Glutamate inhibitors in 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.

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.

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.