Glutamine synthetase in the central nervous system is not confined to astrocytes.

Glutamine synthetase (GS) immunoreactivity is frequently used as an astroglial 'marker'. However, when sections of adult rat spinal cords were immunostained with antibodies against sheep glutamine synthetase, intense immunofluorescence was observed in cells resembling oligodendrocytes. In white matter in the rat brain GS immunostaining was also found in ovoid oligodendrocyte-like cells, whereas in gray matter in the same tissue sections GS immunostaining was found in astrocytes. Like the antibodies against sheep GS, antibodies against rat GS also immunostained putative oligodendrocytes, and colocalization with 2',3'-cyclic nucleotide-3'-phosphohydrolase in spinal cord supported the designation of the GS-positive cells as oligodendrocytes.