The Journal of Neuroscience, August 13, 2003, 23(19):7311-7316
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BRIEF COMMUNICATION
The Role of the Extracellular Signal-Regulated Kinase Signaling Pathway in Mood Modulation
Haim Einat,1
Peixiong Yuan,1,2
Todd D. Gould,1
Jianling Li,2
JianHua Du,2
Lei Zhang,1
Husseini K. Manji,1 and
Guang Chen1
1Laboratory of Molecular Pathophysiology, Mood
and Anxiety Disorders Program, National Institute of Mental Health, National
Institutes of Health, Department of Health and Human Services, Bethesda,
Maryland 20892, and 2Laboratory of Molecular
Pathophysiology, Department of Psychiatry, Wayne State University School of
Medicine, Detroit, Michigan 48201
 |
Abstract
|
|---|
The neurobiological underpinnings of mood modulation, molecular
pathophysiology of manic-depressive illness, and therapeutic mechanism of mood
stabilizers are largely unknown. The extracellular signal-regulated kinase
(ERK) pathway is activated by neurotrophins and other neuroactive chemicals to
produce their effects on neuronal differentiation, survival, regeneration, and
structural and functional plasticity. We found that lithium and valproate,
commonly used mood stabilizers for the treatment of manic-depressive illness,
stimulated the ERK pathway in the rat hippocampus and frontal cortex. Both
drugs increased the levels of activated phospho-ERK44/42, activated
phospho-ribosomal protein S6 kinase-1 (RSK1) (a substrate of ERK),
phospho-CREB (cAMP response element-binding protein) and phospho-B cell
lymphoma protein-2 antagonist of cell death (substrates of RSK), and BDNF.
Inhibiting the ERK pathway with the blood-brain barrier-penetrating
mitogen-activated protein kinase (MAP kinase)/ERK kinase (MEK) kinase
inhibitor SL327, but not with the nonblood-brain barrier-penetrating MEK
inhibitor U0126, decreased immobility time and increased swimming time of rats
in the forced-swim test. SL327, but not U0126, also increased locomotion time
and distance traveled in a large open field. The behavioral changes in the
open field were prevented with chronic lithium pretreatment. SL327-induced
behavioral changes are qualitatively similar to the changes induced by
amphetamine, a compound that induces relapse in remitted manic patients and
mood elevation in normal subjects. These data suggest that the ERK pathway may
mediate the antimanic effects of mood stabilizers.
Key words: mood; affect; bipolar disorder; mania; depression; lithium; valproate; ERK; RSK; CREB; RSK; BAD; BDNF
 |
Introduction
|
|---|
Severe mood alteration as manifested in mood disorders (e.g.,
manic-depressive illness and unipolar depression) affects one's sense of well
being, as well as self esteem, judgment, attention, motivation, learning and
memory, sleep, appetite, and overall psychomotor activity
(Goodwin and Jamison, 1990
).
Family, twin, and adoption studies strongly suggest that genetic factors are
involved in these disorders. Mood disorders have traditionally been
conceptualized as neurochemical disorders of certain neurotransmitter systems
(such as serotonergic, adrenergic, and, more recently, glutamatergic and
GABAergic systems). However, recent brain imaging and morphometric-postmortem
studies demonstrate regional changes in CNS volume, as well as changes in the
number and/or size of glia and neurons in discrete brain areas of patients
with mood disorders. These recent data suggest neurotrophic function, cellular
growth, death, and resilience as possible factors that contribute to these
disorders (for review see Manji et al.,
2001
; Nestler et al.,
2002
; Coyle and Duman,
2003
).
Mood stabilizers are a small group of agents used in the treatment of
manic-depressive illness. These agents have diverse chemical structures and
different direct targets (Gould et al.,
2002
; Manji and Chen,
2002
). Lithium and valproate are the most commonly used mood
stabilizers. Lithium is a monovalent cation, and valproate is a branched chain
fatty acid. The full clinical beneficial effects of lithium and valproate
require weeks of treatment and blood drug levels maintained within a
therapeutic window. Given that the pathophysiology of mood disorders have not
been well established (Manji and Chen,
2002
), it is difficult to link any given mood stabilizer-produced
direct or indirect biological action to their therapeutic effect. To guide our
research on the mechanism of action of mood stabilizer, we use five
therapeutically relevant biological action(s) validating criteria: (1)
identity: the action itself or its downstream effects must be shared by
structurally dissimilar mood stabilizers; (2) dosing: the action must occur
within the therapeutic concentration window; (3) temporal: the action must
occur in a time frame consistent with the clinical effects; (4) regional: the
action must occur in one or more brain regions implicated in mood modulation;
and (5) behavioral: the functional manipulation of the molecules involved in
the action(s) of mood stabilizers must induce mood-like behavioral change or
alter mood stabilizer-induced behavioral response in experimental animal
models.
Many common (or shared) indirect actions of mood stabilizers have been
identified (Gould et al.,
2002
). Among these common actions, the lithium- and
valproate-induced increases in activity of activator protein-1 (AP-1)
transcription factors and B cell lymphoma protein-2 (Bcl-2) levels meet three
or more therapeutically relevant action(s) validating criteria
(Gould et al., 2002
;
Manji and Chen, 2002
). In an
effort to elucidate the mechanisms by which lithium and valproate affect AP-1
and Bcl-2, we previously investigated the extracellular signal-regulated
kinase (ERK) pathway and found that vaproate not only activated the ERK
pathway in cultured cells, but also promotes cellular functions associated
with the ERK pathway including neurite outgrowth and cell survival
(Yuan et al., 2001
).
The ERK pathway is activated by neurotrophins and other neuroactive
chemicals and is involved in differentiation, survival, and structural and
functional plasticity of neurons (Huang
and Reichardt, 2001
;
Marinissen and Gutkind, 2001
;
Dawson and Ginty, 2002
;
Weeber and Sweatt, 2002
). It
has been established that phosphorylation of ERK44 at Thr202/Tyr204 and ERK42
at Thr183/Tyr185 by MAP/ERK kinase (MEK) results in ERK activation; in turn,
phospho-ERKs activate ribosomal protein S6 kinase-1 (RSK) by phosphorylation
(Frodin and Gammeltoft, 1999
).
Phospho-RSKs phosphorylate Bcl-2 antagonist of cell death (BAD) at Ser112,
thereby inactivating its proapoptotic activity. Phospho-RSKs also
phosphorylate cAMP response element-binding protein (CREB) at Ser 133, which
increases CREB transcriptional activity. CREB regulates Bcl-2 and BDNF gene
expression. The ERK pathway also regulates AP-1 transcription factors directly
or through other kinases, such as glycogen synthase kinase-3
(Karin, 1996
).
The present study investigated the hypothesis that the ERK pathway is a
common therapeutically relevant target of the mood stabilizers.
 |
Materials and Methods
|
|---|
Animal and drug treatments. Male rats (weight, 150-250 gm) were
housed three to four per cage with ad libitum access to water and
food, maintained under a 12 hr light/dark cycle, and allowed a 1 week
accommodation period before initiation of experiments. All experiments were
performed during the light phase of the light/dark cycle and were approved by
the National Institutes of Health (NIH) Animal Care and Use Committee in
accordance with NIH guidelines on the care and use of animals. Rats were fed
with regular rodent chow (control) or lithium carbonate (2.4 gm/kg) or sodium
valproate (20 gm/kg) containing rodent chow (custom produced by Bio-Serv,
Frenchtown, NJ) for 9 d or 28 d. In addition to tap water, a bottle of saline
was available for rats receiving lithium chow to minimize any electrochemical
imbalance that may occur because of the diuretic properties of the drug. At
the end of treatments, rats were euthanized between 9:00 A.M. and 12:00 P.M.,
trunk blood samples were collected for monitoring the drug concentration, and
frontal cortex and hippocampus were removed, frozen in liquid nitrogen, and
stored at -80°C. The MEK inhibitors SL327 (Bristol-Myers Squibb,
Princeton, NJ) and U0126 (LC Laboratories, Woburn, MA) were dissolved in DMSO
and injected intraperitoneally at an injection volume of 1 ml/kg and at doses
ranging from 25 to 100 mg/kg. On the basis of a previous report
(Selcher et al., 1999
) as well
as our initial dose-finding experiments of SL327-induced inhibition of ERK and
RSK1 phosphorylation in hippocampus and frontal cortex, the 50 mg/kg dose was
chosen for all behavioral experiments. After the injection, rats were kept in
their home cage for 1 hr before the behavioral experiments. Amphetamine
sulfate was dissolved in saline and injected intraperitoneally at an injection
volume of 1 ml/kg and at a dose of 0.5 mg/kg just before the behavioral
tests.
Immunoblotting. Frontal cortex and hippocampus tissues were
homogenized by brief sonication in an extraction buffer containing 20
mM Tris-HCl, pH 7.5, 150 mM NaCl, 1 mM EDTA,
1 mM EGTA, 1% Triton X-100, 2.5 mM sodium pyrophosphate,
1 mM
-glycerophosphate, pro-tease inhibitor mixture (Sigma,
St. Louis, MO), and phosphatase inhibitor mixture I and II (Sigma).
Homogenates were centrifuged at 14,000 x g for 15 sec to remove
undissolved debris. For immunoblots, total protein contents were adjusted to
the same level for all samples, and the amount of total protein loaded on the
gel was at levels within the linear range of detection. The blots were first
probed with antibodies against the phosphorylated forms of the protein (except
for BDNF) and then stripped and probed with antibodies against total proteins
of same type. Antibodies were from Cell Signaling (Beverly, MA) (ERK, RSK1,
BAD and CREB, phospho- and total-protein except total RSK1) and Santa Cruz
Biotechnology [Santa Cruz, CA; BDNF and RSK1 (total protein)] and were used
according the manufacturer's instructions. The immuno-complexes were detected
using the ECL method (Amersham Biosciences, Piscataway, NJ). Quantitation of
the immunoblot was by densitometric scanning using an image analysis system
with NIH Image 1.55 software or a Kodak Image Station.
Large open field test. The large open field is a 120 x 120
cm transparent Plexiglas platform, without walls, elevated 80 cm above the
ground. This type of arena is effective in the evaluation of exploratory
behavior in rats (Decker et al.,
2000
). Rats were introduced to the center of the open field for a
30 min session. Behavior was videotaped for later analysis with the Ethovision
(Noldus, VA) video tracking system. The locomotion time, distance traveled,
center time, and number of rearings were recorded.
Forced swim test. The forced swim test was conducted according to
previous reports (Porsolt,
1979
; Einat et al.,
2001
). In brief, the test involves two exposures to a cylindrical
tank of water in which rats cannot touch the bottom of the tank or escape. The
tank is made of transparent Plexiglas, 80 cm tall, 30 cm in diameter, and
filled with water (22-23°C) to a depth of 40 cm. Water in the tank was
changed after each rat. For the first exposure, rats were placed in the water
for 10 min. Twenty-four hours later and 1 hr after SL327, U0126, or DMSO
injection, rats were placed in the water again for a 5 min session (test
session). Behavior was videotaped for later analysis, and the periods of
immobility, swimming, and struggle time were recorded.
Statistical analysis. Data for all experiments were analyzed using
parametric statistics with Student's t test, ANOVA, or multiple
factors ANOVA, as appropriate. ANOVA analysis was followed by post
hoc tests or planned comparisons as projected from the design of each
experiment.
 |
Results
|
|---|
Effects of lithium on the ERK pathway in rat hippocampus and frontal
cortex
We first tested whether lithium stimulates the ERK pathway in regions of
brain involved in human mood modulation in the intact animal. Rats were
treated with lithium carbonate chow for 9 or 28 d, achieving final blood
concentrations within the therapeutic range (
0.8 mEq/l). The activation
of the ERK pathway was monitored by measuring the multiple proteins in their
functional (phosphorylated) forms along sequential steps of the ERK pathway.
The 9 d lithium treatment did not significantly alter the levels of
phospho-proteins or proteins (data not shown). The 28 d lithium treatment
significantly increased phospho-ERK44/42 levels in hippocampus
(Fig. 1a,b) and
phospho-ERK42 level in frontal cortex (Fig.
1c,d). The 28 d lithium treatment also significantly
increased phospho-RSK1 levels and phospho-BAD levels in hippocampus
(Fig. 1a,b) and
frontal cortex (Fig.
1c,d), significantly increased phospho-CREB level in
hippocampus (Fig.
1a,b), and significantly increased BDNF levels in
hippocampus (Fig.
1a,b) and frontal cortex
(Fig. 1c,d).
Lithium-induced increases in phospho-proteins were not likely attributable to
increases in the total amount of the proteins
(Fig. 1a,c).

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Figure 1. Effects of chronic lithium treatment on the ERK pathway. The photographs
are representative immunoblots of the same sample loaded in duplicate. Similar
immunoblots were also obtained from eight additional sets of rats. Bar graphs
are mean ± SE from nine sets of rats; *p < 0.05. a,
b, Results from hippocampal samples. c, d, Results from frontal
cortex samples.
|
|
Effects of valproate on the ERK pathway in rat hippocampus and
frontal cortex
We next tested whether the effects of lithium on the ERK pathway were
shared by valproate, one of the most commonly used mood stabilizers in recent
years. Rats were treated with sodium valproate chow for 9 or 28 d, achieving
final blood concentrations at the low end of the therapeutic range (
40
µg/ml). The 9 d valproate treatment did not significantly alter the levels
of phospho-proteins or total protein (data not shown), but, similar to
lithium, the 28 d valproate treatment significantly increased phospho-ERK44
levels in the hippocampus (Fig.
2a,b) and frontal cortex
(Fig. 2c,d). The
valproate treatment significantly increased phospho-RSK1 levels and
phospho-BAD levels in hippocampus (Fig.
2a,b) and frontal cortex
(Fig. 2c,d). Valproate
treatment also significantly elevated phospho-CREB levels in hippocampus
(Fig. 2a,b) and
frontal cortex (Fig.
2c,d). The treatment-induced increases in
phospho-proteins were not likely attributable to increases in total protein
levels of these proteins (Fig.
2a,c).

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Figure 2. Effects of chronic valproate (VPA) treatment on the ERK pathway. The
photographs are representative immunoblots of the same sample loaded in
duplicate. Similar immunoblots were also obtained from eight additional sets
of rats. Bar graphs are mean ± SE from nine sets of rats; *p
< 0.05. a, b, Results from hippocampal samples. c, d,
Results from frontal cortex samples.
|
|
Effects of the CNS blockade of the ERK pathway on animal behavior in
forced swim test
SL327 is a specific MEK inhibitor that penetrates the blood-brain barrier.
U0126 is the parent compound of SL327 that does not cross the blood-brain
barrier. As reported previously (Selcher
et al., 1999
), we found that a peripheral injection of SL327, but
not U0126, dose dependently (0-100 mg/kg) lowered activated ERK44/42 levels in
rat hippocampus and frontal cortex, effects that reached a plateau at the 50
mg/kg dose (data not shown). Injection of SL327 (50 mg/kg)
(Fig. 3a,b), but not
U0126 (Fig. 3c,d),
significantly decreased immobility time and increased swim time in the forced
swim test. These data suggest that the blockade of the ERK pathway in the CNS
induces antidepressant-like and/or amphetamine-like behavioral effects (see
Discussion).

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Figure 3. Effects of SL327 and U0126 on animal behavior in the forced swim test. Bar
graphs represent mean ± SE from seven to eight rats; *p <
0.05. a, Forced swim test immobility time for SL327/DMSO comparison
[t(14) = 2.84; p < 0.02]. b, Forced
swim test swimming time for SL327/DMSO comparison [t(14) =
3.9; p < 0.01]. Forced swim test immobility time (c) and
forced swim test swimming time (d) for U0126/DMSO comparison
(NS).
|
|
Effects of the CNS blockade of the ERK pathway on locomotion in an
open field
Amphetamine-induced hyperactivity in the open field test has been used as a
model of mania (Einat et al.,
2000
). To further explore the effects of SL327, we studied its
effects on behavior in a large open field. Injection of SL327
(Fig. 4a,b), but not
U0126 (Fig. 4c,d),
significantly increased locomotion time and distance traveled in a large open
field. Interestingly, co-injection of SL327 with amphetamine did not produce
additive effects (data not shown).

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Figure 4. Effects of SL327 and U0126 on animal behavior in a large open field.
a, Large open field locomotion time for the lithium (Li)/SL327
interaction experiment (F(3,28) = 3.7; p <
0.03). b, Large open field distance traveled for the Li/SL327
interaction experiment (F(3,28) = 6.15; p <
0.01; post hoc test, SL327 group different than all other groups).
Large open field locomotion time (c) and large open field distance
(d) traveled for the U0126/DMSO comparison (NS).
|
|
Effects of lithium pretreatment on SL327-induced increase in
locomotion
Lithium pretreatment for 28 d prevented the SL327-induced increase in
locomotor activity (Fig.
4a,b), further supporting the notion that SL327-induced
hyperactivity may be relevant to mania.
 |
Discussion
|
|---|
Lithium and valproate activate the ERK pathway and facilitate ERK
pathway function at therapeutic doses
MEK phosphorylates ERK44 at Thr202/Tyr204 and ERK42 at Thr183/Tyr185; this
phosphorylation results in severalfold increases in ERK44/42 activity
(Frodin and Gammeltoft, 1999
).
We found that chronic treatments with therapeutic concentrations of lithium
and valproate increased the levels of activated phospho-ERK44/42 in rat
hippocampus and frontal cortex (Figs.
1,
2), suggesting that these
treatments stimulated ERK activity. We also found that phosphorylation of the
ERK substrate RSK1 was also increased concurrently with phospho-ERK44/42
(Figs. 1,
2). Taken together, these data
suggest lithium and valproate stimulate the ERK pathway.
Bcl-2, BAD, CREB, and BDNF are targets downstream of ERK/RSK and play key
roles in neuronal development, neuronal survival, and long-term neuronal
plasticity (Huang and Reichardt,
2001
; Dawson and Ginty,
2002
; Weeber and Sweatt,
2002
). Impairments of neuroplasticity and cellular resilience have
been postulated to play a role in the pathophysiology of manic-depressive
illness (Manji et al., 2001
;
Coyle and Duman, 2003
). Chronic
lithium and valproate treatments increased phospho-BAD and phospho-CREB levels
(Figs. 1,
2). Consistent with an earlier
report (Fukumoto et al.,
2001
), both treatments also increased BDNF levels in the rat brain
(Figs. 1,
2). The neuroprotective ability
of lithium has been well documented (Chuang
et al., 2002
). In a preliminary study, we found that valproate
activates the ERK pathway in cortical neurons and protects cortical neurons
against
-amyloid toxicity; furthermore, this protection is dependent on
activation of the ERK pathway activation (our unpublished observations). These
cell signaling and neurobiological data support the hypothesis that lithium
and valproate activate the ERK/RSK pathway and enhance associated
functions.
In addition to hippocampus and prefrontal cortex, clinical and postmortem
studies also reveal abnormalities in other brain regions of patients with mood
disorders (Manji et al.,
2001
). The possible effects of mood stabilizers on the ERK pathway
in other potentially relevant areas of the brain, including the nucleus
accumbens and amygdala, are yet to be investigated.
Blockade of ERK pathway in CNS-induced hyperactivity that may be
relevant to mania
Lithium and valproate are primarily used for treatment and prophylaxis of
mania. We found that both drugs activate the ERK pathway in the rat frontal
cortex and hippocampus. In line with these findings, we found that blockade of
the ERK pathway induced behavioral changes similar to the change induced by
amphetamine in the forced swim test (Fig.
3) and in the open field test
(Fig. 4). Antidepressants
relieve depression and can result in a switch to mania in some depressed or
euthymic patients with manic-depressive illness. Amphetamine can induce manic
relapse in recently remitted patients and various degrees of mood elevation in
normal subjects (for review, see Goodwin
and Jamison, 1990
). Both agents induce reduction of immobility in
the forced swim test, and amphetamine, but not most antidepressants, induces
hyperactivity in the open field in rats. Amphetamine-induced hyperactivity in
the open field is attenuated by chronic pretreatment with lithium
(Lerer et al., 1984
). Similar
to this phenomenon, we found that chronic pretreatment with lithium attenuated
SL327-induced hyperactivity (Fig.
4). Taken together, the data supports the notion that blockade of
the ERK pathway in the CNS results in behavioral changes that are relevant to
facets of mania.
Our results need to be discussed in the context of the body of data
demonstrating that antidepressants increase BDNF expression
(Nestler et al., 2002
;
Coyle and Duman, 2003
). Because
BDNF is one upstream regulator of the ERK cascade, those findings may appear
inconsistent with our results at first glance. However, it is clear that BDNF
and ERK bring about distinct biochemical effects, and a growing body of data
suggests that region-specific manipulation of these plasticity cascades may
bring about quite distinct affective behavioral phenotypes
(Newton et al., 2002
).
Lithium and valproate do not cause depression in patients or healthy
subjects. The effects of acute and subchronic (15 d) treatments of lithium
have been studied in the forced swim test in rats, finding that neither
treatment has significant effects
(Kitamura et al., 2002
;
Wegener et al., 2003
). Whether
these two agents block the SL327-induced effects in the forced swim test is
yet to be investigated.
Genetic alterations in rodents of the ERK pathway and its downstream
targets cause behavioral changes
Genetic disruptions within the ERK pathway and the downstream targets
demonstrate behavioral alterations and drug responses that support the
hypothesis that the ERK pathway modulates manic-depressive illness-related
behavior in rodent. BDNF-deficient mice exhibit increased spontaneous
locomotion (Kernie et al.,
2000
), increased sensitivity to acute amphetamine, altered
response to chronic cocaine, increased aggression, an increase in risk-taking
behavior, and changes in eating patterns
(Horger et al., 1999
;
Lyons et al., 1999
;
Kernie et al., 2000
;
Dluzen et al., 2001
). ERK-1
knock-out mice also exhibit an increased locomotor activity
(Selcher et al., 2001
;
Mazzucchelli et al., 2002
).
CREB-deficient mice exhibit reduced immobility in the forced swim and tail
suspension tests (Conti et al.,
2002
). cAMP response element modulator-deficient mice are
hyperactive and show less anxiety-related behavior
(Maldonado et al., 1999
).
Direct injection of viral vector encoding mutant CREB (HVS-mCREB) into nucleus
accumbens results in decreased anxiety-related behavior in the elevated plus
maze and open field test (Barrot et al.,
2002
) and reduction in depression-related behavior in the forced
swim test (Pliakas et al.,
2001
) and in the learned helplessness paradigm
(Newton et al., 2002
).
Overexpression of CREB in the region produces opposite effects. These data
appear to support our current findings and implicate the involvement of the
ERK pathway in the modulation of behaviors relevant to manic-depressive
illness.
In summary, the present study demonstrates that two structurally distinct
mood stabilizers stimulate the ERK pathway at therapeutic concentrations, in a
clinically relevant time frame, and in brain regions implicated in mood
modulation. The study further demonstrates that inhibiting the ERK pathway
produces behavioral effects in rats that are normalized by mood stabilizer
treatment. These pharmacological, intracellular cell signaling, and behavioral
data are consistent with data from the genetic manipulations of the ERK
pathway. Together, these data strongly suggest that the ERK pathway is a
common therapeutically relevant target of antimanic mood stabilizers.
 |
Footnotes
|
|---|
Received Feb. 4, 2003;
revised Jun. 23, 2003;
accepted Jun. 23, 2003.
This work was supported by the Intramural Research Program of the National
Institute of Mental Health, a grant from Stanley Medical Research Institute,
and the National Alliance for Research on Schizophrenia and Depression.
Correspondence should be addressed to Dr. Guang Chen, Laboratory of
Molecular Pathophysiology, Mood and Anxiety Disorders Program, National
Institute of Mental Health, National Institutes of Health, Department of
Health and Human Services, Building 49, Room B1EE16, 49 Convent Drive, MSC
4405, Bethesda, MD 20892-4405. E-mail:
cheng{at}intra.nimh.nih.gov.
Copyright © 2003 Society for Neuroscience
0270-6474/03/237311-06$15.00/0
 |
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