Description of the sub-miniature endplate potential distribution, determination of subunit size and number of subunits in the adult frog neuromuscular bell-miniature endplate potential

Statistics for studying quanta at synapses: resampling and confidence limits on histograms

This paper describes some statistical methods for working with data on quantal sizes. Since quantal sizes often do not fit to normal probability distribution functions, statistics based on the normal distribution are inappropriate. Resampling methods can be used to determine confidence limits and to test whether two sets of data differ by chance. Some hypotheses about the nature of quanta are based on apparent peaks and valleys in histograms. Confidence limits can be placed on the bins in the histogram by using the Kolomorogov-Smirnov statistic or by resampling. The confidence limits should assist in the evaluation of the significance of the valleys.

Hypertonic treatment reversibly increases the ratio of giant skew-miniature endplate potentials to bell-miniature endplate potentials

Miniature endplate potentials were recorded from single frog muscle fibers before, during and after treatment with hypertonic saline (200-500 mM NaCl or Na gluconate added to frog saline). Miniature endplate potential amplitude distributions were plotted from small muscle fibers so that the modes and ratios of the skew-miniature endplate potential to bell-miniature endplate potential classes could be defined. Muscle fibers were voltage clamped with two electrodes to determine the input resistance before, during and after treatment. Input resistance increased from two to 100 times during treatment and rapidly fell towards control values (no more than 30% greater) when preparations were returned to normal frog saline. Short duration treatments with 200-300 mM hypertonic salines immediately increased frequencies (100-fold) of both skew-miniature endplate potential and bell-miniature endplate potential classes. Preparations when returned to normal frog saline after a few minutes of treatment showed control miniature endplate potential distributions within minutes. One to two hour treatments left only the skew-miniature endplate potential class and with hour-long recovery periods bell-miniature endplate potentials reappeared and ratios of skew-miniature endplate potential to bell-miniature endplate potential classes returned to control values. Treatment with 500 mM NaCl added to frog saline immediately increased the percentage of skew-miniature endplate potentials (from 2 to 50%) with little or no increase in overall miniature endplate potential frequencies. The mode of the skew-miniature endplate potential class was unchanged after hypertonic treatment, whereas that of the bell-miniature end plate potential class either remained about the same size or decreased depending on the duration of treatment. The number and percentage of giant-miniature endplate potentials belonging to the skew-miniature endplate potential class increased as a function of the duration of 200-300 mM hypertonic saline treatments. Most giant-miniature endplate potentials had a slow rising phase with a foot and/or breaks demonstrating a composite structure. Sequentially recorded giant-miniature endplate potentials had similar initial slopes indicating either repetitive releases from single sites or releases from cooperative sites. After hypertonic treatment the bell-miniature endplate potential size was never more than that expected with the increase (under 30%) in input resistance. The results presented here are completely different from those of Yu and Van der Kloot [(1991) J. Physiol. 433, 677-704] who reported that the bell-miniature endplate potential amplitude was increased two- to four-fold after hypertonic treatment. The wide range of results in the ratio of skew-miniature endplate potential to bell-miniature endplate potential classes is discussed in regards to the quantal hypothesis which is based on a single class of immutable amounts of transmitter; and, a hypothesis based on a dynamical process that meters transmitter in subunit amounts to control miniature endplate potential size and class during release.

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.

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.

Characteristics of slow-miniature endplate currents show a subunit composition

The normal neuromuscular junction shows two classes of spontaneous miniature endplate potentials. These classes are based on a discontinuity in the profile of miniature endplate potential amplitude distributions. The amplitude of one class of miniature endplate potentials from a bell-shaped amplitude distribution and the remaining miniature endplate potentials compose a population which forms a left-hand skew distribution with a mode 1/7 to 1/10 that of the bell-miniature endplate potentials [Kriebel M. E. and Gross C. E. (1974) J. gen. Physiol, 64, 85-103]. Some skew-miniature endplate potentials have a slow time-to-peak and show breaks on the rising phase. Most treatments that alter the miniature endplate potential frequency change the ratio of skew-miniature endplate potentials/bell-miniature endplate potentials [Kriebel M. E. et al. (1976) J. Physiol. 262, 553-581]. The time characteristics of miniature endplate currents were readily altered in the isolated frog and mouse neuromuscular junctions with several agents known to increase the percentage of slow-miniature endplate potentials (heat, botulinum toxin, 4-aminoquinoline and increases in bath osmolarity). The slow-miniature endplate potential amplitudes were a continuum of amplitudes from skew- to giant miniature endplate potentials. The rising phases of miniature endplate potentials were a continuum from smooth to many with breaks and offsets. In a series of sequentially recorded slow-miniature endplate currents, many had congruent rising phases of constant slope regardless of amplitude or of time-to-peak. The rising phases of congruent slow-miniature endplate currents which showed a change in slope deviated at similar amplitudes. The least value of the slope of a slow-miniature endplate current was that of the sub-miniature endplate current; and, miniature endplate currents with overall lower slope values showed a wave pattern and/or irregular breaks which suggests summation of sequentially delayed sub-miniature endplate currents. Plots of the amplitude vs time-to-peak of miniature endplate currents from identified junctions demonstrated that the normal percentage of slow-miniature endplate currents was greatly increased with the treatments used here and that the time-to-peak of giant miniature endplate currents usually was longer than that of normally occurring bell-miniature endplate currents. Giant miniature endplate currents with short time-to-peak values are probably from two miniature endplate currents occurring, by chance, almost simultaneously. During and/or after treatments, miniature endplate currents formed clusters of similar size miniature endplate currents, not randomly distributed in time, which graded from distinct miniature endplate currents to giant miniature endplate currents.(ABSTRACT TRUNCATED AT 400 WORDS)

The regulation of quantal size

Quantal size can be altered experimentally by numerous treatments that seem to lack any common thread. The observations may seem haphazard and senseless unless clear distinctions are made from the outset. Some treatments shift the size of the entire population of quanta. These quanta are released by nerve stimulation. Other treatments add quanta of abnormal size or shape--monstrosities--to the population (4.0). Usually, perhaps even invariably, the monstrosities are not released by nerve stimulation. 6.1. POPULATION SIZE INCREASES. 6.1.1. Quantal size must be regulated. The size of the entire quantal population can be experimentally shifted to a larger size, with the mean rising two- or even four-fold. Before these observations, it was reasonable to suppose that quantal size was relatively fixed, with little room for maneuver. A logical picture is that synaptic vesicles have a maximum transmitter capacity, and usually they are filled to the brim. This picture is wrong. The quantity of transmitter packaged in the quantum must be regulated by the neuron, so depending on circumstances, quantal size can be increased or decreased. Figure 18 makes the case for regulation more strongly than words. We are beginning to identify some of the signals for up and down regulation, and the first steps have been made in discovering the signal transduction pathways, but we are far from a true understanding. This is hardly surprising, because our information about how transmitter molecules are assembled into quantal packages is still imperfect. Until we understand the engine, it may be difficult to picture the accelerator or the brake. 6.1.2. Signals that up regulate size. Stimulation of the presynaptic neuron increases quantal size at the NMJ, at synapses in autonomic ganglia and in hippocampus. The stimulus parameters necessary to elicit the quantal size increase have not been explored sufficiently in any of these cases, and all deserve further investigation. At both frog and mouse NMJs quantal size is roughly doubled following exposure to hypertonic solutions, which elevate the rate of spontaneous quantal release. This discovery, coupled with the increases caused by tetanic stimulation, suggested that the signal for up regulation is a period of greatly enhanced quantal output. The size increase takes about 15 min in hypertonic solution in mouse and about 60 min in frog. Highly hypertonic solutions do not increase the rate of quantal release in frog; they also do not increase quantal size. This supported the idea that quantal release rate is the signal for up regulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Spontaneous and evoked transmitter releases after concanavalin A treatment are affected differently by hypertonic low calcium solutions at frog neuromuscular junction

Adding sucrose to the low calcium bathing solution made with no added calcium but containing Mg2+ EGTA to increase the tonicity elevated the basal frequency of miniature end-plate potentials (MEPPs) in the frog. The hypertonic low calcium solution also increased the rate at which the MEPP frequency rose in response to tetanic stimulation and elevated the level of the maximum frequency, which approached an asymptote. Pretreatment with concanavalin A (con A) greatly reduced the elevation in the basal frequency produced by hypertonic solutions. However, tetanic stimulation gave the same results as in untreated preparations when the tonicity was increased. Pretreatment with colchicine before the con A treatment eliminated the blocking action of con A on the MEPP frequency when the preparation was exposed to hypertonic solutions. Tetanic stimulation produced increases in MEPP frequency similar to those observed in normal junctions immersed in hypertonic solutions. Caffeine elevated the basal level of the MEPP frequency. Tetanic stimulation in the caffeine solution caused the increase in the MEPP frequency; the higher the basal level rose, the higher the maximum level became. However, the rate at which the MEPP frequency rose remained unchanged. The present results indicate that hypertonicity increases not only the basal frequency of MEPPs but also the slope at which the MEPP frequency is elevated by tetanic stimulation, both mechanisms being different and that the rate and magnitude of the tetanic potentiation of MEPP frequency are not simply determined by the pre-tetanus frequency.

Statistical and graphical methods for testing the hypothesis that quanta are made up of subunits

It has been proposed that at the neuromuscular junction quanta are made up of subunits, because regularly spaced peaks appear in some histograms of miniature end-plate potential (MEPP) amplitudes. Many sets of MEPP sizes fit lognormal probability distribution functions. I compared sets of MEPP amplitudes from the literature to lognormal distributions, using cumulative plots and a robust test for significance. Some data sets fit a single lognormal distribution, but most required two lognormal subpopulations. This bimodal model is a parsimonious alternative to the subunits hypothesis. I also used graphs in which cumulative MEPP sizes were plotted on the ordinate against probability on the abscissa. Model data sets, generated on the subunit assumptions, produced concave plots. End-plate potential sizes at low quantal outputs--in which we know there are subunits--show the same concave shape. Most of the tested data from the subunit literature produced convex curves.

Two classes of spontaneous miniature excitatory junction potentials and one synaptic vesicle class are present in the ray electrocyte

Cross sections (1-2 mm thick) of the ray (Raja) tail were secured to a dish and immersed in elasmobranch saline. Spontaneous miniature excitatory junction potentials (MEJPs) were recorded by advancing a 50 k omega, KCl filled electrode into the electric organ (20 microV peak-to-peak baseline noise). Data were filmed, and/or recorded on magnetic tape for computer analyses. Intracellularly recorded MEJP amplitude histograms showed a peak at 60 microV and had a right-hand skew with MEJPs up to 0.5 mV. The small peak amplitude and the skewed amplitude distribution of intracellularly recorded MEJPs result from the relatively low input resistance and the short space constant of the electrocyte coupled with the dispersed synapses on the electrocyte. At 23 degrees C the intracellularly recorded MEJP frequency ranged from 1-10 MEJPs/s. The MEJPs became larger and became focally recorded as the electrode was advanced against the intracellular surface of the innervated membrane of the electrocyte. Focal extracellular MEJPs (reversed polarity) were also recorded with the electrode positioned against the outside surface of the innervated side of the electrocyte. The frequency of focally recorded intracellular MEJPs was increased (up to 40/s) when the electrode was pushed against the membrane. Focal MEJP frequencies decreased to a few/min within 5-10 min but the mean amplitude of 3-5 mV remained constant. Decreases in amplitude and frequency in focally recorded intracellular MEJPs are attributed to changes in electrode pressure against the membrane. Amplitude histograms were constructed from focally recorded intracellular or extracellular MEJPs which showed the same time characteristics. The focal MEJP amplitude histograms have two distinct classes, each forming a bell-shaped distribution. It is concluded that both classes are generated at the electrode tip. The smaller class of MEJPs has a mean 1/10th that of the larger class and composes about 2% of the MEJPs. The small class is analogous to the sub-MEPP class found in the frog sartorius (Kriebel and Gross 1974) and mouse diaphragm (Kriebel et al. 1976, 1982). Distributions of synaptic vesicle diameters are slightly log normal (right hand skew) such that the mean diameter (57 nm) is slightly larger than the modal value (52 nm). Vesicles touching the membrane were of the same size and diameter distribution as the entire vesicle population. The profiles of the distributions are smooth and suggest only 1 class of synaptic vesicle based on diameter.