2.1. Glutamate-glutamine cycle

Although astrocyte glutamine synthetase has the ability to remove ammonia, this is not the major function of this enzyme in the brain. It plays a key role in the glutamate-glutamine cycle (which is also called “glutamine-glutamate/GABA cycle” since GABA is produced by decarboxylation of glutamate). Glutamate, the most abundant excitatory neurotransmitter of the human brain, needs to be removed rapidly from the synaptic cleft by this cycle, when released from the pre-synapse after stimulation, to prevent postsynaptic over-excitation, which might result in cell death. In a first step, glutamate is taken up rapidly by astrocytes via excitatory amino acids transporters (EAAT) 1-3. EAATs are sodium-dependent and thus rely on a co-transport of glutamate and sodium. In other words, the sodium gradient is the driving force of this transport and needs to be continuously regenerated by energy-dependent Na+/K+-ATPase. In astrocytes, 1 mol glutamate is converted to 1 mol glutamine by use of 1 mol ATP and 1 mol ammonia. Glutamine is then transported back to neurons via amino acid transporter systems N and L (astrocytes) and system A (neurons). Systems N and A are also sodium-dependent and thus also depend on the proper function of Na+/K+-ATPase. In presynaptic neurons, ammonia is released from glutamine by phosphate-activated glutaminase. Glutamate is then stored in synaptic vesicles and can be released again to the synaptic cleft. Released ammonia can be recycled by astrocytes and can be used for the amidation of glutamate by glutamine synthetase thus forming glutamine.

This cycle is the key mechanism for control of glutamatergic neurotransmission in the human brain. By this mechanism the steep gradient between high intracellular glutamate concentration (up to 12 mmol/L) in neurons and low glutamate concentration in the synaptic cleft (1-3 µmol/L) can be maintained. Furthermore, this cycle is important for neuronal energy metabolism. Glutamate (and GABA) are synthesized de novo in the glutamatergic neurons by the use of 2-oxoglutarate causing a constant drain of intermediates of the tricarboxylic acid cycle. This is a cataplerotic mechanism (catapleroism = reactions using TCA cycle intermediates and thus limiting the flux through the TCA cycle) which would cause energy impairment and even cell death if it is not compensated. Neurons are metabolically handicapped in that they have a low pyruvate carboxylase activity. Pyruvate carboxylase forms oxaloacetate from pyruvate after glycolytic breakdown of glucose. This is the most important anaplerotic mechanism (anapleroism = reactions forming TCA cycle intermediates). However, since neurons have a low pyruvate carboxylase activity, they are not capable to completely restore the loss of 2-oxoglutarate induced by glutamate de novo synthesis. Therefore, the glutamate-glutamine cycle should be seen as an important bio-energetic and metabolic coupling between astrocytes and neurons which allows the bi-directional transfer of carbon and nitrogen units between these cells.