Enhanced release of inhibitory and excitatory transmitter quanta in the crayfish neuromuscular junction by glycine and GABA.

High concentrations of glycine (greater than or equal to 0.1 mol/l) applied to the bathing medium of voltage-clamped muscle fibres elicited high rates of spontaneous inhibitory miniature currents (sIPSCs) which were identified by means of the noise analysis technique. The rate of spontaneous excitatory miniature currents (sEPSCs) was not raised appreciably in presence of these high concentrations of glycine. Contrary to the effect of glycine, high concentrations of GABA (greater than or equal to 0.1 mol/l) desensitized inhibitory postsynaptic receptors and induced high rates of sEPSCs. When, in addition to GABA, glycine was also applied at high concentrations, the discharge of sEPSCs was suppressed. Moreover, glycine reduced significantly the current noise intensity elicited by activation of excitatory postsynaptic membrane channels with 1.10(-4) mol/l glutamate. The experiments suggest an inhibitory action of glycine on the excitatory receptor--channel complex in the postsynaptic membrane.