Use of NMR spectroscopy for the study of ammonia metabolism in astrocytes and neurons: role of glutamine synthesis in astrocytes

Abstract

The molecular mechanisms of cerebral ammonia toxicity and their relationship to the pathogenesis of hepatic encephalopathy (HE) are still incompletely understood. One hypothesis suggests that toxic effects may be mediated by the metabolism of ammonia to glutamine rather than by ammonia itself. The conversion of glutamate to glutamine by the activity of astrocytic glutamine synthetase (GS) is thought to be the brain’s primary mechanism of ammonia detoxification. Glutamine is taken up by neurons as a substrate for synthesis of the neurotransmitter glutamate. Glutamate, which is released from synaptic vesicles, is taken up by astrocytes again and converted to glutamine. This “glutamine-glutamate cycle” meets the two major requirements for glutamatergic neurotransmission, i.e. rapid removal of abundant glutamate from the synaptic cleft, and the restoration of glutamate to neurons in form of glutamine. Thus, an increase in glutamine synthesis due to ammonia detoxification may affect astrocyte function and the normal metabolic balance between astrocytes and neurons. Nuclear magnetic resonance (NMR) spectroscopy, which provides a satisfactory method of analysing cellular metabolites, high-energy phosphates and the metabolism of glucose in astrocytes and neurons, can be applied to clarify the effects of glutamine on cell-specific metabolism.