Impact of the selective estrogen receptor modulator, raloxifene, on neuronal survival and outgrowth following toxic insults associated with aging and Alzheimer's disease.

The current study investigated the estrogen agonist-antagonist properties of the selective estrogen receptor modulator, raloxifene (Ral), on neuroprotection and neuronal markers of memory function. Low concentrations of raloxifene significantly reduced basal markers of membrane damage and had no deleterious effect on neuronal survival. However, high concentrations of raloxifene (1000-5000 ng/ml) induced a significant increase in markers of membrane damage and a significant decrease in neuronal survival. At subtoxic concentrations, raloxifene induced significant neuroprotection against beta amyloid(25-35)-, hydrogen peroxide- and glutamate-induced toxicity. Results of analyses to determine whether raloxifene acted competitively or synergistically with 17 beta-estradiol revealed that a postmenopausal level of 17 beta-estradiol exerted a significantly greater increase in neuronal survival against beta-amyloid- and glutamate-induced toxicity compared to 50 ng/ml raloxifene. The combined presence of raloxifene and 17 beta-estradiol was significantly neuroprotective against beta amyloid(25-35)- and glutamate-induced excitotoxicity but was significantly lower than 17 beta-estradiol alone while not significantly different than raloxifene alone. Morphologic analyses demonstrated that raloxifene significantly increased neuronal outgrowth of hippocampal neurons within a narrow dose range that was blocked by a glutamate NMDA receptor antagonist. Raloxifene did not promote the outgrowth of basal forebrain or cortical neurons. Results of this study indicate that raloxifene exerted partial estrogen agonist action in the absence of 17 beta-estradiol whereas in the presence of 17 beta-estradiol, raloxifene exerted a mixed estrogen agonist-antagonist effect.