Possible role of the Müller cell in uptake and metabolism of glutamate in the mammalian outer retina.

It is not known how glutamate released synaptically in the outer plexiform layer of the retina is inactivated. Since there is no extracellular enzymatic degradation, glutamate released from photoreceptors is taken up intracellularly by at least one of the three transporters known, and metabolized by a glutamate degrading enzyme. In order to elucidate which of the transporters and enzymes are involved in this process, immuno-electron microscopy was carried out on retinal sections of adult albino rats, applying antiserum against either glutamine synthetase (GS) or L-glutamate-L-aspartate transporter (GLAST). Both stainings revealed immunoreactivity in Müller cells and particularly in their processes tightly ensheathing rod photoreceptor terminals. Thus, although this remains to be tested functionally, transmitter uptake and subsequent degradation at photoreceptor terminals might be preferentially controlled by GLAST and GS expressed in the fine Müller cell processes.