Hemispheric comparisons of neuron density in the planum temporale of schizophrenia and nonpsychiatric brains

Semantic priming and neurobiology in schizophrenia: A theoretical review

In this theoretical review we bridge the cognitive and neurobiological sciences to shed light on the neurocognitive foundations of the semantic priming effect in schizophrenia. We review and theoretically evaluate the neurotransmitter systems (dopaminergic, GABAergic and glutamatergic) and neurobiological underpinnings of behavioural and electrophysiological (N400) semantic priming in the pathology, and the main hypotheses on their geneses: a disinhibition of the semantic spread of activation, a disorganised semantic storage or noisy lexical-semantic associations, a psychomotor artefact, an artefact of relatedness proportions, or an inability to mobilise contextual information. We further assess the literature on the endophenotype of Formal Thought Disorder from multiple standpoints, ranging from neurophysiology to cognition: considerations are weaved on neuronal (PV basket cell, SST, VIP) and receptor deficits (DRD1, NMDA), neurotransmitter imbalances (dopamine), cortical and dopaminergic lateralisation, inter alia. In conclusion, we put forth novel postulates on the underlying causes of controlled hypopriming, automatic hyperpriming, N400 reversals (larger amplitudes for close associations), indirect versus direct hyperpriming, and the endophenotype of lexical-semantic disturbances in schizophrenia.

The enigma and implications of brain hemispheric asymmetry in neurodegenerative diseases

The lateralization of the human brain may provide clues into the pathogenesis and progression of neurodegenerative diseases. Though differing in their presentation and underlying pathologies, neurodegenerative diseases are all devastating and share an intriguing theme of asymmetrical pathology and clinical symptoms. Parkinson’s disease, with its distinctive onset of motor symptoms on one side of the body, stands out in this regard, but a review of the literature reveals asymmetries in several other neurodegenerative diseases. Here, we review the lateralization of the structure and function of the healthy human brain and the common genetic and epigenetic patterns contributing to the development of asymmetry in health and disease. We specifically examine the role of asymmetry in Parkinson’s disease, Alzheimer’s disease, amyotrophic lateral sclerosis, and multiple sclerosis, and interrogate whether these imbalances may reveal meaningful clues about the origins of these diseases. We also propose several hypotheses for how lateralization may contribute to the distinctive and enigmatic features of asymmetry in neurodegenerative diseases, suggesting a role for asymmetry in the choroid plexus, neurochemistry, protein distribution, brain connectivity and the vagus nerve. Finally, we suggest how future studies may reveal novel insights into these diseases through the lens of asymmetry.

Reduced GABA neuron density in auditory cerebral cortex of subjects with major depressive disorder

Although major depressive disorder (MDD) and schizophrenia (SZ) are closely associated with disrupted functions in frontal and limbic areas of cerebral cortex, cellular pathology has also been found in other brain areas, including primary sensory cortex. Auditory cortex is of particular interest, given the prominence of auditory hallucinations in SZ, and sensory deficits in MDD. We used stereological sampling methods in auditory cortex to look for cellular differences between MDD, SZ and non-psychiatric subjects. Additionally, as all of our MDD subjects died of suicide, we evaluated the association of suicide with our measurements by selecting a SZ sample that was divided between suicide and non-suicide subjects. Measurements were done in primary auditory cortex (area A1) and auditory association cortex (area Tpt), two areas with distinct roles in sensory processing and obvious differences in neuron density and size. In MDD, densities of GABAergic interneurons immunolabeled for calretinin (CR) and calbindin (CB) were 23-29% lower than non-psychiatric controls in both areas. Parvalbumin (PV) interneurons (counted only in area Tpt) showed a nominally smaller (16%) reduction that was not statistically significant. Total neuron and glia densities measured in Nissl stained sections did not show corresponding reductions. Analysis of suicide in the SZ sample indicated that reduced CR cell density was associated with suicide, whereas the densities of CB and other cells were not. Our results are consistent with previous studies in MDD that found altered GABA-associated markers throughout the cerebral cortex including primary sensory areas.

Neurophysiological differences between patients clinically at high risk for schizophrenia and neurotypical controls--first steps in development of a biomarker

BACKGROUND

Schizophrenia is a severe, disabling and prevalent mental disorder without cure and with a variable, incomplete pharmacotherapeutic response. Prior to onset in adolescence or young adulthood a prodromal period of abnormal symptoms lasting weeks to years has been identified and operationalized as clinically high risk (CHR) for schizophrenia. However, only a minority of subjects prospectively identified with CHR convert to schizophrenia, thereby limiting enthusiasm for early intervention(s). This study utilized objective resting electroencephalogram (EEG) quantification to determine whether CHR constitutes a cohesive entity and an evoked potential to assess CHR cortical auditory processing.

METHODS

This study constitutes an EEG-based quantitative neurophysiological comparison between two unmedicated subject groups: 35 neurotypical controls (CON) and 22 CHR patients. After artifact management, principal component analysis (PCA) identified EEG spectral and spectral coherence factors described by associated loading patterns. Discriminant function analysis (DFA) determined factors' discrimination success between subjects in the CON and CHR groups. Loading patterns on DFA-selected factors described CHR-specific spectral and coherence differences when compared to controls. The frequency modulated auditory evoked response (FMAER) explored functional CON-CHR differences within the superior temporal gyri.

RESULTS

Variable reduction by PCA identified 40 coherence-based factors explaining 77.8% of the total variance and 40 spectral factors explaining 95.9% of the variance. DFA demonstrated significant CON-CHR group difference (P <0.00001) and successful jackknifed subject classification (CON, 85.7%; CHR, 86.4% correct). The population distribution plotted along the canonical discriminant variable was clearly bimodal. Coherence factors delineated loading patterns of altered connectivity primarily involving the bilateral posterior temporal electrodes. However, FMAER analysis showed no CON-CHR group differences.

CONCLUSIONS

CHR subjects form a cohesive group, significantly separable from CON subjects by EEG-derived indices. Symptoms of CHR may relate to altered connectivity with the posterior temporal regions but not to primary auditory processing abnormalities within these regions.

Auditory dysfunction in schizophrenia: integrating clinical and basic features

Schizophrenia is a complex neuropsychiatric disorder that is associated with persistent psychosocial disability in affected individuals. Although studies of schizophrenia have traditionally focused on deficits in higher-order processes such as working memory and executive function, there is an increasing realization that, in this disorder, deficits can be found throughout the cortex and are manifest even at the level of early sensory processing. These deficits are highly amenable to translational investigation and represent potential novel targets for clinical intervention. Deficits, moreover, have been linked to specific structural abnormalities in post-mortem auditory cortex tissue from individuals with schizophrenia, providing unique insights into underlying pathophysiological mechanisms.

The glutamate hypothesis of schizophrenia: evidence from human brain tissue studies

A number of studies have indicated that antagonists of the N-methyl-d-aspartate subtypes of glutamate receptors can cause schizophrenia-like symptoms in healthy individuals and exacerbate symptoms in individuals with schizophrenia. These findings have led to the glutamate hypothesis of schizophrenia. Here we review the evidence for this hypothesis in postmortem studies of brain tissue from individuals affected by schizophrenia, summarizing studies of glutamate neuron morphology, of expression of glutamate receptors and transporters, and of the synthesizing and metabolizing enzymes for glutamate and its co-agonists. We found consistent evidence of morphological alterations of dendrites of glutamatergic neurons in the cerebral cortex of subjects with schizophrenia and of reduced levels of the axon bouton marker synaptophysin. There were no consistent alterations of mRNA expression of glutamate receptors, although there has been limited study of the corresponding proteins. Studies of the glutamate metabolic pathway have been limited, although there is some evidence that excitatory amino acid transporter-2, glutamine synthetase, and glutaminase have altered expression in schizophrenia. Future studies would benefit from additional direct examination of glutamatergic proteins. Further advances, such as selective testing of synaptic microdomains, cortical layers, and neuronal subtypes, may also be required to elucidate the nature of glutamate signaling impairments in schizophrenia.

Specific calpain inhibition by calpastatin prevents tauopathy and neurodegeneration and restores normal lifespan in tau P301L mice

Tau pathogenicity in Alzheimer's disease and other tauopathies is thought to involve the generation of hyperphosphorylated, truncated, and oligomeric tau species with enhanced neurotoxicity, although the generative mechanisms and the implications for disease therapy are not well understood. Here, we report a striking rescue from mutant tau toxicity in the JNPL3 mouse model of tauopathy. We show that pathological activation of calpains gives rise to a range of potentially toxic forms of tau, directly, and by activating cdk5. Calpain overactivation in brains of these mice is accelerated as a result of the marked depletion of the endogenous calpain inhibitor, calpastatin. When levels of this inhibitor are restored in neurons of JNPL3 mice by overexpressing calpastatin, tauopathy is prevented, including calpain-mediated breakdown of cytoskeletal proteins, cdk5 activation, tau hyperphosphorylation, formation of potentially neurotoxic tau fragments by either calpain or caspase-3, and tau oligomerization. Calpastatin overexpression also prevents loss of motor axons, delays disease onset, and extends survival of JNPL3 mice by 3 months to within the range of normal lifespan. Our findings support the therapeutic promise of highly specific calpain inhibition in the treatment of tauopathies and other neurodegenerative states.

Morphometric differences in planum temporale in schizophrenia and bipolar disorder revealed by statistical analysis of labeled cortical depth maps

Differences in cortical thickness in the lateral temporal lobe, including the planum temporale (PT), have been reported in MRI studies of schizophrenia (SCZ) and bipolar disorder (BPD) patients. Most of these studies have used a single-valued global or local measure for thickness. However, additional and complementary information can be obtained by generating labeled cortical distance maps (LCDMs), which are distances of labeled gray matter (GM) voxels from the nearest point on the GM/white matter (WM) (inner) cortical surface. Statistical analyses of pooled and censored LCDM distances reveal subtle differences in PT between SCZ and BPD groups from data generated by Ratnanather et al. (Schizophrenia Research, http://dx.doi.org/10.1016/j.schres.2013.08.014). These results confirm that the left planum temporale (LPT) is more sensitive than the right PT in distinguishing between SCZ, BPD, and healthy controls. Also confirmed is a strong gender effect, with a thicker PT seen in males than in females. The differences between groups at smaller distances in the LPT revealed by pooled and censored LCDM analysis suggest that SCZ and BPD have different effects on the cortical mantle close to the GM/WM surface. This is consistent with reported subtle changes in the cortical mantle observed in post-mortem studies.

Hemispheric asymmetry of primary auditory cortex and Heschl's gyrus in schizophrenia and nonpsychiatric brains

Heschl's gyrus (HG) is reported to have a normal left>right hemispheric volume asymmetry, and reduced asymmetry in schizophrenia. Primary auditory cortex (A1) occupies the caudal-medial surface of HG, but it is unclear if A1 has normal asymmetry, or whether its asymmetry is altered in schizophrenia. To address these issues, we compared bilateral gray matter volumes of HG and A1, and neuron density and number in A1, in autopsy brains from male subjects with or without schizophrenia. Comparison of diagnostic groups did not reveal altered gray matter volumes, neuron density, neuron number or hemispheric asymmetries in schizophrenia. With respect to hemispheric differences, HG displayed a clear left>right asymmetry of gray matter volume. Area A1 occupied nearly half of HG, but had less consistent volume asymmetry, that was clearly present only in a subgroup of archival brains from elderly subjects. Neuron counts, in layers IIIb-c and V-VI, showed that the A1 volume asymmetry reflected differences in neuron number, and was not caused simply by changes in neuron density. Our findings confirm previous reports of striking hemispheric asymmetry of HG, and additionally show evidence that A1 has a corresponding asymmetry, although less consistent than that of HG.

Altered volume and lateralization of language-related regions in first-episode schizophrenia

Neuroanatomical abnormalities are considered to be related to the pathogenesis of schizophrenia. Reversal or reduction of normal structural cerebral asymmetries in schizophrenia is particularly striking. The current study investigated the alteration of gray matter volume and cerebral asymmetry in early stage of first-episode schizophrenia (FESZ), and their correlations with clinical measures. Magnetic resonance imaging scans were obtained from a total of 89 participants. Thirty-three FESZ patients and 41 matched healthy controls were included in the analysis. Compared to healthy controls, the FESZ patients showed decreased gray matter volume (GMV) in the frontal cortex, anterior cingulate cortex, temporal cortex, parahippocampal, fusiform, insula, and lingual; and increased GMV in cerebellum. Both male and female patients displayed an increased rightward lateralization in frontal and temporal cortex, which was significantly correlated with the severity of symptoms and social functioning. These findings may provide the neurological substrate for the etiology and clinical manifestations of the illness.

Updating the mild encephalitis hypothesis of schizophrenia

Schizophrenia seems to be a heterogeneous disorder. Emerging evidence indicates that low level neuroinflammation (LLNI) may not occur infrequently. Many infectious agents with low overall pathogenicity are risk factors for psychoses including schizophrenia and for autoimmune disorders. According to the mild encephalitis (ME) hypothesis, LLNI represents the core pathogenetic mechanism in a schizophrenia subgroup that has syndromal overlap with other psychiatric disorders. ME may be triggered by infections, autoimmunity, toxicity, or trauma. A 'late hit' and gene-environment interaction are required to explain major findings about schizophrenia, and both aspects would be consistent with the ME hypothesis. Schizophrenia risk genes stay rather constant within populations despite a resulting low number of progeny; this may result from advantages associated with risk genes, e.g., an improved immune response, which may act protectively within changing environments, although they are associated with the disadvantage of increased susceptibility to psychotic disorders. Specific schizophrenic symptoms may arise with instances of LLNI when certain brain functional systems are involved, in addition to being shaped by pre-existing liability factors. Prodrome phase and the transition to a diseased status may be related to LLNI processes emerging and varying over time. The variability in the course of schizophrenia resembles the varying courses of autoimmune disorders, which result from three required factors: genes, the environment, and the immune system. Preliminary criteria for subgrouping neurodevelopmental, genetic, ME, and other types of schizophrenias are provided. A rare example of ME schizophrenia may be observed in Borna disease virus infection. Neurodevelopmental schizophrenia due to early infections has been estimated by others to explain approximately 30% of cases, but the underlying pathomechanisms of transition to disease remain in question. LLNI (e.g. from reactivation related to persistent infection) may be involved and other pathomechanisms including dysfunction of the blood-brain barrier or the blood-CSF barrier, CNS-endogenous immunity and the volume transmission mode balancing wiring transmission (the latter represented mainly by synaptic transmission, which is often described as being disturbed in schizophrenia). Volume transmission is linked to CSF signaling; and together could represent a common pathogenetic link for the distributed brain dysfunction, dysconnectivity, and brain structural abnormalities observed in schizophrenia. In addition, CSF signaling may extend into peripheral tissues via the CSF outflow pathway along brain nerves and peripheral nerves, and it may explain the peripheral topology of neuronal dysfunctions found, like in olfactory dysfunction, dysautonomia, and even in peripheral tissues, i.e., the muscle lesions that were found in 50% of cases. Modulating factors in schizophrenia, such as stress, hormones, and diet, are also modulating factors in the immune response. Considering recent investigations of CSF, the ME schizophrenia subgroup may constitute approximately 40% of cases.

Neuroimaging schizophrenia: a picture is worth a thousand words, but is it saying anything important?

Schizophrenia is characterized by neurostructural and neurofunctional aberrations that have now been demonstrated through neuroimaging research. The article reviews recent studies that have attempted to use neuroimaging to understand the relation between neurological abnormalities and aspects of the phenomenology of schizophrenia. Neuroimaging studies show that neurostructural and neurofunctional abnormalities are present in people with schizophrenia and their close relatives and may represent putative endophenotypes. Neuroimaging phenotypes predict the emergence of psychosis in individuals classified as high-risk. Neuroimaging studies have linked structural and functional abnormalities to symptoms; and progressive structural changes to clinical course and functional outcome. Neuroimaging has successfully indexed the neurotoxic and neuroprotective effects of schizophrenia treatments. Pictures can inform about aspects of the phenomenology of schizophrenia including etiology, onset, symptoms, clinical course, and treatment effects but this assertion is tempered by the scientific and practical limitations of neuroimaging.

MTR abnormalities in subjects at ultra-high risk for schizophrenia and first-episode schizophrenic patients compared to healthy controls

BACKGROUND

Neuroimaging studies have suggested gray (GM) and white matter (WM) abnormalities in early stages of schizophrenia. We aimed at evaluating subtle parenchymal alterations in individuals at ultra-high risk (UHR) for transition into psychosis and first-episode schizophrenic (FES) patients by measuring the magnetization transfer ratio (MTR).

METHODS AND MATERIAL

In a cross-sectional study magnetization transfer images and high-resolution volumetric T1-weighted images were acquired in 70 age- and gender-matched subjects (25 UHR subjects, 16 FES patients and 29 controls) in a 1.5Tesla scanner. Following normalization of MTR-maps the intensity histograms were analyzed by performing a Kruskal-Wallis-test.

RESULTS

Gray matter MTR decreases were depicted in UHR subjects solely, involving the cingulate gyrus and precentral cortex. WM MTR alterations were more pronounced in FES than in UHR patients and exclusively affected the frontal lobe bilaterally. In addition, UHR subjects showed bilateral MTR decreases at the stria terminalis though statistically significant only on the left side (p=0.018.)

CONCLUSION

Our results indicate GM affection earlier on during disease progression as well as cumulative WM affection within frontal lobes during transition from UHR to FES. MTR reductions at the stria terminalis of UHR patients points to the involvement of the extended amygdala in the prodromal disease stage.

Cortical thickness, neuron density and size in the inferior parietal lobe in schizophrenia

The inferior parietal lobe (IPL) is a center of multisensory integration, and both functional and structural MRI studies have found evidence that pathology in this region may contribute to disrupted sensory perception in schizophrenia. To further define this pathology, we used postmortem samples from the left and right IPL, to compare the thickness and volume of the upper (I-III) and lower (IV-VI) cortical layers. The samples were divided into supramarginal and angular gyri, and neuron density and size were measured in the supramarginal gyrus. The laminar thickness and volume measurements did not demonstrate significant changes in schizophrenia, but did show that the angular gyrus was thinner than the supramarginal gyrus, due to a difference mainly in the lower layers. Measurements of cortical thickness, neuron size and neuron density all showed some evidence of previously reported normal hemispheric differences. These asymmetries were reduced in schizophrenia, but the small changes were at the threshold of detection, and are discussed in the context of the sensitivity of the methods applied.

Targeting glutamate synapses in schizophrenia

Although early clinical observations implicated dopamine dysfunction in the neuropathology of schizophrenia, accumulating evidence suggests that multiple neurotransmitter pathways are dysregulated. The psychotomimetic actions of NMDA receptor antagonists point to an imbalance of glutamatergic signaling. Encouragingly, numerous preclinical and clinical studies have elucidated several potential targets for increasing NMDA receptor function and equilibrating glutamatergic tone, including the metabotropic glutamate receptors 2, 3 and 5, the muscarinic acetylcholine receptors M(1) and M(4), and the glycine transporter GlyT1. Highly specific allosteric and orthosteric ligands have been developed that modify the activity of these novel target proteins, and in this review we summarize both the glutamatergic mechanisms and the novel compounds that are increasing the promise for a multifaceted pharmacological approach to treat schizophrenia.