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SPECIAL ARTICLE
World Psychiatry 19:1 - February 2020 15
Dopamine and glutamate in schizophrenia: biology, symptoms
and treatment
Robert A. McCutcheon1-3, John H. Krystal4-6, Oliver D. Howes1-3
1Institute of Psychiatry, Psychology and Neuroscience, King’s College London, London, UK; 2MRC London Institute of Medical Sciences, Imperial College London, Hammer-
smith Hospital, London, UK; 3South London and Maudsley Foundation NHS Trust, Maudsley Hospital, London, UK; 4Department of Radiology and Biomedical Imaging, Yale
University School of Medicine, New Haven, CT, USA; 5Department of Psychiatry, Yale University School of Medicine, New Haven, CT, USA; 6VA National Center for PTSD,
VA Connecticut Healthcare System, West Haven, CT, USA
Glutamate and dopamine systems play distinct roles in terms of neuronal signalling, yet both have been proposed to contribute signicantly to
the pathophysiology of schizophrenia. In this paper we assess research that has implicated both systems in the aetiology of this disorder. Weex-
amine evidence from post-mortem, preclinical, pharmacological and in vivo neuroimaging studies. Pharmacological and preclinical studies
implicate both systems, and in vivo imaging of the dopamine system has consistently identied elevated striatal dopamine synthesis and release
capacity in schizophrenia. Imaging of the glutamate system and other aspects of research on the dopamine system have produced less consistent
ndings, potentially due to methodological limitations and the heterogeneity of the disorder. Converging evidence indicates that genetic and
environmental risk factors for schizophrenia underlie disruption of glutamatergic and dopaminergic function. However, while genetic inuences
may directly underlie glutamatergic dysfunction, few genetic risk variants directly implicate the dopamine system, indicating that aberrant do-
pamine signalling is likely to be predominantly due to other factors. We discuss the neural circuits through which the two systems interact, and
how their disruption may cause psychotic symptoms. We also discuss mechanisms through which existing treatments operate, and how recent
research has highlighted opportunities for the development of novel pharmacological therapies. Finally, we consider outstanding questions for
the eld, including what remains unknown regarding the nature of glutamate and dopamine function in schizophrenia, and what needs to be
achieved to make progress in developing new treatments.
Key words: Psychosis, schizophrenia, dopamine, glutamate, antipsychotics, striatum, NMDA receptors, D2 receptors, D1 receptors, dorsolateral
prefrontal cortex, GABA interneurons, amphetamine, ketamine, cognitive symptoms
(World Psychiatry 2020;19:15–33)
Schizophrenia is a severe mental disorder characterized by
positive symptoms such as delusions and hallucinations, nega-
tive symptoms including amotivation and social withdrawal,
and cognitive symptoms such as decits in working memory and
cognitive exibility1. e disorder accounts for signicant health
care costs, and is associated with a reduced life expectancy of
about 15 years on average2.
Antipsychotics were serendipitously discovered over fty years
ago, but it took another decade or so until dopamine antagonism
was demonstrated as central to their clinical eectiveness3. Fur-
ther evidence implicating the dopamine system in the patho-
physiology of schizophrenia has subsequently accumulated, and
it remains the case that all licensed rst-line treatments for schiz-
ophrenia operate primarily via antagonism of the dopamine D2
receptor4.
However, despite the central role that dopamine plays in our
understanding of schizophrenia, it has also become increasingly
clear that dysfunction of this system may not be sucient to ex-
plain several phenomena. In particular, dopamine blockade is
not an eective treatment for negative and cognitive symptoms
and, in a signicant proportion of patients, it does not improve
positive symptoms either. As a result, attention has turned to ad-
ditional neurochemical targets. Glutamate is the major excita-
tory neurotransmitter of the central nervous system. e nding
that antagonists of a specic glutamate receptor, the N-methyl-
D-aspartate (NMDA) receptor, induce psychotic symptoms has
led to a wealth of research implicating the glutamate system in
the pathophysiology of schizophrenia.
In this paper we review the evidence regarding dopaminer-
gic and glutamatergic functioning in schizophrenia. We survey
indirect ndings from preclinical, genetic and pharmacologi-
cal studies, evidence from post-mortem research, and results of
neuroimaging studies that characterize functioning in living pa-
tients. We discuss how dysregulation of these systems may lead
to the symptoms of the disorder, and the therapeutic possibilities
associated with their pharmacological modulation. We then ex-
plore what may underlie this dysregulation, and the interaction
between the two systems, before concluding by considering out-
standing questions for the eld.
DOPAMINE
Dopamine was initially thought to be a biologically inactive
intermediary compound on the synthetic pathway between tyro-
sine and noradrenaline. Work by A. Carlsson and others, however,
demonstrated that dopamine depletion inhibited movement,
and that this eect could be reversed following the administra-
tion of the dopamine precursor L-DOPA. is established that
the molecule was of major biological importance in its own right5,
and discrete dopaminergic projections were subsequently identi-
ed.
at dopaminergic dysfunction might play a role in the de-
velopment of psychotic symptoms is one of the longest stand-
ing hypotheses regarding the pathophysiology of schizophrenia.
Below, we discuss the evidence for dopamine dysfunction in
16 World Psychiatry 19:1 - Februar y 2020
schizophrenia, before considering how this may lead to psychot-
ic symptoms, and the mechanisms through which dopamine
modulating treatments exert their eects.
Indirect evidence for dopamine dysfunction in
schizophrenia
Animal models
Rodent models of schizophrenia are useful for investigating
molecular mechanisms that may be of pathophysiological rel-
evance, and for testing novel therapeutic interventions.
One well characterized model of dopaminergic hyperactiv-
ity involves administering repeated doses of amphetamine. is
has been shown to induce events that are also observed in indi-
viduals with schizophrenia, such as reduced prepulse inhibition,
stereotyped behaviours, and impaired cognitive exibility and
attention6. Given that amphetamine results in dopamine release,
and that the above eects can be ameliorated with the adminis-
tration of dopamine antagonists, this provides indirect evidence
for a role of dopamine in behaviour thought to be a proxy for psy-
chotic symptoms.
Another example is that of mice genetically modied to over-
express dopamine D2 receptors in the striatum, which also dis-
play a wide range of schizophrenia-like behaviours7. Similarly,
transgenic insertion of tyrosine hydroxylase and guanosine tri-
phosphate (GTP) cyclohydrase 1 into the substantia nigra in
early adolescence increases dopamine synthesis capacity, and
has been associated with a schizophrenia-like behavioural phe-
notype8.
Other examples do not target the dopamine system directly,
but are still associated with dopaminergic abnormalities. e
methylazoxymethanol acetate (MAM) model involves inducing
neurodevelopmental disruption of the hippocampus via the ad-
ministration of MAM to pregnant rats, and is accompanied by in-
creased ring rates of mesostriatal dopamine neurons9. A model
of environmental risk factors in which rats were socially isolated
post weaning has also been associated with increased striatal pre -
synaptic dopamine function10.
In summary, multiple methods have been used to induce in-
creased striatal dopamine signalling in animal models, and these
consistently produce behaviours analogous to those observed in
individuals with schizophrenia.
Cerebrospinal fluid and post-mortem studies
Studies examining levels of dopamine and its metabolites
– 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic
acid (HVA) – in schizophrenia, both peripherally and in cere-
brospinal uid (CSF), have given inconsistent results11-13. is
may be due to the fact that these levels are a state dependent
marker, and to the eects of antipsychotic treatment. Studies
have found that levels of dopamine, HVA and DOPAC in CSF are
only increased in those receiving antipsychotic treatment13,14,
and that reductions occur following the withdrawal of antipsy-
chotics15,16.
Some17-19, but not all20, studies of HVA have found higher lev-
els in both CSF and plasma of acutely relapsed patients com-
pared to stable patients. ere have also been suggestions that
baseline plasma HVA levels may predict subsequent response to
antipsychotics, which shows some parallels with imaging nd-
ings considered below21.
is approach to studying the dopamine system, however, has
declined in popularity over recent years. A major weakness is
that the measurement occurs distal from the dopamine neurons
of interest. Since both hypo- and hyperdopaminergic function
may exist within an individual simultaneously, a technique that
allows for anatomical specicity is required to understand the
nature and localization of changes.
Early post-mortem investigations suggested that striatal D2
receptor levels might be raised in individuals with psychosis22,
and a meta-analysis of seven post-mortem studies suggested
that receptor levels were increased with a large eect size23. How-
ever, no studies of antipsychotic naïve individuals exist, and
themajority are of individuals chronically treated with antipsy-
chotic medications, which have been found to lead to D2 recep-
tor upregulation24,25.
Post-mortem studies have also examined the substantia nigra.
In these studies evidence regarding dopamine function is incon-
sistent, with some studies suggesting an increase in tyrosine hy-
droxylase levels in patients26, but others nding no dierence27,28.
Other studies have found abnormal nuclear morphology of sub-
stantia nigra neurons29, reduced dopamine transporter (DAT)
and vesicular monoamine transporter (VMAT) gene expression,
and increased monoamine oxidase A expression28.
Recent collaborative eorts in amassing signicantly larger
post-mortem sample sizes, and applying more sophisticated
methods of analysis, may improve our understanding in the near
future30. However, even with these developments, the drawbacks
of post-mortem studies include heterogenous tissue quality, the
fact that the majority of samples are from older patients with a
long history of antipsychotic use, limited information regarding
clinical phenotype, and that death itself leads to a wide range
of neurobiological changes that may obscure important dier-
ences.
Studies in living participants have greater potential to include
younger individuals, drug-free subjects, and also the ability to
look at within-individual changes in symptoms and how these
relate to pharmacological manipulation.
Psychopharmacology of dopaminergic agonists and
antagonists
The discovery that chlorpromazine and reserpine were ef-
fective in the treatment of schizophrenia occurred prior to the
identication of dopamine as a neurotransmitter. It was not until
the 1970s that the clinical potency of antipsychotics was incon-
World Psychiatry 19:1 - February 2020 17
trovertibly linked to blockade of the dopamine D2 receptor31,32.
In addition, selective D2 antagonists show equivalent ecacy to
drugs with a broad spectrum of activity33, indicating that D2 an-
tagonism is sucient for antipsychotic ecacy.
It was also noted that drugs such as amphetamine that in-
crease dopaminergic neurotransmission could induce psychotic
symptoms in healthy individuals, and exacerbate psychotic
symptoms in individuals with schizophrenia34,35. Similarly, L-
DOPA treatment in Parkinson’s disease has been found to in-
duce psychotic symptoms in some individuals36. However, while
amphetamine-induced psychosis is marked by hallucinations,
delusions, paranoia, and conceptual disorganization, it is not
typically associated with negative and cognitive symptoms of the
same form as those observed in schizophrenia37. is relative
specicity to positive psychotic symptoms contrasts with gluta-
matergic models of schizophrenia (see later).
Summary of indirect findings
e ndings discussed above provide evidence that aberrant
function of the dopamine system contributes to psychotic symp-
toms (see Table1). However, these methods are unable to iden-
tify where within the brain this dysfunction is localized to and,
for the most part, cannot provide a direct link to symptoms. We
next discuss methods for in vivo imaging of the dopamine sys-
tem, which has the potential to overcome these obstacles.
Imaging dopamine in vivo
Both magnetic resonance imaging (MRI) and positron emis-
sion tomography (PET) have been used to characterize the dopa-
mine system in vivo (Table2). PET provides molecular specicity
to the dopamine system, but this comes at the cost of lower tem-
poral and spatial resolution compared to MRI.
MRI
Although MRI lacks the ability to directly image the dopa-
mine system, recent work imaging neuromelanin has shown
some promise in quantifying the dopamine system in vivo. Neu-
romelanin is synthesized via iron dependent oxidation of cyto-
solic dopamine, and accumulates in dopamine neurons of the
substantia nigra. It has been demonstrated that the neuromela-
nin MRI signal is associated with integrity of dopamine neurons,
with dopamine release capacity in the striatum, and with the se-
verity of psychosis in schizophrenia49.
Functional MRI (fMRI) has also been used in attempts to infer
functioning of the dopamine system. Task-based fMRI has been
adopted to quantify the striatal response to reward, and this has
been linked to dopamine function, although the precise relation-
ship is complex50. ere is consistent evidence of reduced ventral
striatal activation to reward in schizophrenia51. We consider how
this is consistent with the hypothesis of an overactive dopamine
system in the section discussing psychotic symptoms below.
PET: dopamine receptors
Dopamine receptors have been studied using a wide range of
radioligands. e majority of studies have used ligands specic
for D2-type (i.e., D2, D3 and D4) dopamine receptors, although
several studies have also examined D1-type (i.e., D1 and D5) re-
ceptors.
Table 1 Summary of indirect evidence for dysfunction of dopamine and glutamate systems in schizophrenia
Dopamine Glutamate
Animal models Amphetamine administration, striatal D2 overexpression, and
transgenically increased dopamine synthesis capacity are
associated with schizophrenia-like behaviours. Models of
neurodevelopmental and social risk factors are associated
with increased striatal dopamine function.
Administration of NMDA antagonists induces a wide variety
of schizophrenia-like behaviours. Genetic models that
disrupt NMDA signalling (by reducing levels of D-serine,
inactivating D-amino oxidase or decreasing dysbindin) show
behavioural and neurobiological changes similar to those
observed in schizophrenia.
Cerebrospinal fluid (CSF) Studies of DOPAC and HVA both peripherally and in CSF
have been inconsistent.
Studies of glutamate levels are inconsistent, but kynurenic acid
(an NMDA antagonist) levels appear consistently raised.
Post-mortem studies Increased D2 receptor densities have been observed, but may
result from medication use.
Glutamate neurons show reduced dendrite arborization, spine
density and synaptophysin expression. Glutamate trans-
porter EAAT2 protein and mRNA levels appear reduced
in frontal and temporal areas. There is some evidence that
glutaminase expression is increased in patients, and also that
GRIN1 abnormalities exist.
Pharmacological studies Clinical potency of antipsychotics is strongly linked to their
affinity for the D2 receptor. Amphetamines can induce
positive psychotic symptoms in healthy controls and worsen
symptoms in patients.
NMDA antagonists induce positive, negative and cognitive
psychotic symptoms in healthy controls. Chronic ketamine
users show subthreshold psychotic symptoms.
NMDA – N-methyl-D-aspartate, DOPAC – 3,4-dihydroxyphenylacetic acid, HVA – homovanillic acid, EAAT – excitatory amino acid transporter
18 World Psychiatry 19:1 - Februar y 2020
Striatum
It has been proposed that excessive dopaminergic neuro-
transmission in schizophrenia results from upregulation of stri-
atal postsynaptic D2-type receptors. However, meta-analyses of
studies using PET show only a small increase in receptor den-
sity at most in schizophrenia, and there is no signicant dier-
ence between patients and controls in analyses restricted to
medication naïve patients52. When combined with evidence that
antipsychotic treatment appears to lead to D2 receptor upregula-
tion24,25, it appears possible that any patient-control dierences
may be secondary to confounding by treatment.
ere are caveats, however, to the above inference. First, the
majority of studies are unable to measure the absolute density
of receptors, because a proportion of receptors will be occupied
by endogenous dopamine. If schizophrenia is associated with in-
creased synaptic dopamine levels, this could mask a concurrent
increase in receptor densities. Indeed, one study where dopa-
mine depletion was undertaken prior to PET scanning showed
significantly increased dopamine receptor availability in pa-
tients, although this increase was not signicant in another study
using this approach53,54.
Second, the majority of ligands are selective for D2 over D3
and D4 receptors. e studies that have employed butyrophe-
none tracers (that have an anity for D4 receptors in addition to
D2 and D3 receptors) have tended to show raised receptor den-
sities compared to those studies employing ligands that do not
have D4 anity52. In addition to potential dierences in D2/3/4
subtype proportions, D2 receptors exist in both high and low af-
nity states, and some evidence suggests that schizophrenia may
be associated with an increased proportion of receptors in the
high anity state55-58.
Furthermore, following receptor internalization, some tracers
remain bound, while others dissociate. So, if receptor internali-
zation is increased in one group, this would register as reduced
ligand binding if using a tracer that dissociates on internaliza-
tion, but not if using a tracer that remains bound59,60.
Finally, it has recently been shown that the variability of stri-
atal D2 receptor levels is greater in patients than controls61, sug-
gesting that dierences in D2 receptor density may exist, but only
within a subgroup of patients, although whether this reects a
primary pathology or an eect of prior antipsychotic treatment
in some patients remains unclear.
D1-type receptors have not been studied frequently in the
striatum, and the studies that have been undertaken do not show
any clear patient-control dierences52,62.
Extra-striatal regions
e measurement of dopamine receptors in extra-striatal re-
gions is complicated by the lower receptor densities, which means
Table 2 Summary of imaging studies of the dopamine and glutamate systems in schizophrenia
Striatal Extra-striatal
DOPAMINE
Dopamine receptors
D1 Few studies, and no differences
consistently noted.
Studies using [3H]SCH 23390 have reported
decreased binding in patients; those using [11C]
NNC 112 reported an increase in patients.
D2 No patient/control differences in
unmedicated cohorts. Variability
increased in patients.
Generally poor signal-to-noise ratio. No consistent
patient/control differences.
Presynaptic dopamine function
Consistently increased in both previously
medicated and antipsychotic naïve
patients (g=0.7). Patient-control
differences appear greatest in the
dorsal striatum.
Two studies have found increased synthesis capacity
in the substantia nigra (although not observed
in another). One amphetamine challenge study
found reduced release in patients in prefrontal
cortex. Psychological challenges have produced
less clear results. Findings of challenge studies in
the substantia nigra are inconsistent.
Dopamine transport
DAT No patient-control differences in mean
binding, but variability increased in
patients.
Fewer studies. Some suggestion that thalamic levels
may be raised in patients.
VMAT Two studies have found increased levels in the
ventral brainstem of patients.
GLUTAMATE
Basal
ganglia
Glx (g=0.4) and glutamate (g=0.6) levels raised in patients.
Thalamus Glutamine levels raised in patients (g=0.6).
Medial temporal
lobe
Glx levels raised in patients (g=0.3).
DAT – dopamine transporter, VMAT – vesicular monoamine transporter, Glx – glutamate + glutamine
World Psychiatry 19:1 - February 2020 19
that the signal-to-noise ratio is much lower than in the striatum.
Studies of thalamic, temporal cortex and substantia nigra D2/3
receptor availability have not consistently shown patient-control
dierences63. Other cortical regions have rarely been studied, and
have not shown consistent changes63.
D1 receptors have been more thoroughly examined in cortical
regions than in the striatum. Two studies using [11C]NNC 112 re-
ported an increase in patients64,65, while one reported a decrease66.
Four studies using [3H]SCH 23390 have reported a decrease62,66-68,
while two found no signicant dierences69,70. e interpretation
of these ndings is complicated by the fact that dopamine deple-
tion paradoxically decreases the binding of [3H]SCH 23390, while
it has no eect upon [11C]NNC 112 binding. Furthermore, anti-
psychotic exposure decreases D1 receptor expression, and both
the above ligands also show anity for the 5-HT2A receptor71-73.
PET: dopamine transport mechanisms
DAT is involved in reuptake of dopamine from the synaptic
cleft, and is often interpreted in PET studies as a measure of the
density of dopamine neurons. Studies examining DAT density in
the striatum have found no consistent dierences between pa-
tients and controls52, although, as with D2 receptors, variability is
increased in schizophrenia, suggesting that dierences may exist
within a subgroup61. A more recent study did nd signicantly
raised striatal DAT levels in patients, but this was observed in those
with a chronic illness with long-term antipsychotic exposure74.
ere have been fewer studies examining extra-striatal regions,
although the ones that have been undertaken do suggest that tha-
lamic DAT levels may be raised in patients74,75.
VMAT2 transports intracellular monoamines into synaptic
vesicles. Two PET studies have found that its levels were increased
in the ventral brainstem of individuals with schizophrenia, but
found no dierences compared to controls in the striatum or thal-
amus76,77. is is in contrast to the post-mortem studies discussed
above28, but in keeping with a study showing increased VMAT2
density within platelets from individuals with schizophrenia78.
PET: presynaptic dopamine function
Multiple methods exist for quantifying aspects of presynaptic
dopamine function.
Several studies have investigated dopamine release capacity
by studying the reaction of the dopamine system to an acute chal-
lenge, be that pharmacological such as amphetamine, or psycho-
logical such as a reward or stress task79. Animal studies have shown
that comparing ligand binding during the challenge to binding at
baseline allows one to infer the amount of dopamine release in-
duced by the task80.
Alternatively, one can obtain a measure of baseline synaptic
dopamine levels by comparing a baseline scan with a scan ob-
tained following the administration of a dopamine depleting agent
such as alpha-methylparatyrosine.
Finally, radiolabelled L-DOPA can be used to quantify dopa-
mine synthesis capacity. Radiolabeled L-DOPA is taken up by do-
pamine neurons, where it is converted by aromatic L-amino acid
decarboxylase to dopamine, which is then sequestered in vesicles
within nerve terminals81. e rate of uptake provides an index of
dopamine synthesis capacity.
Striatum
Studies have consistently demonstrated raised presynaptic do-
pamine function in schizophrenia, with Hedges’ g=0.7 (Hedges’ g
is a measure of eect size, and values of 0.2 are typically considered
small, those of 0.5 medium, and those of 0.8 large82). e studies
using a challenge paradigm show larger eect sizes (g=1.0) com-
pared to those quantifying dopamine synthesis capacity (g=0.5)83.
e hyperdopaminergic state associated with schizophrenia ap-
pears greatest within the dorsal striatum83.
Further evidence for pathophysiological relevance comes from
studies showing a direct association between synthesis capac-
ity and the severity of positive psychotic symptoms84-86. e re-
lationship with other symptom domains is less clear: an inverse
relationship with depressive symptoms87 and a lower synthesis ca-
pacity associated with worse cognitive performance88 have been
reported.
Extra-striatal regions
Outside of the striatum, dopamine synthesis capacity can only
be reliably measured in a limited number of brain regions, such as
the substantia nigra and the amygdala, using current techniques89.
Two studies have found increased dopamine synthesis capac-
ity in the substantia nigra90,91, although this was not observed in
another92. One study also found raised dopamine turnover in the
amygdala91.
Although cortical dopamine receptors are predominantly D1-
type, D1 receptor ligands are not reliably displaceable, and there-
fore not suitable for challenge or displacement studies. Cortical D2
receptors do exist, but studies are complicated by their sparsity93.
Furthermore, although challenge paradigms have demonstrated
validity in the striatum, the results of cortical studies are harder to
interpret, with displacement not always observed94.
One study using amphetamine challenge in combination with
the high-anity ligand FLB-457 found reduced dopamine release
in the prefrontal cortex in individuals with schizophrenia95. Two
other recent FLB-457 studies adopted psychological challenges.
One of these used a psychological stressor, which did not induce
cortical tracer displacement in either patients or controls96. e
other used a cognitive test of executive function, which did show
lower tracer displacement in patients, but interpretation was com-
plicated by the fact that, again, the task did not consistently induce
dopamine release97. A study using 18F-fallypride found no dier-
ences between patients and controls in terms of stress-induced
cortical dopamine release98.
20 World Psychiatry 19:1 - Februar y 2020
Two studies have examined dopamine release in the substan-
tia nigra. One used a stress challenge and found an increased re-
lease in patients99; the other adopted an amphetamine challenge
and found a non-signicant reduction95.
PET: dopamine across the psychosis spectrum
Several studies have investigated dopamine function in sub-
jects at clinical high risk for psychosis. Initial studies showed
evidence of raised presynaptic dopamine function in these in-
dividuals100-102. However, this was not seen in the largest study
to date103. is may potentially result from the fact that raised
presynaptic striatal dopamine function appears to be limited to
those subjects who subsequently develop psychosis104, and tran-
sition rates have declined in recent years.
A study of healthy individuals that experience auditory hallu-
cinations also found no dierence in striatal dopamine synthesis
capacity compared to healthy controls without hallucinations105.
Studies in individuals at increased genetic risk for schizophrenia,
such as patients’ relatives and individuals with 22q11.2 deletion
syndrome, have also not shown consistent dierences from con-
trols in terms of presynaptic dopamine function106-108.
Studies in psychotic individuals with diagnoses other than
schizophrenia, such as bipolar disorder and temporal lobe epi-
lepsy86,109, have found raised striatal dopamine synthesis capac-
ity. is nding, along with the inconsistent evidence in people
at increased clinical or genetic risk, may suggest that increased
striatal dopamine synthesis capacity is associated with psycho-
sis across psychiatric diagnoses, rather than being an underlying
risk factor for schizophrenia.
Studies of dopamine receptor densities in individuals at both
clinical99,100,110 and genetic107,110-113 high risk are similar to those
in individuals with schizophrenia, in that they have shown no
clear dierences from controls.
Summary of PET findings
e studies reviewed above provide consistent evidence of a
striatal presynaptic hyperdopaminergic state in schizophrenia
(see Table2), and little consistent evidence of altered D2/3 re-
ceptor levels. It remains uncertain as to whether abnormalities
exist with regard to other dopamine receptors, or with cortical
dopamine function.
Consequences of dopaminergic dysfunction
Prediction errors, salience and positive symptoms
After its role in movement was established, preclinical nd-
ings suggested that dopamine also played a role in signalling
reward114. Later work demonstrated that signalling more speci-
cally related to the discrepancy between expected and received
reward – a reward prediction error115. More recently, it has been
demonstrated that ring is not exclusively tied to reward predic-
tion, but rather can occur in response to a wide range of salient
stimuli116-120, and that in more dorsal regions of the striatum do-
pamine signalling is particularly associated with threat-related
stimuli118,119.
Several related theories have proposed how disruption to nor-
mal dopamine function could underlie positive psychotic symp-
toms such as delusions and hallucinations121-123. Dysregulated
dopamine neuron ring will aberrantly signal that irrelevant stim-
uli are of importance, thereby imbuing percepts and thoughts with
abnormal salience, in turn leading to inappropriate associations
and causal attributions124. ere are also mechanisms through
which uncoordinated dopamine signalling may contribute not
only to the generation of delusional beliefs, but also to the impervi-
ously rigid form of delusional thought123,125.
Recent work has attempted to identify more precisely the
mechanisms through which dopaminergic dysfunction may
contribute to symptoms. e experience of a stimulus depends
not only on the sensory inputs resulting from that stimulus, but
also on prior expectations regarding the probability of a percept.
Auditory hallucinations appear to result from a stronger inu-
ence of prior expectations upon sensory percepts126, and this
increased weighting of priors has been associated with greater
levels of amphetamine-induced dopamine release in the stria-
tum127.
In terms of understanding the development of delusions, a
combined PET and MRI experiment found that dopamine re-
lease was related to neural signalling of belief updates rather
than just sensory surprise128. is suggests that aberrant dopa-
mine signalling may lead to irrelevant stimuli being understood
as meaningful, the clinical relevance of which is supported by
the nding that participants who displayed more aberrant belief
updating showed greater subclinical paranoid ideation128.
In addition to mesostriatal dopamine signalling, several cor-
tical regions have also been implicated in salience process-
ing129,130. e salience network comprises the anterior cingulate
cortex and bilateral insula, and abnormalities of this network
have been proposed as a core feature of schizophrenia patho-
physiology131. e network has a key role in orchestrating dy-
namic switching between brain states, for example between a
resting state and states associated with performing cognitively
demanding tasks132. It is of relevance that dopamine signalling
also plays a role in dynamic reorganization of brain states133,134.
Recent work has demonstrated that mesostriatal dopamine sig-
nalling and salience network connectivity are tightly linked135,
although whether this relationship is disrupted in schizophrenia
is not known.
Reward, motivation and negative symptoms
Reward and punishment are fundamental drivers of behav-
iour, and reinforcement learning models formalize the relation-
ship between reward, states and behaviour. Prediction errors
World Psychiatry 19:1 - February 2020 21
allow the value of states and actions to be learnt, and are a key
signal in many reinforcement learning models. Given the central
role of dopamine in both coding prediction errors and in the cor-
tical representation of environmental states, several studies have
used this framework to explore the behavioural consequences of
disrupted dopamine signalling136.
Cortical D1 receptors play a central role in shaping the accu-
rate neural representation of environmental states, by allowing
precise inhibition of neural activity137. Reduced cortical dopa-
mine signalling means that stimuli associated with reward can-
not be accurately encoded, eectively foreclosing their ability to
guide behaviour137. Furthermore, reduced cortical dopamine
signalling may mean that reward-related representations are
short-lived, with the consequence that, even if correctly repre-
sented, the motivational properties of reward-associated stimuli
have a briefer impact137.
Dopamine neurons re in response to stimuli that have been
previously associated with reward, and guide behaviour towards
actions associated with previous reward138. A striatal hyperdo-
paminergic state may mean that reward-associated stimuli have
reduced motivational inuence, as aberrantly high background
levels of dopamine signalling reduce the signal-to-noise ratio
of adaptive phasic signalling139. is mechanism also has the
potential to reduce the appetitive properties of a given reward,
thereby reducing its impact to shape future behaviour, and ac-
counting for negative symptoms such as anhedonia and amoti-
vation140-142. is reduced signal-to-noise ratio may account for
the reduced striatal activation to reward observed with fMRI in
individuals with schizophrenia51.
Cortical dopamine and cognitive symptoms
Cognitive symptoms of schizophrenia include deficits in
working memory, executive function, and information process-
ing. ey occur prior to the onset of frank psychosis and account
for a signicant proportion of the morbidity associated with the
illness143-145.
e dorsolateral prefrontal cortex is central to many cognitive
processes, and both functional and structural pathology of the
region has been linked to the decits seen in schizophrenia146.
e molecular changes underlying cognitive symptoms, howev-
er, are unknown. Given the importance of D1 receptor signalling
for cognition147, reduced cortical dopamine signalling has long
been hypothesized to contribute to the cognitive symptoms ob-
served in schizophrenia. As discussed above, however, evidence
regarding D1 receptors in schizophrenia is inconclusive, and
there has been only a single study demonstrating reduced corti-
cal dopamine release95.
On the basis of preclinical work using a model of striatal D2
overexpression, it has also been proposed that excessive striatal
dopamine signalling may lead to reductions in cortical dopa-
mine and associated cognitive symptoms7. erefore, it appears
that both reduced and excessive dopamine signalling can have
deleterious eects on cognition, which may contribute to the fact
that there has been minimal success in developing dopamine
modulating treatments for cognitive symptoms of schizophre-
nia4,145.
GLUTAMATE
As reviewed above, there is signicant evidence that dysfunc-
tion of the dopamine system is involved in the pathogenesis of
schizophrenia. is may, however, occur downstream of other
pathophysiological processes. Furthermore, schizophrenia is a
heterogenous disorder, and dopaminergic dysfunction may not
play a signicant role in some individuals, while in others it may
be only one of several pathological mechanisms.
Substantial evidence has accumulated implicating the gluta-
mate system in the pathogenesis of schizophrenia. Glutamate
is the major excitatory neurotransmitter in the central nervous
system, and, in contrast to the anatomically localized cell bod-
ies of dopamine neurons, glutamatergic neurons are widespread
throughout the brain.
Glutamate receptors show considerable variety, and are clas-
sied as either ionotropic or metabotropic. Ionotropic receptors
include the NMDA and the non-NMDA receptors – kainate and
alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid
(AMPA). NMDA and non-NMDA receptors are normally co-lo-
calized on neurons, and act synergistically in that the NMDA re-
ceptor has slower kinetics and tends to enhance depolarization
initiated by non-NMDA receptors.
Following the discovery of the psychotomimetic effects of
NMDA antagonists, the NMDA receptor has become a primary
focus when considering potential glutamatergic dysfunction
in schizophrenia. Below we examine the evidence associating
schizophrenia with glutamatergic abnormalities, consider how
these abnormalities may lead to symptoms, and review the po-
tential of glutamate modulating agents as treatments.
Indirect evidence for glutamatergic dysfunction in
schizophrenia
Animal models
The administration of NMDA antagonists to non-human
primates and rodents has been shown to induce a variety of
schizophrenia-like behaviours, such as sensorimotor gating im-
pairments, increased locomotion, abnormal repetitive move-
ments, and cognitive and social decits38. NMDA antagonism
has also been shown to lead to hippocampal hypermetabolism,
similar to that which has been observed in schizophrenia148,149.
A genetic animal model that reduced levels of the NMDA co-
agonist D-serine was associated with neurobiological changes
similar to those observed in schizophrenia, such as reduced
dendritic spine density and hippocampal volume39. Several
other genetic mice models, such as those involving inactivation
of D-amino oxidase40 and reduction of the synaptic protein dys-
22 World Psychiatry 19:1 - Februar y 2020
bindin41, also show NMDA receptor hypofunction accompanied
by behavioural and neurobiological changes analogous to those
observed in schizophrenia.
CSF and post-mortem studies
Initial small studies of CSF found reduced glutamate levels
in patients, but these ndings were not replicated42,43. However,
CSF and post-mortem brain levels of kynurenic acid, an NMDA
receptor antagonist, have consistently found to be raised, al-
though not plasma levels44.
Post-mortem studies investigating structural alterations of
glutamate neurons have generally found reductions in den-
drite arborization, spine density, and synaptophysin expression
across frontal and temporal regions45.
mRNA expression of specic glutamate receptors and their
subunits has also been investigated across multiple brain re-
gions. Although several studies have found reduced expres-
sion of NMDA receptor subunits such as GRIN1, GRIN2A and
GRIN2C, these have not been consistently replicated across
brain regions45, although – when regions are examined individu-
ally – there is evidence of reduced GRIN1 expression in the hip-
pocampus45. A recent study examining samples from over 500
individuals with schizophrenia found increased exon skipping in
GRIN1, that would aect the extracellular ligand binding site30.
Fewer studies have examined protein expression of these subu-
nits, and these are also inconsistent in their ndings45.
ere have also been a small number of studies investigating
enzymes involved in glutamate metabolism. Excitatory amino
acid transporters (EAAT) remove glutamate from the synaptic
cleft. After reuptake from the synapse, glutamine synthetase con-
verts glutamate to glutamine. When glutamine is delivered to neu-
rons, it is converted back to glutamate by glutaminase. ere is
some preliminary evidence that both mRNA and protein levels of
EAAT2, the transporter responsible for the majority of glutamate
uptake, are reduced in frontal and temporal areas of patients with
schizophrenia, while glutaminase mRNA levels and enzymatic
activity have been found to be raised in two studies examining,
respectively, the thalamus and dorsolateral prefrontal cortex45.
Although the caveats regarding post-mortem studies dis-
cussed above remain, several of the ndings covered here are
consistent with impaired functioning of the NMDA receptor.
In addition to the direct pathology suggested by the findings
relating to GRIN1, the reduced expression of EAAT2 and the
increased expression of glutaminase may be understood as com-
pensatory responses attempting to increase synaptic glutamate
levels.
Psychopharmacology of NMDA antagonists
e administration of NMDA receptor antagonists, such as
phencyclidine and ketamine, to healthy controls has been re-
peatedly shown to induce manifestations similar to the posi-
tive, negative and cognitive symptoms of schizophrenia46. It
has been demonstrated that NMDA receptor blockade by these
compounds is both necessary and sucient for their psychoto-
mimetic eects150.
ese drugs have also been shown to exacerbate a similarly
wide spectrum of symptoms in individuals with schizophrenia,
in contrast to amphetamines, which predominantly worsen
positive symptoms151. Furthermore, it has been shown that anti-
NMDA receptor encephalitis may be associated with psychiatric
presentations that resemble schizophrenia in some individu-
als152. NMDA antagonism has, therefore, been proposed to be a
superior pharmacological model of schizophrenia, compared to
amphetamines, given its ability to more reliably induce negative
as well as positive symptoms153,154.
Acute NMDA antagonist administration has typically been
employed in experimental settings, since chronic administration
to healthy controls cannot be ethically undertaken. However,
chronic ketamine users show subthreshold psychotic symptoms
across positive, negative and cognitive domains47,48, and a sub-
group of these chronic users develop a persisting psychosis that
is more similar to schizophrenia than that induced by acute sin-
gle dose ketamine administration155.
Imaging glutamate in vivo
Proton magnetic resonance spectroscopy
Proton magnetic resonance spectroscopy (1H-MRS) is the
most frequently used technique for investigating the glutamate
system in vivo. It does not require the use of ionizing radiation,
and is signicantly cheaper than PET.
1H-MRS does, however, have several drawbacks, including the
fact that it is unable to distinguish between intra- and extracel-
lular compartments156. Glutamate does not act solely as a neu-
rotransmitter, but is involved in protein synthesis and nitrogen
metabolism, and is a precursor to GABA157. is means that it is
not possible to infer whether dierences between patients and
controls relate to synaptic glutamate concentrations, as opposed
to alterations in these other functions of glutamate. Even if one as-
sumes that detectable abnormalities relate to dierences in syn-
aptic neurotransmission, it is not possible to determine whether
this is secondary to dierences in synaptic levels, as opposed to
presynaptic dysfunction, or altered reuptake of glutamate.
At higher eld strengths, glutamate and its metabolite glu-
tamine can be distinguished, while at lower strengths the con-
centration of both, often abbreviated to Glx, is all that can be
accurately quantied. Glutamine is synthesized from glutamate
following the uptake of synaptic glutamate by astrocytes, and
glutamine levels have been taken to be a marker of glutamate
neurotransmission. However, as with glutamate, glutamine takes
part in multiple cell process, which complicates interpretation158.
ere have been over fty 1H-MRS studies of the glutamate
system in schizophrenia. A synthesis of their ndings is com-
plicated by methodological dierences, notably in the imaging
World Psychiatry 19:1 - February 2020 23
sequences used, the brain regions studied, and the patient co-
horts enrolled. Notwithstanding these issues, a meta-analysis of
these studies has found several relatively consistent ndings159.
ere is evidence that Glx (g=0.4) and glutamate (g=0.6) levels
are raised in the basal ganglia, that glutamine concentration is
increased in the thalamus (g=0.6), and that Glx levels are raised
in the medial temporal lobe (g=0.3) in patients with schizophre-
nia (Table2).
Although these eect sizes are in some instances comparable
to those observed for presynaptic dopamine function, they rep-
resent considerably less studies. In the case of both glutamate in
the basal ganglia and glutamine in the thalamus, only three stud-
ies have been performed, and as such these ndings should be
regarded as preliminary. e increased Glx levels observed in the
medial temporal lobe represent 18 studies (13 in schizophrenia
and 5 in clinical high-risk individuals).
If taken to reect synaptic glutamate levels, the temporal lobe
ndings are consistent with the post-mortem ndings discussed
above, and the PET study discussed below, which suggest that
receptor dysfunction is accompanied by compensatory changes
that increase synaptic glutamate levels. is is also consistent
with studies that have found hippocampal hyperactivity in psy-
chosis, potentially resulting from dysfunction of NMDA recep-
tors located on GABAergic interneurons9,148,149.
In recent years, several studies have been performed using high-
er magnetic eld strengths of up to 7 Tesla. Higher eld strength
has greater sensitivity, enabling greater separation of glutamate
and glutamine peaks, and allowing for more accurate quantica-
tion. Two studies in rst-episode patients have shown reduced glu-
tamate concentrations in the anterior cingulate cortex160,161, while
another only found this in a subset of patients with predominantly
negative symptoms162. No dierence has been observed in several
other investigations163-166. Overall this suggests that, even at high
eld strengths, group dierences are inconsistent.
Another recent development is functional MRS. is involves
acquiring multiple measures during a task or other stimulus, to
investigate dynamic changes in metabolite measures. Changes
in 1H-MRS measures during a stimulus are proposed to result
from compartmental shifts, as glutamate located extracellularly
may contribute more to the signal. is results from the fact that,
within presynaptic vesicles, metabolite movement is restricted,
and therefore may have a faster T2 relaxation rate167.
A study using a heat pain stress found a reduced anterior cin-
gulate cortex glutamate response in individuals with schizophre-
nia compared to healthy controls168, although interpretation is
complicated by the fact that baseline glutamate levels were high-
er in patients. A similar pattern was seen in a study using a cog-
nitive task in which patients showed reduced anterior cingulate
glutamate response compared to controls166.
Other neuroimaging techniques
Given the limitations of 1H-MRS in determining the precise
nature of glutamatergic abnormalities, several attempts have
been made to develop radioligands capable of directly measur-
ing glutamate receptors.
One study found reduced NMDA receptor binding in the left
hippocampus of patients with schizophrenia, but no further
studies have been attempted, in part due to concerns regarding
a lack of specicity of the tracer. A recent study using a tracer for
the metabotropic glutamate receptor 5 found no patient-control
dierences169. New tracers are under development, but lack of
specicity remains an ongoing problem when attempting to im-
age glutamate receptors170.
13C-MRS is another non-invasive imaging technique. It has
lower sensitivity compared to 1H-MRS, but, when combined with
a 13C labelled infusion, it has the potential to overcome some of
the limitations associated with 1H-MRS, specically as regards to
characterizing the glutamate-glutamine cycle171. is technique
has been adopted to show that the majority of energy production
in the brain supports glutamatergic activity and, although stud-
ies are yet to be performed in schizophrenia, it has recently been
used in humans to show that ketamine increases glutamate-glu-
tamine cycling172,173.
Glutamate chemical exchange saturation transfer (GluCEST)
is another novel technique for measuring glutamate in vivo. In
addition to improved sensitivity, it allows for whole brain imag-
ing without the need to specify a single voxel174. It has so far been
used in a single investigation, which reported reduced glutamate
levels in individuals with schizophrenia and those at clinical
high risk, compared to healthy controls175.
Glutamate across the psychosis spectrum
As with imaging of the dopamine system, eorts have been
made to characterize glutamatergic function in individuals at clin-
ical and genetic high risk for psychosis. Similarly, although a me-
ta-analysis suggested that increased Glx levels might exist in the
medial frontal cortex in individuals at clinical high risk of psycho-
sis165, recent studies have been negative176-178, and there appear to
be no clear dierences between at-risk individuals and controls.
Given that 1H-MRS measures of glutamate have been consist-
ently shown to decline with age across the frontal cortex, anterior
cingulate, hippocampus and basal ganglia179-181, it may be that
schizophrenia is associated with a distinct trajectory of change, and
so dierences only become detectable later in the time course of
the illness.
Consequences of glutamatergic dysfunction
Unlike dopamine neurons, which are restricted to relatively
well circumscribed anatomical pathways, glutamate signalling
occurs ubiquitously throughout the brain and, as a result, dys-
function of this system has the potential to account for a wide
range of impairments.
However, given the limitations regarding techniques for direct-
ly quantifying the glutamate system in vivo, there is a paucity of
24 World Psychiatry 19:1 - Februar y 2020
direct evidence regarding the precise nature of glutamatergic
dysfunction in schizophrenia, and studies looking at the rela-
tionship between 1H-MRS measures of glutamate and symptom
severity have produced inconsistent ndings182. Indeed, both
increased and decreased level of glutamate as measured by 1H-
MRS have been proposed to support a hypothesis of NMDA hy-
pofunction in schizophrenia183,184.
NMDA receptors, sparse coding and memory
NMDA receptors play a vital role in orchestrating several cog-
nitive processes, including working memory185. One of the mech-
anisms involved in the ecient cortical representation of in-
formation is that of sparse coding.
Dierent cortical cell classes encode information dierently.
More supercial cell layers code “sparsely”. is means that only
a small proportion of cells within a region will be spiking, and in-
formation is thus encoded spatially186. is is in contrast to deeper
layers, where the majority of cells may be ring, and information
is encoded by variations in the rate of spiking186. Sparse coding in-
volves great spatial precision in terms of the area of excitation, and
this is mediated by strong lateral inhibition secondary to dense
GABAergic interneuron networks within the supercial layers187.
Sparse coding allows the maintenance of multiple mnemonic
networks, and also the protection of encoded memories from
distractors137. Disruption to sparse coding mechanisms has been
shown to lead to the development of false memories in animal
models188. Both individuals with schizophrenia and healthy
controls administered NMDA antagonists display phenomena
that may result from impaired sparse coding – a smaller work-
ing memory buer, decreased mnemonic precision, and false
alarms in working memory137,189. Decreased inhibition second-
ary to hypofunction of NMDA receptors on GABAergic interneu-
rons could account for a disruption to the spatial precision of
supercial layer ring, but this remains to be denitively tested.
Excitatory/inhibitory balance and neuronal oscillations
Synchronized neuronal oscillations are associated with a wide
range of cognitive processes, such as working memory. ese
oscillations result from a tightly maintained balance between
excitatory and inhibitory populations of neurons, and can be
measured in vivo using electroencephalogram (EEG).
e balance between excitation and inhibition is crucial for
normal physiological function, and NMDA receptors play a criti-
cal role here. Disruption to this balance has been proposed to
result in the EEG abnormalities observed in schizophrenia. is
disorder is associated with increased resting gamma oscillations,
that have been linked to cognitive symptoms190. is disruption
of normal oscillatory activity mirrors what has been observed
with ketamine administration in healthy volunteers.
A range of molecular alterations have been proposed to po-
tentially lead to excitatory/inhibitory imbalance in schizophre-
nia, including excessive pruning of dendritic spines, and intrinsic
GABAergic abnormalities191. Another candidate mechanism is
that of NMDA hypofunction. NMDA antagonists preferentially
act on GABA interneurons, because these neurons have a more
depolarized membrane potential. It has also been proposed that,
in schizophrenia, NMDA hypofunction may preferentially aect
interneurons192, which would in turn lead to greater activity of
pyramidal neurons. is uncoordinated increased activity may
underlie disruptions to normal oscillatory activity mentioned
above, and act as noise, impairing the ability of coordinated ac-
tivity to be passed down to subcortical regions150.
FACTORS UNDERLYING GLUTAMATERGIC AND
DOPAMINERGIC DYSFUNCTION
Genetic factors
Over a hundred risk loci have been associated with schizophre-
nia by large scale genome-wide association studies (GWAS)195.
Given the evidence implicating dopamine in the disorder, it is sur-
prising that only one of the loci was found to be associated with
the dopamine system, specically the dopamine D2 receptor.
Analyses specically looking at genes involved in dopamine
synthesis, signalling and metabolism have also been unable to nd
a signal other than for D2 receptors, and this accounts for a negli-
gible proportion of the overall genetic risk196. However, other loci
strongly linked to schizophrenia, such as 10q24.32, have shown
associations with dopamine synthesis capacity197, as have poly-
morphisms of the gene DISC1198. Although relatively small sample
sizes were used, these ndings suggest that genetic factors related
to schizophrenia may indirectly aect the dopamine system.
In the case of glutamate, many genes involved in the develop-
ment and maintenance of glutamatergic synapses are not only
implicated by loci signicantly associated with schizophrenia in
GWAS193,195, but also by studies examining rarer genetic risk fac-
tors199,200. is includes genes that directly code for components
of glutamate receptors, such as GRIN2A, GRIA1 and GRM3, and
genes involved in facilitating glutamatergic neurotransmission
through other means, such as that coding for serine racemase
(SRR). ese results provide some of the strongest support that
disruption of glutamatergic signalling is a fundamental compo-
nent of schizophrenia pathophysiology.
Induced pluripotent stem cells (iPSCs) allow for the genera-
tion of live neurons, in vitro, from somatic cells taken from pa-
tients. ese neurons are best conceptualized as modelling fetal
brain tissue, primarily reecting an underlying genetic architec-
ture, distilled from environmental exposures201. e use of iPSCs
can be particularly valuable in elucidating the eects of genetics
in a polygenic disorder such as schizophrenia, where disease risk
is encoded by complex networks of genes, the functional conse-
quences of which are not easily intuited.
Several studies using this technique have investigated how
the dopamine system may be aected. One study found reduced
DAT expression, indicating immaturity in dopaminergic neurons
derived from individuals with schizophrenia202. Another study,
however, found that dopamine neurons derived from patients
World Psychiatry 19:1 - February 2020 25
tended to develop more rapidly, and showed increased dopa-
mine release203. Small sample sizes and dierences in experi-
mental protocols likely contributed to these discrepancies204.
iPSCs have also been used to study the glutamate system in
neurons derived from individuals with schizophrenia. Two stud-
ies have demonstrated decits in glutamate receptor signalling in
patient-derived cells205,206, and a follow-up study identied spe-
cic gene modules associated with these decits in glutamatergic
signalling201. One investigation found reduced glutamate release
in schizophrenia-derived cells dierentiated to hippocampal den-
tate gyrus cells207, while another found delayed maturation of both
dopaminergic and glutamatergic neurons, and that glutamatergic
neurons displayed a reduced ability to form synaptic contacts202.
Environmental factors
Acute psychosocial stress has been shown to induce striatal
dopamine release208,209. Measures of presynaptic dopamine func -
tion are raised in migrants, and those that have experienced
childhood trauma, both of which are risk factors associated with
schizophrenia210-213. However, another risk factor for schizo-
phrenia, heavy cannabis use, is associated with blunted dopa-
mine synthesis capacity and release214,215.
Although acute stress has been shown to increase cortical glu-
tamate level in preclinical models, this has not been demonstrat-
ed in humans using 1H-MRS216,217. 1H-MRS studies in cannabis
users generally report reduced glutamate levels in both cortical
and subcortical areas, which is in keeping with animal work218.
Of note, a recent iPSC study showed that, in neurons derived
from individuals with schizophrenia, tetrahydrocannabinol ad-
ministration led to depressed glutamate signalling219.
Neural circuits and dopamine-glutamate interactions
e evidence discussed above suggests that, while the dopa-
mine hypothesis can account for the positive symptoms of psy-
chosis, it is less clear whether it can fully account for negative and
cognitive symptoms. Similarly, while glutamatergic models of
psychosis are able to replicate a wide range of symptoms of psy-
chosis, they do not directly account for the nding of increased pre-
synaptic striatal dopamine function, nor the clinical eectiveness
of dopamine antagonists. is suggests that dysfunction in both
systems contributes to the pathophysiology of schizophrenia, and
highlights the need to understand how these two systems may in-
teract.
Much research has investigated dopamine-glutamate rela-
tionships in humans using pharmacological challenges. Am-
phetamine administration has been shown to increase cortical
glutamate levels, as measured using 1H-MRS220, but dopamine
antagonists do not have consistent eects on glutamate levels as
measured using 1H-MRS221. Several, but not all, PET studies have
found that ketamine administration is associated with striatal
dopamine release222. While glutamatergic dysfunction mayen-
courage dopaminergic disinhibition, it is clear that this is not
theonlyroute to symptoms, given that dopamine antagonists do
not entirely ameliorate the eects of NMDA antagonists223.
Recent studies have combined PET and MRI measurements
in the same individuals to investigate this relationship without
the use of pharmacological modulation. One study in healthy
individuals found that increased dopamine synthesis capacity
in the ventral striatum was associated with both reduced corti-
cal and increased striatal levels of glutamate224. is is in keep-
ing with the imaging studies discussed above, in which both
increased dopamine and glutamate measures were observed
in the striatum in schizophrenia, which could potentially result
from increased activity of glutamatergic projection to the stria-
tum. Other studies found the same relationship between cortical
glutamate and striatal dopamine in clinical high-risk and rst-
episode psychosis patients, but not in controls225,226.
eories linking glutamate and dopamine have proposed that
defective NMDA receptors on cortical GABA interneurons result
in inadequate inhibition of glutamatergic projections to the mid-
brain. is, in turn, may overstimulate mesostriatal dopamine
neurons. In order to account for purported cortical dopamine
decits, it has also been proposed that overactive glutamatergic
projections might overstimulate GABA interneurons in the ventral
tegmental area, and thereby overinhibit mesocortical projection
neurons227.
e rst of these proposed mechanisms has support from
studies demonstrating ketamine-induced release of striatal
dopamine. It is also in line with the 1H-MRS studies discussed
above, if the increased basal ganglia glutamate levels observed in
schizophrenia are taken to represent increased activity of cortical
projections. e second mechanism remains speculative222.
It is likely, however, that the relationship between glutamate
anddopamine is not one-way but bidirectional. As mentioneda-
bove, human studies have suggested that modulation of the dopa-
mine system aects cortical glutamate levels. Preclinical studies
have shown that mice genetically modied to have upregulated
dopaminergic signalling display disrupted glutamatergic signal-
ling of thalamocortical neurons228. Furthermore, cortical dopamine
receptors have been shown to inuence local glutamate release229.
e wide range of pathways potentially linking the two sys-
tems, and the potential for opposing eects depending on the
number of interneurons within a circuit, means that it is not pos-
sible to disentangle precise mechanisms with currently available
neuroimaging methods.
TREATMENT
Dopamine modulating treatments
Antipsychotics are eective treatments for positive symptoms
in the majority of patients with a diagnosis of schizophrenia.
However, about one third of patients show persistent positive
symptoms despite treatment, and this has been termed treat-
ment-resistant schizophrenia230.
26 World Psychiatry 19:1 - Februar y 2020
Recent studies have suggested that dopamine synthesis ca-
pacity may predict which patients respond to treatment. An
initial study found that dopamine synthesis capacity was only
raised in those with a treatment-responsive illness84, which is
consistent with a more recent prospective study231.
Even when eective in treating positive symptoms, dopamine
antagonists do not typically show signicant benet for negative
and cognitive symptoms. is is expected if these symptoms do
not primarily relate to the hyperdopaminergic state, and particu-
larly so if they result from decits in dopaminergic signalling.
All currently licensed antipsychotics exert their dopaminergic
eects primarily at postsynaptic D2 receptors, which is down-
stream of the presynaptic hyperdopaminergic state that has
been observed in molecular imaging studies. ere are currently
a number of treatments in development which attempt to correct
dysregulated dopamine function further upstream.
Apomorphine was shown to be an ecacious treatment in an
early clinical trial232. Although later studies did not consistently
replicate this nding4, a recent investigation suggested that this
drug may normalize dopaminergic activity, potentially via ago-
nism of presynaptic autoreceptors233. Another upstream approach
involves agonism of trace amine type 1 receptors. is has been
shown preclinically to both reduce midbrain dopamine neuron
activity, and reduce the locomotor response to amphetamine234.
Finally, the PET studies discussed above have demonstrated
that presynaptic dopaminergic dysfunction is greatest in thedor-
sal striatum83, and muscarinic receptor 4 positive allosteric mod-
ulators specically inhibit dorsal striatal dopamine release, with
ecacy demonstrated in some clinical trials235,236.
e dopamine system may also be more indirectly regulated
via upstream circuits. e potential of glutamate signalling to
modulate dopamine neurotransmission has been discussed a-
bove.Reduced functioning of GABAergic interneurons has also
been suggested to contribute to disinhibition of dopamine neu-
rons, and alpha 5 selective GABA agonists have been proposed
as means of addressing this. Although neuroimaging and pre-
clinical work provides conceptual support, ecacy in patients is
yet to be demonstrated4.
ere is also the chance of intervening downstream of post-
synaptic receptors. Stimulation of D2 receptors inhibits cyclic
adenosine monophosphate (cAMP) production, while phospho-
diesterase inhibitors have an opposing eect by preventing cAMP
breakdown. Phosphodiesterase inhibitors may therefore have the
potential to block the downstream eects of excessive D2 signal-
ling, and also the potential benet of boosting cortical D1 signal-
ling237. Although clinical trials testing these compounds have not
yet been successful4, there is a signicant variability in the region-
al expression of phosphodiesterase inhibitor subtypes, and those
showing the greatest cortical expression remain to be tested238.
Glutamate modulating treatments
Glutamate modulating treatments for schizophrenia fall into
two camps: those that aim to augment NMDA signalling, and
those that aim to reduce levels of synaptic glutamate proposed
to be pathologically raised as a result of NMDA hypofunction.
A general challenge for the development of glutamatergic treat-
ments is that they will typically have relatively global effects,
whereas pathology may be conned to discrete cell types such as
NMDA receptors on specic GABAergic interneurons239,240.
Since directly augmenting synaptic glutamate levels could
have pathologically excitotoxic effects, efforts at augmenting
NMDA signalling have focused on the receptor’s glycine modu-
latory site. For activation of the NMDA receptor, glycine or D-ser-
ine has to bind to the glycine modulatory site on GluN1 subunit,
in addition to glutamate binding at the GluN2 subunit. Agonists
of the glycine modulatory site – including glycine, D-serine and
D-cycloserine – have demonstrated the ability to attenuate the
psychotogenic eects of NMDA antagonists in preclinical stud-
ies, although this has not been clearly demonstrated in humans
150.
A meta-analysis of clinical trials in individuals with schizophre-
nia suggested that D-serine may be eective in the treatment
of negative symptoms241, but a relatively large trial was subse-
quently negative242. e same meta-analysis also suggested that
glycine may have a benet for overall symptoms as well, but large
scale trials are needed for clear conrmation of this eect241. A
meta-analysis of glutamate positive modulators in the treatment
of cognitive symptoms, including D-serine and glycine, did not
demonstrate benet over placebo243.
Poor blood-brain barrier penetration by glycine and some of
the other co-agonists means that occupancy of the glycine mod-
ulatory site may be insucient to exert clinically measurable
eects. To address this, an alternative approach has involved at-
tempts to increase synaptic glycine levels by blocking the glycine
type 1 transporter, thereby inhibiting removal of glycine from the
synapse.
Bitopertin, a glycine type I transporter inhibitor, appeared to
be a promising compound in this regard, showing good blood-
brain barrier penetration, with encouraging results in early clini-
cal trials244. However, later trials were not successful, perhaps
due to the unusually high placebo responses, or the use of chron-
ic patient populations, given that there is some evidence that in-
tervention is likely to be more eective in early illness stages245.
More recently, another glycine type I transporter inhibitor was
shown to increase long-term potentiation (a marker of neuro-
plasticity) in individuals with schizophrenia, and the compound
awaits testing in clinical trials246.
Another potential approach may be to lower levels of kynuren-
ic acid, which is an endogenous antagonist of the glycine modu-
latory site247. Cyclooxygenase-2 inhibitors reduce kynurenic acid
levels, and celecoxib has shown benet as an adjunctive treat-
ment in early psychosis, but not chronic illness248. However, a
recent in vitro study found that celecoxib did not signicantly
reduce kynurenic acid levels, while parecoxib and niumic acid
did249. So, it may be that other cyclooxygenase-2 inhibitors have
the potential for greater ecacy.
Based on ndings that NMDA antagonism increases synaptic
glutamate levels, the second general approach has focused on
World Psychiatry 19:1 - February 2020 27
hypotheses that NMDA hypofunction may result in pathological-
ly raised levels of glutamate, and therefore inhibiting the release
of glutamate from terminals may be therapeutic150,250.
e metabotropic glutamate 2 receptor (mGluR2) is located
presynaptically on glutamate neurons, where it acts as an autore-
ceptor to regulate glutamate release251. Positive allosteric modu-
lators of mGluR2 have been found to be eective in reducing the
cognitive impairments induced by ketamine252. However, eca-
cy in clinical trials has not been consistently shown253. One com-
plication in these trials was the high rate of placebo response.
Riluzole (2-amino-6-triuormethoxy benzothiazole) has also
been shown to reduce synaptic glutamate levels through a wide
range of mechanisms, and an initial trial found it to be eective
in treating negative symptoms in schizophrenia, potentially by
altering striatocortical connectivity254,255. Similarly, lamotrigine
inhibits glutamate release via inhibition of several ion channels,
and attenuates the psychotomimetic eects of ketamine256. La-
motrigine has also shown ecacy as an adjunctive medication
for clozapine-resistant schizophrenia, although studies to date
are small and ndings inconsistent257.
Neuroimaging studies have suggested that treatment-resist-
ant schizophrenia may not show the dopaminergic dysfunction
seen in treatment-responsive schizophrenia84,231,258, and that
glutamatergic abnormalities may be of greater pathophysiologi-
cal relevance in those cases of schizophrenia which do not re-
spond to antipsychotic medications259,260. Supporting this view,
there is evidence that cortical glutamate levels are higher in pa-
tients with treatment-resistant schizophrenia relative to respon-
sive patients261. One of the reasons for unsuccessful clinical trials
may therefore be that glutamate modulating treatments are only
of signicant benet in a subgroup of patients.
OUTSTANDING QUESTIONS AND FUTURE
DIRECTIONS
ere are a number of outstanding issues when it comes to
understanding the role of dopamine and glutamate in schizo-
phrenia. In the case of glutamate, it is not possible to separate
extra- and intracellular compartments using MRS, and we can-
not accurately probe receptors and synaptic glutamate levels in
vivo. As a result, it is not currently possible to precisely character-
ize the nature of glutamate dysfunction in schizophrenia. It re-
mains unclear whether synaptic glutamate levels are abnormal,
whether receptors are altered, and where any alterations might
be localized within the brain.
Because of this, it is not clear whether treatments should aim
to reduce synaptic glutamate levels or augment glutamatergic
neurotransmission150. is likely contributes to the fact that to
date no glutamate modulating agents exist that demonstrate un-
equivocal ecacy in schizophrenia.
ere is a need for radioligands with reliable binding at the
NMDA receptor to allow for investigation of receptor abnormali-
ties in schizophrenia. PET ligands for other proteins involved in
glutamatergic signalling, such as AMPA receptors, enzymes in-
volved in glutamate synthesis and metabolism, and the kynure-
nine pathway, would also represent a considerable advance. In
the meantime, other methods, such as functional MRS, 11C-MRS,
GluCEST and 7T 1H-MRS, may advance our understanding by
allowing more precise inferences regarding the nature of gluta-
matergic abnormalities in schizophrenia.
Imaging studies have provided more information when it
comes to the dopamine system. However, several questionsre-
main unanswered, such as the nature of cortical dopamine
function in schizophrenia, whether a cortical hypodopaminer-
gic state co-exists with the striatal hyperdopaminergic condi-
tion, and how dopaminergic dysfunction evolves across illness
course.
The improved resolution of PET cameras has allowed for
greater anatomical precision in identifying the locus of dopa-
minergic dysfunction in schizophrenia. Early hypotheses had
suggested that this dysfunction might be characterized by aber-
rant mesolimbic function. However, the use of new PET cameras
has demonstrated that the greatest patient-control dierences
are in the dorsal striatum83. e resolution of PET is still relatively
coarse, however, and this limits the precision with which infer-
ences can be made about which specic neuron groups are af-
fected.
In addition to advances in hardware, further progress may be
made by employing novel methods, such as super-resolution
techniques, in which multiple low-resolution images are com-
bined to create a high-resolution image, or deep learning meth-
ods, where anatomical information from an MRI scan is used to
help improve the resolution of the PET image262,263. e limited
resolution of in vivo imaging techniques means that it is dicult
to directly test circuit level hypotheses regarding the nature of
disrupted glutamate-dopamine interactions in schizophrenia.
Instead, hypotheses regarding circuit interactions are largely
based on preclinical, post-mortem and pharmacological studies,
but await direct testing in patients.
Translation of research ndings to clinically useful applica-
tions is not straightforward, and compounds acting though mech-
anisms other than dopamine receptor antagonism have not
consistently shown ecacy in clinical trials. However, there ex-
ists a range of novel mechanisms for manipulation of both glu-
tamate and dopamine signalling that show potential and await
clinical testing. As discussed above, it may be that certain treat-
ments are only of benet in specic subgroups of patients, and
clinical benet may therefore be optimized by stratifying partici-
pants on the basis of underlying neurobiology231,259. Given the
coarseness of current clinical measures264, the development of
imaging biomarkers to evaluate treatment eects at a neurobio-
logical level may assist in moving the eld forward265.
e non-linearity inherent to neural signalling within a com-
plex network means that, even if one disregards potential incon-
sistencies between studies, a coherent integration of existing
ndings is challenging. e combination of large datasets across
illness phases, biophysical networks models to link molecular
pathology to the macroscale dysfunction observed with neuro-
imaging, and carefully designed experiments to test and nesse
28 World Psychiatry 19:1 - Februar y 2020
these models is one route to integrating what at times appears to
be a disparate collection of ndings266,267.
CONCLUSIONS
e hypothesis that dopamine signalling is altered in schizo-
phrenia is supported by animal studies, post-mortem research,
and the clinical eects of drugs that either block or accentuate
dopaminergic neurotransmission. In addition, over the past 25
years, substantial evidence has accumulated from PET studies that
there is increased dopamine synthesis and release capacity in
schizophrenia, that is greatest within the dorsal striatum.
Genetic ndings do not provide strong support for the idea
that dopaminergic dysregulation is a primary abnormality. Rath-
er, it appears that the dopaminergic dysfunction is more likely
to develop downstream of abnormalities in other systems, in-
cluding the glutamatergic system. It also appears that environ-
mental factors may play a signicant role in the development of
dopaminergic dysregulation. Dopamine antagonists remain the
mainstay for pharmacological treatment of schizophrenia, but
there is increasing evidence that these are not eective for all pa-
tients.
Evidence for glutamate playing a role in the pathophysiology
of schizophrenia initially came from the psychotomimetic eects
of NMDA antagonists. While preclinical and post-mortem nd-
ings are consistent with this hypothesis, there is limited support
from imaging studies. However, in contrast to dopamine, recent
genetic ndings do provide support for the view that glutamater-
gic abnormalities may play a major role in schizophrenia patho-
physiology. However, progress is hampered by the challenges
involved in precisely characterizing the system in vivo, and, while
a wide range of glutamate modulating agents have been investi-
gated, none have clear clinical ecacy.
Despite the limitations described, as regards both treatment
ecacy and direct evidence for dysfunction, the dopamine and
glutamate hypotheses of schizophrenia remain inuential and
relevant. is is not least because, as recent data demonstrate,
they possess the exibility to accommodate new ndings, and to
provide ongoing potential avenues for the development of novel
treatments.
ACKNOWLEDGEMENTS
R.A. McCutcheon’s work is funded by a Wellcome trust grant (no. 200102/Z/
15/Z) and UK National Institute for Health Research (NIHR) fellowships.
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DOI:10.1002/wps.20693
