Inhibition of L-homocysteic acid and buthionine sulphoximine-mediated neurotoxicity in rat embryonic neuronal cultures with alpha-lipoic acid enantiomers

In the present report, we have set out to investigate the potential capacit y of both the oxidised and reduced forms of RS-alpha-lipoic acid, and its s eparate R-(+) and S-(-)enantiomers, to prevent cell death induced with L-ho mocysteic acid (L-HCA) and buthionine sulphoximine (BSO) in rat primary cor tical and hippocampal neurons. L-HCA induced a concentration-dependent neur otoxic effect, estimated by cellular 3-(4,5-dimethylthiazol-2-yl)-2,5-diphe nyl-tetrazolium bromide (MTT) reduction, in primary neurons, but was signif icantly more toxic for hippocampal (EC50 = 197 mu M) compared with cortical neurons (EC50 = 1016 mu M) whereas D-HCA demonstrated only moderate (< 20% ) toxicity. On the other hand, cortical and hippocampal cultures were equal ly susceptible (341 and 326 mu M, respectively) to the neurotoxic action of BSO. Antioxidants including butylated hydroxyanisole, propyl gallate and v itamin E protected cells against the neurotoxic effect of L-HCA and BSO. Ho wever, N-acetyl-cysteine and tert-butylphenyl nitrone, although capable of abrogating L-HCA-mediated cell death showed no protective effect against BS O-mediated toxicity. RS-alpha-lipoic acid, RS-alpha-dihydrolipoic acid and the enantiomers R-alpha-lipoic acid and S-alpha-lipoic acid protected cells against L-HCA-mediated toxicity with EC50 values between 3.1-8.3 mu M in p rimary hippocampal neurons and 2.6-16.8 mu M for cortical neurons. However, RS-alpha-lipoic acid, RS-alpha-dihydrolipoic acid, and S-alpha-lipoic acid failed to protect cells against the degeneration induced by prolonged expo sure to BSO, whereas the natural form, R-alpha-lipoic, was partially active under the same conditions. The present results indicate a unique sensitivi ty of hippocampal neurons to the effect of L-HCA-mediated toxicity, and sug gest that RS-alpha-lipoic acid, and in particular the R-alpha-enantiomeric form is capable of preventing oxidative stress-mediated neuronal cell death in primary cell culture. (C) 2000 Elsevier Science B.V. All rights reserve d.