Enhancement of acetylcholine secretion by two sulfhydryl reagents

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.

N-ethylmaleimide increases release probability at GABAergic synapses in layer I of the mouse visual cortex

The sulphydryl alkylating agent N-ethylmaleimide (NEM) has been often used as an uncoupler of pertussis toxin-sensitive G-proteins. However, the effects of NEM on gamma-aminobutyric acid (GABA)ergic synaptic transmission remain controversial. Using the whole-cell patch-clamp technique, GABA(A) receptor-mediated postsynaptic currents (IPSCs) have been recorded from Cajal-Retzius (CR) cells in layer I of the neonatal mouse visual cortex. NEM increased the frequencies of both spontaneous and miniature IPSCs (mIPSCs) without an effect on the median mIPSC amplitudes or mIPSC kinetics. The NEM actions on mIPSCs did not depend on the extracellular Ca(2+), Ca(2+) release from intracellular stores, adenylyl cyclase and protein kinase A activities. NEM increased the mean amplitudes of evoked IPSCs and strongly decreased the paired-pulse ratio. The size of the readily releasable pool of presynaptic vesicles (RRP) was estimated using a high-frequency stimulation protocol. The RRP size was not affected by NEM. In addition, NEM significantly decreased the latency between the stimulus and the onset of GABA release. These results suggest that NEM selectively increases GABA release probability. At postnatal day 2, mIPSCs were observed only in about 30% of CR cells. NEM application revealed, however, that more than 90% of CR cells receive GABAergic inputs. Therefore, NEM seems to be a useful tool to verify the existence of 'silent' GABAergic synapses.

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.

N-ethylmaleimide blocks depolarization-induced suppression of inhibition and enhances GABA release in the rat hippocampal slice in vitro

Regulation of synaptic, GABAA receptor-mediated inhibition is a process of critical importance to normal brain function. Recently, we have described a phenomenon in hippocampus of a transient, yet marked, decrease in spontaneous, GABAA receptor-mediated IPSCs after depolarization activated Ca2+ influx into a pyramidal cell. This process, depolarization-induced suppression of inhibition (DSI), is absent in hippocampal cells that previously had been exposed to pertussis toxin in vivo, implicating a G-protein in the DSI process. To circumvent the problem that a single cell cannot be studied before and after G-protein block using the pertussis toxin pretreatment method, we have used the sulfhydryl alkylating agent N-ethylmaleimide (NEM), which blocks pertussis toxin-sensitive G-proteins, to determine whether acute inhibition of G-proteins can eliminate DSI of spontaneous IPSCs (sIPSCs). In whole-cell recordings from CA1 pyramidal cells that were first determined to express DSI, we have found that NEM does block DSI of sIPSCs. We also report that DSI of monosynaptic, evoked IPSCs is blocked by NEM, suggesting that a similar mechanism underlies both forms of DSI. It was of interest that DSI was abolished at a time when NEM had increased, not decreased, GABA transmission. Indeed, NEM greatly increased quantal GABA release by a Ca(2+)-independent mechanism, an observation with potentially important implications for understanding synaptic GABA release.

Modification of ionic currents underlying action potentials in mouse nerve terminals by the thiol-oxidizing agent diamide

The effect of diamide, a thiol-oxidizing agent, was tested using electrophysiological techniques to determine whether its ability to alter neuromuscular transmission in vitro could be attributed to alterations of ion channels controlling neuronal excitability and/or acetylcholine release. In mouse triangularis sterni preparations, diamide transiently increased the evoked release of acetylcholine and then blocked release. Extracellular recording of perineural waveforms associated with neuronal action potentials at motor nerve terminals showed that diamide reduced the waveforms associated with the delayed rectifier K+ current, a Ca2+ current and a Ca(2+)-activated K+ current (IK,Ca). Inhibition of quantal transmitter release was not associated with failure of action potentials to invade nerve terminals. Thus, diamide modifies the ionic currents underlying the nerve terminal action potential, some of these changes probably account for the complex effects of diamide on quantal transmission.

Acute alterations in murine neuromuscular transmission following exposure to a nonparalytic dose of dithiobiuret

Treatment of rats for 5 to 6 days with dithiobiuret (DTB, 1 mg/kg/day, ip) causes a flaccid, ascending neuromuscular weakness which is associated with a decreased end-plate potential (EPP) amplitude, quantal content, miniature end-plate potential (MEPP) frequency, and prolongation of MEPP and EPP rise and decay times. Whereas small daily doses of DTB reliably cause this paresis, a single large dose, approximating the LD50, and far in excess of the cumulative dose given chronically to induce paralysis, causes no apparent muscle weakness. It was of interest to determine whether subtle changes in neuromuscular transmission are produced by DTB under dosing conditions in which gross muscle weakness is not apparent. As such the present study had two goals: first, to determine whether a single large dose of DTB (25 mg/kg, ip) altered neuromuscular transmission at times when the animal did not exhibit paresis; and second, to determine whether bath application of DTB, at concentrations approximating those in the animal following a single large dose, altered junctional transmission at early exposure times. EPPs and MEPPs were recorded, from hemidiaphragms taken 1, 4, 8, or 24 hr following treatment of rats with a single dose of DTB or vehicle or from untreated rats which were exposed to 200 microM or 1.85 mM DTB by bath application. One hour after a single large dose of DTB, EPP amplitude and MEPP frequency and amplitude were all decreased. Rise and decay times for MEPPs were prolonged in muscles taken 4 hr after treatment. By 4, 8, and 24 hr after treatment, EPP amplitude, MEPP amplitude, and MEPP frequency returned toward control levels. Bath application of DTB initially increased EPP amplitude, MEPP amplitude, and MEPP frequency; however, with continued exposure EPP amplitude decreased to below control levels. Block of EPPs occurred after approximately 10 or 37 min of exposure to 1.85 mM or 200 microM DTB, respectively. MEPP frequency also decreased with continued exposure to DTB, yet remained above control levels for the duration of DTB exposure. Bath application of DTB caused a slowing of decay times of MEPPs similar to that observed following in vivo exposure. These results demonstrate that a single large dose of DTB initially induces neuromuscular effects similar to those observed in rats paralyzed following chronic treatment with DTB but these effects, with the exception of effects on rise and decay times of synaptic potentials, tend to reverse by 24 hr following exposure.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

p-chloromercuribenzoate causes Ca2+-dependent exocytotic catecholamine secretion from cultured bovine adrenal medullary cells

Incubation of cultured bovine adrenal medullary cells with p-chloromercuribenzoate (50-500 microM), a sulfhydryl-reacting agent, caused an increase in the secretion of catecholamines, p-Chloromercuriphenyl sulfonate, a p-chloromercuribenzoate analogue that poorly penetrates the cell membrane, caused a similar increase in catecholamine secretion. In both cases, catecholamine secretion was dependent on extracellular Ca2+. Furthermore, p-chloromercuribenzoate caused both 45Ca2+ influx into the cells and an increase in the intracellular free Ca2+ concentration. The increases in catecholamine secretion and 45Ca2+ influx behaved similarly in relation to p-chloromercuribenzoate concentration. The time courses of the increased secretion, 45Ca2+ influx, and intracellular free Ca2+ concentration by p-chloromercuribenzoate were also quite similar. The stimulation of catecholamine secretion by p-chloromercuribenzoate was reversed by washing the cells with dithiothreitol-containing medium, but not by dithiothreitol-free medium. When the cells were treated with p-chloromercuribenzoate, dopamine-beta-hydroxylase, an enzyme present in the chromaffin granules along with catecholamines, was also released. However, p-chloromercuribenzoate did not cause release of phenylethanolamine-N-methyltransferase, an enzyme present in the cytoplasm. These results indicate that catecholamine secretion due to p-chloromercuribenzoate occurs by Ca2+-dependent exocytosis.

Effects of mercuric chloride on [3H]dopamine release from rat brain striatal synaptosomes

Electrophysiological studies employing amphibian neuromuscular preparations have shown that mercuric chloride (HgCl2) in vitro increases both spontaneous and evoked neurotransmitter release. The present study examines the effect of HgCl2 on the release of [3H]dopamine from synaptosomes prepared from mammalian brain tissue. Mercuric chloride (3-10 microM) produces a concentration-dependent increase in spontaneous [3H]dopamine release from "purified" rat striatal synaptosomes, in both the presence and absence of extra-synaptosomal calcium. The effects of HgCl2 on transmitter release from amphibian neuromuscular junction preparations resemble those produced by the Na+, K+-ATPase inhibitor ouabain. Experiments were performed to determine whether the HgCl2 effects on mammalian synaptosomal dopamine release are a consequence of Na+, K+-ATPase inhibition. Na+, K+-ATPase activity in lysed synaptosomal membranes is inhibited by HgCl2 (IC50 = 160 nM). However, mercuric chloride in the presence of 1 mM ouabain still increased [3H]dopamine release. The specific inhibitor of Na+-dependent, high-affinity dopamine transport, RMI81,182 inhibited ouabain-induced [3H]dopamine release whereas it had no effect on HgCl2-induced [3H]dopamine release. These data suggest that augmentation of spontaneous [3H]dopamine release by HgCl2 probably is not mediated by an inhibition of Na+, K+-ATPase and HgCl2 does not act directly on the dopamine transporter.

Calcium-independent increase of transmitter release at frog end-plate by trinitrobenzene sulphonic acid

1. Application of an amino-residue-modifying reagent, 2,4,6-trinitrobenzene-1-sulphonic acid (TNBS), to the frog neuromuscular junction in high-magnesium Ringer solution rapidly increased both the amplitude of nerve-evoked end-plate potentials (EPPs) and the frequency of miniature end-plate potentials (MEPPs). These showed a similar initial time course and reached a maximum 3-7 min and about 10 min, respectively, after the start of application of 2 mM-TNBS. Then, the EPP amplitude decreased, while the MEPP frequency maintained its plateau value. The increase in transmitter release and the decrease in EPP amplitude by TNBS may have been due to different modes of action. 2. The distribution of MEPP amplitude was unchanged by TNBS treatment. 3. The carbachol-induced postsynaptic potential and the extracellularly recorded presynaptic action current were not affected by TNBS treatment for up to 30 min, indicating that the change in EPP amplitude produced by TNBS was not due to either a postsynaptic effect or a change in action potential at the presynaptic terminal. 4. The frequency of MEPPs was increased by TNBS application even when Ca2+ was omitted from the external Ringer solution or when a specific calcium channel blocker, synthetic omega-conotoxin, was added. This indicates that Ca2+ inflow to the nerve terminal is not necessary for TNBS action. 5. When a calcium chelator, BAPTA, was loaded into the presynaptic nerve terminal, the facilitation of EPPs by trains of nerve stimuli was scarcely observed. This suggested that the cytosolic free Ca2+ in the presynaptic terminal was buffered by BAPTA. Under this condition, the amplitudes of EPPs were increased by TNBS application to the same extent as in the control without BAPTA, but were accompanied by little facilitation. The MEPP frequency was also increased by TNBS to the same extent as in the control. These results suggest strongly that augmentation of transmitter release by TNBS was not due to an increase in cytosolic Ca2+ concentration. 6. These observations suggest that TNBS might react with specific protein(s) on the outer surface of the presynaptic membrane and accelerate the exocytosis of synaptic vesicles.

Evidence for the identification of synaptic transmitters released by photoreceptors of the toad retina

1. When toad retinae were incubated with veratrine, kainic acid, and L-alpha-aminoadipic acid, photoreceptor cells survived and most other neurones died. This preparation of 'isolated' photoreceptor cells accumulated radioactive molecules from the incubation medium and metabolized these into labelled compounds. When a preparation was placed on a filter and superfused, radioactive molecules which were released into the superfusion fluid could be collected and later analysed. Several procedures were used for inducing the release of possible transmitter compounds. Each released compound was chemically identified. 2. Three compounds, aspartic acid, glutamic acid, and N-acetyl histidine, were released when the potassium concentration was increased in media that lacked calcium and contained cobalt. 3. The release of these compounds was further increased when cobalt was removed and calcium returned to the extracellular medium. 4. Two additional compounds, putrescine and cadavarine, were also released during depolarization when calcium was present. 5. The efflux of each of the compounds listed in Section 2 was also increased by homo- and hetero-exchange. For at least aspartate, exchange was sodium-dependent. 6. The post-synaptic effect of released compounds was tested by their ability to increase the efflux of [3H]GABA from 'isolated' horizontal cells. 0 . 1 mM-L-aspartate, or L-glutamate produced an increase in GABA efflux. N-acetyl histidine, putrescine, and cadavarine were ineffective. 7. Isolated photoreceptors and intact retinae were incubated with [3H]aspartate, or [3H]putrescine. Subsequent histology and autoradiography demonstrated that both compounds were selectively accumulated by cones.

Diamide, temperature and spontaneous transmitter release at the neuromuscular junction: stimulation of exocytosis by a direct effect on membrane fusion?

The thiol-oxidizing agent diamide markedly increases m.e.p.p. frequency at the frog neuromuscular junction, even at low [Ca2+]0 and also when the mitochondria are uncoupled with DNP. The effect is reversed by dithioerythritol and is very temperature-sensitive, with a marked transition at 16 degrees C; m.e.p.p. frequency is raised 2- to 5-fold at 13-15 degrees C and 55- to 60-fold at 17-20 degrees C. Diamide increases the frequency of large amplitude m.e.p.p.s, the effect being explicable as the fusion of two or more vesicles. It is concluded that (a) diamide does not act at the Ca2+ channels of the plasma membrane, nor at the mitochondria. It affects the release system directly via an alteration of membrane protein --SH groups; (b) the eventual decline in m.e.p.p. frequency after DNP treatment is because of the exhaustion of mitochondrial Ca2+ rather than a depletion of quanta; (c) the major effect of temperature is on the release mechanism, perhaps via a phase-change in the phospholipoproteins of the plasmalemma or vesicles, rather than an elevation of [Ca2+]i; (d) either diamide or temperatures above 16 degrees C make Ca2+ more effective in promoting vesicle-plasmalemma fusion.

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.