Stochastic properties of spontaneous transmitter release at the crayfish neuromuscular junction

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

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

Stationary and non-stationary occurrences of miniature end plate potentials are well described as stationary and non-stationary Poisson processes in the mollusc Navanax inermis

Protractor muscles in the gastropod mollusc Navanax inermis exhibit typical spontaneous miniature end plate potentials with mean amplitude 1.71 +/- 1.19 (standard deviation) mV. The evoked end plate potential is quantized, with a quantum equal to the miniature end plate potential amplitude. When their rate is stationary, occurrence of miniature end plate potentials is a random, Poisson process. When non-stationary, spontaneous miniature end plate potential occurrence is a non-stationary Poisson process, a Poisson process with the mean frequency changing with time. This extends the random Poisson model for miniature end plate potentials to the frequently observed non-stationary occurrence. Reported deviations from a Poisson process can sometimes be accounted for by the non-stationary Poisson process and more complex models, such as clustered release, are not always needed.

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.

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.

Synaptic depression related to presynaptic axon conduction block

1. The depression of synaptic transmission, which occurs during prolonged repetitive activation, was examined in the opener muscle of the crayfish walking leg. 2. Excitatory post-synaptic potentials (e.p.s.p.s) initially facilitated but then declined to low amplitudes after about 4000 stimulus pulses had been delivered; this depression is presynaptic in origin; 3. Axon conduction blocks occured at points of bifurcation along the entire length of the presynaptic nerve. This resulted in failure of the nerve impulse to invade some branches of the terminal arborization. 4. Nerve terminal invasion failure caused either intermittent or complete inactiviation of some synaptic release sites; this was associated with depression of the post-synaptic response. 5. The statistics of transmitter release during prolonged repetitive stimulation were examined by focal extracellular recording methods. Transmitter release could be described by binomial statistics, and depression involved a drop in m, n and p. 6. The rate of spontaneous quantal release did not decrease, however, arguing against transmitter depletion. 7. It is concluded that repetitive stimulation eventually leads to depolarization of the axon membrane. This causes impulse propagation failure which reduces the number of synaptic release sites that are activated and mimics a drop in the effective stimulation rate; both effects cause synaptic depression.

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.