LOCALLY EVOKED-POTENTIALS IN SLICES OF THE RAT NUCLEUS-ACCUMBENS - NMDA AND NON-NMDA RECEPTOR MEDIATED COMPONENTS AND MODULATION BY GABA

In a slice preparation of the rat nucleus accumbens (Acb), local electrical stimulation elicited a field potential composed of two negative peaks, followed by a positive wave. The early negative peak was identified as a non-synaptic compound action potential, the late negative peak as a monosynaptic population spike (PS) and the positive wave as a mixture of an excitatory and an inhibitory postsynaptic potential (PSP). Both the PS and the PSP exhibited a marked degree of paired-pulse facilitation. The quisqualate/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 2 μM) and the broadly acting glutamate receptor antagonist kynurenic acid (300 μM) reversibly abolished or reduced both the PS and PSP. In contrast, nicotinic, muscarinic and N-methyl-d-aspartate (NMDA) receptor antagonists had no suppressive action. Washout of Mg2+ from the superfusion medium reversibly enhanced and prolonged the PSP and this effect was blocked by the NMDA receptor antagonist d(-)-2-amino-5-phosphonopentanoic acid (d-AP-5). The γ-aminobutyric acid antagonist picrotoxin (60 μM) enhanced the PS and induced secondary spikes which were superimposed on a prolonged PSP. Most of this prolongation was abolished by d-AP-5. It is concluded that locally evoked synaptic responses in the Acb are mediated by glutamate or aspartate, and that NMDA receptor mediated activity evoked by low-frequency stimulation is substantial in Mg2+-free medium or during reduced GABAA receptor activity, but not under normal conditions. © 1990.