NMDA receptors at excitatory synapses in the hippocampus: test of a theory of magnesium block.

Excitatory postsynaptic currents (EPSCs) were studied using the whole-cell patch-clamp technique to record from rat hippocampal pyramidal neurons in both slices and dissociated cultures. The voltage- and magnesium-dependence of the N-methyl-D-aspartate (NMDA) component of the EPSCs was quantified by measuring the amplitude of this component at a range of holding potentials (-80 mV to 60 or 80 mV) in several bath magnesium concentrations (0.2 microM, 0.1, 1 and 10 mM). All of our results were well-fitted by a theory of magnesium block developed from single-channel studies of extrajunctional NMDA channels in culture. It is concluded that extrasynaptic and subsynaptic NMDA channels both in culture and in situ have identical properties of magnesium block.