The mechanism of action and pharmacological specificity of the anticonvulsant NMDA antagonist MK‐801: a voltage clamp study on neuronal cells in culture

Some possible molecular mechanisms of action of the anxiolytic, anticonvulsant and neuroprotective agent MK‐801 have been examined in ‘whole‐cell’ voltage clamp recordings performed on rat hippocampal and cortical neurones, bovine adrenomedullary chromaffin cells and N1E‐115 neuroblastoma cells maintained in cell culture. Transmembrane currents recorded from rat hippocampal and cortical neurones in response to locally applied N‐methyl‐d‐aspartate (NMDA) were antagonized by MK‐801 (0.1–3.0 μm). Blockade was use‐dependent, and little influenced by transmembrane potential. MK‐801 (3 μm) had no effect on currents evoked by kainate (100 μm). The antagonism of NMDA‐induced currents by MK‐801 was only slowly and incompletely reversed when the cell membrane potential was clamped at −60mV during washout. Prolonged applications of NMDA at +40, but not −60 mV during washout, markedly accelerated recovery from block. In contrast to MK‐801, ketamine (10 μm) blocked NMDA‐induced currents in a voltage‐dependent manner. Blockade increased with membrane hyperpolarization and was completely reversible upon washout. MK‐801 (1–10 μm) produced a voltage‐ and concentration‐dependent block of membrane currents elicited by ionophoretically applied acetylcholine (ACh) recorded from bovine chromaffin cells. The block was readily reversible upon washout. γ‐Aminobutyric acid_A (GABA_A) receptor‐mediated chloride currents of chromaffin cells were unaffected by MK‐801 (1–100 μm). In contrast, such currents were potentiated by diazepam (1 μm). MK‐801 (100 μmm) had no effect on currents evoked by GABA on hippocampal neurones. MK‐801 (10 μm) had little effect on membrane currents recorded from N1E‐115 neuroblastoma cells in response to ionophoretically applied 5‐hydroxytryptamine (5‐HT). Such currents were antagonized by the 5‐HT₃ receptor antagonist GR 38032F (1 nm) and also by MK‐801 at high concentration (100 μm). Voltage‐activated, tetrodotoxin‐sensitive, sodium currents of chromaffin cells were unaffected by 10 μm MK‐801. However, at a relatively high concentration (100 μm), MK‐801 reduced the amplitude of such currents to approximately 77% of control. The relevance of the present results to the central actions of MK‐801 is discussed. 1989 British Pharmacological Society