The effects of estradiol on estrogen receptor and glutamate transporter expression in organotypic hippocampal cultures exposed to oxygen--glucose deprivation.

The molecular basis of estrogen-mediated neuroprotection against brain ischemia remains unclear. In the present study, we investigated changes in expression of estrogen receptors (ERs) alpha and beta and excitatory amino acid transporters (EAAT) 1 and 2 in rat organotypic hippocampal slice cultures treated with estradiol and subsequently exposed to oxygen--glucose deprivation (OGD). Pretreatment with 17beta-estradiol (10 nM) for 7 days protected the CA1 area of hippocampus against OGD (60 min), reducing cellular injury by 46% compared to the vehicle control group. Levels of ERalpha protein were significantly reduced by 20% after OGD in both vehicle- and estradiol-treated cultures, whereas ERbeta was significantly up-regulated by 25% in the estradiol-treated cultures. In contrast, EAAT1 and EAAT2 levels were unchanged in response to estradiol treatment in this model of OGD. These findings suggest that estrogen-induced neuroprotection against ischemia might involve regulation of ERbeta and, consequently, of the genes influenced by this receptor.

Hypoxic preconditioning in neonatal rat brain involves regulation of excitatory amino acid transporter 2 and estrogen receptor alpha.

Exposure of the brain to a sublethal insult can protect against a subsequent brain injury. Hypoxic preconditioning induces tolerance to hypoxic--ischemic injury in neonatal rat brain and is associated with changes in gene and protein expression. To study the involvement of excitatory amino acid transporters (EAAT1 and EAAT2) and estrogen receptors (ERalpha and ERbeta) in neonatal hypoxia--induced ischemic tolerance, we examined changes in expression of these proteins in the cortex, hippocampus and striatum of newborn rats at different time points after exposure to sublethal hypoxia (8% O(2), 3h). Preconditioning with hypoxia 24h before hypoxia-ischemia afforded marked brain protection compared with littermate control animals as determined by morphological assessment. Immunoblot analysis showed that EAAT2 and ERalpha were significantly increased by 55% and 49%, respectively, in cortex at 24h after hypoxic-preconditioning. Surprisingly, at the same time point, a significant decrease of EAAT2 by 48% in striatum was observed. In contrast, hypoxic preconditioning had no effect on the levels of EAAT1 and ERbeta in any of the brain regions studied at any of the time points analyzed. The similar pattern of changes in EAAT2 and ERalpha levels suggests that ERalpha might interact with EAAT2 in producing preconditioning. The endogenous molecular mechanisms modulated by hypoxia preconditioning may contribute to the development of hypoxia-induced ischemic tolerance, and may provide novel therapeutic targets for the treatment of cerebral ischemia.