Nonvesicular release of neurotransmitter from astrocytes

Several types of neurotransmitters, including glutamate, ATP and aspartate, can be released from astrocytes through plasmalemmal routes, namely via reversed activity of neurotransmitter transporters or via plasmalemmal channels (Figure 5.14).

Reversed activity of transporters can result in glutamate release from astrocytes (and maybe also from oligodendrocytes); this can happen in conditions of increased intracellular concentrations of Na+ or glutamate, combined with cell depolarization caused by an increased extracellular concentration of K+. These conditions are clearly pathological and may occur in ischaemia and upon various insults to CNS, which are accompanied by neuronal damage and massive release of glutamate and

Passage Neurotransmitter

Figure 5.14 Mechanisms of nonvesicular 'gliotransmitter' release from astrocytes. Nonvesicular release of gliotransmitters can occur through volume-sensitive chloride channels, through hemichannels or through P2X7 receptors; all these channels can allow passage of relatively large molecules with m.w. up to 1 kDa

Figure 5.14 Mechanisms of nonvesicular 'gliotransmitter' release from astrocytes. Nonvesicular release of gliotransmitters can occur through volume-sensitive chloride channels, through hemichannels or through P2X7 receptors; all these channels can allow passage of relatively large molecules with m.w. up to 1 kDa

K+ into the extracellular space. Glutamate in turn is accumulated by astrocytes together with Na+.

Volume-activated anion channels provide a route for the release of glutamate and other negatively charged amino acids, such as taurine. These volume-activated anion channels are opened upon hypo-osmotic shock, i.e. by a decrease in extracellular osmotic pressure, which in turn leads to rapid swelling of astroglial cells. This type of release is physiologically important in the supraoptic nerve of the hypothalamus, which is the primary centre of body osmoregulation. Minor changes (several percentages) of extracellular osmotic pressure stimulate (hypo-osmotic conditions) or inhibit (hyper-osmotic conditions) taurine release from hypophysal glial cells (pituicytes); taurine in turn interacts with glycine receptors present in the terminals of vasopressin/oxytocin neurones, thereby regulating secretion of these two neuro-hormones, which are important in regulation of overall body osmotic homeostasis.

Hemichannels that do not form gap junctions (see Chapter 5.4) provide a further route for glutamate, aspartate and ATP release from astroglial cells. Release of neurotransmitters through this pathway may be regulated, for example, by physiological changes in intra- or extracellular Ca2+ concentration.

P2X7 purinoreceptors are large enough to be permeable to amino acid molecules (see Chapter 5.3.2) and may also constitute a pathway by which neurotransmitters can be released from astrocytes, although it must be noted that P2X7 receptors are most likely activated under pathological conditions when extracellular ATP rises above ~1 mM.

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