β-Estradiol, Dehydroepiandrosterone, and Dehydroepiandrosterone Sulfate Protect against N-Methyl-d-aspartate-Induced Neurotoxicity in Rat Hippocampal Neurons by Different Mechanisms

Abstract

We examined neuroprotective effects of β-estradiol, dehydroepiandrosterone (DHEA), and dehydroepiandrosterone sulfate (DHEA-S) against N-methyl-d-aspartate (NMDA)-induced neurotoxicity in primary cultured rat hippocampal neurons. All three steroids demonstrated neuroprotective effects. Time-course studies revealed that steroid cotreatment for only 15 min at the same time as exposure to NMDA, but neither pretreatment nor addition of steroids for 24 h after NMDA-mediated neuroprotective effects. This indicates that short-term actions of these steroids are critical for this process. Acute treatment with β-estradiol dose dependently inhibited NMDA-induced intracellular Ca²⁺ increases, which strongly correlated with its neuroprotective effect via L-type voltage-gated calcium channels. Acute treatment with DHEA, but not with DHEA-S, significantly inhibited nitric oxide (NO) production and Ca²⁺-sensitive NO synthase (NOS) activity caused by NMDA stimulation. An NOS inhibitor, N^G-monomethyl-l-arginine acetate was also protective against NMDA-induced neurotoxicity. These data indicate that β-estradiol may exert neuroprotective effects mainly by reducing Ca²⁺ increases but that DHEA may act by inhibiting NOS activity. Treatment with the σ-1 receptor (Sig-1R) antagonists rimcazole or BD1063 (1-[2-(3,4-dichlorophenyl)ethyl]-4-methylpiperazine dihydrochloride) partially, but significantly, reversed the neuroprotective effect of DHEA-S against NMDA-induced neurotoxicity, whereas muscimol, a GABA-A-receptor agonist, did not. This suggests that the neuroprotective effect of DHEA-S may be mediated via Sig-1R, at least in part. Together, our data suggest that the neurosteroid family members β-estradiol, DHEA, and DHEA-S exert neuroprotective effects through different nongenomic mechanisms.