Isoform-specific expression of sodium channels in astrocytes in vitro: immunocytochemical observations

The expression of sodium channel alpha-subunit isoforms in astrocytes cultured from P-0 rat spinal cord and P-7 rat optic nerve was examined utilizing immunocytochemical methods with antibodies generated against conserved and isoform-specific amino acid sequences of the rat brain sodium channel. In spinal cord cultures at 5 days in vitro (DIV), both stellate and flat astrocytes were immunostained with antibody SP20, which recognizes a conserved sequence common to sodium channel types I, II/IIA, and III. Antibody SP11-I, which is directed against a subtype-specific sequence in sodium channel I, did not yield detectable staining in spinal cord astrocytes. Antibody SP11-II, which is directed against a subtype-specific sequence in sodium channel II, immunostained both stellate and flat spinal cord astrocytes, although with less intensity than SP20. Antibody SP32-III, which is directed against a subtype-sequence in sodium channel III, immunostained stellate but not flat spinal cord astrocytes. SP20, SP11-II, and SP32-III staining persisted in stellate spinal cord astrocytes through 14-21 DIV, while SP20 and SP11-II immunostaining in flat spinal cord astrocytes was attenuated with time in culture. In optic nerve cultures at 5 DIV, SP20 staining was present in both stellate and flat astrocytes, but at reduced levels compared to spinal cord astrocytes. With increased time in culture SP20 staining was maintained in stellate optic nerve astrocytes but was gradually lost in flat optic nerve astrocytes. Stellate optic nerve astrocytes exhibited low levels of staining with SP11-I, SP11-II, and SP32-III. Flat optic nerve astrocytes lacked or displayed very low SP11-II staining, and SP11-I and SP32-III staining was not detectable. These observations demonstrate that cultures astrocytes are immunoreactive to antibodies generated against conserved and isotype-specific peptide sequences of rat brain sodium channels, and further suggest that there are different patterns of sodium channel expression between flat vs. stellate astrocytes and in astrocytes derived from different regions of the CNS.