Signalling from astrocytes to neurones

Calcium signals, which occur in astroglial cells either spontaneously or in response to activation of neighbouring neurones, are capable of triggering the release of neurotransmitters from the glial cells (see Chapter 5.6). It is now firmly established that these 'glio' transmitters can directly affect the neurones residing in the vicinity of the glial cells. This glial to neurone signalling is mediated by either ionotropic or metabotropic receptors present in the neuronal membrane. In particular, astroglial release of glutamate can directly depolarize (and hence excite) neurones through ionotropic glutamate receptors of AMPA and NMDA types. Glutamate can also activate metabotropic receptors residing in presynaptic terminals. Astrocytes can also modulate neuronal excitability through the release of ATP; the latter acting either directly, through stimulation of neuronal purinoreceptors, or indirectly, by degrading to adenosine and activating adenosine receptors.

Astrocyte Closes Synapse

Figure 6.6 An overview of neuronal-glial interactions in the grey matter; scheme indicating the way in which reciprocal neuronal-glial communication may occur. Neurotransmitters released at 'neurone-glial' and 'tripartite' synapses act on astrocytes, to initiate Ca2+ signals that are propagated from astrocyte to astrocyte via gap junctions and the release of gliotrans-mitters, which in turn can act on distant synapses. In addition, signals may be communicated via glial-neuronal gap junctional contacts

Figure 6.6 An overview of neuronal-glial interactions in the grey matter; scheme indicating the way in which reciprocal neuronal-glial communication may occur. Neurotransmitters released at 'neurone-glial' and 'tripartite' synapses act on astrocytes, to initiate Ca2+ signals that are propagated from astrocyte to astrocyte via gap junctions and the release of gliotrans-mitters, which in turn can act on distant synapses. In addition, signals may be communicated via glial-neuronal gap junctional contacts

Signalling from astrocytes to neurones has been demonstrated in a number of preparations, and it has been shown that neurotransmitters released from astrocytes can modulate neuronal activity. However, the significance of astrocyte-neuronal signalling in the integrative activity of the brain is at present speculative. Nonetheless, the machinery is present. It is clear that the release of glutamate from astrocytes is regulated by the frequency of Ca2+ oscillations - each single [Ca2+]j peak triggers a pulsatile release of glutamate. Since the frequency of astroglial Ca2+ oscillations is governed by the intensity of synaptic activity, then an increase in the latter would substantially increase glutamate secretion from astrocytes, which would strengthen and amplify the original synaptic signal. Moreover, astrocytes release glutamate not only into the synaptic cleft, but elsewhere, which would induce activation of extrasynaptic receptors, which in turn could affect neuronal processing of incoming information. Finally, glial Ca2+ oscillations are conveyed through the astroglial network, which could induce the release of neurotransmitter in locations distant to the initial neuronal-glial contacts, therefore allowing a parallel spread of information in the grey matter (Figure 6.6). Furthermore, glutamate released from a single astrocyte may act on several adjacent neurones thus producing synchronous excitation or inhibition of the latter.

How To Reduce Acne Scarring

How To Reduce Acne Scarring

Acne is a name that is famous in its own right, but for all of the wrong reasons. Most teenagers know, and dread, the very word, as it so prevalently wrecks havoc on their faces throughout their adolescent years.

Get My Free Ebook


Post a comment