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The mechanisms of schizophrenia that underlie the development of schizophrenia, a chronic mental disorder are complex. A number of theories attempt to explain the link between altered brain function and schizophrenia, including the dopamine hypothesis and the glutamate hypothesis. These theories are separate from the causes of schizophrenia,
which deal with the factors that lead to schizophrenia. The current
theories attempt to explain how changes in brain functioning can
contribute to symptoms of the disease.
Pathophysiology
The exact pathophysiology of schizophrenia remains poorly understood. The most commonly supported theories are the dopamine hypothesis and the glutamate hypothesis.
Other theories include the specific dysfunction of interneurons,
abnormalities in the immune system, abnormalities in myelination and
oxidative stress.
Dopamine dysfunction
The first formulations of the dopamine hypothesis of schizophrenia
came from post-mortem studies finding increased striatal availability of
D2/D3
receptors in the striatum, as well as studies finding elevated CSF
levels of dopamine metabolites. Subsequently, most antipsychotics were
found to have affinity for D2 receptors. More modern investigations of
the hypothesis suggest a link between striatal dopamine synthesis and
positive symptoms, as well as increased and decreased dopamine
transmission in subcortical and cortical regions respectively.
A meta analysis of molecular imaging studies observed increased
presynaptic indicators of dopamine function, but no difference in the
availability of dopamine transporters or dopamine D2/D3 receptors. Both studies using radio labeled L-DOPA, an indicator of dopamine synthesis, and studies using amphetamine
release challenges observed significant differences between those with
schizophrenia and control. These findings were interpreted as increased
synthesis of dopamine, and increased release of dopamine respectively.
These findings were localized to the striatum, and were noted to be
limited by the quality of studies used. A large degree of inconsistency has been observed in D2/D3 receptor binding, although a small but nonsignificant reduction in thalamic availability has been found.
The inconsistent findings with respect to receptor expression has been
emphasized as not precluding dysfunction in dopamine receptors, as many
factors such as regional heterogeneity and medication status may lead
to variable findings. When combined with findings in presynaptic
dopamine function, most evidence suggests dysregulation of dopamine in
schizophrenia.
Exactly how dopamine dysregulation can contribute to
schizophrenia symptoms remains unclear. Some studies have suggested that
disruption of the auditory thalamocortical projections give rise to
hallucinations, while dysregulated corticostriatal circuitry and reward circuitry in the form of aberrant salience can give rise to delusions.
Decreased inhibitory dopamine signals in the thalamus have been
hypothesized to result in reduced sensory gating, and excessive activity
in excitatory inputs into the cortex.
One hypothesis linking delusions in schizophrenia to dopamine
suggests that unstable representation of expectations in prefrontal
neurons occurs in psychotic states due to insufficient D1 and NMDA
receptor stimulation. This, when combined with hyperactivity of
expectations to modification by salient stimuli is thought to lead to
improper formation of beliefs.
Glutamate abnormalities
Beside the dopamine hypothesis, interest has also focused on the neurotransmitter glutamate and the reduced function of the NMDA glutamate receptor in the pathophysiology of schizophrenia. This has largely been suggested by lower levels of glutamate receptors found in postmortem brains of people previously diagnosed with schizophrenia and the discovery that glutamate blocking drugs such as phencyclidine and ketamine can mimic the symptoms and cognitive problems associated with the condition.
The fact that reduced glutamate function is linked to poor performance on tests requiring frontal lobe and hippocampal function and that glutamate can affect dopamine
function, all of which have been implicated in schizophrenia, have
suggested an important mediating (and possibly causal) role of glutamate
pathways in schizophrenia. Positive symptoms fail however to respond to glutamatergic medication.
Reduced mRNA and protein expression of several NMDA receptor
subunits has also been reported in postmortem brains from people with
schizophrenia.
In particular, the expression of mRNA for the NR1 receptor subunit, as
well as the protein itself is reduced in the prefrontal cortex in
post-mortem studies of those with schizophrenia. Fewer studies have
examined other subunits, and results have been equivocal, except for a
reduction in prefrontal NRC2.
The large genome-wide association study mentioned above has
supported glutamate abnormalities for schizophrenia, reporting several
mutations in genes related to glutamatergic neurotransmission, such as GRIN2A, GRIA1, SRR, and GRM3.
Interneuron dysfunction
Another
hypothesis concerning the pathophysiology of schizophrenia, closely
relates to the glutamate hypothesis, and involves dysfunction of interneurons in the brain. Interneurons in the brain are inhibitory GABAergic and local, and function mainly through the inhibition of other cells. One type of interneuron, the fast-spiking, parvalbumin-positive interneuron, has been suggested to play a key role in schizophrenia pathophysiology.
Early studies have identified decreases in GAD67 mRNA and protein in post-mortem brains from those with schizophrenia compared to controls.
These reductions were found in only a subset of cortical interneurons.
Furthermore, GAD67 mRNA was completely undetectable in a subset of
interneurons also expressing parvalbumin.
Levels of parvalbumin protein and mRNA were also found to be lower in
various regions in the brain. Actual numbers of parvalbumin interneurons
have been found to be unchanged in these studies, however, except for a
single study showing a decrease in parvalbumin interneurons in the
hippocampus.
Finally, excitatory synapse density is lower selectively on parvalbumin
interneurons in schizophrenia and predicts the activity-dependent
down-regulation of parvalbumin and GAD67. Together, this suggests that parvalbumin interneurons are somehow specifically affected in the disease.
Several studies have tried to assess levels in GABA in vivo in those with schizophrenia, but these findings have remained inconclusive.
EEG studies have indirectly also pointed to interneuron dysfunction in schizophrenia (see below).
These studies have pointed to abnormalities in oscillatory activity in
schizophrenia, particularly in the gamma band (30–80 Hz). Gamma band
activity appears to originate from intact functioning parvalbumin-positive interneuron.
Together with the post-mortem findings, these EEG abnormalities point
to a role for dysfunctional parvalbumin interneurons in schizophrenia.
The largest meta-analysis on copy-number variations
(CNVs), structural abnormalities in the form of genetic deletions or
duplications, to date for schizophrenia, published in 2015, was the
first genetic evidence for the broad involvement of GABAergic
neurotransmission.
Myelination abnormalities
Another hypothesis states that abnormalities in myelination are a core pathophysiology of schizophrenia.
This theory originated from structural imaging studies, which found
that white matter regions, in addition to grey matter regions, showed
volumetric reductions in people with schizophrenia. In addition, gene
expression studies have shown abnormalities in myelination and
oligodendrocytes in the post-mortem brains. Furthermore, oligodendrocyte
numbers appear to be reduced in several post-mortem studies.
It has been suggested that myelination abnormalities could originate from impaired maturation of oligodendrocyte precursor cells, as these have been found to be intact in schizophrenia brains.
Immune system abnormalities
Another hypothesis postulates that inflammation and immune system abnormalities could play a central role in the disease. The immune hypothesis is supported by findings of high levels of immune markers in the blood of people with schizophrenia. High levels of immune markers have also been associated with having more severe psychotic symptoms. Furthermore, a meta-analysis of genome-wide association studies discovered that 129 out of 136 single-nucleotide polymorphisms (SNP) significantly associated with schizophrenia were located in the major histocompatibility complex region of the genome.
A systematic review investigating neuroinflammatory markers in
post-mortem schizophrenia brains has shown quite some variability, with
some studies showing alterations in various markers but others failing
to find any differences.
Oxidative stress
Another theory that has gained support is that a large role is played in the disease by oxidative stress.
Redox dysregulation in early development can potentially influence
development of different cell types that have been shown to be impaired
in the disease.
Oxidative stress has also been indicated through genetic studies into schizophrenia.
Oxidative stress has been shown to affect maturation of oligodendrocytes, the myelinating cell types in the brain, potentially underlying the white matter abnormalities found in the brain (see below).
Furthermore, oxidative stress could also influence the development of GABAergic interneurons, which have also been found to be dysregulated in schizophrenia (see above).
Evidence that oxidative stress and oxidative DNA damage are increased in various tissues of people with schizophrenia has been reviewed by Markkanen et al.
The presence of increased oxidative DNA damage may be due, in part, to
insufficient repair of such damages. Several studies have linked polymorphisms in DNA repair genes to the development of schizophrenia. In particular, the base excision repair protein XRCC1 has been implicated.
Neuropathology
The most consistent finding in post-mortem examinations of brain tissue is a lack of neurodegenerative lesions or gliosis. Abnormal neuronal organization and orientation (dysplasia) has been observed in the entorhinal cortex,
hippocampus, and subcortical white matter, although results are not
entirely consistent. A more consistent cytoarchitectural finding is
reduced volume of purkinje cells and pyramidal cells in the hippocampus.
This is consistent with the observation of decreased presynaptic
terminals in the hippocampus, and a reduction in dendritic spines in the
prefrontal cortex. The reductions in prefrontal and increase in striatal spine densities seem to be independent of antipsychotic drug use.
Sleep disorders
It has been suggested that sleep problems may be a core component of the pathophysiology of schizophrenia.
Structural abnormalities
Beside
theories concerning the functional mechanism underlying the disease,
structural findings have been identified as well using a wide range of
imaging techniques. Studies have tended to show various subtle average
differences in the volume of certain areas of brain structure between
people with and without diagnoses of schizophrenia, although it has
become increasingly clear that no single pathological neuropsychological
or structural neuroanatomic profile exists.
Morphometry
Structural
imaging studies have consistently reported differences in the size and
structure of certain brain areas in schizophrenia.
The largest combined neuroimaging study with over 2000 subjects and 2500 controls has replicated these previous findings. Volumetric increases were found in the lateral ventricles (+18%), caudate nucleus and pallidum, and extensive decreases in the hippocampus (-4%), thalamus, amygdala and nucleus accumbens. Together, this indicates that extensive changes do occur in the brains of people with schizophrenia.
A 2006 meta-analysis of MRI studies found that whole brain and hippocampal volume are reduced and that ventricular
volume is increased in those with a first psychotic episode relative to
healthy controls. The average volumetric changes in these studies are
however close to the limit of detection by MRI methods, so it remains to
be determined whether schizophrenia is a neurodegenerative process that
begins at about the time of symptom onset, or whether it is better
characterised as a neurodevelopmental process that produces abnormal
brain volumes at an early age.
In first episode psychosis typical antipsychotics like haloperidol were
associated with significant reductions in gray matter volume, whereas
atypical antipsychotics like olanzapine were not. Studies in non-human primates found gray and white matter reductions for both typical and atypical antipsychotics.
Abnormal findings in the prefrontal cortex, temporal cortex and anterior cingulate cortex
are found before the first onset of schizophrenia symptoms. These
regions are the regions of structural deficits found in schizophrenia
and first-episode subjects. Positive symptoms, such as thoughts of being persecuted, were found to be related to the medial prefrontal cortex, amygdala, and hippocampus region. Negative symptoms were found to be related to the ventrolateral prefrontal cortex and ventral striatum.
Ventricular and third ventricle enlargement, abnormal functioning
of the amygdala, hippocampus, parahippocampal gyrus, neocortical
temporal lobe regions, frontal lobe, prefontal gray matter,
orbitofrontal areas, parietal lobs abnormalities and subcortical
abnormalities including the cavum septi pellucidi,
basal ganglia, corpus callosum, thalamus and cerebellar abnormalities.
Such abnormalities usually present in the form of loss of volume.
Most schizophrenia studies have found average reduced volume of the left medial temporal lobe and left superior temporal gyrus, and half of studies have revealed deficits in certain areas of the frontal gyrus, parahippocampal gyrus and temporal gyrus. However, at variance with some findings in individuals with chronic schizophrenia significant group differences of temporal lobe and amygdala volumes are not shown in first-episode people on average.
Finally, MRI studies
utilizing modern cortical surface reconstruction techniques have shown
widespread reduction in cerebral cortical thickness (i.e., "cortical
thinning") in frontal and temporal regions and somewhat less widespread cortical thinning in occipital and parietal regions
in people with schizophrenia, relative to healthy control subjects.
Moreover, one study decomposed cortical volume into its constituent
parts, cortical surface area and cortical thickness, and reported
widespread cortical volume reduction in schizophrenia, mainly driven by
cortical thinning, but also reduced cortical surface area in smaller
frontal, temporal, parietal and occipital cortical regions.
CT scans of the brains of people with schizophrenia show several pathologies. The brain ventricles are enlarged as compared to normal brains. The ventricles hold cerebrospinal fluid (CSF) and enlarged ventricles indicate a loss of brain volume. Additionally, the brains have widened sulci as compared to normal brains, also with increased CSF volumes and reduced brain volume.
Using machine learning, two neuroanatomical subtypes of schizophrenia have been described.
Subtype 1 shows widespread low grey matter volumes, particularly in the thalamus, nucleus accumbens, medial temporal, medial prefrontal, frontal, and insular cortices. Subtype 2 shows increased volume in the basal ganglia and internal capsule, with otherwise normal brain volume.
White matter
Diffusion tensor imaging (DTI) allows for the investigation of white matter more closely than traditional MRI. Over 300 DTI imaging studies have been published examining white matter abnormalities in schizophrenia.
Although quite some variation has been found pertaining to the specific
regions affected, the general consensus states a reduced fractional anisotropy
in brains from people with schizophrenia versus controls. Importantly,
these differences between subjects and controls could potentially be
attributed to lifestyle effects, medication effects etc. Other studies
have looked at people with first-episode schizophrenia that have never
received any medication, so-called medication-naive subjects. These
studies, although few in number, also found reduced fractional anisotropy
in subject brains compared to control brains. As with earlier findings,
abnormalities can be found throughout the brain, although the corpus callosum seemed to be most commonly effected.
Functional abnormalities
During executive function tasks, people with schizophrenia demonstrate decreased activity relative to controls in the bilateral dorsolateral prefrontal cortex(dlPFC), right anterior cingulate cortex(ACC), and left mediodorsal nucleus of the thalamus. Increased activation was observed in the left ACC and left inferior parietal lobe. During emotional processing tasks, reduced activations have been observed in the Medial prefrontal cortex, ACC, dlPFC and amygdala. A meta analysis of facial emotional processing observed decreased activation in the amygdala, parahippocampus, lentiform nuclei, fusiform gyrus and right superior frontal gyrus, as well as increased activation in the left insula.
One meta analysis of functional neuroiamging during acute
auditory verbal hallucinations has reported increased activations in
areas implicated in language, including the bilateral inferior frontal and post central gyri, as well as the left parietal operculum.
Another meta analysis during both visual and auditory verbal
hallucinations, replicated the findings in the inferior frontal and
postcentral gyri during auditory verbal hallucinations, and also
observed hippocampal, superior temporal, insular and medial prefrontal
activations. Visual hallucinations were reported to be associated with
increased activations in the secondary and associate visual cortices.
PET
PET scan findings in people with schizophrenia indicate cerebral blood flow decreases in the left parahippocampal region. PET scans also show a reduced ability to metabolize glucose in the thalamus and frontal cortex. PET scans also show involvement of the medial part of the left temporal lobe and the limbic
and frontal systems as suffering from developmental abnormality. PET
scans show thought disorders stem from increased flow in the frontal and
temporal regions while delusions and hallucinations were associated
with reduced flow in the cingulate, left frontal, and temporal areas.
PET scans carried out during active auditory hallucinations revealed
increased blood flow in both thalami, left hippocampus, right striatum,
parahippocampus, orbitofrontal, and cingulate areas.
In addition, a decrease in NAA uptake has been reported in the hippocampus and both the grey and white matter of the prefrontal cortex
of those with schizophrenia. NAA may be an indicator of neural activity
of number of viable neurons. however given methodological limitations
and variance it is impossible to use this as a diagnostic method. Decreased prefrontal cortex connectivity has also been observed. DOPA PET studies have confirmed an altered synthesis capacity of dopamine in the nigrostriatal system demonstrating a dopaminergic dysregulation.