Endocrine glia: roles of glial cells in the brain actions of steroid and 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 tissue modulates internal homeostasis, not only by electrical signaling, but also by regulating the release of endocrine messengers. However, the mechanisms underlying these processes are not fully understood. Recent evidence indicates that glia may play a significant role in the link between the endocrine and nervous systems. Glial cells express nuclear receptors for both thyroid and steroid hormones and participate in the metabolism of these hormones, resulting in the production of neuroactive metabolites. Furthermore, glial cells synthesize endogenous neuroactive steroids, including pregnenolone and progesterone, from cholesterol. Thyroid hormones, glucocorticoids, gonadal steroids, and neurosteroids affect myelinization by acting on oligodendroglia and modulate astroglia morphology, differentiation, and gene expression in different brain areas. Under physiological conditions, hormonal effects on glia may have important consequences for neuronal development, metabolism, and activity and for the formation and plasticity of synaptic connections. In addition, glucocorticoids, gonadal steroids, and neurosteroids may affect regenerative processes in neurons by modulating glial responses after injury. These effects include the activation of microglia, which is regulated by glucocorticoids, and the proliferation of reactive astroglia, which is regulated by gonadal hormones and neurosteroids. Glial cells are also involved in the modulation of hormone release. Pituicytes and microglia in the neurohypophysis may influence hormonal secretion by regulating neurovascular contacts, while astroglia in the hypothalamus regulate the number of synaptic inputs to specific neuronal populations involved in pituitary hormone release, such as LHRH and oxytocinergic neurons. Furthermore, tanycytes and astrocytes in the arcuate nucleus and median eminence release trophic factors that regulate hormone secretion by hypothalamic neurons.