Hypoglycemic coma and brain injury are potential complications of insulin therapy. Certain neurons in the hippocampus and cerebral cortex are uniquely vulnerable to hypoglycemic cell death, and oxidative stress is a key event in this cell death process. Here we show that hypoglycemia-induced oxidative stress and neuronal death are attributable primarily to the activation of neuronal NADPH oxidase during glucose reperfusion. Superoxide production and neuronal death were blocked by the NADPH oxidase inhibitor apocynin in both cell culture and in vivo models of insulin-induced hypoglycemia. Superoxide production and neuronal death were also blocked in studies using mice or cultured neurons deficient in the p47 phox subunit of NADPH oxidase. Chelation of zinc with calcium disodium EDTA blocked both the assembly of the neuronal NADPH oxidase complex and superoxide production. Inhibition of the hexose monophosphate shunt, which utilizes glucose to regenerate NADPH, also prevented superoxide formation and neuronal death, suggesting a mechanism linking glucose reperfusion to superoxide formation. Moreover, the degree of superoxide production and neuronal death increased with increasing glucose concentrations during the reperfusion period. These results suggest that high blood glucose concentrations following hypoglycemic coma can initiate neuronal death by a mechanism involving extracellular zinc release and activation of neuronal NADPH oxidase.

Severe hypoglycemia causes neuronal death and cognitive impairment. Evidence suggests that hypoglycemic neuronal death involves excitotoxicity and DNA damage. Poly(ADP-ribose) polymerase-1 (PARP-1) normally functions in DNA repair, but promotes cell death when extensively activated by DNA damage. Cortical neuron cultures were subjected to glucose deprivation to assess the role of PARP-1 in hypoglycemic neuronal death. PARP-1-/- neurons and wild-type, PARP-1+/+ neurons treated with the PARP inhibitor 3,4-dihydro-5-[4-(1-piperidinyl)butoxy]-1(2H)-isoquinolinone both showed increased resistance to glucose deprivation. A rat model of insulin-induced hypoglycemia was used to assess the therapeutic potential of PARP inhibitors after hypoglycemia. Rats subjected to severe hypoglycemia (30 min EEG isoelectricity) accumulated both nitrotyrosine and the PARP-1 product, poly(ADP-ribose), in vulnerable neurons. Treatment with PARP inhibitors immediately after hypoglycemia blocked production of poly(ADP-ribose) and reduced neuronal death by >80% in most brain regions examined. Increased neuronal survival was also achieved when PARP inhibitors were administered up to 2 hr after blood glucose correction. Behavioral and histological assessments performed 6 weeks after hypoglycemia confirmed a sustained salutary effect of PARP inhibition. These results suggest that PARP-1 activation is a major factor mediating hypoglycemic neuronal death and that PARP-1 inhibitors can rescue neurons that would otherwise die after severe hypoglycemia.

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