Mixed-agonist action of excitatory amino acids on mouse spinal cord neurones under voltage clamp

Neurones from the ventral half of mouse embryo spinal cord were grown in tissue culture and voltage clamped with two micro-electrodes. The current-voltage relation of responses evoked by brief pressure applications of excitatory amino acids was examined over a membrane potential range of -100 to +70 mV. Three types of current-voltage relation were observed. Responses to kainic and quisqualic acids were relatively linear within +/- 20 mV of the resting potential. N-methyl-D-aspartate (NMDA) and L-aspartic acid responses had a negative slope conductance at membrane potentials more negative than -30 mV. In contrast, over the same potential range the slope conductance of responses evoked by L-glutamic and L-homocysteic acids was close to zero. The membrane potential-chord conductance relation of the ionic mechanism activated by excitatory amino acids, derived using the driving force for ionic current, showed two types of behaviour. The conductance linked to NMDA receptors was highly voltage sensitive and increased on depolarization; a much weaker voltage sensitivity was observed for responses evoked by kainic and quisqualic acids. L-glutamic and L-homocysteic acid responses behaved as though due to simultaneous activation of both NMDA and either kainate or quisqualate receptors. In the presence of the NMDA receptor antagonist (+/-)-2-aminophosphonovaleric acid (2-APV) the response to L-glutamate became less voltage sensitive and resembled responses evoked by kainate or quisqualate. Simultaneous activation of both conductance mechanisms by mixtures of kainate and NMDA produced current-voltage and membrane potential-chord conductance relations similar to those of L-glutamate. The voltage sensitivity of the L-glutamate response was inversely related to the dose; for low doses of L-glutamate the slope conductance of responses recorded near the resting potential was close to zero. However, larger doses of L-glutamate evoked responses with a voltage sensitivity similar to that of kainate. We suggest that L-glutamate acts as a mixed agonist at both NMDA and non-NMDA receptors. This can explain the results of previous experiments that failed to demonstrate a membrane resistance change during L-glutamate-induced depolarizations.