Schizophrenia Bulletin vol. 33 no. 6 pp. 1263–1269, 2007
doi:10.1093/schbul/sbm106
Advance Access publication on September 28, 2007
Ether-a-go-go–Related Gene Potassium Channels: What’s All the Buzz About?
Paul D. Shepard1,2, Carmen C. Canavier3, and Edwin S.
Levitan4
Antipsychotic drugs are thought to exert their therapeutic
action by antagonizing dopamine receptors but are also
known to produce side effects in the heart by inhibiting cardiac ether-a-go-go–related gene (ERG) K1 channels. Recently, it has been discovered that the same channels are
present in the brain, including midbrain dopamine neurons.
ERG channels are most active after the cessation of intense
electrical activity, and blockade of these channels prolongs
plateau potentials in bursting dopamine neurons. This
change in excitability would be expected to alter dopamine
release. Therefore, the therapeutic action of antipsychotic
drugs may depend on inhibition of both postsynaptic dopamine receptors and presynaptic ERG K1 channels.
Key words: schizophrenia/bursting/dopamine/
antipsychotic drugs/review
Introduction
Efforts to identify the pharmacological basis for the
therapeutic actions of antipsychotic drugs have focused
primarily on the interaction of these compounds with
G-protein–coupled receptors. Early observations that
antipsychotics block dopamine receptors in proportion
to their clinical potency contributed significantly to formulation of the dopamine hypothesis of schizophrenia.1,2
Subsequent findings showing that many secondgeneration antipsychotic drugs potently block serotonin
receptors gave rise to the notion that these sites are somehow involved in conferring the atypical properties of
these drugs.3 In both instances, the existence of a distinct
pharmacological action (eg, dopamine or serotonin re1
To whom correspondence should be addressed; tel: 410-4027753, fax: 410-402-6066, e-mail: pshepard@mprc.umaryland.edu.
Biophysical Properties of ERG K1 Channels
ERG Kþ channels, like other ion channels, are macromolecular protein complexes that gate the flow of ions, in
this case Kþ, across cell membranes. Functional ERG
channels are comprised of 4 alpha subunits, each consisting of a protein comprised of 6 transmembrane-spanning
domains (S1–S6). The first 4 domains of each subunit
(S1–S4) comprise the voltage sensor which controls channel gating, while S5 and S6 collectively form the pore
region of the channel.7 Voltage-dependent changes in
the conformation of the protein complex give rise to 3
Ó The Author 2007. Published by Oxford University Press on behalf of the Maryland Psychiatric Research Center. All rights reserved.
For permissions, please email: journals.permissions@oxfordjournals.org.
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2
Maryland Psychiatric Research Center and Department of Psychiatry, University of Maryland School of Medicine, PO Box 21247,
Baltimore, MD 21228; 3Neuroscience Center for Excellence and
Department of Ophthalmology, Louisiana State University Health
Sciences Center, New Orleans, LA 70112; 4Department of
Pharmacology, University of Pittsburgh, Pittsburgh, PA 15261
ceptor blockade), shared by a group of compounds
with disparate chemical structures, provided important
clues as to how these drugs might exert their therapeutic
effects in individuals with schizophrenia.
Recent clinical and experimental evidence suggests that
members of the phenothiazine, butyrophenone, dibenzazepine, and benzamide classes of antipsychotics as well
as other structurally heterogeneously neuroleptic drugs
share another common pharmacological property—
specifically, the ability to block ether-a-go-go–related
gene (ERG) potassium channels.4 ERG Kþ channels
(also referred to as Kv11 channels following International
Union of Pure and Applied Chemistry nomenclature) belong to a superfamily of voltage-activated Kþ channels
encoded by 3 distinct gene subfamilies, including ethera-go-go (eag), ether-a-go-go-like (elk), and ether-ago-go–related (erg) genes.5 The first member of the
family to be identified was discovered in a Drosophila
melanogaster mutant and named for the characteristic
leg-shaking behavior exhibited when the flies were anesthetized with ether.6 Several years later, a human homolog of eag was identified from low-stringency screening of
a human hippocampal cDNA library and subsequently
localized to chromosome 7.5 Designated the humaneag–related gene, herg (or KCNH2) shares less than
50% sequence homology with other eag genes and is
thus considered to represent a distinct subfamily in
humans. Erg genes resembling herg have been identified
in mice (merg) and rats (rerg) and the channels they encode
share a common set of functional properties that set them
apart from other EAG Kþ channels. Accordingly, these
genes are referred to simply as erg throughout the remainder of the text.
P. D. Shepard et al.
conductance states: closed (nonconductive), open (conductive), and inactivated (nonconductive) (see figure
1). ERG channels, which are closed at hyperpolarized
membrane potentials, open slowly in response to membrane depolarization but inactivate so quickly that
very little outward Kþ current flows at the peak of the
action potential. As the membrane potential begins to
repolarize, the channels rapidly recover from inactivation
and must once again enter an open (conductive) state
prior to closing. Because the rate of recovery from
inactivation greatly exceeds the rate at which the channels
deactivate (ie, reenter a close state), a large albeit transient ‘‘resurgent’’ current is generated as the membrane
potential repolarizes. Thus, ERG channels act as strong
inward rectifiers, preferentially conducting outward currents at relatively hyperpolarized membrane potentials.
However, unlike classical inward rectifiers, they must
be initially induced to open with depolarization. During
repeated spiking or prolonged depolarization, the sum
of the fraction of channels in the activated and inactivated states can summate temporally,8,9 enhancing the
inward rectification effect. These properties ensure that
ERG channels are most active at the cessation of intense
activity.
and ERG1b) that differ slightly in their amino acid composition and kinetic properties.13–15 The unique gating
characteristics of hERG Kþ channels, including their
strong inward rectification, limit the amount of outward
current occurring during the initial phase of the cardiac
action potential which supports the development of the
plateau phase and allows adequate time for Ca2þ entry
and proper excitation contraction coupling. On the other
hand, the ability of the channels to rapidly recover from
inactivation, yet deactivate slowly, results in a large outward current that, together with other voltage-dependent
Kþ channels, leads to rapid repolarization of the cardiac
action potential. The role of hERG Kþ channels in normal cardiac function was revealed in studies showing that
missense mutations in herg are responsible for long QT
syndrome (LQTS), a cardiac repolarization disorder that
predisposes affected individuals to life-threatening ventricular arrhythmias such as torsade de pointes.16 (The
QT interval of the electrocardiogram is a measure of cardiac action potential duration and thus depends on
hERG channel function.) Inherited LQTS can be caused
by nearly 200 different mutations in herg, many of which
appear to interfere with the normal trafficking of the
channel to the cell membrane.7,17
hERG Channels in the Heart
hERG K1 Channels and Antipsychotic Drugs
The discovery and characterization of a Kþ current with
the biophysical properties described above preceded
identification of the gene that we now know encodes
the channel.10,11 Previously designated IKr, the function
of this channel is best understood in the heart where it
plays a prominent role in repolarization of cardiac action
potentials.7,12 In mammalian heart, native hERG Kþ
channels are comprised of 2 erg-1 transcripts (ERG1a
QT prolongation and an increased risk of torsade de
pointes are potential side effects of drugs that block
hERG Kþ channels. Terfenadine (seldane) and cisapride
(propulsid) were both withdrawn from the market because of their propensity to cause fatal arrhythmias particularly when combined with other drugs that decrease
cardiac repolarization reserve. A large number of firstand second-generation antipsychotic drugs also potently
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Fig. 1. Voltage-Dependent Characteristics of ether-a-go-go–Related Gene (ERG) Kþ Channels. Upper Panel: Cartoon schematic illustrating
the 3 conductance states of ERG Kþ channels. Lower Panel: Macroscopic current corresponding to the conductance states as illustrated in the
upper panel. At hyperpolarized membrane potentials, ERG channels are closed (C). Depolarizing the membrane potential from 80 to 0 mV
(lower trace) slowly opens the channels (C/O) resulting in an initial outward current as Kþ ions diffuse out of the cell. However, ERG channels
rapidly inactivate (O/I) by entering a second nonconductive state that limits the outward current produced by the initial depolarization. Partial
repolarization of the membrane potential induces a rapid transition from an inactive to an open conformation (I/O). Because the rate of
deinactivation (I/O) exceeds the rate at which the channels can close, a large resurgent current is generated as the membrane potential
repolarizes.
ERG K+ Channels
Table 1. Comparative Affinity of Antipsychotic Drugs for
Dopamine D2 Receptors and hERG Kþ Channels
Drug
hERG Kþ
a
D2 Receptor , Channel,b
Ki nmol/l
IC50, nmol/l
18d–55e
27e–1000g
3h,i–15e
21 600j
148e–167h
33e–191h
231e–6013h
125e–169h
320e–2500l
5765h
26–78
38–1429
2–12
16 615
87–98
14–83
36–940
15–20
4–30
37
a 3
[ H]-raclopride or [3H]-spiroperidol.
Displacement from cloned human dopamine D2 receptors,
hERG channel current.
c
Malmberg et al. (1993).25
d
CHO cells, Kang et al.22
e
HEK293 cells, Ekins et al.18
f
Kapur and Seeman (2000).26
g
Xenopus oocytes, Suessbrich et al.21
h
CHO cells, Kongsamut et al.19
i
HEK293 cells, Rampe et al.24
j
Xenopus oocytes, Thomas et al.20
k
Seeger et al. (1995).27
l
HEK293 cells, Lee et al.23
hERG K1 Channels in the Brain
b
block hERG Kþ channels in vitro18–27 (table 1). Blockade
is voltage dependent, suggesting that the binding of these
drugs to the channel occurs in the open or inactivated
state.21,22,24 Sertindole and pimozide, both potent D2 receptor blockers (Ki ; 1.2 and 0.7 nM, respectively), exhibit high affinity for hERG Kþ channels expressed in
Chinese Hamster ovary (CHO) cells (IC50 3 and 18
nM, respectively). Both drugs are known to prolong
QT interval,28,29 although only sertindole was regarded
to pose sufficient threat to warrant its temporary removal
from the market. Clozapine, ziprasidone, quetiapine, and
thioridizine also block hERG Kþ channels with nanomolar to low micromolar affinity although they are more potent as D2 dopamine receptor antagonists (table 1). Of
these drugs, thioridizine has been shown to exert a disproportionate effect on QT prolongation and as a result carries a black box warning.30 Despite its favorable hERG to
D2 receptor affinity ratio, chlorpromazine has been
reported to prolong QT interval and increase the risk
of ventricular arrhythmia.31 The absence of a direct correlation between the affinity of an antipsychotic drug for
hERG Kþ channels and its liability for producing QT
prolongation is incompletely understood but likely to involve factors such as drug-protein interactions,19 drug
metabolism,32 and the degree to which individual drugs
In addition to being ubiquitously distributed in cardiac
tissue, the gene encoding Kv11.1 Kþ channels in the heart
(erg-1) is also strongly expressed in mammalian brain.37
Two additional erg genes (erg-2 and erg-3) and their respective Kþ channels (Kv11.2 and Kv11.3) have recently
been identified and shown to be distributed exclusively
within the central nervous system (CNS).38 Individual
members of the erg family are expressed in different levels
and patterns throughout the CNS. mRNA encoding erg1, erg-2, and erg-3 is distributed in the cerebral cortex,
hippocampus, reticular nucleus of the thalamus, paraventricular nucleus of the hypothalamus, cerebellum, and
several brain stem nuclei.39,40 Moderate to high levels
of the erg-1 transcript and all 3 ERG proteins are also
expressed in neurons within the pars compacta of the substantia nigra.41 In most regions, the level of expression of
erg-1 and erg-3 exceeds that of erg-2. While all 3 transcripts appear to be weakly expressed in pyramidal neurons, in the rat, erg-1 is expressed alone and in high levels
in parvalbumin-labeled interneurons in the cingulate and
retrosplenial cortex, selective interneuronal populations
in the hippocampus, and in aspiny interneurons in the
caudate.39,40 The spatial distribution of ERG Kþ channels in the CNS generally parallels the distribution of
erg transcripts.41 Notably, ERG proteins are not only
expressed in neuronal cell bodies but also in dendritic
and axonal compartments,41 and the high degree of overlap in the expression of erg-1, erg-2, and erg-3 mRNA in
the mammalian brain suggests that native channels are
likely to be heteromultimeric. This is an important consideration given that the kinetic properties of Kv11.3
channels are appreciably different from those associated
with channels encoded by erg-1 and erg-2.38
The contribution of native ERG Kþ current (IERG) to
the intrinsic electrical properties of CNS neurons that
express the conductance is for the most part poorly
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Pimozide
0.7c
Haloperidol
0.7f
Sertindole
1.2f
Chlorpromazine
1.3f
Risperidone
1.7c
Thioridizine
2.3c
Olanzapine
6.4f
Ziprasidone
8.4k
Clozapine
82f
Quetiapine
155f
Relative
Potency,
hERG
IC50/D2 Ki
accumulate in myocardium.33 While there are clear differences in the propensity of individual antipsychotics
to cause LQTS,34 it has also been difficult to establish
a causal relationship between drug-induced QT prolongation and torsade de pointes or sudden cardiac death.35
Approximately 8%–10% of individuals treated with atypical antipsychotic drugs exhibit clinical evidence of QT
prolongation, while the incidence of arrhythmogenic disorders in the same population occurs with a frequency of
only 1 in 10 000 individuals.30,34 Deaths attributed to
drug-induced torsade de pointes amount to less than 1
in 100 000 patients.30 With the possible exception of thioridizine, most instances of antipsychotic drug-induced
torsade de pointes are associated with high dosing regimens, intentional drug overdose, synergistic drug interactions, or other predisposing factors, including diabetes,
hypokalemia, or cardiac ischemia.4,30,35,36
P. D. Shepard et al.
Antipsychotic Drugs and IERG in the CNS: Therapeutic
Implications
The discovery of ERG Kþ channels in CNS neurons
together with preliminary evidence that they can
influence neuronal excitability raises the intriguing
question of whether blockade of these channels by antipsychotic drugs contributes in some way to their therapeutic actions or neurological side effects. Although the
ERG-blocking capabilities of antipsychotic drugs have
focused almost exclusively on Kv11.1 (hERG) channels,
there is evidence to suggest that these drugs also block additional ERG channel subtypes in the CNS (Kv11.2 and
Kv11.3). Cloned human Kv11.3 channels expressed in
CHO cells produce an ERG current that is inhibited by
sertindole and pimozide at concentrations comparable
to their Ki values at D2 dopamine receptors.42 Rispiridone,
haloperidol, thioridazine, and clozapine but not metoclopramide block native ERG currents in cloned pituitary
GH3 cells.43 Notably, these channels play a prominent
role in controlling burst duration and prolactin release
from pituitary lactotrophs.44,45 Haloperidol has also
been shown to block IERG in cerebellar Purkinje neurons.9
Midbrain dopamine-containing neurons, cells strongly
implicated in both the therapeutic and side effects of antipsychotic drugs, also appear to express functional
ERG Kþ channels that are blocked by these compounds.
Using intracellular recording techniques in conjunction
with a brain slice preparation, Steen Nedergaard46 identified an ERG-like slow, voltage-activated Kþ current
responsible for a long-lasting afterhyperpolarization
termed AHPs. AHPs in dopamine neurons is calcium independent, activated by prolonged depolarization, and
inhibited by local application of low micromolar concentrations of haloperidol. The effects of haloperidol could
not be attributed to blockade of dopamine or sigma
receptors as neither ()sulpiride, (þ)SKF10047, nor pentazocine had any effect on AHPs. By contrast, terfenadine, a potent blocker of ERG Kþ channels, effectively
inhibited AHPs in dopamine neurons.
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The most prominent physiological effect associated
with blockade of the putative ERG current in dopamine
neurons was a reduction in the duration of a pause in
spontaneous firing that occurs at the end of a train of
stimulus-evoked spiking.46 This ‘‘poststimulus inhibitory
period,’’ resembles the characteristic pause in spontaneous activity that follows a burst of spikes.47,48 Bursting
activity in dopamine neurons has been implicated in a variety of physiological processes, including the encoding
of reward, modulation of dopamine release, and the induction of ‘‘depolarization block’’ following chronic
administration of antipsychotic drugs.49–51 Although the
neurobiological basis of bursting activity in dopamine
neurons is not yet understood, it is likely to involve an interaction between afferent inputs and the intrinsic properties of the cell, including the voltage and ligand-gated ion
channels expressed in the soma and dendrites of these neurons.52,53 The ability of haloperidol and terfenadine, both
potent ERG Kþ channel blockers, to reduce the inhibition
in activity following episodes of high-frequency firing, suggested that these channels could contribute to the process
underlying burst termination. In support of this hypothesis, Canavier et al.8 demonstrated that addition of an ERG
Kþ conductance to a computational model of oscillatory
activity in dopamine neurons faithfully reproduced the
time course of membrane potential changes and somatic
calcium oscillations observed during plateau potentials
induced experimentally by the calcium chelator 1,2bis(2-aminophenoxy)ethane N,N,N’,N’-tetraacetic acid
(BAPTA). These data suggested that IERG is capable of
terminating plateau potentials in dopamine neurons
and implied that the current may be important in the
generation of bursting activity by preventing induction
of acute depolarization inactivation.8 The predictions of
the computational model were tested experimentally by
obtaining intracellular recordings from spontaneously active dopamine neurons under conditions similar to those
used to simulate plateau potential oscillations in the model.
As illustrated in figure 2A, and in accordance with previous
studies,54,55 bursting was induced by partially blocking an
SK-type Ca2þ-activated Kþ conductance. The plateau potential oscillations underlying this type of bursting activity
are clearly visible when tetrodotoxin is applied to block
spiking (figure 2B and C, black traces). Addition of haloperidol, at a concentration previously shown to block the
putative IERG in dopamine cells (5 lM), prolonged the plateau potentials exhibited by these neurons in the presence of
tetrodotoxin (TTX)8 (figure 2C, gray trace). Comparable
results were not observed in response to sulpiride (2 lM,
figure 2B, gray trace). At these concentrations, both drugs
wouldhaveeffectivelyantagonizedD2 dopaminereceptors;
however, only haloperidol would have also reduced IERG
activated during the prolonged plateau depolarization.
While it has yet to be determined whether plateau
potentials triggered by blockade of SK channels in vitro
are involved in ‘‘natural’’ bursting activity exhibited by
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understood. One exception is the cerebellar cortex, where
Purkinje cells have been shown to exhibit an inwardly
rectifying Kþ current with the biophysical and pharmacological characteristics of an ERG channel.9 IERG in
these neurons is characterized by slow deactivation,
a fast recovery from inactivation, and is potently suppressed by the ERG-selective blocker WAY-123,398.
Blockade of IERG in Purkinje cells increased neuronal excitability and suppressed spike frequency adaptation without altering the duration of individual action potentials.
Loss of IERG also prolonged the duration of complex spikes
elicited by activation of climbing fiber inputs. These data
support the proposition that pharmacological blockade of
IERG in the CNS could result in a change in neuronal excitability that would alter synaptic integration.
ERG K+ Channels
Funding
Fig. 2. Effect of Haloperidol and Sulpiride on Plateau Potentials
Exhibited by Nigral Dopamine-Containing Neurons In Vitro.
(A) Intracellular recording obtained from a spontaneously active
dopamine neuron in vitro in the presence of the negative SK channel
modulator, NS8593.64 Note the ‘‘burst’’ of spikes on the rising phase
of a depolarizing plateau potential. Addition of TTX blocks spiking
but has no effect on the underlying plateau potential (B and C, black
traces). Bath application of haloperidol (C, gray trace) but not
sulpiride (B, gray trace) prolongs the duration of plateau potentials
recorded in the presence of the SK channel pore blocker, apamin.
Modified from Canavier et al.8
dopamine neurons in vivo, plateau properties are importantly involved in regulating the activity of a variety of
CNS neurons. The unique kinetic properties of ERG
channels including their rapid inactivation and large resurgent outward current would appear to make them
uniquely suited for limiting the duration of plateau
potentials. It is interesting to note in this regard that
many of the CNS neurons expressing high levels of erg
transcripts, including subthalamic,56 hypothalamic paraventricular,57 cerebellar Purkinje,58 cortical pyramidal,59
and subicular neurons60 also exhibit regenerative plateau
potentials.
In summary, ERG Kþ channels exhibit a unique set of
biophysical properties that are well suited for their prominent physiological role in repolarizing plateau potentials
both in the heart and the brain. First- and secondgeneration antipsychotic drugs block ERG channels in
the heart resulting in LQTS and an increased liability
for developing life-threatening arrhythmias. However,
the discovery of ERG Kþ channels in brain suggests
This work was supported by National Institutes of
Health grants R01NS037963 (CCC). and by funds provided by the Office of the Dean and the Department
of Psychiatry, University of Maryland School of Medicine (PDS).
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