Glial cells have heart: rH1 Na+ channel mRNA and protein in spinal cord astrocytes

Astrocytes in vitro express several distinct voltage-sensitive sodium currents, including tetrodotoxin (TTX)-resistant in non-stellate astrocytes and TTX-sensitive currents in stellate astrocytes. However, the molecular identity of the underlying channels, and the mechanisms that regulate their expression, have yet to be identified. Since spinal cord astrocytes in vitro express sodium currents that are nearly ten-fold greater that those of astrocytes derived from other regions, we used reverse transcription polymerase chain reaction (RT-PCR), in situ hybridization, and immunocytochemistry to search for a sodium channel mRNA and protein corresponding to a TTX-resistant channel in these cells. RT-PCR did not detect transcripts for SNS, which is known to encode a TTX-resistant current in dorsal root ganglion neurons. However, RT-PCR demonstrated the presence of rH1 mRNA in cultured spinal cord astrocytes derived from postnatal day 0 (P0) Sprague Dawley rats at 7 days in vitro and in also intact spinal cords of P0 and P7 rats. Hybridization signal for rH1 mRNA was detected by in situ hybridization cytochemistry in most non-stellate and, at varying levels, in stellate astrocytes in these cultures. Immunocytochemical studies, utilizing a polyclonal antibody (R-12) generated against a conserved polypeptide sequence of sodium channels, demonstrated sodium channel immunoreactivity in non-stellate and stellate astrocytes in these cultures. Spinal cord cultures reacted with a rH1-specific polyclonal antibody also showed rH1 immunostaining in non-stellate and stellate astrocytes, although the intensity of the rH1 immunoreactivity in both astrocyte morphologies was attenuated compared to that observed with the R-12 generic sodium channel antibody. The presence of rH1 mRNA and protein in non-stellate astrocytes in vitro provides a possible correlate for the TTX-resistant current that has been recorded in these cells. Since TTX-resistant current is not present in stellate astrocytes, the presence of rH1 mRNA and protein in these cells suggests, in addition, that post-translational mechanisms participate in the control of sodium channel expression in these cells.