Electrogenic glutamate uptake is a major current carrier in the membrane of axolotl retinal glial cells

Abstract

Glutamate is taken up avidly by glial cells in the central nervous system. Glutamate uptake may terminate the transmitter action of glutamate released from neurons, and keep extracellular glutamate at concentrations below those which are neurotoxic. We report here that glutamate evokes a large inward current in retinal glial cells which have their membrane potential and intracellular ion concentrations controlled by the whole-cell patch-clamp technique. This current seems to be due to an electrogenic glutamate uptake carrier, which transports at least two sodium ions with every glutamate anion carried into the cell. Glutamate uptake is strongly voltage-dependent, decreasing at depolarized potentials: when fully activated, it contributes almost half of the conductance in the part of the glial cell membrane facing the retinal neurons. The spatial localization, glutamate affinity and magnitude of the uptake are appropriate for terminating the synaptic action of glutamate released from photoreceptors and bipolar cells. These data challenge present explanations of how the b-wave of the electroretinogram is generated, and suggest a mechanism for non-vesicular voltage-dependent release of glutamate from neurons.