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Cardiovascular Psychiatry and Neurology
Volume 2009, Article ID 475108, 8pages
doi:10.1155/2009/475108
Hypothesis
Serotonin 5-HT2A Receptor Function as a Contributing Factor to
Both Neuropsychiatric and Cardiovascular Diseases
Charles D. Nichols
Department of Pharmacology and Experimental Therapeutics, LSU Health Sciences Center, New Orleans, LA 70112, USA
Correspondence should be addressed to Charles D. Nichols, cnich1@lsuhsc.edu
Received 1 June 2009; Revised 7 August 2009; Accepted 14 August 2009
Recommended by Hari Manev
There are high levels of comorbidity between neuropsychiatric and cardiovascular disorders. A key molecule central to both
cognitive and cardiovascular function is the molecule serotonin. In the brain, serotonin modulates neuronal activity and is
actively involved in mediating many cognitive functions and behaviors. In the periphery, serotonin is involved in vasoconstriction,
inflammation, and cell growth, among other processes. It is hypothesized that one component of the serotonin system, the 5-HT2A
receptor, is a common and contributing factor underlying aspects of the comorbidity between neuropsychiatric and cardiovascular
disorders. Within the brain this receptor participates in processes such as cognition and working memory, been implicated in
effective disorders such as schizophrenia, and mediate the primary effects of hallucinogenic drugs. In the periphery, 5-HT2A
receptors have been linked to vasoconstriction and hypertension, and to inflammatory processes that can lead to atherosclerosis.
Copyright © 2009 Charles D. Nichols. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Neuropsychiatric disorders have high levels of comorbidity
with cardiovascular disease. A recent retrospective study
indicates that metabolic syndrome was reported in about
40% of schizophrenic patients, 35% of bipolar patients, and
25% of patients with recurrent depression [1]. Environ-
mental factors, including medications, likely underlie some
of the metabolic dysfunction associated with schizophrenia
and depression, however, studies in unmedicated drug na¨
ıve
first episode schizophrenics indicate that a pathological
association exists [2]. Significantly, many other studies have
also linked metabolic syndrome, cardiovascular disease, and
psychiatric disorders [3–7], and specific aspects of cardio-
vascular disease like atherosclerosis and hypertension are
associated with psychiatric disorders [8–10]. Patients with
schizophrenia have an average reduction in life expectancy
of 15 years, largely due to coronary heart disease [11].
Unfortunately, many therapeutics used to treat psychiatric
disorders can have significant negative influences on aspects
of cardiovascular function and have thus clouded the nature
of these links with regard to cause and effect. Antipsychotic
medications, as well as therapeutics for other psychiatric dis-
orders, can have dramatic effects on metabolic processes and
can induce metabolic syndrome, weight gain, and diabetes,
which are all significant risk factors for the development
of cardiovascular diseases [12–15]. Furthermore, prolon-
gation of the interval between ventricular depolarization
and repolarization (QT interval) also has been associated
with antipsychotic medications [16]. Overall, metabolic and
cardiovascular dysfunction associated with neuropsychiatric
disorders, therefore, likely represent a mixture of environ-
mental, medication, and pathological factors.
Whereas the exact biochemical nature of the links
between cardiovascular disease and psychiatric disorders
remains elusive, it is evident that there is a strong association
between these biological processes. The fact that medications
used to treat one condition can influence, and even induce,
the other condition underscores these associations. With
respect to depression, models have been proposed that
largely invoke an underlying dysregulation of the HPA axis,
which through modulation of factors such as cortisol and
CRF influence mood, affect, immunity, and cardiovascular
function [6,17,18].
Aspects of cardiovascular disease including endothelial
dysfunction and atherosclerosis are acutely mediated by
inflammatory mechanisms. For example, adipose tissues can
release proinflammatory cytokines into the circulation. As
more adipose tissues are present in an individual, represented
by a higher body mass index, more cytokines can be released.
2Cardiovascular Psychiatry and Neurology
These cytokines, primarily Tumor Necrosis Factor-α(TNF-
α) and IL6, can directly induce inflammation in cardio-
vascular tissues, as well as activate the HPA axis, which in
turn can lead to metabolic syndrome. Metabolic syndrome
can subsequently lead to oxidative stress and generation
of free radicals that together induce further production of
proinflammatory cytokines, and the two processes of inflam-
mation and metabolic syndrome can interact synergistically
to elevate levels of proinflammatory cytokines and promote
further endothelial dysfunction and atherosclerosis [19].
A detailed review of the development and progression of
atherosclerosis itself will not be given here, and the reader
is referred to other reviews and references therein [20,21].
A key mediator of the development of atherosclerosis is
the cytokine TNF-αwhich, acting through its receptors on
the surface of macrophage, endothelial, and smooth muscle
cells of the vasculature, induces signal transduction cascades
leading to NOS activity, activation of transcription factors
such as Nuclear Factor kappa B (NF-κB), and production
of proinflammatory adhesion molecules and cytokines such
as ICAM-1, VCAM-1, and IL6. Together, these processes
facilitate macrophage infiltration of the arterial wall, differ-
entiation of macrophages to lipid-accumulating foam cells,
and migration of arterial smooth muscle cells to form a
fibrous cap, together constituting the atherosclerotic plaque.
Severe cases cause significant blockage of the artery, and
eventual rupture of the plaque and thrombosis.
Recently, cytokine-mediated inflammation has been
implicated in the development and presentation of psychi-
atric disorders that include depression and psychosis [22–
24]. In major depression and bipolar disease, increases in
TNF-α, and other proinflammatory cytokines (e.g., IL6,
and other proinflammatory molecules such as ICAM-1 and
MCP-1), have been found within the CNS [23,25]. Although
the association of inflammation with depression does not
necessarily imply causality, certain symptoms of depression
have been shown in both clinical studies and animal models
to be alleviated by anti-inflammatory therapeutics [26].
Interestingly, knockout mice lacking TNF-αreceptors exhibit
antidepressant-like behaviors in several types of assays [27].
Neuroinflammation leading to dysfunction of the adult
CNS as well as inflammatory events in utero leading to
perturbation of normal synaptic development has been
proposed as possible factors contributing to psychiatric
disorders [23,24].
It has long been recognized that 5-hydroxytryptamine
(serotonin; 5-HT), and its biosynthetic precursor tryp-
tophan, play an important role in regulating immune
functions through non-5-HT receptor interactions involving
circulating tryptophan and kynurenine levels [28–30]. Indi-
vidual serotonin receptors, however, are expressed in many
immune-related tissues, and interactions at specific receptors
are also known to modulate aspects of the immune response
and inflammation [31–33]. Within the CNS, serotonin
and serotonin receptors have been strongly associated with
normal function. Certain neuropsychiatric disorders that
include depression, bipolar disorder, OCD, anorexia, and
schizophrenia have been linked to dysregulation of CNS
serotonin [34,35]. Indeed, therapeutics for these disorders
often include inhibition of the serotonin transporter (SERT)
with selective serotonin reuptake inhibitor (SSRI) medica-
tions, or blockade of specific serotonin receptor subtypes.
SSRIs can also show an efficacy in treating aspects of
cardiovascular disease associated with depression [36], and
have been demonstrated in animal models to have an anti-
inflammatory effect [37]. The mechanisms underlying the
protective effect of antidepressants are not precisely known,
but are predicted by some researchers to involve activation
of the pituitary-adrenocortical system via increased central
serotonin levels [38], by modulation of cytokine levels in
peripheral tissues [39,40], and by suppression of platelet
activation [41]. Furthermore, acute SSRI administration has
been shown to have a vasodilatory effect on the coronary
artery that may be cardioprotective [42]. Interestingly, TNF-
α, as well as certain other cytokines, have been shown
to influence both expression and transport activity of the
serotonin transporter. In neuronally derived cells and chori-
ocarcinoma cells, TNF-α,INF-γ,andIL1βincrease function
[43–45], whereas in B lymphocytes, IL4 decreases function
[46], and in intestinal epithelial derived Caco-2 cells, TNF-
αhas been found to decrease both expression and transport
activity of SERT [47]. Whereas the nature of the influence of
cytokines on SERT function (e.g., facilitation or repression)
likely depends on the cytokine and tissue, modulation
of synaptic serotonin levels in various brain regions by
inflammatory cytokines would certainly be anticipated to
have some effect on neuronal function relevant to psychiatric
disorders like depression. In summary, there appears to be
a strong link between proper functioning and regulation of
the serotonin system and factors underlying cardiovascular
disease and neuropsychiatric disorders.
We hypothesize that a particular aspect of the serotonin
system, the 5-HT2A receptor, is a common and contributing
factor underlying aspects of normal cardiovascular and CNS
function, and that dysfunction of this receptor results in
certain characteristics of cardiovascular and neuropsychiatric
disorders. There are seven families of serotonin receptors
comprised of fourteen distinct subtypes [48]. With the
exception of the 5-HT3receptor, which is a ligand-gated ion
channel, all are seven transmembrane-spanning G-protein-
coupled receptors. Of all the serotonin receptors, the 5-
HT2A receptor has been the one most closely linked to
complex behaviors and neuropsychiatric disorders. The 5-
HT2A receptor is highly expressed within the frontal cortex,
with lower expression levels throughout the brain [48]. There
has been extensive research performed to establish the role
of 5-HT2A receptors within the brain, where they have been
shown to participate in processes such as cognition and
working memory [49], mediate the primary effects of hal-
lucinogenic drugs [50], and been implicated in mechanisms
underlying schizophrenia [51,52]. Furthermore, abnormal
expression of 5-HT2A receptors has also been linked to
depression. For example, some studies have shown that
receptor protein expression is increased in certain cortical
areas of patients with major depression [53,54], as well
as suicide victims [55,56]. 5-HT2A receptor expression
decreases, however, have been found in brain limbic regions
of patients with major depressive disorder [57].
Cardiovascular Psychiatry and Neurology 3
Significantly, 5-HT2A receptors are found outside the
CNS in many diverse tissues, including those related to
cardiovascular function. Their role in the periphery, how-
ever, is less clear. Also, 5-HT2A receptor mRNA is expressed
within vascular smooth muscle and endothelial cells, and
cardiomyocytes, where the receptors are believed to mediate
aspects of vasoconstriction and cellular proliferation [58–
60]. Not only can 5-HT2A receptor activity modulate cardio-
vascular function in the periphery, but it has been found to
act centrally: activation of 5-HT2A receptors in the nucleus
tractus solitarius of the brain dramatically lowers both blood
pressure and heart rate [61].
Recently, we have found that selective activation of 5-
HT2A receptors in primary aortic smooth muscle inhibits
TNF-α-mediated inflammatory markers with extraordinary
potency. With an IC50 value of about 10 picomolar, 5-
HT2A receptor activation with the drug (R)-DOI inhibits
NOS activity, the activation and nuclear translocation of the
p65 subunit of NF-κB, as well as the production of mRNA
for the proinflammatory cell adhesion proteins ICAM-1
and VCAM-1, and mRNA for the cytokine IL6 [33]. Other
chemically diverse molecules that activate 5-HT2A recep-
tors, including the hallucinogen lysergic acid diethylamide
(LSD), also have potent anti-inflammatory effects on aortic
smooth muscle in vitro [33], indicating that this is a
property of 5-HT2A receptor activation and not specific to
a particular drug. Significantly, we have found potent anti-
inflammatory effects in primary aortic endothelial cells as
well as macrophages (unpublished data). TNF-αsignaling in
these three cell types, aortic smooth muscle, endothelial, and
macrophage, is believed to be a major contributing factor
to the inflammatory processes underlying the development
and progression of atherosclerosis. As such, drugs acting
at 5-HT2A receptors, like (R)-DOI, may represent a novel
class of superpotent small molecule inhibitors of TNF-α
pathway signaling with therapeutic potential for treating not
only atherosclerosis but also other inflammatory conditions
involving TNF-α, that are more then 100-fold more potent
than the more potent steroidal anti-inflammatories currently
on the market. Importantly, we have also found potent
anti-inflammatory effects of 5-HT2A receptor activation in
CNS-related cell culture systems, including C6 glioma, and
SH-SY5Y neuroblastoma cells (unpublished data), indicat-
ing that the role of 5-HT2A receptors in mediating anti-
inflammatory pathways is not limited to cardiovascular
tissues, but is likely relevant in the CNS.
As mentioned previously, drugs that interact with or
influence 5-HT2A receptor function can dramatically affect
aspects of cardiovascular function. Some, including atypical
antipsychotic, medications have a negative influence, while
others, including ketanserin and certain antidepressants, are
reported to have a beneficial cardiovascular effect. How do
these effects fit within the framework of our hypothesis?
Ketanserin has been effective in the clinic as an anti-
hypertensiveagentaswellasanantiarrhythmic.Itcan
also sometimes induce proarrhythmias, and was withdrawn
from the market largely for this reason. Recent reports
suggest that the antiarrhythmic effects of ketanserin may be
due to direct interactions with certain potassium channels,
including the HERG channel, and not to blockade of the 5-
HT2A receptor per se [62–64]. With regards to ketanserin’s
use as an antihypertensive, the underlying mechanisms are
not entirely clear as ketanserin has significant affinity for
the alpha-1 adrenergic receptor, and many reports have
cited this as the putative antihypertensive therapeutic target
rather than antagonism of the 5-HT2A receptor [58,65,
66]. Nevertheless, many in vitro studies of 5-HT2A receptor
antagonists have clearly demonstrated that 5-HT-induced
vasoconstriction in isolated vascular tissue preparations is
in large part mediated by 5-HT2A receptors [60]. Although
blockade of 5-HT2A receptors can potently inhibit serotonin-
mediated vasoconstriction in isolated vascular preparations,
aside from ketanserin, other 5-HT2A receptor antagonists
show little to no antihypertensive effect in vivo [66,67].
Indeed, newer highly selective 5-HT2A receptor antagonists,
like M100907 (volinanserin), ACP-103 (primavanserin), and
SR46349B (eplivanserin), are currently in clinical trials as
novel therapeutics to treat insomnia [68] and there are
no reports in literature describing effects on hypertension,
inflammation, or other cardiovascular processes. One report,
however, examining the physiological and pharmacokinetics
of ACP-103 in a small study comprised of normal human
subjects has been published that concluded that there were
no significant changes in vital signs or ECG associated with
treatment for up to fourteen days [69].
An interesting study recently published detailed the
effects of chronic increases in circulating serotonin levels, as
opposed to large bolus doses. It was predicted that, as occurs
with a bolus dose of serotonin, blood pressure would increase
due to the vasoconstrictive effects of increased 5-HT acting
at 5-HT2receptors. It was found that increased circulating
5-HT levels actually significantly decreased blood pressure
[70,71]. The author of this study stated that it was unlikely
that direct activation of vasoralaxant 5-HT receptors was
responsible for this effect, and that further studies are needed
to elucidate underlying mechanisms [70]. If antagonism
of 5-HT2A receptorsisexpectedtoproducehypotension
and affect cardiac rhythmicity, then activation would be
anticipated to produce hypertension and potentially affect
rhythmicity. This has not been the case. In humans, the
5-HT2A receptor agonist, psilocybin, which also has high
affinity for 5-HT1A receptors, produces only mild and tran-
sient cardiovascular effects at high doses when administered
systemically. Highly hallucinogenic doses (e.g., 30 mg) only
produce minor and transient increases in baseline heart
rate (+10 bpm) and blood pressure (∼15%) and do not
influence heart function as measured by electrocardiogram
[72–74]. Lower non-hallucinogenic doses of psilocybin do
not produce significant changes in heart rate, blood pressure,
or heart function [72–74]. Another 5-HT2A receptor agonist
dimethyltryptamne (DMT) has been given to humans at
highly hallucinogenic doses [75]. In that study, intravenous
injection of DMT was found to only elicit minor and very
transient increases in heart rate and blood pressure [75]. It
should be noted that some of these increase can probably
be attributed to psychological stress and anxiety produced
by the hallucinogenic effects of psilocybin and DMT at high
doses, and not by a direct pharmacological action on blood
4Cardiovascular Psychiatry and Neurology
pressure or heart rate. There have been no studies reported
examining the effects of chronic administration of 5-HT2A
receptor agonists in mammals. It will be interesting to see
in future experiments if chronic administration of these
agents affects inflammation-related cardiovascular diseases
or other aspects of cardiovascular function. Our data indicate
that potential anti-inflammatory effects of agonists like (R)-
DOI would be evident at doses far below that necessary
to elicit behavioral effects like hallucinations. Interestingly,
there are antidepressant-like effects associated with single
hallucinogenic doses of psilocybin [73,76].
Atypical antipsychotic medications like olanzapine,
clozapine, and risperidone belong to a newer class of drug
that are believed to have a component of their therapeutic
effect mediated by antagonism of 5-HT2A receptors [77].
Unlike traditional antipsychotic medications like haloperidol
that act primarily as antagonists at dopamine D2 receptors,
atypical antipsychotics have some efficacy at treating the
negative, or more cognitive, symptoms of schizophrenia, and
this may be due to their effects on 5-HT2A receptors. As pre-
viously mentioned, pathological associations exist between
schizophrenia and metabolic syndrome and cardiovascular
disorders, however, the use of atypical antipsychotics is,
unfortunately, strongly associated with the development
of significant weight gain, metabolic, and cardiovascular
disorders [14,15,78]. The substantial weight gain asso-
ciated with atypical antipsychotics is believed to partially
involve antagonist or inverse agonist activity of these drugs
at 5-HT2C receptors [79]. Indeed, the 5-HT2C knockout
mouse is severely obese [80], and agonists of this receptor
can produce hypophagia [81]. Although many aspects
of metabolic and cardiovascular disorders associated with
atypical antipsychotics are likely a direct consequence of
weight gain, other aspects may be mediated by blockade of
5-HT2A receptor function. For example, 5-HT2A receptors
have been implicated in regulation of glucose homeostasis
[82,83], and antagonism of the 5-HT2A receptor may
influence insulin sensitivity [84,85]. Within the framework
of our hypothesis, aberrant 5-HT2A receptor function may
contribute to both psychosis and pathological association
of metabolic and cardiovascular disorders. This dysfunction
could result in hyperacticvity in the CNS, and contribute
to psychosis. In the periphery, receptor dysfunction may
promote processes leading to metabolic disorder and car-
diovascular disease through largely unexplored mechanisms.
Whereas blockade of 5-HT2A receptor hyperfunction in
the CNS may be therapeutic for treating psychosis, recep-
tor blockade, both in the CNS and periphery, may also
interfere with endogenous anti-inflammatory processes and
synergistically act with the effects of induced weight gain
to produce significant metabolic and cardiovascular disor-
ders.
Another class of medication that affects psychiatric
disorders, inflammatory processes, and cardiovascular func-
tion is selective serotonin reuptake inhibitor antidepressants
(SSRIs). Interestingly, SSRI antidepressant medications have
a biphasic effect on serotonin within the brain. Acute
treatment leads to decreased serotonin release, and chronic
treatment leads to increased release [86,87]. The acute
decrease in 5-HT release results from autoreceptor acti-
vation and subsequent inhibition of release and synthesis
of serotonin. As these receptors desensitize with chronic
SSRI treatment, however, overall 5-HT transmission is
facilitated. Chronic treatment with SSRI antidepressants also
has been shown to produce significant downregulation and
desensitization of 5-HT2A receptors both in vitro and in
vivo similar to chronic treatment with atypical antipsychotics
[88]. The effects of SSRI induced receptor desensitization
and downregulation would be anticipated to mimic the
effects of chronic treatment with atypical antipsychotics,
and reduce overall 5-HT2A receptor function. Within the
framework of our model, these effects would be predicted
to produce a deficit in receptor function, and increases in
proinflamatory mechanisms potentially leading to cardiovas-
cular disease, metabolic disorders, and neuroinflammation.
SSRI antidepressants, however, have been shown to have
anti-inflammatory activity and to be cardioprotective when
given both acutely and chronically. It is conceivable that
the acute anti-inflammatory and cardioprotective effects of
SSRI antidepressants are mediated by mechanisms other
than manipulation of 5-HT2A receptor function, as discussed
previously, and the beneficial effects of chronic treatment
may involve enhanced 5-HT tone at 5-HT2A receptors.
Although chronic treatment with SSRI antidepressants
produces desensitization and downregulation of 5-HT2A
receptors, our results demonstrate that 5-HT2A receptors in
this state are actually more sensitive to the anti-inflammatory
effects of activation by the agonist (R)-DOI by an order
of magnitude [33]. Together, the anti-inflammatory and
cardioprotective effects of SSRI antidepressants are, there-
fore, likely a combination of direct modulation of cytokines,
central action within the CNS, and modulation of 5-
HT2A receptor function, with each component contributing
differently as therapy progresses to achieve a steady state.
Here, we propose that deficits in 5-HT2A receptor
function underlie at least part of the comorbidity of
cardiovascular disease and neuropsychiatric disorders. If
5-HT2A receptor activation normally appears to exert a
powerful anti-inflammatory influence on a variety of cells,
especially vascular tissues, dysfunction may be anticipated
to lead to a repression of anti-inflammatory influences and
to the expression of proinflammatory markers, sensitiza-
tion of the cell to inflammatory stimuli, or both, leading
to an increased risk of inflammation and atherosclerosis.
Similarly, 5-HT2A receptor dysfunction also may contribute
to increased risk of hypertension, and cardiac hypertro-
phies within the cardiovascular system. Unfortunately, there
are few, if any, studies reported in literature examining
expression levels of 5-HT2A receptors in diseased cardio-
vascular related tissues. This simply may be due to the
fact that no one has looked. If so, then examination
of receptor levels in diseased cardiovascular-related tis-
sues may be a productive avenue of exploration. In rat
models of congestive heart failure, there are two reports
demonstrating increased levels of 5-HT2A receptor mRNA
[59,89]. It remains to be determined if the increased
expression is causative, or a compensatory response to other
factors.
Cardiovascular Psychiatry and Neurology 5
Within the CNS, the same receptor dysfunction may
result in or contribute to the development of neuropsy-
chiatric disorders including depression, bipolar disease,
and psychosis. This dysfunction may either come from
alterations in regulation due to promoter polymorphisms
or other regulatory mechanisms influencing expression, or
polymorphisms or mutations affecting the protein itself
that could influence responsiveness and downstream signal
transduction pathways. Polymorphisms in the promoter
region of the human HTR2A locus have been shown
to alter receptor expression levels [90], and these same
polymorphisms have been linked to response to antisychotics
and certain SSRIs [91,92], and in some studies positively
associated with various CNS conditions including major
depression, bipolar disorder, and schizophrenia [93–96].
Significantly, positive associations also have been detected
for these polymorphisms and symptoms of cardiovascular-
related disorders [97]. Polymorphisms within the coding
regions of the HTR2A locus have been found in some studies
to be positively associated with neuropsychiatric disorders,
as well as to rheumatoid arthritis [98], circulating cholesterol
levels [99], hypertension [100], myocardial infarction [101],
as well as blood pressure and metabolic syndrome [102].
There is significant opportunity for future research to
investigate how 5-HT2A receptor function mediates certain
aspects of both neuropsychiatric and cardiovascular-related
disorders. Greater clarification of the role of receptor antag-
onists in vivo is needed. This could involve examining the
effects of the new highly selective receptor antagonists in
rodent models of cardiovascular disease and atherosclerosis,
as well as careful examination of clinical trial data for the use
of these drugs as sleep aids and continued analysis for the
effects of chronic use on cardiovascular-related issues after
these therapeutics come to market. Not only could results
from these types of studies be informative about the effects of
selective receptor blockade on cardiovascular-related diseases
but they could also help to address the question of whether
or not the negative cardiovascular and metabolic effects of
atypical antipsychotics have a significant 5-HT2A receptor-
mediated component. If they did, then perhaps long-term
therapy with these new highly selective receptor antagonists
would produce metabolic and cardiovascular disorders. In
our laboratory, we are continuing to study the effects of
agonists on inflammation-related cardiovascular processes,
and attempting to elucidate the molecular mechanisms
underlying their anti-inflammatory effects. An additional
resource that would beneficial to explore is the 5-HT2A
receptor knockout mouse model. Amazingly, given the
widespread expression and importance of the 5-HT2A recep-
tor, the knockout animal appears overtly normal. There are,
however, certain behavioral effects associated with loss of this
receptor [103,104]. Interestingly, some observed behaviors
are opposite to the effects of receptor antagonists [105],
indicating that caution should be exercised in the interpre-
tation of knockout studies using this model. Nevertheless,
studies utilizing this mouse in models of cardiovascular-
related diseases will likely be of value. A better understanding
of the relationship between 5-HT2A receptor function and its
roles in both the CNS and cardiovascular system should lead
to development of improved therapeutics to treat diseases
affecting each of these systems either separately or together.
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