Mg2+ dependence of membrane resistance increases evoked by NMDA in hippocampal neurones

The response of granule cells and CA1 pyramidal neurones to NMDA was studied in the presence and absence of Mg2+ using an in vitro slice preparation. In the absence of Mg2+ the depolarizing response of hippocampal neurones to NMDA is accompanied by a decrease in input resistance. In the presence of Mg2+ ions, however, the response to NMDA is always associated with an apparent increase in input resistance. These results indicate that the action of NMDA is by a classical mechanism of conductance increase and are in agreement with the suggestion that the apparent increase in input resistance associated with NMDA depolarizations is the result of voltage-dependent channel block by Mg2+ of the NMDA evoked current.