The ontogeny and localization of glutamine synthetase gene expression in rat brain.

A 2.4 kb cDNA clone containing the coding sequence for glutamine synthetase (GS) was isolated from a rat brain cDNA library, and a probe constructed from this cDNA was utilized in Northern analysis of total RNA to study the tissue distribution and the ontogeny of GS mRNA expression in the rat brain from embryonic day 14 (E14) to adulthood. The levels of GS mRNA were highest in the brain, followed by kidney and liver. In the brain, the GS message was detected as early as E14, earlier than it can be detected by either enzymatic assays of GS activity or by immunocytochemical localization of GS. The relatively low levels of GS mRNA seen at E14 increase to a peak around the time of birth, and in the second postnatal week another rise in GS message occurs approaching adult levels by P15. Localization of GS to astrocytes in the brain was confirmed using both immunocytochemistry and in situ hybridization.