Neurosteroids block the increase in intracellular calcium level induced by Alzheimer’s β-amyloid protein in long-term cultured rat hippocampal neurons

The neurotoxicity of β-amyloid protein (AβP) is implicated in the etiology of Alzheimer’s disease. We previously have demonstrated that AβP forms Ca(2+)-permeable pores on neuronal membranes, causes a marked increase in intracellular calcium level, and leads to neuronal death. Here, we investigated in detail the features of AβP-induced changes in intracellular Ca(2+) level in primary cultured rat hippocampal neurons using a multisite Ca(2+)-imaging system with fura-2 as a fluorescent probe. Only a small fraction of short-term cultured hippocampal neurons (ca 1 week in vitro) exhibited changes in intracellular Ca(2+) level after AβP exposure. However, AβP caused an acute increase in intracellular Ca(2+) level in long-term cultured neurons (ca 1 month in vitro). The responses to AβP were highly heterogeneous, and immunohistochemical analysis using an antibody to AβP revealed that AβP is deposited on some but not all neurons. Considering that the disruption of Ca(2+) homeostasis is the primary event in AβP neurotoxicity, substances that protect neurons from an AβP-induced intracellular Ca(2+) level increase may be candidates as therapeutic drugs for Alzheimer’s disease. In line with the search for such protective substances, we found that the preadministration of neurosteroids including dehydroepiandrosterone, dehydroepiandrosterone sulfate, and pregnenolone significantly inhibits the increase in intracellular calcium level induced by AβP. Our results suggest the possible significance of neurosteroids, whose levels are reduced in the elderly, in preventing AβP neurotoxicity.