Spinal cord astrocytes in vitro: phenotypic diversity and sodium channel immunoreactivity

The expression of sodium channels in morphologically and antigenically distinct astrocytes derived from neonatal rat spinal cords was examined at various times in culture. During the course of this study [2-40 days in vitro (DIV)], nine morphologies of glial fibrillary acidic protein (GFAP)+ cells were distinguished: 1a) flat, fibroblast-like; 1b) elongated, with generally few, short processes; 1c) triangular soma with three short, stubby processes; 1d) bipolar with long, slender processes; 1e) bipolar with broad, flared processes; 1f) stellate with radially oriented slender processes extending from a small to moderate-sized soma; 1g) multiple short, stubby processes extending from a moderate-sized soma; 1h) flat, roundish shape with either a smooth edge ("pancake"-like) or numerous very short processes; and 1i) broad, elongated cell body with orthogonally oriented short, spike-like processes. Not all cell types were present at all times in culture. Each type of astrocyte displayed sodium channel immunoreactivity at some time in culture; however, different types of astrocytes exhibited different patterns, over time, of sodium channel staining. Sodium channel immunoreactivity in all astrocyte types was reduced to low levels by 14 DIV, and was not detectable at 40 DIV. Except for types 1b and 1e, A2B5 staining was present on all astrocyte morphologies at some time in culture, and was generally attenuated with longer times in vitro; in contrast to cultures derived from neonatal rat optic nerve, A2B5 staining does not distinguish unequivocally between the various classes of morphologically different astrocytes derived from spinal cord. O4 immunoreactivity was consistently observed only on bipolar, elongated, and process-bearing astrocytes, though not all process-bearing astrocytes were O4+. These results demonstrate that astrocytes derived from neonatal spinal cord are morphologically and antigenically heterogeneous. Moreover, while spinal cord astrocytes express sodium channels, these astrocytes exhibit a time-course of channel expression that is different from astrocytes derived from several other CNS regions where sodium channel staining is maintained even for extended times in culture, suggesting a regional modulation of astrocyte function.