REGULATION OF EXCITATORY AMINO-ACID RELEASE BY N-METHYL-D-ASPARTATE RECEPTORS IN RAT STRIATUM - INVIVO MICRODIALYSIS STUDIES

The microdialysis technique was utilized to study the effects of N-methyl-D-aspartate (NMDA) receptor ligands on the in vivo release of endogenous glutamate (Glu) and aspartate (Asp) from the rat striatum. Addition of NMDA (250 and 500-mu-M) to the dialysis perfusion solution resulted in a striking dose-dependent increase in extracellular concentrations of Glu and Asp in the striatum. The NMDA-induced effects were reduced in a dose-related way by prior perfusion with 75-mu-M dizocilpine (MK-801), a non-competitive NMDA receptor antagonist. MK-801, at 75-mu-M, produced no changes on basal levels of Glu and Asp. However, 100-mu-M MK-801 did increase Glu and Asp extracellular concentrations. Local infusion with 500-mu-M D-serine, an agonist at the glycine site associated to the NMDA receptor, significantly increased basal level of Glu, but not Asp. Such D-serine-induced effects were reduced by 7-Cl-kynurenic acid (200-mu-M), a selective blocker of the glycine site present in the NMDA receptor. It is proposed that activation of NMDA receptors by endogenous Glu and Asp enhances the subsequent release of these excitatory amino acids in the striatum. Part of these NMDA receptors might be located presynaptically on cortico-striatal nerve endings. In addition, postsynaptic NMDA receptors present in the striatum may also indirectly modulate the release of Glu and Asp, through trans-synaptic mechanism.