Aromatase expression by astrocytes after brain injury: Implications for local estrogen formation in brain repair

Recent evidence indicates that 17 beta-estradiol may have neuroprotective a nd neuroregenerative properties. Estradiol is formed locally in neural tiss ue from precursor androgens. The expression of aromatase, the enzyme that c atalyses the conversion of androgens to estrogens, is restricted, under nor mal circumstances, to specific neuronal populations. These neurons are loca ted in brain areas in which local estrogen formation may be involved in neu roendocrine control and in the modulation of reproductive or sex dimorphic behaviours. In this study the distribution of aromatase immunoreactivity ha s been assessed in the brain of mice and rats after a neurotoxic lesion ind uced by the systemic administration of kainic acid. This treatment resulted in the induction of aromatase expression by reactive glia in the hippocamp us and in other brain areas that are affected by kainic acid. The reactive glia were identified as astrocytes by co-localization of aromatase with gli al fibrillary acidic protein and by ultrastructural analysis. No immunoreac tive astrocytes were detected in control animals. The same result, the de n ovo induction of aromatase expression in reactive astrocytes on the hippoca mpus, was observed after a penetrating brain injury. Furthermore, using a ( H2O)-H-3 assay, aromatase activity was found to increase significantly in t he injured hippocampus. These findings indicate that although astrocytes do not normally express ar omatase, the enzyme expression is induced in these glial cells by different forms of brain injury. The results suggest a role for local astroglial est rogen formation in brain repair. (C) 1998 IBRO. Published by Elsevier Scien ce Ltd.