Glutamate metabolic pathways and retinal function

Abstract

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, α‐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.

The Neuroprotective Potential of Rho-Kinase Inhibition in Promoting Cell Survival and Reducing Reactive Gliosis in Response to Hypoxia in Isolated Bovine Retina

“…The rapid clearance and recycling of glutamate from the synapses is therefore essential for the normal neuronal functioning. The Müller cells are mainly responsible for the glutamate uptake via high affinity transporters, which rely on a very negative membrane potential [50,51]. However, in retinal ischemia and diabetes, the glutamate uptake into the Müller cells is reduced due to the depolarization of the Müller cells in response to elevated extracellular potassium.…”

Functional and neurochemical development in the normal and degenerating mouse retina

“…Cell classification criteria employed in this study were as in our previous amino acid quantitative studies (e.g., Fletcher andKalloniatis, 1996, 1997a,b;Bui et al, 2004bBui et al, , 2009Sun et al, 2007a,b). Similar to other rodent retinas (Fletcher andKalloniatis, 1996, 1997a,b), the mouse retina displays distinct cellular location during development, identified using pattern recognition of amino acid labeling patterns (Chua, 2008;Kalloniatis et al, 2012).…”

“…The earlier change in amino acid levels in Müller cells suggest that altered amino acid homeostasis is an earlier indicator of impending glial dysfunction. Further, the earlier reduction in glutamine levels in Müller cells (by $P8) also presages the impending changes in neurons, as glutamine is an important precursor for neuronal amino acid production (Erecinska and Silver, 1990;Robin and Kalloniatis, 1992;Kalloniatis and Tomisich, 1999;Bui et al, 2004aBui et al, , 2009.…”

Sildenafil alters retinal function in mouse carriers of Retinitis Pigmentosa

“…The b-wave amplitude was determined as value from the a-wave to maximum peak (Bui et al, 2009;Saszik et al, 2002). Data for each treatment group (n ¼ 6) was compared with the pooled ERG values for all animals pre-treatment (n ¼ 24 for Fig.…”

“…Full-field scotopic ERGs were performed as described previously (Bui et al, 2009;Sun et al, 2007). Mice were dark-adapted overnight then deeply anesthetized with an intraperinatal injection of ketamine (0.75 mg/kg, Parnell Australia, Alexandria, NSW, Australia) and domitor (5 mg/kg, Orion Pharma, Espoo, Finland) diluted in sterile saline.…”