Convulsant-induced depression of amino acid responses in cultured mouse spinal neurones studied under voltage clamp.

A two-microelectrode voltage-clamp technique was used to record membrane current responses induced by the application of the neutral amino acids, gamma-aminobutyric acid (GABA) and glycine, to mouse spinal neurones maintained in cell culture. Membrane current responses to GABA were depressed by the convulsant agents, bicuculline, picrotoxin and pentylenetetrazole. Membrane current responses to glycine were depressed by the convulsant compound, strychnine. Analysis of the fluctuations in membrane current about the mean response during prolonged applications of GABA revealed that neither the conductance nor the average open-time of GABA-activated ion-channels was altered in the presence of picrotoxin or bicuculline. Pentylenetetrazole caused a reduction in the open-time of GABA-activated ion-channels but this effect alone was too small to account for the depression of the current response. A similar analysis of glycine-induced responses in the presence of strychnine revealed that the depression of glycine responses by strychnine was not associated with any changes in the conductance or average open-time of glycine-activated ion-channels. We therefore conclude that the convulsant compounds act by reducing the rate of activation of ion-channels by the neutral amino acids either through a reduction in the number of free receptors or by a change in the kinetics of channel activation.