Retrieval of glutamate from extracellular sites in the retina
involves at least five excitatory amino acid transporters.
Immunocytochemical analysis of the cat retina indicates that each of
these transporters exhibits a selective distribution which may reflect
its specific function. The uptake of glutamate into Müller cells
or astrocytes appears to depend upon GLAST and EAAT4, respectively.
Staining for EAAT4 was also seen in the pigment epithelium. The
remaining transporters are neuronal with GLT-1α localized to a
number of cone bipolar, amacrine, and ganglion cells and GLT-1v in cone
photoreceptors and several populations of bipolar cells. The EAAC1
transporter was found in horizontal, amacrine, and ganglion cells.
Staining for EAAT5 was seen in the axon terminals of both rod and cone
photoreceptors as well as in numerous amacrine and ganglion cells.
Although some of the glutamate transporter molecules are positioned for
presynaptic or postsynaptic uptake at glutamatergic synapses, others
with localizations more distant from such contacts may serve in
modulatory roles or provide protection against excitoxic or oxidative
damage.