Glutamate Neurotoxicity and the Inhibition of Protein Synthesis in the Hippocampal Slice

Abstract

In some animal models of ischemia, neuronal degeneration can be prevented by the selective antagonism of the N-methyl-D-aspartate (NMDA) glutamate receptor subtype, suggesting that glutamate released during ischemia causes injury by activating NMDA receptors. The rat hippocampal slice preparation was used as an in vitro model to study the pharmacology of glutamate toxicity and investigate why NMDA receptors are critical in ischemic injury. Acute toxicity was assessed by quantifying the inhibition of protein synthesis, which we confirmed by autoradiography to be primarily neuronal. The effect of NMDA was prevented by the specific antagonists MK-801 and ketamine, as well as by the less selective antagonist kynurenic acid. The less selective antagonists kynurenic acid and 6,7-dinitroquinoxaline-2,3-dione antagonized the effects of quisqualate and NMDA. In contrast to previous observations with dissociated neurons in tissue culture, the toxicity of glutamate was unaffected by antagonists, regardless of the glutamate concentration, the duration of exposure, or the presence of magnesium. The high concentration of glutamate required to inhibit protein synthesis and the inability of receptor antagonists to block the effect of glutamate suggest that either glutamate acts through a non-receptor-mediated mechanism, or that the receptor-mediated nature of glutamate effects are masked in the slice preparation, perhaps by the glial uptake of glutamate. The altered physiology induced by ischemia must potentiate the neurotoxicity of glutamate, because we observed with a brain slice preparation that only high concentrations of glutamate caused neurotoxicity in the presence of oxygen and glucose and that these effects were not reversed by glutamate receptor antagonists.

Gene therapy effectiveness differs for neuronal survival and behavioral performance

“…Those neurons that are protected from metabolic decline sooner may take less time to return to normal function. By similar logic, an earlier intervention may also make for less cumulative oxidative damage and/or inhibition of protein synthesis 25,26 allowing for more rapid recovery after insult. Little is known, however, about the time-course of repair of oxidative damage or the restitution of protein synthesis in the hippocampus in vivo following an excitotoxic insult.…”

Glutamate-Dependent Phosphorylation of Elongation Factor-2 and Inhibition of Protein Synthesis in Neurons

“…First, the massive release of glutamate occurring during cerebral ischemia (Benveniste et al, 1984) seems to be responsible for a persistent inhibition of neuronal protein synthesis, which precedes delayed neuronal death (Thilmann et al, 1986;Raley-Susman and Lipton, 1990). Second, the exposure of brain slices and cultured cerebellar granule cells to glutamate leads to a marked decrease in radioactive amino acid incorporation into proteins (Orrego and Lipmann, 1967;Vornov and Coyle, 1991;Dessi et al, 1994). However, the mechanism of this inhibition of protein synthesis is not known.…”

Delayed protection by MK-801 and tetrodotoxin in a rat organotypic hippocampal culture model of ischemia.

“…Our previous work using acute hippocampal slices from adult animals demonstrated that adult brain tissue differs in its sensitivity to the neurotoxic effects of glutamate when compared with slices from neonatal animals or dissociated cultures prepared from embryonic brain. In hippocampal slices prepared from adult rats, the toxicity of glutamate cannot be blocked by NMDA receptor antagonists 15 unless ischemia is modeled by removing oxygen and glucose or the slices are placed in ionic conditions intended to reproduce the high-potassium, low-sodium conditions that occur in the ischemic brain. 16 This suggested that the physiological changes of ischemia influence the mechanisms of neuronal injury caused by excitatory amino acid release.…”