Mg 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 of 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.