The stochastic properties of spontaneous quantal release of transmitter at the frog neuromuscular junction

Group I metabotropic glutamate receptor-triggered temporally patterned action potential-dependent spontaneous synaptic transmission in mouse MNTB neurons

Rhythmic action potentials (AP) are generated via intrinsic ionic mechanisms in pacemaking neurons, producing synaptic responses of regular inter-event intervals (IEIs) in their targets. In auditory processing, evoked temporally patterned activities are induced when neural responses timely lock to a certain phase of the sound stimuli. Spontaneous spike activity, however, is a stochastic process, rendering the prediction of the exact timing of the next event completely based on probability. Furthermore, neuromodulation mediated by metabotropic glutamate receptors (mGluRs) is not commonly associated with patterned neural activities. Here, we report an intriguing phenomenon. In a subpopulation of medial nucleus of the trapezoid body (MNTB) neurons recorded under whole-cell voltage-clamp mode in acute mouse brain slices, temporally patterned AP-dependent glycinergic sIPSCs and glutamatergic sEPSCs were elicited by activation of group I mGluRs with 3,5-DHPG (200 µM). Auto-correlation analyses revealed rhythmogenesis in these synaptic responses. Knockout of mGluR5 largely eliminated the effects of 3,5-DHPG. Cell-attached recordings showed temporally patterned spikes evoked by 3,5-DHPG in potential presynaptic VNTB cells for synaptic inhibition onto MNTB. The amplitudes of sEPSCs enhanced by 3,5-DHPG were larger than quantal size but smaller than spike-driven calyceal inputs, suggesting that non-calyceal inputs to MNTB might be responsible for the temporally patterned sEPSCs. Finally, immunocytochemical studies identified expression and localization of mGluR5 and mGluR1 in the VNTB-MNTB inhibitory pathway. Our results imply a potential central mechanism underlying the generation of patterned spontaneous spike activity in the brainstem sound localization circuit.

Probabilities of evoked and spontaneous synaptic transmission at individual active zones: Lessons from Drosophila

Nerve terminals release neuronal transmitters at morphological specializations known as active zones (AZs). Synaptic vesicle fusion at individual AZs is probabilistic, and this property is fundamental for the neuronal information transfer. Until recently, a lack of appropriate tools limited the studies of stochastic properties of neuronal secretion at individual AZs. However, Drosophila transgenic lines that express postsynaptically tethered Ca²⁺ sensor GCaMP enabled the visualization of single exocytic event at individual AZs. The present mini-review discusses how this powerful approach enables the investigation of the evoked and spontaneous transmission at single AZs and promotes the understanding of the properties of both release components.

Nonextensivity and self-affinity in the mammalian neuromuscular junction

We study time series and the spontaneous miniature end-plate potentials (MEPPs) of mammals recorded at neuromuscular junctions using two different approaches: generalized thermostatistics and detrended fluctuation analysis (DFA). Classical concepts establish that the magnitude of these potentials is characterized by Gaussian statistics and that their intervals are randomly displayed. First we show that MEPP distributions adequately satisfy the q-Gaussian distributions that maximize the Tsallis entropy, indicating their nonextensive and nonequilibrium behavior. We then examine the intervals between the miniature potentials via DFA, where the profile of the intervals between events configures a deviation from the expected random behavior. Some possible physiological substrates for these findings are discussed.

The intervals between miniature end-plate potentials in the frog are unlikely to be independently or exponentially distributed

1. It has been suggested that spontaneous quantal release of transmitter at the neuromuscular junction is a Poisson process. One logical argument against accepting the Poisson hypothesis is that so far relatively few intervals between miniature end-plate potentials (min.e.p.p.s) have been studied in any single experiment. Release is known to occur from many sites on the nerve terminal, so many intervals must be studied before drawing any conclusions about the timing of release from the individual sites. Moreover, the statistical methods that have been used are relatively insensitive to deviations from Poisson predictions.2. The Poisson hypothesis is evaluated with respect to three major criteria:(a) The fit to the exponential distribution is analysed by five goodness of fit tests which were applied to eleven sets of data, showing that it is unlikely that the data sets were generated by an exponential distribution.(b) The independence of intervals is assessed in two ways. First, the autocorrelogram of intervals is constructed. This shows an excess of significant positive correlations beyond the 5% limits of the Poisson expectation. Secondly, the unsmoothed power spectrum is calculated, and compared to the Poisson prediction by means of the modified mean test. Again, most sets deviate significantly from the Poisson expectation. It is unlikely that the intervals are independent.(c) The possibility of simultaneous occurrences is evaluated by construction of the amplitude histogram of min.e.p.p.s. In all sets the Poisson prediction for the frequency of multiples of the unit height was exceeded by the empirical data sets. The over-all conclusion is that the process which generates spontaneous releases is unlikely to be Poisson.

Stochastic properties of spontaneous transmitter release at the crayfish neuromuscular junction

1. Miniature excitatory junctional potentials (min.e.j.p.s) were recorded with an intracellular electrode from the adductor muscle of the dactyl of the first or second walking leg of the crayfish, Orconectes virilis.2. The intervals between the min.e.j.p.s were compared to the exponential prediction by five goodness of fit tests. The results indicate that the intervals are not exponentially distributed.3. The autocorrelogram of intervals shows that the intervals are unlikely to be independent.4. A stochastic analysis that includes the power spectrum of intervals, the variance-time curve, and the ln-survivor curve suggest that there is a clustering of min.e.j.p.s. The results are similar to those on the frog neuromuscular junction.5. An autocorrelogram of the min.e.j.p. amplitudes suggests that sizes are not independently distributed.6. These results, which are similar to those previously reported from the frog neuromuscular junction, support the use of the branching Poisson process as a theoretical model for the stochastic properties of spontaneous quantal release of transmitter.

Characterization of Spontaneous Depolarizations in Smooth Muscle Cells of the Guinea Pig Prostate

PURPOSE

We characterized the electrical events recorded in small segments of the dorsal lobe of the prostate of immature male guinea pigs and examined some mechanisms underlying their generation.

MATERIAL AND METHODS

Membrane potential recordings were made in the stroma of the guinea pig prostate using conventional single microelectrode techniques.

RESULTS

Three distinct, spontaneously occurring electrical events were recorded in guinea pig prostate, namely slow waves, consisting of a depolarizing transient 14 mV in amplitude with 1 to 6 nifedipine sensitive spikes superimposed, pacemaker potentials, consisting of a larger depolarization 40 mV in amplitude, and STDs 1 to 10 mV in amplitude. Only spikes on slow waves were inhibited by nifedipine. The depolarizing transient of slow waves, pacemaker potentials and STDs were abolished by cyclopiazonic acid, a blocker of the SERCA pump, and the mitochondrial uncoupler cyanide m-chlorophenyl hydrazone as well as upon exposure to Ca(2+)-free saline or the Cl(-) channel blockers niflumic acid and anthracene-9-carboxylic acid (Sigma Chemical Co., St. Louis, Missouri). Examination of the stochastic properties of STDs revealed that they were not well modeled by Poisson statistics, but rather they occurred in a clustered manner, such they may well underlie pacemaker potential generation.

CONCLUSIONS

Guinea pig prostate shows STD and pacemaker potentials that arise from the release of Ca(2+) from intracellular stores and the activation of Ca(2+) activated Cl(-) channels. We speculate that the depolarizing transient of prostatic slow waves is the propagated response of pacemaker potentials evoked at sites electrically distant from the recording electrode.

Timing of quantal release from the retinal bipolar terminal is regulated by a feedback circuit

In isolation, a presynaptic terminal generally releases quanta according to Poisson statistics, but in a circuit its release statistics might be shaped by synaptic interactions. We monitored quantal glutamate release from retinal bipolar cell terminals (which receive GABA-ergic feedback from amacrine cells) by recording spontaneous EPSCs (sEPSCs) in their postsynaptic amacrine and ganglion cells. In about one-third of these cells, sEPSCs were temporally correlated, arriving in brief bursts (10-55 ms) more often than expected from a Poisson process. Correlations were suppressed by antagonizing the GABA(C) receptor (expressed on bipolar terminals), and correlations were induced by raising extracellular calcium or osmolarity. Simulations of the feedback circuit produced "bursty" release when the bipolar cell escaped intermittently from inhibition. Correlations of similar duration were present in the light-evoked sEPSCs and spike trains of sluggish-type ganglion cells. These correlations were suppressed by antagonizing GABA(C) receptors, indicating that glutamate bursts from bipolar terminals induce spike bursts in ganglion cells.

Multimodal quantal release at individual hippocampal synapses: evidence for no lateral inhibition

Most CNS synapses investigated thus far contain a large number of vesicles docked at the active zone, possibly forming individual release sites. At the present time, it is unclear whether these vesicles can be discharged independently of one another. To investigate this problem, we recorded miniature excitatory currents by whole-cell and single-synapse recordings from CA3-CA1 hippocampal neurons and analyzed their stochastic properties. In addition, spontaneous release was investigated by ultrastructural analysis of quickly frozen synapses, revealing vesicle intermediates in docking and spontaneous fusion states. In these experiments, no signs of inhibitory interactions between quanta could be detected up to 1 msec from the previous discharge. This suggests that exocytosis at one site does not per se inhibit vesicular fusion at neighboring sites. At longer intervals, the output of quanta diverged from a random memoryless Poisson process because of the presence of a bursting component. The latter, which could not be accounted for by random coincidences, was independent of Ca2+ elevations in the cytosol, whether from Ca2+ flux through the plasma membrane or release from internal stores. Results of these experiments, together with the observation of spontaneous pairs of omega profiles at the active zone, suggest that multimodal release is produced by an enduring activation of an integrated cluster of release sites.

Spontaneous and neurally activated depolarizations in smooth muscle cells of the guinea-pig urethra

1. Membrane potential recordings were made from longitudinal smooth muscle cells of the guinea-pig urethra using conventional microelectrode techniques. 2. Smooth muscle cells of the urethra developed spontaneous transient depolarizations (STDs) and slow waves. Single unit STDs had amplitudes of approximately 5 mV and slow waves seemed to occur as amplitude multiples of single unit STDs. 3. STDs and slow waves were abolished by niflumic acid or low chloride solution and also by cyclopiazonic acid (CPA), BAPTA or high concentrations of caffeine. Lower concentrations of caffeine abolished slow waves but not STDs. Nifedipine inhibited slow waves but not STDs. 4. When stochastic properties of STDs were examined, it was found that the intervals between occurrences were not well modelled by Poisson statistics, instead the STDs appeared to be clustered. 5. Transmural stimulation evoked excitatory junctional potentials (EJPs) and triggered slow waves which were abolished by either alpha,beta-methylene-ATP or tetrodotoxin. Evoked slow waves were also abolished by caffeine, co-application of caffeine and ryanodine or by CPA which left EJPs unaffected. 6. In conclusion, smooth muscle cells of urethra exhibit STDs which are clustered rather than random events, and are the result of spontaneous Ca2+ release from intracellular stores and subsequent activation of Ca2+-activated chloride channels. STDs sum to activate L-type Ca2+ channels which contribute to the sustained phase of slow waves. Stimulation of purinoceptors by neurally released ATP initiates EJPs and also causes the release of Ca2+ from intracellular stores to evoke slow waves.

Quantal neurotransmitter secretion rate exhibits fractal behavior

The rate of exocytic events from both neurons and non-neuronal cells exhibits fluctuations consistent with fractal (self-similar) behavior in time, as evidenced by a number of statistical measures. We explicitly demonstrate this for neurotransmitter secretion at Xenopus neuromuscular junctions and for rat hippocampal synapses in culture; the exocytosis of exogenously supplied neurotransmitter from cultured Xenopus myocytes and from rat fibroblasts behaves similarly. The magnitude of the fluctuations of the rate of exocytic events about the mean decreases slowly as the rate is computed over longer and longer time periods, the periodogram decreases in power-law manner with frequency, and the Allan factor (relative variance of the number of exocytic events) increases as a power-law function of the counting time. These features are hallmarks of self-similar behavior. Their description requires models that exhibit long-range correlation (memory) in event occurrences. We have developed a physiologically plausible model that accords with all of the statistical measures that we have examined. The appearance of fractal behavior at synapses, as well as in systems comprising collections of synapses, indicates that such behavior is ubiquitous in neural signaling.

Quantal neurotransmitter secretion rate exhibits fractal behavior

The rate of exocytic events from both neurons and non-neuronal cells exhibits fluctuations consistent with fractal (self-similar) behavior in time, as evidenced by a number of statistical measures. We explicitly demonstrate this for neurotransmitter secretion at Xenopus neuromuscular junctions and for rat hippocampal synapses in culture; the exocytosis of exogenously supplied neurotransmitter from cultured Xenopus myocytes and from rat fibroblasts behaves similarly. The magnitude of the fluctuations of the rate of exocytic events about the mean decreases slowly as the rate is computed over longer and longer time periods, the periodogram decreases in power-law manner with frequency, and the Allan factor (relative variance of the number of exocytic events) increases as a power-law function of the counting time. These features are hallmarks of self-similar behavior. Their description requires models that exhibit long-range correlation (memory) in event occurrences. We have developed a physiologically plausible model that accords with all of the statistical measures that we have examined. The appearance of fractal behavior at synapses, as well as in systems comprising collections of synapses, indicates that such behavior is ubiquitous in neural signaling.

Spontaneous synaptic activities in rat nucleus tractus solitarius neurons in vitro: evidence for re-excitatory processing

The pattern of synaptic interactions between neurons of the nucleus tractus solitarius (NTS) has been analyzed using whole cell recording in rat brainstem slices. Following tractus solitarius (TS) stimulation 15/55 neurons presented a prolonged (up to 300 ms) increased excitability (PIE neurons) and 40/55 neurons presented a prolonged (up to 200 ms) reduced excitability (PRE neurons). In the absence of afferent sensory input all neurons showed spontaneous synaptic activity. Ongoing synaptic activity in PIE cells was glutamatergic and characterized by the absence of detectable inhibitory potentials while in PRE cells it was 90% GABAergic and 10% glutamatergic. Glutamatergic synaptic currents in PIE cells and GABAergic synaptic currents in PRE were studied using probability density and intensity functions. Distribution of time intervals between synaptic events indicated the latter were generated, in both PIE and PRE cells, by two simultaneous processes: (1) a close to Poisson process generating independent events; and (2) a subsidiary re-excitatory process generating synaptic events separated by intervals shorter than 20 ms. Blockade of glutamatergic transmission by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) or blockade of action potentials by tetrodotoxin (TTX; 1 microM) suppressed the subsidiary process. In conclusion, we propose that PIE cells (1) form a re-excitatory network contributing to generation of excitatory activity in the NTS and (2) are located presynaptically with respect to PRE cells.

Effects of calcium on the dynamic process of transmitter release which generates either skew- or bell-MEPPS

Miniature endplate potential (MEPP) amplitudes, MEPP frequencies and ratios of skew:bell-MEPPs were determined as well as synaptic vesicle diameters and densities at the mouse diaphragm neuromuscular endplate during exposure to elevated calcium concentrations. Additions of external Ca2+ had variable effects on MEPP frequencies and percentages of skew-MEPPs, regardless of concentrations used (1-25 mM). Nevertheless, changes in MEPP amplitudes were most sensitive (4-fold decrease) to low value increases of Ca2+. Changes in MEPP frequencies produced by an increase in Ca2+ were very sensitive to initial frequencies as well as the initial calcium concentration. An increase in Ca2+ usually increased MEPP frequency (providing skew-MEPPs were measured). Changes in the percentage of skew-MEPPs were extremely variable (4-90%) and these changes depended on initial frequencies, initial skew- to bell-MEPP ratios and age of the mouse. With a change in Ca2+ concentration, synaptic vesicle diameters and densities remained constant during changes in MEPP frequencies and large changes in the skew:bell-MEPP ratios; and, vesicle numbers were sometimes slightly increased. Because of the wide range in MEPP frequencies and amplitudes, this study demonstrates that the effect of various treatments should be evaluated on identified endplates and that analyses of randomly selected endplates must consider the large variability between endplates. These results show that the skew-MEPP class must not be ignored in studies of spontaneous MEPP release, and that initial frequencies and age of the mouse are also important in evaluating changes in skew-MEPP to bell-MEPP ratios. The rapid changes in skew- to bell-MEPP classes indicate that MEPP class and size are determined at the moment of release by the state of the release process as proposed by Kriebel et al. (1990). Because changes in calcium concentration can immediately alter the ratio of skew- to bell-MEPPs we conclude that the release process has two states to generate the two classes of MEPPs, and that the release process is very sensitive to conditions so that states are easily changed. We propose that the release process meters transmitter in subunit amounts to form both classes of MEPPS and that the calcium ions modulate the process.

Phenthonium, a quaternary derivative of (-)-hyoscyamine, enhances the spontaneous release of acetylcholine at rat motor nerve terminals

1. A quaternary derivative of (-)-hyoscyamine, phenthonium (Phen) induced a concentration-dependent increase in the rate of spontaneous quantal release of acetylcholine (ACh) at the mammalian neuromuscular junction, as shown by intracellular recordings of the miniature endplate potentials (m.e.p.ps) in rat diaphragm muscles. 2. The prejunctional effect of Phen (10-50 microM) was reversible, unrelated to temperature (22 degrees-35 degrees C), unaltered by either changes in [Ca2+]o or by high [Mg2+]o, and was not induced by membrane depolarization. 3. Simultaneously, Phen reduced the amplitude of m.e.p.ps by a postjunctional action. 4. The muscarinic agonist oxotremorine did not prevent the increase in m.e.p.p. frequency induced by Phen. Cholinesterase inhibition with neostigmine potentiated the prejunctional effect induced by a low (20 microM) but not a high (50 microM) concentration of Phen. 5. The increase in m.e.p.p. frequency induced by Phen was not influenced by previous incubation with either atropine (0.01-10 microM) or (+)-tubocurarine (0.05-0.1 microM). Each antagonist however, intensified the postjunctional effect of Phen. 6. Phen (20 microM) did not influence the quantal contents of e.p.ps in cut-muscle preparations or in the presence of high [Mg2+]o. A high concentration of Phen (50 microM) increased the rundown of e.p.p. trains evoked at 10-50 Hz. 7. The results indicate that the facilitatory prejunctional action of Phen cannot be explained by an antimuscarinic activity. A possible interaction of the antagonist with putative prejunctional nicotinic cholinoceptors however, was not excluded.

Spontaneous release of multiquantal miniature excitatory junction potentials induced by a Drosophila mutant

1. Intracellular recordings were made from muscle fibre No. 6 of the dorsal longitudinal flight muscle (DLM) of Drosophila melanogaster in both wild-type flies and the temperature-sensitive paralytic mutant, shibirets-1 (shi). 2. Continuous recordings of the miniature excitatory junction potentials (MEJPs) in this fibre were made as the temperature was changed from 19 to 29 degrees C, and back to 19 degrees C. In shi flies, synapses become depleted of vesicles at 29 degrees C due to a temperature-dependent blockage in the recycling process, while transmitter release proceeds normally. When the temperature is lowered to 19 degrees C, recycling is allowed to proceed and recovery of the full complement of synaptic vesicles gradually occurs in about 20 min. 3. It was observed that the MEJP amplitude distribution in shi flies was unimodal at 19 degrees C prior to heating (as was wild-type), but during recovery from 8 min exposure to 29 degrees C became multimodal, with peaks at roughly integral multiples of the original peak prior to heating. This effect was never seen in wild-type flies. 4. Also, during recovery, the MEJP did not occur randomly, but rather occurred in a clustered fashion. 5. It is concluded that during recovery from depletion in shi neuromuscular junctions, a condition exists which causes the synchronization of spontaneous release, causing multiquantal MEJPs or clustering of MEJPs, depending on the degree of synchronization. 6. The possible role of Ca2+ in this phenomenon is discussed.

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+.

A method for testing an extended poisson hypothesis of spontaneous quantal transmitter release at neuromuscular junctions

A statistical method for testing the Poisson hypothesis of spontaneous quantal transmitter release at neuromuscular junctions has been proposed. The notion of the Poisson hypothesis is extended so as to allow for nonstationarity in the data, since nonstationarity is commonly seen in the occurrence of spontaneous miniature potentials. Special emphasis has been put on the nonstationary analysis of the quantal release. A time scaling technique has been introduced and is discussed for the analysis. Artificially generated data, which simulate three types of nonstationary spontaneous quantal release, i.e., Poisson, non-Poisson-clustered, and non-Poisson-ordered types, were analyzed to demonstrate the effectiveness of the method. Some sets of miniature endplate potentials, intracellularly recorded at frog sartorius neuromuscular junctions in low Ca++ and high Mg++ solutions showing apparent nonstationarities, were analyzed as illustrative examples. The proposed method will extend the range of applicable data for the statistical analysis of spontaneous quantal transmitter release.

Bursts of miniature end-plate potentials can be released from localized regions of the frog motor nerve terminal

We examined spontaneous transmitter release by simultaneously recording from end-plates with both an intracellular electrode and a focal extracellular electrode. In normal Ringer's solution the focal releases appear to be a near random portion of the pooled output. But in elevated [Sr2+]out bursts of miniature end-plate potentials (m.e.p.p.s.) occur, and these can be localized at a portion of the presynaptic terminal. The bursts are not artifacts caused by the focal recording technique. In some conditions, sections of the nerve terminal can exhibit an enhanced probability of quantal release that persists for seconds. We occasionally observe extracellular events that must be generated by m.e.p.p.s. in adjacent fibers.

Effects of two synaptic activators, calcium and ethanol, on MEPP distribution in time

Miniature end-plate potentials (MEPPs) were recorded intracellularly from sartorius muscle of Rana esculenta. Tracings were divided into time bins whose duration approximated one-fifth of the mean interval between consecutive potentials. The observed number of bins containing 0, 1, 2, ... MEPPs was compared, by the X2 test, with the number calculated from the Poisson equation. MEPP timing was analyzed in the absence as well in the presence of Ca2+ (1 mM, 2.5 MM, and 15 mM). In half of the experiments, 0.5% ethanol was added to the bathing solution. In the absence of Ca2+, MEPP timing fitted the Poisson predictions. On adding Ca2+, the fit became poor and MEPPs showed the tendency to cluster. At 15 mM Ca2+, no experiment proved to be Poissonian. Though increasing the frequency of MEPPs similarly to Ca2+, ethanol maintained a Poissonian release of transmitter at any concentration of Ca2+. It is suggested that ethanol masks the effects of Ca2+ on MEPP timing by also inducing the discharge of transmitter outside the Ca2+-dependent sites of exocytosis in the presynaptic membrane.

Transmitter release from normal and degenerating locust motor nerve terminals

1. An analysis has been made of spontaneous and evoked transmitter release from terminals of 'fast' excitatory motor axons on locust muscle fibres using intra- and extracellular recording together with a Ca-electrode technique for activating transmitter release from single nerve terminals on multiterminally innervated muscle fibres. 2. Spontaneous intracellular miniature excitatory junction potentials (m.e.j.p.s), recorded at active spots on these muscle fibres, occurred non-randomly with frequent bursts of m.e.j.p.s. 3. M.e.j.p.s of subnormal amplitude were also seen but contributed only a small fraction of the minature discharge. 4. The amplitude distribution of intracellularly recorded excitatory junction potentials (e.j.p.s) evoked during ionophoretic application of Ca onto single nerve terminals was adequately predicted by Poisson statistics. 5. During the course of nerve terminal degeneration m.e.j.p.s of subnormal amplitude became more frequent and eventually formed the major part of the miniature discharge. Transmitter quanta responsible for 'small' m.e.j.p.s did not contribute to evoked release either at normal or degenerating terminals. Evoked transmitter release from degenerating axon terminals before excitation-secretion coupling failure conformed to Poisson statistics. 6. It is concluded that more than one release mechanism operates on the transmitter pool or pools in locust motor nerve terminals.

Spontaneous multiquantal release at synapses in guinea-pig hypogastric ganglia: evidence that release can occur in bursts

1. A study was made of some properties of the spontaneous synaptic potentials recorded in cells of the hypogastric ganglia of guinea-pigs. 2. The distribution of the amplitudes of the spontaneous synaptic potentials arising from a single preganglionic fibre was found to be multimodal, with peaks at roughly integral multiples of a unit peak. 3. It was found that the amplitudes of the larger spontaneous potentials were consistent with them being the result of synchronous or near-synchronous release of two or more unit-sized quanta (multiquantal release). 4. The proportion of multiquantal potentials observed was found to be dependent on the extracellular calcium ion concentration. 5. When the stochastic properties of the spontaneous potentials were examined, it was found that the spontaneous release process was not random and independent but appeared to be clustered. Indeed, the probability of occurrence of a unit spontaneous synaptic potential was greatly enhanced during the 40--60 msec immediately following any given spontaneous synaptic potential. 6. When unit spontaneous potentials were excluded from the analysis, the multiquantal potentials were still found to be clustered although the clustering was less marked than that seen in the over-all process. 7. These results suggested that the multiquantal spontaneous potentials arose from the release of unit quanta in short high frequency bursts and it was found that a mathematical model incorporating such a mechanism could describe the spontaneous quantal release process.

Spontaneous subminature end-plate potentials in mouse diaphragm muscle: evidence for synchronous release

1. Miniature end-plate potentials (min.e.p.p.s) were recorded from small muscle cells of mouse diaphragms. Min.e.p.p. amplitude histograms showed successive peaks which were integral multiples of the smallest peak. The smallest potentials (submin.e.p.p.s) averaged 0-3-0-6mV and the mean of the larger min.e.p.p.s averaged 3-7 mV, depending on the muscle cell diameter. There was a positive correlation between time-to-peak and min.e.p.p. amplitude. Time-to-peak of the submin.e.p.p.s fell slightly below the regression line through the larger min.e.p.p.s. 2. Sometimes min.e.p.p. amplitude distributions changed spontaneously such that the mean of the major mode min.e.p.p.s decreased twofold during which time the mean of the submin.e.p.p.s did not change. Spontaneous decreases were most pronounced during low frequencies of release (10/min) achieved at 32 degrees C. 3. Small changes in temperature (2 degrees C steps in the range 32-40 degrees C) greatly altered the number of peaks of min.e.p.p. amplitude histograms without noticeably changing the position of the submin.e.p.p. peak. At 32 degrees C submin.e.p.p.s composed 5-20% of the histograms and the amplitude of the major mode peak was twelve to fifteen-times that of the submin.e.p.p.s. Over-all bell-shaped distributions were obtained at 37 degrees C which showed up to eight peaks with the major peak at the fourth to sixth peak. Temperatures slightly above 37 degrees C gave a flat distribution with the mean amplitude at the third peak. Min.e.p.p. amplitude histograms were initially skewed (mostly small min.e.p.p.s) after a 40 degrees C heat challenge. 4. Two to eight-times the normal concentration of Ca2+ in the saline reversibly increased the min.e.p.p. frequency and also decreased the mean of the major mode min.e.p.p.s (two to nine-times) without noticeably changing the mean of the submin.e.p.p.s. 5. Botulinum toxin A, 10(5) X intraperitoneal median lethal dose (10(5)I.P.LD50)/ml., almost abolished min.e.p.p.s in 30-90 min. The relative proportion of submin.e.p.p.s increased and the mean of the major mode min.e.p.p.s usually did not change during the initial decrease in frequency. Major mode min.e.p.p.s essentially ceased after 200-1000 were generated and remaining min.e.p.p.s of some cells showed skewed distributions with three small peaks that were integral multiples of the submin.e.p.p. peak. Smaller min.e.p.p.s were more resistant to block than the larger min.e.p.p.s and, although frequencies were low, small min.e.p.p.s were recorded after 4 hr of botulinum toxin incubation. 6. Colchicine (5 X 10(-4)M) within minutes reduced the major mode min.e.p.p.s by half (mean of major peak reduced to sixth or seventh peak). Additional colchicine (10(-3)M reduced the major mode min.e.p.p. amplitude to a fifth of that of control (mean of major mode min.e.p.p.s at the third peak) with no change in position of the submin.e.p.p. peak. Min.e.p.p. amplitudes slowly recovered to half control values after washing. 7...

Electrophysiological studies of normal and degenerating mouse neuromuscular junctions

A quantitative study of the functional changes occurring at end-plates of phrenicotomised mouse hemidiaphragms was made. Analysis of the frequency of spontaneous transmitter release revealed large scale deviations from a Poisson based process commencing 6-7 h post-phrenicotomy. Short term bursts of miniature end-plate potentials (minEPPs) lasting 0.5-1.0 sec frequently occurred and also long term bursts of minEPPs which lasted from several minutes to 0.5 h or more. Following the long term bursts there was regularly a lasting silence at the end-plates. MinEPP amplitudes were analysed. Probability analyses suggest that the minEPPs are often normally distributed amongst two or more populations. This relationship was maintained during degeneration of the end-plate. No significant differences between the distribution of mean minEPP amplitudes of degenerating end-plates and normal end-plates were found. This suggests that no change in the quantal unit of transmitter release occurs during degeneration of the end-plate.

The formation of synapses in amphibian striated muscle during development

1. A study has been made of the formation of synapses in developing reinnervated and cross-reinnervated amphibian twitch muscles which receive either a focal (iliofibularis) or a distributed (sartorius) innervation from 'en plaque' nerve terminals using histological, ultrastructural and electrophysiological techniques. 2. During the development of the tadpole through metamorphosis to the adult frog, the sartorius myofibres increased in length at about twice the rate of the iliofibularis myofibres, due to a fast rate of growth at their insertions on to the pelvic tendon. 3. The short iliofibularis and sartorius myofibres of young tadpoles (800 mum long) possessed only a single synapse and the iliofibularis myofibres did not receive any further innervation during development. However the sartorius myofibres received further transient innervation on the new muscle laid down during development at the fast growing pelvic insertion, until the distance between the original synapse formed on the myofibres and the synapse at the pelvic end of the muscle was about 12 mm. 4. During development synapses possessed either skewed, multimodal, or unimodal m.e.p.p. amplitude-frequency distributions; the intervals between m.e.p.p.s. were not distributed randomly according to a Poisson process, as m.e.p.p.s. of similar amplitudes tended to be separated by very short intervals; the unit-size e.p.p. had a similar amplitude-frequency distribution as the m.e.p.p.s. if these had a unimodal distribution. 5. Reinnervation or cross-reinnervation of the sartorius and the iliofibularis muscles in adults or at a late stage of development simply reconstituted the normal focal and distributed innervation patterns of the muscles, as found in the control muscles of the contralateral and unoperated legs. 6. These observations on synapse formation in amphibia are consistent with the hypothesis that during development the axon making the initial synaptic contact on the muscle cells induces a property over a length of muscle membrane adjacent to this site which makes it refractory to synapse formation; thus during reinnervation or cross-reinnervation of adult muscles this refractory property constrains synapse formation to these sites.

A computer program for the graphical and iterative fitting of probability density functions to biological data

A tutorially-assisted, interactive program, written for a Digital Equipment Corporation LAB-11 minicomputer (PDP-11/20, is described which allows a user to fit (with or without automatic estimation of initial parameter values), by a method of nonlinear least squres, any one of seven different types of probability density functions (p.d.f.'s) to an empirical frequency distribution; the latter of which may be input to the program or formed by the program whenever it is furnished a series of times between events. The iteratively-obtained, "best fit" p.d.f. is displayed on a two color, point-plot display against the background of a point-plot histogram. By selecting any one of nine output modes, the user is allowed: (1) to view histograms successively on the point-plot display, (2) to generate selected p.d.f.'s (3) to "force" p.d.f.'s having known parameters through the histogram data, (4) to obtain Chi-square (x2) and Kolmogorov-Smionov estimates of the goodness of fit to the data, and (5) to apply a special test [Williams and Kloot, 1953] in order to determine whether the least squares estimates of two candidate models are statistically different. The resident driver program and the four overlayable program segments are written in standard FORTRAN IV; except for two plot routines, which are written in PDP-11 assembly language.