ENDOCRINE GLIA - ROLES OF GLIAL-CELLS IN THE BRAIN ACTIONS OF STEROIDAND THYROID-HORMONES AND IN THE REGULATION OF HORMONE-SECRETION

The development and functioning of the nervous system are known to be influenced in various ways by endocrine signals. In turn, neural tissu e modulates internal homeostasis, not only by electrical signaling, bu t also by regulating the release of endocrine messengers. However, the mechanisms underlying these processes are not fully understood. Recen t evidence indicates that glia may play a significant role in the link between the endocrine and nervous systems. Glial cells express nuclea r receptors for both thyroid and steroid hormones and participate in t he metabolism of these hormones, resulting in the production of neuroa ctive metabolites. Furthermore, glial cells synthesize endogenous neur oactive steroids, including pregnenolone and progesterone, from choles terol. Thyroid hormones, glucocorticoids, gonadal steroids, and neuros teroids affect myelinization by acting on oligodendroglia and modulate astroglia morphology, differentiation, and gene expression in differe nt brain areas. Under physiological conditions, hormonal effects on gl ia may have important consequences for neuronal development, metabolis m, and activity and for the formation and plasticity of synaptic conne ctions. In addition, glucocorticoids, gonadal steroids, and neurostero ids may affect regenerative processes in neurons by modulating glial r esponses after injury. These effects include the activation of microgl ia, which is regulated by glucocorticoids, and the proliferation of re active astroglia, which is regulated by gonadal hormones and neuroster oids. Glial cells are also involved in the modulation of hormone relea se. Pituicytes and microglia in the neurohypophysis may influence horm onal secretion by regulating neurovascular contacts, while astroglia i n the hypothalamus regulate the number of synaptic inputs to specific neuronal populations involved in pituitary hormone release, such as LH RH and oxytocinergic neurons. Furthermore, tanycytes and astrocytes in the arcuate nucleus and median eminence release trophic factors that regulate hormone secretion by hypothalamic neurons. (C) 1996 Academic Press, Inc.