Glutamate toxicity in neuron-enriched and neuron-astrocyte co-cultures: effect of the glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylate.

Exposure of neuron-enriched cultures of the embryonic rat cerebral cortex to glutamate (Glu) resulted in a concentration-dependent (EC50 = 50 microM) increase in the release of lactate dehydrogenase (LDH) into the bathing medium. Glu-induced neurotoxicity appeared to be mediated predominantly by the activation of N-methyl-D-aspartate receptors because its effects were almost completely reversed by 1 microM (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine hydrogen maleate (MK-801). The potency of Glu was increased by ca eight-fold (EC50 = 6 microM) when neuronal cultures were incubated with Glu in the presence of the uptake inhibitor L-trans-pyrrolidine-2, 4-dicarboxylate (L-trans-PDC). Increased LDH release was also observed when neuron-enriched cultures were exposed to an ineffective concentration of Glu in the presence of increasing concentrations of L-trans-PDC, moreover, the uptake inhibitor itself became neurotoxic at concentrations above 100 microM. Glu also stimulated the release of LDH from neuron-astrocyte co-cultures in a concentration-dependent manner, however, it was found to be less potent (EC50 = 200 microM) in these cultures than it was in the neuron-enriched cultures. Incubation of the cocultures with Glu in the presence of the uptake inhibitor increased the potency of Glu (EC50 = 100 microM) but to a much lesser extent than that found in neuron-enriched cultures. Cultured astrocytes were found to be resistant to injury by Glu even in the presence of the uptake inhibitor.