The idea that astrocytes, like neurons, might have diverse, and region-specific, roles to play in development and function of the central nervous system is slowly gaining recognition. This implies that it might not be useful to talk about astrocytic function in general anymore than with neurons. Examining neurons, their synapses, and astrocytes as a single local functional unit involving mutual interactions will help in the elucidation of local network modulation by these glial cells. That astrocytes form an integral and active part of synaptic transmission is a currently accepted concept.
The physiological role of astrocytes in maintaining and modulating synaptic function is yet to be fully understood. Current evidence suggests that astrocytes might act as local controllers of synaptic function determined by the extent of influence for a single astrocyte. As one astrocyte can contact, or be in proximity, of multiple synapses, the idea that these cells might act as integrators of local network activity is tempting.
Such an idea requires the astrocyte to fulfill three requirements (Fig. ): (1) They must be able to respond to synaptic activity. (2) They need to transmit this information to relatively distant synapses. (3) They need to have mechanisms for communicating back to synapses.
The idea that astrocytes can actively respond to neuronal activity is now quite well accepted. A number of functional receptors for various neurotransmitters that might be released at a synapse have been reported (29
). These include purinergic receptors (29
), AMPA receptors (AMPARs; 31
), metabotropic glutamate receptors (mGluRs; 33
), GABAA receptors (GABARs; 34
), nicotinic acetylcholine receptors (nAChRs; 35
), muscarinic receptors (mAChRs; 39
), and α1- as well as β2-noradrenergic receptors (40
), possibly among others. In some cases where ultrastructural evidence exists, the astrocytic receptors have been localized close to synaptic release sites (42
). In oligodendrocyte progenitor cells, ‘synaptic’ AMPAR currents have been seen in response to vesicular release of glutamate (44
While each of these receptor types is likely to have specific functional roles, for the purpose of this review, it suffices to state that there is clear evidence that astrocytes can respond to synaptic release of various neurotransmitters in the brain.
Propagation of Signals by Astrocytes
While it is known that astrocytes possess low densities of sodium channels and, therefore, do not propagate signals via action potentials (APs), it is now recognized that these cells have a form of excitability based on sophisticated calcium-signaling mechanisms. This signal manifests in the form of intra- and intercellular calcium waves that can mediate communication across significant distances (Fig. c).
Complex calcium dynamics in astrocytes
The existence of propagating calcium signals along the length of an astrocyte provides credence to the idea of chemical propagation of signaling. From our current understanding of published literature, we can postulate one operative mechanism. Activation of surface metabotropic receptors (mGluRs, for instance) activates the phospholipase C (PLC) signaling cascade leading to the production of inositol trisphosphates (IP3
) and the activation of their receptors (IP3
Rs) on the endoplasmic reticulum (ER). Activation of IP3
Rs triggers a regenerative propagation of calcium changes along an astrocyte (Fig. c). This propagation involves the balance of a number of finely tuned processes. IP3
Rs are regulated in a biphasic manner by calcium, where higher local concentration of the ion inhibits the receptor channel open probability (46
). Therefore, local accumulation of calcium around the channel would inhibit further release of ER calcium from that microdomain. In addition, there is tuning of the receptor opening by calcium binding to the luminal side of the IP3
R within the ER (48
). This tight positive and negative control of the receptor opening accounts for the dynamics of local calcium changes in the cytosol around this microdomain. In addition, diffusion of IP3
itself and calcium-dependent modulation of PLC also serve as effective means of signal propagation.
The control of calcium levels around these release sites, in turn, is dependent on the clearance of the ion. There is rapid calcium buffering in all cell types which in turn maintains a tight spatial and temporal control over calcium signal transduction. The domain of influence of calcium signals is thus determined by these buffers as well as the affinity of downstream calcium-signaling pathways for the ion. At synapses, where transmitter release requires micromolar calcium concentrations, incoming calcium flux via voltage-gated calcium channels (VGCCS) would exert a sphere of influence in the 100–200-nm range from influx sites (49
Both mobile and immobile calcium buffering mechanisms play a significant role in these processes. Around the ER, calcium is pumped backed into the stores by the family of sarcoplasmic reticulum Ca/Mg ATPases (SERCA pumps). Another potentially high capacity buffer as well as a source of calcium release is the astrocytic mitochondria. Potential spatial organization of the ER and mitochondria (51
) as well as the crosstalk of calcium signals between the two organelles (52
) affects downstream consequences of astrocytic calcium waves, e.g., glutamate release. In addition, significant concentrations of high affinity diffusible calcium buffers (53
) in astrocytes would serve to rapidly attenuate local calcium increases as well as increase diffusion and sphere of influence of calcium signals (54
Signals via IP3Rs can thus be modulated via complex local and global calcium-signaling mechanisms in order to propagate signals down astrocytic processes as an effective form of signal propagation in response to synaptic activity.
In addition, astrocytes also possess ER ryanodine receptors (RyRs), which might play a role in astrocytic calcium signaling (51
) both by linking calcium influx (e.g., via ionotropic receptor activation) to store calcium release via calcium-induced calcium release (CICR) as well as by spatial and temporal control of IP3
Controlled changes in calcium levels in the astrocytic cytosol, therefore, have a distinct physiological role to play. Consistent with this view is the finding that astrocytes in more intact preparations show spontaneous, IP3
-mediated, and well-defined calcium oscillations (21
). These oscillations, via downstream signaling mechanisms, might play a role in modulating synaptic functions. Current evidence suggests that the frequency of calcium oscillations in astrocytes is controlled within each cell and not across astrocytes. The data suggest that these calcium oscillations might be involved in local regulation of synaptic activity rather than having a large field of influence (58
). However, our data (Grybko and Vijayaraghavan, unpublished results) suggest that, under certain conditions, like that in the presence of nicotine, calcium oscillations across astrocytes can be synchronized, suggesting that these cells might function as ‘pacemakers’ for regulating short- and long-term excitability of local neuronal networks over a larger area than currently assumed (Fig. ). The discovery that changes in astrocytic calcium lead to corresponding alterations in calcium signals from proximal neurons (also see below) is consistent with this view (59
Fig. 2 Putative mechanism for the action of nicotine via astrocytes. a Activation of calcium signaling by nicotine. Astrocytes possess functional α7-nAChRs. Activation of these receptors by nicotine results in calcium influx into the astrocyte which, (more ...)
Propagation of calcium signals by extracellular mechanisms: a role for ATP In addition to the complex control of calcium release and diffusion discussed above, astrocytes can also use ATP for an effective extracellular propagation of synaptic signals. Astrocytes are known to release ATP as a second messenger in response to activity (see below). While the mechanism of ATP release from astrocytes are not yet fully understood, nor is its extent of influence in intact brain tissue, the nucleotide remains an attractive candidate for extracellular propagation of calcium signals along the length of an astrocyte and, perhaps, across a limited population of glia and neurons.
Intercellular calcium waves: a relevant mechanism for astrocytic communication?
Evidence for communication across astrocytes has long been known in purified cell culture preparations (60
) and in brain slice cultures (63
). Both gap junction-mediate mechanisms (65
) as well as propagation via external purinergic mechanisms (62
) have been postulated to contribute to these intercellular waves. The initial idea that calcium released via IP3
R activation directly spreads by diffusion through glial gap junctions appears unlikely due to the limited diffusion of the ion, as discussed above. A likely candidate for such a propagation across gap junctions is the generated IP3
) which not only has a longer range of diffusion but can also be regenerative due to its production by local calcium-dependent mechanisms.
The idea that gap junctions play a significant role in propagating calcium waves is undercut by the fact that: (1) most gap junction blockers used in these studies are notoriously non-specific; (2) significant reduction in the number of gap junctions in connexin-43 knockout mice does not show dramatic effects on wave propagation (70
), even though it has been suggested that expression of connexin hemichannels plays a role in ATP-mediated wave propagation (68
Lastly, while intercellular calcium waves have been demonstrated in more intact preparations like the retina (60
), occurrences of long distance communication in brain tissue appear to be rare (for review see 72
). Intercellular signaling by astrocytes may have other potential roles like long distance transport of metabolites (73
), but its function as a direct means for modulation of calcium-dependent synaptic plasticity might be limited. Instead, astrocyte neuron communication might be restricted to more local domains and intercellular calcium waves might have a role to play only in certain pathological conditions. However, under these pathological conditions, when astrocytes become reactive and undergo significant changes in their properties, such long distance communication may exacerbate symptoms (74
While the extent of propagation of calcium signals by astrocytes might be debated, one conclusion can be derived from the two sections described above. Astrocytes possess both an ability to detect synaptic activity and to transmit these signals across significant distances, albeit restricted in space.
Communicating Back to Neurons: Gliotransmitters
A third requirement to the idea of an active role for astrocytes in modulating synaptic plasticity is the existence of mechanisms in these cells to talk back to the synapse (Fig. d). The evidence, obtained over the last decade, for the existence and release of potential transmitters from astrocytes clearly points to their ability to directly modulate neuronal activity (59
). A few of the known neurotransmitters shown to be released by astrocytes in the brain are discussed below.
The release of ATP from astrocytes has been known for a long time and was initially proposed as a mechanism for the propagation of intercellular calcium waves (see above). Equally documented is ability of ATP acting via the ionotropic P2X and the metabotropic P2Y receptors to mediate and modulate synaptic transmission. ATP receptors are ubiquitous in the nervous system and act as fast transmitters in the CNS (78
), postsynaptic modulators (79
), as well as powerful modulators of transmitter release affecting the release of glutamate (81
), dopamine (82
), norepinephrine (83
), and GABA (84
), to name a few. The receptors have been implicated in a number of CNS functions ranging from modulation of learning and memory to roles in CNS pathology (reviewed in 86
There is a significant debate on the mechanisms underlying ATP release from astrocytes. A number of mechanisms have been proposed. ATP has been suggested to be released via volume-regulated anion channels (87
). Another potential mechanism is that ATP is released via connexin hemi-junctions expressed on the astrocytic plasma membrane (68
). More recent evidence also points to vesicular exocytic mechanisms for the release of ATP (92
). As at least some of these mechanisms are calcium dependent, release of ATP from astrocytes is regulated by neuronal activity and consequent changes in both extra- and intracellular calcium levels. Release of the nucleotide can be further regenerated by ATP-mediated ATP release via the activation of astrocytic P2X7 class of purinergic receptors (93
The presence of high levels of ectonucleotidases also raises the possibility of astrocytic signaling byproducts of ATP hydrolysis. Adenosine, the end product of this process, is known to be a powerful modulator of synaptic activity via its actions on the A1, A2, and A3 adenosine receptors. ATP release from glia has been shown to inhibit synaptic activity via adenosine production (94
Whatever the mechanism for its release, the consensus is that ATP is an important component of astrocytic signaling and plays an important role in glial–neuronal interaction.
In astrocytes, the presence of all components for exocytic release (96
) has generated a lot of interest in release of transmitters in a manner analogous to the presynaptic terminals. Considerable attention has been focused over the last few years on the vesicular release of glutamate from astrocytes. The existence of calcium- and SNARE protein-dependent glutamate release from astrocytes is well documented (for review see 98
). Further, the released transmitter produces slow and large glutamatergic currents on proximal neurons, mainly via the activation of extrasynaptic NMDA receptors, which then synchronizes neuronal activity (101
). This large extrasynaptic current raises the possibility that glutamate release from astrocytes might involve larger vesicles than those at nerve terminals (102
The demonstration of the presence of serine racemase in astrocytes resulting in the production of d
-serine has led to the idea that the amino acid might be a significant player in the regulation of NMDA receptors by acting as the natural substrate for the glycine binding site on the receptor (103
). Though the trigger for its release and mechanisms underlying its control are largely unknown, recent evidence suggests that d
-serine might be released by calcium-dependent exocytosis (105
The existence of glial-mediated transmitter release provides evidence that astrocytes can signal back to neurons thus fulfilling all criteria necessary for glia-mediated transmission in the CNS. The presence of astrocyte-dependent NMDA receptor currents on neurons is intriguing. How these receptors might be activated in the absence of a depolarization component to relieve the magnesium block at NMDA receptor-only regions is still unclear. The promiscuity of individual astrocytes in their transmitter release capabilities might provide a clue for how this might be achieved, e.g., by the co-release of ATP and glutamate at local sites.
Astrocyte-driven modulation of postsynaptic activity by the extrasynaptic release of gliotransmitters might be an important means of regulating synaptic strength and plasticity in the CNS (106