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

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 (Ca2+) and generation of free radicals such as peroxynitrite (ONOO-). Estrogen may protect against glutamate-induced cell death by reducing the excitotoxic Ca2+ 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 17beta-estradiol (17beta-E2) and a more potent antioxidant Delta8, 17beta-estradiol (Delta8, 17beta-E2) on cell viability and intracellular Ca2+ ([Ca2+]i), following treatment of rat cortical cells with glutamate, was investigated.

Glutamate-induced cell death in cortical cultures can occur through NMDAR and NOS-linked mechanisms by increasing nitric oxide and ONOO-. Equine estrogens: 17beta-E2 and Delta8, 17beta-E2, significantly protected cortical cells against glutamate-induced excitotoxicity by a mechanism that appears to be independent of Ca2+ 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.

Primary rat cortical cells were cultured for 7-12 days in Neurobasal medium containing B27 supplements. Addition of glutamate (200 microM) 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 17beta-E2 and Delta8, 17beta-E2 (0.01 to 10 microM) significantly reduced cell death. 17beta-E2 was more potent than Delta8, 17beta-E2. Glutamate caused a rapid 2.5 fold increase in [Ca2+]i. Treatment with 0.001 to 10 microM MK-801 reduced the initial Ca2+ influx by 14-41% and increased cell viability significantly. Pretreatment with 17beta-E2 and Delta8, 17beta-E2 had no effect on Ca2+ influx but protected the cortical cells against glutamate-induced cell death.