An accumulation of amyloidβpeptide (Aβ) due to an imbalance between anabolism and catabolism triggers Alzheimer's disease (AD). Neprilysin is a rate-limiting peptidase, which participates in the catabolism of Aβin brain. We investigated whether rats continuously infused with thiorphan, a specific inhibitor for neprilysin, into the cerebral ventricle cause cognitive dysfunction, with an accumulation of Aβin the brain. Thiorphan-infused rats displayed significant cognitive dysfunction in the ability to discriminate in the object recognition test and spatial memory in the water maze test, but not in other hippocampus-dependent learning and memory tasks. Thiorphan infusion also elevated the Aβ40 level in the insoluble fraction of the cerebral cortex, but not that of the hippocampus. There was no significant difference in the nicotine-stimulated release of acetylcholine in the hippocampus between vehicle-and thiorphan-infused rats. These results indicate that continuous infusion of thiorphan into the cerebral ventricle causes cognitive dysfunction by raising the level of Aβin the cerebral cortex, and suggest that a reduction of neprilysin activity contribute to the deposition of A and development of AD.

Oxidative stress plays an important role in neuronal cell death associated with many different neurodegenerative conditions, and it is reported that 4-hydroxynonenal (HNE), an aldehydic product of membrane lipid peroxidation, is a key mediator of neuronal cell death induced by oxidative stress. Previously, we have demonstrated that interleukin-6 (IL-6) protects PC12 cells from serum deprivation and 6-hydroxydopamine-induced toxicity. Therefore, in the present study, we examined the effects of interleukins on HNE toxicity in PC12 cells. Exposure of PC12 cells to HNE resulted in a decrease in levels of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction, which was due to necrotic and apoptotic cell death. Addition of IL-6 24 h before HNE treatment provided a concentration-dependent protection against HNE toxicity, whereas neither IL-1y nor IL-2 had any effect. Addition of glutathione (GSH)-ethyl ester, but not superoxide dismutase or catalase, before HNE treatment to the culture medium protected PC12 cells from HNE toxicity. We found that IL-6 increases intracellular GSH levels and the activity of y-glutamylcysteine synthetase (y-GCS) in PC12 cells. Buthionine sulfoximine (BSO), an inhibitor of y-GCS, reversed the protective effect of IL-6 against HNE toxicity. These results suggest that IL-6 protects PC12 cells from HNEinduced cytotoxicity by increasing intracellular levels of GSH.