Calcium Signaling In The Astrocyte Network And Between Astrocytes And Neurons

A seminal observation was made in the early 1990s by Cornell Bell and collaborators, who found that in the presence of glutamate, Ca2+ waves propagate in vitro at a velocity of about 15-20 mm/sec following complex routes of hundreds of micrometers through the astrocyte syncytium. Cornell Bell and collaborators proposed that networks of astrocytes may constitute a long-range signaling system within the brain. Since then, several critical steps involved in intercellular calcium signaling have been identified, and it is likely that gap junction communications play a key role in this process. Nevertheless, alternative mechanisms for Ca2+ wave propagation may involve extracellular messengers, such as ATP, released by stimulated cells. Propagation of calcium signals from astrocytes to neurons has recently been observed in astrocyte and neuron cocultures as well as in situ on acute rat brain slices. Elevation of cytosolic calcium in astrocytes induces glutamate release from neurons, which in turn signals back to neurons. Although different from neuronal excitability, astrocytes do indeed exhibit a form of excitability mediated by intracellular calcium spikes and oscillations as well as intercellular calcium waves that propagate through the glial syncytium in response to firing of glutamatergic neurons. The astrocytic Ca2+ excitability may be functionally analogous to the action potential that neurons use for communication, thereby enabling a bidirectional communicating network between neurons and astrocytes.

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