The metabotropic glutamate receptor agonist 1 S,3 R-ACPD stimulates and modulates NMDA receptor mediated excitotoxicity in organotypic hippocampal slice cultures

Abstract

The potential toxic effects of the metabotropic glutamate receptor agonist (1 S,3 R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) and its interactions with the N-methyl- d-aspartate (NMDA) receptor were studied in hippocampal brain slice cultures, using densitometric measurements of the cellular uptake of propidium iodide (PI) to quantify neuronal degeneration. Cultures exposed to ACPD, showed a concentration (2–5 mM) and time (1–4 days) dependent increase in PI uptake in CA1, CA3 and dentate subfields after 24 h and 48 h of exposure, with CA1 pyramidal cells being most sensitive. The neurodegeneration induced by 2 mM ACPD was completely abolished by addition of 10 μM of the NMDA receptor antagonist (5 R,10 S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[ a, d]cyclohepten-5,10-imine (MK-801), while 20 μM of the 2-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainic acid receptor antagonist 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[ f]quinoxaline-7-sulfonamide (NBQX) had no effect. Co-exposing cultures to a subtoxic dose of 300 μM ACPD together with 10 μM NMDA, which at this dose is known to induce a fairly selective degeneration of CA1 pyramidal cells, significantly increased the PI uptake in both CA1 and CA3, compared to cultures exposed to 10 μM NMDA only. Adding the 300 μM ACPD as pretreatment for 30 min followed by a 30 min wash in normal medium before the ACPD/NMDA co-exposure, eliminated the potentiation of NMDA toxicity. The potentiation was also blocked by addition of 10 or 100 μM 2-methyl-6-(phenylethynyl)pyridine (MPEP) (mGluR5 antagonist) during the co-exposure, while a corresponding addition of 10 or 100 μM 7-(hydroxyimino)cyclopropa[ b]chromen-1a-carboxylate ethyl ester (CPCCOEt) (mGluR1 antagonist) had no effect. We conclude that, stimulation of metabotropic glutamate receptors with ACPD at concentrations of 2 mM or higher induces a distinct subfield-related and time and concentration dependent pattern of hippocampal degeneration, and that ACPD at subtoxic concentrations modulates NMDA-induced excitotoxicity through the mGluR5 receptor in a time dependent way.