Glutamate metabolic pathways and retinal function.
Glutamate is a major neurotransmitter in the CNS but is also a key metabolite intimately coupled to amino acid production/degradation. We consider the effect of inhibition of two key glutamate metabolic enzymes: glutamine synthetase (GS) and aspartate aminotransferase on retinal function assessed using the electroretinogram to consider photoreceptoral (a-wave) and post-receptoral (b-wave) amplitudes. Quantitative immunocytochemistry was used to assess amino acid levels within photoreceptors, ganglion and Müller cells secondary to GS inhibition. Intravitreal injections of methionine sulfoximine reduced GS immunoreactivity in the rat retina. Additionally, glutamate and its precursor aspartate was reduced in photoreceptors and ganglion cells, but elevated in Müller cells. This reduction in neuronal glutamate was consistent with a deficit in neurotransmission (-75% b-wave reduction). Exogenous glutamine supply completely restored the b-wave, whereas other amino acid substrates (lactate, pyruvate, alpha-ketoglutarate, and succinate) only partially restored the b-wave (16-20%). Inhibition of the aminotranferases using aminooxyacetic acid had no effect on retinal function. However, aminooxyacetic acid application after methionine sulfoximine further reduced the b-wave (from -75% to -92%). The above data suggest that de novo glutamate synthesis involving aspartate aminotransferase can partially sustain neurotransmission when glutamate recycling is impaired. We also show that altered glutamate homeostasis results in a greater change in amino acid distribution in ganglion cells compared with photoreceptors.
Bui
Bang V
BV
Department of Optometry and Vision Sciences, University of Melbourne, Parkville, Victoria, Australia. bvb@unimelb.edu.au
Hu
Rebecca G
RG
Acosta
Monica L
ML
Donaldson
Paul
P
Vingrys
Algis J
AJ
Kalloniatis
Michael
M
D000818:Animals; D001219:Aspartate Aminotransferases / Q000037:antagonists & inhibitors / Q000378:metabolism; D004596:Electroretinography; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005974:Glutamate-Ammonia Ligase / Q000037:antagonists & inhibitors / Q000378:metabolism; D018698:Glutamic Acid / Q000096:biosynthesis* / Q000378:metabolism*; D005973:Glutamine / Q000378:metabolism*; D006706:Homeostasis / Q000502:physiology; D007150:Immunohistochemistry; D007656:Ketoglutaric Acids / Q000378:metabolism; D019344:Lactic Acid / Q000378:metabolism; D008717:Methionine Sulfoximine / Q000031:analogs & derivatives / Q000494:pharmacology; D020419:Photoreceptor Cells, Vertebrate / Q000378:metabolism*; D019289:Pyruvic Acid / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D019802:Succinic Acid / Q000378:metabolism; D009435:Synaptic Transmission / Q000502:physiology; D014785:Vision, Ocular / Q000502:physiology*
D004791:Enzyme Inhibitors; D007656:Ketoglutaric Acids; D005973:Glutamine; C054437:methionine sulfoximine phosphate; D008717:Methionine Sulfoximine; D019344:Lactic Acid; D018698:Glutamic Acid; D019289:Pyruvic Acid; D019802:Succinic Acid; D001219:Aspartate Aminotransferases; D005974:Glutamate-Ammonia Ligase