Ciliary neurotrophic factor stimulates astroglial hypertrophy in vivo and in vitro

After insult or trauma, astrocytes become activated and endeavor to restore the brain's delicately balanced microenvironment. An index of their activated state is that they become enlarged or hypertrophic. Ciliary neurotrophic factor (CNTF), a member of the alpha helical family of cytokines, is synthesized by astrocytes and is generally regarded to be an autocrine and paracrine injury signal. To determine whether CNTF might be an endogenous signal that stimulates astrocyte hypertrophy in vivo, we intracerebrally injected 200 ng of recombinant human CNTF into the adult rat neocortex. To study the astrocytes their cytosol was stained with antibodies against S100beta and their nuclei were stained with propidium iodide (PI). Fluorescent images of astrocytic nuclei and somas were acquired using a confocal laser-scanning microscope and their areas were measured using the NIH image software. Within 24 h of treatment, CNTF induced a volume increase of the somas and nuclei of protoplasmic and fibrous astrocytes in vivo, and this effect persisted for at least 48 h. To determine whether CNTF activates astrocytes directly, glial cultures were treated with CNTF (10 ng/ml) and were evaluated by measuring the area of PI stained nuclei. CNTF stimulation increased the size of both polygonal and process-bearing astroglia. Since our studies in vivo have shown that CNTF induces other key aspects of gliosis (S. W. Levison et al., 1996; Exp. Neurol. 141, 256), we conclude that CNTF is a powerful activator of astrocytes and that it is likely responsible for the persistent glial hypertrophy observed following injuries and diseases of the CNS.