Differential effects of diltiazem on glutamate potentials and excitatory junctional potentials at the crayfish neuromuscular junction

Anomalous voltage dependence of channel blockade at a crustacean glutamate-mediated synapse

1. The voltage dependence and concentration dependence of blockade of glutamate-activated currents by the diquaternary amine, chlorisondamine, were examined in a marine crustacean muscle. 2. Chlorisondamine results in the splitting of focally recorded synaptic current decays into two exponential components. The fast component becomes faster with increases in drug concentration and with hyperpolarization. The slow decay rate is unchanged or faster with hyperpolarization and the relative amplitude of the slow component is increased with hyperpolarization. 3. The alteration of synaptic current decay rates by chlorisondamine over the range of 5 to 100 microM and -80 to -140 mV is quantitatively consistent with a simple channel blockade model with a zero-voltage blocking rate of 6 x 10(5) M-1 s-1 at 12 degrees C with a voltage dependence of about 40 mV per e-fold change. The unblocking rate is about 5 s-1 at 0 mV and increases with hyperpolarization with a voltage dependence of about 30 mV per e-fold change. 4. The dose dependence and voltage dependence of blockade of ionophoretically activated glutamate currents by chlorisondamine are qualitatively consistent with the kinetic estimates. 5. The anomalous voltage dependence of the unblocking process is considered in terms of the possibility that the relief from blockade by chlorisondamine occurs by transit of chlorisondamine through the ion channel opened by glutamate.

Stizolobic acid, a competitive antagonist of the quisqualate-type receptor at the crayfish neuromuscular junction

Stizolobic acid and stizolobinic acid are amino acids isolated from a plant, Stizolobium hassjoo. Both amino acids reduced responses to glutamate and quisqualate in a competitive manner at the crayfish neuromuscular junction, without affecting responses to GABA and acromelic acid. Excitatory junctional potentials were decreased in the presence of stizolobic or stizolobinic acid in a concentration dependent manner. Stizolobinic acid was about 5 times less potent than stizolobic acid.

Three tests of the hypothesis that glutamate is the sensory hair cell transmitter in the frog semicircular canal

Three series of experiments were devised to test the hypothesis that glutamate is the transmitter released by sensory hair cells of the frog semicircular canal. These three tests were: 1 - The Tolerance experiment (i.e. making the preparation tolerant to injected Glu yet still capable of responding to endogenous transmitter). 2 - The Glu Decarboxylase experiment (i.e. bathing the preparation in sufficient enzyme to prevent the effects of exogenous Glu by degrading it without affecting the response to endogenous transmitter) and; 3 - The Diltiazem experiment (i.e. using the calcium channel antagonist, diltiazem, to prevent the effect of exogenous Glu and yet not to interfere with endogenous transmitter release and action). The Tolerance and Diltiazem experiments produced results indicative of a clear dissociation between exogenous Glu and natural transmitter. The Glu decarboxylase experiment results were not so clear, producing both evidence for and against the hypothesis.

Acromelic acid, a novel excitatory amino acid from a poisonous mushroom: effects on the crayfish neuromuscular junction

A novel amino acid, acromelic acid, which is one of kainoids isolated from a poisonous mushroom, markedly depolarizes the crayfish opener muscle fiber in a dose-dependent manner, its potency being much greater than that of kainoids such as kainic or domoic acid. Moreover, acrolemic acid markedly potentiates the glutamate response, in spite of the fact that it reduces the quisqualate response in a dose-dependent manner. The amplitude of excitatory junctional potentials was slightly reduced by acromelic acid.

A new potent channel blocker: effects on glutamate responses at the crayfish neuromuscular junction

The glutamate blocking action of 5-methyl-1-phenyl-2-(3-piperidinopropylamino)-hexane-1-ol (MLV-5860) was studied at the crayfish neuromuscular junction using electrophysiological techniques. The opener muscle of the dactyl in the first leg of the crayfish was used to examine the action of the drug on the glutamate response. MLV-5860 reduced the amplitude of repetitively-induced glutamate potentials in a dose-dependent manner at the crayfish neuromuscular junction and this reduction was time- and activity-dependent. The minimum effective concentration of MLV-5860 to reduce the glutamate response was estimated to be lower than 50 nM, and therefore MLV-5860 is the most powerful glutamate blocker known at the crayfish neuromuscular junction. Pretreatment of the muscle fiber with concanavalin A did not affect the action of MLV-5860. MLV-5860 reduced the amplitude of excitatory junctional potentials (EJPs) and increased the decay rate of extracellularly-recorded EJPs in a dose-dependent manner. Quisqualate responses were also reduced by this drug but the conductance increase of the muscle membrane induced by GABA was not affected. MLV-5860 did not cause a significant change in the input resistance of the opener muscle fiber at concentrations less than 10 microM. The action of the drug is possibly explained in part by the open channel block of the glutamate-activated ion channels. The forward rate constant for channel blockade was estimated from the difference between the decay rate constants of extracellular EJPs in the absence and presence of the drug and the estimated value was 6.5 +/- 1.4 X 10(7) M-1 s-1.

Phencyclidine (PCP) blocks glutamate-activated postsynaptic currents

Phencyclidine (PCP) was tested on the metathoracic tibialis muscles of Locusta migratoria. In physiological solution, the peak amplitude of the excitatory postsynaptic currents (EPSCs) evoked by nerve stimulation was linearly related to membrane potential between -50 and -150 mV. The decay time constant of the EPSC (tau EPSC) was exponentially dependent on voltage and decreased with hyperpolarization. The membrane potential change required to produce an e-fold change in tau EPSC was 315 mV. PCP (5-40 microM) produced a concentration-dependent depression of both EPSC peak amplitude and tau EPSC. A slight nonlinearity in the current-voltage relationship could be discerned at high concentrations of PCP. The shortening of the decay time constant of EPSC (tau EPSC) occurred without significant change in the voltage sensitivity observed under control conditions. Under all experimental conditions, the decay of the EPSCs remained a single exponential of time. Fluctuation analysis indicated that 5 microM PCP shortens the lifetime of the glutamate-activated channels by 25.7 +/- 3%. PCP (10-80 microM) did not induced desensitization of the glutamate receptors. These results suggest that PCP interacts with the open conformation of ion channels activated by the glutamate receptor.

TI-233 as a glutamate channel blocker at the crayfish neuromuscular junction

Effects of TI-233 (4-isopropyl-1-[N2-(5,6-dimethyl-aminonaphthalene-1-sulphonyl)-L-arginyl ]- piperidine) on glutamate-induced responses and nerve-evoked synaptic responses were compared at the crayfish neuromuscular junction. Intracellularly recorded excitatory junctional potentials (e.j.ps) were markedly augmented by TI-233 when they were evoked at long intervals, whereas the unit size of extracellular e.j.ps was hardly affected by TI-233 and, at that stage, the glutamate-induced current was markedly reduced by TI-233. The decay rate of extracellular e.j.ps was slightly increased 3 min after the addition of TI-233 at concentrations higher than 0.05 mM. Repetitive stimulation of the excitatory axon at a high frequency caused a gradual decrease in the amplitudes of extracellular e.j.ps in the presence of TI-233. After prolonged application of TI-233 with repetitive nerve stimulation, the glutamate-induced response became significantly smaller than the control. TI-233 increased the input resistance of the crayfish muscle fibre and facilitated transmitter release at the excitatory neuromuscular junction. These two effects would entirely explain the augmentation of intracellular e.j.ps by TI-233. TI-233 (greater than 3 microM) reduced the amplitude of current responses to trains of glutamate pulses in a dose-dependent manner, but this reduction by TI-233 was time- and activity-dependent. The effect of TI-233 on glutamate-induced responses was voltage-dependent and hyperpolarization increased this effect. Pretreatment of the muscle fibre with concanavalin A did not affect the gradual decline, caused by TI-233, of the successive currents evoked by a train of glutamate pulses. The apparent differences between the glutamate-induced current and nerve-evoked synaptic response revealed by TI-233 can be explained by open-channel block of the glutamate-activated ion-channel, and do not confute the hypothesis that glutamate is the natural transmitter substance at this junction.

Inhibitory actions of tuberostemonine on the excitatory transmission at the crayfish neuromuscular junction

At the crayfish neuromuscular junction, tuberostemonine, an alkaloid from Stemona japonica, reduced the amplitude of both the excitatory junctional potential (e.j.p.) and the glutamate response in a dose-dependent manner at concentrations above 0.1 mM. Tuberostemonine acted presynaptically on the crayfish neuromuscular junction to reduce a quantal content of extracellularly recorded e.j.p.s, and postsynaptically to reduce their unit size. The decay of the excitatory synaptic current was accelerated by tuberostemonine. The gradual decline of the successive glutamate currents induced by a train was facilitated by the presence of tuberostemonine even in the muscle fibre pre-treated with concanavalin A. The rate of recovery from the refractory form of the glutamate receptor to the free reactive one was slightly affected by tuberostemonine when it was determined by using a paired pulse method. The inhibitory action of tuberostemonine on glutamate responses was voltage-dependent and hyperpolarization increased the drug action. These results indicate that tuberostemonine acts in part as an open-channel blocker at the crayfish neuromuscular junction.

Biochemical identification of a putative glutamate receptor in housefly thoracic membranes

Specific stereoselective binding of [3H]L-glutamate was detected to membranes prepared from housefly thorax to which were added several antiproteases. A single high affinity binding site was detected (KD 0.5 +/- 0.04 microM), but total binding varied from preparation to preparation (5-60 pmoles/mg protein). Specific binding was inhibited by preincubation of the membranes with trypsin, chymotrypsin or protease, or by exposure to 70 degrees C for 5 min. It was also inhibited by several compounds, the most potent being L-glutamate and L-aspartate, followed by L-glutamate diethylester, then D-glutamate, N-methyl-D-aspartate and ibotenate. Quisqualate had little effect, while kainate, proctolin and D-aspartate had none. d-Tubocurarine stimulated [3H]L-glutamate binding. The data suggest that [3H]L-glutamate is binding to an L-glutamate receptor in housefly thoracic muscle membranes.

Effects of caroverine and diltiazem on synaptic responses, L-glutamate-induced depolarization and potassium efflux in the frog spinal cord

The frog spinal cord was used to determine the characteristics of the actions of caroverine and diltiazem, two organic Ca2+-antagonists, on synaptic responses and L-glutamate-induced depolarization. Caroverine and diltiazem (10(-4)M) depressed the dorsal root potential (DR-DRP) induced by electrical stimulation of an adjacent dorsal root. Diltiazem also depressed the ventral root potential (DR-VRP), whereas caroverine augmented both the polysynaptic component in the ventral root reflex and the size of the DR-VRP. The root potentials induced by high frequency stimulation (20 Hz, for 1 s) were markedly depressed by these Ca2+-antagonists at a concentration of 10(-4)M. When the preparation was perfused with normal medium, the compounds depressed L-glutamate-induced depolarizations in ventral and dorsal roots. In preparations treated with tetrodotoxin (TTX) (2 X 10(-7)M), the antagonizing actions of the drugs against L-glutamate-induced depolarizations in the ventral root were markedly reduced or abolished, while significant antagonizing actions on the depolarization in the dorsal root were still observed. The increase in extracellular K+ activity induced by L-glutamate in the TTX-treated preparation was significantly reduced by the compounds. Caroverine and diltiazem had no effect on the presynaptic nerve spike and on the focal synaptic potential induced by a single stimulation of a dorsal root; however, the focal synaptic potential induced by high frequency stimulation (20 Hz, 1 s) was attenuated. Motoneuronal action potentials were abolished by the drugs, while the excitatory postsynaptic potential remained unaffected. 9 The present results suggest that caroverine and diltiazem are not specific L-glutamate antagonists in the frog spinal cord, but that they block the initiation of an action potential without affecting presynaptic nerve conduction, transmitter release or transmitter-receptor interactions. The inhibitory effects of these compounds on L-glutamate-induced K+-efflux are discussed with reference to their Ca2+-antagonizing actions.

Baclofen has a presynaptic action at the crayfish neuromuscular junction

The action of baclofen, a GABA analog, was studied at the crayfish neuromuscular junction (NMJ). Baclofen depressed the amplitude of excitatory junction potentials (ejps) without affecting muscle input resistance and reduced the frequency of spontaneous miniature ejps without affecting their size. Thus, baclofen may mediate presynaptic inhibition by depressing transmitter release from the excitatory nerve. The site of baclofen's effect at the crayfish NMJ may parallel its site of action in the vertebrate nervous system.

The effect of calcium ions on the glutamate response and its desensitization in crayfish muscle fibres

The responses of crayfish muscle fibres to bath application or long ionophoresis of L-glutamate were studied in normal and low Ca2+ solutions. The smaller responses recorded in low Ca2+ solutions have characteristics suggesting a faster desensitization. Desensitization and recovery have complex kinetics. Desensitization is faster and recovery slower when external Ca2+ concentration is reduced. Both components of the recovery phase, which can be fitted by the sum of two exponentials, are affected by the external Ca2+ concentration. Recovery can be accelerated by external Ca2+ ionophoresis onto desensitized glutamate receptors. Responses to brief glutamate pulses of low intensity are not affected by Ca2+ reduction. For higher intensities, signs of desensitization are detectable early in the rising phase of the response. Concanavalin A (Con A) blocks both desensitization and Ca2+ dependence with similar time courses. Whether or not the preparation has been treated with Con A, the slowly rising responses recorded in isotonic Ca2+ do not show signs of desensitization. Con A causes a partial blockade of the glutamate response. The Ca2+ dependence of the glutamate response can be explained by the Ca2+ dependence of the desensitization process, the cation acting at ectocellular sites of the muscle membrane.

Glutamate inhibitory action of matrine at the crayfish neuromuscular junction

The effect of some alkaloids from Sophora flavescens (matrine, oxymatrine and N-methylcytisine) on glutamate-induced responses was investigated using electrophysiological techniques at the crayfish neuromuscular junction. At concentrations greater than 0.1 mM, matrine depressed both glutamate-induced responses and neurally evoked excitatory junctional potentials. Oxymatrine was less powerful than matrine, and N-methylcytisine was not effective. Matrine also depressed quisqualate-induced responses at this site, but did not have an effect on gamma-aminobutyric acid (GABA)-induced responses. Matrine had no influence upon either the resting membrane potential or the input resistance of the crayfish opener muscle. The inhibition of the glutamate-induced response by matrine was not affected by the membrane potential of the muscle fibre. Matrine reduced the size of extracellularly recorded excitatory junctional potentials without affecting their quantal content. Matrine did not affect the decay of extracellular excitatory junctional currents at the resting membrane potential. The results presented clearly demonstrate that matrine has an inhibitory action on glutamate-induced responses.

The effects of diltiazem on Periplaneta americana

The effects of the calcium antagonist diltiazem on nerve and muscle in the cockroach Periplaneta americana were examined. Diltiazem was observed to inhibit myogenic and glutamate-induced contractions of the visceral muscle while having either a potentiating, an inhibiting or a biphasic effect against proctolin-induced contractions. Against the isolated nervous system, diltiazem induced an increase in spontaneous discharge activity, followed by nerve block. Injection of diltiazem into cockroaches produced behavioral and toxic effects.

Excitatory junctional responses and glutamate responses at the crayfish neuromuscular junction in the presence of chlorisondamine

At the crayfish neuromuscular junction chlorisondamine reduced the amplitude of both the excitatory junctional potential and the glutamate current in a dose-dependent manner in concentrations above 3 microM, and it is suggested that the drug is a powerful non-competitive antagonist for glutamate. Chlorisondamine did not act presynaptically on the crayfish neuromuscular junction. A double exponential decay of excitatory synaptic currents was observed in the presence of chlorisondamine, suggesting that this drug is an open channel blocker for the excitatory neurotransmitter. The glutamate current tail was prolonged in the presence of chlorisondamine. This prolongation increased with increasing iontophoretic current of glutamate. The rate of recovery from the refractory form of the glutamate receptor to the free reactive one was hardly affected by chlorisondamine. The inhibitory action of chlorisondamine on glutamate responses was voltage-dependent and hyperpolarization reduced the drug action. Chlorisondamine depressed the glutamate current even in Na-free, Ca-rich solution.

Trimethaphan as a glutamate inhibitor at the crayfish neuromuscular junction

At the crayfish neuromuscular junction trimethaphan reduced the amplitude of both the glutamate-induced synaptic current and the excitatory junctional current in a dose-dependent manner at concentrations greater than 5 microM. These effects were dependent on membrane potential. Trimethaphan did not affect the inhibitory junctional potential and the input resistance of the opener muscle. The dose-response curves for inhibition of glutamate responses by trimethaphan suggest that trimethaphan is not a competitive glutamate antagonist. A quantum analysis of extracellularly recorded excitatory junctional potentials showed that trimethaphan decreased both quantum content and average unit size. Trimethaphan also prolonged the glutamate currents evoked by both short and prolonged ionophoretic currents, but the decay of nerve-evoked synaptic currents was accelerated by the drug. Three explanations worthy of consideration to explain the action of trimethaphan are the responses of extra-junctional receptors, the sudden release and short actions of the neurotransmitter in contrast with the progressive application and long exposure of exogenous agonists to receptors, and discrimination of glutamate and excitatory transmitter in the crayfish neuromuscular junction. The second of these possibilities is mainly discussed at length.

The effects of cyclic dicarboxylic acids on spontaneous and amino acid-evoked activity of rat cortical neurones

1 A series of cyclic dicarboxylic acids were applied by microiontophoresis to neurones in the cerebral cortex of rats anaesthetized with urethane. The object was to examine effects on spontaneous firing rates and any ability to antagonize responses to excitatory amino acids. 2 At relatively low ejecting currents (10-25 nA) cis-2,3-piperidine dicarboxylic acid (cis-2,3-PDA) had no effect on spontaneous firing but selectively antagonized the excitation evoked by n-methyl-D-aspartate (NMDA) without affecting responses to quisqualaife or kainate. At higher ejecting currents (60-100 nA) responses to all three agonists were reduced. 3 Other cis-piperidine dicarboxylic acids and piperazine-2,3-dicarboxylic acid had only weak and variable effects on cell firing and responses to NMDA, quisqualate, kainate, glutamate and aspartate. 4 2, 3-Pyridine dicarboxylic acid (quinolinic acid) produced excitation of all cortical neurones tested. 5 2-Amino-5-phosphono-valeric acid, an NMDA antagonist, reduced responses to quinolinate, implying that this compound can act at NMDA receptors. 6 It is suggested that quinolinic acid may be of physiological interest as a potential endogenous excitant in the nervous system and that cis-2,3-PDA and its N-methyl derivative may be of use in studies of receptor pharmacology and the identification of synaptic transmitters.

Postsynaptic potential and postsynaptic current in muscle fibres with large time-constant. EPSP amplitude is independent of membrane resistance

1. Equations have been developed for a computer programmed calculation of synaptic current from synaptic potentials (epsps). 2. The method permits separation of presynaptic and postsynaptic effects in experiments involving drugs which affect synaptic transmission. It is particularly applicable where the postsynaptic cells are large as in the case of commonly employed crustacean muscle fibres. 3. Contrary to a widely held view the theoretical approach used predicts that epsp-amplitude is relatively independent of membrane resistance. 4. Confirmation is provided by experiments involving the application of barium which increases, and of GABA which decreases membrane resistance.

Glutamate inhibitors in the crayfish neuromuscular junction

1. The effects of chlorisondamine and TI-233 on the crayfish neuromuscular junction were investigated in order to compare the action of glutamate with that of the excitatory transmitter. 2. The glutamate-induced synaptic current was inhibited by both of these two drugs. Excitatory junctional potentials were significantly reduced by chlorisondamine, whereas they were increased by TI-233. 3. It is suggested that chlorisondamine and TI-233 are powerful non-competitive antagonists for glutamate. 4. A quantum analysis of extracellular EJPs demonstrated that chlorisondamine did not possess presynaptic action in the crayfish neuromuscular junction. Chlorisondamine shortened the decay phase of extracellular EJPs, and the decay was frequently fitted by a double exponential in relatively low concentrations. 5. Semilogarithmic plots of the decay phase of the glutamate current evoked by a short glutamate pulse were nearly linear, but they shifted from linearity to some extent in the presence of chlorisondamine, showing prolongation of the glutamate current tails. 6. When TI-233 was added to the bathing solution at a concentration of 0.1 mM, the quantum content of extracellular EJPs was increased by about two times, but the average unit size was not changed. 7. There was no change in the rise time and the decay phase of the glutamate potential in the presence of TI-233. 8. Pharmacological difference between glutamate responses and EJPs was revealed in the presence of chlorisondamine and TI-233. Unless this difference can be explicated with a reasonable explanation on the glutamate transmitter hypothesis, it is difficult to confirm that glutamic acid is an excitatory transmitter at the crayfish neuromuscular junction.

Effects of anisatin on the GABA action in the crayfish neuromuscular junction

Anisatin depressed the conductance increase produced by GABA in a non-competitive manner and decreased the amplitude of the inhibitory junctional potentials. The presynaptic inhibition was depressed by anisatin in the similar concentrations to those of picrotoxin. The blockade of the presynaptic inhibition by anisatin was irreversible, whereas the effect of anisatin on postsynaptic inhibition subsided after prolonged washing of the preparation.

The recovery from desensitization of the glutamate receptor in the crayfish neuromuscular junction

The rate of constant of recovery from desensitization of the glutamate receptor in the crayfish neuromuscular junction was determined using conditioning and test responses for iontophoretically applied glutamate. The rate constant was independent on the extent of desensitization. The extent of desensitization was a function of applied glutamate current.