From "bedside" to "bench" and back: A translational approach to studying dopamine dysfunction in schizophrenia

THPP-1 PDE10A inhibitor reverses the cognitive deficits and hyperdopaminergic state in a neurodevelopment model of schizophrenia

Schizophrenia (SCZ) is a complex neuropsychiatric disorder characterized by positive, negative, and cognitive symptoms. The neurodevelopmental methylazoxy-methanol acetate (MAM) rodent model replicates key neurobiological features of SCZ which includes hyperdopaminergic states in the ventral tegmental area (VTA) and cognitive deficits. Typical and atypical antipsychotics are primarily effective in treating the positive symptoms of SCZ but often fall short of addressing cognitive deficits. A promising therapeutic approach for treating all symptoms of SCZ has emerged through the inhibition of phosphodiesterase 10 A (PDE10A). Our study aim was to investigate the impact of acute and chronic THPP-1 (PDE10A inhibitor) treatment, in MAM rats, focusing on cognitive deficits and VTA dopamine (DA) activity. Adult offspring of pregnant rats treated with Saline or MAM (20 mg/kg) on gestational day 17 were treated with THPP-1 acutely (male/female rats; 3 mg/kg) at postnatal day (PD) 70-80 or chronically (males; 3 weeks; 2-3 mg/kg) from PD 70-91 and tested in the novel object recognition test and electrophysiological recording of DA neurons in the VTA. Acute THPP-1 treatment reversed cognitive impairments and normalized the increased number of active DA neurons in the VTA of male and female MAM rats, without affecting control rats. Also, chronic THPP-1 treatment reversed cognitive deficits and normalized DA hyperactivity in the VTA of male MAM rats. The efficacy of THPP-1 in reversing MAM-induced impairments underscores its ability to target disease-specific circuitry without affecting normal regulated systems in control rats. Our findings highlight the therapeutic potential of THPP-1 for addressing cognitive deficits and DA dysregulation in SCZ.

Enhanced cognitive flexibility and phasic striatal dopamine dynamics in a mouse model of low striatal tonic dopamine

The catecholamine neuromodulators dopamine and norepinephrine are implicated in motor function, motivation, and cognition. Although roles for striatal dopamine in these aspects of behavior are well established, the specific roles for cortical catecholamines in regulating striatal dopamine dynamics and behavior are less clear. We recently showed that elevating cortical dopamine but not norepinephrine suppresses hyperactivity in dopamine transporter knockout (DAT-KO) mice, which have elevated striatal dopamine levels. In contrast, norepinephrine transporter knockout (NET-KO) mice have a phenotype distinct from DAT-KO mice, as they show elevated extracellular cortical catecholamines but reduced baseline striatal dopamine levels. Here we evaluated the consequences of altered catecholamine levels in NET-KO mice on cognitive flexibility and striatal dopamine dynamics. In a probabilistic reversal learning task, NET-KO mice showed enhanced reversal learning, which was consistent with larger phasic dopamine transients (dLight) in the dorsomedial striatum (DMS) during reward delivery and reward omission, compared to WT controls. Selective depletion of dorsal medial prefrontal cortex (mPFC) norepinephrine in WT mice did not alter performance on the reversal learning task but reduced nestlet shredding. Surprisingly, NET-KO mice did not show altered breakpoints in a progressive ratio task, suggesting intact food motivation. Collectively, these studies show novel roles of cortical catecholamines in the regulation of tonic and phasic striatal dopamine dynamics and cognitive flexibility, updating our current views on dopamine regulation and informing future therapeutic strategies to counter multiple psychiatric disorders.

Consensus molecular environment of schizophrenia risk genes in coexpression networks shifting across age and brain regions

Schizophrenia is a neurodevelopmental brain disorder whose genetic risk is associated with shifting clinical phenomena across the life span. We investigated the convergence of putative schizophrenia risk genes in brain coexpression networks in postmortem human prefrontal cortex (DLPFC), hippocampus, caudate nucleus, and dentate gyrus granule cells, parsed by specific age periods (total N = 833). The results support an early prefrontal involvement in the biology underlying schizophrenia and reveal a dynamic interplay of regions in which age parsing explains more variance in schizophrenia risk compared to lumping all age periods together. Across multiple data sources and publications, we identify 28 genes that are the most consistently found partners in modules enriched for schizophrenia risk genes in DLPFC; twenty-three are previously unidentified associations with schizophrenia. In iPSC-derived neurons, the relationship of these genes with schizophrenia risk genes is maintained. The genetic architecture of schizophrenia is embedded in shifting coexpression patterns across brain regions and time, potentially underwriting its shifting clinical presentation.

The effects of social isolation stress and discrimination on mental health

Social isolation and discrimination are growing public health concerns associated with poor physical and mental health. They are risk factors for increased morbidity and mortality and reduced quality of life. Despite their detrimental effects on health, there is a lack of knowledge regarding translation across the domains of experimental research, clinical studies, and real-life applications. Here, we review and synthesize evidence from basic research in animals and humans to clinical translation and interventions. Animal models indicate that social separation stress, particularly in early life, activates the hypothalamic-pituitary-adrenal axis and interacts with monoaminergic, glutamatergic, and GABAergic neurotransmitter systems, inducing long-lasting reductions in serotonin turnover and alterations in dopamine receptor sensitivity. These findings are of particular importance for human social isolation stress, as effects of social isolation stress on the same neurotransmitter systems have been implicated in addictive, psychotic, and affective disorders. Children may be particularly vulnerable due to lasting effects of social isolation and discrimination stress on the developing brain. The effects of social isolation and loneliness are pronounced in the context of social exclusion due to discrimination and racism, during widespread infectious disease related containment strategies such as quarantine, and in older persons due to sociodemographic changes. This highlights the importance of new strategies for social inclusion and outreach, including gender, culture, and socially sensitive telemedicine and digital interventions for mental health care.

Using animal models for the studies of schizophrenia and depression: The value of translational models for treatment and prevention

Animal models of psychiatric disorders have been highly effective in advancing the field, identifying circuits related to pathophysiology, and identifying novel therapeutic targets. In this review, we show how animal models, particularly those based on development, have provided essential information regarding circuits involved in disorders, disease progression, and novel targets for intervention and potentially prevention. Nonetheless, in recent years there has been a pushback, largely driven by the US National Institute of Mental Health (NIMH), to shift away from animal models and instead focus on circuits in normal subjects. This has been driven primarily from a lack of discovery of new effective therapeutic targets, and the failure of targets based on preclinical research to show efficacy. We discuss why animal models of complex disorders, when strongly cross-validated by clinical research, are essential to understand disease etiology as well as pathophysiology, and direct new drug discovery. Issues related to shortcomings in clinical trial design that confound translation from animal models as well as the failure to take patient pharmacological history into account are proposed to be a source of the failure of what are likely effective compounds from showing promise in clinical trials.

Chronic N-Acetylcysteine Treatment Prevents Amphetamine-Induced Hyperactivity in Heterozygous Disc1 Mutant Mice, a Putative Prodromal Schizophrenia Animal Model

Symptoms of schizophrenia (SZ) typically emerge during adolescence to young adulthood, which gives a window before full-blown psychosis for early intervention. Strategies for preventing the conversion from the prodromal phase to the psychotic phase are warranted. Heterozygous (Het) Disc1 mutant mice are considered a prodromal model of SZ, suitable for studying psychotic conversion. We evaluated the preventive effect of chronic N-acetylcysteine (NAC) administration, covering the prenatal era to adulthood, on the reaction following the Amph challenge, which mimics the outbreak or conversion of psychosis, in adult Het Disc1 mice. Biochemical and morphological features were examined in the striatum of NAC-treated mice. Chronic NAC treatment normalized the Amph-induced activity in the Het Disc1 mice. Furthermore, the striatal phenotypes of Het Disc1 mice were rescued by NAC including dopamine receptors, the expression of GSK3s, MSN dendritic impairments, and striatal PV density. The current study demonstrated a potent preventive effect of chronic NAC treatment in Disc1 Het mice on the acute Amph test, which mimics the outbreak of psychosis. Our findings not only support the benefit of NAC as a dietary supplement for SZ prodromes, but also advance our knowledge of striatal dopamine receptors, PV neurons, and GSK3 signaling pathways as therapeutic targets for treating or preventing the pathogenesis of mental disorders.

Activity-State Dependent Reversal of Ketamine-Induced Resting State EEG Effects by Clozapine and Naltrexone in the Freely Moving Rat

Ketamine is a non-competitive N-Methyl-D-aspartate receptor (NMDAR) antagonist used in the clinic to initiate and maintain anaesthesia; it induces dissociative states and has emerged as a breakthrough therapy for major depressive disorder. Using local field potential recordings in freely moving rats, we studied resting state EEG profiles induced by co-administering ketamine with either: clozapine, a highly efficacious antipsychotic; or naltrexone, an opioid receptor antagonist reported to block the acute antidepressant effects of ketamine. As human electroencephalography (EEG) is predominantly recorded in a passive state, head-mounted accelerometers were used with rats to determine active and passive states at a high temporal resolution to offer the highest translatability. In general, pharmacological effects for the three drugs were more pronounced in (or restricted to) the passive state. Specifically, during inactive periods clozapine induced increases in delta (0.1-4 Hz), gamma (30-60 Hz) and higher frequencies (>100 Hz). Importantly, it reversed the ketamine-induced reduction in low beta power (10-20 Hz) and potentiated ketamine-induced increases in gamma and high frequency oscillations (130-160 Hz). Naltrexone inhibited frequencies above 50 Hz and significantly reduced the ketamine-induced increase in high frequency oscillations. However, some frequency band changes, such as clozapine-induced decreases in delta power, were only seen in locomoting rats. These results emphasise the potential in differentiating between activity states to capture drug effects and translate to human resting state EEG. Furthermore, the differential reversal of ketamine-induced EEG effects by clozapine and naltrexone may have implications for the understanding of psychotomimetic as well as rapid antidepressant effects of ketamine.

It's in the Timing: Reduced Temporal Precision in Neural Activity of Schizophrenia

Studies of perception and cognition in schizophrenia (SCZ) show neuronal background noise (ongoing activity) to intermittently overwhelm the processing of external stimuli. This increased noise, relative to the activity evoked by the stimulus, results in temporal imprecision and higher variability of behavioral responses. What, however, are the neural correlates of temporal imprecision in SCZ behavior? We first report a decrease in electroencephalography signal-to-noise ratio (SNR) in two SCZ datasets and tasks in the broadband (1-80 Hz), theta (4-8 Hz), and alpha (8-13 Hz) bands. SCZ participants also show lower inter-trial phase coherence (ITPC)-consistency over trials in the phase of the signal-in theta. From these ITPC results, we varied phase offsets in a computational simulation, which illustrated phase-based temporal desynchronization. This modeling also provided a necessary link to our results and showed decreased neural synchrony in SCZ in both datasets and tasks when compared with healthy controls. Finally, we showed that reduced SNR and ITPC are related and showed a relationship to temporal precision on the behavioral level, namely reaction times. In conclusion, we demonstrate how temporal imprecision in SCZ neural activity-reduced relative signal strength and phase coherence-mediates temporal imprecision on the behavioral level.

Effects of Radiofrequency Electromagnetic Radiation on Neurotransmitters in the Brain

With the rapid development of electronic information in the past 30 years, technical achievements based on electromagnetism have been widely used in various fields pertaining to human production and life. Consequently, electromagnetic radiation (EMR) has become a substantial new pollution source in modern civilization. The biological effects of EMR have attracted considerable attention worldwide. The possible interaction of EMR with human organs, especially the brain, is currently where the most attention is focused. Many studies have shown that the nervous system is an important target organ system sensitive to EMR. In recent years, an increasing number of studies have focused on the neurobiological effects of EMR, including the metabolism and transport of neurotransmitters. As messengers of synaptic transmission, neurotransmitters play critical roles in cognitive and emotional behavior. Here, the effects of EMR on the metabolism and receptors of neurotransmitters in the brain are summarized.

The chemical tools for imaging dopamine release

Dopamine is a modulatory neurotransmitter involved in learning, motor functions, and reward. Many neuropsychiatric disorders, including Parkinson's disease, autism, and schizophrenia, are associated with imbalances or dysfunction in the dopaminergic system. Yet, our understanding of these pervasive public health issues is limited by our ability to effectively image dopamine in humans, which has long been a goal for chemists and neuroscientists. The last two decades have witnessed the development of many molecules used to trace dopamine. We review the small molecules, nanoparticles, and protein sensors used with fluorescent microscopy/photometry, MRI, and PET that shape dopamine research today. None of these tools observe dopamine itself, but instead harness the biology of the dopamine system-its synthetic and metabolic pathways, synaptic vesicle cycle, and receptors-in elegant ways. Their advantages and weaknesses are covered here, along with recent examples and the chemistry and biology that allow them to function.

Interaction of Ligands for PET with the Dopamine D3 Receptor: In Silico and In Vitro Methods

[¹⁸F]Fallypride and [¹⁸F]Fluortriopride (FTP) are two different PET radiotracers that bind with sub-nanomolar affinity to the dopamine D3 receptor (D₃R). In spite of their similar D₃ affinities, the two PET ligands display very different properties for labeling the D₃R in vivo: [¹⁸F]Fallypride is capable of binding to D₃R under "baseline" conditions, whereas [¹⁸F]FTP requires the depletion of synaptic dopamine in order to image the receptor in vivo. These data suggest that [¹⁸F]Fallypride is able to compete with synaptic dopamine for binding to the D₃R, whereas [¹⁸F]FTP is not. The goal of this study was to conduct a series of docking and molecular dynamic simulation studies to identify differences in the ability of each molecule to interact with the D₃R that could explain these differences with respect to competition with synaptic dopamine. Competition studies measuring the ability of each ligand to compete with dopamine in the β-arrestin assay were also conducted. The results of the in silico studies indicate that FTP has a weaker interaction with the orthosteric binding site of the D₃R versus that of Fallypride. The results of the in silico studies were also consistent with the IC50 values of each compound in the dopamine β-arrestin competition assays. The results of this study indicate that in silico methods may be able to predict the ability of a small molecule to compete with synaptic dopamine for binding to the D₃R.

Innovative screening models for the discovery of new schizophrenia drug therapies: an integrated approach

Introduction: Schizophrenia is a severe psychiatric disorder affecting millions worldwide. However, available treatment options do not fully address the disease. Whereas current antipsychotics may control psychotic symptoms, they seem notoriously ineffective in improving negative and cognitive symptoms or in preventing functional decline. As the etiology of schizophrenia eludes us, the development of valid animal models for screening new drug targets appears to be a strenuous task.Areas covered: In this review, the authors present the key concepts that validate animal models of schizophrenia, as well as the different screening approaches for novel schizophrenia treatments. The models covered are either based on major neurotransmitter systems or neurodevelopmental, immune, and genetic approaches.Expert opinion: Sadly, due to inertia, research focuses on developing 'anti-psychotics', instead of 'anti-schizophrenia' drugs that would tackle the entire syndrome of schizophrenia. Whereas no perfect model may ever exist, combining different experimental designs may enhance validity, as the over-reliance on a single model is inappropriate. Multi-model approaches incorporating vulnerability, the 'two-hit' hypothesis, and endophenotypes offer a promise for developing new strategies for schizophrenia treatment. Forward and reverse translation between preclinical and clinical research will increase the probability of success and limit failures in drug development.

Stem cell-based models and therapies: a key approach into schizophrenia treatment

Psychiatric disorders such as schizophrenia can generate distress and disability along with heavy costs on individuals and health care systems. Different genetic and environmental factors play a pivotal role in the appearance of the mentioned disorders. Since the conventional treatment options for psychiatric disorders are suboptimal, investigators are trying to find novel strategies. Herein, stem cell therapies have been recommended as novel choices. In this context, the preclinical examination of stem cell-based therapies specifically using appropriate models can facilitate passing strong filters and serious examination to ensure proper quality and safety of them as a novel treatment approach. Animal models cannot be adequately helpful to follow pathophysiological features. Nowadays, stem cell-based models, particularly induced pluripotent stem cells reflected as suitable alternative models in this field. Accordingly, the importance of stem cell-based models, especially to experiment with the regenerative medicine outcomes for schizophrenia as one of the severe typing of psychiatric disorders, is addressed here.

Retrograde Labeling Illuminates Distinct Topographical Organization of D1 and D2 Receptor-Positive Pyramidal Neurons in the Prefrontal Cortex of Mice

The cortex plays an important role in regulating motivation and cognition, and does so by regulating multiple subcortical brain circuits. Glutamatergic pyramidal neurons in the prefrontal cortex (PFC) are topographically organized in different subregions such as the prelimbic, infralimbic (IL), and orbitofrontal and project to topographically-organized subcortical target regions. Dopamine D1 and D2 receptors are expressed on glutamatergic pyramidal neurons in the PFC. However, it is unclear whether D1 and D2 receptor-expressing pyramidal neurons in the PFC are also topographically organized. We used a retrograde adeno-associated virus (AAVRG)-based approach to illuminate the topographical organization of D1 and D2 receptor-expressing neurons, projecting to distinct striatal and midbrain subregions. Our experiments reveal that AAVRG injection in the nucleus accumbens (NAcc) or dorsal striatum (dSTR) of D1Cre mice labeled distinct neuronal subpopulations in medial orbitofrontal or prelimbic PFC, respectively. However, AAVRG injection in NAcc or dSTR of D2Cre mice labeled medial orbitofrontal, but not medial prelimbic PFC, respectively. Additionally, D2R+ but not D1R+ PFC neurons were labeled on injection of AAVRG in substantia nigra pars compacta (SNpc). Thus, our data are the first to highlight a unique dopamine receptor-specific topographical pattern in the PFC, which could have profound implications for corticostriatal signaling in the basal ganglia.

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.

Hypofunctional Dopamine Uptake and Antipsychotic Treatment-Resistant Schizophrenia

Antipsychotic treatment resistance in schizophrenia remains a major issue in psychiatry. Nearly 30% of patients with schizophrenia do not respond to antipsychotic treatment, yet the underlying neurobiological causes are unknown. All effective antipsychotic medications are thought to achieve their efficacy by targeting the dopaminergic system. Here we review early literature describing the fundamental mechanisms of antipsychotic drug efficacy, highlighting mechanistic concepts that have persisted over time. We then reconsider the original framework for understanding antipsychotic efficacy in light of recent advances in our scientific understanding of the dopaminergic effects of antipsychotics. Based on these new insights, we describe a role for the dopamine transporter in the genesis of both antipsychotic therapeutic response and primary resistance. We believe that this discussion will help delineate the dopaminergic nature of antipsychotic treatment-resistant schizophrenia.