7 A number of in vitro and in vivo studies demonstrate the primary importance of astrocytic glutamate uptake in preventing glutamate-induced exciloloxicily.20-23 A good example is provided by the phenotypical changes displayed byknockout mice for the various glutamate transporters. Indeed, knockout mice for GLT-1, considered the main astrocytic glutamate transporter, suffer lethal Inhibitors,research,lifescience,medical spontaneous

seizures and selective hippocampal neuronal degeneration,24 whereas knockout mice for the neuronal EAAC1 display no apparent neurodegeneration.25 Interestingly, beta-lactam antibiotics have been shown to upregulate the expression of GLT-1 and to prevent neuronal loss both in vitro and in vivo in models involving excitotoxicity.26 This suggests that modulation of the glutamate uptake capacity of astrocytes may be achievable in vivo with classical Inhibitors,research,lifescience,medical pharmacological tools, thus representing a promising therapeutic target for pathologies involving excitotoxicity. Astrocytes also play a central role in the transfer of glutamate back to neurons following its uptake at the synapse. Failure to do so would result in the rapid depletion Inhibitors,research,lifescience,medical of the glutamate

pool in presynaptic neurons and subsequent disruption of excitatory neurotransmission. This transfer is achieved by the well-described glutamate-glutamine cycle (Figure 2, pink box).27,28 In short, glutamate is converted to glutamine by the astrocytespecific enzyme glutamine synthetase (GS).29 Glutamine is then transferred to neurons in a process most Inhibitors,research,lifescience,medical likely involving the amino acid transport systems N, L, and ASC in astrocytes and system A in neurons.27 Glutamine is then converted back to glutamate via deamination by phosphate-activated glutaminase

which is enriched in the neuronal compartment. The ammonia produced in the process is thought Inhibitors,research,lifescience,medical to be shuttled back to astrocytes following its incorporation into leucine and/or alanine.27 It is important to note that glutamate can be metabolized in a number of different pathways in astrocytes and neurons, including oxidation in the tricarboxylic acid (TCA) cycle.28 Astrocytes are responsible for the replenishment of brain glutamate, GPX6 as they are the only neural cell type selleck inhibitor expressing pyruvate carboxylase, a key enzyme in the main anaplerotic pathway in the brain, effectively allowing them to synthesize glutamate from glucose.30,31 This represents another level of cooperation between astrocytes and neurons. Figure 2. Simplified representation of the main roles of astrocytes in brain homeostasis. Pink box: glutamate-glutamine cycle. Astrocytic excitatory amino acid transporters (EAATs) are responsible for the uptake of a large fraction of glutamate at the synapse. …