THE ROLE OF EXCITATORY AMINO-ACIDS IN THE GENERATION OF SPONTANEOUS HIPPOCAMPAL OSCILLATIONS

We examined the role of N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors in generating spontaneous CA3 field potential oscillations in the hippocampal slice. Non-NMDA EPSPs are responsible for a portion of the spontaneous activity recorded in standard perfusion medium. NMDA receptors are not activated when inhibition is intact, however, recurrent excitation via NMDA receptors accounts for an increasing proportion of the low frequency (2-4 Hz) rhythms produced as inhibition is progressively blocked by low concentrations of the GABA(A) antagonist, penicillin. Production of these rhythms involves complex interactions among NMDA, non-NMDA and GABA(A) receptors. NMDA EPSPs can drive the rhythm in the absence of non-NMDA receptors only when inhibition is suppressed by more than 50%. Otherwise non-NMDA EPSPs appear to be necessary to depolarize neurons before NMDA EPSPs can be activated.