Abstract

Activation of N-methyl-d-aspartate (NMDA) receptors is known to produce arachidonic acid release, which has been implicated in excitotoxicity. Antagonists and partial agonists at the glycine site of the NMDA receptor, despite exhibiting functional differences in electrophysiological studies, inhibit glutamate-induced neurotoxicity and ischemia-induced neurodegeneration. The objective of this study was to investigate the effects of both glycine site antagonists and partial agonists on NMDA receptor-mediated [³H]arachidonic acid (AA) release evoked by glutamate, NMDA or a competitive inhibitor of the glutamate/aspartate uptake carrier. The [³H]AA release evoked by a maximally effective concentration of glutamate (100 μM) was blocked by the glycine site antagonists 7-chlorokynurenic acid (7-CKYN) and 5,7- dichlorokynurenic acid (5,7-DCKYN) and by a low intrinsic efficacy glycine partial agonist (+)-1-hydroxy-3-aminopyrrolid-2-one [(+)-HA-966]. 1-Aminocyclopropanecarboxylic acid (ACPC), a high intrinsic efficacy glycine partial agonist, did not modify [³H]AA release evoked by 100 μM glutamate. However, ACPC blocked (in a glycine reversible manner) the [³H]AA release induced by NMDA (100 μM) with an IC₅₀ of 131 ± 2 μM. Furthermore,l-trans-pyrrolidine-2,4-dicarboxylate (PDC), a competitive inhibitor of the glutamate transporter, also released [³H]AA (E_max and EC₅₀ of 127 ± 4% and 30 ± 1 μM, respectively). ACPC, 7-CKYN and (±)-2-amino-7-phosphonoheptanoic acid (AP-7), a competitive NMDA receptor antagonist, inhibited [³H]AA release evoked by PDC. These results demonstrate that both glycine site antagonists and partial agonists can inhibit NMDA receptor-mediated [³H]AA release in cerebellar granule cells, an action consistent with the neuroprotective effects of these compounds.