Table 2.
Channels in astrocytes
| Current or Channel Type/Subunit | Experimental Preparation/Technique | Pharmacology | Biophysical Properties and Functional Relevance | Reference Nos. |
|---|---|---|---|---|
| Voltage-gated sodium channels | ||||
| INa | Rat/cultured primary astroglia/whole cell voltage clamp | TTX (KD ~0.5 mM), STX (KD ~30 nM) | First detection of fast voltage-activated Na+ currents in astrocytes; INa amplitudes were ~1 nA. Authors suggested that astrocytes synthesize Na+ (and K+) channels for subsequent insertion into the neighboring axon. | 166 |
| INa | Rat/cultured primary astroglia/optic nerve/whole cell voltage clamp | TTX (KD ~2.8 nM) | INa was detected in the majority of type 2 astrocytes and in many type 1 cells; respective current amplitudes were 400–3,100 pA and 10–300 pA. Glial INa had slower kinetics when compared with neuronal currents. In type 1 astrocytes, the steady-state inactivation curve was significantly shifted into hyperpolarizing direction with h∞ ~-80 mV. | 105, 106 |
| INa fast, INa slow | Rat/cultured primary astroglia/optic nerve, hippocampus/whole cell voltage clamp | Two types of INa with distinct (fast and slow) kinetics and steady-state inactivation were dissected. With increasing time in culture, slow current became dominant in hippocampal cultures. | 1649, 1650 | |
| INa TTX sensitive, INa TTX resistant | Rat/cultured primary astroglia/optic nerve, hippocampus/whole cell voltage clamp | TTX (KD ~5.7 nM), TTX (KD ~1,007 nM) | The TTX-sensitive INa was confined to stellate astrocytes, whereas TTX-resistant INa dominated in fibroblast-like astrocytes. | 1647, 1652 |
| INa slow | Rat/tissue prints/optic nerve, hippocampus/whole cell voltage clamp | TTX (10 μM) | In tissue prints of astrocytes at P2, INa was not detected; in contrast, at P10, all astrocytes demonstrated slow INa. | 107 |
| INa | Rat/acute slices/hippocampus/whole cell voltage clamp | INa was recorded from a minor subpopulation (5 of 40) of GFAP-positive astrocytes. | 1651 | |
| INa | Rat/acute slices/spinal cord/whole cell voltage clamp | INa with amplitudes of 20–55 pA was recorded from a subpopulation of cells with small somata and long processes with faint GFAP staining. | 337 | |
| Nav1.5, Nav1.2, Nav1.3 | Rat/cultured primary astroglia/spinal cord/RT-PCR, in situ hybridization, immunocytochemistry | The Nav1.5 identified as the predominant type of astroglial Na+ channels. | 185, 189 | |
| Nav1.6 | Rat/cultured primary astroglia/spinal cord/immunocytochemistry | The Nav1.6 expression was detected in embryonic cultures; immunoreactivity was confined to stellate cells only. | 1444 | |
| Non-voltage-gated sodium channels | ||||
| Nax | Rat | The Nax channel is expressed in astrocytes and ependymocytes in circumventricular organs. The channel is activated by an increase in [Na+]o above 150 mM; the activation threshold is reduced to ~140 mM in the presence of endothelin-3 acting through ETBRs. | 1234, 1613, 1864 | |
| ENaC | Rat/tissue section/immunocytochemistry | The ENaC γ-subunits were found in GFAP-positive astrocytes using specific antibodies. | 1116 | |
| Voltage-gated calcium channels | ||||
| L-type Ca2+ currents | Rat/cultured primary astroglia/cortex/two-electrode patch clamp | Co2+ (2–10 mM), Cd2+ (1 mM), nifedipine (10 μM) | Ba2+-dependent action potentials or Ba2+ current were measured from individual astrocytes; at 10 mM Ba2+, maximal amplitudes were at nA range. Norepinephrine or dbt-cAMP enhanced or induced Ba2+ currents. | 1038, 1041 |
| L-type Ca2+ currents | Rat/cultured primary astroglia/cortex/whole cell patch clamp | Nifedipine (100 μM) | Ca2+ currents with maximal amplitudes ~100–200 pA were recorded only after cell preincubation for 1–2 h with forskolin or 8-bromo-cAMP. | 104 |
| L- and T-type of Ca2+ currents | Rat/cultured primary astroglia/acutely isolated astrocytes/tissue prints/optic nerve/whole cell patch clamp | High (L) and low (T) Ca2+ currents with maximal amplitudes of 50–200 pA were recorded from the majority of neonatal astrocytes. With age (at P30), T currents disappeared and L currents were detected only in 37% of cells. | 106, 107 | |
| L-type Ca2+ currents | Rat/cultured primary astroglia/acutely isolated astrocytes/hippocampus/ [Ca2+]i microfluorimetry | Co2+ (1 mM), verapamil (>30 μM), nifedipine (100 μM), BAY K 8644 (10 μM) | High K+ depolarization triggered transient elevations in [Ca2+]i reaching ~1 mM; these were strictly dependent on [Ca2+]o, where inhibited by Ca2+ blockers and potentiated by BAY K 8644. | 460, 1040 |
| L-, N-, and R-type Ca2+ currents | Rat/cultured primary astroglia/cortex/whole cell or perforated patch clamp | Nifedipine (5 μM), ω-conotoxin GVIA (3 μM), SNX-482 (100 nM) | Ba2+ whole cell and single-channel currents showed complex pharmacological behavior. Whole cell currents were partially sensitive to L-, N-, and R-type Ca2+ blockers. RT-PCR revealed expression of relevant α1 subunits. | 374 |
| L-type Ca2+ currents | Rat/cultured primary astroglia/hippocampus/ [Ca2+]i microfluorimetry | Cd2+ (100 μM) | High K+ depolarization triggered transient elevations in [Ca2+]i, which instigated vesicular release of glutamate. RT-PCR revealed expression of mRNAs for α1B, α1C, α1D, and α1E VGCC subunits. | 1909 |
| Cav1.2, Cv1.3 channels | Mouse/primary culture/cortex/[Ca2+]i microfluorimetry | Verapamil (5 μM), nifedipine (5 μM) | High K+ depolarization induced [Ca2+]i transients, which disappeared after removal of extracellular Ca2+. Expression of mRNA for Cav1.2 and Cav1.3 was detected at mRNA and protein levels. In Cav1.2 KO mice, [Ca2+]i transients were reduced by 80%. | 314 |
| L-type Ca2+ channels | Rat/acute slices/ventrobasal thalamus/[Ca2+]i microfluorimetry | Nifedipine (1 μM), BAY K 8644 | Spontaneous astroglial [Ca2+]i oscillations were potentiated by BAY K 8644 and inhibited by nifedipine. | 1331, 1332 |
| Ca2+ release-activated Ca2+ channels (ICRAC/ORAI) | ||||
| ORAI1 | Rat/primary culture/cortex/[Ca2+]i microfluorimetry, RT-PCR, immunocytochemistry | Expression of ORAI1 and STIM1 was detected at mRNA and protein level; overexpression of ORA1/STIM1 increased, whereas siRNA KO decreased thrombin-induced SOCE. | 1150 | |
| ORAI3, ORAI2 | Rat/primary culture/hippocampus/[Ca2+]i microfluorimetry, RT-PCR | Pyr6 (5μM) | Expression of ORAI3 mRNA was predominant, exceeding expression of ORAI2 6 times; ORAI1 was not detected at all. | 1492 |
| TRP channels | ||||
| TRPA1 | Rat/tissue sections/trigeminal caudal nucleus/immunogold electron microscopy | TRPA1 immunoreactivity was detected in the somata and processes of astrocytes; immunoreactivity increased significantly in the fine process of astrocyte in rats with experimental inflammation of the temporomandibular joint. | 965 | |
| TRPA1 | Rat/primary cultures, acute slices/hippocampus/[Ca2+]i microfluorimetry, whole cell voltage clamp | Gd3+, Li3+, TRP antagonist HC030031 (40 μM), TRPA1 agonist AITC (1 μM) | Constitutive activity of TRPA1 channels was found to trigger “spotty” near-membrane Ca2+ microdomains and contributed to resting [Ca2+]i. | 1611 |
| TRPV1 | Rat/brain, spinal cord/immunocytochemistry, immuno-EM | TRPV1 channels were identified in astrocytes throughout the CNS. TRPV1 protein was preferentially localized in astroglial processes. | 442, 1755 | |
| TRPV1 | Rat/primary culture/whole cell patch clamp, immunocytochemistry | Capsazepine (30 μM) | TRPV1 channels were detected immunocytochemically; ~90% of all cell demonstrated TRPV1-mediated currents. | 746 |
| TRPV4 | Rat/primary culture/whole cell patch clamp/[Ca2+]i microfluorimetry, immunocytochemistry, RT-PCR | TRPV4 agonist 4α-phorbol 12,13-didecanoate, 4αPDD (3 μM) | TPPV4 were detected at transcript and protein levels. Astrocytes generated TRPV4-mediated currents and [Ca2+]i transients. | 135 |
| TRPV4 | Rat/hippocampus/acute slices, primary cultures/whole cell patch clamp/[Ca2+]i microfluorimetry, immunocytochemistry, Q-PCR | TRPV4 agonist 4αPDD (5 μM) | Postischemic astrogliosis was associated with an increase in TRPV4 expression, TRPV4-mediated currents, and [Ca2+]i transients | 265 |
| TRPC1, TRPC4, TRPC5, | Mouse embryo/primary culture, acutely isolated astrocytes/cortex/[Ca2+]i microfluorimetry, immunocytochemistry, RT-PCR | Members of TRPC family were detected at transcript and protein level. Treatment with anti-TRPC1 inhibiting antibody suppressed astroglial [Ca2+]i transients, likely due to an inhibition of SOCE | 587, 606, 1055, 1393, 1462 | |
| TRPC6 | Mouse/primary culture/[Ca2+]i microfluorimetry | In vitro knockdown of TPRC6 reduced receptor-induced Ca2+ entry. Treatment of astrocytes with IL-1β for 24–48 h increased TRPC6 expression and disrupted Ca2+ homeostasis. | 163 | |
| Acid-sensitive ion channels, ASICs | ||||
| ASIC1, ASIC 2a, ASIC3 | Rat/primary culture/cortex/[Ca2+]i microfluorimetry, whole cell voltage clamp | ASICs expression was confirmed with immunocytochemistry and Western blotting; ASICs were mainly localized to the nuclei. | 746 | |
| Inward rectifying potassium channels | ||||
| Inward rectifying K+ current | Rat/acutely isolated astrocytes/hippocampus/whole cell patch clamp | Ba2+ (10 mM) | The inwardly rectifying K+ currents were isolated after inhibiting other K+ conductances with a mixture of TEA and 4-AP. | 1763 |
| Inward rectifying K+ current | Rat/primary cultures/spinal cord/whole cell patch clamp, cell-attached patch clamp | Cs+ (KD 189 μM), Ba2+ (KD 3.5 μM), TEA (90% block at 10 mM) | Two types of single-channel currents with conductances of 28 and 80 pS were detected. The open probability was largest at EK. | 1434 |
| Kir4.1 channels | Rat/primary culture/acute slices/whole cell patch clamp; RT-PCR, immunocytochemistry | Ba2+ (100 μM) | The Kir4.1 channels (detected at mRNA and protein levels) were principally responsible for resting K+ conductance. Genetic deletion of Kir4.1 protein resulted in depolarizing shift in resting Vm and in a 4.5 times increase in the input resistance. | 1265 |
| Kir4.1 homomeric, Kir4.1/Kir5.1 heteromeric channels | Mouse/brain sections/cortex, hippocampus, olfactory bulb, thalamus/immunocytochemistry/immunoelectron microscopy | Both Kir4.1 and Kir5.1 are expressed specifically in astrocytes; the Kir4.1/Kir5.1 colocalization was detected in cortex, hippocampus, and pontine nucleus. The Kir4.1/Kir5.1 heteromers were mainly confined to perivascular and perisynaptic processes. | 694 | |
| Kir2.1, Kir2.2, Kir 2.3 | Mouse/acute slices/hippocampus/whole cell patch clamp, RT-PCR | Ba2+ (100 μM) | Strongly rectifying Kir currents reflected the expression (at mRNA level) of Kir2.1, Kir2.2, Kir2.3 channel isoforms. These current were inhibited following activation of AMPA receptors; inhibition was due to an increased Na+ influx. | 1576 |
| Kir6.1 | Rat, mouse/brain sections, acute slices/cerebellum | Tolbutamide (100 μM) | Kir6.1 expression was found in astrocytes from hippocampus, cortex, and cerebellum; Kir6.2 was detected in neurons only. Whole cell recordings revealed tolbutamide-sensitive KATP currents in Bergmann glial cells. | 1745 |
| Voltage-independent two-pore domain (K2P) K+ channels | ||||
| TREK-1,TWIK-1 | Rat/Acute slices/Hippocampus/Whole-cell patch clamp, Immunocytochemistry | Quinine (200 μM) | TREK-1,TWIK-1 channels were detected immunocytochemically and whole-cell currents sensitive to quinine were considered to reflect their operation. | 1955 |
| TREK-1,TREK-2,TWIK-1 | Mouse/freshly isolated astrocytes/hippocampus/whole cell voltage clamp, singe-cell RT-PCR | Quinine (200 μM), bupivacaine (200 μM) | Transcripts for TREK-1,TREK-2, and TWIK-1 were identified in single astrocytes. K2P currents (with almost linear I–V relationship) were isolated as quinine- or bupivacaine-sensitive components after inhibition of Kir and A-currents with Ba2+ and 4-AP. | 1588 |
| TWIK-1/TREK-1 heterodimer | Mouse/primary cultures/brain sections/whole cell voltage clamp, immunocytochemistry | The TREK1/K2P2.1(KCNK2) and TWIK1/K2P1.1(KCNK1) channels were localized in hippocampal astrocytes by immunocytochemistry. Acidification increased currents mediated by TREK1 channels. Further analysis in cultured cortical astrocytes and in astrocytes in hippocampal slices identified TWIK-1/TREK-1 heterodimer (formed by disulfide bridge between cysteine-cysteine residuals of both subunits) as the predominant form of astroglial channel. | 756, 1874, 1955 | |
| Voltage-gated delayed rectifying K+ currents | ||||
| KSI (slow inactivating), KD, KA | Rat/primary cultures/spinal cord/whole cell patch clamp | 4-AP (100 μM to 8 mM) | At 100 μM, 4-AP inhibited slowly inactivating outward current KSI; at 2 mM, KA and KD; at 8 mM, KIR. | 214 |
| Kv1.5 | Rat/primary cultures, acute slices/hippocampus, cerebellum, spinal cord/immunocytochemistry, whole cell patch clamp | TEA (0.4–40 mM) | Specific Kv1.5 antibodies revealed widespread staining of GFAP-positive astrocytes, with particularly high immunoreactivity in perivascular endfeet. Treatment of astrocytes with Kv1.5 siRNA halved the amplitudes of delayed rectifying whole cell currents. | 1513 |
| Kv1.4 | Rat/tissue sections/spinal cord/immunocytochemistry, Western blot, in situ hybridization | Both mRNA and protein for Kv1.4 were identified in astrocytes; Kv1.4 expression increased 6 wk after spinal cord injury | 466 | |
| Kv11.1, or ERG1 (ether-à-go-go-related gene) | Rat/acute slices/hippocampus/immunocytochemistry, whole cell patch clamp | Dofetilide (100–1,000 nM), E4031 (100 nM) | ERG1 channels were localized to astrocytes by immunocytochemistry and characterized electrophysiologically | 475 |
| Rapidly inactivated K+ current (A-current) | ||||
| A-current | Rat/hippocampus/acutely isolated astrocytes/whole cell patch clamp | 4-AP (1 mM) | The A-current has a characteristic fast kinetics and steady-state inactivation with V0.5 at −60 mV | 1763 |
| Kv4.3, Kv3.2, Kv1 | Rat/primary cultures/hippocampus/whole cell patch clamp, RT-PCR, immunocytochemistry | 4-AP (4 mM), TEA (10 mM), ETYA (nonhydrolyzable analog of arachidonic acid, 10 μM) | Pharmacological analysis indicated that Kv4, Kv3, and Kv1 account for 70, 10, and <5% of A-current, respectively. | 124 |
| Ca2+-activated K+ channels | ||||
| BK, IK | Rat/primary culture/cortex, hippocampus/cell-attached and whole cell patch clamp | TEA (1 mM) | Two types of unitary KCa currents with conductances of 71 ± 6 and 161 ± 11 pS were detected. KCa currents were potentiated by activation of mGluRs. | 562 |
| SK3 (KCa2.3) | Rat, mice/supraoptic nucleus/sections/immunocytochemistry | Immunoreactivity for SK3(KCa2.3) was mainly confined to astroglial processes. | 65 | |
| BK | Rat/cortex/acute slices/perforated whole cell patch clamp, cell-attached patch clamp | Iberiotoxin, IbTX (200 nM), TEA (1 mM) | In astroglial endfeet, both whole cell and single-channel (unitary conductance 225 pS) BK currents were observed. | 511 |
| KCa3.1 | Mouse/acute slices/cortex, whole cell patch clamp, immunocytochemistry, RT-PCR | KCa3.1 blocker TRAM-34 (1 mM), KCa2.1–2.3 and KCa3.1 agonist NS309 | Astrocytes express KCa2.3 and KCa3.1 mRNA; immunoreactivity of KCa3.1 was restricted to processes. KCa2.1/KCa3/1 opener increased, whereas KCa3.1 inhibitor decreased whole cell currents. | 1006 |
| Intracellular Ca2+ channels | ||||
| InsP3R type 2 | Mouse, rat/primary cultures, brain sections/RT-PCR, immunocytochemistry, transgenic techniques | The InsP3R2 are considered to be the main type of InsP3 receptors in astroglia. Genetic deletion of InsP3 R2 leads to an almost complete disappearance of global astrocytic Ca2+ signals. | 678, 718, 820, 1381, 1598, 1602, 1817 | |
| InsP3R type 1, InsP3R type 2 | Mouse/primary cultures/organotypic and freshly isolated hipocampal slices/RT-PCR, immunocytochemistry, /[Ca2+]i microfluorimetry | InsP3 R1 transcripts and proteins were detected in astrocytes cultured from hippocampus and entorhinal cortex. Evidence for InsP3R1/2-mediated Ca2+ release was obtained in imaging experiments. | 607, 986, 1604 | |
| RyR3 | Mouse/primary cultures/[Ca2+]i microfluorimetry | Ryanodine (200 mM), chloro-m-cresol (4 mM) | RYR agonist chloro-m-cresol triggered [Ca2+]i elevation. | 1089 |
| TPC1, TPC2 | Rat/primary culture/cortex/[Ca2+]i microfluorimetry, RT-PCR | NED-19 (TPC channel blocker, 10 μM), bafilomycin 1A (100 nM) | Injection of 100 nM NAADP triggered [Ca2+]i transients, sensitive to NED-19 and preincubation with bafilomycin 1A. RT-PCR revealed transcripts for TPC1 and TPC2 channels. | 1370 |
| Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels | ||||
| Hyperpolarization-activated cyclic nucleotide-gated channels, HCN1–4 | Mouse/freshly isolated, FACS sorted astrocytes, acute slices/RT-qPCR, whole cell patch clamp | NCN 1–4 were detected at mRNA level; the whole cell HNC-mediated currents have been characterized. | 719, 1520 | |
| Anion channels | ||||
| Voltage-gated Cl− channels, ClC-1, ClC-2, ClC3 | Mouse/primary cultures, acute slices/whole cell voltage clamp | ClC channels were detected at mRNA and protein levels. Inwardly rectifying Cl− currents (i.e., promoting Cl− efflux) have been characterized. | 456, 1054, 1326, 1489 | |
| Best1 | Mouse/primary culture, acute slices/cortex, hippocampus/[Ca2+]i microfluorimetry, whole cell perforated patch clamp, RT-PCR, immunocytochemistry | Niflumic acid (100 μM), flufenamic acid (100 μM), NPPB (100 μM) | Best1 was detected at mRNA (single-cell RT-PCR) and protein levels. Whole cell currents were characterized in isolated astrocytes and in astrocytes in slices. Silencing of Best1 gene with shRNA significantly suppressed Cl− currents. | 1324 |
| Volume-regulated anion channels, VRAC/SWELL1 | Mouse/primary culture, acute slices/whole cell voltage clamp | Channels have been mainly characterized in vitro; VRAC channels have been shown to mediate release of excitatory amino acids, which have been detected in vitro and in situ. Increase in [Na+]i suppressed VRAC activity. | 5, 138, 867, 868, 1122, 1333 | |
| Aquaporins | ||||
| Aquaporin: AQP1, AQP4, AQP9 | Mouse, rat/primary culture, brain sections/RT-PCR, immunocytochemostry | The main astroglial type is AQP4, which is primarily concentrated in the endfeet; are important water pathways, form the molecular basis for the glymphatic system. | 1184, 1749 | |
| Connexons and pannexons | ||||
| Cx43 | Mouse/primary culture/Northern blot, immunocytochemistry, double-cell patch clamp | The Cx43 expression was found at cRNA and protein level with immunoreactivity; mainly concentrated at intercellular contact areas. Single-channel conductance was determined at ~56 pS. | 570 | |
| Cx26 | Rat, mouse/brain/tissue sections/Northern blot, Western blot, immunocytochemistry, EM | Expression of Cx26 was detected in astroglial gap junctions. | 1187 | |
| Cx30 | Rat/whole brain/primary cultures, tissue sections, acute brain slices/immunocytochemistry, EM | Cx30 protein was detected in astrocytes in situ and in vitro; Cx30 expression increased postnatally attaining maximum at 4 wk after birth. Cx30 was colocalized with Cx43 at gap junctions. | 923, 1189 | |
| Cx26-Cx32 or Cx43/32-Cx47 | Mouse/whole brain/brain sections/immunocytochemistry | Heteromeric Cx26-Cx32 or Cx43/32-Cx47 gap junctional channels were found to connect astrocytes with oligodendrocytes. | 28 | |
| Panx1 | Mouse/primary cultures/RT-PCR, whole cell perforated patch clamp | CBX (50 mM), MFQ (100 nM) | Detected at mRNA level and by voltage clamp on cultured embryonic astrocytes. | 749, 758, 1443 |