Edited by: Alessandro Tozzi, University of Perugia, Italy
Reviewed by: C. Peter Bengtson, University of Heidelberg, Germany; László Héja, Hungarian Academy of Sciences, Hungary
*Correspondence: Balázs Pál
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
In the last few decades, knowledge about astrocytic functions has significantly increased. It was demonstrated that astrocytes are not passive elements of the central nervous system (CNS), but active partners of neurons. There is a growing body of knowledge about the calcium excitability of astrocytes, the actions of different gliotransmitters and their release mechanisms, as well as the participation of astrocytes in the regulation of synaptic functions and their contribution to synaptic plasticity. However, astrocytic functions are even more complex than being a partner of the “tripartite synapse,” as they can influence extrasynaptic neuronal currents either by releasing substances or regulating ambient neurotransmitter levels. Several types of currents or changes of membrane potential with different kinetics and via different mechanisms can be elicited by astrocytic activity. Astrocyte-dependent phasic or tonic, inward or outward currents were described in several brain areas. Such currents, together with the synaptic actions of astrocytes, can contribute to neuromodulatory mechanisms, neurosensory and -secretory processes, cortical oscillatory activity, memory, and learning or overall neuronal excitability. This mini-review is an attempt to give a brief summary of astrocyte-dependent extrasynaptic neuronal currents and their possible functional significance.
It was extensively demonstrated in the last decades that glial cells, especially astrocytes are not passive elements of the brain, but active partners of the neurons in signal processing (Araque et al.,
Astrocytes have complex functions in the central nervous system (CNS), including maintenance of the extracellular milieu for optimal neuronal function, regulation of synaptic plasticity, and participation in neuromodulatory actions. They contribute to sleep homeostasis, synchronization of neuronal networks, cortical oscillatory activity, chemosensitivity, and regulation of brain metabolism (Gourine et al.,
As several aspects of astrocytic functions were thoroughly covered by numerous reviews, in this mini-review, I would like to focus on a distinct form of astrocyte-neuron communication, namely astrocytic actions on neuronal extrasynaptic currents.
Extrasynaptic currents can be detected in several areas of the CNS and show large heterogeneity in their kinetics, direction, neuro/gliotransmitters, and receptors responsible for them. These currents are elicited by ambient neurotransmitters acting on extrasynaptic neuro/gliotransmitter receptors. Ambient neurotransmitter levels are under control of both neurons and astrocytes, and have several potential sources such as spillover from synaptic clefts, neuronal volume transmission or somatodendritic release by neurons, and uptake or release of neuro/gliotransmitters by astrocytes (e.g., Semyanov et al.,
The following sections will focus on the astrocytic influences on extrasynaptic currents which will be grouped according to their time scale and direction.
Neuronal slow inward currents (SICs) are phasic extrasynaptic excitatory events distinguished from excitatory postsynaptic currents (EPSCs), due to differences in amplitude, rise time and decay time. The amplitude of these currents was 18–477 pA, with slow rise (13–332 ms) and decay times (72–1630 ms), fit by a single exponential function. In contrast, the amplitude of the miniature EPSCs was 19–40 pA, the rise time was significantly shorter (1–6 ms), whereas decay had a double exponential fit (τ1 = 6.6−27.6 ms, τ2 = 83−146 ms; Fellin et al.,
These currents are generated by activation of extrasynaptic NMDA receptors containing NR2B subunits, as SICs were prevented by general or NR2B subunit selective NMDA receptor antagonists. The involvement of NMDA receptors is also supported by the observation that appearance of SICs is largely facilitated in Mg2+-free extracellular solution. D-serine, a co-activator of the NMDA receptor also contributed to generation of SICs (Angulo et al.,
SICs were found in several areas of the CNS, such as in the hippocampus (Angulo et al.,
It has been extensively demonstrated that SICs are consequences of astrocytic activity. Stimulation of astrocytes in astrocyte-neuron co-cultures led to the appearance of SICs on neurons in their neighborhood, and inhibition of astrocytic calcium signaling prevented the development of these events (Araque et al.,
SICs recorded from different areas of the CNS have largely variable characteristics (Table
Nucleus accumbens | D'Ascenso et al., |
120.5 ± 9.3 | 0.05–0.2 | 81.4 ± 5.8 | 451 ± 42.2 | DHPG, ATP, baclofen, low Ca2+ ACSF, uncaging Ca2+ | D-AP5, ifenprodil | Yes | Stimulation of glutamatergic afferent; 10 trains with 30 Hz, repeated with 1 s intervals; MPEP prevented frequency increase elicited by stimulation |
Olfactory bulb, granule cell | Kozlov et al., |
Approx. 30–100 | 0.24 ± 0.1 | MK-801 | |||||
Ventrobasal thalamus | Pirttimaki et al., |
124.7 ± 0.5 | 0.07 ± 0.01 | 117.2 ± 20.8 | 831.4 ± 336.8 | t-ACPD | D-AP5, ifenprodil | Lemniscal or cortical inputs; 10–20 stimuli with 50 Hz in every 5–10 s for 60–120 s. The frequency increase is prevented by group I. mGluR antagonists, and the effect remained for at least further 1 h. | |
Medial nucleus of the trapezoid body | Reyes-Haro et al., |
89.3 ± 9.7 | 0.275 ± 0.056 | 166.6 ± 16.3 | Strychnine, gabazine, TTX after astrocyte stimulation | BAPTA dialysis of astrocytes, ifenprodil, MK-801+APV, DAAO | Rarely | Local electrical stimulation (amplitude of SICs increased) | |
Primary visual cortex | Chen et al., |
Approx. 26 and 15 pA | 3.2 ± 1.1 | 13.08; 18.64 ± 2.31 | 116.47; 46.04 ± 3.42 | Acetylcholine, photostimulation of ChR2-expressing astrocytes | BAPTA perfusion of astrocytes, D-AP5 | ||
Hippocampus | Fellin et al., |
95 ± 36.7 | 0.16 ± 0.04 | 92.3 ± 29 | 538.5 ± 176 | DHPG, uncaging Ca2+ | D-AP5, ifenprodil, MK-801, 1 mM Mg2+ | Yes | Stimulation of Schaffer collateral (100–200 ms long trains with 25–30 Hz frequency, 0.3–1 Hz repetition frequency) |
Angulo et al., |
104 ± 13 | 0.82 ± 0.15 | 135.5 ± 20 | 608.2 ± 216.45 | DHPG, PGE2 | D-AP5, MK-801 | Yes | ||
Perea and Araque, |
18.3 ± 1.4 (spontaneous); 77.7 ± 1.3 (evoked) | 0.79 ± 0.09 | 13.9 ± 1.7 | 72.5 ± 11.1 | D-AP-5, 2 mM Mg2+ |
Stimulation of Schaffer collateral | |||
Shigetomi et al., |
29 ± 1 | 0.03 ± 0.005 | 46 ± 18 | 196 ± 92 | thrombin, TFLLR-NH2 | BAPTA or fluoroacetate incubation, ifenprodil, D-AP5 | |||
Spinal cord | Nie et al., |
477 ± 43.42 | 0.04 ± 0.003 | 332.46 ± 31.46 | 1630.61 ± 153.87 | TBOA | TTX + TBOA; fluorocitrate + TBOA; D-AP5; Ro 25-6981 (amplitude reduction) | Stimulation of spinal dorsal root entry zone, in the presence of TBOA | |
Bardoni et al., |
80.3 ± 12.8 | 0.01 ± 0.01 (12.5% of all tested neurons displayed SICs) | 83.5 ± 16.1 | 423.1 ± 65.9 | BzATP, low Ca2+ | Peripherial inflammation (by intraplantar zymosan injection) | |||
Thalamic nuclei (dorsal lateral geniculate nucleus, nucleus reticularis thalami, ventrobasal complex) | Jiménez-González et al., |
111.3 ± 31.27 | 0.02 ± 0.01 | 108.9 ± 34.53 | Hypo-osmotic stimulus, vigabatrin | Gabazine (SR95531) | |||
Olfactory bulb, mitral cell | Kozlov et al., |
266 ± 26 | 0.95 ± 0.16 | 47.2 ± 4.2 | 350 ± 25 | Calcium-free ACSF, hypo-osmotic stimulus | Gabazine (SR95531), picrotoxin, bicuculline (partial) | Yes | Mechanical stimulation of astrocytes or blood vessels |
Hippocampus | Le Meur et al., |
71.05 ± 14.99 (CA3) 46.95 ± 6.93 (DG) | 0.04 ± 0.02 (CA1) 0.15 ± 0.04 (CA3) 0.34 ± 0.04 (DG) | 35.7 ± 4.85 (CA3) 26.19 ± 8.41 (DG) | Hypo-osmotic stimulus | Gabazine (SR95531) |
The functional role of SICs is likely the synchronization of neighboring neurons after longer excitatory stimulation by a significant input of their area. Longer stimulation of inputs increased the frequency of SICs in the nucleus accumbens, the hippocampus, and the ventrobasal thalamus. This synchronization was mostly demonstrated in forebrain structures: astrocytes can elicit SICs synchronously on pairs of neurons in the nucleus accumbens and on the hippocampal CA1 pyramidal neurons (Angulo et al.,
SICs can be affected by a special form of plasticity: prolonged stimulation of afferents of the ventrobasal thalamus resulted a longer lasting (at least 60 min long) increase of SIC frequency, representing an astrocyte-dependent nonsynaptic plasticity (Pirttimaki et al.,
Slow outward currents (SOCs) are rarely described phasic inhibitory events. Distinguishing them from neuronal inhibitory postsynaptic currents (IPSCs) is not as clear as in the case of SICs: although with their rise time of 26–109 ms and decay time of 350 ms they are significantly slower than GABAA receptor-mediated fast IPSCs (3–8 ms decay time), their kinetic properties are closer to slow IPSCs mediated by GABAA (30–70 ms decay time; Capogna and Pearce,
GABA acting on (δ-subunit-containing) GABAA receptors is responsible for generation of SOCs (Kozlov et al.,
SOCs were recorded from the ventrobasal thalamus (Jiménez-González et al.,
The astrocytic origin of SOCs was demonstrated in the olfactory bulb, where mechanical stimulation of astrocytes or blood vessels was capable of eliciting these events (Kozlov et al.,
Although not discussed as SOCs, astrocytic activity might raise other inhibitory events with slow kinetics. Rhythmical, hyperpolarizing, A1 adenosine-receptor mediated events were found in thalamic nuclei of the cat. The possible source of adenosine was the ATP released by non-neuronal structures (Lörincz et al.,
The significance of the rarely identified SOCs is not known in great detail. However, they might have a role in neuronal synchronization, as they appeared synchronously on neurons of the olfactory bulb (Kozlov et al.,
Neuronal tonic excitatory currents were detected in several brain structures, but the relationship between these currents and astrocytic activity is less clear than with SICs. These currents are predominantly mediated by ambient glutamate and the NMDA receptor co-activator glycine (Le Meur et al.,
Tonic excitatory currents might appear together with SICs. The glutamate transporter inhibitor TBOA increased the amplitude of SICs together with activation of an NMDA-receptor dependent tonic current. It is likely that glutamate originating from the same astrocytic source elicited SICs on its closer targets, but activated a larger number of NMDA receptors with a lower concentration on distant activation sites on neurons, thus eliciting tonic inward current (Jabaudon et al.,
Tonic excitatory currents were detected in the hippocampus (Jabaudon et al.,
Astrocytic contribution to tonic inward currents is supported by several indirect and direct observations. Inhibition of astrocytic functions by gliotoxins, EAAT1 and 2 glutamate transporters or blockade of astrocytic glutamine synthase increased the amplitude of the tonic inward current (Jabaudon et al.,
Hippocampus | Fellin et al., |
−80 ± 23 pA | 98 ± 9 s | BzATP (100 μM), potentiated by 0 Ca2+ | D-AP5 (50–100 μM), OxATP (300 μM), BBG (2–4 μM) | Both were elicited by BzATP, but SICs were unaffected by OxATP or BBG |
Angulo et al., |
378.9 ± 87.8 pS (spontaneous, inhibited by D-AP5); 377.6 ± 54.9 pA (induced by TBOA, at +40 mV) | 0 Mg2+, TBOA (100 μM) | D-AP5 (50 μM) | Triggered by overlapping mechanisms, appear together | ||
Jabaudon et al., |
331 ± 60 pA at +40 mV | TBOA (200 μM), MSO (1.5 mM) | D-AP5 (70 μM) | |||
Le Meur et al., |
50.8 ± 13.4 pA at +40 mV | TBOA (100 μM), TBOA after preincubation with MSO | D-AP5 (50 μM), MK-801 (40 μM), 7-Cl-KYN (10 μM), PPDA (0.1 μM), NVP-AAM077 (0.1 μM) | |||
Papouin et al., |
26.8 ± 3.1 pA at +40 mV (inhibited by D-AP5) | D-AP5 (50 μM), reduced by Ro25-6981 (2 μM) and BsGO | ||||
Spinal dorsal horn | Nie et al., |
−75.5±11.25 pA at −70 mV | TBOA (100 μM) | Both triggered by TBOA, appear together | ||
Supraoptic nucleus | Fleming et al., |
31.8 ± 4.8 pA at +40 mV; −7.4±1.3 pA at −70 mV | Dihydrokainate (300 μM), TBOA (100 μM), α-AA (2 mM) | Kynurenic acid (2 mM), ifenprodil (10 μM), D-AP5 (100 μM), memantine (30 μM) | ||
Cerebellum, Purkinje-cells | Sasaki et al., |
Optogenetic stimulation of Bergmann glia; TBOA (100 μM) | GYKI 53655 (100 μM), NBQX (10 μM), DIDS (1 mM) | |||
Beppu et al., |
Optogenetic stimulation of Bergmann glia; oxygen and glucose deprivation | GYKI 53655 (100 μM), DIDS (1 mM) | ||||
Pedunculopontine nucleus | Kõszeghy et al., |
−24.5±4.4 pA at −60 mV | ACEA (5 μM) | Only tonic depolarization was investigated; thapsigargin (1 μM), LY 341495 (10 μM) | ||
Olfactory bulb (mitral, external tufted cells) | Belluzzi et al., |
Approx. −300 pA at −90 mV, +200 pA at −60 mV (10 mM taurine); biphasic at −60 mV | 1.98 ± 0.23 s (10–90% rise time) | Taurine (2.5–10 mM), GABA (rapid decay; 200 μM) | Bicuculline (10 μM), picrotoxin (10 μM) | |
Hypothalamic paraventricular nucleus | Park et al., |
32.79 ± 5.04 pA (300 μM nipecotic acid) | Nipecotic acid (100, 300 μM); β-alanine (100 μM) | Bicuculline (20 μM; if elicited by nipecotic acid) bicuculline (20 μM) and strychnine (10 μM), if elicited by β-alanine | ||
89.25 ± 22.66 pA (100 μM β-alanine) | ||||||
Inward current with symmetrical Cl− concentrations | ||||||
Cerebellar granule cells | Lee et al., |
35.7 ± 4.1 pA at -60 mV with symmetrical Cl− concentrations, blocked with SR95531 | SR95531 (10 μM); NPPB (50 μM), NFA, DIDS (100 μM) | |||
Neocortex | Lalo et al., |
39.9 ± 8.3 pA at −80 mV, with symmetrical Cl− concentrations, blocked by bicuculline | Impairment of SNARE in astrocytes (dn-SNARE) | Bicuculline (50 μM), TFLLR (10 μM), TFLLR + PPADS (10 μM) | ||
Pedunculopontine nucleus | Kõszeghy et al., |
19 ± 1.9 pA at −60 mV | ACEA (5 μM) | Tonic hyperpolarization was blocked by MPEP (10 μM) + CPCCOEt (100 μM), thapsigargin (1 μM) | ||
Hypoglossal motoneurons | Gomeza et al., |
62.2 pA at −70 mV with symmetrical Cl− concentrations | Disruption of GlyT1 gene | Strychnine (10 μM) | ||
Spinal cord, lamina X neurons | Bradaïa et al., |
−20 to −50 pA at −60 mV, with symmetrical Cl− concentrations | ORG24598 (10 μM); also potentiated by ORG25543 (10 μM) | Strychnine (1 μM) |
With their regulatory role on tonic neuronal excitatory currents, astrocytes participate in protection of neurons against hyperexcitability, are involved in the pathogenesis of seizures, contribute to long term synaptic plasticity and learning, and regulate neuromodulatory actions.
Administration of the glutamate transporter inhibitor TBOA generated seizures (Montiel et al.,
In cooperation with neuronal processes, astrocytic regulation of these extrasynaptic currents contributes to learning and synaptic plasticity. Mice heterozygous for the EAAT2 gene (with moderate loss of EAAT2 protein) exhibited altered learning abilities compared to wild type animals (improvement in cue-based fear conditioning, but worse context-based fear conditioning; Kiryk et al.,
Tonic excitatory currents elicited with the contribution of astrocytic activity are neuromodulatory mechanisms. Tonic excitatory currents appeared after stimulation of astrocytic muscarinic acetylcholine receptors and induced acetylcholine-dependent cortical plasticity (Chen et al.,
Neuronal tonic inhibitory currents mainly originate from activation of extrasynaptic GABAA receptors (reviewed in: Semyanov et al.,
Similar to tonic inward currents, tonic GABAergic currents are not exclusively of glial origin. Ambient GABA can originate from neurotransmitter spillover or volume transmission as well, although astrocytes can also release GABA as a gliotransmitter via different mechanisms (through bestrophin channels, Lee et al.,
Tonic GABA currents can be regulated by astrocytes via ATP release. ATP promoted the down-regulation of GABAA receptors via activation of the P2X4 receptor, and thus attenuated the effects of extrasynaptic GABA in the neocortex (Lalo et al.,
In contrast to astrocyte-dependent tonic excitation, it has not been shown that optogenetic activation of astrocytes elicits hyperpolarization of neurons. However, based on
Tonic inhibitory currents can be elicited by neuro/gliotransmitters other than GABA. Tonic glycinergic currents were generated on spinal cord neurons by either blockade of glial glycine transporter (Bradaïa et al.,
The tonic outward current has variable roles in regulation of neuronal functions, such as setting neuronal excitability, contribution to network oscillations, and has developmental functions like inhibition of cell proliferation and stimulation of cell migration (Semyanov et al.,
Astrocyte-dependent neuronal extrasynaptic currents seem to be a general feature of the CNS. These phenomena are not always consequences of astrocytic actions on neurons, but often represent interplay between neuronal and astrocytic activity. The degree of astrocytic contribution is different: SICs have unambiguous astrocytic origin, but the astrocytic component of tonic inward and outward currents can't be always clearly separated.
The (patho)physiological roles of these currents are also heterogeneous and sometimes contradictory: tonic inward and outward currents are generally thought to contribute to synaptic plasticity and memory, although similar neuronal tonic inward currents can be observed in models of Alzheimer's disease (Talantova et al.,
Furthermore, the function of the generally observed SICs is also not fully resolved. Appearing on neighboring neurons at the same time and being elicited by well-defined afferentation of the investigated brain area, they can synchronize neuronal activity, but their role in synaptic plasticity, to the best of my knowledge, has not been thoroughly investigated yet.
Taken together, neuronal extrasynaptic currents influenced by astrocytes contribute to several pathophysiological processes, but the uniform role of these phenomena and the significance of pure astrocytic mechanisms in eliciting them still remain an exciting field for further investigations.
BP wrote the paper.
The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
This work was supported by the János Bolyai Research Scholarship of the Hungarian Academy of Sciences, the Szodoray Fellowship of the University of Debrecen and the National Brain Research Program (KTIA_13_NAP-A-I/10.). I am indebted to Dr. Attila Szöllõsi (University College Dublin, Ireland) for the thorough reading of the manuscript and for his valuable comments.
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artificial cerebrospinal fluid
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excitatory amino acid transporter type 1-4
excitatory postsynaptic current
GABA transporter type 2-3
glycine transporter type 1
1-(4-aminophenyl)-3-methylcarbamyl-4-methyl-3,4-dihydro-7,8-methylenedioxy-5
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[(S)-{[(1S)-1-(4-bromophenyl)ethyl]amino}(2,3-dioxo-1,2,3,4-tetrahydroquinoxalin-5-yl)methyl]phosphonic acid
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purinergic receptor type 2X4
purinergic receptor type 2Y1
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