Amino acid neurochemistry of the vertebrate retina

The dominant neurochemicals involved in encoding sensory information are th e amino acid neurotransmitters, glutamate, gamma-aminobutyrate (GABA) and g lycine, which mediate fast point-to-point synaptic transmission in the reti na and other parts of the central nervous system. The relative abundance of these neurochemicals and the existence of neuronal and glial uptake mechan isms as well as a plethora of receptors support the key role these neuroche micals play in shaping neural information. However, in addition to subservi ng neurotransmitter roles, amino acids subserve normal metabolic/cellular f unctions, may be precursors for other amino acids, and may also be associat ed with protein synthesis. Post-embedding immunocytochemistry of small mole cules has allowed the characterization of multiple amino acid profiles with in subpopulations of neurons in the vertebrate retina. The general theme em erging from these studies is that the retinal through pathway uses glutamat e as its neurotransmitter, and the lateral elements, GABA and/or glycine. C o-localization studies using quantitative immunocytochemistry have shown th at virtually all neuronal space can be accounted for by the three dominant amino acids. In addition, co-localization studies have demonstrated that th ere are no purely aspartate, glutamine, alanine, leucine or ornithine immun oreactive neurons and thus these amino acids are likely to act as metabolit es and may sustain glutamate production through a multitude of enzymatic pa thways. The mapping of multiple cellular metabolic profiles during developm ent or in degenerating retinas has shown that amino acid neurochemistry is a sensitive marker for metabolic activity. In the degenerating retina, (RCS retina), neurochemical anomalies were evident early in development (from b irth), even before photoreceptors mature at PND6-8 implying a generalized m etabolic dysfunction. Identification of metabolic anomalies within subpopul ation of neurons is now possible and can be used to investigate a multitude of retinal functions including amino acid metabolic and neurochemical chan ges secondary to external insult as well as to expand our understanding of the intricate interrelationship between neurons and glia. (C) 1998 Elsevier Science Ltd. All rights reserved.