8/18/2019 IKCa Channels Are a Critical Determinant of the Slow AHP in CA1 Pyramidal Neurons
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Report
IKCa Channels Are a Critical Determinant of the Slow AHP in CA1 Pyramidal Neurons
Graphical Abstract
Highlights
d CA1 pyramidal cells express intermediate-conductance
Ca2+-activated K + channels
d An sAHP exhibits a pharmacological profile specific to IKCa
channels
d IKCa channels reduce temporal summation of EPSPs and
mediate spike accommodation
d IKCa channels are a key determinant of the sAHP in CA1
pyramidal cells
Authors
Brian King, Arsalan P. Rizwan, ...,
Gerald W. Zamponi, Ray W. Turner
Correspondence
In Brief
The molecular identity of a Ca2+-
dependent slow afterhyperpolarization
(sAHP) that controls cortical neuronal
excitability has gone unsolved for over 30
years. King et al. now show that IKCa
(KCa3.1) channels underlie the sAHP in
CA1 pyramidal cells to suppress temporal
summation of EPSPs and mediate spike
accommodation.
King et al., 2015, Cell Reports 11, 175–182 April 14, 2015 ª2015 The Authors
http://dx.doi.org/10.1016/j.celrep.2015.03.026
mailto:[email protected]://dx.doi.org/10.1016/j.celrep.2015.03.026http://crossmark.crossref.org/dialog/?doi=10.1016/j.celrep.2015.03.026&domain=pdfhttp://dx.doi.org/10.1016/j.celrep.2015.03.026mailto:[email protected]
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Cell Reports
Report
IKCa Channels Are a Critical Determinantof the Slow AHP in CA1 Pyramidal Neurons
Brian King,1,2 Arsalan P. Rizwan,1,2 Hadhimulya Asmara,1 Norman C. Heath,1 Jordan D.T. Engbers,1 Steven Dykstra,1
Theodore M. Bartoletti,1 Shahid Hameed,1 Gerald W. Zamponi,1 and Ray W. Turner 1,*1Hotchkiss Brain Institute, University of Calgary, Calgary, AB T2N 4N1, Canada2Co-first author
*Correspondence: [email protected]
http://dx.doi.org/10.1016/j.celrep.2015.03.026
This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ).
SUMMARY
Control over the frequency and pattern of neuronal
spike discharge depends on Ca
2+
-gated K
+
channelsthat reduce cell excitability by hyperpolarizing
the membrane potential. The Ca2+-dependent slow
afterhyperpolarization (sAHP) is one of the most
prominent inhibitory responses in the brain, with
sAHP amplitude linked to a host of circuit and
behavioral functions, yet the channel that underlies
the sAHP has defied identification for decades.
Here, we show that intermediate-conductance
Ca2+-dependent K + (IKCa) channels underlie the
sAHP generated by trains of synaptic input or post-
synaptic stimuli in CA1 hippocampal pyramidal cells.
These findings are significant in providing a molecu-
lar identity for the sAHP of central neurons that willidentify pharmacological tools capable of potentially
modifying the several behavioral or disease states
associated with the sAHP.
INTRODUCTION
Of the many classes of K+ channels recognized in central neu-
rons, few have as key a role in regulating the frequency and
pattern of spike discharge as Ca2+-gated K+ channels. Despite
this, only two classes of Ca2+-gated K+ channels were recog-
nized to control signal processing and spike output in central
neurons: big-conductance (BK) and small-conductance (SK)channels ( Adelman et al., 2012; Berkefeld et al., 2010 ). The prop-
erties of BK and SK channels differ in key respects that mediate
repolarization over relatively short time frames of activity, with
BK channels typically generating a fast afterhyperpolarization
of 10 ms and SK channels a medium afterhyperpolarization
of 100 ms. However, other Ca2+-gated K+ channels exert
even greater control over membrane excitability than BK or SK
channels. One of the longest-standing examples of this is the
Ca2+-gated slow afterhyperpolarization (sAHP) of seconds dura-
tion ( Andrade et al., 2012 ). The sAHP is unique in being highly
amenable to modulation by neurotransmitters, with sAHP ampli-
tude linked to circuit functions that include synaptic plasticity,
electroencephalography, aging, and several disease states ( Dis-
terhoft et al., 2004; Haug and Storm, 2000; Madison and Nicoll,
1986; Martı́n et al., 2001; Zhang et al., 2013 ).
Despite the wealth of information on sAHP properties, the mo-
lecular identity of the Ca2+-dependent K+ channel(s) underlying
the sAHP has defied explanation. A third possible Ca2+-depen-
dent channel is the ‘‘intermediate-conductance Ca2+-gated K+
channel’’ (SK4, IKCa, KCa3.1) ( Ishii et al., 1997; Joiner et al.,
1997; Logsdon et al., 1997 ). While these were not originally
thought to be expressed in CNS neurons (reviewed in Wulff
et al., 2007 ), recent work in cerebellar Purkinje cells and on
IKCa protein distribution suggests a wide potential expression
pattern in central neurons ( Engbers et al., 2012; Turner et al.,
2015 ). The current study tested the hypothesis that IKCa chan-
nels contribute to the Ca2+-dependent component of the sAHP
in CA1 pyramidal cells. We report that K+ channels of intermedi-
ate conductance with the unique pharmacological profile of IKCa
channels indeed underlie the sAHP, identifying the molecular ba-sis for one of the largest inhibitory responses in central neurons.
RESULTS
The sAHP of CA1 Pyramidal Cells
The sAHP in CA1 hippocampal pyramidal cells is known to
mediate spike accommodation, in which spike frequency is pro-
gressively reduced during an injected current pulse and followed
by a post-stimulus afterhyperpolarization ( Madison and Nicoll,
1986 ). The sAHP can also be evoked by synaptic inputs in stra-
tum radiatum (SR) using short stimulus bursts (5–30 pulses,
50 Hz), with the sAHP apparent during and immediately after
the stimulus train ( Figures 1 and 2 ). To test the functional roleof IKCa channels, we first blocked SK channels using 100 nM
apamin and Kv7 channels with 10 mM XE-991, given a role for
Kv7 channels in other hippocampal neurons ( Tzingounis et al.,
2010 ). It is known that IKCa channels are apamin insensitive
( Wulff et al., 2007 ), and we confirmed that 10 mM XE-991 had
no effect on IKCa channels ( Figure S1 A) and did not impede
SR-evoked synaptic transmission ( Vervaeke et al., 2006 ). Any
contribution from the Na-K pump was minimized by recording
at 32C ( Gulledge et al., 2013 ). To focus on excitatory synaptic
potentials, we applied 50 mM picrotoxin to block GABAergic
transmission. The combination of apamin, XE-991, and picro-
toxin produced little qualitative change in the firing response to
Cell Reports 11, 175–182, April 14, 2015 ª2015 The Authors 175
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current pulse injection or SR stimulus trains (see the Supple-
mental Experimental Procedures and Figures S2 A and S2B).
The role of IKCa channels can be distinguished using the se-
lective blockers TRAM-34, Senicapoc, or NS-6180 ( Stocker
et al., 2003; Strøbæk et al., 2013; Wulff et al., 2001 ). The selec-
tivity of TRAM-34 has been thoroughly established, with IKCa
channels exhibiting far greater sensitivity to 1 mM TRAM-34
than SK or BK channels ( Wulff et al., 2000 ). Control tests
confirmed that 1 mM TRAM-34 had no significant effect on BK,
Kv7.3, or TMEM16B channels expressed in tsA-201 cells or
the SR-evoked excitatory postsynaptic current paired-pulse ra-
tio in tissue slices ( Figures S1B–S1E). Since TRAM-34 must firstbe internalized to block IKCa channels, we found that the fastest
and most reliable block was obtained through internal perfusion
of 1 mM TRAM-34 through the electrode (see the Experimental
Procedures ).
IKCa Channels Contribute to Spike Accommodation and
Temporal Summation
Internal perfusion of 1 mM TRAM-34 reduced spike accommoda-
tion and the sAHP in rat CA1 pyramidal cells evoked by current
injection or a 30-pulse SR stimulus train ( Figures 1 A and 1B).
The initial suppression of excitatory postsynaptic potential
(EPSP) summation by the sAHP during a 50-Hz SR stimulus train
C
5
mV
5
mV
200 ms
TRAM-34
SR Stimulation, 30 pulses, 50 Hz
GABAergic inhibition intact
BA
TRAM-34
1st 500 ms last sec
500 1000 500 1000 F o l d F r e q u e n c y
F o l d N u m b e r
F o l d F r e q u e n c y
F o l d N u m b e r
20
mV
500 ms
024
6
8
0
2
4
Time frame (ms)
10
mV100 ms 1st 200 ms last 400 ms
1
2
(7)
(7)
00
2
4
6
200 400 200 400
10
mV
Time frame (ms)
SR Stimulation, 30 pulses, 50 Hz
10
mV
200 ms
** * * *
50 ms
200 ms
Figure 1. TRAM-34-Sensitive Mechanisms
Contribute to Spike Accommodation and
the sAHP
(A) Spike accommodation during depolarizing
current injection is reduced by 1 mM TRAM-34 to
increase spike number and frequency. Mean barplotsindicate thefold change in spike frequencyor
spike number over the times indicated by hori-
zontal bars following TRAM-34.
(B) Repetitive SR stimulation (30 pulses, 50 Hz) in
thesamecellas in (A)is associated with prominent
EPSPsuppression and spike accommodationthat
is reduced by infusion of 1 mM TRAM-34. Open
arrows and inset illustrate the effects of TRAM-34
on thesAHP that follows a synaptic train. Meanbar
plotsindicate thefold change in spike frequencyor
spike number over the times indicated by hori-
zontal bars following TRAM-34.
(C) Repetitive SR stimulation (30 pulses, 50 Hz) at
sub- or supra-threshold intensity in the absence of
picrotoxin to preserve inhibitory inputs reveals a
significant effect by TRAM-34 (1 mM) on EPSP
summation and spike output.
All recordings in (A) and (B) were obtained in
100 nM apamin, 10 mM XE-991, and 50 mM
picrotoxin, while those in (C) did not include
picrotoxin. In all cases, TRAM-34 was internally
perfused through the electrode. Values are mean
± SEM; * p < 0.05. See also Figures S1–S3.
could be extensive, with the membrane
potential often approaching or even fall-
ingbelow therestingpotential ( Figure1B).
Block of the sAHP by TRAM-34 led to
a maintained depolarization during SR-
evoked synaptic trains and a loss of spike
accommodation evident in an increase in the frequency and
number of spikes ( Figures 1 A and 1B). The magnitude of a
post-stimulus TRAM-34-sensitive sAHP was more prevalent
for SR-evoked trains compared to just threshold current-evoked
spike discharge ( Figures 1 A and 1B). This is expected, given a
known relation between sAHP amplitude and spike frequency
and number ( Madison and Nicoll, 1984 ), with threshold current
injection evoking spike output at 12.7 ± 2.3 Hz (n = 7) compared
to 36.1 ± 5.3 Hz for 50-Hz SR stimuli. Each of these results was
obtained in rats of age postnatal day 18 (P18) to P23 (n = 8) or
P90 (n = 2) and in mice ranging from P25 to P50 (n = 8) or P60
(n = 2). We further delivered SR stimulus trains when picrotoxinwasexcluded(n = 3) to determineif theeffectsof TRAM-34 could
be detected when inhibitory GABAergic inputs were intact.
These tests confirmed that TRAM-34-sensitive currents are
effective when GABAergic inputs are intact by modulating
temporal summation and spike accommodation for either sub-
or supra-threshold SR stimulation ( Figure 1C).
The Synaptically Evoked sAHP Is Reduced in KCa3.1 /
Mice
We next examined the ability to record the sAHP in KCa3.1 /
mice compared to wild-type (WT). We found that in response
to depolarizing current injection, pyramidal cells in KCa3.1 /
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MTx reduced outward current by 40% ± 7.6% (n = 5, p < 0.01)
and reversed at 94.1 ± 6.7 mV (n = 5) ( Figure 3C). Finally, inter-
nal perfusion of the electrodewith the catalytic subunit of protein
kinase A (PKA cat , 100 U/ml) reduced outward current by 35% ±
4.3% in five out of six cells (p < 0.001) that reversed at 97.6 ±
4. 7m V( n= 5) ( Figure3D).A comparison of these results revealed
that the degree of block by each of the compounds (TRAM-34,
ChTx, MTx, and PKA cat ) was not significantly different (one-
way ANOVA, p > 0.05). The results again reveal a K+ current in
CA1 pyramidal cells with the unique pharmacological profile of
IKCa channels.
CA1 Pyramidal Cells Exhibit Ca2+-Dependent
Intermediate Conductance K + Channels
We next determined if Ca2+-activated K+ channels of intermedi-
ate conductance could be recorded in pyramidal cells. For this,
we obtained somatic on-cell patch recordings using a HEPES-
buffered aCSF-based electrode solution to preserve intracellular
contents and native Ca2+ buffering mechanisms. IKCa current
was isolated using a set of blockers of voltage- or Ca 2+-gated
ion channels in both the electrode and external aCSF (see the
Supplemental Experimental Procedures and Table S1 ). Calcium
channels were not blocked in order to promote Ca2+ conduc-
tance acrossthe patch by delivering a setof spike-like depolariz-
ing commands through the electrode (50 Hz, 20 pulses, 5 ms,
80 mV). The membrane potential was subsequently stepped to
a set of steady-state potentials from
60 to +30mV with respectto the resting state to examine evoked currents. When recording
under conditions of 3.25 mM [K]o in the on-cell electrode, we
assumed that the resting potential under the patch approached
the value obtained in a separate set of perforated-patch record-
ings of 64 ± 1.5 mV (n = 12). To simplify interpretation, outward
current is presented with respect to the cell interior as an upward
deflection in all on-cell recordings.
We found that the train of spike-like commands was followed
by activation of non-inactivating single channels (n = 4) or even
macroscopic current (n = 6) ( Figures 4 A and 4B). Single-channel
amplitude revealed a mean conductance of 30.4 ± 5.8 pS (n = 4)
( Figure 4 A), a value within the range for IKCa channels reported
in other cell types ( Wulff et al., 2007 ). Both single channels
(n = 4) and macroscopic currents (n = 6) were rapidly reduced
by bath perfusion of 1 mM TRAM-34, which will cross the cell
membrane in regions outside of the on-cell patch ( Figures 4 A
and 4B). The I-V plot for the TRAM-34-sensitive currents evoked
by the spike-like stimulus train reversed 10 mV more negative
than the native resting potential ( Figure 4 A) and could exhibit
slight inward rectification at more hyperpolarized potentials
(n = 4/6).
To determine the ion selectivity of macroscopic currents, we
obtained on-cell recordings using an electrode solution contain-
ing 140 mM KCl to establish equimolar [K] across the membrane
patch, thus setting EK to 0 mV. By comparison, ECl under these
conditions was predicted to rest at 90 mV across the patch.
All other blockers in the electrode and bath are described in
the Supplemental Experimental Procedures. To apply the series
of spike-like depolarizing commands over a range that would
promote Ca2+ influx through the patch, the holding potential
was set to 65 mV and the membrane subsequently stepped
to potentials over a range of 40 mV to +120 mV ( D mV) with
respect to 65 mV. As before, we recorded non-inactivating
macroscopic currents that were reduced by 1 mM TRAM-34
(n = 6) ( Figure 4B). Importantly, the TRAM-34-sensitive currents
were confirmed as representing a K+ conductance by reversing
through 0 mV (EK ) on the I-V plot ( Figure 4B).
The Ca2+ sensitivity of these currents was established when
0.1 mM DC-EBIO increased peak macrocurrent in six out of nine cells ( Figure 4C). Subsequent perfusion of the membrane
permeable BAPTA-AM (10 mM) fully blocked evoked currents
(n = 3) ( Figure 4C). PKA-mediated phosphorylation is well known
to block the sAHP of CA1 pyramidal cells ( Lancaster et al., 1991;
Pedarzani et al., 1998 ) as well as IKCa channels ( Wong and
Schlichter, 2014 ). We found that 8-bromo-cyclic AMP (100 mM)
rapidly reduced channel activity or macroscopic outward current
in on-cell recordings (n = 5) ( Figure 4D). These data reveal that a
Ca2+-dependent intermediate-conductance K+ channel can be
recorded that is sensitive to TRAM-34, an internal Ca2+ chelator,
or elevation of cyclic AMP, all properties consistent with IKCa
channels.
Control
50
pA100 ms
Control
20
pA100 ms
-80 -60 -40 -20 20 40
10
30
mV
60
Ap
20
-10
10
20
30
40
50
-80 -60 -40 -20 20 40 60
A
B
Control
50
pA100 ms
-80 -60 -40 -20 20 40 60-20
20
40
60
80
Control
50
pA100 ms
-110 mV
60 mV
Subtraction
-5
10
20
30
-80 -60 -40 -20 20 40 60
C
D
Subtraction
-110 mV
60 mV
TRAM-34 (n = 7)
PKAcat (n = 5)ChTx (n = 7)
Subtraction
Subtraction
MTx (n = 5)
mV
Ap
mV
Ap
mV
Ap
Figure 3. CA1 Pyramidal Cells Exhibit an
Outward Current with a Pharmacological
Profile Consistent with IKCa Channels
Shown are outside-out recordings from pyramidal
cell somata with 1 mM [Ca]i in response to a
500-ms rampcommand from110 mVto +60 mV.Mean I-V plots reflect currents blocked by the
indicated agents following subtraction of test
fromcontrol recordings. IKCa current was isolated
using blockers described in the Supplemental
Experimental Procedures.
(A–D) In each case, outward current is voltage in-
dependent and reduced by (A) TRAM-34 (1 mM),
(B) ChTx(100 nM),(C)MTx(100nM), or(D)PKA cat
(100 U/ml). ChTx and MTx were focally pressure
ejected, while TRAM-34 and PKA cat were inter-
nally perfused in the patch electrode.
SEMs for mean values in I-V plots are indicated by
the shaded area.
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Synaptic Activation of IKCa Channels and the sAHP
To test for synaptic activation of IKCa channels in relation to the
sAHP, we again used on-cell recordings and short trains of
supra-threshold SR stimuli (five pulses, 50 Hz). We furtherapplied a 60-mV steady-state holding potential above the resting
membrane potential to the patch to increase the driving force for
K+ current, and we applied the same blockers as for single chan-
nels above ( Figure 4 ).
In 5 of27 cases, on-cell recordings yielded singlechannels that
were activated during or by the end of a five-pulse SR stimulus
train( Figure 5 A). A high rate of opening was apparent immediately
following the train followed by a persistent flickering state for
several seconds ( Figure 5 A). Alternatively, a macroscopic out-
ward current was recorded that had a rapid onset following the
SR stimulus train and remained open for a variable duration of
2–5s before spontaneously terminating (n = 5) ( Figure5B). Calcu-
lating the ensembleaverageof SR-evokedmacroscopiccurrents
revealed an outward current that peaked immediately following
the SR stimulus train and then dissipated over 4–5 s ( Figure 5B).
Bath application of 1 mM TRAM-34 rapidly reduced or blockedSR-evoked single channels (n = 5) or macrocurrents (n = 5) ( Fig-
ures 5 A and 5B). These results are important in providing the first
evidence for synaptic activation of an identified K+ channel with
activity that correlates to the sAHP of CA1 pyramidal cells.
The Ca2+-activated outward current underlying sAHP current
(I sAHP ) is most often recorded under voltage clamp following a
depolarizing step command of 1 s. We directly compared the
sensitivity of the step-evoked and SR-evoked I sAHP to TRAM-
34 application. Perforated-patch whole-cell recordings were
used to preserve intracellular Ca2+ buffering and outward cur-
rents evoked by five SR stimuli (50 Hz) or by a step command
to +60 mV (500 ms) ( Figures 5C and 5D). An outward current
A
∆ mV Control
TRAM-34
C
O2
O1
O2O3
O1C
-60
-40
-20
+20
2
pA
200 msec
+30
+30
0
(-65)
∆ 30 mV
RMP
∆ 80 mV
∆ -60 mV
[K] o = 3.25 mM
C u r r e n t ( p A )
C u r r e n t ( p A )
Applied voltage f rom rest (∆ mV)
20RMP-20-40-60
-1.5
-1.0
-0.5
RMP
0.5
1.0
1.5
γ = 30 pS
C
Control
BAPTA-AM
DC-EBIO
50
pA1 sec
∆ -60 mV
∆ 30 mV
∆ 30 mV
RMP
∆ 80 mV
∆ -60 mV
-60
∆ mV
+30
-60
+30
D
Control
-60
∆ mV
5
pA1 sec∆ 30 mV
RMP
∆ 80 mV
∆ -60 mV
+30
-60
+30 8-bromo-cAMP
B∆ mV
n = 6-8
-6
-4
-2
0
2
4
6
C u r r e n t ( p A )
-100 -80 -60 -40 -20 20 40 600
∆ 120 mV
-65 mV
∆ 80 mV
∆ -40 mV
ECl EK
TRAM-34
Subtraction
Control
5pA
2 sec
[K] o = 140 mM
+120
+120
-40
-40
C u r r e n t ( p A )
Predicted transmembrane
potential (mV)
Figure 4. IKCa Channels Are Expressed in CA1 Pyramidal Cells
Shown are on-cell somatic recordings using a HEPES-buffered aCSF internal solution, with Ca2+
influx evoked by a repetitive spike-like command (50 Hz,
20 pulses, 5 ms, 80 mV) followed by steps to different steady-state potentials. The resting membrane potential (RMP) across the patch is presumed to be
approximately
65 mV for records in (A), (C), and (D) with 3.25 mM [K]o, while that for 140 mM [K]o in (B) was set at
65 mV. Voltage commands reflect the stepapplied to the electrode and displayed with net depolarizing commands upward. Current is displayed with respect to the cell interior with outward current as an
upward deflection. The mean I-V plot in (B) reflects TRAM-34-sensitive currents calculated by subtraction of test from control records.
(A)On-cell recordingusing 3.25 mM[K]oin theelectrode reveals single channel activityfollowing thepulse train that israpidlyreducedor blocked by1mM TRAM-
34. Current reversed approximately 10 mV from the resting state.
(B) On-cell recordings of macroscopic current using equimolar [K]o exhibits reversal of TRAM-34-sensitive current through 0 mV (EK ).
(C) On-cell recorded macroscopic current is enhanced by the IKCa agonist DC-EBIO (0.1 mM) and blocked by 10 mM BAPTA-AM.
(D) On-cell macroscopic current is reduced by 100 mM 8-bromo-cyclic AMP. Records in (B) show every second record for clarity.
The electrode and bath perfusate in all cases containedblockers as described in theSupplemental Experimental Procedures. Traces were filtered at400–500 Hz
(8-pole Bessel). Values are mean ± SEM.
Cell Reports 11, 175–182, April 14, 2015 ª2015 The Authors 179
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was recorded following the SR stimulus train in seven outof eight
cells with an amplitude of 47 ± 5 pA and duration of 6.8 ± 1.5 s
(n = 7). In the same cells a step command to +60 mV evoked
an I sAHP of 28 ± 11 pA and 6.3 ± 1.2 s (n = 7). Perfusion of
1 mM TRAM-34 reduced both the SR- and step-evoked outward
current (n = 7, p < 0.01) ( Figure 5C). If IKCa channels underlie
I sAHP , then the outward current should also be blocked by
ChTx. We thus repeated these tests in the additional presence
of 5 mM TEA to first block BK channels and found that pressure
ejection of 100 nM ChTx reduced the I sAHP evoked by either SR
stimulation (n = 6, p < 0.05) or a step command (n = 7, p < 0.05)( Figures 5C and 5D).
DISCUSSION
The current study reveals that IKCa channels contribute sub-
stantially to the Ca2+-dependent sAHP in CA1 hippocampal py-
ramidal cells. Support for this was obtained through recordings
of intermediate-conductance K+ channels evoked in relation to
the sAHP with the unique pharmacological profile for IKCa chan-
nels. These data are thus important in resolving the molecular
basis for one of the largest Ca2+-dependent inhibitory postsyn-
aptic responses in central neurons.
Ionic Contributions to the sAHP
Studies on the ionic basis for the sAHP of CA1 pyramidal cells
have considered several channel subtypes. Fluctuation analysis
implicated a K+ channel of 2–5 pS ( Sah and Isaacson, 1995 ).
The SK1 isoform was thus considered to underlie the sAHP,
but this was later discounted ( Bond et al., 2004 ). A role for
TMEM16B channels in mediating various aspects of repolariza-
tion has been proposed for CA1 cells using the drugs NFA and
NPPB ( Huang et al., 2012 ), but these drugs also block IKCa
channels ( Fioretti et al., 2004; Wulff et al., 2007 ). We further
found no block of TMEM16B channels by TRAM-34 when ex-pressed in tsA-201 cells, and the current isolated here reversed
through EK and not ECl. A role for voltage-gated Kv7 channels
(IM ) in generating an sAHP has been reported, but these are
less involved in CA1 cells ( Tzingounis et al., 2010 ) and we
consistently recorded a TRAM-34-sensitive sAHP in XE-991.
While sodium-dependent K+ channels can contribute to an
sAHP in other cells ( Zhang et al., 2010 ), the sAHP and I sAHP
recorded here is Ca2+ dependent ( Figures S4 A–S4C). A role
for the Na-K pump was proposed based on the effects of
ouabain at 35C using high-frequency current pulses ( Gulledge
et al., 2013 ). We found that the m ajority of the sAHP evoked by
this protocol at 35C was blocked by internal infusion of 1 mM
5
pA
1 sec
SR stimulation, 5 pulses, 50 Hz
n = 9
A
c
o
c
o
C
D
2
pA100 msec
SR stimulation, 5 pulses, 50 Hz
Control TRAM-342
pA200 msec
Control
TRAM-3410pA
1 sec
Control
TRAM-3420
pA1 secSR
-65 mV
-65 mV
60 mV
5
pA1 sec
Control
ChTx
-65 mV
60 mV
10
pA1 sec
SR stimulation
+60 mV
Control
ChTx
4
pA1 sec
SR
-65 mV
B
Control TRAM-34
Figure 5. IKCa Channels Are Evoked in CA1
Pyramidal Cells by Synaptic Stimulation
(A and B) On-cell recordings with a HEPES-buff-
ered aCSF electrode solution (3.25 mM [K]). Cur-
rents are illustrated with respect to the cell interior
(outward current upward). Currents were evokedusing a 50 Hz, five-pulse SR stimulus train with a
net 60 mV depolarized holding potential to in-
crease driving force for K+
across the patch. The
electrode and bath perfusate contained blockers
as described in the Supplemental Experimental
Procedures. (A) A single channel is activated dur-
ing and followinga five-pulse SR stimulustrain and
is blocked by bath perfusion of 1 mM TRAM-34.
Dashed lines depict open (o) and closed (c) states,
with the same example expanded below. (B) On-
cellrecordings of an outward macroscopic current
that opens for prolonged but variable periods of
time following SR stimulus trains (arrows) and is
reduced or blocked by TRAM-34. An ensemble
average from 9 SR stimulus trains (lowest trace)
reveals an average time course equivalent to
an sAHP. Transients in (B) reflect capacitive
transients from spontaneous spike discharge in
the cell.
(C and D) Comparison of outward current evoked
by SR stimulation to the I sAHP evoked by a step
command in perforated-patch recordings in the
presence of 100 nM apamin, 10 mM XE-991, and
50 mM picrotoxin (C and D), with 5 mM TEA further
included in (D) to block BK channels for tests with
ChTx. Currents were evoked by five supra-
threshold SR stimuli (50 Hz) or a 500-ms step to
60mV toevokeI sAHP andbathapply 1mM TRAM-
34 or pressure eject 100 nM ChTx. Results in (C)
and (D) are from separate cells.
Tracesin (A)and (B) were filtered at500 Hz andthe
expanded trace in (A) at 1 kHz (8-pole Bessel).
Values are mean ± SEM.
180 Cell Reports 11, 175–182, April 14, 2015 ª2015 The Authors
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TRAM-34, with a small remaining TRAM-34-insensitive compo-
nent subsequently blocked by 20 mM ouabain ( Figures S4D
and S4E).
Interestingly, Ca2+-dependent single channels of 19 pS
conductance were earlier reported in both tissue slices ( Limaand Marrion, 2007 ) and cultured neurons ( Lancaster et al.,
1991 ). However, in neither case was the molecular identity of
these channels attained. The sAHP in cultured hippocampal
neurons was further reduced by clotrimazole ( Shah et al.,
2001 ), a drug related to TRAM-34 but later recognized as
non-specific. The sAHP and spike accommodation was also
reported to be insensitive to ChTx ( Lancaster and Nicoll,
1987; Shah and Haylett, 2000 ). By comparison, we found
ChTx-sensitive current in outside-out and perforated-patch re-
cordings. The reason for this difference is unknown but might
reflect differences in internal Ca2+ buffering. Indeed, our most
stable recordings of IKCa channels and I sAHP were obtained
using on-cell or perforated-patch recordings that minimize a
disruption of [Ca]i.
IKCa Channels and the sAHP
Our current interpretation for the ionic basis of the sAHP in CA1
pyramidal cells is that the majority of the large-amplitude, Ca2+-
dependent early phase (4–5 s) of the sAHP is mediated by IKCa
channels.An additional smaller contribution canbe made by Kv7
channels or by the Na-K pump, although the Na-K pump is ex-
pected to be most active following intense activation. Delin-
eating IKCa channels as a key contributor to the sAHP in a
cortical pyramidal neuron is significant in that IKCa channels
were believed to be restricted to endothelial cells and activated
microglia in central regions ( Wulff et al., 2007 ). The reasons why
early probes for in situ hybridization did not detect KCa3.1 signal
in brain is unknown, since a signal from at least endothelial
KCa3.1 would be expected. However, recent work has shown
that IKCa channels can be recorded in cerebellar Purkinje cells
( Engbers et al., 2012 ) and GFP expression driven by the
KCa3.1 promoter suggested an even wider expression pattern
of IKCa ( Turner et al., 2015 ). Indeed, numerous central neurons
express an sAHP that shares many properties with the sAHP in
CA1 pyramidal cells, including Ca2+ dependence, insensitivity
to BK or SK channel blockers, and rapid block by kinase path-
ways, with a very close parallel found in enteric and myenteric
neurons ( Neylon et al., 2004; Vogalis et al., 2002 ). The finding
that IKCa-mediated suppression of temporal summation is still
active under conditions of intact GABAergic inhibition further en-
sures that the influence of IKCa channels will be present underphysiological conditions. We thus expect the role for IKCa chan-
nels in generating an sAHP to be widely applicable to other clas-
ses of central neurons.
EXPERIMENTAL PROCEDURES
Animal Care and Slice Preparation
Experiments wereconducted on P18–P24 maleSprague-Dawley rats(Charles
River) raised from timed-pregnant dams or on breeding colonies of P25–P50
C57BL/6 WT mice or P25–P60 KCa3.1 / mice (UC Davis) (as described
in Turner et al., 2015 ) according to approved procedures by the Canadian
Council of Animal Care. Details on in vitro slice preparation are provided in
the Supplemental Experimental Procedures.
Electrophysiology
A suite of patch-clamp recording techniques was used to record from rat or
mouse hippocampal CA1 pyramidal cells under current- or voltage-clamp
conditions in the in vitro slice preparation (3234C) and from tsA-201 cells
(22C). Details on recording equipment and patch recording configurations
can be found in the Supplemental Experimental Procedures.
Solutions and Drug Applications
The lipophilic drugs TRAM-34, Senicapoc, and NS-6180 must be internalized
to target an internal binding site ( Stocker et al., 2003; Strøbæk et al., 2013;
Wulff et al., 2001 ). Using bath perfusion of these drugs, channel activity was
reduced in on-cell recordings in 10–15 min while whole-cell recordings could
take 20–30 minto achieve full block. Themostreliable blockwas obtainedwith
TRAM-34 and Senicapoc, with infusion of 1 mM TRAM-34 by exchanging the
electrodesolution (ALAScientific Instruments)achieving a block of IKCachan-
nelsin
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182 Cell Reports 11, 175–182, April 14, 2015 ª2015 The Authors
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Cell Reports
Supplemental Information
IKCa Channels Are a Critical Determinant
of the Slow AHP in CA1 Pyramidal Neurons Brian King, Arsalan P. Rizwan, Hadhimulya Asmara, Norman C. Heath, Jordan D.T.
Engbers, Steven Dykstra, Theodore M. Bartoletti, Shahid Hameed, Gerald W. Zamponi,
and Ray W. Turner
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1
Figure S1, related to Figure 1. Pharmacological isolation of IKCa channels
Whole-cell recordings obtained with alpha subunits of the indicated proteins transiently expressed in tsA-
201 cells and recorded at room temperature.(A) The Kv7 channel blocker XE-991 (10 µM) has no significant effect on IKCa channels. IKCa was
coexpressed with calmodulin and currents recorded in 1 µM [Ca]i in response to a ramp command.
(B - D) TRAM-34 (1 µM) has no significant effect on BK channels (B), Kv7.3 channels (C), orTMEM16B calcium-gated chloride channels recorded with 100 nM [Ca]i (D). TMEM16B currents are
rapidly blocked by 300 µM niflumic acid (NFA) in (D).
(E) TRAM-34 (1 µM) does not significantly affect the mean SR-evoked EPSC paired pulse ratio (20 msinterstimulus interval).
Values are mean ± SEM with sample values shown in brackets.
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2
Figure S2, related to Figures 1 and 2. Background channel blockers have minimal effects on thesAHP and I sAHP (A) Perfusion of 100 nM apamin, 10 µM XE-991, and 50 µM picrotoxin to block SK and Kv7 channels
and GABAergic transmission slightly reduce but do not eliminate spike accommodation during current pulse injection.
(B) The early phase of the sAHP following a train of SR input is blocked ( arrow) and the sAHP is more
evident after perfusion of apamin, XE-991, and picrotoxin (inset ).
(C) The sAHP evoked by a train of SR stimuli is progressively reduced by block of NMDA receptors (DL-AP5, 25 µM) and AMPA receptors (DNQX, 10 µM).(D, E) Recordings of IsAHP evoked as a tail current under whole-cell voltage clamp using a depolarizing
step command (D) or following a SR stimulus train (E). Perfusion of apamin, XE-991, and picrotoxin
selectively reduces the early component of outward current consistent with a block of SK channels. Spikesand stimulus artefacts in (B, C, E) and currents during the command pulse in (D) are truncated.
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4
Figure S4, related to the Discussion. The majority of the sAHP is calcium-dependent and TRAM-34-
sensitive(A, B) Representative whole-cell recordings of IsAHP evoked by a step command to +60 mV at 34ºC is blocked by perfusion of low extracellular calcium (a), as is an inward tail current recorded in another cell
in the presence of internal 1 µM TRAM-34 (B).
(C) A perforated patch whole-cell recording at the soma records an IsAHP of 6 sec duration following a
step command to +60 mV at 22 ºC. The IsAHP is amplified upon generation of a calcium current duringthe step command, visible as an all-or-none unclamped calcium spike (inset ). Recordings in (B) were
conducted using a KMeSO4-based internal solution with 0.1 EGTA, ATP, GTP, creatine, and 1 µM
TRAM-34.
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(D, E) Representative whole-cell recording from a rat CA1 pyramidal cell applying a stimulus protocol
consisting of 1 nA, 2 ms current pulses delivered at 50 Hz for 3 sec to test the role for the Na-K pump ingenerating the sAHP at 35ºC (as in Gulledge et al. 2013). Internal infusion of TRAM-34 (1 µM) blocks a
large component of the sAHP. Subsequent addition of bath perfused 20 µM ouabain blocked the small
remaining TRAM-insensitive component of the sAHP. All recordings in (D, E) were conducted in the
presence of 100 nM apamin, 10 µM XE-991, and 50 µM picrotoxin.(E) Bar plot of mean sAHP area following the end of the current pulse train for records such as shown in
(D).Values are mean ± SEM. *, p < 0.05. Sample numbers for mean values are shown in brackets.
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6
Table S1, related to Figures 3, 4 and 5. Values of Kd/IC50/EC50 and the actual applied concentrations of
agonists and antagonists used.
Inhibitors Actions Kd IC50 /EC50 Applied
External
Applied
Internal
References
Tetrodotoxin Nav1.1-1.4,
1.6-1.7
1-24 nM 1 μM (Catterall et al., 2005a;
Zimmer, 2010)
Apamin SK1-3 2-12 nM 100 nM (Sah and Faber, 2002)
TEA BK
Kv1.1
Kv1.6
Kv3.x
80-330 µM
0.5 mM
1.7-7 mM
0.09-0.3 mM
5 mM See (Coetzee et al., 1999)for pharmacology of TEA
4-AP Kv1.x
Kv2.x
Kv3.x
Kv4.2
0.1-1.5 mM
0.5-4.5 mM
0.02-1.2mM
2-5 mM
5 mM 2 mM (Coetzee et al., 1999)
CsCl HCN1-4 0.16-0.20 mM 2 mM (Stieber et al., 2005)
TRAM-34 KCa3.1 20-25 nM 20 nM 1 µM 1 µM (Jenkins et al., 2013; Wulffet al., 2001; Wulff et al.,
2000)
Senicapoc KCa3.1 11 nM 100 nM (Maezawa et al., 2012)
ChTx KCa3.1
Kv1.2
Kv1.3
Kv1.6
10 nM
1.7-17 nM
0.5-2.6 nM
1 nM
100 nM (Van Renterghem et al.,
1995)
(Coetzee et al., 1999)
Maurotoxin KCa3.1
Kv1.2
1 nM
0.1 nM
100 nM (Castle et al., 2003)
Ni2+ Cav3.x 50-300 µM 300 µM (Lee et al., 1999)
Cd2+ Cav1.x
Cav2.3
2.14 µM
0.8 µM
30 µM (Hobai et al., 1997)
(Catterall et al., 2005b)8-bromo-cAMP PKA-I and-
II
0.015-
0.019 nM
100 µM (Schwede et al., 2000)
(Hoffman and Johnston,
1998)
PKACat KCa3.1CaM-
bindingdomain
100 U/ml (Wong and Schlichter, 2014)
XE-991 Kv7.x 2.2 µM 10 µM (Schwarz et al., 2006)
Activators
DC-EBIO 1 µM 0.1 µM (Wulff and Castle, 2010)
SKA-31 KCa3.1 260 nM 1 µM (Sankaranarayanan et al.,
2009)
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Supplemental Experimental Procedures:
Slice preparation:
Animals were anaesthetized by isoflurane inhalation and 270 µm dorsal hippocampal slices cut in ice-cold
solution (mM): 215 sucrose, 25 NaHCO3, 20 D-glucose, 2.5 KCl, 0.5 CaCl2, 1.25 NaH2PO4 and 3 MgCl2
bubbled with carbogen gas. The slices were incubated for 10-15 min (34°C) in artificial cerebrospinal fluid
(aCSF) composed of (mM): 125 NaCl, 3.25 KCl, 1.5 CaCl2, 1.5 MgCl2, 25 NaHCO3, and 25 D-glucose
bubbled with carbogen gas. Slices were stored at room temperature until recordings conducted at 32-34 ºC
as a submerged preparation.
Evoking the sAHP
As earlier reported (Wu et al., 2004), the amplitude of the evoked sAHP could differ between cells, with
up to 20% exhibiting little or no detectable sAHP. The current study focused on those cells exhibiting a
detectable sAHP. The combination of apamin, XE-991, and picrotoxin slightly reduced but did not block
spike accommodation evoked during current pulse injection (Figure S2A; n = 8). There was also little
qualitative change in the firing response to SR stimulus trains, although the early phase of the subsequent
AHP (SK-mediated) was reduced and the sAHP was slightly more prominent, likely due to an increase in
calcium influx in the absence of SK channels (Figure S2B; n = 8). The ability to detect both AMPA- and
NMDA-mediated components of the SR-evoked sAHP was also maintained in the presence of apamin, XE-
991, and picrotoxin (Figure S2C; n = 8). Recording the IsAHP following a step command or a burst of SR
stimuli established that these compounds reduced primarily an early component of outward current
consistent with an SK-mediated response (Figures S2D and S2E; n = 8). Recordings of IsAHP using
whole-cell or perforated patch configurations were stable over 30 min time (Figure S3, n = 7), revealing
minimal influence of washout or a change in access resistance that could account for the actions of applied
drugs.
Patch recordingsPatch recordings were made using Multiclamp 700B amplifiers and Digidata 1440A with DC-10 kHz
bandpass filter and pClamp software. Pipettes were constructed from 1.5 mm O.D. fiber-filled glass (A-M
Systems) with resistance of 4-8 MΩ. Series resistance was compensated with bridge balance circuitry for
current clamp recordings and during voltage clamp recordings by up to 80% compensation. Negative bias
current of < 200 pA was applied during current clamp recordings to maintain a subthreshold resting
potential at ~-65 mV. For whole-cell experiments, control recordings were made >5 min after break in to
promote full stabilization with the internal solution. During whole-cell recording, cells were rejected for
any drift in access resistance of > 20%. On-cell recordings were obtained with a 10 kHz cutoff filter and
processed offline by filtering at 240-500 Hz (Bessel 8-pole).
Solutions and drug applications
Chemicals were obtained from Sigma unless noted. The following drugs were used to block the identified
ion channels to isolate IKCa: BK (TEA, 5 mM; IbTx, 100 nM), SK (apamin, 100 nM), Kv7 (XE-991, 10
µM), Kv4.x (4-AP, 5 mM external, 2 mM internal), Kv1.x (TEA, 5 mM; ChTx, 100 nM; MTx, 100 nM),
sodium (TTX, 200 nM - 1 µM), HVA calcium (CdCl2, 30 µM), LVA calcium (NiCl2, 300 µM), HCN
(external CsCl, 2 mM). TRAM-34 (1 µM) was applied most often by internal perfusion of the electrode
and by bath perfusion when indicated. Synaptic responses were blocked by: GABA-A (picrotoxin, 50 µM),
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8
GABA-B (CGP55485, 1 µM, Tocris), NMDA (DL-AP5, 25 µM, Ascent Scientific), and AMPA/KA
(DNQX, 10 µM, Tocris Scientific). NiCl2, CdCl2, CsCl, TEA, 4-AP, and picrotoxin were prepared daily
from stock solutions. DNQX, DC-EBIO, and SKA-31 were first dissolved in dimethylsulfoxide (DMSO)
(final DMSO < 0.1%). Senicapoc was a gift of H. Wulff (UC Davis). Pressure ejection of toxins were
carried out in a HEPES (10)-buffered aCSF of (mM): 150 NaCl, 3.25 KCl, 1.5 CaCl 2, 1.5 MgCl2, 10
HEPES, 20 D-glucose, pH 7.3. Pressure electrodes included BSA (0.1%) to reduce drug binding, and food
coloring (1:100) to visualize the region of drug ejection.
Patch recording configurations and media
External solution for all patch configurations: External solution (aCSF) was composed of (mM): 125
NaCl, 3.25 KCl, 1.5 CaCl2, 1.5 MgCl2, 25 NaHCO3, and 25 D-glucose bubbled with carbogen. All slice
recordings were conducted at 32 -34 º C.
Whole-cell internal: Current-clamp whole-cell recording solution consisted of (mM): 130 KMeSO3, 0.1
EGTA, 10 HEPES, 7 NaCl, 0.3 MgCl2, pH 7.3 with KOH, providing an ECl of -75 mV and EK of -97 mV; .
5 di-tris-creatine phosphate , 2 Tris-ATP and 0.5 Na-GTP were added daily from frozen stocks to whole-
cell recording solutions.
Perforated-Patch internal: Gramicidin-perforated patch recordings solution contained (mM): 10 KCl, 135
K-Gluconate, 10 HEPES, 1 MgCl2, 75 µg/ml gramicidin prepared in DMSO (< 0.01% DMSO in the
internal).
Outside-out internal: Outside-out voltage clamp recordings to isolate IKCa currents used an electrode
solution of (mM): 140 KCl, 2.83 MgCl2, 10 HEPES, 5 EGTA, 2 4-AP, 4.25 CaCl2 (1 µM [Ca]i,
Maxchelator Ca/Mg/ATP/EGTA Calculator, 0.165 ionic strength), pH 7.3 (EK -99 mV); 5 di-tris-creatine
phosphate, 2 Tris-ATP and 0.5 Na-GTP were added daily from frozen stocks to outside-out internal
solution. The external solution further contained synaptic blockers and TTX, apamin, XE-991, TEA, 4-AP,and CsCl to block SK, BK, Na, Kv1, Kv4, Kv7, LVA and HVA Ca
2+ channels, and HCN channels (as
above, and detailed in Table S1).
On-cell internal: On-cell recording electrodes to isolate IKCa channels contained HEPES-buffered aCSF
with (mM): 150 NaCl, 3.25 KCl, 1.5 CaCl2, 1.5 MgCl2, 10 HEPES and 20 D-glucose, pH 7.3. The electrode
solution further contained synaptic blockers and TTX, apamin, XE-991, TEA, 4-AP, and CsCl to block SK,
BK, Na, Kv1, Kv4, Kv7 channels (as above, and detailed in Table S1). The external bath solution for on-
cell recordings contained the same drugs as the electrode solution but did not contain excitatory synaptic
blockers (DL-AP5 and DNQX) or TTX to allow for synaptic excitation.
tsA-201 cells
tsA-201 cells were maintained as previously described (Engbers et al., 2012; Rehak et al., 2013) and
transiently transfected with cDNA (5 µg/µl) of Kv7.3, BK, TMEM16B or IKCa and calmodulin together
with cDNA for GFP to identify cells successfully transfected. Currents were recorded at room temperature
(22 ºC) in aCSF consisting of (mM): 120 NaCl, 3 NaHCO3, 4.2 KCl, 1.2 KH2PO4, 1.5 MgCl2, 10 D-
Glucose, 10 HEPES and 1.5 CaCl2 , pH adjusted to 7.3 with NaOH. The following internal solutions were
used for voltage-clamp recordings in tsA-201 cells. For recording IKCa channels, electrodes were filled
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9
with (mM): 140 KCl, 5 EGTA, 5 HEPES, 2.83 MgCl2, 4.25 CaCl2 (1.1 µM free [Ca]i, Maxchelator
Ca/Mg/ATP/EGTA Calculator, http://maxchelator.stanford.edu/CaMgATPEGTA-TS.htm, 0.165 ionic
strength), pH adjusted to 7.3 with KOH. For recording BK currents, electrodes were filled with (mM): 140
KCl, 5 HEPES, 0.1 EGTA, 0.5 MgCl, 5 ATP, 1 GTP, pH adjusted to 7.3. For recording Kv7.3 currents,
electrodes were filled with (mM): 140 KCl, 0.1 EGTA, 2.5 MgCl2, 10 HEPES, pH adjusted to 7.3. For
recording TMEM16B currents, electrodes were filled with (mM): 15.22 CsCl, 124.78 CsMeSO3, 5 EGTA,
10 HEPES, 1.73 MgCl2, 1.68 CaCl2 (100 nM free [Ca]i, Maxchelator Ca/Mg/ATP/EGTA Calculator,http://maxchelator.stanford.edu/CaMgATPEGTA-TS.htm), with ECl = -45 mV, and pH adjusted to 7.3.
TMEM16B currents were activated by giving 200 ms voltage steps from a holding potential of -40 mV.
Stimulation
Synaptic input was evoked using a concentric bipolar electrode (Frederick Haer, CBCMX75(JL2))
positioned in the mid SR and driven by a stimulus isolation unit (Digitimer , 0.1-0.2 ms pulse width).
Data Analysis and Statistical Methods
sAHP areas were measured as the area under (in current clamp mode) or over (in voltage clamp mode)
baseline, defined as the mean voltage current or voltage level preceding the current or voltage protocol,from 200 ms after the protocol to 7 secs after the protocol. Amplitude of the sAHP was measured as the
voltage level 200 ms after the current or stimulus protocol. Mean single channel conductance was
calculated for hyperpolarizing potentials where channel amplitude was best delineated. Data were analyzed
using Clampfit 10 software and custom Matlab R2007B scripts, and statistical analysis in OriginPro 8.
Paired-sample Student t-tests were used unless otherwise indicated.
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Wulff, H., Gutman, G.A., Cahalan, M.D., and Chandy, K.G. (2001). Delineation of the clotrimazole/TRAM-34
binding site on the intermediate conductance calcium-activated potassium channel, IKCa1. J. Biol. Chem. 276 ,
32040-32045.
Wulff, H., Miller, M.J., Hansel, W., Grissmer, S., Cahalan, M.D., and Chandy, K.G. (2000). Design of a potent and
selective inhibitor of the intermediate-conductance Ca2+-activated K+ channel, IKCa1: a potential
immunosuppressant. Proc. Natl. Acad. Sci. U S A 97 , 8151-8156.
Zimmer, T. (2010). Effects of tetrodotoxin on the mammalian cardiovascular system. Mar. Drugs 8 , 741-762.
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