Marked hypofrontality in clozapine-responsive patients

Brain glucose metabolism in schizophrenia: a systematic review and meta-analysis of 18FDG-PET studies in schizophrenia

BACKGROUND

Impaired brain metabolism may be central to schizophrenia pathophysiology, but the magnitude and consistency of metabolic dysfunction is unknown.

METHODS

We searched MEDLINE, PsychINFO and EMBASE between 01/01/1980 and 13/05/2021 for studies comparing regional brain glucose metabolism using 18FDG-PET, in schizophrenia/first-episode psychosis v. controls. Effect sizes (Hedges g) were pooled using a random-effects model. Primary measures were regional absolute and relative CMRGlu in frontal, temporal, parietal and occipital lobes, basal ganglia and thalamus.

RESULTS

Thirty-six studies (1335 subjects) were included. Frontal absolute glucose metabolism (Hedge's g = -0.74 ± 0.54, p = 0.01; I2 = 67%) and metabolism relative to whole brain (g = -0.44 ± 0.34, p = 0.01; I2 = 55%) were lower in schizophrenia v. controls with moderate heterogeneity. Absolute frontal metabolism was lower in chronic (g = -1.18 ± 0.73) v. first-episode patients (g = -0.09 ± 0.88) and controls. Medicated patients showed frontal hypometabolism relative to controls (-1.04 ± 0.26) while metabolism in drug-free patients did not differ significantly from controls. There were no differences in parietal, temporal or occipital lobe or thalamic metabolism in schizophrenia v. controls. Excluding outliers, absolute basal ganglia metabolism was lower in schizophrenia v. controls (-0.25 ± 0.24, p = 0.049; I2 = 5%). Studies identified reporting voxel-based morphometry measures of absolute 18FDG uptake (eight studies) were also analysed using signed differential mapping analysis, finding lower 18FDG uptake in the left anterior cingulate gyrus (Z = -4.143; p = 0.007) and the left inferior orbital frontal gyrus (Z = -4.239; p = 0.02) in schizophrenia.

CONCLUSIONS

We report evidence for hypometabolism with large effect sizes in the frontal cortex in schizophrenia without consistent evidence for alterations in other brain regions. Our findings support the hypothesis of hypofrontality in schizophrenia.

Altered Patterns of Brain Glucose Metabolism Involve More Extensive and Discrete Cortical Areas in Treatment-resistant Schizophrenia Patients Compared to Responder Patients and Controls: Results From a Head-to-Head 2-[18F]-FDG-PET Study

BACKGROUND AND HYPOTHESIS

Treatment resistant schizophrenia (TRS) affects almost 30% of patients with schizophrenia and has been considered a different phenotype of the disease. In vivo characterization of brain metabolic patterns associated with treatment response could contribute to elucidate the neurobiological underpinnings of TRS. Here, we used 2-[18F]-fluorodeoxyglucose (FDG) positron emission tomography (PET) to provide the first head-to-head comparative analysis of cerebral glucose metabolism in TRS patients compared to schizophrenia responder patients (nTRS), and controls. Additionally, we investigated, for the first time, the differences between clozapine responders (Clz-R) and non-responders (Clz-nR).

STUDY DESIGN

53 participants underwent FDG-PET studies (41 patients and 12 controls). Response to conventional antipsychotics and to clozapine was evaluated using a standardized prospective procedure based on PANSS score changes. Maps of relative brain glucose metabolism were processed for voxel-based analysis using Statistical Parametric Mapping software.

STUDY RESULTS

Restricted areas of significant bilateral relative hypometabolism in the superior frontal gyrus characterized TRS compared to nTRS. Moreover, reduced parietal and frontal metabolism was associated with high PANSS disorganization factor scores in TRS (P < .001 voxel level uncorrected, P < .05 cluster level FWE-corrected). Only TRS compared to controls showed significant bilateral prefrontal relative hypometabolism, more extensive in CLZ-nR than in CLZ-R (P < .05 voxel level FWE-corrected). Relative significant hypermetabolism was observed in the temporo-occipital regions in TRS compared to nTRS and controls.

CONCLUSIONS

These data indicate that, in TRS patients, altered metabolism involved discrete brain regions not found affected in nTRS, possibly indicating a more severe disrupted functional brain network associated with disorganization symptoms.

Clozapine's multiple cellular mechanisms: What do we know after more than fifty years? A systematic review and critical assessment of translational mechanisms relevant for innovative strategies in treatment-resistant schizophrenia

Almost fifty years after its first introduction into clinical care, clozapine remains the only evidence-based pharmacological option for treatment-resistant schizophrenia (TRS), which affects approximately 30% of patients with schizophrenia. Despite the long-time experience with clozapine, the specific mechanism of action (MOA) responsible for its superior efficacy among antipsychotics is still elusive, both at the receptor and intracellular signaling level. This systematic review is aimed at critically assessing the role and specific relevance of clozapine's multimodal actions, dissecting those mechanisms that under a translational perspective could shed light on molecular targets worth to be considered for further innovative antipsychotic development. In vivo and in vitro preclinical findings, supported by innovative techniques and methods, together with pharmacogenomic and in vivo functional studies, point to multiple and possibly overlapping MOAs. To better explore this crucial issue, the specific affinity for 5-HT₂R, D1R, α_2c, and muscarinic receptors, the relatively low occupancy at dopamine D2R, the interaction with receptor dimers, as well as the potential confounder effects resulting in biased ligand action, and lastly, the role of the moiety responsible for lipophilic and alkaline features of clozapine are highlighted. Finally, the role of transcription and protein changes at the synaptic level, and the possibility that clozapine can directly impact synaptic architecture are addressed. Although clozapine's exact MOAs that contribute to its unique efficacy and some of its severe adverse effects have not been fully understood, relevant information can be gleaned from recent mechanistic understandings that may help design much needed additional therapeutic strategies for TRS.

Clozapine induces astrocyte-dependent FDG-PET hypometabolism

PURPOSE

Advances in functional imaging allowed us to visualize brain glucose metabolism in vivo and non-invasively with [¹⁸F]fluoro-2-deoxyglucose (FDG) positron emission tomography (PET) imaging. In the past decades, FDG-PET has been instrumental in the understanding of brain function in health and disease. The source of the FDG-PET signal has been attributed to neuronal uptake, with hypometabolism being considered as a direct index of neuronal dysfunction or death. However, other brain cells are also metabolically active, including astrocytes. Based on the astrocyte-neuron lactate shuttle hypothesis, the activation of the glutamate transporter 1 (GLT-1) acts as a trigger for glucose uptake by astrocytes. With this in mind, we investigated glucose utilization changes after pharmacologically downregulating GLT-1 with clozapine (CLO), an anti-psychotic drug.

METHODS

Adult male Wistar rats (control, n = 14; CLO, n = 12) received CLO (25/35 mg kg^-1) for 6 weeks. CLO effects were evaluated in vivo with FDG-PET and cortical tissue was used to evaluate glutamate uptake and GLT-1 and GLAST levels. CLO treatment effects were also assessed in cortical astrocyte cultures (glucose and glutamate uptake, GLT-1 and GLAST levels) and in cortical neuronal cultures (glucose uptake).

RESULTS

CLO markedly reduced in vivo brain glucose metabolism in several brain areas, especially in the cortex. Ex vivo analyses demonstrated decreased cortical glutamate transport along with GLT-1 mRNA and protein downregulation. In astrocyte cultures, CLO decreased GLT-1 density as well as glutamate and glucose uptake. By contrast, in cortical neuronal cultures, CLO did not affect glucose uptake.

CONCLUSION

This work provides in vivo demonstration that GLT-1 downregulation induces astrocyte-dependent cortical FDG-PET hypometabolism-mimicking the hypometabolic signature seen in people developing dementia-and adds further evidence that astrocytes are key contributors of the FDG-PET signal.

Differential neural reward mechanisms in treatment-responsive and treatment-resistant schizophrenia

BACKGROUND

The significant proportion of schizophrenia patients refractory to treatment, primarily directed at the dopamine system, suggests that multiple mechanisms may underlie psychotic symptoms. Reinforcement learning tasks have been employed in schizophrenia to assess dopaminergic functioning and reward processing, but these have not directly compared groups of treatment-refractory and non-refractory patients.

METHODS

In the current functional magnetic resonance imaging study, 21 patients with treatment-resistant schizophrenia (TRS), 21 patients with non-treatment-resistant schizophrenia (NTR), and 24 healthy controls (HC) performed a probabilistic reinforcement learning task, utilizing emotionally valenced face stimuli which elicit a social bias toward happy faces. Behavior was characterized with a reinforcement learning model. Trial-wise reward prediction error (RPE)-related neural activation and the differential impact of emotional bias on these reward signals were compared between groups.

RESULTS

Patients showed impaired reinforcement learning relative to controls, while all groups demonstrated an emotional bias favoring happy faces. The pattern of RPE signaling was similar in the HC and TRS groups, whereas NTR patients showed significant attenuation of RPE-related activation in striatal, thalamic, precentral, parietal, and cerebellar regions. TRS patients, but not NTR patients, showed a positive relationship between emotional bias and RPE signal during negative feedback in bilateral thalamus and caudate.

CONCLUSION

TRS can be dissociated from NTR on the basis of a different neural mechanism underlying reinforcement learning. The data support the hypothesis that a favorable response to antipsychotic treatment is contingent on dopaminergic dysfunction, characterized by aberrant RPE signaling, whereas treatment resistance may be characterized by an abnormality of a non-dopaminergic mechanism - a glutamatergic mechanism would be a possible candidate.

Hypofrontality and Posterior Hyperactivity in Early Schizophrenia: Imaging and Behavior in a Preclinical Model

BACKGROUND

Schizophrenia is a debilitating neuropsychiatric disorder typically diagnosed from late adolescence to adulthood. Subthreshold behavioral symptoms (e.g., cognitive deficits and substance abuse) often precede the clinical diagnosis of schizophrenia. However, these prodromal symptoms have not been consistently associated with structural and functional brain biomarkers, limiting the chance of early diagnosis of schizophrenia.

METHODS

Using an extensively multimodal range of magnetic resonance methods (for anatomy, metabolism, and function), we screened early biomarkers in a methylazoxymethanol acetate (MAM) rat model of schizophrenia and saline-treated control (SHAM) rats, in conjunction with immunohistochemistry, myelin staining, and a novel three-choice, reversal-learning task to identify early behavioral markers corresponding the subthreshold symptoms.

RESULTS

MAM (vs. SHAM) rats had lower/higher structural connectivity in anterior/posterior corpus callosum. The orbitofrontal cortex of MAM rats showed lower resting-state functional magnetic resonance imaging functional connectivity in conjunction with lower neuronal density, lower glucose oxidation, and attenuated neurotransmission (hypofrontality). In contrast, these measures were all higher in visual cortex of MAM rats (posterior hyperactivity), which might parallel perceptual problems in schizophrenia. In behavioral studies, MAM (vs. SHAM) rats displayed abnormal orbitofrontal cortex-mediated decision-making processes, resulting in a novel reward-sensitive hyperflexible phenotype, which might reflect vulnerability of prodromal patients to substance abuse.

CONCLUSIONS

We identified two novel biomarkers of early schizophrenia in a preclinical rat model: hypofrontality associated with the hyperflexible phenotype, and posterior hyperactivity. Because each of these magnetic resonance methods is clinically translatable, these markers could contribute to early diagnosis and the development of novel therapies of schizophrenia.

Brain-imaging studies of treatment-resistant schizophrenia: a systematic review

Around 30% of patients with schizophrenia show an inadequate response to antipsychotics-ie, treatment resistance. Neuroimaging studies can help to uncover the underlying neurobiological reasons for such resistance and identify these patients earlier. Additionally, studies examining the effect of clozapine on the brain can help to identify aspects of clozapine that make it uniquely effective in patients with treatment resistance. We did a systematic search of PubMed between Jan 1, 1980, and April 13, 2015, to identify all neuroimaging studies that examined treatment-resistant patients or longitudinally assessed the effects of clozapine treatment. We identified 330 articles, of which 61 met the inclusion criteria. Replicated differences between treatment-resistant and treatment-responsive patients include reductions in grey matter and perfusion of frontotemporal regions, and increases in white matter and basal ganglia perfusion, with effect sizes ranging from 0·4 to greater than 1. Clozapine treatment led to reductions in caudate nucleus volume in three separate studies. The available evidence supports the hypothesis that some of the neurobiological changes seen in treatment-resistant schizophrenia lie along a continuum with treatment-responsive schizophrenia, whereas other differences are categorical in nature and have potential to be used as biomarkers. However, further replication is needed, and for neuroimaging findings to be clinically translatable, future studies need to focus on a-priori hypotheses and be adequately powered.

Emotion processing in treatment-resistant schizophrenia patients treated with clozapine: An fMRI study

OBJECTIVES

To examine the neural correlates of emotion processing in treatment-resistant patients with schizophrenia (SCZ-TR).

METHODS

Twenty-two SCZ-TR patients on clozapine, 24 schizophrenia patients on antipsychotics other than clozapine, and 39 healthy controls were scanned using functional neuroimaging while viewing positive, negative and neutral images.

RESULTS

Emotionally-laden images (positive and negative) elicited hyper-activations in the dorso-medial prefrontal cortex and left cerebellum in SCZ-TR patients, compared to the two other groups. Similarly, neutral images prompted hyper-activations in the cingulate gyrus in SCZ-TR patients, relative to the two other groups.

CONCLUSIONS

Treatment resistance is associated with neuro-functional hyper-activations in schizophrenia patients during emotion processing.

Structural and functional neuroimaging findings associated with the use of clozapine in schizophrenia: a systematic review

OBJECTIVE

Schizophrenia is one of the most severe psychiatric disorders, and its current treatment relies on antipsychotic medications with only partial effectiveness. Clozapine is an atypical antipsychotic with a specific profile of action indicated for treatment-resistant schizophrenia. Neuroimaging studies assessing the effects of clozapine could help shed light on the neural underpinnings of the effects of this drug in the brain. The objective of this study was to review the available literature on the structural and functional neuroimaging findings associated with use of clozapine.

METHOD

We conducted a systematic review of the indexed literature using the PubMed, BIREME, and ISI Web of Knowledge search engines and the following keywords: clozapine, neuroimaging, computed tomography, MRI, functional magnetic resonance, PET, SPECT, and DTI.

RESULTS

A total of 23 articles were included in the review. In structural studies, the use of clozapine was associated with volume reductions in the basal ganglia, especially the caudate nucleus, where functional neuroimaging studies also found decreased perfusion. In the frontal lobe, clozapine treatment was associated with increased gray matter volume and reduced perfusion.

CONCLUSION

The results of the studies reviewed suggest that the use of clozapine is associated with distinctive structural and functional neuroimaging findings that are not shared with other antipsychotics.

Neuroimaging findings in treatment-resistant schizophrenia: A systematic review: Lack of neuroimaging correlates of treatment-resistant schizophrenia

BACKGROUND

Recent developments in neuroimaging have advanced the understanding of biological mechanisms underlying schizophrenia. However, neuroimaging correlates of treatment-resistant schizophrenia (TRS) and superior effects of clozapine on TRS remain unclear.

METHODS

Systematic search was performed to identify neuroimaging characteristics unique to TRS and ultra-resistant schizophrenia (i.e. clozapine-resistant [URS]), and clozapine's efficacy in TRS using Embase, Medline, and PsychInfo. Search terms included (schizophreni*) and (resistan* OR refractory OR clozapine) and (ASL OR CT OR DTI OR FMRI OR MRI OR MRS OR NIRS OR PET OR SPECT).

RESULTS

25 neuroimaging studies have investigated TRS and effects of clozapine. Only 5 studies have compared TRS and non-TRS, collectively providing no replicated neuroimaging finding specific to TRS. Studies comparing TRS and healthy controls suggest that hypometabolism in the prefrontal cortex, hypermetabolism in the basal ganglia, and structural anomalies in the corpus callosum contribute to TRS. Clozapine may increase prefrontal hypoactivation in TRS although this was not related to clinical improvement; in contrast, evidence has suggested a link between clozapine efficacy and decreased metabolism in the basal ganglia and thalamus.

CONCLUSION

Existing literature does not elucidate neuroimaging correlates specific to TRS or URS, which, if present, might also shed light on clozapine's efficacy in TRS. This said, leads from other lines of investigation, including the glutamatergic system can prove useful in guiding future neuroimaging studies focused on, in particular, the frontocortical-basal ganglia-thalamic circuits. Critical to the success of this work will be precise subtyping of study subjects based on treatment response/nonresponse and the use of multimodal neuroimaging.

Antipsychotic medication and prefrontal cortex activation: a review of neuroimaging findings

Decreased prefrontal activation (hypofrontality) in schizophrenia is thought to underlie negative symptoms and cognitive impairments, and may contribute to poor social outcome. Hypofrontality does not always improve during treatment with antipsychotics. We hypothesized that antipsychotics, which share antagonism at dopamine receptors, with a relatively low dopamine receptor affinity and high serotonin receptor affinity may have a sparing effect on prefrontal function compared to strong dopamine receptor antagonists. We systematically investigated the relation between serotonin and dopamine antagonism of antipsychotics and prefrontal functioning by reviewing neuroimaging studies. The weight of the evidence was consistent with our hypothesis that antipsychotics with low dopaminergic receptor affinity and moderate to high serotonergic affinity were associated with higher activation of the prefrontal cortex. However, clozapine, a weak dopamine and strong serotonin antagonist, was associated with decrease in prefrontal activation. Future studies should further elucidate the link between prefrontal activation and negative symptoms using prospective designs and advanced neuroimaging techniques, which may ultimately benefit the development of treatments for disabling negative symptoms.

Relative risk of probabilistic category learning deficits in patients with schizophrenia and their siblings

BACKGROUND

Although patients with schizophrenia display an overall probabilistic category learning performance deficit, the extent to which this deficit occurs in unaffected siblings of patients with schizophrenia is unknown. There are also discrepant findings regarding probabilistic category learning acquisition rate and performance in patients with schizophrenia.

METHODS

A probabilistic category learning test was administered to 108 patients with schizophrenia, 82 unaffected siblings, and 121 healthy participants.

RESULTS

Patients with schizophrenia displayed significant differences from their unaffected siblings and healthy participants with respect to probabilistic category learning acquisition rates. Although siblings on the whole failed to differ from healthy participants on strategy and quantitative indexes of overall performance and learning acquisition, application of a revised learning criterion enabling classification into good and poor learners on the basis of individual learning curves revealed significant differences between percentages of sibling and healthy poor learners: healthy (13.2%), siblings (34.1%), patients (48.1%), yielding a moderate relative risk.

CONCLUSIONS

These results clarify previous discrepant findings pertaining to probabilistic category learning acquisition rate in schizophrenia and provide the first evidence for the relative risk of probabilistic category learning abnormalities in unaffected siblings of patients with schizophrenia, supporting genetic underpinnings of probabilistic category learning deficits in schizophrenia. These findings also raise questions regarding the contribution of antipsychotic medication to the probabilistic category learning deficit in schizophrenia. The distinction between good and poor learning might be used to inform genetic studies designed to detect schizophrenia risk alleles.

The effect of clozapine on regional cerebral blood flow and brain metabolite ratios in schizophrenia: relationship with treatment response

The purpose of this study was to investigate the effect of clozapine on regional cerebral blood flow (rCBF) and its relationship with response to treatment. In addition, we aimed to study the influence of clozapine on proton magnetic resonance spectroscopy ((1)H-MRS) findings in the dorsolateral prefrontal cortex (DLPFC) in a subgroup of patients. Psychopathology, neurocognitive functioning, and SPECT imaging of 22 patients were assessed at the baseline and 8 weeks after the initiation of clozapine treatment. In 10 of these patients intermediate-echo (TE: 135 ms) single-voxel (1)H-MRS was also performed at the baseline and after 8 weeks. Clozapine treatment increased the right frontal (superior and medial)/caudate perfusion ratio in the whole group, while it increased bilateral frontal (superior and medial)/caudate perfusion ratios in treatment responders. In addition, percentage changes in left and right frontal (superior and medial)/caudate perfusion ratios compared to the baseline were higher in treatment responders than in non-responders. The improvement in attention was related to the increase in percentage change in the right frontal (superior and medial)/caudate perfusion ratio, while the improvement in verbal fluency was related to the increase in percentage changes in both right and left frontal (superior and medial)/caudate perfusion ratios and to right frontal (superior and medial)/thalamus perfusion. Baseline frontal (superior and medial)/thalamus perfusion could explain 32% of the variability of percentage improvements in psychopathology. (1)H-MRS showed that the baseline PANSS general psychopathology score was inversely correlated with the baseline NAA/Cre ratio. An increased NAA/Cre ratio in DLPFC after 8 weeks of clozapine treatment was also revealed by (1)H-MRS. Our SPECT imaging results suggest the presence of an imbalance in fronto-striato-thalamic circuitry that changes with clozapine, especially in the responders, while (1)H-MRS results indicate a supportive effect of clozapine on neuronal integrity.

Remission from mania is associated with a decrease in amygdala activation during motor response inhibition

OBJECTIVES

Neuroimaging studies of bipolar disorder (BD) have provided evidence of brain functional abnormalities during both the states of mania and remission. However, the differences in brain function between these two states are still poorly known. In the current study, we aimed to use a longitudinal design to examine the functional changes associated with symptomatic remission from mania within the brain network underlying motor response inhibition.

METHODS

Using event-related functional magnetic resonance imaging (fMRI), 10 BD patients and 10 healthy subjects were imaged twice while performing a Go/NoGo task. Patients were in a manic state when they underwent the first scan and fully remitted during the second scan. A mixed-effect ANOVA was used to identify brain regions showing differences in activation change over time between the two groups.

RESULTS

The left amygdala was the only brain region to show a time-dependent change in activation that was significantly different between BD patients and healthy subjects. Further analyses revealed that this difference arose from the patient group, in which amygdala activation was decreased between mania and subsequent remission.

CONCLUSIONS

This finding suggests that a decrease in left amygdala responsiveness is a critical phenomenon associated with remission from mania. It emphasizes the relevance of longitudinal approaches for identifying neurofunctional modifications associated with mood changes in BD.

Neural correlates of probabilistic category learning in patients with schizophrenia

Functional neuroimaging studies of probabilistic category learning in healthy adults report activation of cortical-striatal circuitry. Based on previous findings of normal learning rate concurrent with an overall performance deficit in patients with schizophrenia, we hypothesized that relative to healthy adults, patients with schizophrenia would display preserved caudate nucleus and abnormal prefrontal cortex activation during probabilistic category learning. Forty patients with schizophrenia receiving antipsychotic medication and 25 healthy participants were assessed on interleaved blocks of probabilistic category learning and control tasks while undergoing blood oxygenation level-dependent functional magnetic resonance imaging. In addition to the whole sample of patients with schizophrenia and healthy adults, a subset of patients and healthy adults matched for good learning was also compared. In the whole sample analysis, patients with schizophrenia displayed impaired performance in conjunction with normal learning rate relative to healthy adults. The matched comparison of patients and healthy adults classified as good learners revealed greater caudate and dorsolateral prefrontal cortex activity in the healthy adults and greater activation in a more rostral region of the dorsolateral prefrontal, cingulate, parahippocampal and parietal cortex in patients. These results demonstrate that successful probabilistic category learning can occur in the absence of normal frontal-striatal function. Based on analyses of the patients and healthy adults matched on learning and performance, a minority of patients with schizophrenia achieve successful probabilistic category learning and performance levels through differential activation of a circumscribed neural network which suggests a compensatory mechanism in patients showing successful learning.

The thalamus and schizophrenia: current status of research

The thalamus provides a nodal link for multiple functional circuits that are impaired in schizophrenia (SZ). Despite inconsistencies in the literature, a meta analysis suggests that the volume of the thalamus relative to that of the brain is decreased in SZ. Morphometric neuroimaging studies employing deformation, voxel-based and region of interest methodologies suggest that the volume deficit preferentially affects the thalamic regions containing the anterior and mediodorsal nuclei, and the pulvinar. Postmortem design-based stereological studies have produced mixed results regarding volume and neuronal deficits in these nuclei. This review examines those aspects of thalamic circuitry and function that suggest salience to SZ. Evidence for anomalies of thalamic structure and function obtained from postmortem and neuroimaging studies is then examined and directions for further research proposed.