Stimulation of Sigma Receptors with Afobazole Blocks Activation of Microglia and Reduces Toxicity Caused by Amyloid-β25-35

Alzheimer's disease (AD) is a progressive neurodegenerative disease and the leading cause of senile dementia in the United States. Accumulation of amyloid-beta (A beta) and the effects of this peptide on microglial cells contribute greatly to the etiology of AD. Experiments were carried out to determine whether the pan-selective sigma-receptor agonist afobazole can modulate microglial response to the cytotoxic A beta fragment, A beta(25-35). Treatment with afobazole decreased microglial activation in response to A beta, as indicated by reduced membrane ruffling and cell migration. The effects of afobazole on A beta(25-35)-evoked migration were concentration dependent and consistent with sigma-receptor activation. When afobazole was coapplied with either BD-1047 [N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino) ethylamine dihydrobromide] or rimcazole, which are sigma-1- and sigma-2-selective antagonists, respectively, the inhibition of A beta(25-35)-induced migration by afobazole was reduced. Prolonged exposure of microglia to A beta(25-35) resulted in glial cell death that was associated with increased expression of the proapoptotic protein Bax and the death protease caspase-3. Coapplication of afobazole with A beta(25-35) decreased the number of cells expressing both Bax and caspase-3 and resulted in a concomitant enhancement in cell survival. Although afobazole inhibited activation of microglia cells by A beta(25-35), it preserved normal functional responses in these cells after exposure to the amyloid peptide. Intracellular calcium increases induced by ATP were depressed in microglia after 24-hour exposure to A beta(25-35). However, coincubation in afobazole returned these responses to near control levels. Therefore, stimulation of sigma-1 and sigma-2 receptors by afobazole prevents A beta(25-35) activation of microglia and inhibits A beta(25-35)-associated cytotoxicity, suggesting that afobazole may be useful for AD therapeutics.