The Integration of the Glutamatergic and the White Matter Hypotheses of Schizophrenia's Etiology

Transdiagnostic role of glutamate and white matter damage in neuropsychiatric disorders: A Systematic Review

Neuropsychiatric disorders including generalized anxiety disorder (GAD), obsessive-compulsive disorder (OCD), major depressive disorder (MDD), bipolar disorder (BD), and schizophrenia (SZ) have been considered distinct categories of diseases despite their overlapping characteristics and symptomatology. We aimed to provide an in-depth review elucidating the role of glutamate/Glx and white matter (WM) abnormalities in these disorders from a transdiagnostic perspective. The PubMed online database was searched for studies published between 2010 and 2021. After careful screening, 401 studies were included. The findings point to decreased levels of glutamate in the Anterior Cingulate Cortex in both SZ and BD, whereas Glx is elevated in the Hippocampus in SZ and MDD. With regard to WM abnormalities, the Corpus Callosum and superior Longitudinal Fascicle were the most consistently identified brain regions showing decreased fractional anisotropy (FA) across all the reviewed disorders, except GAD. Additionally, the Uncinate Fasciculus displayed decreased FA in all disorders, except OCD. Decreased FA was also found in the inferior Longitudinal Fasciculus, inferior Fronto-Occipital Fasciculus, Thalamic Radiation, and Corona Radiata in SZ, BD, and MDD. Decreased FA in the Fornix and Corticospinal Tract were found in BD and SZ patients. The Cingulum and Anterior Limb of Internal Capsule exhibited decreased FA in MDD and SZ patients. The results suggest a gradual increase in severity from GAD to SZ defined by the number of brain regions with WM abnormality which may be partially caused by abnormal glutamate levels. WM damage could thus be considered a potential marker of some of the main neuropsychiatric disorders.

A Comparative Multimodal Meta-analysis of Anisotropy and Volume Abnormalities in White Matter in People Suffering From Bipolar Disorder or Schizophrenia

Schizophrenia (SZ) and bipolar disorder (BD) share some similarities in terms of genetic-risk genes and abnormalities of gray-matter structure in the brain, but white matter (WM) abnormalities have not been studied in depth. We undertook a comparative multimodal meta-analysis to identify common and disorder-specific abnormalities in WM structure between SZ and BD. Anisotropic effect size-signed differential mapping software was used to conduct a comparative meta-analysis of 68 diffusion tensor imaging (DTI) and 34 voxel-based morphometry (VBM) studies comparing fractional anisotropy (FA) and white matter volume (WMV), respectively, between patients with SZ (DTI: N = 1543; VBM: N = 1068) and BD (DTI: N = 983; VBM: N = 518) and healthy controls (HCs). The bilateral corpus callosum (extending to the anterior and superior corona radiata) showed shared decreased WMV and FA in SZ and BD. Compared with BD patients, SZ patients showed remarkable disorder-specific WM abnormalities: decreased FA and increased WMV in the left cingulum, and increased FA plus decreased WMV in the right anterior limb of the internal capsule. SZ patients showed more extensive alterations in WM than BD cases, which may be the pathophysiological basis for the clinical continuity of both disorders. The disorder-specific regions in the left cingulum and right anterior limb of the internal capsule provided novel insights into both disorders. Our study adds value to further understanding of the pathophysiology, classification, and differential diagnosis of SZ and BD.

Metabolite Alterations in Adults With Schizophrenia, First Degree Relatives, and Healthy Controls: A Multi-Region 7T MRS Study

Proton magnetic resonance spectroscopy (MRS) studies in schizophrenia have shown altered GABAergic, glutamatergic, and bioenergetic pathways, but if these abnormalities are brain region or illness-stage specific is largely unknown. MRS at 7T MR enables reliable quantification of multiple metabolites, including GABA, glutamate (Glu) and glutamine (Gln), from multiple brain regions within the time constraints of a clinical examination. In this study, GABA, Glu, Gln, the ratio Gln/Glu, and lactate (Lac) were quantified using 7T MRS in five brain regions in adults with schizophrenia (N = 40), first-degree relatives (N = 11), and healthy controls (N = 38). Metabolites were analyzed for differences between groups, as well as between subjects with schizophrenia with either short (<5 years, N = 19 or long (>5 years, N = 21) illness duration. For analyses between the three groups, there were significant glutamatergic and GABAergic differences observed in the anterior cingulate, centrum semiovale, and dorsolateral prefrontal cortex. There were also significant relationships between anterior cingulate cortex, centrum semiovale, and dorsolateral prefrontal cortex and cognitive measures. There were also significant glutamatergic, GABAergic, and lactate differences between subjects with long and short illness duration in the anterior cingulate, centrum semiovale, dorsolateral prefrontal cortex, and hippocampus. Finally, negative symptom severity ratings were significantly correlated with both anterior cingulate and centrum semiovale metabolite levels. In summary, 7T MRS shows multi-region differences in GABAergic and glutamatergic metabolites between subjects with schizophrenia, first-degree relatives and healthy controls, suggesting relatively diffuse involvement that evolves with illness duration. Unmedicated first-degree relatives share some of the same metabolic characteristics as patients with a diagnosis of schizophrenia, suggesting that these differences may reflect a genetic vulnerability and are not solely due to the effects of antipsychotic interventions.

The use of antioxidant compounds in the treatment of first psychotic episode: Highlights from preclinical studies

Recent evidence highlighted a pathogenetic link between redox dysregulation and the early stages of psychosis. Indeed, an increasing number of studies have pointed toward an association between oxidative stress, both at central and peripheral levels, and first psychotic episode. Moreover, basal low antioxidant capacity has been shown to directly correlate with cognitive impairment in the early onset of psychosis. In this context, the possibility to use antioxidant compounds in first psychotic episode, especially as supplementation to antipsychotic therapy, has become the focus of numerous investigations on rodents with the aim to translate data on the possible effects of antioxidant therapies to large populations of patients, with a diagnosis of the first psychotic episode. In this review, we will discuss studies, published from January 1st, 2007 to July 31st, 2017, investigating the effects of antioxidant compounds on neuropathological alterations observed in different rodent models characterized by a cluster of psychotic-like symptoms reminiscent of what observed in human first psychotic episode. A final focus on the effective possibility to directly translate data obtained on rodents to humans will be also provided.

CaMKIIα expression in a mouse model of NMDAR hypofunction schizophrenia: Putative roles for IGF-1R and TLR4

Schizophrenia (SCZ) is a neuropsychiatric disorder that is linked to social behavioral deficits and other negative symptoms associated with hippocampal synaptic dysfunction. Synaptic mechanism of schizophrenia is characterized by loss of hippocampal N-Methyl-d-Aspartate Receptor (NMDAR) activity (NMDAR hypofunction) and dendritic spines. Previous studies show that genetic deletion of hippocampal synaptic regulatory calcium-calmodulin dependent kinase II alpha (CaMKIIα) cause synaptic and behavioral defects associated with schizophrenia in mice. Although CaMKIIα is involved in modulation of NMDAR activity, it is equally linked to inflammatory and neurotropin signaling in neurons. Based on these propositions, we speculate that non-neurotransmitter upstream receptors associated with neurotropic and inflammatory signaling activities of CaMKIIα may alter its synaptic function. Besides, how these receptors (i.e. inflammatory and neurotropic receptors) alter CaMKIIα function (phosphorylation) relative to hippocampal NMDAR activity in schizophrenia is poorly understood. Here, we examined the relationship between toll-like receptor (TLR4; inflammatory), insulin-like growth factor receptor 1 (IGF-1R; neurotropic) and CaMKIIα expression in the hippocampus of behaviorally deficient schizophrenic mice after we induced schizophrenia through NMDAR inhibition. Schizophrenia was induced in WT (C57BL/6) mice through intraperitoneal administration of 30mg/Kg ketamine (NMDAR antagonist) for 5days (WT/SCZ). Five days after the last ketamine treatment, wild type schizophrenic mice show deficiencies in sociability and social novelty behavior. Furthermore, there was a significant decrease in hippocampal CaMKIIα (p<0.001) and IGF-1R (p<0.001) expression when assessed through immunoblotting and confocal immunofluorescence microscopy. Additionally, WT schizophrenic mice show an increased percentage of phosphorylated CaMKIIα in addition to upregulated TLR4 signaling (TLR4, NF-κB, and MAPK/ErK) in the hippocampus. To ascertain the functional link between TLR4, IGF-1R and CaMKIIα relative to NMDAR hypofunction in schizophrenia, we created hippocampal-specific TLR4 knockdown mouse using AAV-driven Cre-lox technique (TLR4 KD). Subsequently, we inhibited NMDAR function in TLR4 KD mice in an attempt to induce schizophrenia (TLR4 KD SCZ). Interestingly, IGF-1R and CaMKIIα expressions were preserved in the TLR4 KD hippocampus after attenuation of NMDAR function. Furthermore, TLR4 KD SCZ mice showed no prominent defects in sociability and social novelty behavior when compared with the control (WT). Our results show that a sustained IGF-1R expression may preserve the synaptic activity of CaMKIIα while TLR4 signaling ablates hippocampal CaMKIIα expression in NMDAR hypofunction schizophrenia. Together, we infer that IGF-1R depletion and increased TLR4 signaling are non-neurotransmitter pro-schizophrenic cues that can reduce synaptic CaMKIIα activity in a pharmacologic mouse model of schizophrenia.

Altered Glutamate and Regional Cerebral Blood Flow Levels in Schizophrenia: A 1H-MRS and pCASL study

The neurobiology of schizophrenia (SZ) may be altered in older versus younger adults with SZ, as less frequent episodes of symptom exacerbation and increased sensitivity to medications are observed in older age. The goal of this study was to examine the effect of age and diagnosis on glutamate and cerebral blood flow (rCBF) in adults with SZ and healthy controls. Young and older adults with SZ and healthy controls were recruited to participate in this study. Participants completed a neuropsychological battery and neuroimaging that included optimized magnetic resonance spectroscopy to measure anterior cingulate (AC) glutamate (Glu) and glutamine (Gln) and arterial spin labeling evaluation for rCBF. Regression analyses revealed significant effects of age with Glu, Gln, Gln/Glu, and AC white matter (WM) rCBF. Glu and WM rCBF decreased linearly with age while Gln and Gln/Glu increased linearly with age. Glu was lower in adults with SZ compared with healthy controls and in older adults versus younger adults but there was no interaction. Glu and WM rCBF were correlated with the UCSD Performance-Based Skills Assessment (UPSA) and processing speed, and the correlations were stronger in the SZ group. In the largest sample to date, lower Glu and elevated Gln/Glu levels were observed in adults with SZ and in older subjects. Contrary to expectation, these results do not show evidence of accelerated Glu aging in the anterior cingulate region in SZ compared with healthy controls.