ABSTRACT
This chapter discusses glial metabolism and considers the locations and functions of glutamate, aspartate, and gamma aminobutryic acid (GABA) in the retina. Biopolars carry visual information centripetally through the tissue from photoreceptor to the ganglion cells, whose axons form the optic nerve, whereas horizontal and amacrine intemeurons mediate modulating, lateral interactions in the outer and inner synaptic layers, respectively. Photoreceptors transduce light to nervous energy in their outer limbs. In line with the diverse locations in which GABA is found, functions are numerous. In lower-order species, where it is employed by horizontal cells, it provides feedback modulation of photoreceptor activity, thereby heightening contrast features in the visual image, and potentially is involved in color processing. Since glutamine is continuously being formed from other amino acids that enter the extracellular space and then are cleared by the glia, and since it can also enter the central nervous system from the blood-stream, it is, potentially, a ubiquitous substrate.
