Müller cell localisation of glutamine synthetase in rat retina.

Abstract

IN retina as in brain, the metabolism of glutamic acid and γ-aminobutyric acid (GABA) is compartmentalised in biochemically defined pools1–4. When retinas are incubated in vitro with radioactive glutamate and GABA, the specific activity of the glutamine produced is greater than the specific activity of the total tissue glutamate. This indicates that glutamine is formed from a small glutamate pool of higher specific activity than the total glutamate pool. To understand better the metabolic organisation of these amino acids in retina, the anatomic localisation of the small glutamate pool is essential. There is evidence that Muller cells (retinal glia) may be the site of the small glutamate pool. Autoradiographic studies indicate that mammalian Muller cells take up GABA and glutamate5–8. Moreover, these cells contain succinic semialdehyde dehydrogenase9 and GABA-transaminase10 which are enzymes involved in the degradative metabolism of GABA. In a number of species, glutamine synthetase (GS), a key enzyme of the small glutamate pool11, shows a striking increase in activity12–14 which parallels the morphological development of the Muller cell15–17. We describe here the localisation of GS to Muller cells in rat retina by immunohistochemical techniques.