A comparison of the effects of various sulfhydryl reagents on neuromuscular transmission

Munc13-1 and Munc18-1 together prevent NSF-dependent de-priming of synaptic vesicles

Synaptic transmission requires a stable pool of release-ready (primed) vesicles. Here we show that two molecules involved in SNARE-complex assembly, Munc13-1 and Munc18-1, together stabilize release-ready vesicles by preventing de-priming. Replacing neuronal Munc18-1 by a non-neuronal isoform Munc18-2 (Munc18-1/2SWAP) supports activity-dependent priming, but primed vesicles fall back into a non-releasable state (de-prime) within seconds. Munc13-1 deficiency produces a similar defect. Inhibitors of N-ethylmaleimide sensitive factor (NSF), N-ethylmaleimide (NEM) or interfering peptides, prevent de-priming in munc18-1/2SWAP or munc13-1 null synapses, but not in CAPS-1/2 null, another priming-deficient mutant. NEM rescues synaptic transmission in munc13-1 null and munc18-1/2SWAP synapses, in acute munc13-1 null slices and even partially in munc13-1/2 double null synapses. Together these data indicate that Munc13-1 and Munc18-1, but not CAPS-1/2, stabilize primed synaptic vesicles by preventing NSF-dependent de-priming.

The molecular mechanism underlying the generation and maintenance of the readily releasable pool composed of primed synaptic vesicles is only partially known. Here the authors show that in mouse primary neurons, Munc13-1 and Munc18-1 stabilize primed synaptic vesicles by preventing NSF-dependent de-priming.

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

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

Different effects of two gold compounds on muscle contraction, membrane potential and ryanodine receptor

Effects of gold sodium thiomalate and NaAuCl4 on skeletal muscle function were studied using intact single fibres of frog skeletal muscle and fragmented sarcoplasmic reticulum prepared from frog and rabbit skeletal muscles. Gold sodium thiomalate at a concentration of 500 microM decreased tension amplitude by 27% and resting membrane potential by 5.3% after 30 and 22 min, respectively. The duration of tetanus tension was markedly shortened by 500 microM gold sodium thiomalate. When 10 microM NaAuCl4 was applied to gold sodium thiomalate-pretreated fibres, the fibres lost the ability to contract upon electrical stimulation, similar to the effects of 10 microM NaAuCl4 alone. In the presence of thiomalic acid, on the other hand, NaAuCl4 did not completely block tetanus tension even at 50 microM. Thiomalic acid also inhibited NaAuCl4-induced membrane depolarization. These findings suggest that thiomalate masks the effects of gold ion on muscle function. When sarcoplasmic reticulum vesicles were incorporated into lipid bilayers, exposure of the cis side of the Ca2+-release channel to 100 microM gold sodium thiomalate rapidly increased the open probability of the channel 3.3-fold, from 0.032 in controls to 0.105, with an increase in number of open events and a decrease in mean closed time. The ability of NaAuCl4 to activate the Ca2+-release channel was much stronger than that of gold sodium thiomalate. Only 1 microM NaAuCl4 was enough to activate the channel and this gold was effective from either side of the channel. These results suggest that gold sodium thiomalate could be used as an antirheumatic drug without considering severe side-effects on skeletal muscle. Coexistent thiomalate probably contributes to protection of muscle function from side-effects of gold ion.

Gold ion induces contraction in frog skeletal muscle fibers

In Ringer solution, gold ions (Au3+) at concentrations more than 50 microM produced a phasic and subsequent tonic contraction spontaneously in single toe muscle fiber of frog. When 1.8 mM Ca2+ in Ringer solution was replaced by 3 mM Mg2+, tonic contraction was no longer provoked in response to Au3+. Only phasic contraction was potentiated by 10 mM perchlorate (an L-type Ca2+ channel activator) irrespective of external Ca2+, and both phasic and tonic contractions were blocked by 10 microM nifedipine (an L-type Ca2+ channel blocker). Upon application of 5 mM dithiothreitol to the contracting fiber, the Au(3+)-induced tension disappeared rapidly. The fiber pretreated with 0.05% H2O2 for 10 min did not respond to Au3+ with visible contraction. Treatment of H2O2-paralyzed fibers with dithiothreitol (to reduce oxidized sulfhydryl groups) fully restored the Au(3+)-induced contraction. These results suggest that the phasic contraction induced by Au3+ probably is mediated through sulfhydryl groups in the L-type Ca2+ channel (dihydropyridine receptor) on the transverse tubular membrane. Sustained contraction was produced by Ca2+ application to Au(3+)-treated fibers in Mg(2+)-Ringer solution, and Au3+ caused membrane depolarization in a dose-dependent manner. These effects of Au3+ may explain tonic contraction development.

Effects of the sulphydryl inhibitor N-ethyl-maleimide on the phrenic nerve and diaphragm muscle of the rat

N-Ethyl-maleimide (NEM, 2.5 x 10(-5) M) inhibited the compound action potential of the phrenic nerve and increased the spontaneous release of transmitter from the nerve terminals, recorded as miniature endplate potentials. The first effect was the cause of a blockade of the phrenic nerve diaphragm preparation, during indirect stimulation. The left phrenic nerve was more susceptible to inhibition than the right. An increase of the threshold was observed during the progression of the inhibition. The inhibition was not use-dependent and there was no synergistic interaction with the local anaesthetic drug, tetracaine. The inhibition was partly antagonized by di-thio-threitol (3.0 x 10(-3) M). The increase of spontaneous release of transmitter was not accompanied by an increase of the stimulus-evoked release since the amplitude of the endplate potential was not increased and partial inhibition caused by d-tubocurarine or magnesium chloride was not antagonized. When the concentration of NEM was increased to 2.75 x 10(-4) M, the directly-elicited twitches were inhibited, and the baseline tension was increased. This increase of tension was slightly reduced in a preparation depolarized with potassium chloride; a small depolarization could partly explain this effect. It was not reduced by dantrolene or in a calcium-free solution. The inhibition of the twitch and the increased baseline tension (probably a rigor) might be caused by a reduced sensitivity of the contractile proteins for calcium ions and an inhibition of the myosin ATPase activity, respectively.

The effects of tetraethylammonium during twitch and tetanic stimulation of the phrenic nerve diaphragm preparation in the rat

Tetraethylammonium (TEA) (2.6 x 10(-3) M) potentiated the twitches of the indirectly- or directly-stimulated phrenic nerve diaphragm of the rat at 37 degrees C by prolonging the action potential of the sarcolemma, due to an inhibition of the repolarizing K+ current. With indirect stimulation, TEA caused a use-dependent inhibition of tetanic contractions, induced every 10 min by 10 sec of 50 Hz stimulation, and a post-tetanic depression of the twitches was observed after about 40 min. Recording of the electromyogram (EMG) and compound action potentials of the phrenic nerve, localized the two inhibitory effects to the neuromuscular junction. They were caused by different mechanisms of action. Choline (3.6 x 10(-4) M) antagonized the depression of the twitch but not the use-dependent inhibition. Lowering the temperature to 20 degrees C reduced the depression of the twitch, whereas the use-dependent inhibition was enhanced. The release of transmitter was probably normal during tetanic stimulation; a post-synaptic desensitization of acetylcholine (ACh) receptors caused the inhibition. Microelectrode recordings of endplate potentials supported this conclusion. The depression of the twitch was due to a presynaptic depletion of transmitter. This was confirmed by inducing an additional depletion and depression of the twitch with N-ethyl-maleimide (2.5 x 10(-5) M). Since the depression of the twitch was antagonized by choline, the depletion was probably due to an inhibited uptake of choline into the nerve terminals.

Silver ion-induced tension development and membrane depolarization in frog skeletal muscle fibres

Silver ions elicit dose-dependently a transient contracture in single fibres of bull-frog toe muscle placed in 0-Ca2+, Cl- -free MOPS solution containing 3 mM Mg2+ and NO3-. To elucidate the mechanisms involved, changes in membrane potential and in tension development were continuously measured following exposure to Ag+. The effect of Ag+ on contraction in fibres in which the membrane had been depolarized by elevating the external K+ concentration was also examined. The major findings of this investigation are as follows. (1) The mechanical threshold was shifted towards more negative potentials by 5 mV (-51 to -56 mV), when Ca2+ and Cl- in the Ringer's solution were replaced with Mg2+ and NO3-, respectively. (2) On the exposure of the fibres to 5 microM Ag+, the membrane potential decreased by 1.6 mV from -87.8 mV and tension was developed. (3) In fibres soaked in a solution containing 10 mM K+ (corresponding to a membrane potential of -69.5 mV), 5 microM Ag+ produced a large contracture similar to that seen in the control solution. (4) The Ag+-induced contracture was inactivated when more than 20 mM K+ was used. (5) The membrane depolarization evoked by either 20 or 50 microM Hg2+ did not produce contraction. (6) Muscle fibres which had been exposed to 20 microM Hg2+ for 5 min responded to 5 microM Ag+ by a transient tension development. These findings strongly suggest that Ag+-induced tension development is not associated with depolarization of the surface membrane but rather is caused by specific actions of Ag+ on membrane proteins in the T-tubules.

The aminoglycoside antibiotic, gentamicin, fails to block increases in miniature endplate potential frequency induced by the sulfhydryl reagent, N-ethylmaleimide, in low calcium solutions

N-ethylmaleimide (NEM) increases the frequency of miniature endplate potentials (MEPPs) at the adult rat hemidiaphragm. This sulfhydryl-alkylating agent produces comparable effects in the absence of added calcium (2 mM EGTA), suggesting that the drug releases calcium from internal stores, or promotes calcium-independent release by depolarizing the nerve terminal or interacting more directly with the release mechanism. These increases in frequency are not blocked by the aminoglycoside antibiotic, gentamicin; although the latter agent reduces quantal content and the elevations in MEPP frequency induced by high potassium solutions. The results suggest that gentamicin and NEM act at different sites at the presynaptic terminal, and that the aminoglycosides block voltage-dependent presynaptic calcium influx.

Myotonia in the rat diaphragm preparation caused by the sulfhydryl inhibiting para-substituted mercuribenzoates

The sulfhydryl (SH) inhibiting para-substituted mercuribenzoates like pOHMB caused a myotonia which appeared as a smooth myotonic profile during recording of the response to twitch stimulation (0.1/s) of isolated rat diaphragm preparations. The maximum myotonic tension varied from about 10% to 200% of the twitch tension at pOHMB addition, and the myotonic repetitive action potential activity varied from a few to more than one hundred action potentials in different cells of the same preparation. The myotonia did not appear after pretreatment with the SH-reducing agent dithiothreitol, and both dithiothreitol and N-ethyl-maleimide inhibited the motonia. The myotonia increased with temperature, appearing at about 32 degrees C. Increased twitch frequency and tetanic stimulation decreased the myotonia. No change of threshold was observed in myotonic preparations. The myotonia was depressed in K+-free solution, and it was blocked by K+ concentrations exceeding 1.5 X normal. The myotonia was reduced when the NaCl was replaced by sucrose or choline chloride. In Ca2+-free solution the time to maximal myotonic tension and the variability of the maximal myotonia were reduced. CA2+ concentrations above normal inhibited the myotonia. No myotonia was observed in the slow twitch soleus muscle. pOHMB also caused a twitch depression during indirect and direct stimulation. The depression was observed in soleus muscle and in the diaphragm below 30 degrees C. The depression was thus independent of the myotonia.

Diphenylhydantoin-induced block of the rat phrenic nerve-diaphragm preparation pretreated with p-hydroxymercuribenzoate

Pretreatment of the rat phrenic nerve-diaphragm preparation with the sulfhydryl-(SH) blocking agent p-hydroxymercuribenzoate (pOHMB) increased the blocking efficiency of the antiepileptic drug diphenylhydantoin (DPH) during indirect stimulation. Another SH-blocking agent, N-ethyl-maleimide (NEM) did not potentiate the block, but SH-group protection with dithiothreitol (DTT) abolished the effect of pOHMB. SH-binding was thus necessary but not sufficient for enhancement of DPH-block. High Ca2+-concentration potentiated the block. Well-maintained response of the isolated phrenic nerve, and of the diaphragm during direct stimulation, located the block at the neuromuscular junction. Microelectrode records in preparations which were curarized, cut or Mg2+ paralyzed to abolish action potential activity, disclosed an abrupt cessation of end-plate potentials (EPPs) by DPH, and pOHMB pretreatment reduced the time period to abrupt EPP fallout in the curarized preparation, suggesting depressed nerve terminal excitability as the cause of the block and its potentiation. Observation of miniature EPPs beyond the time of EPP cessation excluded a postsynaptic block. The pOHMB-treated preparation is suggested as a model for testing antiepileptic drugs.

Effects of nickel ion on the conduction of action potentials in non-myelinated nerve fibre of crayfish

Effects of NiCL2 and PCMB (p-chloromercuribenzoate) on the action potential were examined by the method of extra- and intracellular electrodes, using a single nerve fibre of crayfish. The results obtained were as follows : The conduction of the action potential was blocked by treating the nerve fibre with Ni ion or PCMB. The blockade was easily recovered by replacement with cysteine. The process of the blockade and recovery, which could be repeated several times, was fairly characteristic such that the more repetition led the sooner blockade and the harder recovery. No conduction block was observed by treatment with Ni-cysteine mixed solution nor with PCMB-cysteine solution. The critical concentration for blocking was 1.1 x 10(-4)M for NiCL2 and 5.6 x 10(-6) M for PCMB. The action potential was disappeared without any change in the resting potential by treatment with the chemicals, which gave significant effects on the rising and falling phases of the action potential before the blockade.

Enhancement of acetylcholine secretion by two sulfhydryl reagents

Two sulfhydryl reagents (N-ethyl maleimide and p-chloromercuribenzoate), used in a concentration of 0.1 mmol/l, increased both spontaneous and evoked acetylcholine secretion at the toad neuromuscular junction, the former in the absence of extracellular calcium ions.

The thiol-oxidizing agent diamide increases transmitter release by decreasing calcium requirements for neuromuscular transmission in the frog

Diamide, which in concentrations of 10(-5) M and higher oxidizes glutathione intracellularly, produces a dose-related increase in the frequency of miniature end-plate potentials (MEPPs). With high enough doses, quantal release is blocked, apparently through exhaustion. The early phase of MEPP frequency increase is accompanied by an increase in EPP amplitude that may reach more than 10-fold and is therefore not produced by depolarization of axon terminals. Subsequently, EPP amplitude is reduced and falls to zero, associated with failure of invasion of the nerve action into the terminals while the MEPP frequency remains elevated. Both facilitation and PTP follow the time course of change in EPP amplitude. The increase in MEPP frequency with diamide does not require external Ca2+ but raising external Ca2+ increases the MEPP rate in the presence of diamide. External Ca2+ is necessary for EPP appearance and also potentiates the diamide effects. Conversely diamide reduces the requirements for Ca2+ in releasing ACh. Diamide substitutes for external Ca2+ in K+ evoked MEPP release and in the absence of external Ca2+, diamide-evoked MEPP release is increased by raising external Mg2+ levels. The action of diamide may be dependent on the actual release of Ca2+ from intracellular stores or it may work through mimicking some of the actions of Ca2+. The action of diamide bears close resemblance to the effects of prolonged stimulation of the motor axon at 10 Hz.

The chemistry of olfactory reception: stimulus-specific protection from sulfhydryl reagent inhibition

The group-specific protein reagent, N-ethylmaleimide, irreversibly blocks the electrical response of the olfactory receptor organ of the frog to odorous stimuli. If the odorous substance, ethyl n-butyrate, in concentrations high enough to saturate the receptor system, is present in the nasal cavity before and during a brief exposure to N-ethylmaleimide, the nose, after a wash and a recovery period, responds in nearly normal fashion to vapors of ethyl n-butyrate. Responses to other odorous substances, except those closely related to ethyl n-butyrate, are abolished. We propose that we can use this protection technique to identify the properties of the various receptor sites in the nose, and possibly to characterize the receptor substances.