Role of astroglia in estrogen regulation of synaptic plasticity and brain repair

Astroglia are targets for estrogen and testosterone and are apparently invo lved in the action of sex steroids on the brain. Sex hormones induce change s in the expression of glial fibrillary acidic protein, the growth of astro cytic processes, and the degree of apposition of astroglial processes to ne uronal membranes in the rat hypothalamus. These changes are linked to modif ications in the number of synaptic inputs to hypothalamic neurons. These fi ndings suggest that astrocytes may participate in the genesis of androgen-i nduced sex differences in synaptic connectivity and in estrogen-induced syn aptic plasticity in the adult brain. Astrocytes and tanycytes may also part icipate in the cellular effects of sex steroids by releasing neuroactive su bstances and by regulating the local accumulation of specific growth factor s, such as insulin-like growth factor-I, that are involved in estrogen-indu ced synaptic plasticity and estrogen-mediated neuroendocrine control. Astro glia may also be involved in regenerative and neuroprotective effects of se x steroids, since astroglia formation after brain injury or after periphera l nerve axotomy is regulated by sex hormones. Furthermore, the expression o f aromatase, the enzyme that produces estrogen, is induced de novo in astro cytes in lesioned brain areas of adult male and female rodents. Since astro glia do not express aromatase under normal circumstances, the induction of this enzyme may be part of the program of glial activation to cope with the new conditions of the neural tissue after injury. Given the neuroprotectiv e and growth-promoting effects of estrogen after injury, the local producti on of this steroid may be a relevant component of the reparative process. ( C) 1999 John Wiley & Sons, Inc.