Voltage-jump analysis of curare action at a glutamate synapse

Streptomycin blocks the postsynaptic effects of excitatory amino acids on the vestibular system primary afferents

It has been suggested that streptomycin might be an antagonist of the glutamate receptors, and that it selectively blocks quisqualic acid receptors. We studied whether streptomycin blocks the responses to excitatory amino acid agonists on the vestibular system primary afferents, and if it allows us to differentiate between kainate (KA) and quisqualate (QA) receptor mediated responses. The experiments were performed in the axolotl (Ambystoma tigrinum). Intra- and extracellular records of the electrical activity of semicircular canal afferent fibers were obtained. Drugs were applied by pressure ejection in volumes of 20 microliters in a 10 ml bath. Streptomycin (0.01-10 mM), induced a dose dependent reversible inhibition of the basal spike discharge of the afferent fibers. This coincided with a reduction in the amplitude of excitatory postsynaptic potentials (EPSP) recorded intracellularly in the afferent fibers. Streptomycin also blocked the excitatory action produced by KA and QA; increasing concentrations of streptomycin produced a rightward shift in the concentration-response curves for both KA and QA. This action persisted even in a high Mg2+ (10 mM), low Ca2+ (0.09 mM) Ringer solution, indicating its postsynaptic nature. These results show that streptomycin might be a non-selective excitatory amino acid (EAA) receptor antagonist.

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.

Activation of protein kinase C promotes glutamate-mediated transmission at the neuromuscular junction of the mealworm

1. Actions of protein kinase C activators, 1,2-oleoylacetylglycerol (OAG) and 12-O-tetradecanoylphorbol-13-acetate (TPA), on the glutamate-mediated neuromuscular transmission in the mealworm, Tenebrio molitor, were studied by the microelectrode current-clamp and voltage-clamp techniques. 2. The activators OAG and TPA stimulate the evoked and spontaneous transmitter releases from the presynaptic terminal, as evidenced by an increase in the quantum content estimated by the number of failures of extracellular excitatory postsynaptic potentials (EPSPs), and in the frequency of miniature EPSPs. 3. Both OAG and TPA act on the postsynaptic membrane to enhance responses to the transmitter L-glutamate. Protein kinase C activators increased the apparent maximum of the ionophoretic dose-response curve for glutamate-induced depolarization, without affecting the reversal potential and the voltage-dependent decay rate for the excitatory postsynaptic current (EPSC) under voltage-clamp conditions. 4. The postsynaptic effect of OAG and TPA is distinctly different from that of activators of cyclic nucleotide-dependent protein kinases, such as octopamine, forskolin, CPT-cyclic AMP (8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate), and 8-bromo-cyclic GMP (8-bromoguanosine 3',5'-cyclic monophosphate) which decreased the postsynaptic sensitivity to L-glutamate. 5. I suggest that the responsiveness of the receptor to L-glutamate is under the control of these counteracting enzyme systems in the insect 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.

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.