Evidence for a magnesium-insensitive membrane resistance increase during NMDA-induced depolarizations in rat neocortical neurons in vitro

The responses of rat neocortical neurons in vitro to
iontophoretically applied N-methyl-d-aspartate (NMDA) were
investigated by means of intracellular recording in the presence
and absence of extracellular magnesium ions (Mg2+). At
Mg2+-concentrations of 1.3 mM the neurons responded with a
depolarization accompanied by an increase in membrane resistance.
Upon removal of Mg2+ the NMDA-induced depolarization was markedly
potentiated. However, even in neurons recorded from slices which
were incubated in a Mg2+-free solution for 3–7 h, the NMDA response
was still associated with a resistance increase, suggesting that
the voltage-dependence of the NMDA-activated conductance is not
exclusively determined by Mg2+.