The paradoxical effect of NMDA receptor stimulation on electrical activity of the sensorimotor cortex in freely behaving rats: analysis by combined EEG-intracerebral microdialysis.

This study was designed to determine the effects of N-methyl-D-aspartate (NMDA) receptor stimulation on the electrical activity of neocortex in freely behaving rats. Electroencephalogram (EEG) recording and intracerebral microdialysis were conducted simultaneously in the same site of the sensorimotor cortex, where the basal extracellular concentrations of aspartate and glutamate were 2.1 +/- 0.7 microM and 11.5 +/- 2.4 microM, respectively. Microdialysis with NMDA solutions (ranging from 10.0 microM to 10.0 mM) reduced the amplitude of the EEG activity and decreased the power of all frequency bands, with a virtual elimination of the high frequency waves, in a dose-dependent manner. These EEG changes were reversed after washing out the drug from the microdialysis fluid, and could be effectively antagonized with the competitive NMDA receptor antagonist DL-2-amino-5-phosphonovalerate. Remarkably, the NMDA actions were not associated with epileptiform behavioral or electrographic events. Control studies demonstrated that in the same experimental conditions, cholinergic receptor agonist carbachol caused seizures, and microdialysis with NMDA in the hippocampus readily induced epileptiform spikes. Our study shows that NMDA receptor stimulation in the rat sensorimotor cortex, although excitatory at synaptic level, can depress the local EEG activity. This may indicate that the NMDA receptor-mediated signals are processed by the neocortical network in a different way than by many other brain circuitries including hippocampus.