Glucose insufficiency alters neuronal viability and increases susceptibility to glutamate toxicity

Complete glucose deprivation has been shown to induce neuronal apoptosis, but the effect of moderate glucose deprivation under normal and pathological conditions is not fully understood. We investigated the effect of a restricted supply of glucose on neuronal vulnerability to glutamate by assaying cellular ATP levels (cellular energy production), 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) reduction (mitochondrial function), lactate dehydrogenase (LDH) release (cellular viability) and activation of caspase-3 (apoptosis) in rat hippocampal neurons cultured in media (1.7, 5 and 25 mM glucose) with or without 100 microM glutamate. Cellular ATP levels were significantly reduced in neurons cultured in 1.7 mM glucose, while addition of glutamate markedly lowered cellular ATP levels even at the normal glucose concentration. MTT reduction was also significantly inhibited by 1.7 mM glucose; however, unlike cellular ATP levels, glutamate inhibition of MTT reduction was glucose concentration dependent. The LDH assay suggested that neuronal survival declines with decreasing glucose concentration in media, and glutamate potentiates this effect. Since low glucose media caused a decrease in cellular ATP and cell viability, we investigated apoptosis-related changes in cultured neurons by examining activity of caspase-3. Low glucose media (1.7 and 5 mM glucose) increased caspase-3 activity, and glutamate potentiated this effect. Our results suggest that a low glucose supply in culture media activates an apoptosis mediator and markedly increases susceptibility to glutamate toxicity. Thus, even moderate glucose deprivation could be a serious risk factor that potentiates the pathophysiological consequences of certain neurodegenerative diseases.