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.