Quantal Content Of Endplate Potentials Research Articles

Effects of butanedione monoxime on neuromuscular transmission

1. The amplitude of endplate potentials was increased by concentrations of butanedione monoxime (BDM, 5-20 mM) that typically caused muscle paralysis. 2. Although BDM slowed the decay of spontaneous miniature endplate currents, the effect was insufficient to explain most of the large increase in amplitude of endplate potentials. 3. The quantal content of endplate potentials was increased by BDM in a reversible, concentration-dependent manner. 4. The frequency of miniature endplate potentials was not changed by 10 mM BDM in the presence of normal or raised potassium concentrations, indicating that BDM does not change quantal content by a direct effect on calcium channels or on steady-state intracellular calcium concentration. 5. A change in the time course of the extracellularly recorded nerve terminal action potential caused by BDM was similar to the change produced by 4-aminopyridine (4-AP). 6. The increase in quantal content produced by BDM was only slightly reduced in the presence of 1 mM tetraethylammonium (TEA) but was significantly reduced in the presence of 0.5 to 1 mM 4-AP. 7. It was concluded that BDM blocks a 4-AP-sensitive potassium conductance in motor nerve terminals and, by increasing the duration of the action potential in this way, increases evoked transmitter release.

Blocking effects of hypaconitine and aconitine on nerve action potentials in phrenic nerve-diaphragm muscles of mice

The mechanisms of neuromuscular blockade by hypaconitine and aconitine were investigated electrophysiologically in isolated phrenic nerve-diaphragm muscles of mice. Hypaconitine (0.08-2 microM) and aconitine (0.3-2 microM) depressed the nerve-evoked twitch tension, without affecting the contraction evoked by stimulation of the muscle. At the concentrations of hypaconitine (up to 5 microM) and aconitine (up to 2 microM) that depressed the nerve-evoked twitch tension, the resting membrane potential of the muscle cells was unchanged. Hypaconitine (0.1-2 microM) and aconitine (2 microM) blocked the end-plate potential (epp), without affecting the amplitude of the miniature epp (mepp). The quantal content of end-plate potentials was decreased by these agents in parallel with the decrement in amplitude. The nerve compound action potential was inhibited by hypaconitine (5 microM) and aconitine (2-10 microM), as well as by 1 microM tetrodotoxin (TTX). When the nerve compound action potential was completely blocked by 2 microM aconitine, the muscle action potential was unaffected, although 1 microM TTX suppressed both potentials to the same degree. These results indicate the neuromuscular blockade produced by hypaconitine and aconitine were caused by reducing the evoked quantal release. The mechanism of this effect was attributed mainly to blocking of the nerve compound action potential.

Effects of botulinum neurotoxin and Lambert-Eaton myasthenic syndrome IgG at mouse nerve terminals

The interaction between two presynaptically acting agents, Lambert-Eaton myasthenic syndrome (LEMS) immunoglobulin G (IgG) and purified botulinum neurotoxin (BoNT) type A, was studied. Intracellular microelectrode recordings were carried out on mouse muscles after injection with LEMS IgG. BoNT was either injected before recordings were made or applied in vitro. The time course of the in vitro actions of BoNT on miniature end-plate potential and end-plate potential parameters were not affected by pretreatment with LEMS IgG. After in vivo injection of BoNT, end-plate potential quantal content was reduced to less than 2% of control values, whether or not LEMS IgG had also been previously given. Quantitative electron-microscope autoradiographical analysis showed that neither the binding of 125I-BoNT to acceptors on the nerve terminal membrane nor the pattern of its internalisation were affected by pretreatment with LEMS IgG. We conclude that the effects of BoNT are not affected by LEMS IgG, suggesting different presynaptic binding sites for the two agents.

Sodium salicylate facilitates calcium‐dependent release of transmitter at mouse neuromuscular junctions

1. The effects of sodium salicylate on the frequency of miniature endplate potentials (m.e.p.ps) and on the quantal content of endplate potentials (e.p.ps) in mouse diaphragm muscles at 36 degrees C and 24 degrees C were studied by conventional microelectrode techniques. 2. At 36 degrees C, salicylate (10 mM) elevated the frequency of m.e.p.ps in a manner which was insensitive to [Mg2+]0 and independent of the presence of [Ca2+]0. Lowering the temperature to 24 degrees C abolished the stimulating effect. 2,4-Dinitrophenol (10 microM) had similar effects at 36 degrees C and these disappeared at 24 degrees C. All subsequent experiments were performed at 24 degrees C. 3. Salicylate (10 mM) further increased the frequency (F) of m.e.p.ps stimulated by [Ca2+]0 in a depolarizing solution. When [Ca2+]0 was varied in the absence of salicylate, a linear relationship between ln(F) and ln([ CA2+]0) was obtained. Salicylate shifted this relationship to the left, with respect to the control, without altering the slope. 4. Salicylate (5 mM) also increased the quantal content (m) of e.p.ps in a solution that contained 5 mM Mg2+, in a concentration-dependent fashion. As [Ca2+]0 was varied in the absence of salicylate, a linear relationship between ln(m) and ln[(Ca2+]0) was observed. Salicylate shifted this linear relationship to the left, with respect to the control, without altering the slope. Doubling the concentration of [Mg2+]0 antagonized this effect of salicylate on the quantal content of e.p.ps. 5. These results indicate that salicylate enhances the effect of changing [Ca2+]0. Salicylate probably facilitates the entry of Ca2+ into the nerve terminal or sensitizes the process that is regulated by Ca2 , thereby stimulating the release of transmitter. Surface negative charges may have an important role in the effect of [Ca22]0.

Effects of sodium selenite on neuromuscular junction of the mouse phrenic nerve-diaphragm preparation

The effects of sodium selenite on the neuromuscular junction of the phrenic nerve-diaphragm of the mouse were studied. Nerve-evoked twitches of the diaphragm of the mouse, the frequency of miniature endplate potentials, the quantal content of endplate potentials and the compound action potentials of the axon were measured. Sodium selenite induced a slight increase of the amplitude of the twitch, followed by twitch depression. The amplitude of the twitch, increased by selenite, became more prominent after the suppression of the twitch induced by cadmium ions, d-tubocurarine or magnesium ions. It appeared that the increased amplitude of twitch was due to the facilitation of transmitter release, since selenite significantly increased the frequency of miniature endplate potentials, and the amplitude and quantal content of endplate potentials; the amplitude and half decay time of miniature endplate potentials were unaffected. Twitch depression induced by selenite was enhanced by ammonium ions, high potassium and low magnesium and attenuated by high calcium. During the period of gradual depression of the twitch, selenite decreased the amplitude of compound action potentials of the phrenic nerve axon and caused the disappearance of endplate potentials. Ammonium ions enhanced the blockade of axonal conduction induced by selenite. Moreover, the depolarizing agents, ammonium and high potassium also induced an initial increase of twitch amplitude followed by depression of the twitch. These findings indicate that selenite probably alters the release of the transmitter by depolarizing the nerve membrane. The effects of selenite were antagonized by glutathione and cyanide, suggesting that the binding of selenite to sulfhydryl groups of the membrane was essential for inducing its pharmacological actions.

Muscarinic inhibition of quantal transmitter release from the magnesium-paralysed frog sartorius muscle

The existence of presynaptic muscarinic acetylcholine receptors on motor nerve terminals of the isolated frog sartorius muscle was investigated. The modulatory role of these receptors was studied by observing the effects of muscarinic ligands on the frequency of miniature endplate potentials and on the quantal content of endplate potentials. The agonist oxotremorine reduced in concentration-dependent fashion the frequency of spontaneous potentials and the amplitude of evoked potentials. Also, high concentrations of oxotremorine depolarized the postsynaptic membrane and reduced the amplitude of the miniature endplate potentials. The depolarizing action of the drug was blocked by d-tubocurarine. The muscarinic antagonist atropine attenuated agonist-induced reductions in endplate potential amplitude and miniature endplate potential frequency but did not affect the depression in amplitude of the spontaneous potentials evoked by oxotremorine. It is concluded that activation of presynaptic muscarinic receptors inhibits the release of acetylcholine from motor nerve terminals. Atropine itself had no effect on the quantal content of evoked potentials or on the frequency of spontaneous potentials suggesting that the nerve terminal is not affected by non-quantal acetylcholine.

Presynaptic action of uranyl nitrate on the phrenic nerve-diaphragm preparation of the mouse

The contraction of the diaphragm of the mouse, induced by nerve stimulation, can be potentiated by uranyl nitrate [UO 2(NO 3) 2, 0.2–0.8 mM]. At concentrations greater than 0.8 mM, uranyl nitrate also directly enhanced the contractions induced by electrical stimulation of muscle. The effects of uranyl nitrate in potentiating the twitch were augmented by low calcium (0.25–0.5 mM) and antagonized by high levels of calcium (5–10 mM). Electrophysiological studies on the effects of uranyl nitrate on the diaphragm in the mouse have revealed that the frequency of miniature end-plate potentials (MEPP) but not the amplitude was increased; the amplitude and quantal content of end-plate potential (EPPs) were also markedly increased by uranyl nitrate. The most peculiar phenomenon induced by uranyl nitrate was that repetitive end-plate potentials, as well as repetitive action potentials, of the diaphragm of the mouse were triggered by single nerve stimulation in the presence of uranyl nitrate. The duration of muscle action potentials was significantly prolonged. Recordings of the evoked compound action potentials from nerve axons showed that uranyl nitrate not only prolonged the duration of the axonal compound action potentials but also induced antidromic activity with a single stimulation. This latter effect of uranyl nitrate was blocked by d-tubocurarine. All of these findings suggest that uranyl nitrate potentiated the contractions of the diaphragm of the mouse, possibly through the activation of stimulus activated repetitive discharges which resulted in the repetitive end-plate potentials and muscle action potentials.

Multiple potassium conductances at the mammalian motor nerve terminal.

The possibility that multiple K+ conductances are present in mammalian motor nerve terminals was investigated by measuring the differential effects of tetraethylammonium (TEA) and 4-aminopyridine (4-AP) on transmitter release and on nerve terminal excitability in mouse phrenic nerve-diaphram preparations. Neither 4-AP nor TEA alter the spontaneous frequency of miniature end-plate potentials (fmepp) and therefore evidently do not affect calcium channels directly. Both 4-AP and TEA increased the quantal content of end-plate potentials (epps) evoked by nerve stimulation but the effects were not mutually exclusive; TEA continued to act in the presence of a maximally effective concentration of 4-AP. The increase in transmitter release evoked by focal polarisation of the terminal was not affected by 4-AP, whereas TEA exerted an effect consistent with a reduction in membrane conductance. Similarly, threshold for nerve terminal action potential generation was not affected by 4-AP, but TEA caused a reduction in threshold and alteration of the 'accommodation curve' indicative of a reduction of membrane conductance. Under conditions where one would predict no contribution of calcium K+ conductance, i.e., when release was evoked by Ba2+ in the absence of Ca2+, the different and non-competing effects of TEA and 4-AP were still apparent. It is concluded, therefore, that the motor nerve terminal possesses at least two K+ conductances, not including calcium activated gK, which may be distinguished pharmacologically.

The effect of myasthenic syndrome antibody on presynaptic calcium channels in the mouse.

1. The action of immunoglobulin G obtained from patients with Lambert-Eaton myasthenic syndrome (LEMS IgG) was investigated by injecting mice, followed by intracellular recordings from the mouse diaphragm. 2. End-plate potential quantal content was studied over a range of Ca2+ concentrations. Curves of log quantal content versus log Ca2+ concentration were shifted to the right by LEMS IgG. For low Ca2+ concentrations, release continued to follow Poisson statistics after LEMS IgG treatment. 3. Miniature end-plate potential (m.e.p.p.) frequency was measured in solutions containing high K+ concentrations. LEMS IgG significantly reduced m.e.p.p. frequency at each K+ concentration studied. 4. M.e.p.p. frequency was measured at fixed high-K+ concentration (15.9 mM) for a range of Ca2+ concentrations. The log-log plot of m.e.p.p. frequency versus Ca2+ concentration was shifted downwards throughout by LEMS IgG. 5. M.e.p.p. frequency was not affected by LEMS IgG in Ca2+-free solutions (K+ concentration 15.9 mM) or in solutions of low Ca2+ concentration (K+ concentration 5.9 mM). 6. At each Ca2+ concentration studied, m.e.p.p. amplitudes were not affected by LEMS IgG. 7. The data suggest that LEMS IgG acts on presynaptic voltage-dependent Ca2+ channels to cause their loss of function, probably by down-regulation.

Does Endogenous Adenosine Modulate the Release of Acetylcholine from Motor Nerve during Single and Repetitive Stimulations in the Mouse Diaphragm?

In order to elucidate the physiological role of endogenous adenosine in regulating the release of acetylcholine, the effects of 8-phenyltheophylline, an antagonist of adenosine receptors and dipyridamole, an uptake inhibitor of adenosine, on the contractile response and quantal release of acetylcholine during single and repetitive stimulations of isolated mouse phrenic nerve-diaphragm preparations were studied. The curves relating the concentration vs. inhibition of contractile response to added adenosine and ATP were shifted parallel to the left by dipyridamole, but were shifted to the right by 8-phenyltheophylline at concentrations with little Ca2+-mobilization or phosphodiesterase inhibition. In the absence of ex-ogenously added adenosine, 8-phenyltheophylline increased the quantal content of end-plate potentials (1 Hz), whereas dipyridamole decreased the quantal content. Successive decrease of the amplitude of end-plate potentials (e.p.p. run-down) evoked at 50 Hz was not changed either by 8-phenyltheophylline or by dipyridamole, suggesting that adenosine or ATP released from the motor nerve does not accumulate to an effective concentration even after repetitive stimulation for a feed-back regulation of the transmitter release. It is concluded that endogenous adenosine does inhibit the release of acetylcholine from motor nerve. However, the source of adenosine may be mostly from the muscle and is probably not involved in the feedback autoregulation.

Does endogenous adenosine modulate the release of acetylcholine from motor nerve during single and repetitive stimulations in the mouse diaphragm?

In order to elucidate the physiological role of endogenous adenosine in regulating the release of acetylcholine, the effects of 8-phenyltheophylline, an antagonist of adenosine receptors and dipyridamole, an uptake inhibitor of adenosine, on the contractile response and quantal release of acetylcholine during single and repetitive stimulations of isolated mouse phrenic nerve-diaphragm preparations were studied. The curves relating the concentration vs. inhibition of contractile response to added adenosine and ATP were shifted parallel to the left by dipyridamole, but were shifted to the right by 8-phenyltheophylline at concentrations with little Ca2+-mobilization or phosphodiesterase inhibition. In the absence of exogenously added adenosine, 8-phenyltheophylline increased the quantal content of end-plate potentials (1 Hz), whereas dipyridamole decreased the quantal content. Successive decrease of the amplitude of end-plate potentials (e.p.p. run-down) evoked at 50 Hz was not changed either by 8-phenyltheophylline or by dipyridamole, suggesting that adenosine or ATP released from the motor nerve does not accumulate to an effective concentration even after repetitive stimulation for a feed-back regulation of the transmitter release. It is concluded that endogenous adenosine does inhibit the release of acetylcholine from motor nerve. However, the source of adenosine may be mostly from the muscle and is probably not involved in the feedback autoregulation.

Presynaptic modification of neuromuscular transmission by antiacetylcholine receptor antibody: myasthenic serum and monoclonal antibody produced by transformed lymphocytes.

When myasthenic serum or a monoclonal antiacetylcholine receptor (anti-AChR) antibody produced by human transformed lymphocytes and transferable to an animal was applied to rat diaphragms in vitro, presynaptic facilitation was demonstrated by changes in ACh quantal content of endplate potentials. The results correlated with ability of the antibody to block binding of alpha-bungarotoxin to AChR, but not with titers of anti-AChR antibody by immunoprecipitation assay and AChR degradation rate. Antibody to the receptor site near the ACh-binding site may act presynaptically to compensate for the postsynaptic failure in myasthenia gravis.

Mechanisms of the inhibition by neostigmine of tetanic contraction in the mouse diaphragm.

Neostigmine (0.5-2 microM) caused fade of tetanic contractions (Wedensky inhibition) evoked by repetitive nerve stimulation. The mechanism underlying this action was studied in intact and cut isolated phrenic nerve-diaphragm preparations of mice. The fade was brought about by failure to elicit muscle action potentials. During fade, the muscle was unable to conduct directly evoked action potentials across the central endplate zone. Recovery of excitability occurred in 5 s with continued stimulation. In the presence of neostigmine, the resting membrane potential at endplate areas during repetitive stimulation decreased from -80 mV to less than -50 mV within the first 10 pulses at 75-200 Hz and thereafter recovered gradually to about -60 mV in the following 5 s during continuous stimulation. The quantal content of endplate potentials evoked by single stimulation was not reduced by neostigmine whereas that evoked by high frequency stimuli (75 Hz) was reduced to about 1/3 in 10 pulses. It is concluded that the fade of tetanic contraction caused by inhibition of acetylcholinesterase is induced by the inactivation of sodium channels in the area surrounding the endplates and that the sustained fade is due to a decrease of transmitter release. Both effects are the result of acetylcholine accumulation.

Lanthanum as a surrogate for calcium in transmitter release at mouse motor nerve terminals.

The mechanism by which lanthanum (La3+) causes an increased frequency of miniature end-plate potentials (m.e.p.p.s) was studied at the mouse neuromuscular junction. At concentrations as low as 0.25 microM, La3+ caused a progressive rise in m.e.p.p. frequency, to a maximum of several hundred per second. 'Washing' with solution containing EDTA arrested the rise, but did not substantially reduce the raised m.e.p.p. frequency. At partially 'lanthanized' junctions high frequencies of m.e.p.p.s were maintained indefinitely, even in 0 Ca2+/EDTA solutions. The rate of development of high m.e.p.p. frequency was increased by repetitive nerve stimulation or by depolarization of the nerve terminal (high K+ or focally applied current), and appeared to be proportional to the concentration of La3+ over the range of 0.25-5 microM. At low concentrations of La3+ the rise of m.e.p.p. frequency depended upon the co-presence of a small amount of Ca2+ (greater than 10 microM) and was slowed and partially blocked by Cd2+, or by Ca2+ at about 10 microM. The quantal content of end-plate potentials was usually reduced in the presence of La3+, but was increased over control values after removal of La3+ by 'washing' with solution containing EDTA, once a raised m.e.p.p. frequency had developed. At partially lanthanized junctions the absolute increases in m.e.p.p. frequency produced by Ca2+ (in raised K+), ethanol, or by nerve stimulation in the presence of Ba2+, were greater than at control junctions, but in each case the increases in the logarithm of m.e.p.p. frequency were less than at control junctions. It is concluded that La3+ causes transmitter release only after entry into the nerve terminal via voltage-sensitive channels, probably those that normally admit Ca2+, that La3+ and Ca2+ may co-operate at internal sites to induce transmitter release, and that these ions both co-operate and compete at external sites that regulate their entry into the nerve terminal.

Action of Lambert-Eaton myasthenic syndrome IgG at mouse motor nerve terminals.

We have studied the electrophysiological effects of IgG obtained from four patients with Lambert-Eaton myasthenic syndrome (LEMS) (two with small cell carcinoma), using the mouse passive transfer model. Mice received LEMS or control IgG or plasma, 10 to 60 mg daily. Microelectrode intracellular recordings were made from diaphragm muscle. LEMS IgG and plasma decreased end-plate potential quantal content similarly, confirming IgG as the active factor. LEMS IgG was equally effective in C5-deficient mice, indicating that late complement components are not required. The time course of decline and recovery of quantal content closely followed that of the human IgG in the mouse serum, with time to half-maximal effect of about 1.5 days in each case. Binding/dissociation of IgG or down/up regulation of the antigenic determinants, possibly Ca2+ channels, has a half-life of between 2 and 36 hours. The results confirm our concept that IgG antibody to nerve terminal determinants underlies the disorder of transmitter release in LEMS.

Muscle paralyzing effect of the juice from the trunk of the banana tree

Y. N. Sinsgh and W. F. Dryden. Muscle paralyzing effect of the juice from the trunk of the banana tree. Toxicon 23, 973–981, 1985. — The effect of an extract from the trunk of the banana tree ( Musa sapientum) was investigated in isolated skeletal muscle preparations from the chick, mouse and frog using twitch tension and intracellular recording techniques. The extract produced, in the same concentration range and after an initial period of twitch augmentation, paralysis of skeletal muscle in both directly and indirectly stimulated preparations. It also had a dosedependent stimulant effect on the muscle causing a contracture. The neuromuscular blockade was reversed by calcium, but only when added before complete paralysis of the muscle. On the other hand, neostigmine usually hastened the blockade and aggravated the contracture. The frequency of the miniature endplate potential in the mouse phrenic nerve — diaphragm preparation greatly increased initially, declining to an elevated plateau. Effects on quantal content of endplate potentials e.p.p.s were studied in the transected mouse phrenic nerve — hemidiaphragm using trains of e.p.p.s. In the presence of the extract, only a few e.p.p. trains could normally be evoked, probably due to nerve terminal block. When quantal content could be measured at low concentrations of the extract, an increase was usually obtained. Muscle action potentials in the frog sartorius muscle were decreased in amplitude until no further potentials could be generated. The results suggest that the nature of the block produced by the extract resembles that of a potent local anaesthetic with an initial atypical labilizing effect on cell calcium rather than a conventional curariform block.

Dopamine decreases quantal output in mouse muscles

Dopamine was found to reduce the quantal content of end-plate potentials in cutfiber preparations of mouse diaphragm but not in Mg 2+-blocked preparations. Possible mechanisms for this effect include reduction of the presynaptic store of acetylcholine available for release, or blockade by dopamine of a positive feedback on acetylcholine release, possibly mediated by muscarinic receptors on nerve terminals.

Studies on the mode of action of botulinum toxin type A at the frog neuromuscular junction

In frogs poisoned with botulinum toxin type A the quantal content of endplate potentials is greatly reduced. Lowering the temperature of the preparation increases quantum content; between 14 and 4 °C the mean Q 10 for this effect is 6.3. Facilitation of synaptic transmission is marked with pairs of stimuli and cooling further enhances facilitation. The time constant of decay of facilitation is 34 ms at 20 °C and 116 ms at 4 °C. The increase in facilitation and in its time constant of decay at low temperature are presumably not a result of a prolongation of the duration of the nerve terminal action potential since such changes are not seen in the presence of K +-channel blockade by 3,4-diaminopyridine. Electrotonic depolarization of nerve terminals in the presence of tetrodotoxin and 3,4-diaminopyridine induces all-or-none endplate currents. Such endplate currents, at a holding potential of −50 mV, show that the amount of charge entry is about1/3 of that in unpoisoned junctions but still corresponds to5−10 ×10 3 transmitter quanta. Transmitter release at this level is maintained during repetitive stimulation even in the presence of 82 mM Ca 2+ in the extracellular solution. We speculate that the blockade of transmitter release in BoTx-poisoned muscles results from a stimulatory effect of the toxin on metabolic systems of Ca 2+ disposal in the nerve terminal.

Mode of neuromuscular blocking action of ceruleotoxin

Ceruleotoxin is an acidic toxin protein from a Bungarus venom which has recently been clarified as B. fasciatus venom. The toxin at a low concentration (10 −6g/ml) abolished the twitch response of the indirectly stimulated biventer cervicis muscle of the chick, without affecting the response to acetylcholine or carbamylcholine. At a higher concentration (10 -5g/ml), the toxin, in addition to inhibition of the twitch response, caused contracture in the chick biventer cervicis muscle and reduced the response to acetylcholine or carbamylcholine in both the innervated chick cervical and denervated rat diaphragm muscles. Electrophysiological studies on the rat phrenic nerve - diaphragm preparation showed that ceruleotoxin at a low concentration caused an initial inhibition followed by recovery of the quantal content of endplate potentials and then a second phase of inhibition leading to complete failure. At a higher concentration, the toxin gradually reduced the resting membrane potential of the diaphragm muscle to a level lower than 50 mV within 2 hr. In addition, enzyme assay showed that the toxin possessed phospholipase A activity comparable to that of other basic phospholipases A 2 of snake venom origin. It is concluded that ceruleotoxin is a phospholipase A 2 with presynaptic and myotropic actions on vertebrate nerve - muscle system similar to those of notexin from Notechis scutatus scutatus venom.

PRE- AND POSTJUNCTIONAL BLOCKING EFFECTS OF AMINOGLYCOSIDE, POLYMYXIN, TETRACYCLINE AND LINCOSAMIDE ANTIBIOTICS

The effects of seven antibiotics (streptomycin, amikacin, polymyxin B, lincomycin, clindamycin, tetracycline and oxytetracycline) were compared with those of magnesium, tubocurarine and lignocaine in the frog sciatic nerve--sartorius muscle preparation, using intracellular recording techniques. All compounds except tubocurarine decreased end-plate potential quantal content. The prejunctional effects of magnesium, streptomycin, amikacin, polymyxin B and oxytetracycline (but not the other drugs) were well reversed by increasing the calcium concentration. At concentrations which depressed quantal content, only magnesium, tetracycline and oxytetracycline did not reduce postjunctional sensitivity. Further postjunctional effects of the drugs were revealed by alterations in the time-courses of end-plate potentials. All the drugs tested except magnesium, tubocurarine and lincomycin produced changes in muscle action potentials. None of the compounds had anticholinesterase activity. The results confirm that aminoglycoside, polymyxin, tetracycline and lincosamide antibiotics produce neuromuscular block by a combination of both pre- and postjunctional actions.