Neuronal glutamine utilization: glutamine/glutamate homeostasis in synaptosomes

Area of Science:

• Neurochemistry
• Synaptic Metabolism
• Amino Acid Biochemistry

Background:

• Synaptosomes are crucial for studying neurotransmitter metabolism.
• Glutamine is a key metabolic precursor in the brain.
• Understanding glutamine metabolism under depolarization is vital for neuronal function.

Purpose of the Study:

• To investigate the in vitro effects of depolarization on synaptosomal glutamine metabolism.
• To identify the roles of sodium, inorganic phosphate, and calcium in regulating glutaminase activity.
• To elucidate the physiological regulators of glutamine breakdown in neurons.

Main Methods:

• Utilized [2-15N]glutamine as a precursor to trace metabolic pathways.
• Simulated in vivo depolarization using veratridine and high KCl concentrations.
• Manipulated extracellular and intracellular concentrations of sodium, inorganic phosphate, calcium, and pH.

Main Results:

• Depolarization decreased glutamine uptake but increased glutaminase activity.
• Sodium is essential for glutamine influx, suggesting a Na+-amino acid complex.
• Inorganic phosphate and pH were identified as major regulators of glutaminase activity, with calcium exerting indirect effects.

Conclusions:

• Synaptosomal glutamine uptake is inhibited by depolarization via reduced membrane potential and Na+ gradient.
• Glutaminase activity is enhanced by depolarization, primarily through increased intrasynaptosomal inorganic phosphate.
• Phosphate and H+ ions are the principal physiological regulators of glutaminase, influencing neuronal amino acid metabolism.