CNTF and CNTF receptor alpha are constitutively expressed by astrocytes in the mouse brain

Ciliary neurotrophic factor (CNTF) is regarded as one of the signals that lead to astrocyte activation following central nervous system lesion. However, it is not clear if CNTF can directly initiate astrocytic responses to injury because CNTF levels are very low in most areas of the unlesioned brain and CNTF receptor (CNTFRalpha) expression by astrocytes has not yet been demonstrated in the intact brain. In the present study, the expression patterns of CNTF protein and CNTFRalpha mRNA were studied in the intact mouse brain using immunocytochemistry and in situ hybridization, respectively. These procedures were combined with immunocytochemistry for glial fibrillary acidic protein in order to identify CNTF- and CNTFRalpha-expressing astrocytes. CNTF-immunoreactive astrocytes were exclusively found in white matter structures such as the optical tract, the corticospinal tract, and the fimbria-fornix. Gray matter astrocytes did not exhibit CNTF immunoreactivity. In contrast, CNTFRalpha mRNA-expressing astrocytes were found in gray matter areas, preferentially in the molecular layers of the cortex and hippocampus. White matter astrocytes did not show a detectable CNTFRalpha mRNA signal. These data demonstrate that both CNTF and its receptor are constitutively expressed by astrocytes in mouse brain. The nonoverlapping locations of astrocytes expressing detectable levels of CNTF and CNTFRalpha, respectively, may be related to distinct postlesional functions of these two glial cell populations.