Protection of cortical cells by equine estrogens against glutamate-induced excitotoxicity is mediated through a calcium independent mechanism

BACKGROUND: High concentrations of glutamate can accumulate in the brain and may be involved in the pathogenesis of neurodegenerative disorders such as Alzheimer's disease. This form of neurotoxicity involves changes in the regulation of cellular calcium (Ca(2+)) and generation of free radicals such as peroxynitrite (ONOO(-)). Estrogen may protect against glutamate-induced cell death by reducing the excitotoxic Ca(2+ )influx associated with glutamate excitotoxicity. In this study, the inhibition of N-methyl-D-aspartate (NMDA) receptor and nitric oxide synthase (NOS) along with the effect of 17β-estradiol (17β-E(2)) and a more potent antioxidant Δ(8), 17β-estradiol (Δ(8), 17β-E(2)) on cell viability and intracellular Ca(2+ )([Ca(2+)](i)), following treatment of rat cortical cells with glutamate, was investigated. RESULTS: Primary rat cortical cells were cultured for 7–12 days in Neurobasal medium containing B27 supplements. Addition of glutamate (200 μM) decreased cell viability to 51.3 ± 0.7% compared to control. Treatment with the noncompetitive NMDAR antagonist, MK-801, and the NOS inhibitor, L-NAME, completely prevented cell death. Pretreatment (24 hrs) with 17β-E(2 )and Δ(8), 17β-E(2 )(0.01 to 10 μM) significantly reduced cell death. 17β-E(2 )was more potent than Δ(8), 17β-E(2). Glutamate caused a rapid 2.5 fold increase in [Ca(2+)](i). Treatment with 0.001 to 10 μM MK-801 reduced the initial Ca(2+ )influx by 14–41% and increased cell viability significantly. Pretreatment with 17β-E(2 )and Δ(8), 17β-E(2 )had no effect on Ca(2+ )influx but protected the cortical cells against glutamate-induced cell death. CONCLUSION: Glutamate-induced cell death in cortical cultures can occur through NMDAR and NOS-linked mechanisms by increasing nitric oxide and ONOO(-). Equine estrogens: 17β-E(2 )and Δ(8), 17β-E(2), significantly protected cortical cells against glutamate-induced excitotoxicity by a mechanism that appears to be independent of Ca(2+ )influx. To our knowledge, this is a first such observation. Whether the decrease in NOS related products such as ONOO(-), is a mechanism by which estrogens protect against glutamate toxicity, remains to be investigated. Estrogen replacement therapy in healthy and young postmenopausal women may protect against neurodegenerative diseases by these mechanisms.