Functional identification of activity‐regulated, high‐affinity glutamine transport in hippocampal neurons inhibited by riluzole

• Published in: Journal of neurochemistry Vol. 142; no. 1; pp. 29 - 40
• Main Author: Erickson, Jeffrey D.
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01.07.2017
• Subjects: activity‐dependent regulation; Affinity; Amino Acid Transport System X-AG - drug effects; Amino acids; Animals; Astrocytes; Astrocytes - drug effects; Astrocytes - metabolism; Axonal transport; beta-Alanine - analogs & derivatives; beta-Alanine - metabolism; Calcium; Calcium (extracellular); Calcium - metabolism; Calcium Channel Blockers - pharmacology; Calcium channels; Calcium channels (voltage-gated); Calcium transport; Central nervous system; Channels; Circuits; Competition; Displays; Enrichment; Excitatory Amino Acid Antagonists - pharmacology; Excitotoxicity; Female; Functional anatomy; glutamate/glutamine cycle; Glutamatergic transmission; Glutamic acid; Glutamine; Glutamine - metabolism; Hippocampus; Hippocampus - cytology; Hippocampus - drug effects; Hippocampus - metabolism; In vitro methods and tests; Inhibition; Inhibitors; Ions; Low concentrations; Mammals; Maturation; Nervous system; Neural networks; neuronal glutamine transporter; Neurons; Neurons - drug effects; Neurons - metabolism; neuroprotection; Neuroprotective Agents - pharmacology; Neurosciences; neurotransmitter cycling; Physiology; Potassium - pharmacology; Pregnancy; Rats; Rats, Sprague-Dawley; Riluzole - pharmacology; Sodium channels (voltage-gated); Tetrodotoxin; Transport; γ-Aminobutyric acid; neurotransmitter cycling; neuronal glutamine transporter; glutamate/glutamine cycle; excitotoxicity; activity-dependent regulation; neuroprotection
• ISSN: 0022-3042; 1471-4159
• DOI: 10.1111/jnc.14046

Glutamine (Gln) is considered the preferred precursor for the neurotransmitter pool of glutamate (Glu), the major excitatory transmitter in the mammalian CNS. Here, an activity‐regulated, high‐affinity Gln transport system is described in developing and mature neuron‐enriched hippocampal cultures that is potently inhibited by riluzole (IC50 1.3 ± 0.5 μM), an anti‐glutamatergic drug, and is blocked by low concentrations of 2‐(methylamino)isobutyrate (MeAIB), a system A transport inhibitor. K+‐stimulated MeAIB transport displays an affinity (Km) for MeAIB of 37 ± 1.2 μM, saturates at ~ 200 μM, is dependent on extracellular Ca2+, and is blocked by inhibition of voltage‐gated Ca2+ channels. Spontaneous MeAIB transport is also dependent on extracellullar Ca2+ and voltage‐gated calcium channels, but is also blocked by the Na+ channel blocker tetrodotoxin, by Glu receptor antagonists, and by GABA indicating its dependence on intact neural circuits driven by endogenous glutamatergic activity. The transport of MeAIB itself does not rely on Ca2+, but on Na+ ions, and is pH sensitive. Activity‐regulated, riluzole‐sensitive spontaneous and K+‐stimulated transport is minimal at 7–8 days in vitro, coordinately induced during the next 2 weeks and is maximally expressed by days in vitro > 20; the known period for maturation of the Glu/Gln cycle and regulated pre‐synaptic Glu release. Competition analyses with various amino acids indicate that Gln is the most likely physiological substrate. Activity‐regulated Gln/MeAIB transport is not observed in astrocytes. The functional identification of activity‐regulated, high‐affinity, riluzole‐sensitive Gln/MeAIB transport in hippocampal neurons may have important ramifications in the neurobiology of activity‐stimulated pre‐synaptic Glu release, the Glu/Gln cycle between astrocytes and neurons, and neuronal Glu‐induced excitotoxicity. Cover Image for this issue: doi: 10.1111/jnc.13805. This report describes a Ca2+‐regulated ‘system A’ glutamine transport system in hippocampal neuron‐enriched primary cultures that is dependent on neural activity in mature synapses, potently inhibited by riluzole (a blocker of synaptic glutamate release), and that is up‐regulated during the critical postnatal period of functional maturation of the glutamate/glutamine cycle between astrocytes and neurons and synaptic glutamate release. The novel high‐affinity system A transporter described here may have physiological and pathological implications in understanding the neurobiology of excitotoxic synaptic glutamate release in acute and chronic neurodegenerative diseases. Cover Image for this issue: doi: 10.1111/jnc.13805.