Neurones from the ventral half of mouse embryo spinal cord were grown in dissociated culture and voltage clamped. The current‐voltage relation of responses evoked by N‐methyl‐D‐aspartic acid (NMDA), L‐glutamic acid and kainic acid was recorded in media of different ionic composition. On removal of Mg2+ from the extracellular solution, responses to NMDA and L‐glutamate became less voltage sensitive, such that NMDA responses were no longer associated with a region of negative slope conductance. The antagonism of NMDA responses produced by application of Mg2+ to neurones bathed in nominally Mg2+‐free solutions shows voltage dependence and uncompetitive kinetics. Voltage‐jump experiments showed that the voltage‐dependent action of Mg2+ occurred rapidly, and with complex kinetics. Ni2+ and Cd2+, two potent blockers of calcium currents in spinal cord neurones, had significantly different potencies as NMDA antagonists, Ni2+ being of greater potency than Mg2+, and Cd2+ considerably weaker. The voltage‐dependent block of NMDA responses produced by physiological concentrations of Mg2+ is sufficient to explain the apparent increase in membrane resistance produced by NMDA in current‐clamp experiments, and the ability of NMDA to support repetitive firing. Substitution of choline for Na+ produced a hyperpolarizing shift in the reversal potential for responses evoked by kainic acid consistent with an increase in permeability to Na+ and K+. In choline‐substituted solutions, the reversal potential of NMDA responses was more positive than that recorded for kainic acid, and in addition NMDA responses showed enhanced desensitization.
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