The co-ordinate functioning of neurons and glia is required for glutamate-mediated neurotransmission. In this study, we show by immunocytochemical detection of D-aspartate uptake, that functional glutamate transporters are present in the developing CNS of fetal and neonatal rats, including forebrain, midbrain and hindbrain, at least as early as embryonic day 12 (E12). Use of the transport inhibitor dihydrokainic acid revealed a significant role for GLT-1 in the uptake process. Immunolabelling for the glutamate transporters GLAST, GLT-1α and GLT-1v showed that each of these proteins are expressed early in development and appear to be restricted to glial-like cells throughout the development period examined (except in the retina, where neuronal elements were also labelled). Our capacity to detect very early expression of the variant forms of GLT-1 contrasts with other studies, a feature that we attribute to the use of antigen-recovery techniques that unmask protein epitopes that are otherwise undetectable. These studies illustrate the widespread presence of functional glutamate transporters in the developing CNS, in many cases before the onset of periods of synaptogenesis and indicate that regulation of extracellular glutamate by multiple excitatory amino acid transporters might be crucial in early CNS development.

The conventional view that glucose is the substrate for neuronal energy metabolism has been recently challenged by the “lactate shuttle” hypothesis in which glutamate cycling in glial cells drives all neuronal glucose metabolism. According to this view, glutamate released by activated retinal neurons is transported into Müller (glial) cells where it triggers glycolysis. The lactate released by Müller cells serves as the energy substrate for neuronal metabolism. Because the L-Glutamate/aspartate transporter (GLAST) is the predominant, Na+-dependent, glutamate transporter expressed by Müller cells, we have used GLAST-knockout (GLAST−/−) mice to examine the relationship between lactate release and GLAST activity in the retina. We found that glucose uptake and lactate production by the GLAST−/− mouse retina was similar to that observed in the wild type mouse retina. Furthermore, addition of 1 mM glutamate and NH4Cl to the incubation medium did not further stimulate glucose uptake in either case. When lactate release was measured in the presence of the lactate uptake inhibitor, α-cyano-4-hydroxycinnamate, there was no significant change in the amount of lactate released by retinas from GLAST−/− mice compared to the wild type. Finally, lactate release was similar under both dark and light conditions. These results show that lactate production and release is not altered in retinas of GLAST−/− mice, which suggests that metabolic coupling between photoreceptors and Müller cells is not mediated by the glial glutamate transporter, GLAST.