PLASTICITY OF ASTROCYTES

It is becoming apparent that astrocytes carry out a large number of di fferent functions in brain and are able to modify their characteristic s throughout life, that is they exhibit a high degree of plasticity in their phenotype. For example, the morphology of astrocytes changes ma rkedly during neuronal migration, maturation, and degeneration. It is conceivable that these cells must constantly adjust their abilities to meet changes in brain environment. Several examples of astrocytic pla sticity are presented in this review. First, the ability of astrocytes to recognize neuronal signals can change qualitatively as well as qua ntitatively; evidence suggests that the expression of glial receptors may be developmentally regulated by both intrinsic and extrinsic signa ls. Second, the expression of adrenergic receptors by astrocytes in ad ult brain can change in response to neuronal degeneration. The up-regu lation of beta-adrenergic receptors in this case suggests that these r eceptors play a role in function of reactive astrocytes. Finally, glia l morphology can be reciprocally regulated by neurotransmitters such a s norepinephrine and glutamate. This reciprocal regulation may be sign ificant since both beta-adrenergic receptors and glutamate transporter s are found predominantly in astrocytes in the brain. The change in gl ial morphology may also affect neuronal activity by changing the volum e of the extracellular space. (C) 1994 Wiley-Liss, Inc.