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Astrocytes in the hippocampus

by Rachel E. Ventura In synaptic transmission, astrocytes can exert an influence through modulation of the volume, composition, and the concentrations of ions, neurotransmitters, neuromodulators, and energy sources within the extracellular space. The involvement of astrocytes in message transmission motivates a reconsideration of the synaptic unit, as possibly including not only the pre- and post- synaptic partners but also neighboring glia.

Astrocytic syncytium: a means of non-synaptic communication

As non-synaptic message transmitters, astrocytes form a network in the neuropil called the astrocytic syncytium. Diffusion of molecules occurs in the syncytium via gap junctions between astrocytes.

Research has found that calcium waves propagate through the syncytium (Cornell-Bell et al., 1990), and that these waves can be induced by mechanical stimulation and by the neurotransmitter glutamate. The calcium waves could result in long-distance modulation of the composition and concentration of molecules in the extracellular space if the flux of calcium into astrocytes throughout the network leads to calcium-sensitive release and uptake of ions and neuromodulators. The astrocytic syncytium allows a non-synaptic means of communication within the brain. Astrocytes can sense and integrate information from a number of synapses simultaneously and can also receive information about the composition of the extracellular space and within the blood vessels. While astrocytes, unlike neurons, are incapable of being potentiated, they do have a capacity for long-range modulation and integration, and may therefore serve an integrative role in brain functioning (see Blomstrand et. al., 1997).

Providing metabolic factors for neurons

One means by which astrocytes are involved in synaptic function is by providing metabolites for neuronal activity. The main energy source of the brain, glucose, enters the central nervous system through astrocytes in contact with blood vessels. The astrocytic processes which contact blood vessels are known as astrocytic end feet.

Evidence suggests that the glucose is partially metabolized by astrocytes and intermediates are released for neuronal use (reviewed in Pfrieger and Barres, 1996; Tsacopoulos and Magistretti, 1996). In the hippocampus, when glial metabolism is blocked by fluorocitrate, a glia-specific inhibitor of the Krebs cycle, synaptic transmission is inhibited (Keyser and Pellmar, 1994; reviewed in Pfrieger and Barres, 1996). This implies that the neurons are dependent on glia for fuel needed for synaptic function. In addition, evidence suggests that the amount of fuel supplied to neurons by the glia depends on the amount of synaptic activity. According to one finding, glutamate stimulates astrocytic uptake of glucose and subsequent release of lactate into the extracellular space (Pellerin and Magistretti, 1994; Pfrieger and Barres, 1996). Thus the neuronal activity both depends on and results in glial release of energy substrates.

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