve terminal release sites

A study has been made of the effects of calcium ions on the number of quanta secreted from all the release sites at an amphibian motor nerve terminal recorded with an intracellular microelectrode (m) compared with the number secreted simultaneously from a small number of release sites recorded with an extracellular microelectrode (me). If the endplate potential was made subthreshold by lowering the external calcium concentration ([Ca]o less than or equal to 0.4 mM), it was possible to find small groups of release sites for which me was comparable to m, indicating considerable nonuniformity in the probability of release of a quantum at different groups of release sites (Pe) in a given [Ca]o. Increasing [Ca]o in the range from 0.25 to 0.4 mM increased the probability of release of a quantum at groups of release sites (Pe), independent of the initial value of Pe, and the dependence of Pe on [Ca]o followed a fourth power relationship. A conditioning impulse enhanced the probability of release of a quantum by a subsequent test impulse at release sites, if Pe was less than 1.0 during the conditioning impulse. It is shown that the present observations regarding the dependence of Pe on [Ca]o and on conditioning impulses can be quantitatively predicted from previous observations regarding the dependence of the binomial parameters m, p, and n on [Ca]o and on conditioning impulses determined with intracellular electrodes, if the probability of secretion of a quantum at a release site (Pj) is different for different release sites and Pj is distributed as a beta random variable.

The regression of synapses formed by a foreign nerve in a mature axolotl striated muscle

A study has been made of the factors which determine that the terminals of a foreign flexor nerve at synaptic sites in a unrodele extensor muscle regress on return of the original extensor nerve. The quantal content (m) of the endplate potential (EPP) at flexor nerve terminal synapses, during innervation of a previously denervated extensor muscle, increased in about 8 weeks to reach the same size as at normal extensor nerve terminals; the same time was taken for m to reach normal size at extensor nerve terminals when these reinnervated their own muscle. At flexor nerve terminals, m decreased eventually to zero if the extensor nerve terminals returned within about 6 weeks of synapse formation by the flexor nerve terminals to the same or an immediately adjacent synaptic site to that occupied by these terminals. During this decrease in m at flexor nerve terminals, stimulation of the flexor nerves in the presence of horseradish peroxidase (HRP) showed HRP-labelled flexor nerve terminals present only in those regions of the extensor muscle in which the electrical signs of flexor nerve terminals were observed, indicating that the decrease in m at regressing flexor terminals was accompanied by their vacating synaptic sites. However, flexor nerve terminals failed to regress from the extensor muscle on return of the original nerve supply if the flexor nerve allowed to form synapses for more than about 10 weeks before the return of the extensor nerve.

Depression, facilitation, and mobilization of transmitter at the rat diaphragm neuromuscular junction

The characteristics of depression, facilitation, and mobilization of transmitter were examined at the rat diaphragm neuromuscular junction. Intracellular recording techniques were used to monitor end-plate potentials (EPPs), miniature end-plate potentials (MEPPs) and the muscle resting potentials. The cut-muscle technique was used to prevent muscle action potentials. Quantal release was determined by the direct method. The binomial statistical parameters, releasable store (n) and probability of release (p), were examined under various stimulating conditions to determine the basis for depression and facilitation. The present experiments demonstrate that p remains unchanged during repetitive nerve stimulation at low or moderately high frequencies. The experiments demonstrate that depression is due to a decrease in n and facilitation is due to an elevation in n. It is suggested that the increase in n during facilitation is due to a transient recruitment of inactive releasing sites. Substantial replenishment of n by mobilization occurs within a few ms after a stimulus but a slow residual rate of mobilization is needed to replenish n to resting levels.

The effect of calcium ions and temperature on the binomial parameters that control acetylcholine release by a nerve impulse at amphibian neuromuscular synapses

1. A study has been made of the effects of changing the external calcium concentration, [Ca](o), and the temperature on both the number of quanta available for release by the nerve impulse (n) as well as the increase in release probability of a quantum p(t) during the release period (from 0 to T) following a nerve impulse at synapses in amphibian striated muscle.2. When [Ca](o) was increased in the low range from 0.25 to 0.4 mM at 18 degrees C, the average quantal content of the e.p.p. (m) increased as the fourth power of [Ca](o) and this was primarily due to a third power dependence of n on [Ca](o); the dissociation constants and power dependence of n on calcium determined in the [Ca](o) range from 0.25 to 1.0 mM were successfully used to predict the changes in size of the e.p.p. in the very high [Ca](o) range from 1 to 10 mM. When the temperature was increased from 7 to 18 degrees C in a [Ca](o) of 0.6 mM or 0.35 mM, n increased with a Q(10) of 2.5.3. When [Ca](o) was increased in the range from 0.25 to 1.0 mM at 18 degrees C, the probability that a quantum initially available for release is released during the release period (p(T)) was very sensitive to [Ca](o), increasing as the third power of [Ca](o) and with a dissociation constant of 0.13 mM. When the temperature was increased from 7 to 18 degrees C in a [Ca](o) of 0.6 mM or 0.35 mM, p(T) decreased.4. The histograms of latencies of individual quanta following a nerve impulse was very temperature dependent: the time to peak of the histograms (i.e. the interval in which most quanta fell) had a Q(10) of over 4 as did the time constant of decline of the histograms in the temperature range from 7 to 18 degrees C.5. The average number of quanta released up to time t during the release period following a nerve impulse, namely np(t), was well described by a stochastic process in which p(t) was determined by two reactions; one of these reactions released available quanta from the nerve terminal whilst the other made some of the available quanta unavailable for release by the nerve impulse.