The effect of calcium ions on the binomial statistic parameters which control acetylcholine release at synapses in striated muscle

Depressed neuromuscular transmission causes weakness in mice lacking BK potassium channels

Mice lacking functional large-conductance voltage- and Ca²⁺-activated K⁺ channels (BK channels) are viable but have motor deficits including ataxia and weakness. The cause of weakness is unknown. In this study, we discovered, in vivo, that skeletal muscle in mice lacking BK channels (BK^−/−) was weak in response to nerve stimulation but not to direct muscle stimulation, suggesting a failure of neuromuscular transmission. Voltage-clamp studies of the BK^−/− neuromuscular junction (NMJ) revealed a reduction in evoked endplate current amplitude and the frequency of spontaneous vesicle release compared with WT littermates. Responses to 50-Hz stimulation indicated a reduced probability of vesicle release in BK^−/− mice, suggestive of lower presynaptic Ca²⁺ entry. Pharmacological block of BK channels in WT NMJs did not affect NMJ function, surprisingly suggesting that the reduced vesicle release in BK^−/− NMJs was not due to loss of BK channel–mediated K⁺ current. Possible explanations for our data include an effect of BK channels on development of the NMJ, a role for BK channels in regulating presynaptic Ca²⁺ current or the effectiveness of Ca²⁺ in triggering release. Consistent with reduced Ca²⁺ entry or effectiveness of Ca²⁺ in triggering release, use of 3,4-diaminopyridine to widen action potentials normalized evoked release in BK^−/− mice to WT levels. Intraperitoneal application of 3,4-diaminopyridine fully restored in vivo nerve-stimulated muscle force in BK^−/− mice. Our work demonstrates that mice lacking BK channels have weakness due to a defect in vesicle release at the NMJ.

Impaired neuromuscular function by conjoint actions of organophosphorus insecticide metabolites omethoate and cyclohexanol with implications for treatment of respiratory failure

BACKGROUND

Ingestion of agricultural organophosphorus insecticides is a significant cause of death in rural Asia. Patients often show acute respiratory failure and/or delayed, unexplained signs of neuromuscular paralysis, sometimes diagnosed as "Intermediate Syndrome". We tested the hypothesis that omethoate and cyclohexanol, circulating metabolites of one agricultural formulation, cause muscle weakness and paralysis.

METHODS

Acetylcholinesterase activity of insecticide components and metabolites was measured using purified enzyme from eel electroplaque or muscle homogenates. Mechanomyographic recording of pelvic limb responses to nerve stimulation was made in anaesthetized pigs and isometric force was recorded from isolated nerve-muscle preparations from mice. Omethoate and cyclohexanol were administered intravenously or added to physiological saline bathing isolated muscle. We also assessed the effect of MgSO₄ and cooling on neuromuscular function.

RESULTS

Omethoate caused tetanic fade in pig muscles and long-lasting contractions of the motor innervation zone in mouse muscle. Both effects were mitigated, either by i.v. administration of MgSO₄ in vivo or by adding 5 mM Mg²⁺ to the medium bathing isolated preparations. Combination of omethoate and cyclohexanol initially potentiated muscle contractions but then rapidly blocked them. Cyclohexanol alone caused fade and block of muscle contractions in pigs and in isolated preparations. Similar effects were observed ex vivo with cyclohexanone and xylene. Cyclohexanol-induced neuromuscular block was temperature-sensitive and rapidly reversible.

CONCLUSIONS

The data indicate a crucial role for organophosphorus and solvent metabolites in muscle weakness following ingestion of agricultural OP insecticide formulations. The metabolites omethoate and cyclohexanol acted conjointly to impair neuromuscular function but their effects were mitigated by elevating extracellular Mg²⁺ and decreasing core temperature, respectively. Clinical studies of MgSO₄ therapy and targeted temperature management in insecticide-poisoned patients are required to determine whether they may be effective adjuncts to treatment.

Synaptotagmin-2, and -1, linked to neurotransmission impairment and vulnerability in Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is the most frequent genetic cause of infant mortality. The disease is characterized by progressive muscle weakness and paralysis of axial and proximal limb muscles. It is caused by homozygous loss or mutation of the SMN1 gene, which codes for the Survival Motor Neuron (SMN) protein. In mouse models of the disease, neurotransmitter release is greatly impaired, but the molecular mechanisms of the synaptic dysfunction and the basis of the selective muscle vulnerability are unknown. In the present study, we investigated these open questions by comparing the molecular and functional properties of nerve terminals in severely and mildly affected muscles in the SMNΔ7 mouse model. We discovered that synaptotagmin-1 (Syt1) was developmentally downregulated in nerve terminals of highly affected muscles but not in low vulnerable muscles. Additionally, the expression levels of synaptotagmin-2 (Syt2), and its interacting protein, synaptic vesicle protein 2 (SV2) B, were reduced in proportion to the degree of muscle vulnerability while other synaptic proteins, such as syntaxin-1B (Stx1B) and synaptotagmin-7 (Syt7), were not affected. Consistently with the extremely low levels of both Syt-isoforms, and SV2B, in most affected neuromuscular synapses, the functional analysis of neurotransmission revealed highly reduced evoked release, altered short-term plasticity, low release probability, and inability to modulate normally the number of functional release sites. Together, we propose that the strong reduction of Syt2 and SV2B are key factors of the functional synaptic alteration and that the physiological downregulation of Syt1 plays a determinant role in muscle vulnerability in SMA.

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.

Ca2+ dependence of the binomial parameters p and n at the mouse neuromuscular junction

The Ca(2+) dependence of synaptic quantal release is generally thought to be restricted to probability of vesicular release. However, some studies have suggested that the number of release sites (n) at the neuromuscular junction (NMJ) is also Ca(2+) dependent. In this study, we recorded endplate currents over a wide range of extracellular Ca(2+) concentrations and found the expected Ca(2+) dependency of release. A graphical technique was used to estimate p (probability of release) and n using standard binomial assumptions. The results suggested n was Ca(2+) dependent. The data were simulated using compound binomial statistics with variable n (Ca(2+) dependent) or fixed n (Ca(2+) independent). With fixed n, successful simulation of increasing Ca(2+) required that p increase abruptly at some sites from very low to high values. Successful simulation with variable n required the introduction of previously silent release sites (p = 0) with high values of p. Thus the success of both simulations required abrupt, large increases of p at a subset of release sites with initially low or zero p. Estimates of the time course of release obtained by deconvolving evoked endplate currents with average miniature endplate currents decreased slightly as Ca(2+) increased, thus arguing against sequential release of multiple quanta at higher Ca(2+) levels. Our results suggest that the apparent Ca(2+) dependence of n at the NMJ can be explained by an underlying Ca(2+) dependence of a spatially variable p such that p increases abruptly at a subset of sites as Ca(2+) is increased.

Mechanisms of neuromodulation as dissected using Sr2+ at motor nerve endings

The use of binomial analysis as a tool for determining the sites of action of neuromodulators may be complicated by the nonuniformity of release probability. One of the potential sources for nonuniformity of release probability is the presence of multiple forms of synaptotagmins, the Ca2+ sensors responsible for triggering vesicular exocytosis. In this study we have used Sr2+, an ion whose actions may be restricted to a subpopulation of synaptotagmins, in an attempt to obtain meaningful estimates of the binomial parameters p (the probability of evoked acetylcholine [Ach] release) and n (the immediate available store of ACh quanta, whereby m = np). In contrast to results in Ca2+ solutions, binomial analysis of Sr2+-dependent release reveals a dramatically reduced dependence of n on extracellular Sr2+ concentrations. In Sr2+ solutions, blockade of potassium channels with 3,4-diaminopyridine increased m by an exclusive increase in p, whereas treatment with phorbol ester increased m solely by effects on n. The cyclic adenosine monophosphate (cAMP) analogue CPT-cAMP increased m by increasing both n and p. The effect of CPT-cAMP on p but not on n was blocked by protein kinase A (PKA) inhibitors, whereas the effect on n was mimicked by 8-CPT-2'-O-Me-cAMP, a selective agonist for exchange protein directly activated by cAMP, otherwise known as the cAMP-sensitive guanine nucleotide-exchange protein. The results demonstrate both the utility of the binomial distribution in Sr2+ solutions and the dual effects of cyclic AMP on both PKA-dependent and PKA-independent processes at the amphibian neuromuscular junction.

Phorbol esters and adenosine affect the readily releasable neurotransmitter pool by different mechanisms at amphibian motor nerve endings

Phorbol esters and adenosine have been proposed to interact at common sites downstream of calcium entry at amphibian motor nerve endings. We thus studied the actions and interactions of phorbol esters and adenosine using electrophysiological recording techniques in conjunction with both binomial statistical analysis and high-frequency stimulation at the amphibian neuromuscular junction. To begin this study, we confirmed previous observations that synchronous evoked acetylcholine (ACh) release (reflected as endplate potentials, EPPs) is well described by a simple binomial distribution. We then used binomial analysis to study the effects of the phorbol ester phorbol dibutyrate (PDBu, 100 nM) and adenosine (50 microM) on the binomial parameters n (the number of calcium charged ACh quanta available for release) and p (the average probability of release), where the mean level of evoked ACh release (m) = np. We found that PDBu increased m by increasing the parameter n whilst adenosine reduced m by reducing n; neither agent affected the parameter p. PDBu had no effect on either the potency or efficacy of the inhibition produced by adenosine. Subtle differences between these two agents were revealed by the patterns of EPPs evoked by high-frequency trains of stimuli. Phorbol esters increased ACh release during the early phase of stimulation but not during the subsequent plateau phase. The inhibitory effect of adenosine was maximal at the beginning of the train and was still present with reduced efficacy during the plateau phase. When taken together with previous findings, these present results suggest that phorbol esters increase the immediately available store of synaptic vesicles by increasing the number of primed vesicles whilst adenosine acts at a later stage of the secretory process to decrease the number of calcium-charged primed vesicles.

Protein Kinase C Facilitation of Acetylcholine Release at the Rat Neuromuscular Junction

Protein kinase C (PKC) is a Ca2+-dependent enzyme involved in synaptic transmission, which can be experimentally activated by the phorbol ester, phorbol 12-myristate-13-acetate (TPA). We studied the effects of TPA application on acetylcholine (ACh) release at the rat neuromuscular junction by means of the focal recording technique; possible effects of TPA at the postsynaptic site had been ruled out in preliminary studies. In extracellular solutions containing 2 mM Ca2+ and at the stimulation frequency of 0.1 Hz, TPA increased endplate current (EPC) amplitude. In non-stimulated preparations spontaneous current frequency was increased at a similar rate. The similar time course of TPA action on evoked and spontaneous currents suggests that an increased presynaptic Ca2+ efficacy can be considered to be the probable mechanism of action. The interactions of PKC with ACh release were further investigated. In 0.1 mM Ca2+ extracellular solutions, TPA enhanced evoked currents only at stimulation frequencies (e.g. 40 Hz) that were themselves capable of inducing facilitation. This facilitation is classically associated with presynaptic Ca2+ accumulation, indicating that PKC interacts synergistically with Ca2+ to facilitate ACh release. In particular, since mean quantum size and release probability remained almost unchanged during TPA facilitation, it was concluded that PKC acted by enlarging the immediately available store. Interestingly, TPA also increased the presynaptic currents that were observed to be largely brought about by Ca2+-dependent K+ currents: evidence was obtained to suggest that increases in these currents provide negative feedback against excess release activation rather than being an expression of enhanced Ca2+ influx.

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.

Quantal analysis of presynaptic inhibition, low [Ca2+]0, and high pressure interactions at crustacean excitatory synapses

The cellular mechanisms underlying the effects of high pressure, GABAergic presynaptic inhibition, and low [Ca2+]0 on glutamatergic excitatory synaptic transmission were studied in the opener muscle of the lobster walking leg. Excitatory postsynaptic currents (EPSCs) were recorded with or without prior stimulation of the inhibitor using a loose macropatch clamp technique at atmospheric pressure and at 6.9 MPA helium pressure. High pressure reduced the mean EPSC amplitude and variance, decreased the quantal content (m), but did not affect the quantum current (q). Pressure shifted the median of the amplitude histogram to the left by 1-2 q. Under normal pressure conditions, presynaptic inhibition and low [Ca2+]0 induced similar effects. However, quantal analysis using a binomial frequency distribution model revealed that high pressure and low [Ca2+]0 diminished n (available active zones) and slightly increased p (probability of release), but presynaptic inhibition reduced p and slightly increased n. At high pressure, presynaptic inhibition was reduced, at which time the major contributor to the inhibitory process appeared to be reduction in n and not p. The similarity of the alterations in quantal parameters of release at high pressure, low [Ca2+]0, and in some conditions of presynaptic inhibition is consistent with the hypothesis that pressure reduces Ca2+ inflow into the presynaptic nerve terminals to affect the Ca(2+)-dependent quantal release parameters n and p.

Reduced safety factor for neuromuscular transmission and abnormal quantal secretion precede neuromuscular weakness induced by dithiobiuret

Daily treatment of rats with 2,4-dithiobiuret (DTB, 1 mg/kg/day, ip) causes a flaccid neuromuscular weakness first observable in the hindlimbs after 5-6 days of treatment. With continued exposure, neuromuscular weakness appears to encompass the other muscles of the body; death is presumed to result from paralysis of respiratory muscles. The purpose of the present study was to investigate the early effects on neuromuscular transmission which precede neuromuscular weakness caused by DTB, particularly as they relate to the apparent differential muscle sensitivity, using conventional intracellular microelectrode recording techniques. Experiments were conducted using the hemidiaphragm muscle isolated from male rats treated for 7-8 days, with 1 mg/kg/day DTB ip, a regimen which resulted in hindlimb, but not diaphragmatic paralysis, or with 0.9% NaCl (1 ml/kg/day) as control. Analysis of quantal content of end plate potentials (EPPs) from hemidiaphragms of DTB-treated rats indicated no difference from control. Exposure of hemidiaphragm preparations from DTB-paralyzed rats to solutions containing elevated Mg2+ and lowered Ca2+ concentrations (6 and 1 mM, respectively) resulted in a decreased quantal content of the EPP compared to that of similarly treated control preparations. When miniature EPPs (MEPPs) were evoked from nerve terminals by elevating [K+]e, in the presence of 6 mM Mg2+ and 1 mM Ca2+, the mean peak frequency evoked by K(+)-induced depolarization was reduced in the DTB-treated group; however, the time at which peak frequency was attained was the same for the control and DTB-treated group. Mean MEPP amplitude but not resting MEPP frequency was altered in the presence of high [Mg2+] for diaphragms of the DTB-treated group. Neither MEPP amplitude nor frequency was altered in diaphragms exposed to normal concentrations of Ca and Mg. Prolongation of rise and decay times of MEPPs occurred from end plates of DTB-treated rats irrespective of whether low [Ca2+]/high [Mg2+] solutions were used. However, these effects were more pronounced when low [Ca2+]/high [Mg2+] solutions were used. Diaphragm-derived end plates of the DTB-treated group were also characterized frequently by the presence of very large amplitude MEPPs with prolonged decay times. The overall percentage of the total population of MEPPs which these abnormal MEPPs made up in the DTB-treated rats was increased dramatically by exposure to low [Ca2+]/high [Mg2+] solutions. Thus, differential muscle sensitivity occurs during paralysis induced by chronic DTB treatment, with the diaphragm being somewhat more resistant than hindlimbs to neuromuscular weakness.(ABSTRACT TRUNCATED AT 400 WORDS)

Depression, recovery and facilitation of neuromuscular transmission during prolonged tetanic stimulation

The phrenic nerve of an unparalysed "cut" rat diaphragm preparation was stimulated with a rapid sequence of short tetanic trains. The amplitudes of both the first and the last (25th) endplate potentials produced by short tetanic trains progressively decreased with repeated application, but the latter diminished faster. Both relative depression and relative recovery (defined as fractional decrease and fractional recovery of endplate potential amplitudes that occur during and after each short train) became more pronounced with time. Relative facilitation (defined as fractional increase of endplate potential amplitudes observed at the beginning of each train) when initially present, persisted or increased slightly with duration of stimulation. Present results suggest that the "classic" depletion model for depression ought to be modified. They can be explained if it is assumed that during stimulation the capacity of the immediately available store to contain transmitter increases, and/or that newly formed transmitter preferentially replenishes the store of quanta immediately available for release. (An interesting consequence of the former model is that the immediately available store is replenished even when the small store behind is relatively more depleted.

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.

The origin of (+)-tubocurarine resistance in dystrophic mice

Intracellular recording, twitch responses and radio-ligand binding techniques were used to study the causes of resistance to (+)-tubocurarine (curare) of extensor digitorum longus (EDL) muscles from dystrophic mice (129 ReJ/strain). The indirectly evoked twitch response of muscles from dystrophic mice was more resistant to block by curare than the twitch response of muscles from normal littermates. The IC50 (concentration producing 50% inhibition of stimulus-evoked contractions) values for the curare block of muscle twitch were 0.78 +/- 0.03 microM and 1.32 +/- 0.05 microM (mean +/- 95% confidence limits) for muscles from normal and dystrophic mice, respectively. There was no difference between muscles from normal and dystrophic mice in the number of alpha-bungarotoxin binding sites per endplate. The amplitudes of both spontaneous miniature endplate potentials (m.e.p.ps) in unblocked preparations and of evoked endplate potentials (e.p.ps) in 1.91 microM curare were greater in muscles from dystrophic mice than in muscles from normal mice. The ratio dystrophic/normal was greater for the e.p.p. amplitudes than for the m.e.p.p. amplitudes. The quantum content of e.p.ps in magnesium-blocked and in cut-fibre preparations was greater in muscles from dystrophic mice than in muscles from normal littermates. Calculation of the binomial parameters n and p in the cut-fibre preparations indicated that this increased quantum content was caused by an increase in the value of p. It is concluded that at least part of the increased resistance to curare of the indirectly evoked twitch response of muscles from dystrophic mice is due to an increase in the quantum content of e.p.ps in these muscles.

Antagonistic action of uranyl nitrate on presynaptic neurotoxins from snake venoms

Uranyl ion (UO2+2) antagonized the neuromuscular blocking action and phospholipase A2 activity of neurotoxins which act presynaptically [beta-bungarotoxin (beta-BuTX) and crotoxin] but did not affect the action of alpha-bungarotoxin and tetrodotoxin. On the basis of the kinetic analysis of the UO2+2 and strontium ion (Sr2+) antagonism of muscle paralysis induced by beta-bungarotoxin, it was found that they inhibited both the binding of the toxin and the steps following binding that brought about the neuromuscular blocking action of beta-bungarotoxin. Uranyl ion was about 50 times more potent than Sr2+ in antagonizing beta-bungarotoxin. High Ca2+ (10 mM) abolished but low Ca2+ (0.25-1.25 mM) medium enhanced the antagonizing action of UO2+2 and Sr2+. In low Ca2+ medium, UO2+2 markedly potentiated the amplitude of the twitch, subsequent addition of beta-bungarotoxin produced three phases of effects on the twitches, e.g. an initial depression, followed by the second facilitation and finally a rapid depression of twitches; however, approx. 70 min after beta-bungarotoxin the small twitches reached a steady state which persisted for more than 350 min. Therefore, it is evident that UO2+2 is the most potent antagonist of beta-bungarotoxin so far tested.

Presynaptic effects of 4-aminopyridine and streptomycin on the neuromuscular junction

Studies were done to assess the effects of 4-aminopyridine (4AP) and streptomycin (SM) on transmitter release parameters and extracellularly recorded presynaptic action potential. The application of 5 micrometer 4AP resulted in a marked increase in the mean quantal content (m1) associated with an increase in the total number of the store of available quanta (n) but had no effect on the probability of release (p) and the fractional release (P). Focal recording showed that 50 micrometer 4AP modified the shape of the presynaptic action potential from a triphasic configuration to a diphasic one. In contrast, prolongation of the muscle action potential was found only at higher concentrations (greater than 1 mM) of of 4AP. Thus, the increase in n with 4 AP was associated with prolongation of the presynaptic action potential evoked by blocking the K current. SM (172 micrometer mM) had no effect on p and P. Reduction of n by SM was completely reversed by 4AP. As the presynaptic action potential change induced by 4AP was not antagonised by SM, it may be that the decrease of n by SM followed a modification of the voltage-dependent Ca channel.

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

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

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

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

Change of statistical parameters of transmitter release during various kinetic tests in unparalysed voltage-clamped rat diaphragm

1. The statistical nature of transmitter release was studied in unparalysed cut rat diaphragm using a voltage clamp technique at room temperature (23 degrees C). 2. While the binomial distribution described observed amplitude histograms well, the Poisson distribution was clearly inadequate. Values of m ranged from 40 to 45, while values for n varied from 45 to 53 and p was between 0.82 and 0.90. 3. Estimation of the probability of release from the transient decay of e.p.c.s in short tetanic trains (11 pulses, 150 Hz) gave values of p from 0.031 to 0.054, which are more than one order of magnitude lower than statistical estimates. 4. As a result of the short tetanic stimulation (5 pulses, 20, 50 and 100 Hz) there is an initial transient facilitation which afterwards becomes masked by depression. Statistical analysis suggests that the changes in the average numbers of quanta released (m) could be attributed to the change in the immediately available store (n). 5. During long tetanic stimulation (4000 pulses, 10-100 Hz) statistical analysis suggests that the decrease in the average number of quanta released (m) could be attributed almost entirely to the decrease in the immediately available store (n). The probability of release (p) decreased only slightly. The extent of the post-tetanic potentiation indicates that it cannot be explained on the grounds of increased probability of release (p) only. There should be an increase as well in the immediately available store (n). 6. It is suggested that while depression is most likely caused by the depletion of the immediately available store due to insufficient replenishment, the facilitation is probably caused by the increase in the capacity of the immediately available store to contain transmitter.

Effects of 4-aminopyridine on statistical parameters of transmitter release at the neuromuscular junction

4-Aminopyridine (4-AP) potentiates transmitter release from motor nerve terminals by increasing the quantum content (m) of endplate potentials. Estimates of the binomial parameters of transmitter release shows that 4-AP enhances m by increasing n and not p.

Synaptic enhancement in fascia dentata: cooperativity among coactive afferents

Evidence is presented that long-lasting enhancement of perforant path synapses following high-frequency activity is a cooperative process requiring coactivity of a considerable number of fibers. This was demonstrated in two ways: (1) A threshold stimulus intensity during high-frequency stimulation was required before any enhancement was observed. This threshold was considerably above the stimulus threshold for observing a minimal synaptic response in the fascia dentata. Above the enhancement threshold, the amount of enhancement increased as the intensity of high-frequency stimulation increased. (2) Concurrent high-frequency activation of fibres of the medial and lateral perforant path produced enhancement at intensities where identical but independent stimulation of the two pathways either failed or was less effective. Discharge of the granule cells during the high-frequency stimulation was independently manipulated and found not to be physically coupled to the enhancement generating mechanism.

Adaptive rundown of excitatory post-synaptic potentials at synapses between hair cells and eight nerve fibres in the goldfish

1. The excitatory post-synaptic potentials (e.p.s.p.s.) evoked by sound stimuli were recorded intracellularly from large afferent eight nerve fibres in the sacculus of the goldfish (S1 fibres). The fish were anaesthetized with MS-222 and spike potentials were suppressed with locally applied tetrodotoxin. 2. The e.p.s.p.s. successively evoked in response to each wound wave showed a marked rundown in size, while no reduction was observed in the microphonic potentials. The amplitude of successive e.p.s.p.s was reduced keeping approximately a fixed ratio to the preceding ones, suggesting that the rundown is attributable to a depletion of transmitter quanta from the release sites. 3. The rate of rundown of successive e.p.s.p.s, however, remained almost unchanged when the intensity of the stimulus sound was changed. It was also observed that, even after the e.p.s.p.s had been completely adapted to a continuous sound, a vigorous discharge of new e.p.s.p.s was observed when the intensity of the sound was increased. 4. These findings seem to indicate that it is the size of the readily available store and not the release fraction that is changed by a change in the sound intensity. 5. The saccular macula was superfused with solutions different in Ca and Mg ion concentrations. High Ca ion concentration brought about an increase in the size of the readily available store as well as the release fraction. 6. Mechanisms underlying these observations were discussed in terms of the quantal release mechanism as well as the morphology of the release sites.

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

1. A study has been made of the effects of changing the external calcium concentration [Ca](o) on the binomial parameters p and n that control the average quantal content (m) of the end-plate potential (e.p.p.) during trains of nerve impulses at synapses in amphibian striated muscle.2. In high external calcium concentrations (0.4 mM </= [Ca](o) < 1.0 mM) the increase in m of a test impulse following a conditioning impulse at different intervals (< 100 msec) was due to an increase in the number of quanta available for release, n; the increase in m of successive e.p.p.s in a short high frequency train was primarily due to an increase in n.3. In high external calcium concentrations (1.0 mM </= [Ca](o) < 10 mM) there was a decrease in m of a test impulse following a short high frequency conditioning train (4-5 impulses, 20-100 Hz) at different intervals (200 msec < 5 sec) and this was due to a decrease in the number of quanta available for release, n; in a long high frequency train (20 impulses, 20-100 Hz) there was an increase in m for the first few successive e.p.p.s followed by a depression of m which eventually reached a steady state and these changes in m were due to changes in n; the higher the frequency the greater was the depression in n during the steady-state period.4. In high calcium concentrations, the steady-state m reached in the first 20 impulses during continual stimulation at high frequency gave way to a decline in m over several minutes until a new depressed steady-state value of m was reached and this was maintained during the longest periods of stimulation (30 min); this decline in m was primarily due to a decline in the number of quanta available for release.5. These changes in the number of quanta available for release during trains of impulses are predicted in terms of a hypothesis in which facilitation is due to the accumulation of a residual calcium-receptor complex in the nerve terminal that determines the fraction of a pool of quanta which contributes to n, and depression is due to a decrease in the number of quanta in this pool.

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

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

Transmitter release during repetitive stimulation: selective changes produced by Sr2+ and Ba2+

The addition of Sr2+ or Ba2+ to the solution bathing the frog neuromuscular junction leads to an increased release of transmitter by each nerve impulse during and following repetitive stimulation. The mechanisms by which Sr2+ and Ba2+ increase release are not the same. Each ion appears to act selectively on a different process involved in transmitter release.

The effects of strontium and barium ions at synapses in sympathetic ganglia

1. A study has been made of the effects of Sr2+ and Ba2+ ions at synapses in isolated superior cervical ganglia of guinea-pigs. Intracellular recordings of membrane potential were made from ganglion cells in the presence of different concentrations of Ca2+, Sr2+ and Ba2+ ions. 2. The addition of Sr2+ (2-5 mM) caused little change in resting membrane potential; in contrast, Ba2+ (1-6 mM) always depolarized the cells and prolonged the duration of action potentials. 3. The resting frequency of spontaneous miniature excitatory post-synaptic potentials (min. e.p.s.p.s) was briefly accelerated by the addition of either Sr2+ or Ba2+, but subsequently returned to about control levels. 4. Following replacement of Ca2+ by Sr2+, e.p.s.p.s could always be evoked during repetitive stimulation of preganglionic axons at a fixed latency after the nerve impulses ('phasic' transmitter release). Replacement of Ca2+ by Ba2+ produced many asynchronous e.p.s.p.s during trains of impulses ('residual' transmitter release). 5. By analysis of the interaction between Sr2+ and Ca2+, Sr2+ was shown to have a partial agonist action on 'phasic' transmitter release. The same analysis applied to Ba2+ failed to demonstrate either a partial agonist or antagonist action. 6. Both Sr2+ and Ba2+ prolonged e.p.s.p.s. Changes in Sr2+ could mainly be attributed to its effect on cell input resistance; Ba2+ may also prolong the time course of transmitter release. 7. The increased frequency of min. e.p.s.p.s which occurs during repetitive stimulation was potentiated by both Sr2+ and Ba2+, Ba2+ being about twice as potent as Sr2+. This activation of 'residual' transmitter release is independent of the action of these ions on 'phasic' release. 8. It is concluded that the reported maintenance by Ba2+ of acetyl-choline output from perfused ganglia results from the asynchronous release of large numbers of quanta during trains of impulses.

The formation and regression of synapses during the re-innervation of axolotl striated muscles

1. A study has been made of the formation and regression of synapses formed by spinal nerves 16 and 17 in axolotl hind-limb flexor muscles following the severing of nerve 16, using histological, ultrastructural and electrophysiological techniques. 2. Axolotl hind-limb flexor myofibres possessed 'en plaque' end-plates from either spinal nerve 16 or 17 or both at intervals of about 1000 micronm along their length; the myofibre's length constant was about 700 micronm allowing electrophysiological observations of at least two of these synapses during a single impalement; transmitter release at these synapses could be described by binomial statistics and in a given set of ionic conditions the binomial statistic parameter n was directly proportional to the size of the nerve terminals whilst the binomial statistic parameter p was invariant to changes in nerve terminal size. 3. The distribution of synapses formed by spinal nerves 16 and 17 in different sectors of the axolotl hind-limb flexor muscles was determined from a study of evoked end-plate potentials; the middle and proximal sectors of the flexor muscles contained myofibres which received an innervation from nerve 16 only, whereas the sectors surrounding these contained myofibres innervated either by nerve 16 or nerve 17 or by both nerves. 4. Six days following the severing of spinal nerve 16, evoked transmitter release from the synapses formed by this nerve had failed; transmission was subsequently recorded at a few synapses formed by nerve 17 in the middle and proximal sectors of the flexor muscles which are not normally innervated by this nerve and these synapses had a low n; during the succeeding four weeks the value of n at the synapses increased to a size about 70% that of the terminals normally formed by nerve 16 at these sites. 5. Four weeks after severing nerve 16, myofibres which possessed synapses formed by nerve 17 also possessed synapses from re-innervating nerve 16 and these were sometimes formed at the same synaptic sites as those occupied by nerve 17. 6. In the subsequent sixteen weeks, the n value of synapses formed by nerve 17 declined whilst the n values of synapses formed by re-innervating nerve 16 on the same myofibres matured to their control size. 7. It is suggested that on severing nerve 16 collateral sprouting of nearby intact nerve 17 occurs and these collateral sprouts innervate the denervated synaptic sites, although the sprouts arenot as well matched to the denervated synaptic sites as are the original nerve terminals; thus if nerve 16 returns it preferentially forms synapses at its original synaptic sites, and the collateral synapses formed by nerve 17 regress.

Quantal parameters of transmission at the frog neuromuscular junction

Estimates of quantal release parameters at frog neuromuscular junctions showed that alterations in [Ca2+]o affected m (number of quanta), p (probability of quantal release) and n (stores of quanta available for release). The effect of [Ca2+]o depended upon the initial value for p. When p was low, raising [Ca2+]o increased m and p, but not n. However, when p was large, raising [Ca2+]o had no further effect on p but increased m and n. During prolonged repetitive nerve stimulation to cause a decrease in m, n was decreased and p was increased. This finding was attributed to a failure of transmitter mobilization to maintain the stores of transmitter available for release.

Evoked neurotransmitter release: statistical effects of nonuniformity and nonstationarity

Recent studies of the mechanism of quantal neurotransmitter release have assumed that the number of quanta released at each stimulation is binomially distributed and have sought to estimate the binomial parameters n and p. Mathematical analysis and computer simulations show that temporal variation in the number of eligible or filled release sites and either spatial or temporal variation in the probability of release at a site can drastically bias such estimates, while the experimental histograms remain statistically indistinguishable from those predicted by the binomial law. Interpretation of the estimates n and p in terms of ultrastructural or physiological characteristics of the nerve terminal is liable to significant error if departures from the binomial assumptions are not suitably assessed.

Synaptic depression at a synapse in Aplysia californica: analysis in terms of a material flow model of neurotransmitter

When a pair of stimuli separated by an appropriate interval is given to the right visceropleural connective of Aplysia californica the amplitude of the second EPSP elicited in cell R15 is usually smaller than the amplitude of the first EPSP. In the present paper we show that this phenomenon, synaptic depression, can be analyzed in terms of the material flow model of neurotransmitter economics developed in our preceding publications. We specifically show how changes in the 4 model parameters; A, the available pool of transmitter; F, the fraction of the available pool released by a presynaptic action potential; M, the rate of transmitter mobilization into the available pool; and D, the rate constant of demobilization of transmitter from the available pool, all effect synaptic depression. In addition, we show how transient changes in F and M, that are observed immediately and for seconds after a stimulus, influence the time course of synaptic depression. Using this analysis we then tested our previous inferences about changes in the model parameters produced either by pharmacological manipulations or repetitive stimulation, by comparing the observed effects of these manipulations on synaptic depression with the theoretical predictions. The theoretical and experimental findings agreed, thereby strengthening both our previous conclusions of the mode of action of these manipulations and the model itself.

The effect of calcium ions on the binomial statistic parameters that control acetylcholine release at preganglionic nerve terminals

1. A study has been made of the effects of changing [Ca]O and [Mg]O on the binomial statistic parameters p and n that control the average quantal content (m) of the excitatory post-synaptic potential (e.p.s.p.) due to acetylcholine release at preganglionic nerve terminals. 2. When [Ca]O was increased in the range from 0-2 to 0-5 mM, p increased as the first power of [Ca]O whereas n increased as the 0-5 power of [Ca]O; when [Mg]O was increased in the range from 5 to 200 mM, p decreased as the first power of [Mg]O whereas n decreased as the 0-5 power of [Mg]O. 3. The increase in quantal release of a test impulse following a conditioning impulse was primarily due to an increase in n; the increase in quantal content of successive e.p.s.p.s in a short train was due to an increase in n and p, and the increase in n was quantitatively described in terms of the accumulation of a Ca-receptor complex in the nerve terminal. 4. The decrease in quantal content of successive e.p.s.p.s during long trains of impulses over several minutes was primarily due to a decrease in n. These results are discussed in terms of an hypothesis concerning the physical basis of n and p in the release process.

Long term changes in augmentation, potentiation, and depression of transmitter release as a function of repeated synaptic activity at the frog neuromuscular junction

1. End-plate potentials (e.p.p.s) were recorded from frog neuromuscular junctions under conditions of low quantal content to study the long-term effects of repeated synaptic activity on transmitter release. 2. The nerve terminal was presented with 30-100 successive conditioning-testing trials applied once every 7-10 min over a 4-16 hr perod. Each conditioning-testing trial consisted of a 200-600 impulse conditioning train followed by a series of testing impulses. The magnitudes and time constants of decay of augmentation and potentiation following each successive conditioning train were determined by measuring the e.p.p. amplitudes resulting from the testing impulses. 3. The magnitude of augmentation immediately following the conditioning trains increased an average of 3-4 times (range 1-20) with sucessive trials. 4. As the magnitude of augmentation increased with successive trials the decay of augmentation deviated from a simple exponential, decaying faster immediately after the conditioning train. This faster decay led to a 20% decrease with successive trials in estimates of the time constant obtained from the first 10 or 20 sec of the decay of augmentation. The deviation of the decay of augmentation from a simple exponential could be accounted for if augmentation is related to the 4th power of some substance which decays with a simple exponential time course. Some alternative explantations for the non-exponential decay of augmentation are also discussed. 5. The magnitude of potentiation increased or decreased about 25% with successive trials. 6. The time constant characterizing the decay of potentiation inceased an average of 1-5 times (range 0-8-5 times) with successive trials. 7. The increase in the magnitude of augmentation with successive trials was accompanied by a similar increase in the magnitude of the e.p.p. amplitudes during the conditioning trains, suggesting that augmentation develops during the conditioning train. In some preparatons augmentation appeared to be the major factor acting to increase e.p.p. amplitudes during the conditioning train, having a greater effect than facilitation or potentiation. 8. If a sufficiently large number of successive trials were applied, a depression of e.p.p. amplitudes developed during the conditioning trains and estimates of the magnitude of potentiation following the depressed conditioning trains were reduced...

Augmentation: A process that acts to increase transmitter release at the frog neuromuscular junction

1. End-plate potentials (e.p.p.s) were recorded from frog neuromuscular junctions bathed in Ringer solution containing increased Mg and decreased Ca to reduce transmitter release. Conditioning and testing stimulation was applied to the nerve to study a previously uncharacterized process which acts to increase e.p.p. amplitudes. We will refer to this process as augmentation. 2. Following repetitive stimulation augmentation decayed approximately exponentially over most of its time course with a mean time constant of about 7 sec (range 4-10 sec) which is intermediate in duration between the time constants for the decay of facilitation and potentiation. 3 . The magnitude of agumentation increased with the duration of the conditioning stimulation. Assuming a multiplicative relationship between augmentation and potentiation, values of the magnitude of augmentation ranged from 0-3 to 0-6 following 50 impulses at 20/sec to 0-5-7-8 following 600 impulses at 20/sec. (An augmentation of 0-3 and 7-8 would increase e.p.p. amplitudes 1-3 and 8-8 times, respectively.) 4. The time constant characterizing the decay of augmentation remained relatively constant as the duration of the conditioning stimulation was increased. 5. Augmentation as well as facilitation and potentiation resulted from an increase in the number of quanta of transmitter released from the nerve terminal. 6. Augmentation decayed faster at higher temperatures with a mean temperature coefficient, Q10, of about 3-8. The corresponding Q10 for the decay of potentiation was found to be about 2-4. 7. It is concluded that augmentation can be a significant factor in increasing transmitter release and will therefore have to be accounted for when studying the effects of repetitive stimulation on the function of the nerve terminal or when formulating models of transmitter release.

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.

An electrophysiological analysis of the effect of Ca ions on neuromuscular transmission in the mouse vas deferens

1 A study has been made of the effects of changing the external calcium concentration [Ca]0 and the external magnesium concentration [Mg]0 on the synaptic potential due to noradrenaline release. 2 When [Ca]0 was varied in the range 0.7 to 1.8 mM, the synaptic potential increased as about the second power of [Ca]0. 3 Increasing [Mg]0 depressed the synaptic potential; however, variation of [Ca]0 in the presence of high [Mg]0 did not significantly change the power relationship between the synaptic potential and [Ca]0. 4 The facilitated increase in the synaptic potential during short trains of impulses at different frequencies was quantitatively predicted on the assumption that each impulse leaves residual Ca ions bound to release receptors in the nerve terminal.