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

Seasonal comparison of the neuromuscular junction morphology of Bufo marinus

At mammalian neuromuscular junctions (NMJs), prolonged inactivity leads to muscle denervation and atrophy. By contrast, amphibian NMJs do not show such degeneration even though they can remain in a state of drought-imposed dormancy (hibernation) for many years. We have previously reported that during the dry season, toad (Bufo marinus) NMJs display decreased sensitivity to extracellular calcium-dependent neurotransmitter release, which leads to minimal neuromuscular transmission. In the present study, we examined and compared NMJ morphology of toads obtained from the wild during the wet season (February-March) when these toads are active, to toads obtained from dry season (October-November) when toads are inactive. Iliofibularis muscles were isolated and prepared for immunostaining with anti-SV2, a monoclonal antibody that labels synaptic vesicle glycoprotein SV2. The corresponding postsynaptic acetylcholine receptors were stained using Alexa Fluro-555 conjugated α-bungarotoxin. Confocal microscopy and three-dimensional reconstructions were then used to examine the pre-and postsynaptic morphology of toads NMJs from the dry (inactive) and wet (active) seasons. Total axon branch number, the percentage of axon branches with discontinuous distributions of synaptic vesicles, and further the Pearson value of colocalization of pre and postsynaptic elements in each NMJs from both the dry and wet season were compared. While our previous studies on dry toads revealed a significant reduction in evoked neurotransmission, our present findings show that the structure of the NMJs suffered limited level of remodeling, suggesting a mechanism utilized by NMJs in dry season toads to support quick recover from their dormant state after the heavy rain in wet season.

Climatic modulation of neurotransmitter release in amphibian neuromuscular junctions: role of dynorphin-A

Amphibian neuromuscular junctions (NMJs) become relatively more silent during the dry winter season in Australia. During the dry, calcium sensitivity is reduced, whereas calcium dependence remains unchanged. Endogenous opioid peptides play an important role in the regulation of the physiological functions of active and dormant vertebrates. Previous findings suggest that dynorphin-A is more potent than other opiates in decreasing evoked neurotransmission in amphibian NMJs. Dynorphin-A has been shown not to alter the amplitude or the frequency of miniature quantal neurotransmitter release. In the present study, we report that dynorphin-A exerted a more pronounced inhibitory effect on evoked neurotransmitter release during the dry (hibernating period) when compared with the wet (active period) season. Dynorphin-A increased the frequency and decreased the amplitude of miniature neurotransmitter release only at relatively high concentration during the dry season. In the present study, we propose that dynorphin-A suppresses evoked neurotransmitter release and thus contraction of skeletal muscles, while allowing subthreshold activation of the NMJ by miniature neurotransmission, thus preventing any significant neuromuscular remodeling. The inhibitory effect of dynorphin-A on evoked transmitter release is reduced by increasing the extracellular calcium concentration.

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.

Loss of β2-laminin alters calcium sensitivity and voltage-gated calcium channel maturation of neurotransmission at the neuromuscular junction

KEY POINTS

Neuromuscular junctions from β2-laminin-deficient mice exhibit lower levels of calcium sensitivity. Loss of β2-laminin leads to a failure in switching from N- to P/Q-type voltage-gated calcium channel (VGCC)-mediated transmitter release that normally occurs with neuromuscular junction maturation. The motor nerve terminals from β2-laminin-deficient mice fail to up-regulate the expression of P/Q-type VGCCs clusters and down-regulate N-type VGCCs clusters, as they mature. There is decreased co-localisation of presynaptic specialisations in β2-laminin-deficient neuromuscular junctions as a consequence of lesser P/Q-type VGCC expression. These findings support the idea that β2-laminin is critical in the organisation and maintenance of active zones at the neuromuscular junction via its interaction with P/Q-type VGCCs, which aid in stabilisation of the synapse. β2-laminin is a key mediator in the differentiation and formation of the skeletal neuromuscular junction. Loss of β2-laminin results in significant structural and functional aberrations such as decreased number of active zones and reduced spontaneous release of transmitter. In vitro β2-laminin has been shown to bind directly to the pore forming subunit of P/Q-type voltage-gated calcium channels (VGCCs). Neurotransmission is initially mediated by N-type VGCCs, but by postnatal day 18 switches to P/Q-type VGCC dominance. The present study investigated the changes in neurotransmission during the switch from N- to P/Q-type VGCC-mediated transmitter release at β2-laminin-deficient junctions. Analysis of the relationship between quantal content and extracellular calcium concentrations demonstrated a decrease in the calcium sensitivity, but no change in calcium dependence at β2-laminin-deficient junctions. Electrophysiological studies on VGCC sub-types involved in transmitter release indicate N-type VGCCs remain the primary mediator of transmitter release at matured β2-laminin-deficient junctions. Immunohistochemical analyses displayed irregularly shaped and immature β2-laminin-deficient neuromuscular junctions when compared to matured wild-type junctions. β2-laminin-deficient junctions also maintained the presence of N-type VGCC clustering within the presynaptic membrane, which supported the functional findings of the present study. We conclude that β2-laminin is a key regulator in development of the NMJ, with its loss resulting in reduced transmitter release due to decreased calcium sensitivity stemming from a failure to switch from N- to P/Q-type VGCC-mediated synaptic transmission.

Correlation of non-uniform protein expression with variation in transmitter release probability

The strength of synaptic transmission is highly variable between different synapses. The present study examined some factors that may contribute to this variation in the strength of neurotransmission in sympathetic varicosities of the mouse vas deferens. Transmitter release was measured using a focal macropatch electrode placed over pairs of visualised varicosities. By regulating the calcium concentration of the solutions inside the recording electrode and in the bath independently of each other, transmitter release was restricted to one or two surface varicosities at each recording site. Using this technique, transmitter release probability was shown to be highly variable, even between adjacent varicosities on single axon branches. Very little variation was observed in the calcium influx following single impulse nerve stimulation between adjacent Oregon Green BAPTA-1 loaded varicosities. However, the staining intensities of three vesicular proteins, SV2, synaptophysin, and synaptotagmin 1, showed considerable variation between adjacent varicosities on single axon branches. This variation in staining intensity may be partly explained by variation in the density of synaptic vesicles. However, double staining experiments using two vesicular antigens showed some varicosities staining for one vesicular antigen, but not for the second, suggesting that the expression of these release machinery proteins is regulated locally within the varicosities. The results of the present study strengthen suggestions that synaptic strength is at least in part, regulated by variation in the expression of vesicular proteins.

The effect of N-ethylmaleimide on transmitter release from the skeletal neuromuscular junction ofBufo marinus

N-ethylmaleimide (NEM) has been used extensively in biochemical assays as an inhibitor of the NEM sensitive fusion protein (NSF). However, examination of the effect of NEM on transmitter release in more physiologically relevant preparations has proved inconclusive. In the present study, we have examined the effect of low concentrations of NEM on synaptic transmission in intact nerve-muscle preparations from toads (Bufo marinus). Under conditions of low transmitter release probability (0.3 mM calcium, 1 mM magnesium), treatment with NEM (10 microM) caused a significant increase in the amplitude of stimulus-evoked endplate potentials (EPPs) and a significant increase in the frequency of spontaneously occurring miniature EPPS (MEPPS) without affecting the amplitude of MEPPs. When the calcium concentration in the bath was raised to 4 mM, 10 microM NEM had no effect on EPP amplitude. Under these conditions, NEM treatment reduced paired pulse facilitation and increased depression during stimulus trains. Treatment with NEM also resulted in a significant decrease in the synaptic delay. The effects of NEM on transmitter release in the present study were not due to inactivation of G-proteins. The results of the present study show a calcium-dependent facilitation of stimulus-evoked transmitter release by NEM. These results are discussed in terms of the possible sites of NEM action leading to the observed changes in transmitter release.

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 principal component and spectral analyses with the method of moments in studies of quantal transmission

This chapter considers methods for measurements of postsynaptic responses and simple approaches to the estimation of parameters of quantal release in synapses of the central nervous system of vertebrates. The use of these methods is illustrated by the analysis of single-fibre and "minimal" monosynaptic postsynaptic potentials (PSPs) or currents (PSCs) recorded from neurons of the frog spinal cord and rat hippocampus. First, we briefly discuss traditional methods of the response measurements using peak amplitudes or areas, further focusing on a novel method based on multivariate statistical techniques of the principal component analysis (PCA). This approach provides typically better signal-to-noise ratios and is able to separate two or more response components, which can arise due to activation of more than one presynaptic fibre, axon collaterals, receptor subtypes or spatially separated transmission sites. Second, spectral analysis is introduced as the method of choice to verify whether the amplitude fluctuations of the postsynaptic responses have a quantal nature and to obtain estimations of the "basic" quantal parameters, i.e. the quantal size (Q) and mean quantal content (m), without introducing assumptions on release statistics. Third, we show how the method of moments could be applied in the framework of the Poisson and binomial models to estimate the basic quantal parameters and parameters p and n, which reflect the release probability and maximum number of quanta released (or the number of effective release sites), respectively. Fourth, we show that the analysis of the moments can also be instrumental to reveal non-uniformity of release probabilities and compare how several competing models of neurotransmitter release fit to multiple experimental data sets.

Functional analysis of neurotransmission at beta2-laminin deficient terminals

beta2-Laminin is important for the formation of neuromuscular junctions in vertebrates. Previously, we have inactivated the gene that encodes for beta2-laminin in mice and observed predominantly prejunctional structural defects. In this study, we have used both intra- and extracellular recording methods to investigate evoked neurotransmission in beta2-laminin-deficient mice, from postnatal day 8 (P8) through to day 18 (P18). Our results confirmed that there was a decrease in the frequency of spontaneous release, but no change in the postjunctional response to such release. Analysis of evoked neurotransmission showed an increase in the frequency of stimuli that failed to elicit an evoked postjunctional response in the mutants compared to litter mate controls, resulting in a 50 % reduction in mean quantal content at mutant terminals. Compared to littermate controls, beta2-laminin-deficient terminals showed greater synaptic depression when subjected to high frequency stimulation. Furthermore, the paired pulse ratio of the first two stimuli was significantly lower in beta2-laminin mutant terminals. Statistical analysis of the binomial parameters of release showed that the decrease in quantal content was due to a decrease in the number of release sites without any significant change in the average probability of release. This suggestion was supported by the observation of fewer synaptic vesicle protein 2 (SV2)-positive varicosities in beta2-laminin-deficient terminals and by ultrastructural observations showing smaller terminal profiles and increased Schwann cell invasion in beta2-laminin mutants; the differences between beta2-laminin mutants and wild-type mice were the same at both P8 and P18. From these results we conclude that beta2-laminin plays a role in the early structural development of the neuromuscular junction. We also suggest that transmitter release activity may act as a deterrent to Schwann cell invasion in the absence of beta2-laminin.

Stressful animal housing conditions and their potential effect on sympathetic neurotransmission in mice

Although the sympathetic nervous system (SNS) plays a major role in mediating the peripheral stress response, due consideration is not usually given to the effects of prolonged stress on the SNS. The present study examined changes in neurotransmission in the SNS after exposure of mice (BALB/c) to stressful housing conditions. Focal extracellular recording of excitatory junction currents (EJCs) was used as a relative measure of neurotransmitter release from different regions of large surface areas of the mouse vas deferens. Mice were either group housed (control), isolation housed (social deprivation), group housed in a room containing rats (rat odor stress), or isolation housed in a room containing rats (concurrent stress). Social deprivation and concurrent stressors induced an increase of 30 and 335% in EJC amplitude, respectively. The success rate of recording EJCs from sets of varicosities in the concurrent stressor group was greater compared with all other groups. The present study has shown that some common animal housing conditions act as stressors and induce significant changes in sympathetic neurotransmission.

Evidence for two distinct processes in the final stages of neurotransmitter release as detected by binomial analysis in calcium and strontium solutions

The statistical parameters underlying acetylcholine (ACh) release were studied using Ca(2+) and Sr(2+) ions to promote ACh secretion. Experiments were performed at frog neuromuscular junctions using electrophysiological recording techniques. Increases in asynchronous ACh release, reflected as the frequency of occurrence of miniature end-plate potentials (MEPP(f)), were evoked by high potassium depolarization in either Ca(2+) or Sr(2+) solutions. Increases in MEPP(f) mediated by Ca(2+) were of very low probability and well-described by a Poisson distribution whilst similar MEPP(f) increases mediated by Sr(2+) were best described as a simple binomial distribution. From the binomial distribution in Sr(2+) solutions, values for the average probability of release (p) and the number of releasable ACh quanta (n) may be determined (whereby mean MEPP(f) = np). In Sr(2+) solutions, values of p were independent of both bin width and of the value of n, suggesting that both n and p were stationary. Calculations of p using the simple binomial distribution in Sr(2+) solutions gave theoretical values for the third moment of the mean which were indistinguishable from the experimental distribution. These results, in conjunction with Monte Carlo simulations of the data, suggest that spatial and temporal variance do not measurably affect the analysis. Synchronous ACh release evoked by nerve impulses (end-plate potentials, EPPs) follow a simple binomial distribution in both Ca(2+) and Sr(2+) solutions. Similar mean levels of synchronous ACh release (m, where m = np) were produced by lower values of p and higher values of n in Ca(2+) as compared to Sr(2+). The statistical analyses suggest the presence of two different Ca(2+)-dependent steps in the final stages of neurotransmitter release. The results are discussed in accordance with (i) statistical models for quantal neurotransmitter release, (ii) the role of Sr(2+) as a partial agonist for evoked ACh release, and (iii) the specific loci that may represent the sites of Ca(2+) and Sr(2+) sensitivity.

Analysis and implications of equivalent uniform approximations of nonuniform unitary synaptic systems

Real synaptic systems consist of a nonuniform population of synapses with a broad spectrum of probability and response distributions varying between synapses, and broad amplitude distributions of postsynaptic unitary responses within a given synapse. A common approach to such systems has been to assume identical synapses and recover apparent quantal parameters by deconvolution procedures from measured evoked (ePSC) and unitary evoked postsynaptic current (uePSC) distributions. Here we explicitly consider nonuniform synaptic systems with both intra (type I) and intersynaptic (type II) response variability and formally define an equivalent system of uniform synapses in which both uePSC and ePSC amplitude distributions best approximate those of the actual nonuniform synaptic system. This equivalent system has the advantage of being fully defined by just four quantal parameters: ñ, the number of equivalent synapses;p, the mean probability of quantal release; mu, mean; and sigma(2), variance of the uePSC distribution. We show that these equivalent parameters are weighted averages of intrinsic parameters and can be approximated by apparent quantal parameters, therefore establishing a useful analytical link between the apparent and intrinsic parameters. The present study extends previous work on compound binomial analysis of synaptic transmission by highlighting the importance of the product of p and mu, and the variance of that product. Conditions for a unique deconvolution of apparent uniform synaptic parameters have been derived and justified. Our approach does not require independence of synaptic parameters, such as p and mu from each other, therefore the approach will hold even if feedback (i.e., via retrograde transmission) exists between pre and postsynaptic signals. Using numerical simulations we demonstrate how equivalent parameters are meaningful even when there is considerable variation in intrinsic parameters, including systems where subpopulations of high- and low-release probability synapses are present, therefore even under such conditions the apparent parameters estimated from experiments would be informative.

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.

Unveiling synaptic plasticity: a new graphical and analytical approach

Short-term synaptic plasticity has a key role in information processing in the CNS, whereas memories can be formed through long-lasting changes in synaptic strength. Despite the importance of these phenomena, it remains difficult to determine whether a synaptic modulation is expressed at a presynaptic or postsynaptic site. This article describes a new approach that, in its simplest form, can identify the site of expression by direct graphical means. A more-sophisticated form of the technique can quantify functional synaptic properties and determine which of these properties is altered following a modulation of synaptic strength.

Vesicle-associated proteins and quantal release at single active zones of amphibian (Bufo marinus) motor-nerve terminals

A study was made to determine the disposition of vesicle-associated proteins (syntaxin, SV2, SNAP-25) and calcium channels with respect to the spatial extent of spontaneous and evoked quantal release within regions of amphibian motor-nerve terminal branches delineated by FM1-43 stained vesicle clusters (blobs). Discrete concentrations of vesicles revealed approximately 2 microm apart along the length of terminal branches through FM1-43 staining were identical in size and spacing to those identified along terminal branches with SV2 antibody (AbSV2). Fluorescent antibodies to syntaxin 1 (AbS), SNAP-25 (AbS25) and the calcium channel alpha1B subunit (Abalpha1B) were found in relatively high concentrations coincident with the AbSV2 blobs. Three extracellular recording electrodes were placed in the vicinity of individual FM1-43 blobs, and an algorithm was used to determine the spatial origin of miniature endplate potentials (MEPPs) and EPPs together with their relative amplitudes. MEPPs and EPPs originated throughout the region stained by FM1-43 but not elsewhere; amplitude-frequency distributions of MEPPs and EPPs were similar for all FM1-43 blobs with average coefficients of variation of no less than 0.28. A linear relationship existed between the size of an FM1-43 blob, measured as the integrated extent of FM1-43 staining of a blob, and the frequency of MEPPs as well as the probability of EPPs from the blob. There was a proximo-distal gradient in the size of FM1-43 blobs along the length of single terminal branches, suggesting a gradient in release probability along the branches. The frequency distribution of the distances between blobs was approximately Gaussian, whereas the frequency distribution of the size of blobs was highly skewed and was best fitted with a gamma distribution. It is concluded that there are correlations among the extent of labeling of SNAP-25, syntaxin and calcium channels at a release site, the store of vesicles to be found there, and the probability of spontaneous and evoked quantal release.

Decrease of quantal size and quantal content during tetanic stimulation detected by focal recording

End-plate potentials and miniature end-plate potentials were recorded focally (i.e., over a limited area of the end-plate with several or possibly only one active zone) in a cutaneous pectoris from neuromuscular junction during a prolonged (1-6 min) tetanic (20-100 Hz) nerve stimulation. End-plate potential amplitudes decreased and became more variable with prolonged stimulation. Synaptic depression thus occurs even when synaptic output is low, if release is evoked from only a few active zones, suggesting that there is little if any vesicular replenishment between the active zones. The probability density function of the end-plate potential amplitudes has been obtained using the Parzen estimate with a Gaussian weighting function, to reduce the number of end-plate potentials needed for the same accuracy. Quantal size of the end-plate potentials was estimated from the slope of the best fitted line to the prominent and apparently equidistant peaks of probability density functions or from the spectrogram of the probability density function of end-plate potentials. Quantal contents were initially (+/- S.D.):5.7 +/- 2.9, ranged from 2 to 12, and in all cases examined (n = 11) decreased with prolonged tetanic stimulation. The rates of the decrease of end-plate potentials amplitudes (and quantal contents) from different segments of the same nerve terminal were often different, even when they were initially comparable. This suggests that some active zones or some areas of the end-plates become depleted much faster than others. Quantal sizes of the nerve evoked and the spontaneously released quanta were generally similar at low frequencies of stimulation (0.5-2 Hz). Both decreased with high frequency stimulation, but the decrease of the quantal sizes of nerve evoked quanta was usually more pronounced. At different loci of the same end-plate the contribution of lower quantal size to the synaptic depression varied widely (from < 5% to > 80%). In conclusion lower quantal size can contribute significantly to synaptic depression. At uneven decrease of quantal sizes over the whole nerve terminal helps to explain both aspects of synaptic depression (lower synaptic efficacy and greater variability of quantal responses.

The effect of opiates on the terminal nerve impulse and quantal secretion from visualized amphibian nerve terminals

1. Secretion of transmitter from amphibian motor nerve terminal release sites is intermittent, spatially non-uniform and varies considerably throughout the year and during development. The role of opioid receptors in modulating transmitter secretion from amphibian motor nerve terminals is evaluated in this study. 2. Dynorphin-A (24 microM) and morphine (500 microM) did not significantly change the shape of the nerve impulse or the consistency with which it was observed, but decreased evoked quantal secretion by more than 50%. These effects of dynorphin-A and morphine were largely reversed by naloxone (50 microM). 3. Dynorphin-A and morphine did not significantly change either the amplitude or the frequency of spontaneous quantal secretions. 4. There was a uniform decrease in evoked quantal secretion from release sites along terminal branches, irrespective of the quantal content value before drug treatment, indicating no difference in the susceptibility of proximal vs distal release sites to opiates. 5. Increasing the extracellular calcium concentration (0.3 to 0.4 mM) or trains of conditioning-test impulses (25 to 100 Hz) resulted in smaller dynorphin-A or morphine-induced decreases in evoked quantal secretion. 6. The decrease in evoked quantal secretion occurs as a result of a uniform decrease in the probability of quantal secretion from release sites without any affect on the propagation of the nerve terminal impulse. Low probability release sites become effectively silent.

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.

Effects of vesicular acetylcholine uptake blockers on frequency augmentation-potentiation in frog neuromuscular transmission

Vesamicol inhibits the vesicular loading of acetylcholine molecules. The effects of vesamicol and similarly acting compounds on neuromuscular transmission in frogs were investigated to determine whether these inhibitors-inhibit the frequency augmentation-potentiation of transmitter release. Various vesicular acetylcholine transport blockers suppressed the stimulation frequency-related release parameter, k, in a dose-dependent manner. Artane, cetiedil, chloroquine, ethodin, quinacrine, vesamicol and its benzyl-analogue, 2-(4-benzylpiperidino)cyclohexanol, had strong effects, while those of aminacrine, chlorpromazine, fluphenazine, imipramine, pyrilamine and thioridazine were weak. A significant correlation was observed between the biochemically reported values of IC50 and the electrophysiological inhibitory potencies on k at 20 microM. Contrary to expectations from the biochemical data, however, vesamicol and its benzyl-analogue showed equipotent inhibitory actions on the electrophysiological frequency augmentation-potentiation relation. Low sensitivity and low selectivity of the frequency augmentation-potentiation for vesamicol and its benzyl-analogue lead us to conclude that the vesicular acetylcholine transporter is not the site of the electrophysiological action of vesamicol and similarly acting chemicals.

Probabilistic secretion of quanta from visualized sympathetic nerve varicosities in mouse vas deferens

1. Sympathetic varicosities on the surface of smooth muscle cells of the mouse vas deferens were visualized with the fluorescent dye 3-3 Diethyloxardicarbocyanine iodide (DiOC2(5)) and quantal secretion recorded from these with both small diameter (4-6 microns) and large diameter (20-50 microns) microelectrodes. Small diameter electrodes were placed over one to three varicosities and large diameter electrodes over three to seven varicosities. 2. The size and distribution of varicosities along individual terminal branches was about the same when these were fluoresced with DiOC2(5) (length 1.09 +/- 0.40 microns (mean +/- S.D.); intervaricosity distance 5.53 +/- 2.68 microns) as when they were stained for catecholamines using Faglu fluorescence (length 1.05 +/- 0.43 microns; intervaricosity distance 5.12 +/- 2.79 microns) suggesting that DiOC2(5) does allow for identification of the catecholamine-containing varicosities. 3. The spontaneous excitatory junctional currents (EJCs) recorded from visualized varicosities with small diameter electrodes (amplitudes 59-67 microV) were much larger than those recorded with large diameter electrodes (amplitudes 25-29 microV). The frequency of evoked EJCs as well as the amplitude-frequency distribution of these EJCs varied greatly between sets of visualized varicosities recorded along individual branches, either with a small or large diameter electrode. These amplitude-frequency distributions typically followed Poisson statistics, in which the mean quantal content of the EJC (m) varied by over threefold for different sets of varicosities on the same branch (m was 0.07-0.21 for small electrodes whereas m was 1-3 for large electrodes). 4. Although m varied considerably for a constant number of varicosities beneath the electrode at different sites along a single branch, there was an overall correlation between m and the number of varicosities, m increasing on average 0.25 for each additional varicosity in a [Ca2+]o of 4.0 mM. 5. The frequency of evoked EJCs at visualized sets of varicosities along some branches was sufficiently high to allow binomial statistics to predict the amplitude-frequency distributions of evoked EJCs. In these cases m was again shown to vary considerably along single terminal branches, and this was primarily due to variation in the probability of secretion (p) between sets of varicosities and not to variation in binomial parameter n. 6. In one case a relatively isolated varicosity, over 3 microns from adjacent varicosities, was recorded for 30 min with a 4 microns diameter electrode. The mean and variance of the evoked EJC was similar to that of the spontaneous EJCs suggesting that this varicosity secreted at most one quantum on arrival of the nerve impulse.(ABSTRACT TRUNCATED AT 400 WORDS)

Modeling of the quantal release at interneuronal synapses: analysis of permissible values of model moments

A theoretical study of effects of the different factors on fluctuation of post-synaptic potential (PSP) amplitudes was undertaken, using computation of regions of permissible values (RPV) of the ratio between the variance and the mean number of the quanta released (R1) and the ratio between the third moment and the variance (R2). The RPVs of these indexes for the binomial model were compared with regions determined for a number of models incorporating several factors. It has been shown that the involvement of temporal non-uniformity of transmitter release probability, decremental spreading of potentials along dendrites, and failure of spike propagation give the values of skewness index R2 less, compared to the binomial model. Simultaneously, a number of other factors, especially spatial non-uniformity of release probabilities in single release sites, would give amplitude histograms with high positive values of the index. The values of R1 and R2, calculated for 21 samples of sensorimotor EPSP amplitudes, were biased from RPV of these parameters constructed for the binomial model. The scattergram of R1 and R2 can be explained by the presence of two kinds of contacts which release quantum with different probabilities. The same was true for the beta-model based on the assumption that probabilities of quantal release are a sample of values of random variable that has beta-distribution. From analysis of the distribution of individual release probabilities, obtained from evaluation of beta-model parameters, is concluded that a greater part of boutons in the sensorimotor synapses release transmitter with very low probabilities, there being, however, a few boutons with probabilities close to 1.

Analysis of fluctuations of "minimal" excitatory postsynaptic potentials during long-term potentiation in guinea pig hippocampal slices

In previous studies, quantal analysis assuming a simple binomial model has shown that long-term potentiation (LTP) is accompanied by an increase in both mean quantal content (m) and quantal size (v), whereby the increase in m predominates. In the present study, "compound" binomial distributions with variable probabilities were convolved with Gaussian distributions in computer experiments to simulate amplitude histograms of intracellular excitatory postsynaptic potentials (EPSPs). A deconvolution procedure assuming equal "quantal" separation (v) between discrete components, but without assuming binomial statistics, was applied to the simulated distributions to determine v. It was found that with a small ratio of standard deviation of noise to v (Sn/v less than 0.4), a reliable estimate of v can be obtained even for small samples (N = 100). When Sn/v was larger (0.4-0.6), approximate v estimates (within +/- 10-20% of the simulated v) could be obtained by averaging estimates from about 10 small samples (N = 100). "Minimal" EPSPs were recorded in area CA1 of guinea pig hippocampal slices. 37 EPSP amplitude samples of 9 neurones were measured before and up to 55 min after 10 tetanizations of stratum radiatum. In accordance with the previous data, the increase in v accounted for only about 10% of the average post-tetanic increase in EPSP amplitude and was not correlated with the latter. However, for an EPSP subset with small LTP magnitude, the increase in v accounted for an essential part of the LTP magnitude while the increase in m did not correlate with it. The results are in agreement with previous data obtained in the context of the simple binomial model and are interpreted as indicating primarily a presynaptic mechanism of LTP maintenance. The results suggest two types of synaptic mechanism of LTP maintenance related to the increases in m and v, respectively. The latter mechanism is saturated at about 10 to 30% increase in post-tetanic amplitude above the pre-tetanic EPSP amplitude.

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.

Non-uniform responses to Ca2+ along the frog neuromuscular junction: effects on the probability of spontaneous and evoked transmitter release

Spontaneous and evoked transmitter release activity was studied during selective application of Ca2+ in proximal (near the first contact of the axon on the muscle fiber) and distal regions of the frog neuromuscular junction. A new technique called "Microperfusion" was developed, which allowed us to apply a 30-microns-wide Ca2+ stream from an external pipette. The spread of this Ca2+ stream was monitored by adding Blue Dextran (40 mg/ml) to the Ca2+ solution. Microperfusion with a Ca2(+)-free Ringer containing Blue Dextran did not affect the miniature endplate potential frequency or amplitude. Changes of spontaneous transmitter release were studied either during microperfusion of Ringer containing 5 mM Ca2+ or during microperfusion of 2 mM Ca2+ simultaneously with the stimulation of the motor nerve. This second procedure also permitted study of the characteristics of evoked release. Microperfusion of Ca2+ induced a larger and more rapid increase in the miniature endplate potential frequency in proximal than in distal regions. The time required for the miniature endplate potential frequency to return to the control value after Ca2+ microperfusion was longer than the time needed to increase the frequency and this decay period was longer in the proximal region than in the distal one. Moreover, miniature endplate potentials produced in proximal regions, were typically larger and more variable than those produced in distal regions. In five experiments, the endplate potentials produced by 100-200 pulse pairs (interval of 15 ms at every 2 s) were recorded intracellularly during the microperfusion. The quantal content of the first endplate potential of the pair (EPP1) was systematically smaller in distal regions than in proximal regions. The percentage of failures and the coefficients of variation were higher in distal than in proximal regions, indicating a larger variability of quantal content. The frequency facilitation was not different between the two regions, but, however, the second stimuli of the pair usually produced a net increase of transmitter release which was greater in proximal than distal regions. Our experiments demonstrate that both the spontaneous and the evoked release are more responsive to Ca2+ application in the proximal than in the distal regions of the frog neuromuscular junction.

Transmitter release: prepackaging and random mechanism or dynamic and deterministic process

Stepwise variations in end-plate potential amplitudes that are also multiples of spontaneous miniature end-plate potentials (MEPPs) demonstrate a quantal nature of evoked transmitter release at the vertebrate neuromuscular junction. Both the number of quanta which form relatively small end-plate potentials (EPPs) and the time intervals between MEPPs were found to fit Poisson statistics. These observations suggested that the release process randomly liberates uniform quantities of transmitter. Initial studies showed that quantal size remained stable after seemingly high rates of release which was interpreted to indicate that a large store of equally sized, equally available, and independently releasable quanta are present in the nerve terminals. The observation of numerous presynaptic vesicles that contain transmitter provided a morphological basis for prepacked transmitter (i.e., quanta). However, physiological studies over the last 15 years have yielded data that are difficult to incorporate into the quantum-vesicle hypothesis. With normal conditions and during most treatments which increase the rate of release, two classes of MEPPs have been found and both show a substructure. The bell-MEPP class was characterized by Fatt and Katz and the smaller skew-MEPP class has been studied by Kriebel. The ratio of the two classes and substructure compositions of both classes are variable. Short series of MEPPs and unitary EPPs (U-EPPs) show preferred amplitudes and longer series of MEPPs and U-EPPs show stepwise variations in amplitude. Slow-MEPPs and giant MEPPs belong to the skew class and represent nearly synchronous bursts of smaller MEPPs. Transmitter packet formation, preferred amplitudes, stepwise variations in amplitudes, random-like distributions and organized bursts can be simulated by a simple deterministic system, the drop formation process, that is known for its periodic and chaotic behaviors which are determined by the single parameter of flow rate. MEPP intervals, sizes and classes, are also dependent on rates of release which demonstrate that the release process(es) is highly organized and sensitive to different conditions. We demonstrate that the processes of drop formation and release of a packet of transmitter have similar properties and that deterministic characteristics describe MEPP and U-EPP time dependencies and amplitude substructures. The data and model presented here suggest that packet size of acetylcholine may be determined at the moment of release.

The probability of quantal secretion at release sites of different length in toad (Bufo marinus) muscle

1. The evoked quantal secretion recorded with an extracellular microelectrode (me) at selected sites along motor terminal branches visualized with the fluorescent dye 3-3 diethyloxardicarbocyanine iodide (DiOC2(5)) was compared with the maximum length of the synaptic contact at these release sites reconstructed from serial sections examined with the electron microscope. In addition the relationship between the binomial probability of secretion at release sites (pe) and the length of the synaptic contact was determined in an extracellular calcium. [Ca2+]o, of 0.35 mM. 2. Three of the six terminal branches studied in this way showed a decline in synaptic contact length of release sites from near the point of nerve entry (proximal) to the end of the branch (distal). The remaining three branches showed an increase in synaptic contact length near their middle, and in each case this was associated with a Schwann cell nucleus: contact length then declined to the end of the branches. 3. Both me and pe increased linearly with an increase in the maximum length of the synaptic contact over a range from 0.4 to 4.0 microns. This occurred independently of how the synaptic contact length varied along the length of terminal branches. The value of pe increased by about 0.05 for each 1 micron increase in synaptic contact length in a [Ca2+]o of 0.35 mM. 4. The decrease in synaptic contact length along the proximal parts of terminal branches, in which this occurs, is mostly due to a decrease in the length of close opposition (less than 0.2 micron) between the nerve terminal membrane and the postsynaptic membrane: the decrease in more distal parts of branches is due to the progressive encroachment of Schwann cell processes between the presynaptic and postsynaptic membranes as well as a decrease in synaptic contact length.

The probability of quantal secretion at release sites in different calcium concentrations in toad (Bufo marinus) muscle

1. The number of quanta secreted from visualized release sites along terminal branches at toad (Bufo marinus) neuromuscular junctions in different extracellular concentrations of calcium ions. [Ca2+]o, and during facilitation was determined. Terminal branches were visualized by prior staining with the fluorescent dye, 3-3 diethyloxardicarbocyanine iodide (DiOC2(5)). 2. Increasing [Ca2+]o between 0.25 and 0.4 mM gave a similar proportional increase in the mean quantal content of the end-plate potential recorded with an extracellular electrode (me) at all sites along terminal branches. Thus the length constant for the experimental decline in me along terminal branches (the quantal length constant) remained constant with an increase in [Ca2+]o. The increase in m with [Ca2+]o followed a fourth power relation at both proximal and distal release sites. 3. The increase in me with [Ca2+]o was almost entirely due to an increase in the binomial probability for secretion, pe, which increased as the third to fourth power of [Ca2+]o. However, at higher [Ca2+]o there was an increasing tendency for the binomial parameter ne to increase. It is shown that when ne increases by about 1 there is very little change in pe, suggesting that the new release site recruited at high [Ca2+]o has a relatively low probability for secretion. 4. Test impulses gave a similar proportional increase in me following a conditioning impulse at all sites along terminal branches. The quantal length constant remained constant for both conditioning and test values of me. The increase in me for the test impulse increased linearly with an increase in me for the conditioning impulse at all release sites. 5. Facilitation of me declined exponentially with an increase in the test-conditioning interval. The time constant for this decline (30-40 ms) was similar at both proximal and distal release sites. Changes in facilitation of me were almost entirely due to changes in pe except at very short test-conditioning intervals of about 10 ms. At these intervals ne frequently increased by about 1 and there was very little change in pe. Again, this suggests that additional release sites recruited at short intervals have relatively low probability for secretion. 6. The results indicate that relatively low probability release sites exist in close juxtaposition to relatively high probability release sites which themselves decline in probability along the length of terminal branches.

Proximodistal gradients of the postjunctional folds at the frog neuromuscular junction: a scanning electron microscopic study

Frog endplates were studied with the scanning electron microscope following the removal of the presynaptic terminal by collagenase and acid treatments. Endplates had 2-14 branches of primary cleft. The longest branches were parallel to the muscle fiber. Short branches oblique or perpendicular to the muscle fiber were also present near the central region of the endplates. The openings of postjunctional folds in the primary cleft were clearly visible at the bottom of the primary cleft and could be counted and measured. The longest primary cleft branches of each endplate were divided into segments of 20 microns (length corrected for shrinkage). The number of postjunctional folds per micrometer of primary cleft, the average postjunctional fold length (i.e. across the primary cleft) and the total postjunctional fold's length per micrometer of primary cleft were evaluated for each 20-microns segment of primary cleft. Negative proximodistal gradients were observed for these three parameters for the long branches of primary cleft, i.e. values were higher in the proximal region (near the motor axon) than in the distal region. These postsynaptic gradients probably reflect similar or smaller proximodistal presynaptic gradients for the active zones along the nerve.

Frequency and amplitude gradients of spontaneous release along the length of the frog neuromuscular junction

Experiments were done at the neuromuscular junctions (NMJs) of the cutaneous pectoris muscle of the frog to test the validity of the "spatial decay method" which uses simultaneous recording with two intracellular electrodes, each located in the muscle fiber near a distal end of the terminal. The miniature endplate potential (MEPP) peak amplitude recorded by each electrode is used to calculate the position of the release site producing it as well as its initial amplitude in front of its release site. The validity of the method was tested with a third focal electrode located between the two intracellular electrodes, at about 60-100 microns from one of the intracellular electrodes, the probability of spontaneous transmitter release was not uniform along the NMJ either in low or in normal Ca2++ concentration. The release sites located in the region close to the first point of contact between the axon and the muscle fibre, usually close to the center of the NMJ, were spontaneously more active than the distal release sites. The degree of the steepness of proximodistal release gradients varied among different junctions. The less active regions tended to produce smaller MEPPs than did the more active ones. In fact, a correlation was found between the logarithm of MEPP frequency in a given region and the mean corrected MEPP amplitude in that region. These results also show that MEPP frequency was modulated to a greater extent than MEPP amplitude along the frog nerve terminal. The proximodistal gradient in MEPP frequency may be related to corresponding gradients in density and length of release sites along the junction, whereas MEPP amplitude gradients may be related to gradients in the length of release sites and/or postjunctional folds.

Quantal secretion at release sites of nerve terminals in toad (Bufo marinus) muscle during formation of topographical maps

1. The number of quanta secreted from selected sites along terminal branches at suppressed synapses in the developing toad (Bufo marinus) gluteus muscle has been determined. The topographical projection from segmental nerves 8 and 9 to the ventral surface of this muscle matures slowly as toads develop in size from 12 to 40 g. Terminal branches of nerves 8 and 9 were visualized by prior staining with the fluorescent dye, 3-3-diethyloxardicarbocyanine iodide (DiOC2(5]. 2. The evoked quantal release recorded with an extracellular electrode (m(e) at different positions along the length of terminal branches at synaptic sites innervated either by nerve 8 (me,8) or nerve 9 (me,9) was determined in an external Ca2+ concentration, [Ca2+]o, of 0.35-0.45 mM. For over 90% of branches longer than 80 microns, me declined along exponential curves from a relatively large value at the proximal end of branches for both nerve 8 and nerve 9 terminals; the exponent for these exponential curves gave quantal length constants that varied from 26 to 80 microns (48 +/- 4 microns, mean +/- S.E.M.) depending on the length of the branch. 3. The evoked quantal release recorded with an intracellular electrode (m) at synaptic sites dually innervated by nerve 8 and nerve 9 was nearly always (greater than 90%) greater for nerve 8 terminals than for nerve 9 terminals. At singly innervated sites the value of m per 100 microns length of terminal declined approximately exponentially with an increase in total terminal length (length constant 400 microns). However, at dually innervated sites the value of m per 100 microns length of nerve 9 terminal was very low at all total terminal lengths compared with singly innervated sites; this indicates that nerve 9 terminals were suppressed at dually innervated sites. 4. At five dually innervated sites, seven out of nine terminal branches of nerve 8 showed an exponential decline in me,8 along their length, from a relatively large value near the proximal end of the branches (length constant 35 +/- 3 microns, mean +/- S.E.M.). In contrast, all the terminal branches of nerve 9 greater than 80 microns showed a uniformly low value of me,9 along their length. 5. It is suggested that the suppression of nerve 9 terminals at dually innervated sites is primarily due to a decrease in the probability of secretion of normally highly secreting release sites at the proximal end of terminal branches.

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.

Non-uniform release at the frog neuromuscular junction: evidence of morphological and physiological plasticity

The frog neuromuscular junction (NMJ) is a fusiform structure parallel to the muscle fiber with a few secondary and tertiary branches. Both sprouting and regression can occur on the same nerve terminal, suggesting a continuous on-going remodelling of the mature neuromuscular junction. Thus, the frog NMJ is a dynamic structure. Ultrastructural observations of the nerve terminal suggest that the active zones are distributed equally along the mature nerve terminal. Disorganized active zones have however been observed in distal regions. The density of synaptic vesicles is also uniform throughout the whole structure. However, mitochondria appear to be more abundant in the very distal regions of the nerve terminal. The postjunctional folds and the cholinergic receptors are also uniformly distributed along the NMJ. However, during remodelling periods, the distributions of postjunctional folds and of cholinergic receptors are not uniform in the degenerating and regenerating regions. Fig. 1 summarizes these morphological data. The frequency of spontaneous release (MEPPs) at the NMJ is higher in the proximal region than in the distal regions and recent evidence suggests that the mean MEPP amplitude is higher in the proximal than in the distal portions. Evoked transmitter release is also non-uniform along the frog NMJ. As for spontaneous release, it is higher in the proximal regions than in the distal regions. Failures of the active propagation of the PNAP at low safety points, such as the end of the myelinated axon and the branching points, may be one of the mechanisms responsible for unequal evoked release. It is also possible that the PNAP does not actively invade the whole extend of the nerve terminal since Na+ channels are absent from the distal regions. Fig. 2 summarizes these physiological data.

Non-uniform distribution of miniature endplate potential amplitudes along the length of the frog neuromuscular junction

We studied the amplitude distribution of miniature endplate potentials (MEPPs) as a function of the release site position along the frog neuromuscular junction. Following intracellular recordings with two electrodes each located at one distal end, we used the spatial decay method to evaluate the release site position of each MEPP and the MEPP amplitude in front of its release site. Nitroblue-tetrazolium-cholinesterase staining was performed for further morphological correlation. Our results indicate that MEPP amplitude distributions are skewed to the left for distal regions, while MEPP amplitudes are more widely distributed for the proximal regions (i.e. near the last myelinated segment). This suggests that distal regions produce predominantly MEPPs with a small amplitude whereas proximal regions produce MEPPs of all sizes (small to large).

Competition between segmental nerves at end-plates in rat gastrocnemius muscle during loss of polyneuronal innervation

1. The segmental innervation of the rat lateral gastrocnemius (l.g.) muscle is from lumbar nerves L4 and L5. A study has been made of changes in the innervation of the l.g. muscle by nerves L4 and L5 before and after removal of L4 during the loss of polyneuronal innervation which occurs over the first 2 weeks postnatal. 2. The rat l.g. muscle is a complex of three unipennate muscles; the lateral, intermediate and medial heads. Each of these has a characteristic distribution of type I and type II muscle fibres of different diameter by 12 days postnatal. Following the removal of nerve L4 at birth there is no change in the distribution of fibre types in the different heads of the l.g. muscle at 12 days postnatal. 3. The number of fibres in the l.g. muscle doubled between birth and 12 days postnatal and the maximum tetanic force generated by the muscle increased by fourfold. Removal of nerve L4 at birth did not alter the increase in fibre number and there was no difference between the tension generated by the l.g. muscle in the ipsilateral and contralateral limbs of operated animals. 4. The number of detectable motor units in a extracellular calcium concentration [( Ca]o) of 2 mM remained constant in L4 at about fifteen between birth and 12 days postnatal; the number of detectable motor units in L5 declined over this period from about fifteen to six in a [Ca]o of 2 mM. Following the removal of L4 at birth, the number of motor units in L5 increased to twenty by 3 days postnatal and remained there for at least 12 days in a [Ca]o of 2 mM. 5. The mean size of motor units and the range of motor unit sizes declined for both L4 and L5 during the postnatal period. Following the removal of L4, the mean size of motor units and the range of motor unit sizes in L5 failed to decline. 6. The number of motor units detected at birth in a raised [Ca]o of 3-5 mM increased in L4 and L5 to about twenty; this estimate in high [Ca]o remained constant during the postnatal period. Estimates of the size of the large, low threshold motor units declined whether these were made in low or high [Ca]o.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Effects of octopamine on miniature excitatory junction potentials from developing and adult moth muscle

Intracellular recordings of excitatory junction potentials (EJPs) and miniature EJPs (MEJPs) were made from the dorsal longitudinal muscle of Manduca sexta to determine the sites of action of octopamine. MEJPs increased in amplitude and frequency as the moth developed during the 3 days before eclosion. DL-Octopamine (5 X 10(-6) M) increased the amplitude of excitatory junction potentials in both immature moths (one day before eclosion) and adults. Octopamine (10(-5) M) also increased the amplitude and frequency of MEJPs from immature animals (one and two days before eclosion) but had the opposite effect on adults and pharate adults ready to eclose. Treatment with octopamine (10(-5) M) resulted in a decrease in input resistance and a hyperpolarization in both immature and adult muscle fibers. The results suggest that octopamine acts both presynaptically and postsynaptically but that the increase in the amplitude of the evoked response is due primarily to influences on presynaptic processes.

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

The use of end-plate current (e.p.c.) latency measurements to estimate the time course of the stochastic probabilistic process governing evoked release was investigated in the sciatic nerve-sartorius muscle preparation of the frog, Rana pipiens. We also examined the possibility that the release of a quantum depresses or enhances the subsequent release of additional quanta. Muscle end-plates were voltage clamped at 3-4 degrees C. Quantal release was restricted to a short, or localized, region of the nerve terminal using Ca2+-free, EGTA Ringer solution and a Ca2+-filled micropipette. The number of e.p.c.s containing 0, 1, 2, etc. quanta were totalled and compared to numbers predicted using Poisson's theorem. The differences between the actual and predicted numbers of events were not significant at the nineteen junctions studied (P less than 0.05). The latency of the first quantum observed in several hundred e.p.c.s was measured and used to calculate an estimate, alpha 1(t), of the time-dependent, probabilistic process, alpha (t), governing all evoked quantal release (Barrett & Stevens, 1972b). In three experiments, all quantal latencies were measured to obtain the actual alpha (t). The alpha 1(t) function gave an excellent approximation of alpha (t) (P greater than 0.2), in real and simulated latency data. The latency of the second quantum in the e.p.c.s was measured and used to provide another estimate, alpha 2(t), of alpha (t). The alpha 2(t) function was lower (depressed) during the first few milliseconds of the evoked release period, relative to alpha 1(t). The difference was significant (P greater than 0.01) in all experiments. Our measurement procedures were tested using computer-generated 'e.p.c.s' containing randomly occurring 'quanta'. These tests showed that the early depression was due to inadequate detection of the second quantum in the e.p.c.s. The effect of Sr2+ on evoked release was examined using double-barrelled pipettes containing 1 M-SrCl2 and CaCl2 solutions. The major result was that the durations of alpha 1(t) and alpha 2(t) were equally lengthened in Sr2+, relative to Ca2+.

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.

Localizing the site of generation of uni-quantal endplate potentials using two intracellular microelectrodes

By recording with a microelectrode at either end of the endplate, the site of generation of uni-quantal endplate potentials (EPP) can be estimated. One method assumes that the logarithm of the amplitude of the signal declines linearly with distance along the cable [4, 8]. The uncertainties in this method are discussed. We show that even on a muscle cable the logarithm of the time integral of the EPP declines linearly with distance, falling to 1/e at a distance equal to the length constant, lambda. We conclude that the integral approach provides a theoretically sound method for localizing the sites of uniquantal EPP generation with two microelectrodes.