COINCIDENCE OF L-GLUTAMATE L-ASPARTATE TRANSPORTER (GLAST) AND GLUTAMINE-SYNTHETASE (GS) IMMUNOREACTIONS IN RETINAL GLIA - EVIDENCE FOR COUPLING OF GLAST AND GS IN TRANSMITTER CLEARANCE

Our aim was to identify proteins that mediate the uptake and degradati on of synaptically released glutamate, focusing on the rat retina with its well-defined glutamatergic pathways, Immunoreactivity against the L-glutamate/L-aspartate transporter (GLAST) is present in Muller cell s, Ultrastructurally, even the finest glial processes, particularly th ose ensheathing identified structures of glutamatergic transmission (r od spherules), are immunoreactive for GLAST, Further light and electro n microscopic observations revealed that also retinal astrocytes and p igment epithelial cells are immunoreactive for GLAST, No neuronal or m icroglial staining was observed, This is in line with uptake of exogen ous [H-3]glutamate previously localized specifically in Muller cells a nd pigment epithelium (Ehinger and Falck: Brain Res 33:157-172, 1971), Since endogenous glutamate can only be demonstrated in Muller cells i f glutamine synthetase (GS) is inhibited (Pow and Robinson: Neuroscien ce 60:355-366, 1994), the immunocytochemical localization of GS was de termined, GS immunoreactivity was found in all but only those cell typ es immunoreactive for GLAST, The light and electron microscopic patter ns of immunoreactivity were very similar, particularly in the outer pl exiform layer, The three cell types containing both GS and GLAST (Mull er cells, astrocytes, and retinal pigment epithelium) are related deve lopmentally, In the light of the two references quoted the present dat a indicate that the proteins mediating retinal uptake and degradation of synaptically released glutamate may be GLAST and GS, respectively, and that they may operate in concert to terminate the neurotransmitter action of glutamate. (C) 1995 Wiley-Liss, Inc.