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The Journal of Neuroscience, September 1, 2000, 20(17):6347-6354
The TASK-1 Two-Pore Domain K+ Channel Is a
Molecular Substrate for Neuronal Effects of Inhalation Anesthetics
Jay E.
Sirois1,
Qiubo
Lei1,
Edmund M.
Talley1,
Carl
Lynch III2, and
Douglas A.
Bayliss1
Departments of 1 Pharmacology and
2 Anesthesiology, School of Medicine, University of
Virginia, Charlottesville, Virginia 22908
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ABSTRACT |
Despite widespread use of volatile general anesthetics for well
over a century, the mechanisms by which they alter specific CNS
functions remain unclear. Here, we present evidence implicating the
two-pore domain, pH-sensitive TASK-1 channel as a target for specific, clinically important anesthetic effects in mammalian neurons.
In rat somatic motoneurons and locus coeruleus cells, two populations
of neurons that express TASK-1 mRNA, inhalation anesthetics activated a
neuronal K+ conductance, causing membrane
hyperpolarization and suppressing action potential discharge. These
membrane effects occurred at clinically relevant anesthetic levels,
with precisely the steep concentration dependence expected for
anesthetic effects of these compounds. The native neuronal
K+ current displayed voltage- and time-dependent
properties that were identical to those mediated by the open-rectifier
TASK-1 channel. Moreover, the neuronal K+ channel
and heterologously expressed TASK-1 were similarly modulated by
extracellular pH. The decreased cellular excitability associated with
TASK-1 activation in these cell groups probably accounts for specific
CNS effects of anesthetics: in motoneurons, it likely contributes to
anesthetic-induced immobilization, whereas in the locus coeruleus, it
may support analgesic and hypnotic actions attributed to inhibition of
those neurons.
Key words:
two-pore domain K+ channel; TASK-1; pH-sensitive; volatile anesthetic; motor neuron; locus
coeruleus neuron
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INTRODUCTION |
The mechanisms by which inhalation
anesthetics alter the behavior of central neurons have been scrutinized
for many years. Early hypotheses based on nonspecific interactions of
lipid-soluble anesthetics with membrane bilayers have largely given way
to the current idea that membrane-associated proteins, particularly ion channels, are specifically modulated by anesthetics (Franks and Lieb,
1994 ). Indeed, a number of ion channel targets of anesthetics have been
identified, and prominent among these are the inhibitory GABAA and glycine receptor channels (Daniels and
Smith, 1993; Franks and Lieb, 1994; Mihic et al., 1997). It now seems
certain that enhancement of chloride currents gated by GABA and glycine contributes to effects of inhalation anesthetics.
In addition to the well known potentiation of
GABAA and glycine channels, accumulating evidence
indicates that neuronal background K+
channels are also activated by volatile general anesthetics (Nicoll and
Madison, 1982; Takenoshita and Takahashi, 1987; Franks and Lieb, 1988;
Sugiyama et al., 1992; Winegar et al., 1996; Sirois et al., 1998;
Winegar and Yost, 1998a,b; Ries and Puil, 1999). Anesthetic activation
of background K+ channels in central
neurons causes membrane hyperpolarization and increases neuronal input
conductance, providing an additional inhibitory mechanism that could
contribute to the overall central depressant effects of these
compounds. However, the molecular identity of neuronal background
K+ channels, including those native
K+ channels sensitive to volatile
anesthetics, has remained obscure.
Over the last few years a new gene family of background
K+ channels has been identified. Members
of this family exhibit functional properties that suggest their
classification as background K+ channels
and present structural features that suggest a dimeric arrangement,
with each subunit comprising four transmembrane segments and two
pore-forming regions (for review, see Goldstein et al., 1998; Lesage
and Lazdunski, 1999). TASK-1 (also called KCNK3) is a member of
this gene family that generates a pH-sensitive, weakly rectifying
K+ current (Duprat et al., 1997; Kim et
al., 1998, 1999; Leonoudakis et al., 1998; Lopes et al., 2000); it is
called an "open rectifier" because it exhibits no time dependence
(and/or extremely fast kinetics; Lopes et al., 2000), and its weak
rectification in physiological asymmetric
K+ solutions can be accounted for entirely
by constant field considerations (Duprat et al., 1997; Kim et al.,
1998, 1999; Leonoudakis et al., 1998; Lopes et al., 2000). Importantly,
TASK-1 is activated by clinical concentrations of inhalation
anesthetics (i.e., 0.1-0.4 mM) after its heterologous
expression in mammalian cells (Patel et al., 1999).
We recently found that TASK-1 contributes to a prominent background
K+ current with the properties of a
pH-sensitive, open-rectifier in rat motoneurons (Talley et al., 2000),
and we earlier reported that inhalation anesthetics (i.e., halothane,
isoflurane, and sevoflurane) activate a weakly rectifying
K+ current in those same neurons (Sirois
et al., 1998). Together, these convergent observations suggested that
TASK-1 could be an anesthetic-sensitive K+
channel in motoneurons. Here, we have taken advantage of its pH- and
voltage-dependent properties to demonstrate that TASK-1 represents a
native K+ channel activated by clinically
appropriate concentrations of inhalation anesthetics in hypoglossal
motoneurons (HMs). Moreover, we find a similar anesthetic-sensitive
K+ current in locus coeruleus neurons,
where TASK-1 transcripts are also expressed (Talley et al., 2000).
Activation of TASK-1 in these brainstem neurons could account, in part,
for immobilizing and hypnotic effects of anesthetics. Thus, these data
provide a molecular identification of a native neuronal and
anesthetic-activated background K+ channel
that likely contributes to these specific, clinically important
anesthetic actions.
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MATERIALS AND METHODS |
In situ hybridization. In situ hybridization was
performed for detection of TASK-1 mRNA, exactly as described (Talley et
al., 2000). Briefly, transverse brain sections (fresh-frozen, 10 µm) were obtained from Sprague Dawley rats (Hilltop), thaw-mounted on
charged slides, and fixed, dehydrated, and delipidated. Sections were
hybridized to a [33P]UTP-labeled cRNA
probe transcribed from HindIII-digested rTASK1-pcDNA3 (Leonoudakis et al., 1998; Talley et al., 2000) using SP6 RNA polymerase (these and other enzymes obtained from Promega, Madison, WI)
in a hybridization buffer (50 × 106
cpm/ml) containing 50% formamide, 4× SSC (1× SSC: 150 mM NaCl and 15 mM sodium
citrate, pH 7), 1× Denhardt's solution (0.02% each of Ficoll,
polyvinylpyrrolidone, and bovine serum albumin), 10% dextran sulfate,
100 mM DTT, 250 µg/ml yeast tRNA, and 0.5 mg/ml
salmon testes DNA. After hybridization, sections were rinsed in SSC and
treated with RNase A (Boehringer Mannheim, Indianapolis, IN; 0.1 mg/ml
in 10 mM Tris, 500 mM NaCl,
and 1 mM EDTA, pH 7, 30 min at 37°C) and
exposed to film (Hyperfilm  MAX; Amersham, Arlington Heights, IL) for
4 d. Subsequently, they were dipped in liquid emulsion (NTB-2;
Eastman Kodak, Rochester, NY) and exposed for 3 weeks. The specificity
of the hybridization signal obtained with this probe has been
documented (Talley et al., 2000).
Electrical recordings. Transverse brainstem slices from
neonatal rats (7-14 d postnatal) were prepared as described (Sirois et
al., 1998; Talley et al., 2000). Rats were anesthetized with ketamine
and xylazine, brainstems were removed after rapid decapitation, and
transverse slices (200 µm) were cut with a microslicer (DSK 1500E;
Dosaka, Tokyo, Japan) in an ice-cold Ringer's solution consisting of (in mM) 260 sucrose, 3 KCl, 5 MgCl2, 1 CaCl2, 1.25 NaH2PO4, 26 NaHCO3, 10 glucose, and 1 kynurenic acid. After
cutting, slices were incubated for 1 hr at 37°C and subsequently at
room temperature in a Ringer's solution of 130 NaCl, 3 KCl, 2 MgCl2, 2 CaCl2, 1.25 NaH2PO4, 26 NaHCO3, and 10 glucose. Ringer's solutions were
bubbled with 95% O2 and 5%
CO2. Slices were visualized with infrared
differential interference optics, and neurons in the hypoglossal
nucleus and locus coeruleus (LC) were targeted for recording
based on anatomic location and characteristic size and shape (Amaral
and Sinnamon, 1977; Viana et al., 1990). Neurons close to the surface
of the slice were chosen for recording to minimize effects of
endogenous pH buffering within the slice (Voipio and Kaila, 1993;
Chesler et al., 1994).
Human embryonic kidney (HEK) 293 cells were transfected with
rTASK1-pcDNA3 by calcium phosphate precipitation. Cells were cotransfected with a modified green fluorescent protein (GFP; pGreenLantern; Life Technologies, Gaithersburg, MD) at a ratio of
rTASK-1 to GFP of 6:1. One day after transfection, cells were plated
onto glass coverslips; individual transfected cells were visualized
using a standard FITC filter set, and those with green fluorescence
were chosen for recording.
Electrical recordings of neurons and HEK 293 cells were obtained in a
bath solution of (in mM): 130 NaCl, 3 KCl, 2 MgCl2, 2 CaCl2, 10 HEPES, and 10 glucose, with pH adjusted using HCl or NaOH.
Tetrodotoxin (TTX, 0.75-1 µM; Calbiochem, La Jolla, CA) was included in the bathing solution for all voltage-clamp experiments in neurons. Bath solutions were bubbled vigorously with a room air gas
mixture (21% O2/balance
N2); halothane and sevoflurane were added to the
perfusate via calibrated vaporizers (Ohmeda, Austell, GA). Solutions
equilibrated with anesthetic were covered tightly with parafilm and
superfused at ~2 ml/min. Aqueous concentrations of anesthetic
solutions were determined by gas chromatographic analysis from samples
collected at the point of presentation to the preparation (Sirois et
al., 1998).
Patch electrodes with a DC resistance of 1.8-3.0 M were pulled from
borosilicate glass (Warner Instruments) and coated with Sylgard 184 (Dow Corning Corporation). Pipette solution contained (in
mM): 120 KCH3SO3; 4 NaCl; 1 MgCl2; 0.5 CaCl2; 10 HEPES;
10 EGTA; 3 MgATP; 0.3 GTP-Tris, with pH buffered to 7.2. To
block Ih, ZD 7288 (Tocris Cookson) was
included in the pipette solution at concentrations of 12-100
µM. To block GABAA and
glycine receptor channels in some experiments, bicuculline (10 µM) and strychnine (30 µM; both from Research Biochemicals
International, Natick, MA) were added to the perfusate.
Data acquisition and analysis. Recordings were obtained
under whole-cell conditions using an Axopatch 200A amplifier and
digitized with a Digidata 1200 analog-to-digital converter (Axon
Instruments, Foster City, CA). Series resistance (4-15 M) was
compensated by 70-75%, and a liquid junction potential (~10 mV) was
corrected offline. Voltage and current commands were applied, and
recordings were made and analyzed with pClamp software (Axon
Instruments). In current clamp, membrane potential was adjusted by DC
current injection; membrane potential was initially set to  60 mV for determining concentration-dependent effects on membrane potential and
input conductance. Input conductance was calculated from voltage responses to constant amplitude current pulses applied at 5-10 sec
intervals. Under voltage clamp, cells were held at 60 mV, and
membrane currents were recorded at constant intervals of 10-15 sec.
Current-voltage (I-V) relationships were obtained
in neurons using hyperpolarizing voltage steps (to 130 mV, in 10 mV
increments). In HEK 293 cells expressing TASK-1, depolarizing ramps
from 130 to + 40 mV (~0.2 V/sec) were used to obtain
I-V curves; slope conductance was determined from linear
fits to currents between 60 and 80 mV. Data are presented as
mean ± SEM. Curve fitting was accomplished using the least
squares method.
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RESULTS |
Halothane induces a pH-sensitive membrane hyperpolarization in
TASK-1-expressing hypoglossal motoneurons
TASK-1 is a pH-sensitive member of the two-pore domain gene family
of weakly rectifying neuronal background
K+ channels (Duprat et al., 1997; Kim et
al., 1998, 1999; Leonoudakis et al., 1998; Lopes et al., 2000) that is
activated by inhalation anesthetics and inhibited by extracellular
acidification when expressed in mammalian cells (Patel et al., 1999).
As shown in the dark-field autoradiograph from an in situ
hybridization experiment in Figure
1A, TASK-1 transcripts
are present at high levels in hypoglossal motoneurons (see also Talley
et al., 2000). In all hypoglossal motoneurons tested under current
clamp, halothane induced a membrane hyperpolarization that was
associated with increased input conductance, as reported previously
(Sirois et al., 1998). Consistent with the possibility that the
acid-sensitive TASK-1 channel mediates these effects of halothane in
motoneurons, we found that the hyperpolarization was completely
reversed in extracellular solutions acidified to pH 6.5 (Fig.
1B). This level of acidification maximally inhibits
the pH-sensitive resting K+ conductance in
motoneurons (Talley et al., 2000), as it does TASK-1 in heterologous
expression systems (Duprat et al., 1997; Leonoudakis et al., 1998;
Lopes et al., 2000; Talley et al., 2000).
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Figure 1.
Halothane hyperpolarizes motoneurons
at clinically relevant concentrations. A, Dark-field
photomicrograph of emulsion-dipped section hybridized with a
[33P]-labeled cRNA probe specific to TASK-1. Note
the high density of silver grains overlaying neurons in the hypoglossal
nucleus (nXII); labeling is also apparent in the
vagal motor nucleus (dmnX). Scale bar, 250 µm.
B, Current-clamp recording of a hypoglossal motoneuron
exposed to halothane (1.25 mM) and then to an acidified
extracellular solution, pH 6.5, in the continued presence of halothane.
Note that halothane caused a membrane hyperpolarization that was
reversed by acidosis. The resting potential of the motoneuron was 73
mV; it was induced to fire repetitively under control conditions by
current injection (360 pA, DC; action potentials are truncated by the
chart recorder). C, Current-clamp recording of membrane
potential in a hypoglossal motoneuron exposed to increasing
concentrations of halothane. Membrane hyperpolarization was evident
even at 0.1 mM and increased further in a
concentration-dependent manner. The hyperpolarization induced by
halothane was reversed when the acidified bath solution was
introduced during halothane exposure. Control membrane potential in
this motoneuron (and all cells tested with this protocol) was set at
60 mV by DC current injection. D,
Concentration-response curves for membrane effects of halothane. The
halothane-induced hyperpolarization from 60 mV was measured in
individual cells exposed to multiple concentrations of halothane.
Averaged data (± SEM) were plotted as a function of the aqueous
concentrations of halothane solution (± SEM), determined from gas
chromatographic analysis of replicate samples taken from the point of
presentation to the slice. Data were well fitted with the logistic
equation:  Em = max
Em/(1 + ([halothane]/EC50)n), with an
EC50 of 260 µM, a Hill coefficient
(n) of 2.9, and a maximum hyperpolarization
(max Em) of 13 mV.
Inset, Input conductance determined at each halothane
concentration was normalized to control input conductance (G
halothane/G control) and fitted to a logistic equation of similar form,
with an EC50 of 220 µM, n of
3.3, and a maximum G halothane/G control of 1.5 (i.e., 50% increase
in conductance).
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In mammalian cells, activation of recombinant TASK-1 by halothane is
concentration-dependent in the clinical range; current was increased by
~10% at halothane concentrations as low as 0.1 mM, with
an apparent EC50 near ~0.3-0.4 mM
and a maximal increase of ~60% (Patel et al., 1999). Likewise, we
found effects of halothane on membrane potential in motoneurons at 0.1 mM, with increasing concentrations of halothane evoking
additional hyperpolarization (Fig. 1C). The membrane
hyperpolarization and increased conductance induced by halothane were
similarly dose-dependent (Fig. 1D), both with
EC50 values remarkably close to those expected
for anesthetic effects of halothane (~250 µM;
Franks and Lieb, 1994). The fit to these data indicate a maximal
hyperpolarization of ~13 mV from 60 mV, with a 50% increase in
input conductance.
Clinically appropriate concentrations of anesthetics activate a
pH-sensitive K+ current in hypoglossal
motoneurons
Whole-cell voltage-clamp recordings were performed to determine
whether the K+ current in HMs has the
properties expected of the pH-sensitive TASK-1 channel. To study the
anesthetic-sensitive K+ current in
relative isolation, ZD 7288 was included in the pipette solution to
block the hyperpolarization-activated cation current (Ih) that contributes to anesthetic
effects in motoneurons (Sirois et al., 1998) and that may also be
modulated by changes in pH (Munsch and Pape, 1999).
Under these conditions, acidification of the extracellular solution
(from pH 7.3 to pH 6.5) induced an inward shift in holding current at
60 mV (Fig. 2A), as
expected from block of the pH-sensitive TASK-1 channel in motoneurons
(Talley et al., 2000). After wash to normal extracellular pH,
increasing concentrations of halothane applied via the perfusate
induced stepwise outward shifts in holding current; concordant with
current-clamp data, the anesthetic-induced current was completely
reversed by bath acidification to pH 6.5 (Fig. 2A).
At all concentrations tested, the halothane-induced current was
associated with an increase in conductance and displayed a weakly
outwardly rectifying current-voltage (I-V)
relationship that reversed near EK
(Fig. 2B, inset). Activation of this
K+ current was steeply dependent on
halothane concentration (Hill slope, 3.3) with an
EC50 of 230 µM (Fig.
2B), similar to that determined for membrane
hyperpolarization (Fig. 1D) and again nearly
identical to that reported for anesthetic effects of halothane (Franks
and Lieb, 1994).
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Figure 2.
Inhalation anesthetics activate a
pH-sensitive K+ current at clinically relevant
concentrations. A, Voltage-clamp recording of membrane
current in a hypoglossal motoneuron held at 60 mV. Bath acidification
to pH 6.5 caused an inward shift in holding current. After wash into a
neutral pH solution, halothane caused the development of an outward
current in a concentration-dependent manner. The current induced by
halothane was completely suppressed when acidified bath solution
was reintroduced during halothane exposure. B,
Concentration-response curve for current activation by halothane. The
halothane-induced current at 60 mV was measured in individual cells
exposed to multiple concentrations of halothane and normalized to the
current obtained with maximal concentrations of halothane (>0.9
mM). Averaged normalized data (± SEM) were plotted as a
function of measured aqueous halothane concentrations (± SEM). These
concentration-response data were well fitted by using a logistic
equation of the form
I/Imax = 1/(1 + ([halothane]/EC50)n), with an
EC50 of 230 µM and a Hill coefficient
(n) of 3.3. Inset, Note that at
submaximal and supramaximal halothane concentrations, the
I-V relationship of the halothane-induced current
rectified weakly in the outward direction, with a reversal potential
near EK. C, Voltage-clamp
recording of membrane current in a hypoglossal motoneuron held at 60
mV. Sevoflurane caused the development of an outward current in a
concentration-dependent manner; the current was suppressed when
acidified bath solution was introduced during exposure to the
anesthetic. D, Concentration-response curve for current
activation by sevoflurane. The sevoflurane-induced current at 60 mV
was measured in individual cells exposed to multiple concentrations of
sevoflurane and normalized to the current obtained with maximal
concentrations of sevoflurane (~0.7 mM). Averaged
normalized data (± SEM) were plotted as a function of the aqueous
concentrations of sevoflurane (± SEM). These concentration-response
data were well fitted by using a logistic equation of the form
I/Imax = 1/(1 + ([sevoflurane]/EC50)n), with an
EC50 of 290 µM and a Hill coefficient
(n) of 4. Inset, At all
concentrations, I-V relationships of the
sevoflurane-induced current rectified weakly in the outward direction,
with a reversal potential near EK
(inset).
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As expected from our previous work with isoflurane and sevoflurane
(Sirois et al., 1998), activation of this
K+ current was not limited among
inhalational anesthetics to halothane. The fluorinated ether anesthetic
sevoflurane also induced a concentration-dependent and pH-sensitive
outward shift in holding current (Fig. 2C), with weakly
rectifying voltage-dependent characteristics (Fig.
2D, inset) that were indistinguishable
from the halothane-sensitive K+ current.
As shown in Figure 2D, and similar to the halothane current, the sevoflurane-induced current activated with steep concentration dependence (Hill slope, 4) and an
EC50 of 290 µM, very
close to that expected for anesthetic effects of sevoflurane (~280
µM; Park et al., 1996).
Inhalation anesthetics can enhance chloride currents through
GABAA and/or glycine receptor channels,
especially at high concentrations (Daniels and Smith, 1993; Franks and
Lieb, 1994; Mihic et al., 1997). However, we found no evidence for a
contribution from those receptors to observed membrane effects of
anesthetics in motoneurons; after blocking GABAA
and glycine receptors with bicuculline (10 µM) and
strychnine (30 µM), the current induced by high
concentrations of halothane (1.25 mM) was not different in
magnitude, voltage dependence or reversal potential (n = 7, data not shown).
Extracellular acidification blocks a halothane-induced
open-rectifier K+ current in HMs
Extracellular acidification induced an inward shift in holding
current at 60 mV, and that pH-sensitive current was increased in the
continued presence of halothane (Fig. 2A). Because
the increase in pH-sensitive current amplitude was essentially the same
magnitude as the halothane-induced current itself, it appeared that the
same K+ channel activated by halothane was
completely blocked by bath acidification. Indeed, even a supramaximal
concentration of halothane (1.25 mM), which
induced a large outward current under control conditions, was entirely
ineffective when delivered with an extracellular solution titrated to
pH 6.5 (Fig. 3A). At 60 mV,
the current induced by 1.25 mM halothane averaged
160.0 ± 30.7 pA under control conditions and 13.9 ± 13.7 pA in acidified extracellular solution (n = 7;
p < 0.005).
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Figure 3.
The pH-sensitive halothane-activated
K+ current in motoneurons has the properties of an
open-rectifier. A, Voltage-clamp recording of membrane
current in a hypoglossal motoneuron held at 60 mV. A supramaximal
concentration of halothane (1.25 mM) induced an outward
current under control conditions that was completely inhibited after
blocking the pH-sensitive background K+ current with
an acidified extracellular solution. Inset, Sample
current responses to voltage steps (60 to 130 mV) obtained at the
indicated time points; I-V relationships were
determined from currents measured immediately after the capacitive
transient (arrowhead). B, Averaged
I-V relationships (± SEM, n = 5)
of the halothane-induced current in motoneurons at pH 7.3 (diamonds) and pH 6.5 (squares).
C, Averaged data depicting the I-V
relationship of the pH-sensitive component of halothane-induced current
in motoneurons. The data were derived by subtraction of the
I-V data obtained in pH 6.5 from that in pH 7.3 (± SEM, n = 5) and were well fitted by using the GHK
constant field equation. D, Voltage-clamp recording of
membrane current in a hypoglossal motoneuron held at 60 mV. An
acidified solution, pH 6.5, induced an inward shift in current that
reversed after wash, whereas halothane (1.25 mM) evoked an
outward shift. When retested in the continued presence of halothane,
the current induced by acidified solution was enhanced by an amount
essentially identical to the size of the halothane current itself.
E, Averaged I-V relationships (± SEM,
n = 12) of the pH-sensitive currents recorded under
control conditions (diamonds) and in the presence of
halothane (squares). F, Averaged data
depicting the I-V relationship of the halothane-induced
component of pH-sensitive current. The I-V data were
derived by subtraction of currents induced by acidification under
control conditions from those in the presence of halothane (± SEM,
n = 12) and were well fitted by using the GHK
constant field equation.
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In addition to its pH sensitivity, the TASK-1 currents have time- and
voltage-dependent properties that characterize it as an
"open-rectifier" (Duprat et al., 1997; Kim et al., 1998, 1999; Leonoudakis et al., 1998; Lopes et al., 2000; Talley et al., 2000). That is, TASK-1 currents show instantaneous and/or extremely fast activation kinetics, together with a slight outward rectification under
physiological asymmetric K+ concentrations
that is predicted by the Goldman-Hodgkin-Katz (GHK) constant field
equation (Hille, 1992). We took advantage of the pH sensitivity of the
halothane current to determine whether the native anesthetic- and
pH-dependent K+ current in hypoglossal
motoneurons has the properties of TASK-1 (Fig. 3). I-V
relationships were obtained from current responses to voltage steps
applied at the indicated time points (Fig. 3A, inset); currents evoked by hyperpolarizing voltage steps
displayed no appreciable time-dependent activation or inactivation,
indicating that they were instantaneous and persistent. The
I-V relationship of the halothane-induced current at pH 7.3 rectified mildly in the outward direction and reversed near the
expected EK (Fig. 3B,
diamonds); only a small residual halothane-sensitive current remained in pH 6.5 that was not investigated further (Fig.
3B, squares). The component of halothane current
blocked by acidification was derived by subtracting I-V
curves from the two pH conditions (Fig. 3C). Those data were
well fitted by using the GHK constant field equation, indicating that
an open rectifier K+ conductance underlies
the pH-sensitive halothane current in hypoglossal motoneurons.
We performed similar I-V analyses, but with a reverse order
of application, to reveal the voltage-dependent profile of the component of pH-sensitive current that was enhanced by halothane (Fig.
3D-F). I-V relationships of the
pH-sensitive current obtained under control conditions (i.e., before
halothane application; Fig. 3E, diamonds) and in
the presence of halothane (Fig. 3E, squares)
indicated that the inward shift in holding current induced by the pH
6.5 solution was associated with a decrease in a weakly rectifying
conductance that reversed near EK.
Subtracting I-V data in halothane from that in control
yielded the halothane-induced component of pH-sensitive current, which
was well fitted by using the GHK equation (Fig. 3F),
again indicating that halothane activated an open rectifier
K+ conductance that was blocked by acidification.
K+ current activation by halothane and
alkalization are not occlusive
Maximal activation of the motoneuronal
K+ current by inhalation anesthetics did
not prevent further current activation by extracellular alkalization.
As shown in Figure 4A,
an outward shift in current was evoked when the pH of the extracellular
solution was increased (from pH 7.3 to 8.4). In previous work, we found
that further alkalization beyond this level did not cause additional
activation of the pH-sensitive K+
conductance in motoneurons or of TASK-1 in heterologous expression systems (Talley et al., 2000) (see also Duprat et al., 1997;
Leonoudakis et al., 1998; Lopes et al., 2000). Nevertheless, in these
experiments, the current induced by halothane (241.2 ± 37.8 pA at
60 mV) was always larger than that evoked by alkalization to pH 8.4 (51.6 ± 9.0 pA; n = 10; p < 0.0005) and, in the continued presence of halothane, alkalization
always caused a further increase in holding current that was often
enhanced in amplitude (Fig. 4A). The mechanism for
this current enhancement was not explored, but it was clear that the
current induced by alkalization in the presence of halothane once again
had the open-rectifier properties of TASK-1 (Fig.
4B).
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Figure 4.
Currents activated maximally by
extracellular alkalization and halothane are not occlusive in
motoneurons or in rTASK-1-expressing HEK 293 cells. A,
Membrane current in a hypoglossal motoneuron held at 60 mV was
shifted outward by an alkalized solution, pH 8.4, and by halothane
(1.25 mM). In the continued presence of halothane, the
current induced by pH 8.4 solution was still evident (and indeed
enhanced). B, Averaged data (± SEM,
n = 10) depicting the I-V
relationship of the current induced by alkalized solution in the
presence of halothane. The data were derived by subtraction of currents
induced by halothane from those obtained in the pH 8.4 solution in the
continued presence of halothane (inset). This additional
current evoked by extracellular alkalization was well fitted by using
the GHK constant field equation. C, The conductance in
HEK 293 cells expressing rTASK-1, determined by using ramp voltage
commands, was increased by halothane (1.25 mM) when the
pH-sensitive current was enabled by an alkalized extracellular solution
(pH 8.4), but not after blocking the pH-sensitive conductance with an
acidic solution (pH 6.5). Inset, Averaged data (± SEM,
n = 6), depicting the halothane-induced conductance
(G halothane) as a percentage of the total pH-sensitive conductance (G
pH = G8.4 G6.5), show the
complete block of halothane effect in acidified extracellular solution.
D, The I-V relationship of the
halothane-sensitive current in alkalized solution was derived from ramp
voltage commands and was well fitted by using the GHK constant field
equation.
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|
Permissive pH conditions are necessary for halothane activation of
rTASK-1 in HEK 293 cells
The results presented to this point indicate that the
anesthetic-induced K+ channel in
motoneurons has the voltage-dependent properties of TASK-1 and is
sensitive to extracellular pH. Therefore, we investigated the pH
dependence of anesthetic effects on TASK-1 channels expressed heterologously in mammalian cells.
Slow depolarizing voltage ramps were applied to HEK 293 cells
transfected with the rat TASK-1 channel, and the slope conductance was
determined by linear fit to the resultant I-V curve over
the voltage range between 60 and 80 mV. As expected from previous reports with the human TASK-1 channel (Patel et al., 1999), halothane increased conductance in rTASK-1-expressing HEK 293 cells at pH 7.3 (data not shown), confirming the anesthetic sensitivity of the rTASK-1
channel under physiological pH conditions. Alkalization of the
extracellular solution induced an increase in conductance, and under
these conditions of maximal TASK-1 channel activation by alkalization
(Duprat et al., 1997; Leonoudakis et al., 1998; Lopes et al., 2000;
Talley et al., 2000), halothane was able to induce an additional
increase in conductance (Fig. 4C). So, as with the native
current in motoneurons, effects of halothane and alkalization were not
occlusive in these rTASK-1-expressing HEK 293 cells. Note that the
I-V relationship of the halothane-induced current was well
fitted by the overlaid GHK equation (Fig. 4D), reflecting the open rectifier characteristics of the cloned rTASK-1 channel (Leonoudakis et al., 1998). In addition, the rTASK-1
conductance was completely blocked after switching to a pH 6.5 solution
(Leonoudakis et al., 1998; Talley et al., 2000), and as in motoneurons,
halothane was without any effect under this condition of maximal
channel block by extracellular acidification (Fig. 4C).
Thus, rat brainstem motoneurons express TASK-1 and an
anesthetic-activated K+ current with time-
and voltage-dependent properties essentially identical to TASK-1.
Further consistent with a mediating role for TASK-1, we found that both
the motoneuronal anesthetic-sensitive K+
current and the anesthetic effects on rTASK-1 were similarly dependent
on the prevailing pH; in both settings, the anesthetic-sensitive current was blocked in solutions acidified to levels that completely inhibit TASK-1 but enhanced in neutral or alkalized conditions when the
TASK-1 channel is enabled. The fact that channel activation by
permissive pH conditions and by halothane did not obstruct each other
suggests that distinct mechanisms mediate these two forms of
modulation, but this issue was not examined in further detail.
Locus coeruleus neurons express TASK-1 and a halothane-sensitive
current with the properties of TASK-1
To determine whether TASK-1 expression correlates with effects of
inhalation anesthetics on K+ channels in
other neurons, we tested effects of halothane in LC neurons, which
express moderately high levels of TASK-1 mRNA (Fig.
5A). Qualitatively, the
results from LC neurons were remarkably similar to those described
above in motoneurons, although the maximal anesthetic current in LC
neurons was smaller in amplitude. This is consistent with lower levels
of TASK-1 expression in LC neurons, as assessed by high power
microscopic examination of silver grains overlying individual cells
(data not shown; Talley et al., 2000). Despite evoking a smaller
current, halothane was nonetheless able to induce a membrane
hyperpolarization and decrease the spiking activity in every LC neuron
tested (n = 12; Fig. 5B). This inhibition of
neuronal firing was observed at clinical concentrations of halothane
(i.e., 0.3 and 0.4 mM; n = 4 each) and was not a result of activation of GABAA
or glycine receptors because it occurred in the presence of bicuculline
and strychnine (10 and 30 µM). Importantly, as
in motoneurons, the halothane-induced inhibition was reversed when the
extracellular solution was acidified (Fig. 5B).
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|
Figure 5.
Halothane hyperpolarizes neurons of the locus
coeruleus by activating a pH-sensitive, open-rectifier
K+ current. A, Dark-field
photomicrograph of emulsion-dipped section hybridized with a
[33P]-labeled cRNA probe specific to TASK-1. Note
the high density of silver grains overlaying neurons in the nucleus
locus coeruleus (LC); labeling is also apparent in
motoneurons of the trigeminal nucleus (MoV).
Scale bar, 250 µm. B, Current-clamp recording of a
locus coeruleus neuron exposed to halothane (0.3 mM) and
then to an acidified extracellular solution, pH 6.5, in the continued
presence of halothane. Halothane caused a membrane hyperpolarization
that was reversed by acidosis. Cell was recorded in the continuous
presence of bicuculline (10 µM) and strychnine (30 µM), to block GABAA and glycine receptors.
C, Voltage-clamp recording of net membrane current in a
locus coeruleus cell held at 60 mV. An acidified solution, pH 6.5, induced an inward shift in current that reversed after wash, whereas
halothane evoked an outward shift. When retested in the continued
presence of halothane, the change in current induced by acidified
solution was enhanced. D, Effects of halothane on
membrane current in a locus coeruleus neuron held at 60 mV under
voltage clamp. Halothane induced an outward current that was completely
blocked in solution acidified to pH 6.5. E, Averaged
data (± SEM, n = 4) showing the pH sensitivity of
halothane-induced current in locus coeruleus neurons.
I-V data were derived by subtraction of the halothane
current obtained in pH 6.5 from that in pH 7.3, and were well fitted by
using the GHK constant field equation. Inset, Halothane
current at 60 mV in control, pH 7.3, and acidified, pH 6.5, solutions.
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|
Under voltage clamp, extracellular acidification evoked an inward shift
in holding current. Unlike motoneurons, in which this pH-sensitive
current was essentially entirely attributable to a
K+ conductance with properties of TASK-1
(Fig. 2E) (see also Talley et al., 2000), it appeared
that multiple ionic conductances were involved in LC neurons (data not
shown). Nevertheless, similar to motoneurons, halothane induced an
outward current in LC neurons, and this additional halothane-sensitive
current was completely suppressed in acidified bath solution (Fig.
5C). This was also apparent when the order of presentation
was reversed; the current induced by halothane under control pH
conditions was completely blocked in acidified extracellular solutions
(Fig. 5D,E, inset). The averaged I-V
relationships of the pH-sensitive component of halothane current in LC
neurons rectified weakly in the outward direction and reversed near
EK (Fig. 5E); again, as in
motoneurons and HEK 293 cells expressing rTASK-1, the rectification of
the halothane current was well described by the overlaid GHK constant field equation (Fig. 5E).
| |
DISCUSSION |
We have characterized a pH-sensitive background
K+ current in neurons that is activated by
inhalation anesthetics at precisely the concentrations that produce
clinical effects of these compounds. This native neuronal
K+ current has properties that identify it
as TASK-1, or a channel with properties of TASK-1. Its activation in
motoneurons and LC neurons suggest that TASK-1 represents a molecular
substrate for specific clinical actions of volatile anesthetics
mediated by these neurons.
Properties of anesthetic-activated K+ currents
in motoneurons and LC neurons are those of TASK-1
In motoneurons and LC neurons, which express native TASK-1, and in
HEK 293 cells expressing the cloned rTASK-1 channel, the halothane-sensitive K+ currents were
dependent on the prevailing extracellular pH; halothane was able to
activate K+ currents in neutral and
alkalized conditions, when the pH-sensitive K+ channel was enabled, but was
ineffective in solutions acidified to levels that completely block
TASK-1. Furthermore, in both native and heterologous systems, the
pH-sensitive component of halothane current had the properties of an
open-rectifier. Of all K+ channels yet
identified by molecular cloning, including other anesthetic-sensitive
members of the two-pore domain K+ channel
family (see below), the combined properties of pH sensitivity in the
physiological range and open rectification are unique to TASK-1 (Duprat
et al., 1997; Kim et al., 1998, 1999; Leonoudakis et al., 1998; Lopes
et al., 2000; Talley et al., 2000).
It is noteworthy that, of the eight functional mammalian two-pore
domain background K+ channels identified
to date, three are known to be activated by inhalation anesthetics:
TASK-1, TASK-2, and TREK-1 (Patel et al., 1999; Gray et al., 2000). Of
these, however, only TASK-1 has the properties of pH sensitivity and
open rectification described for the anesthetic-sensitive background
K+ current in motoneurons and LC neurons.
The recently characterized rat TASK-3 channel is a pH-sensitive
open-rectifier, but it has a pK of ~6.7 (Kim et al., 2000), clearly
lower than TASK-1 or the pH-sensitive current we found in motoneurons
(pK, ~7.2-7.4; Duprat et al., 1997; Leonoudakis et al., 1998; Lopes
et al., 2000; Talley et al., 2000) and its anesthetic sensitivity has
not been reported. The TREK-1 two-pore domain channel is activated by
inhalation anesthetics, but only at higher concentrations (Patel et
al., 1999). Furthermore, it displays outward rectification (Fink et al., 1996), is sensitive to intracellular rather than extracellular pH
and is activated, not inhibited, by acidification (Maingret et al.,
1999). The anesthetic-sensitive TASK-2 channel is inhibited by
extracellular acidification and activated by alkalization (Gray et al.,
2000), but TASK-2 currents have a pK of ~8.5 (Reyes et al., 1998),
much higher than TASK-1 or the pH-sensitive current in motoneurons
(Duprat et al., 1997; Leonoudakis et al., 1998; Lopes et al., 2000;
Talley et al., 2000). Moreover, TASK-2 generates a time-dependent
current that rectifies outwardly (Reyes et al., 1998), unlike TASK-1
and the anesthetic current in motoneurons and LC neurons (Duprat et
al., 1997; Kim et al., 1998, 1999; Leonoudakis et al., 1998; Lopes et
al., 2000; Talley et al., 2000). Nevertheless, both TREK-1 and TASK-2
are expressed in the mammalian CNS (Fink et al., 1996; Reyes et al.,
1998) and it is therefore possible that they could contribute to
anesthetic-activated K+ currents described
in other neuronal cell groups. Furthermore, given the size of this
K+ channel gene family in other species
(i.e., >50 genes in Caenorhabditis elegans and 11 in
Drosophila melanogaster; Wang et al., 1999; Littleton and
Ganetzky, 2000), it is likely that additional members of this gene
family will be identified in mammalian species, and some of those could
also be anesthetic-sensitive.
Clinical relevance of TASK-1 activation by anesthetics in
motoneurons and LC neurons
The activation of native neuronal pH-sensitive
K+ currents by anesthetics occurs with
exactly the concentration dependence expected for anesthetic effects
(Franks and Lieb, 1994) and is of particular interest because of the
well described physiology of motoneurons and LC neurons and their
potential involvement in mediating specific anesthetic effects. TASK-1
transcripts are expressed at high levels in cranial and spinal
motoneurons (Talley et al., 2000). Motoneurons directly control muscle
activity, and it is therefore likely that TASK-1 activation contributes
to the inhibition of motoneuron excitability and immobilization known to accompany inhalation anesthesia (Zhou et al., 1997, 1998). LC
neurons are the major source of norepinephrine in the CNS and have long
been implicated in control of vigilance and arousal (Aston-Jones et
al., 1991). Halothane depresses LC neuronal activity in
vivo (Camproux et al., 1996), and in halothane-anesthetized rats,
activation of LC neurons converts EEG activity from a sleep-like, large
amplitude, slow-wave pattern to the small-amplitude, high-frequency pattern associated with waking (Berridge and Foote, 1991). Moreover, LC
neurons mediate hypnotic and supraspinal analgesic effects of
 2-adrenoceptor agonist anesthetic compounds
such as clonidine and dexmedetomidine (Correa-Sales et al., 1992;
Aantaa and Scheinin, 1993; Guo et al., 1996; Lakhlani et al., 1997),
which suppress LC firing by activation of a distinct, G-protein-coupled
inwardly rectifying K+ (GIRK) conductance
(North, 1989; Lakhlani et al., 1997). The inhibition of LC firing that
occurs after halothane-induced TASK-1 activation would be expected to
produce similar hypnotic and analgesic effects. Furthermore,
coactivation in LC neurons of these two different
K+ channels TASK-1 and GIRKcould
account, at least in part, for the interactive effects of
2 agonists and inhalation anesthetics [i.e.,
the ability of clonidine and dexmedetomidine to lower the minimum
alveolar concentration (MAC) of halothane; Aantaa and Scheinin, 1993;
Lakhlani et al., 1997].
Because TASK-1 is activated by halothane, even in excised patches
(Patel et al., 1999), it seems likely that it could contribute to
previous reports of anesthetic-sensitive background
K+ channels in other neurons (e.g.,
cerebellar granule neurons, sensory relay neurons of the intralaminar
and ventrobasal thalamic nuclei; Sugiyama et al., 1992; Winegar and
Yost, 1998a; Ries and Puil, 1999) in which TASK-1 is also expressed
(Leonoudakis et al., 1998; Millar et al., 2000; Talley et al., 2000).
The intrinsic pH sensitivity of TASK-1 with a pK~7.2-7.4 (Duprat et
al., 1997; Leonoudakis et al., 1998; Lopes et al., 2000; Talley et al.,
2000), together with the pH dependence of TASK-1 activation by
anesthetics that we have demonstrated, provides a means to determine
its contribution to anesthetic-sensitive
K+ channels in these other neurons. This
pH dependence also suggests that anesthetic effects mediated via this
channel in TASK-1-expressing neurons will be enhanced by alkalization
and inhibited by acidification in the physiological range. However,
interpreting effects of global changes in CSF pH on anesthetic
sensitivity solely in the context of TASK-1 modulation is problematic,
not only because many other neural processes will potentially be
affected by changes in pH, but also because simply lowering pH has
narcotic effects on its own (Eisele et al., 1967). This pH-dependent
narcosis, which occurs with a MAC at a pH of ~6.9 (Eisele et al.,
1967), would clearly confound any analysis of effects of pH on
anesthetic sensitivity caused by TASK-1, which is completely inhibited
only at a pH of ~6.5. Identification of the precise contribution of
TASK-1 to effects of anesthetic compounds will likely require
experiments using mice with targeted disruptions of TASK-1 or mice that
express TASK-1 channels lacking anesthetic sensitivity. Nevertheless, if the presence of TASK-1 indeed predicts inhibitory effects of inhalation anesthetics, as our data indicate, then its differential expression in neurons that subserve unique functional roles provides the potential for specificity in anesthetic actions mediated by TASK-1.
| |
FOOTNOTES |
Received March 27, 2000; revised June 6, 2000; accepted June 20, 2000.
This work was supported by Grant NS33583 (D.A.B.) and Fellowships
HL10271 (J.E.S.) and MH12091 (E.M.T.) from the National Institutes of
Health. We thank Dr. M. B. Harrison for imaging equipment and
support and Dr. A. T. Gray for the gift of TASK-1 cDNA. We also
thank Dr. Marcel Durieux for providing helpful comments while this
study was ongoing, Ms. Jacqueline Washington for help with gas
chromatography, and Dr. Albert Berger for comments on this manuscript.
Correspondence should be addressed to Douglas A. Bayliss, Department of
Pharmacology, University of Virginia Health System, P.O. Box 800735, 1300 Jefferson Park Avenue, Charlottesville, VA 22908-0735. E-mail:
dab3y{at}virginia.edu.
| |
REFERENCES |
-
Aantaa R,
Scheinin M
(1993)
Alpha2-adrenergic agents in anaesthesia.
Acta Anaesthesiol Scand
37:433-448[ISI][Medline].
-
Amaral DG,
Sinnamon HM
(1977)
The locus coeruleus: neurobiology of a central noradrenergic nucleus.
Prog Neurobiol
9:147-196[ISI][Medline].
-
Aston-Jones G,
Chiang C,
Alexinsky T
(1991)
Discharge of noradrenergic locus coeruleus neurons in behaving rats and monkeys suggests a role in vigilance.
Prog Brain Res
88:501-520[ISI][Medline].
-
Berridge CW,
Foote SL
(1991)
Effects of locus coeruleus activation on electroencephalographic activity in neocortex and hippocampus.
J Neurosci
11:3135-3145[Abstract].
-
Camproux AC,
Saunier F,
Chouvet G,
Thalabard JC,
Thomas G
(1996)
A hidden Markov model approach to neuron firing patterns.
Biophys J
71:2404-2412[Abstract/Free Full Text].
-
Chesler M,
Chen JC,
Kraig RP
(1994)
Determination of extracellular bicarbonate and carbon dioxide concentrations in brain slices using carbonate and pH-selective microelectrodes.
J Neurosci Methods
53:129-136[ISI][Medline].
-
Correa-Sales C,
Rabin BC,
Maze M
(1992)
A hypnotic response to dexmedetomidine, an 2 agonist, is mediated in the locus coeruleus in rats.
Anesthesiology
76:948-952[ISI][Medline].
-
Daniels S,
Smith EB
(1993)
Effects of general anaesthetics on ligand-gated ion channels.
Br J Anaesth
71:59-64[Free Full Text].
-
Duprat F,
Lesage F,
Fink M,
Reyes R,
Heurteaux C,
Lazdunski M
(1997)
TASK, a human background K+ channel to sense external pH variations near physiological pH.
EMBO J
16:5464-5471[ISI][Medline].
-
Eisele JH,
Eger EI,
Muallem M
(1967)
Narcotic properties of carbon dioxide in the dog.
Anesthesiology
28:856-865[ISI][Medline].
-
Fink M,
Duprat F,
Lesage F,
Reyes R,
Romey G,
Heurteaux C,
Lazdunski M
(1996)
Cloning, functional expression and brain localization of a novel unconventional outward rectifier K+ channel.
EMBO J
15:6854-6862[ISI][Medline].
-
Franks NP,
Lieb WR
(1988)
Volatile general anaesthetics activate a novel neuronal K+ current.
Nature
333:662-664[Medline].
-
Franks NP,
Lieb WR
(1994)
Molecular and cellular mechanisms of general anaesthesia.
Nature
367:607-614[Medline].
-
Goldstein SA,
Wang KW,
Ilan N,
Pausch MH
(1998)
Sequence and function of the two P domain potassium channels: implications of an emerging superfamily.
J Mol Med
76:13-20[ISI][Medline].
-
Gray AT,
Zhao BB,
Kindler CH,
Winegar BD,
Mazurek MJ,
Xu J,
Chavez RA,
Forsayeth JR,
Yost CS
(2000)
Volatile anesthetics activate the human tandem pore domain baseline K+ channel KCNK5.
Anesthesiology
92:1722-1730[ISI][Medline].
-
Guo TZ,
Jiang JY,
Buttermann AE,
Maze M
(1996)
Dexmedetomidine injection into the locus coeruleus produces antinociception.
Anesthesiology
84:873-881[ISI][Medline].
-
Hille B
(1992)
In: Ionic channels of excitable membranes. Sunderland, MA: Sinauer.
-
Kim D,
Fujita A,
Horio Y,
Kurachi Y
(1998)
Cloning and functional expression of a novel cardiac two-pore background K+ channel (cTBAK-1).
Circ Res
82:513-518[Abstract/Free Full Text].
-
Kim Y,
Bang H,
Kim D
(1999)
TBAK-1 and TASK-1, two-pore K+ channel subunits: kinetic properties and expression in rat heart.
Am J Physiol
277:H1669-H1678.
-
Kim Y,
Bang H,
Kim D
(2000)
TASK-3, a new member of the tandem pore K+ channel family.
J Biol Chem
275:9340-9347[Abstract/Free Full Text].
-
Lakhlani PP,
MacMillan LB,
Guo TZ,
McCool BA,
Lovinger DM,
Maze M,
Limbird LE
(1997)
Substitution of a mutant 2a-adrenergic receptor via "hit and run" gene targeting reveals the role of this subtype in sedative, analgesic, and anesthetic-sparing responses in vivo.
Proc Natl Acad Sci USA
94:9950-9955[Abstract/Free Full Text].
-
Leonoudakis D,
Gray AT,
Winegar BD,
Kindler CH,
Harada M,
Taylor DN,
Chavez RA,
Forsayeth JR,
Yost CS
(1998)
An open rectifier potassium channel with two pore domains in tandem cloned from rat cerebellum.
J Neurosci
18:868-877[Abstract/Free Full Text].
-
Lesage F,
Lazdunski M
(1999)
Potassium channels with two P domains.
In: Potassium channels: molecular structure, function, and diseases (Kurachi Y,
Jan LY,
Lazdunski M,
eds), pp 199-222. San Diego: Academic.
-
Littleton JT,
Ganetzky B
(2000)
Ion channels and synaptic organization: analysis of the Drosophila genome.
Neuron
26:35-43[ISI][Medline].
-
Lopes CMB,
Gallagher PG,
Buck ME,
Butler MH,
Goldstein SAN
(2000)
Proton block and voltage gating are potassium-dependent in the cardiac leak channel Kcnk3.
J Biol Chem
275:16969-16978[Abstract/Free Full Text].
-
Maingret F,
Patel AJ,
Lesage F,
Lazdunski M,
Honore E
(1999)
Mechano- or acid stimulation, two interactive modes of activation of the TREK-1 potassium channel.
J Biol Chem
274:26691-26696[Abstract/Free Full Text].
-
Mihic SJ,
Ye Q,
Wick MJ,
Koltchine VV,
Krasowski MD,
Finn SE,
Mascia MP,
Valenzuela CF,
Hanson KK,
Greenblatt EP,
Harris RA,
Harrison NL
(1997)
Sites of alcohol and volatile anaesthetic action on GABAA and glycine receptors.
Nature
389:385-389[Medline].
-
Millar JA,
Baratt L,
Southan AP,
Page KM,
Fyffe REW,
Robertson B,
Mathie A
(2000)
A functional role for the two-pore domain potassium channel TASK-1 in cerebellar granule neurons.
Proc Natl Acad Sci USA
97:3614-3618[Abstract/Free Full Text].
-
Munsch T,
Pape H-C
(1999)
Modulation of the hyperpolarization-activated cation current of rat thalamic relay neurones by intracellular pH.
J Physiol (Lond)
519:493-504[Abstract/Free Full Text].
-
Nicoll RA,
Madison DV
(1982)
General anesthetics hyperpolarize neurons in the vertebrate central nervous system.
Science
217:1055-1057[Abstract/Free Full Text].
-
North RA
(1989)
Drug receptors and the inhibition of nerve cells.
Br J Pharmacol
98:13-28[ISI][Medline].
-
Park WK,
Pancrazio JJ,
Suh CK,
Lynch C
(1996)
Myocardial depressant effects of sevoflurane. Mechanical and electrophysiologic actions in vitro.
Anesthesiology
84:1166-1176[ISI][Medline].
-
Patel AJ,
Honore E,
Lesage F,
Fink M,
Romey G,
Lazdunski M
(1999)
Inhalational anesthetics activate two-pore-domain background K+ channels.
Nat Neurosci
2:422-426[ISI][Medline].
-
Reyes R,
Duprat F,
Lesage F,
Fink M,
Salinas M,
Farman N,
Lazdunski M
(1998)
Cloning and expression of a novel pH-sensitive two pore domain K+ channel from human kidney.
J Biol Chem
273:30863-30869[Abstract/Free Full Text].
-
Ries CR,
Puil E
(1999)
Ionic mechanism of isoflurane's actions on thalamocortical neurons.
J Neurophysiol
81:1802-1809[Abstract/Free Full Text].
-
Sirois JE,
Pancrazio JJ,
Lynch C,
Bayliss III DA
(1998)
Multiple ionic mechanisms mediate inhibition of rat motoneurones by inhalation anaesthetics.
J Physiol (Lond)
512:851-862[Abstract/Free Full Text].
-
Sugiyama K,
Muteki T,
Shimoji K
(1992)
Halothane-induced hyperpolarization and depression of postsynaptic potentials of guinea pig thalamic neurons in vitro.
Brain Res
576:97-103[ISI][Medline].
-
Takenoshita M,
Takahashi T
(1987)
Mechanisms of halothane action on synaptic transmission in motoneurons of the newborn rat spinal cord in vitro.
Brain Res
402:303-310[ISI][Medline].
-
Talley EM,
Lei Q,
Sirois JE,
Bayliss DA
(2000)
TASK-1, a two pore domain K+ channel, is modulated by multiple neurotransmitters in motoneurons.
Neuron
25:399-410[ISI][Medline].
-
Viana F,
Gibbs L,
Berger AJ
(1990)
Double- and triple-labeling of functionally characterized central neurons projecting to peripheral targets studied in vitro.
Neuroscience
38:829-841[ISI][Medline].
-
Voipio J,
Kaila K
(1993)
Interstitial PCO2 and pH in rat hippocampal slices measured by means of a novel fast CO2/H+-sensitive microelectrode based on a PVC-gelled membrane.
Pflügers Arch
423:193-201[ISI][Medline].
-
Wang ZW,
Kunkel MT,
Wei A,
Butler A,
Salkoff L
(1999)
Genomic organization of nematode 4TM K+ channels.
Ann NY Acad Sci
868:286-303[Abstract/Free Full Text].
-
Winegar BD,
Owen DF,
Yost CS,
Forsayeth JR,
Mayeri E
(1996)
Volatile general anesthetics produce hyperpolarization of Aplysia neurons by activation of a discrete population of baseline potassium channels.
Anesthesiology
85:889-900[ISI][Medline].
-
Winegar BD,
Yost CS
(1998a)
Activation of single potassium channels in rat cerebellar granule cells by volatile anesthetics.
Toxicol Lett
100-101:287-291.
-
Winegar BD,
Yost CS
(1998b)
Volatile anesthetics directly activate baseline S K+ channels in Aplysia neurons.
Brain Res
807:255-262[ISI][Medline].
-
Zhou HH,
Mehta M,
Leis AA
(1997)
Spinal cord motoneuron excitability during isoflurane and nitrous oxide anesthesia.
Anesthesiology
86:302-307[ISI][Medline].
-
Zhou HH,
Jin TT,
Qin BS,
Turndorf H
(1998)
Suppression of spinal cord motoneuron excitability correlates with surgical immobility during isoflurane anesthesia.
Anesthesiology
88:955-961[ISI][Medline].
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ABSTRACT
INTRODUCTION
