Recent years have witnessed a growing appreciation for functional roles astrocytes play within the CNS. It increasingly appears to be the case that astrocytes are integral to brain energy utilization. For example, at glutamate synapses astrocytes take up the glutamate that is released into the synaptic space by the presynaptic neuron. The glutamate is co-imported into the astrocyte along with a sodium cation. The sodium cation, in turn, is removed from the astrocyte by the action of the plasma membrane adenosine triphosphate (ATP)-dependent Na⁺-K⁺ pump. This consumes ATP, which activates astrocyte glycolysis, and, in turn, this stimulates glucose uptake from neighboring capillaries. By consuming more glucose through glycolysis, the astrocyte restores its energy supply but, in the process, also generates lactate. Astrocytegenerated lactate is then exported to the recently activated synapse neurons to help meet its increased energy needs. This relationship defines what is called the “astrocyte-neuron lactate shuttle hypothesis,” and suggests the classic bipartite synapse of a presynaptic and postsynaptic neuron might more accurately be thought of as a tripartite synapse consisting of a presynaptic neuron, postsynaptic neuron, and associated astrocyte. It is also important to note that glutamate absorbed by the synaptic astrocyte is recycled back to the presynaptic neuron. This is accomplished by converting it to glutamine before releasing it into the extracellular space. The presynaptic neuron is able to take up the glutamine, and once it is back inside the neuron, the glutamine is converted back to glutamate.

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