Impaired Prefrontal Cortical Dopamine Release in Schizophrenia During a Cognitive Task: A [11C]FLB 457 Positron Emission Tomography Study

Effect of cariprazine on attention and quality of life in patients with predominant negative symptoms of schizophrenia: A post-hoc analysis

Background

Cariprazine, a potent dopamine D3-preferring D3/D2 receptor partial agonist, has demonstrated benefits on negative symptoms among patients with schizophrenia. Secondary endpoint and post-hoc analyses have also suggested a benefit of cariprazine on quality of life (QoL) and attention.

Methods

Data for this post-hoc analysis were pooled from two 6-week, placebo-controlled phase 3 trials evaluating cariprazine among patients with acute exacerbations of schizophrenia. One study included an aripiprazole active-control arm for assay sensitivity.Two populations were analyzed: pooled intention-to-treat (ITT) population (N = 1043), and the pooled subgroup with predominant negative symptoms (PNS, n = 215), as defined by the Positive and Negative Syndrome Scale (PANSS) subscale and item cut-off criteria at baseline. Analyses of interest were: Schizophrenia Quality of Life Scale Revision 4 (SQLS-R4) total score; Cognitive Drug Research (CDR) power of attention (PoA), and continuity of attention (CoA).

Results

Among study completers, cariprazine and aripiprazole were associated with significant SQLS-R4 improvements in the ITT and PNS populations. Differences in CDR-PoA scores were significant for cariprazine vs. placebo in the ITT and PNS populations, but not for aripiprazole in the ITT or PNS analyses. Differences in CDR-CoA scores were significant for cariprazine vs. placebo in the ITT and PNS analyses; and was significant for aripiprazole vs. placebo in the PNS analysis, but not in the ITT analysis.

Conclusions

This post-hoc analysis suggests that cariprazine may be associated with beneficial effects on measures of attention and QoL among patients with schizophrenia, and these effects could be more pronounced among individuals with PNS.

The Role of Fatty Acid Amide Hydrolase, a Key Regulatory Endocannabinoid Enzyme, in Domain-Specific Cognitive Performance in Psychosis

BACKGROUND AND HYPOTHESIS

Cognitive impairments are particularly disabling for patients with a psychotic disorder and often persist despite optimization of antipsychotic treatment. Thus, motivating an extension of the research focus on the endocannabinoid system. The aim of this study was to evaluate group differences in brain fatty acid amid hydrolase (FAAH), an endocannabinoid enzyme between first-episode psychosis (FEP), individuals with clinical high risk (CHR) for psychosis and healthy controls (HCs). Furthermore, to test the hypothesis that FAAH is linked with cognition using positron emission tomography (PET).

STUDY DESIGN

We analyzed 80 PET scans with the highly selective FAAH radioligand [11C]CURB, including 30 patients with FEP (6 female), 15 CHR (5 female), and 35 HC (19 female). The Repeatable Battery for the Assessment of Neuropsychological Status (RBANS) and the Berg Card Sorting Test (BCST) were applied to test cognitive performance.

STUDY RESULTS

There was no difference in FAAH activity between groups (F2, 75 = 0.75, P = .48; Cohen's f = 0.141; small effect). Overall, there was a difference in the association between groups regarding FAAH activity and the domain visuospatial construction (F2, 72 = 4.67, P = .01; Cohen's f = .36; medium effect). Furthermore, across the sample, lower FAAH activity was associated with a higher percentage of perseverative responses (F1, 66 = 5.06, P = .03; Cohen's f = 0.28, medium effect).

CONCLUSIONS

We report evidence for associations between endocannabinoid alterations in FEP and CHR with specific domains of cognition (visuospatial construction and perseverative response), not overall cognition.

NIMH perspectives on future directions in neuroimaging for mental health

NIMH's mission is to transform the understanding and treatment of mental illnesses through basic and clinical research, paving the way for prevention, recovery, and cure. New imaging techniques hold great promise for improving our understanding of the pathophysiology of mental illnesses, stratifying patients for treatment selection, and developing a personalized medicine approach. Here, we highlight emerging and promising new technologies that are likely to be vital in helping NIMH accomplish its mission, the potential for utilizing multimodal approaches to study mental illness, and considerations for data analytics and data sharing.

New Drug Treatments for Schizophrenia: A Review of Approaches to Target Circuit Dysfunction

Schizophrenia is a leading cause of global disease burden. Current drug treatments are associated with significant side effects and have limited efficacy for many patients, highlighting the need to develop new approaches that target other aspects of the neurobiology of schizophrenia. Preclinical, in vivo imaging, postmortem, genetic, and pharmacological studies have highlighted the key role of cortical GABAergic (gamma-aminobutyric acidergic)-glutamatergic microcircuits and their projections to subcortical dopaminergic circuits in the pathoetiology of negative, cognitive, and psychotic symptoms. Antipsychotics primarily act downstream of the dopaminergic component of this circuit. However, multiple drugs are currently in development that could target other elements of this circuit to treat schizophrenia. These include drugs for GABAergic or glutamatergic targets, including glycine transporters, D-amino acid oxidase, sodium channels, or potassium channels. Other drugs in development are likely to primarily act on pathways that regulate the dopaminergic system, such as muscarinic or trace amine receptors or 5-HT2A receptors, while PDE10A inhibitors are being developed to modulate the downstream consequences of dopaminergic dysfunction. Our review considers where new drugs may act on this circuit and their latest clinical trial evidence in terms of indication, efficacy, and side effects. Limitations of the circuit model, including whether there are neurobiologically distinct subgroups of patients, and future directions are also considered. Several drugs based on the mechanisms reviewed have promising clinical data, with the muscarinic agonist KarXT most advanced. If these drugs are approved for clinical use, they have the potential to revolutionize understanding of the pathophysiology and treatment of schizophrenia.

Trends in non-focal neurological chief complaints and CT angiography utilization among adults in the emergency department

Prudent imaging use is essential for cost reduction and efficient patient triage. Recent efforts have focused on head and neck CTA in patients with emergent concerns for non-focal neurological complaints, but have failed to demonstrate whether increases in utilization have resulted in better care. The objective of this study was to examine trends in head and neck CTA ordering and determine whether a correlation exists between imaging utilization and positivity rates. This is a single-center retrospective observational study at a quaternary referral center. This study includes patients presenting with headache and/or dizziness to the emergency department between January 2017 and December 2021. Patients who received a head and neck CTA were compared to those who did not. The main outcomes included annual head and neck CTA utilization and positivity rates, defined as the percent of scans with attributable acute pathologies. Among 24,892 emergency department visits, 2264 (9.1%) underwent head and neck CTA imaging. The percentage of patients who received a scan over the study period increased from 7.89% (422/5351) in 2017 to 13.24% (662/5001) in 2021, representing a 67.4% increase from baseline (OR, 1.14; 95% CI 1.11-1.18; P < .001). The positivity rate, or the percentage of scans ordered that revealed attributable acute pathology, dropped from 16.8% (71/422) in 2017 to 10.4% (69/662) in 2021 (OR, 0.86; 95% CI 0.79-0.94; P = .001), a 38% reduction in positive examinations. Throughout the study period, there was a 67.4% increase in head and neck CTA ordering with a concomitant 38.1% decrease in positivity rate.

Therapeutic potential of gamma entrainment using sensory stimulation for cognitive symptoms associated with schizophrenia

Schizophrenia is a complex neuropsychiatric disorder with significant morbidity. Treatment options that address the spectrum of symptoms are limited, highlighting the need for innovative therapeutic approaches. Gamma Entrainment Using Sensory Stimulation (GENUS) is an emerging treatment for neuropsychiatric disorders that uses sensory stimulation to entrain impaired oscillatory network activity and restore brain function. Aberrant oscillatory activity often underlies the symptoms experienced by patients with schizophrenia. We propose that GENUS has therapeutic potential for schizophrenia. This paper reviews the current status of schizophrenia treatment and explores the use of sensory stimulation as an adjunctive treatment, specifically through gamma entrainment. Impaired gamma frequency entrainment is observed in patients, particularly in response to auditory and visual stimuli. Thus, sensory stimulation, such as music listening, may have therapeutic potential for individuals with schizophrenia. GENUS holds novel therapeutic potential to improve the lives of individuals with schizophrenia, but further research is required to determine the efficacy of GENUS, optimize its delivery and therapeutic window, and develop strategies for its implementation in specific patient populations.

New clues for the role of cerebellum in schizophrenia and the associated cognitive impairment

Schizophrenia (SZ) is a complex neuropsychiatric disorder associated with severe cognitive dysfunction. Although research has mainly focused on forebrain abnormalities, emerging results support the involvement of the cerebellum in SZ physiopathology, particularly in Cognitive Impairment Associated with SZ (CIAS). Besides its role in motor learning and control, the cerebellum is implicated in cognition and emotion. Recent research suggests that structural and functional changes in the cerebellum are linked to deficits in various cognitive domains including attention, working memory, and decision-making. Moreover, cerebellar dysfunction is related to altered cerebellar circuit activities and connectivity with brain regions associated with cognitive processing. This review delves into the role of the cerebellum in CIAS. We initially consider the major forebrain alterations in CIAS, addressing impairments in neurotransmitter systems, synaptic plasticity, and connectivity. We then focus on recent findings showing that several mechanisms are also altered in the cerebellum and that cerebellar communication with the forebrain is impaired. This evidence implicates the cerebellum as a key component of circuits underpinning CIAS physiopathology. Further studies addressing cerebellar involvement in SZ and CIAS are warranted and might open new perspectives toward understanding the physiopathology and effective treatment of these disorders.

65 years of research on dopamine's role in classical fear conditioning and extinction: A systematic review

Dopamine, a catecholamine neurotransmitter, has historically been associated with the encoding of reward, whereas its role in aversion has received less attention. Here, we systematically gathered the vast evidence of the role of dopamine in the simplest forms of aversive learning: classical fear conditioning and extinction. In the past, crude methods were used to augment or inhibit dopamine to study its relationship with fear conditioning and extinction. More advanced techniques such as conditional genetic, chemogenic and optogenetic approaches now provide causal evidence for dopamine's role in these learning processes. Dopamine neurons encode conditioned stimuli during fear conditioning and extinction and convey the signal via activation of D1-4 receptor sites particularly in the amygdala, prefrontal cortex and striatum. The coordinated activation of dopamine receptors allows for the continuous formation, consolidation, retrieval and updating of fear and extinction memory in a dynamic and reciprocal manner. Based on the reviewed literature, we conclude that dopamine is crucial for the encoding of classical fear conditioning and extinction and contributes in a way that is comparable to its role in encoding reward.

Putative neural mechanisms underlying release-mode-specific abnormalities in dopamine neural activity in a schizophrenia-like model: The distinct roles of glutamate and serotonin in the impaired regulation of dopamine neurons

Abnormalities in dopamine function might be related to psychiatric disorders such as schizophrenia. Even at the same concentration, dopamine exerts opposite effects on information processing in the prefrontal cortex depending on independent dopamine release modes known as tonic and phasic releases. This duality of dopamine prevents a blanket interpretation of the implications of dopamine abnormalities for diseases on the basis of absolute dopamine levels. Moreover, the mechanisms underlying the mode-specific dopamine abnormalities are not clearly understood. Here, I show that the two modes of dopamine release in the prefrontal cortex of a schizophrenia-like model are disrupted by different mechanisms. In the schizophrenia-like model established by perinatal exposure to inflammatory cytokine, epidermal growth factor, tonic release was enhanced and phasic release was decreased in the prefrontal cortex. I examined the activity of dopamine neurons in the ventral tegmental area (VTA), which sends dopamine projections to the prefrontal cortex, under anaesthesia. The activation of VTA dopamine neurons during excitatory stimulation (local application of glutamate or N-methyl-d-aspartic acid [NMDA]), which is associated with phasic activity, was blunt in this model. Dopaminergic neuronal activity in the resting state related to tonic release was increased by disinhibition of the dopamine neurons due to the impairment of 5HT2 (5HT2A) receptor-regulated GABAergic inputs. Moreover, chronic administration of risperidone ameliorated this disinhibition of dopaminergic neurons. These results provide an idea about the mechanism of dopamine disturbance in schizophrenia and may be informative in explaining the effects of atypical antipsychotics as distinct from those of typical drugs.

Assessing NH300094, a novel dopamine and serotonin receptor modulator with cognitive enhancement property for treating schizophrenia

Background: Schizophrenia is a serious psychiatric disorder that significantly affects the quality of life of patients. The objective of this study is to discover a novel antipsychotic candidate with highly antagonistic activity against both serotonin and dopamine receptors, demonstrating robust efficacy in animal models of positive, negative, and cognitive symptoms of schizophrenia. Methods: In the present study, we examined the activity of antipsychotic drug (NH300094) on 5-HT2A, 5-HT2C, 5-HT1A, 5-HT1B, 5-HT7, H1, M1, Alpha1A, D2L, D2S, Alpha2A, D3 receptor functional assay in vitro. In addition, multiple animal models, including dizocilpine (MK-801) induced hyper-locomotion; APO induced climbing; Conditioned Avoidance Response (CAR); DOI-Induced Head Twitch; Forced swimming test; Scopolamine induced cognitive impairment model, were used to verify the antipsychotic activity of NH300094 in preclinical. Results: In vitro functional assays have indicated that NH300094 is a potent antagonist of 5-HT receptors and dopamine receptors, with higher relative antagonistic activity against 5-HT2A receptor (5-HT2A IC50 = 0.47 nM) than dopamine receptors (D2L IC50 = 1.04 nM; D2S IC50 = 11.71 nM; D3 IC50 = 31.55 nM). Preclinical in vivo pharmacological study results showed that NH300094 was effective in multiple models, which is more extensive than the clinic drug Risperidone. Furthermore, the safety window for extrapyramidal side effects of NH300094 is significantly wider than that of Risperidone (For NH300094, mice catalepsy model ED50/ Mice MK-801 model ED50 = 104.6-fold; for Risperidone, mice catalepsy model ED50/ Mice MK-801 model ED50 = 12.9-fold), which suggests a potentially better clinical safety profile for NH300094. Conclusion: NH300094 is a novel potent serotonin and dopamine receptors modulator, which has good safety profile and therapeutic potential for the treatment of schizophrenia with cognition disorders.

Schizophrenia: from neurochemistry to circuits, symptoms and treatments

Schizophrenia is a leading cause of global disability. Current pharmacotherapy for the disease predominantly uses one mechanism - dopamine D2 receptor blockade - but often shows limited efficacy and poor tolerability. These limitations highlight the need to better understand the aetiology of the disease to aid the development of alternative therapeutic approaches. Here, we review the latest meta-analyses and other findings on the neurobiology of prodromal, first-episode and chronic schizophrenia, and the link to psychotic symptoms, focusing on imaging evidence from people with the disorder. This evidence demonstrates regionally specific neurotransmitter alterations, including higher glutamate and dopamine measures in the basal ganglia, and lower glutamate, dopamine and γ-aminobutyric acid (GABA) levels in cortical regions, particularly the frontal cortex, relative to healthy individuals. We consider how dysfunction in cortico-thalamo-striatal-midbrain circuits might alter brain information processing to underlie psychotic symptoms. Finally, we discuss the implications of these findings for developing new, mechanistically based treatments and precision medicine for psychotic symptoms, as well as negative and cognitive symptoms.

Maternal Immune Activation Induces Cortical Catecholaminergic Hypofunction and Cognitive Impairments in Offspring

BACKGROUND

Impairment of specific cognitive domains in schizophrenia has been associated with prefrontal cortex (PFC) catecholaminergic deficits. Among other factors, prenatal exposure to infections represents an environmental risk factor for schizophrenia development in adulthood. However, it remains largely unknown whether the prenatal infection-induced changes in the brain may be associated with concrete switches in a particular neurochemical circuit, and therefore, if they could alter behavioral functions.

METHODS

In vitro and in vivo neurochemical evaluation of the PFC catecholaminergic systems was performed in offspring from mice undergoing maternal immune activation (MIA). The cognitive status was also evaluated. Prenatal viral infection was mimicked by polyriboinosinic-polyribocytidylic acid (poly(I:C)) administration to pregnant dams (7.5 mg/kg i.p., gestational day 9.5) and consequences were evaluated in adult offspring.

RESULTS

MIA-treated offspring showed disrupted recognition memory in the novel object recognition task (t = 2.30, p = 0.031). This poly(I:C)-based group displayed decreased extracellular dopamine (DA) concentrations compared to controls (t = 3.17, p = 0.0068). Potassium-evoked release of DA and noradrenaline (NA) were impaired in the poly(I:C) group (DA: Ft[10,90] = 43.33, p < 0.0001; Ftr[1,90] = 1.224, p = 0.2972; Fi[10,90] = 5.916, p < 0.0001; n = 11); (NA: Ft[10,90] = 36.27, p < 0.0001; Ftr[1,90] = 1.841, p = 0.208; Fi[10,90] = 8.686, p < 0.0001; n = 11). In the same way, amphetamine-evoked release of DA and NA were also impaired in the poly(I:C) group (DA: Ft[8,328] = 22.01, p < 0.0001; Ftr[1,328] = 4.507, p = 0.040; Fi[8,328] = 2.319, p = 0.020; n = 43); (NA: Ft[8,328] = 52.07; p < 0.0001; Ftr[1,328] = 4.322; p = 0.044; Fi[8,398] = 5.727; p < 0.0001; n = 43). This catecholamine imbalance was accompanied by increased dopamine D1 and D2 receptor expression (t = 2.64, p = 0.011 and t = 3.55, p = 0.0009; respectively), whereas tyrosine hydroxylase, DA and NA tissue content, DA and NA transporter (DAT/NET) expression and function were unaltered.

CONCLUSIONS

MIA induces in offspring a presynaptic catecholaminergic hypofunction in PFC with cognitive impairment. This poly(I:C)-based model reproduces catecholamine phenotypes reported in schizophrenia and represents an opportunity for the study of cognitive impairment associated to this disorder.

Clinical response to treatment with a partial dopamine agonist is related to changes in reward processing

Aberrant neuronal coding of reward processing has been linked to psychosis. It remains unresolved how treatment with a partial dopamine agonist affects reward processing, and whether treatment affects reward processing differently in patients responding and not responding to treatment. Here, 33 antipsychotic-naïve psychosis patients and 33 matched healthy controls underwent functional magnetic resonance imaging before and after patients received aripiprazole monotherapy for six weeks. Processing of motivational salient events and negative outcome evaluation (NOE) was examined using a monetary incentive delay task. Psychopathology was assessed with the Positive and Negative Syndrome Scale, and responders were identified by having ≥30% reduction in positive symptoms (N=21). At baseline, patients displayed an increased NOE signal in the caudate and dorsolateral prefrontal cortex compared to healthy controls. In the caudate, the NOE signal was normalized at follow-up, and normalization was driven by responders. In responders only, there was a significant improvement in the motivational salience signal in the caudate at follow-up. Motivational salience and NOE signals in the caudate may be associated with a dopaminergic mechanism in patients characterized as responders which may not be the case in non-responders. Likewise, non-dopaminergic mechanism may underly abnormal NOE processing in dorsolateral prefrontal cortex.

Cognitive impairment in schizophrenia: aetiology, pathophysiology, and treatment

Cognitive deficits are a core feature of schizophrenia, account for much of the impaired functioning associated with the disorder and are not responsive to existing treatments. In this review, we first describe the clinical presentation and natural history of these deficits. We then consider aetiological factors, highlighting how a range of similar genetic and environmental factors are associated with both cognitive function and schizophrenia. We then review the pathophysiological mechanisms thought to underlie cognitive symptoms, including the role of dopamine, cholinergic signalling and the balance between GABAergic interneurons and glutamatergic pyramidal cells. Finally, we review the clinical management of cognitive impairments and candidate novel treatments.

Psychosis spectrum illnesses as disorders of prefrontal critical period plasticity

Emerging research on neuroplasticity processes in psychosis spectrum illnesses-from the synaptic to the macrocircuit levels-fill key gaps in our models of pathophysiology and open up important treatment considerations. In this selective narrative review, we focus on three themes, emphasizing alterations in spike-timing dependent and Hebbian plasticity that occur during adolescence, the critical period for prefrontal system development: (1) Experience-dependent dysplasticity in psychosis emerges from activity decorrelation within neuronal ensembles. (2) Plasticity processes operate bidirectionally: deleterious environmental and experiential inputs shape microcircuits. (3) Dysregulated plasticity processes interact across levels of scale and time and include compensatory mechanisms that have pathogenic importance. We present evidence that-given the centrality of progressive dysplastic changes, especially in prefrontal cortex-pharmacologic or neuromodulatory interventions will need to be supplemented by corrective learning experiences for the brain if we are to help people living with these illnesses to fully thrive.

Current advancements of modelling schizophrenia using patient-derived induced pluripotent stem cells

Schizophrenia (SZ) is a severe psychiatric disorder, with a prevalence of 1-2% world-wide and substantial health- and social care costs. The pathology is influenced by both genetic and environmental factors, however the underlying cause still remains elusive. SZ has symptoms including delusions, hallucinations, confused thoughts, diminished emotional responses, social withdrawal and anhedonia. The onset of psychosis is usually in late adolescence or early adulthood. Multiple genome-wide association and whole exome sequencing studies have provided extraordinary insights into the genetic variants underlying familial as well as polygenic forms of the disease. Nonetheless, a major limitation in schizophrenia research remains the lack of clinically relevant animal models, which in turn hampers the development of novel effective therapies for the patients. The emergence of human induced pluripotent stem cell (hiPSC) technology has allowed researchers to work with SZ patient-derived neuronal and glial cell types in vitro and to investigate the molecular basis of the disorder in a human neuronal context. In this review, we summarise findings from available studies using hiPSC-based neural models and discuss how these have provided new insights into molecular and cellular pathways of SZ. Further, we highlight different examples of how these models have shown alterations in neurogenesis, neuronal maturation, neuronal connectivity and synaptic impairment as well as mitochondrial dysfunction and dysregulation of miRNAs in SZ patient-derived cultures compared to controls. We discuss the pros and cons of these models and describe the potential of using such models for deciphering the contribution of specific human neural cell types to the development of the disease.

Prefrontal and Striatal Dopamine Release Are Inversely Correlated in Schizophrenia

BACKGROUND

The dopamine (DA) hypothesis postulates hyperactivity of subcortical DA transmission and hypoactivity of cortical DA in schizophrenia (SCH). Positron emission tomography provides the ability to assess this hypothesis in humans. However, no studies have examined the relationship between cortical DA and striatal DA in this illness.

METHODS

D_2/3 receptor radiotracer [¹¹C]FLB457 BP_ND (binding potential relative to nondisplaceable uptake) was measured in 14 off-medication subjects with SCH and 14 healthy control (HC) subjects at baseline and after the administration of 0.5 mg/kg oral d-amphetamine. The amphetamine-induced change in BP_ND (ΔBP_ND) was calculated as the difference between BP_ND in the postamphetamine condition and BP_ND in the baseline condition and expressed as a percentage of BP_ND at baseline. DA release in the striatum using the radiotracer [¹¹C]NPA was also measured in these subjects.

RESULTS

[¹¹C]FLB457 ΔBP_ND was greater in the HC group compared with the SCH group (F_1,26 = 5.7; p = .02) with significant differences in [¹¹C]FLB457 ΔBP_ND seen across cortical brain regions. Only in the SCH group was a significant negative correlation observed between [¹¹C]FLB457 ΔBP_ND in the dorsolateral prefrontal cortex and [¹¹C]NPA ΔBP_ND in the dorsal caudate (r = -0.71, p = .005).

CONCLUSIONS

Subjects with SCH demonstrated deficits of DA release in cortical brain regions relative to HC subjects. Examining both cortical and striatal DA release in the same subjects demonstrated an inverse relationship between cortical DA release and striatal DA release in SCH not present in HC subjects, providing support for the current DA hypothesis of SCH.

Self-Entropic Broadening Theory: Toward a New Understanding of Self and Behavior Change Informed by Psychedelics and Psychosis

The extremes of human experiences, such as those occasioned by classic psychedelics and psychosis, provide a rich contrast for understanding how components of these experiences impact well-being. In recent years, research has suggested that classic psychedelics display the potential to promote positive enduring psychologic and behavioral changes in clinical and nonclinical populations. Paradoxically, classic psychedelics have been described as psychotomimetics. This review offers a putative solution to this paradox by providing a theory of how classic psychedelics often facilitate persistent increases in well-being, whereas psychosis leads down a "darker" path. This will be done by providing an overview of the overlap between the states (i.e., entropic processing) and their core differences (i.e., self-focus). In brief, entropic processing can be defined as an enhanced overall attentional scope and decreased predictability in processing stimuli facilitating a hyperassociative style of thinking. However, the outcomes of entropic states vary depending on level of self-focus, or the degree to which the associations and information being processed are evaluated in a self-referential manner. We also describe potential points of overlap with less extreme experiences, such as creative thinking and positive emotion-induction. Self-entropic broadening theory offers a heuristically valuable perspective on classic psychedelics and their lasting effects and relation to other states by creating a novel synthesis of contemporary theories in psychology. SIGNIFICANCE STATEMENT: Self-entropic broadening theory provides a novel theory examining the psychedelic-psychotomimetic paradox, or how classic psychedelics can be therapeutic, yet mimic symptoms of psychosis. It also posits a framework for understanding the transdiagnostic applicability of classic psychedelics. We hope this model invigorates the field to provide more rigorous comparisons between classic psychedelic-induced states and psychosis and further examinations of how classic psychedelics facilitate long-term change. As a more psychedelic future of psychiatry appears imminent, a model that addresses these long-standing questions is crucial.

Striatal dopamine D2/3 receptors in medication-naïve schizophrenia: an [123I] IBZM SPECT study

BACKGROUND

The hyper-function of the striatal dopamine system has been suggested to underlie key pathophysiological mechanisms in schizophrenia. Moreover, patients have been observed to present a significant elevation of dopamine receptor availability compared to healthy controls. Although it is difficult to measure dopamine levels directly in humans, neurochemical imaging techniques such as single-photon emission computed tomography (SPECT) provide indirect indices of in vivo dopamine synthesis and release, and putative synaptic levels.

METHODS

We focused on the role of dopamine postsynaptic regulation using [¹²³I] iodobenzamide (IBZM) SPECT. We compared D_2/3 receptor availability between 53 healthy controls and 21 medication-naive patients with recent-onset schizophrenia.

RESULT

The mean specific striatal binding showed no significant difference between patients and controls (estimated difference = 0.001; 95% CI -0.11 to 0.11; F = 0.00, df = 1, 69; p = 0.99). There was a highly significant effect of age whereby IBZM binding declined with advancing age [estimated change per decade of age = -0.01(binding ratio); 95% CI -0.01 to -0.004; F = 11.5, df = 1, 69; p = 0.001]. No significant correlations were found between the mean specific striatal binding and psychopathological or cognitive rating scores.

CONCLUSIONS

Medication-naïve patients with recent-onset schizophrenia have similar D_2/3 receptor availability to healthy controls. We suggest that, rather than focusing exclusively on postsynaptic receptors, future treatments should target the presynaptic control of dopamine synthesis and release.

The potent α2-adrenoceptor antagonist RS 79948 also inhibits dopamine D2 -receptors: Comparison with atipamezole and raclopride

Neurochemical, electrophysiological and behavioral evidence indicate that the potent α₂-adrenoceptor antagonist RS 79948 is also a dopamine (DA) D₂ receptor antagonist. Thus, results from ligand binding and adenylate cyclase activity indicate that RS 79948 binds to D₂ receptors and antagonized D₂ receptor-mediated inhibition of cAMP synthesis at nanomolar concentrations. RESULTS: from microdialysis indicated that RS 79948 shared with the selective α₂-adrenergic antagonist atipamezole the ability to increase the co-release of DA and norepinephrine (NE) from noradrenergic terminals in the medial prefrontal cortex (mPFC), except that RS 79948-induced DA release persisted after noradrenergic denervation, unlike atipamezole effect, indicating that RS 79948 releases DA from dopaminergic terminals as well. Similarly to the D₂ antagonist raclopride, but unlike atipamezole, RS 79948 increased extracellular DA and DOPAC in the caudate nucleus. Electrophysiological results indicate that RS 79948 shared with raclopride the ability to activate the firing of ventral tegmental area (VTA) DA neurons, while atipamezole was ineffective. RESULTS: from behavioral studies indicated that RS 79948 exerted effects mediated by independent, cooperative and contrasting inhibition of α₂-and D₂ receptors. Thus, RS 79948, but not atipamezole, prevented D₂-autoreceptor mediated hypomotility produced by a small dose of quinpirole. RS 79948 potentiated, more effectively than atipamezole, quinpirole-induced motor stimulation. RS 79948 antagonized, less effectively than atipamezole, raclopride-induced catalepsy. Future studies should clarify if the dual α₂-adrenoceptor- and D₂-receptor antagonistic action might endow RS 79948 with potential therapeutic relevance in the treatment of schizophrenia, drug dependence, depression and Parkinson's disease.

Effect of antipsychotic dose reduction on cognitive function in schizophrenia

Cognitive deficits are predictors of social functioning but remain an unmet therapeutic challenge. While lowering the antipsychotics carries a risk of relapse, it possibly has a beneficial effect on cognitive function. However, this has not been examined in a real-world setting. A prospective naturalistic 6-month follow-up study (n = 71) was conducted with patients between 18 and 45 years in their first five years of illness and the maintenance phase of the treatment. Brief Assessment of Cognition in Schizophrenia (BACS) was administered to assess cognitive functions. Patients were divided into three groups based on the change in dose of antipsychotics. The data were analyzed using linear mixed-effects modeling (LMEM) to examine the group differences. At the end of six months, those with decreased antipsychotic dose had significant improvement in BACS total score, token test, and symbol coding compared to those with no change in the dose of antipsychotic. Reducing the dose of antipsychotics during the maintenance phase was associated with improved cognitive functions without an increased risk of relapse. Antipsychotic dose reduction may be better than discontinuation as the relapse risk is higher with the discontinuation strategy. Clinicians must balance the improvements in cognitive functions and relapse risk.

Elevated endogenous GDNF induces altered dopamine signalling in mice and correlates with clinical severity in schizophrenia

Presynaptic increase in striatal dopamine is the primary dopaminergic abnormality in schizophrenia, but the underlying mechanisms are not understood. Here, we hypothesized that increased expression of endogenous GDNF could induce dopaminergic abnormalities that resemble those seen in schizophrenia. To test the impact of GDNF elevation, without inducing adverse effects caused by ectopic overexpression, we developed a novel in vivo approach to conditionally increase endogenous GDNF expression. We found that a 2-3-fold increase in endogenous GDNF in the brain was sufficient to induce molecular, cellular, and functional changes in dopamine signalling in the striatum and prefrontal cortex, including increased striatal presynaptic dopamine levels and reduction of dopamine in prefrontal cortex. Mechanistically, we identified adenosine A2a receptor (A_2AR), a G-protein coupled receptor that modulates dopaminergic signalling, as a possible mediator of GDNF-driven dopaminergic abnormalities. We further showed that pharmacological inhibition of A_2AR with istradefylline partially normalised striatal GDNF and striatal and cortical dopamine levels in mice. Lastly, we found that GDNF levels are increased in the cerebrospinal fluid of first episode psychosis patients, and in post-mortem striatum of schizophrenia patients. Our results reveal a possible contributor for increased striatal dopamine signalling in a subgroup of schizophrenia patients and suggest that GDNF-A_2AR crosstalk may regulate dopamine function in a therapeutically targetable manner.

Microinjection of Reelin into the mPFC prevents MK-801-induced recognition memory impairment in mice

Reelin, a large extracellular matrix protein, helps to regulate neuronal plasticity and cognitive function. Several studies have shown that Reelin dysfunction, resulting from factors such as mutations in gene RELN or low Reelin expression, is associated with schizophrenia (SCZ). We previously reported that microinjection of Reelin into cerebral ventricle prevents phencyclidine-induced cognitive and sensory-motor gating deficits. However, it remains unclear whether and how Reelin ameliorates behavioral abnormalities in the animal model of SCZ. In the present study, we evaluated the effect of recombinant Reelin microinjection into the medial prefrontal cortex (mPFC) on abnormal behaviors induced by MK-801, an N-methyl-D-aspartate receptor antagonist. Microinjection of Reelin into the mPFC prevented impairment of recognition memory of MK-801-treated mice in the novel object recognition test (NORT). On the other hand, the same treatment had no effect on deficits in sensory-motor gating and short-term memory in the pre-pulse inhibition and Y-maze tests, respectively. To establish the neural substrates that respond to Reelin, the number of c-Fos-positive cells in the mPFC was determined. A significant increase in c-Fos-positive cells in the mPFC of MK-801-treated mice was observed when compared with saline-treated mice, and this change was suppressed by microinjection of Reelin into the mPFC. A K2360/2467A Reelin that cannot bind to its receptor failed to ameliorate MK-801-induced cognitive deficits in NORT. These results suggest that Reelin prevents MK-801-induced recognition memory impairment by acting on its receptors to suppress neural activity in the mPFC of mice.

The utility of PET imaging in the diagnosis and management of psychosis: a brief review

Purpose

Advances in the pathophysiological characterization of psychosis has led to a newfound role of biomarkers in diagnostic and prognostic contexts. Further, advances in the accuracy and sensitivity of nuclear medicine imaging techniques, and specifically positron emission tomography (PET), have improved the ability to diagnose and manage individuals experiencing first-episode psychosis or those at greater risk for developing psychosis.

Methods

Literature searches were performed in PubMed, Google Scholar, and Web of Science to identify papers related to the use of PET imaging in the diagnosis or management of psychosis. Search terms used included “positron emission tomography”, “PET imaging”, “psychosis”, “disorders of psychosis”, “schizophrenia”, “biomarkers”, “diagnostic biomarkers”, “prognostic biomarker”, “monitoring biomarker”, “outcome biomarker”, and “predictive biomarker.”

Results

Studies included fell into three categories: those examining microglia, those studying dopamine synthesis capacity, and those examining acetylcholine receptor activity. Microglial imaging has been shown to be ineffective in all patients with psychosis, but some believe it shows promise in a subset of patients with psychosis, although no defining characteristics of said subset have been postulated. Studies of dopamine synthesis capacity suggest that presynaptic dopamine is reliably elevated in patients with psychosis, but levels of dopamine active transporter are not. Further, positron emission tomography (PET) with [18F]fluoro-l-dihydroxyphenylalanine ([18F]FDOPA)-PET has been recently used successfully as a predictive biomarker of dopaminergic treatment response, although more work is needed to validate such findings. Finally, existing studies have also documented lower levels of binding to the α7 nicotinic cholinergic receptor (α7-nAChR) via [18F]-ASEM PET in patients with psychosis, however there is a dearth of prospective, randomized studies evaluating the efficacy of [18F]-ASEM as a diagnostic or monitoring biomarker of any kind.

Conclusion

Molecular imaging has become a useful tool in the diagnosis and management of psychosis. Further work must be done to improve the comparative prognostic value and diagnostic accuracy of different radiotracers.

No association between cortical dopamine D2 receptor availability and cognition in antipsychotic-naive first-episode psychosis

Cognitive impairment is an important predictor of disability in schizophrenia. Dopamine neurotransmission in cortical brain regions has been suggested to be of importance for higher-order cognitive processes. The aim of this study was to examine the relationship between extrastriatal dopamine D2-R availability and cognitive function, using positron emission tomography and the high-affinity D2-R radioligand [¹¹C]FLB 457, in an antipsychotic-naive sample of 18 first-episode psychosis patients and 16 control subjects. We observed no significant associations between D2-R binding in the dorsolateral prefrontal cortex or hippocampus (β = 0.013–0.074, partial r = −0.037–0.273, p = 0.131–0.841). Instead, using Bayesian statistics, we found moderate support for the null hypothesis of no relationship (BF_H0:H1 = 3.3–8.2). Theoretically, our findings may suggest a lack of detrimental effects of D2-R antagonist drugs on cognition in schizophrenia patients, in line with clinical observations.

Exploring the relation between BOLD fMRI and cognitive performance using a computer-based quantitative systems pharmacology model: Applications to the COMTVAL158MET genotype and ketamine

BOLD fMRI is increasingly used mostly in an observational way to probe the effect of genotypes or therapeutic intervention in normal and diseased subjects. We use a mechanism-based quantitative systems pharmacology computer model of a human cortical microcircuit, previously calibrated for the 2-back working memory paradigm, adding established biophysical principles, of glucose metabolism, oxygen consumption, neurovascular effects and the paramagnetic impact on blood oxygen levels to calculate a readout for the voxel-based BOLD fMRI signal. The objective was to study the effect of the Catechol-O-methyl Transferase Val158Met (COMT) genotype on performance and BOLD fMRI. While the simulation suggests that on average virtual COMTVV genotype subjects perform worse, subjects with lower GABA, lower 5-HT₃ and higher 5-HT_1A activation can improve cognitive performance to the level of COMTMM subjects but at the expense of higher BOLD fMRI signal. In a schizophrenia condition, increased NMDA, GABA tone and noise levels, and lower D₁R activity can improve cognitive outcome with greater BOLD fMRI signal in COMT Val-carriers. We further generate hypotheses about why ketamine in healthy controls increases the BOLD fMRI signal but reduces cognitive performance. These simulations suggest a strong non-linear relationship between BOLD fMRI signal and cognitive performance. When validated, this mechanistic approach can be useful for moving beyond the descriptive nature of BOLD fMRI imaging and supporting the proper interpretation of imaging biomarkers in CNS disorders.

Linking mPFC circuit maturation to the developmental regulation of emotional memory and cognitive flexibility

The medial prefrontal cortex (mPFC) and its abundant connections with other brain regions play key roles in memory, cognition, decision making, social behaviors, and mood. Dysfunction in mPFC is implicated in psychiatric disorders in which these behaviors go awry. The prolonged maturation of mPFC likely enables complex behaviors to emerge, but also increases their vulnerability to disruption. Many foundational studies have characterized either mPFC synaptic or behavioral development without establishing connections between them. Here, we review this rich body of literature, aligning major events in mPFC development with the maturation of complex behaviors. We focus on emotional memory and cognitive flexibility, and highlight new work linking mPFC circuit disruption to alterations of these behaviors in disease models. We advance new hypotheses about the causal connections between mPFC synaptic development and behavioral maturation and propose research strategies to establish an integrated understanding of neural architecture and behavioral repertoires.

Resting-state dopaminergic cell firing in the ventral tegmental area negatively regulates affiliative social interactions in a developmental animal model of schizophrenia

Hyperdopaminergic activities are often linked to positive symptoms of schizophrenia, but their neuropathological implications on negative symptoms are rather controversial among reports. Here, we explored the regulatory role of the resting state-neural activity of dopaminergic neurons in the ventral tegmental area (VTA) on social interaction using a developmental rat model for schizophrenia. We prepared the model by administering an ammonitic cytokine, epidermal growth factor (EGF), to rat pups, which later exhibit the deficits of social interaction as monitored with same-gender affiliative sniffing. In vivo single-unit recording and microdialysis revealed that the baseline firing frequency of and dopamine release from VTA dopaminergic neurons were chronically increased in EGF model rats, and their social interaction was concomitantly reduced. Subchronic treatment with risperidone ameliorated both the social interaction deficits and higher frequency of dopaminergic cell firing in this model. Sustained suppression of hyperdopaminergic cell firing in EGF model rats by DREADD chemogenetic intervention restored the event-triggered dopamine release and their social behaviors. These observations suggest that the higher resting-state activity of VTA dopaminergic neurons is responsible for the reduced social interaction of this schizophrenia model.

Strategies to solve the reverse inference fallacy in future MRI studies of schizophrenia: a review

Few advances in schizophrenia research have been translated into clinical practice, despite 60 years of serum biomarkers studies and 50 years of genetic studies. During the last 30 years, neuroimaging studies on schizophrenia have gradually increased, partly due to the beautiful prospect that the pathophysiology of schizophrenia could be explained entirely by the Human Connectome Project (HCP). However, the fallacy of reverse inference has been a critical problem of the HCP. For this reason, there is a dire need for new strategies or research "bridges" to further schizophrenia at the biological level. To understand the importance of research "bridges," it is vital to examine the strengths and weaknesses of the recent literature. Hence, in this review, our team has summarized the recent literature (1995-2018) about magnetic resonance imaging (MRI) of schizophrenia in terms of regional and global structural and functional alterations. We have also provided a new proposal that may supplement the HCP for studying schizophrenia. As postulated, despite the vast number of MRI studies in schizophrenia, the lack of homogeneity between the studies, along with the relatedness of schizophrenia with other neurological disorders, has hindered the study of schizophrenia. In addition, the reverse inference cannot be used to diagnose schizophrenia, further limiting the clinical impact of findings from medical imaging studies. We believe that multidisciplinary technologies may be used to develop research "bridges" to further investigate schizophrenia at the single neuron or neuron cluster levels. We have postulated about future strategies for overcoming the current limitations and establishing the research "bridges," with an emphasis on multimodality imaging, molecular imaging, neuron cluster signals, single transmitter biomarkers, and nanotechnology. These research "bridges" may help solve the reverse inference fallacy and improve our understanding of schizophrenia for future studies.

Dopamine and Neuroinflammation in Schizophrenia - Interpreting the Findings from Translocator Protein (18kDa) PET Imaging

Schizophrenia is a complex disease whose pathophysiology is not yet fully understood. In addition to the long prevailing dopaminergic hypothesis, the evidence suggests that neuroinflammation plays a role in the pathophysiology of the disease. Recent studies using positron emission tomography (PET) that target a 18kDa translocator protein (TSPO) in activated microglial cells in an attempt to measure neuroinflammation in patients have shown a decrease or a lack of an increase in TSPO binding. Many biological and methodological considerations have been formulated to explain these findings. Although dopamine has been described as an immunomodulatory molecule, its potential role in neuroinflammation has not been explored in the aforementioned studies. In this review, we discuss the interactions between dopamine and neuroinflammation in psychotic states. Dopamine may inhibit neuroinflammation in activated microglia. Proinflammatory molecules released from microglia may decrease dopaminergic transmission. This could potentially explain why the levels of neuroinflammation in the brain of patients with schizophrenia seem to be unchanged or decreased compared to those in healthy subjects. However, most data are indirect and are derived from animal studies or from studies performed outside the field of schizophrenia. Further studies are needed to combine TSPO and dopamine imaging to study the association between microglial activation and dopamine system function.

Orbitofrontal-Striatal Structural Alterations Linked to Negative Symptoms at Different Stages of the Schizophrenia Spectrum

Negative symptoms such as anhedonia and apathy are among the most debilitating manifestations of schizophrenia (SZ). Imaging studies have linked these symptoms to morphometric abnormalities in 2 brain regions implicated in reward and motivation: the orbitofrontal cortex (OFC) and striatum. Higher negative symptoms are generally associated with reduced OFC thickness, while higher apathy specifically maps to reduced striatal volume. However, it remains unclear whether these tissue losses are a consequence of chronic illness and its treatment or an underlying phenotypic trait. Here, we use multicentre magnetic resonance imaging data to investigate orbitofrontal-striatal abnormalities across the SZ spectrum from healthy populations with high schizotypy to unmedicated and medicated first-episode psychosis (FEP), and patients with chronic SZ. Putamen, caudate, accumbens volume, and OFC thickness were estimated from T1-weighted images acquired in all 3 diagnostic groups and controls from 4 sites (n = 337). Results were first established in 1 discovery dataset and replicated in 3 independent samples. There was a negative correlation between apathy and putamen/accumbens volume only in healthy individuals with schizotypy; however, medicated patients exhibited larger putamen volume, which appears to be a consequence of antipsychotic medications. The negative association between reduced OFC thickness and total negative symptoms also appeared to vary along the SZ spectrum, being significant only in FEP patients. In schizotypy, there was increased OFC thickness relative to controls. Our findings suggest that negative symptoms are associated with a temporal continuum of orbitofrontal-striatal abnormalities that may predate the occurrence of SZ. Thicker OFC in schizotypy may represent either compensatory or pathological mechanisms prior to the disease onset.

Noradrenergic Source of Dopamine Assessed by Microdialysis in the Medial Prefrontal Cortex

Previous results indicate that dopamine (DA) release in the medial prefrontal cortex (mPFC) is modified by α₂ adrenoceptor- but not D2 DA receptor- agonists and antagonists, suggesting that DA measured by microdialysis in the mPFC originates from noradrenergic terminals. Accordingly, noradrenergic denervation was found to prevent α₂-receptor-mediated rise and fall of extracellular DA induced by atipamezole and clonidine, respectively, in the mPFC. The present study was aimed to determine whether DA released by dopaminergic terminals in the mPFC is not detected by in vivo microdialysis because is readily taken up by norepinephrine transporter (NET). Accordingly, the D2-antagonist raclopride increased the electrical activity of DA neurons in the ventral tegmental area (VTA) and enhanced extracellular DOPAC but failed to modify DA in the mPFC. However, in rats whose NET was either inactivated by nisoxetine or eliminated by noradrenergic denervation, raclopride still elevated extracellular DOPAC and activated dopaminergic activity, but also increased DA. Conversely, the D2-receptor agonist quinpirole reduced DOPAC but failed to modify DA in the mPFC in control rats. However, in rats whose NET was eliminated by noradrenergic denervation or inhibited by locally perfused nisoxetine, quinpirole maintained its ability to reduce DOPAC but acquired that of reducing DA. Moreover, raclopride and quinpirole, when locally perfused into the mPFC of rats subjected to noradrenergic denervation, were able to increase and decrease, respectively, extracellular DA levels, while being ineffective in control rats. Transient inactivation of noradrenergic neurons by clonidine infusion into the locus coeruleus, a condition where NET is preserved, was found to reduce extracellular NE and DA in the mPFC, whereas noradrenergic denervation, a condition where NET is eliminated, almost totally depleted extracellular NE but increased DA. Both transient inactivation and denervation of noradrenergic neurons were found to reduce the number of spontaneously active DA neurons and their bursting activity in the VTA. The results indicate that DA released in the mPFC by dopaminergic terminals is not detected by microdialysis unless DA clearance from extracellular space is inactivated. They support the hypothesis that noradrenergic terminals are the main source of DA measured by microdialysis in the mPFC during physiologically relevant activities.

Aberrant Salience, Information Processing, and Dopaminergic Signaling in People at Clinical High Risk for Psychosis

The aberrant salience hypothesis proposes that striatal dopamine dysregulation causes misattribution of salience to irrelevant stimuli leading to psychosis. Recently, new lines of preclinical evidence on information coding by subcortical dopamine coupled with computational models of the brain's ability to predict and make inferences about the world (predictive processing) provide a new perspective on this hypothesis. We review these and summarize the evidence for dopamine dysfunction, reward processing, and salience abnormalities in people at clinical high risk of psychosis (CHR) relative to findings in patients with psychosis. This review identifies consistent evidence for dysregulated subcortical dopamine function in people at CHR, but also indicates a number of areas where neurobiological processes are different in CHR subjects relative to patients with psychosis, particularly in reward processing. We then consider how predictive processing models may explain psychotic symptoms in terms of alterations in prediction error and precision signaling using Bayesian approaches. We also review the potential role of environmental risk factors, particularly early adverse life experiences, in influencing the prior expectations that individuals have about their world in terms of computational models of the progression from being at CHR to frank psychosis. We identify a number of key outstanding questions, including the relative roles of prediction error or precision signaling in the development of symptoms and the mechanism underlying dopamine dysfunction. Finally, we discuss how the integration of computational psychiatry with biological investigation may inform the treatment for people at CHR of psychosis.

Dopamine Receptor Subtypes, Physiology and Pharmacology: New Ligands and Concepts in Schizophrenia

Dopamine receptors are widely distributed within the brain where they play critical modulator roles on motor functions, motivation and drive, as well as cognition. The identification of five genes coding for different dopamine receptor subtypes, pharmacologically grouped as D1- (D1 and D5) or D2-like (D2S, D2L, D3, and D4) has allowed the demonstration of differential receptor function in specific neurocircuits. Recent observation on dopamine receptor signaling point at dopamine-glutamate-NMDA neurobiology as the most relevant in schizophrenia and for the development of new therapies. Progress in the chemistry of D1- and D2-like receptor ligands (agonists, antagonists, and partial agonists) has provided more selective compounds possibly able to target the dopamine receptors homo and heterodimers and address different schizophrenia symptoms. Moreover, an extensive evaluation of the functional effect of these agents on dopamine receptor coupling and intracellular signaling highlights important differences that could also result in highly differentiated clinical pharmacology. The review summarizes the recent advances in the field, addressing the relevance of emerging new targets in schizophrenia in particular in relation to the dopamine - glutamate NMDA systems interactions.

Looking into a Deluded Brain through a Neuroimaging Lens

Delusions are irrational, tenacious, and incorrigible false beliefs that are the most common symptom of a range of brain disorders including schizophrenia, Alzheimer's, and Parkinson's disease. In the case of schizophrenia and other primary delusional disorders, their appearance is often how the disorder is first detected and can be sufficient for diagnosis. At this time, not much is known about the brain dysfunctions leading to delusions, and hindering our understanding is that the complexity of the nature of delusions, and their very unique relevance to the human experience has hampered elucidation of their underlying neurobiology using either patients or animal models. Advances in neuroimaging along with improved psychiatric and cognitive modeling offers us a new opportunity to look with more investigative power into the deluded brain. In this article, based on data obtained from neuroimaging studies, we have attempted to draw a picture of the neural networks involved when delusion is present and evaluate whether different manifestations of delusions engage different regions of the brain.

Dopamine and glutamate in schizophrenia: biology, symptoms and treatment

Glutamate and dopamine systems play distinct roles in terms of neuronal signalling, yet both have been proposed to contribute significantly to the pathophysiology of schizophrenia. In this paper we assess research that has implicated both systems in the aetiology of this disorder. We examine 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 identified 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 findings, 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 influences may directly underlie glutamatergic dysfunction, few genetic risk variants directly implicate the dopamine system, indicating that aberrant dopamine 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 field, 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.

Heterogeneity of Striatal Dopamine Function in Schizophrenia: Meta-analysis of Variance

BACKGROUND

It has been hypothesized that dopamine function in schizophrenia exhibits heterogeneity in excess of that seen in the general population. However, no previous study has systematically tested this hypothesis.

METHODS

We employed meta-analysis of variance to investigate interindividual variability of striatal dopaminergic function in patients with schizophrenia and in healthy control subjects. We included 65 studies that reported molecular imaging measures of dopamine synthesis or release capacities, dopamine D_2/3 receptor (D2/3R) or dopamine transporter (DAT) availabilities, or synaptic dopamine levels in 983 patients and 968 control subjects. Variability differences were quantified using variability ratio (VR) and coefficient of variation ratio.

RESULTS

Interindividual variability of striatal D2/3R (VR = 1.26, p < .0001) and DAT (VR = 1.31, p = .01) availabilities and synaptic dopamine levels (VR = 1.38, p = .045) but not dopamine synthesis (VR = 1.12, p = .13) or release (VR = 1.08, p = .70) capacities were significantly greater in patients than in control subjects. Findings were robust to variability measure. Mean dopamine synthesis (g = 0.65, p = .004) and release (g = 0.66, p = .03) capacities, as well as synaptic levels (g = 0.78, p = .0006), were greater in patients overall, but mean synthesis capacity did not differ from that of control subjects in treatment-resistant patients (p > .3). Mean D2/3R (g = 0.17, p = .14) and DAT (g = -0.20, p = .28) availabilities did not differ between groups.

CONCLUSIONS

Our findings demonstrate significant heterogeneity of striatal dopamine function in schizophrenia. They suggest that while elevated dopamine synthesis and release capacities may be core features of the disorder, altered D2/3R and DAT availabilities and synaptic dopamine levels may occur only in a subgroup of patients. This heterogeneity may contribute to variation in treatment response and side effects.

D1-Dopamine Receptor Availability in First-Episode Neuroleptic Naive Psychosis Patients

BACKGROUND

Positron emission tomography studies examining differences in D1-dopamine receptor binding between control subjects and patients with schizophrenia have been inconsistent, reporting higher, lower, and no difference in the frontal cortex. Exposure to antipsychotic medication has been suggested to be a likely source of this heterogeneity, and thus there is a need for studies of patients at early stages of the disorder who have not been exposed to such drugs.

METHODS

Here, we compared 17 healthy control subjects and 18 first-episode neuroleptic naive patients with schizophrenia or schizophreniform psychosis using positron emission tomography and the D1-dopamine receptor radioligand [11C]SCH23390.

RESULTS

We observed a statistically significant difference in the dorsolateral prefrontal cortex. Contrary to our expectations, patients had less D1-dopamine receptor availability with a moderate effect size. In a Bayesian analysis, we show that the data are over 50 times more likely to have occurred under the decrease as opposed to the increase hypothesis. This effect was not global, as our analysis showed that the null hypothesis was preferred over either hypothesis in the striatum.

CONCLUSIONS

This investigation represents the largest single sample of neuroleptic-naive patients examined for D1-dopamine receptor availability using PET and suggests a reduction of prefrontal D1-dopamine receptor density in the pathophysiology of schizophrenia. However, further work will be required to reach a consensus.