Neuroprotective and neurotoxic effects of estrogens

The ovarian hormone 17β-estradiol (E2) is neuroprotective in animal models of neurodegenerative diseases. Some studies suggest that the neuroprotective effects of 17β-estradiol are a consequence of its antioxidant activity that depend on the hydroxyl group in the C3 position of the A ring. As in other tissues, 17β-estradiol is metabolized in the brain to 2-hydroxyestradiol (2OHE2) and 2-methoxyestradiol (2MEOHE2). These two molecules present the hydroxyl group in the A ring and have a higher antioxidant activity than 17β-estradiol. To test the hypothesis that conversion to 2-hydroxyestradiol and 2-methoxyestradiol may mediate neuroprotective actions of 17β-estradiol in vivo, we have assessed whether these molecules protect hilar hippocampal neurons from kainic acid toxicity. Ovariectomized Wistar rats received an i.p. injection of 1, 10 or 100 μg 17β-estradiol, 2-hydroxyestradiol or 2-methoxyestradiol followed by an i.p. injection of kainic acid (7 mg/kg) or vehicle. Treatment with kainic acid resulted in a significant loss of hilar neurons. Only the highest dose tested of 17β-estradiol (100 μg/rat) prevented kainic acid-induced neuronal loss. 2-Hydroxyestradiol and 2-methoxyestradiol did not protect hilar neurons from kainic acid, suggesting that the mechanism of neuroprotection by 17β-estradiol in vivo is not mediated by its metabolism to catecholestrogens or methoxycatecholestrogens. Furthermore, 2-methoxyestradiol (100 μg/rat), by itself, resulted in a significant neuronal loss in the hilus that was detected 96 h after the treatment with the steroid. This finding suggests that endogenous metabolism of 17β-estradiol to 2-methoxyestradiol may counterbalance the neuroprotective effects of the hormone.