Glutamate metabolic pathways in displaced ganglion cells of the chicken retina.
Glutamate (E) is the putative amino acid neurotransmitter used by ganglion cells, photoreceptors, and bipolar cells. Aspartate (D) and glutamine (Q) are potential precursors of glutamate, and glutamate-utilizing neurons may use one or more of these amino acids to sustain production of glutamate. We used post-embedding immunocytochemistry for several amino acid neurotransmitters to characterize the amino acid signatures for displaced ganglion cells of the avian retina. We found two neurochemical signatures for displaced ganglion cells, EQ and EDQ, in mid-peripheral and far-peripheral retina, respectively. Differences in neurochemical signatures cannot be explained by the existence of two ganglion cell populations, and we propose that the two signature categories for the large-diameter displaced ganglion cells reflect variations in the aspartate precursor pool. The transamination reaction involved in glutamate production, aspartate/oxaloacetate and alpha-ketoglutarate/glutamate, requires an active TCA cycle, since the carbon skeleton of glutamate is derived from alpha-ketoglutarate, a TCA intermediary. We hypothesized that aspartate levels vary in the normal chicken retina because eccentricity-dependent differences in oxygen availability result in changes of alpha-ketoglutarate levels, and hence, alterations in the equilibrium of the transamination reaction. We tested this hypothesis by incubating isolated chicken retinas in anaerobic conditions and found elevated aspartate immunoreactivity in subpopulations of glutamate-utilizing neurons in the central retina. Under aerobic conditions, or in retinas placed directly into fixative, retinal samples from the central edge of the pecten did not show differential cellular staining for aspartate. We have, therefore, identified differences in neurochemical signatures for retinal neurons involving changes in active maintenance of precursor pools.
Kalloniatis
M
M
Department of Optometry and Vision Sciences, University of Melbourne, Parkville, Victoria, Australia.
Napper
G A
GA
D000332:Aerobiosis / Q000502:physiology; D000693:Anaerobiosis / Q000502:physiology; D000818:Animals; D001224:Aspartic Acid / Q000378:metabolism; D048429:Cell Size; D002645:Chickens / Q000378:metabolism*; D018698:Glutamic Acid / Q000378:metabolism*; D005973:Glutamine / Q000378:metabolism; D007091:Image Processing, Computer-Assisted; D007150:Immunohistochemistry; D012165:Retinal Ganglion Cells / Q000378:metabolism* / Q000648:ultrastructure; D016707:Tissue Fixation; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
D005973:Glutamine; D001224:Aspartic Acid; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid