Glutamate metabolic pathways in displaced ganglion cells of the chicken retina

Abstract

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.

Mapping photoreceptor and postreceptoral labelling patterns using a channel permeable probe (agmatine) during development in the normal and RCS rat retina

“…The tissue collection, isolation, and retinal incubation procedures are as outlined in earlier studies but is briefly outlined below (Fletcher & Kalloniatis, 1996, 1997aKalloniatis & Napper, 1996;Napper et al, 1999). The tissue collection, isolation, and retinal incubation procedures are as outlined in earlier studies but is briefly outlined below (Fletcher & Kalloniatis, 1996, 1997aKalloniatis & Napper, 1996;Napper et al, 1999).…”

“…The modified Ames0Nesbett medium contained 6 mM agmatine (1-amino-4-guanidobutane), substituted for an equimolar concentration of sodium chloride. The tissue processing and postembedding immunocytochemical procedures used for this study, and antibody specificity, have been described previously (Marc et al, 1990(Marc et al, , 1995Kalloniatis & Fletcher, 1993;Fletcher & Kalloniatis, 1996, 1997aKalloniatis & Napper, 1996;Marc, 1999a,b). The data reported here are all taken from retinas that had been incubated for 30 min.…”

Glutamate metabolic pathways and retinal function

“…Indeed, anaplerotic reactions involving amino acids such as glutamate, aspartate, and alanine are important to replenish carbon skeletons by producing the TCA cycle intermediates α‐ketoglutarate, oxaloacetate, and pyruvate, via reactions catalyzed by the aminotransferase group of enzymes (Ereciñska and Silver 1990). These pathways appear to be important during metabolic stress, where amino acid carbon skeletons are seconded to maintain metabolism (Kalloniatis and Napper 1996; Zeevalk et al. 1998; Acosta and Kalloniatis 2005), at the expense of neurotransmission (Bui et al.…”

Retinal amino acid neurochemistry in health and disease

“…Virtually all retinal cell types showed reduced glutamate and glutamine and amacrine cells showed reduced GABA and glycine and a concurrent increase of aspartate . Aspartate levels are also influenced by physiological changes in retinal metabolic activity and can be dramatically altered in more extreme insult, when the retina is incubated under anoxic conditions …”

“…147 Aspartate levels are also influenced by physiological changes in retinal metabolic activity and can be dramatically altered in more extreme insult, when the retina is incubated under anoxic conditions. 148 Preventing glutamine production through enzymatic inhibition of glutamine synthetase resulted in markedly altered amino acid levels and retinal dysfunction. 101,149,150 The provision of glutamine completely restored function secondary to glutamine synthetase enzyme inhibition, 149,150 whereas the provision of other substrates such as a-ketoglutarate, succinate, lactate or pyruvate only partially restored function (16 to 20 per cent).…”