Cholesterol does not affect the toxicity of amyloid β fragment but mimics its effect on MTT formazan exocytosis in cultured rat hippocampal neurons

It has recently been reported that methyl-beta-cyclodextrin-solubilized cholesterol protects PC12 cells from amyloid beta protein (A beta) toxicity. To ask if this is the case in brain neurons, we investigated its effect in primary cultured rat hippocampal neurons. In basal culture conditions with no addition of A beta, methyl-beta-cyclodextrin-solubilized cholesterol at concentrations of 30-100 mu M was toxic to neurons, but at concentrations of 1-10 mu M promoted neuronal survival. Methyl-beta-cyclodextrin-solubilized cholesterol at 1-10 mu M was also effective in protecting neurons from toxicity of 20 mu M A beta. However, these effects were all mimicked by methyl-beta-cyclodextrin alone, but not by cholesterol solubilized by dimethylsulfoxide or ethanol. The effects of methyl-beta-cyclodextrin-solubilized cholesterol on neuron al survival and A beta toxicity are probably attributed to the action of methyl-beta-cyclodextrin, but not cholesterol. Alternatively, we found that methyl-beta-cyclodextrin-solubilized cholesterol at lower concentrations (> 10 nM) inhibited cellular reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) by promoting the exocytosis of MTT formazan. This effect was shared by dimethylsulfoxide- or ethanol-solubilized cholesterol, but not by methyl-beta-cyclodextrin, supporting that it is attributed to the action of cholesterol. These results suggest that cholesterol does not protect neurons from A beta toxicity, or rather inhibits cellular MTT reduction in a similar manner to A beta. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.