Neurotransmitter release from astroglial cells

Neurotransmitters such as glutamate, ATP, GABA etc. are the material substrates of synaptic transmission, and their regulated release in the CNS was, for many years, believed to be a sole prerogative of neurones and neuro-endocrine cells. Experimental investigations of glial cells performed over the last 15 years, however, have clearly demonstrated that at least some neurotransmitters can be released from astroglial cells (Table 5.3). Thus, chemical transmission is the universal mechanism for communication between both neurones and astrocytes (it has not yet been established that oligodendrocytes, microglia or NG2-glia release neurotransmitters, but they clearly respond to neurotransmitters released by

Table 5.3 'Glio' transmitters

Neurotransmitter

Mechanism of release

Function

Glutamate

Vesicular, hemichannels,

Activation of neuronal glutamate

P2X7 receptors

receptors

Aspartate

Hemichannels

Activation of neuronal glutamate

receptors

ATP

Vesicular, hemichannels

Activation of neuronal and glial

purinoreceptors; initiation of

propagating calcium waves

though glial syncytium

D-serine

Transporter (?); vesicular (?)

Activation of neuronal NMDA

receptors

Homocysteine

??

Activation of neuronal NMDA

receptors

Taurine

Volume-activated chloride

Activation of glycine receptors in

channels

supraoptic nuclei and

circumventricular organ

neurones and astrocytes). Neurotransmitters can be released from astrocytes via several pathways, generally classified as nonvesicular and vesicular, the difference being determined by whether transmitter was accumulated into specialized vesicles before release, or, alternatively, was released directly from the cytosol.

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