Dysfunctional brain networks and genetic risk for schizophrenia: specific neurotransmitter systems

Shared Genetic Determinants of Schizophrenia and Autism Spectrum Disorder Implicate Opposite Risk Patterns: A Genome-Wide Analysis of Common Variants

BACKGROUND AND HYPOTHESIS

The synaptic pruning hypothesis posits that schizophrenia (SCZ) and autism spectrum disorder (ASD) may represent opposite ends of neurodevelopmental disorders: individuals with ASD exhibit an overabundance of synapses and connections while SCZ was characterized by excessive pruning of synapses and a reduction. Given the strong genetic predisposition of both disorders, we propose a shared genetic component, with certain loci having differential regulatory impacts.

STUDY DESIGN

Genome-Wide single nucleotide polymorphism (SNP) data of European descent from SCZ (N cases = 53 386, N controls = 77 258) and ASD (N cases = 18 381, N controls = 27 969) were analyzed. We used genetic correlation, bivariate causal mixture model, conditional false discovery rate method, colocalization, Transcriptome-Wide Association Study (TWAS), and Phenome-Wide Association Study (PheWAS) to investigate the genetic overlap and gene expression pattern.

STUDY RESULTS

We found a positive genetic correlation between SCZ and ASD (rg = .26, SE = 0.01, P = 7.87e-14), with 11 genomic loci jointly influencing both conditions (conjFDR <0.05). Functional analysis highlights a significant enrichment of shared genes during early to mid-fetal developmental stages. A notable genetic region on chromosome 17q21.31 (lead SNP rs2696609) showed strong evidence of colocalization (PP.H4.abf = 0.85). This SNP rs2696609 is linked to many imaging-derived brain phenotypes. TWAS indicated opposing gene expression patterns (primarily pseudogenes and long noncoding RNAs [lncRNAs]) for ASD and SCZ in the 17q21.31 region and some genes (LRRC37A4P, LINC02210, and DND1P1) exhibit considerable variation in the cerebellum across the lifespan.

CONCLUSIONS

Our findings support a shared genetic basis for SCZ and ASD. A common genetic variant, rs2696609, located in the Chr17q21.31 locus, may exert differential risk regulation on SCZ and ASD by altering brain structure. Future studies should focus on the role of pseudogenes, lncRNAs, and cerebellum in synaptic pruning and neurodevelopmental disorders.

Schizophrenia and disruption of circadian rhythms: An overview of genetic, metabolic and clinical signs

A molecular clock in the suprachiasmatic nucleus of the anterior hypothalamus, which is entrained by the dark-light cycle and controls the sleep-wake cycle, regulates circadian rhythms. The risk of developing mental disorders, such as schizophrenia, has long been linked to sleep abnormalities. Additionally, a common aspect of mental disorders is sleep disturbance, which has a direct impact on the intensity of the symptoms and the quality of life of the patient. This relationship can be explained by gene alterations such as CLOCK in schizophrenia which are also important components of the physiological circadian rhythm. The function of dopamine and adenosine in circadian rhythm should also be noted, as these hypotheses are considered to be the most popular theories explaining schizophrenia pathogenesis. Therefore, determining the presence of a causal link between the two can be key to identifying new potential targets in schizophrenia therapy, which can open new avenues for clinical research as well as psychiatric care. We review circadian disruption in schizophrenia at the genetic, metabolic, and clinical levels. We summarize data about clock and clock-controlled genes' alterations, neurotransmitter systems' impairments, and association with chronotype in schizophrenia patients. Our findings demonstrate that in schizophrenia either homeostatic or circadian processes of sleep regulation are disturbed. Also, we found an insufficient number of studies aimed at studying the relationship between known biological phenomena of circadian disorders and clinical signs of schizophrenia.

Paeonol, the active component of Cynanchum paniculatum, ameliorated schizophrenia-like behaviors by regulating the PI3K-Akt-GSK3β-NF-κB signalling pathway in MK-801-treated mice

ETHNOPHARMACOLOGICAL RELEVANCE

Cynanchum paniculatum (Bunge) Kitag. ex H. Hara (Asclepiadaceae) have been traditionally used in East Asia as analgesic or antiviral agents. Interestingly, some Chinese and Korean traditional medicinal books reported that the use of C. paniculatum in the treatment of psychotic symptoms, such as hallucinations and delusions.

AIM OF THE STUDY

In this study, we aimed to investigate whether C. paniculatum could improve sensorimotor gating disruption in mice with MK-801-induced schizophrenia-like behaviors. We also aimed to identify the active component of C. paniculatum that could potentially serve as a treatment for schizophrenia and found that paeonol, the major constituent compound of C. paniculatum, showed potential as a treatment for schizophrenia.

MATERIALS AND METHODS

To assess the effect of paeonol on mice with MK-801-induced schizophrenia-like behaviors, we carried out a series of behavioral tests related with symptoms of schizophrenia. In addition, we utilized Western blotting and ELISA techniques to investigate the antipsychotic actions of paeonol.

RESULT

C. paniculatum extract (100 or 300 mg/kg) and paenol (10 or 30 mg/kg) significantly reversed MK-801-induced prepulse deficits in acoustic startle response test. In addition, paeonol (10 or 30 mg/kg) attenuated social novelty preference and novel object recognition memory on MK-801-induced schizophrenia-like behaviour in mice. Furthermore, the phosphorylation levels of PI3K, Akt, GSK3β and NF-κB, as well as related pro-inflammatory cytokine, such as IL-1β and TNF-α, were significantly reversed by the administration of paeonol (10 or 30 mg/kg) in the prefrontal cortex of MK-801-treated mice.

CONCLUSIONS

Collectively, these data show that paeonol can potentially be used as an agent for treating sensorimotor gating deficits, negative symptoms, and cognitive deficits, such as those observed in schizophrenia with few adverse effects.

Are Essential Trace Elements Effective in Modulation of Mental Disorders? Update and Perspectives

The emergence of mental disorders is associated with several risk factors including genetic and environmental susceptibility. A group of nutrients serves an especially important role in a number of essential neurodevelopmental processes through brain areas promoting the high degree of brain metabolism during early life, although almost all nutrients are needed. These include macronutrients and micronutrients (e.g., iron, magnesium, zinc, copper, selenium). Numerous nutritional psychiatry trials have been performed to examine the correlation of many individual nutrients with mental health, such as essential trace elements. The increased accumulation or lack of such components will facilitate an alternative metabolic pathway that can lead to many diseases and conditions of neurodevelopment. Mental functions have biochemical bases, so the impairment of such neurochemical mechanisms due to lack of trace elements can have mental effects. In psychological conditions such as depression, anxiety, schizophrenia, and autism, scientific studies demonstrate the putative role of trace element deficiency. Therefore, given the critical roles played by essential trace elements in the neurodevelopment and mental health, the effect of these elements' intake on the modulation of psychological functioning is reviewed.

Familial abnormalities of endocannabinoid signaling in schizophrenia

OBJECTIVES

Epidemiological and experimental evidence suggests that the endocannabinoid system plays a pathophysiological role in schizophrenia. This is reflected by elevated cerebrospinal levels of the endocannabinoid anandamide in schizophrenia and its initial prodromal states.

METHODS

We analyzed plasma concentrations of anandamide, 2-arachidonoyl-sn-glycerol, palmitoylethanolamide and oleoylethanolamide from 25 twin pairs discordant for schizophrenia, six discordant for bipolar disorder and eight healthy twin pairs to determine hereditary traits.

RESULTS

Twin pairs discordant for schizophrenia or bipolar disorder had significantly higher levels of anandamide and palmitoylethanolamide compared to healthy twins (both P < 0.002). Non-affected twins discordant for schizophrenia, who developed a psychotic disorder within 5 years follow-up showed lower anandamide (P = 0.042) and 2-arachidonoyl-sn-glycerol levels (P = 0.049) than twins who remained healthy.

CONCLUSIONS

We suggest that the protective upregulation of endocannabinoid signalling reflects either a hereditary trait or mirrors a modulating response to genetically influenced cerebral function involving, e.g., other neurotransmitters or energy metabolism.

Mass spectrometry imaging of amino neurotransmitters: a comparison of derivatization methods and application in mouse brain tissue

The detection of small polar compounds such as amino neurotransmitters by MALDI mass spectrometry imaging has been hindered by low-detection sensitivity and background interferences. Recently, several of on-tissue chemical derivatization strategies have been independently reported that enable their detection. Here, we present a comparison between these methods, and demonstrate the visualization of the distributions of up to 23 amino metabolites in tissue. We applied this methodology to detect alterations of these compounds after inducing an experimental cortical spreading depression in mouse brain, which causes profound transient alterations in key neurotransmitters in one hemisphere and is relevant for migraine and various other neurological disorders.

GC-MS based metabolomics identification of possible novel biomarkers for schizophrenia in peripheral blood mononuclear cells

Schizophrenia is a debilitating mental disorder. Currently, the lack of disease biomarkers to support objective laboratory tests constitutes a bottleneck in the clinical diagnosis of schizophrenia. Here, a gas chromatography-mass spectrometry (GC-MS) based metabolomic approach was applied to characterize the metabolic profile of schizophrenia subjects (n = 69) and healthy controls (n = 85) in peripheral blood mononuclear cells (PBMCs) to identify and validate biomarkers for schizophrenia. Multivariate statistical analysis was used to visualize group discrimination and to identify differentially expressed metabolites in schizophrenia subjects relative to healthy controls. The multivariate statistical analysis demonstrated that the schizophrenia group was significantly distinguishable from the control group. In total, 18 metabolites responsible for the discrimination between the two groups were identified. These differential metabolites were mainly involved in energy metabolism, oxidative stress and neurotransmitter metabolism. A simplified panel of PBMC metabolites consisting of pyroglutamic acid, sorbitol and tocopherol-α was identified as an effective diagnostic tool, yielding an area under the receiver operating characteristic curve (AUC) of 0.82 in the training samples (45 schizophrenia subjects and 50 healthy controls) and 0.71 in the test samples (24 schizophrenic patients and 35 healthy controls). Taken together, these findings help to develop diagnostic tools for schizophrenia.

Pleckstrin homology domain containing 6 protein (PLEKHA6) polymorphisms are associated with psychopathology and response to treatment in schizophrenic patients

Pleckstrin homology domain (PH domain) comprises approximately 120 amino acids and is integrated in a wide range of proteins involved in intracellular signaling or as constituents of the cytoskeleton. This domain can bind phosphatidylinositol (3,4,5)-triphosphate and phosphatidylinositol (4,5)-biphosphate and proteins such as the βγ-subunits of heterotrimeric G proteins and protein kinase C. Associations with psychiatric diseases have not been investigated yet. To identify genes involved in response to antipsychotics, mice were treated with haloperidol (1mg/kg, n = 11) or saline (n = 12) for one week. By analyzing microarray data, we observed an increase of pleckstrin homology domain containing 6 (PLEKHA6) gene expression. Furthermore, we genotyped 263 schizophrenic patients, who were treated monotherapeutically with different antipsychotics within randomized-controlled trials. Psychopathology was measured weekly using the PANSS for a minimum of four and a maximum of twelve weeks. Correlations between PANSS subscale scores at baseline and PANSS improvement scores after four weeks of treatment and genotypes were calculated by using a linear model for all investigated SNPs. We found associations between four PLEKHA6 polymorphisms (rs17333933 (T/G), rs3126209 (C/T), rs4951338 (A/G) and rs100900571 (T/C)) and different PANSS subscales at baseline. Furthermore two different polymorphisms (rs7513240 (T/C), rs4951353 (A/G)) were found to be associated with therapy response in terms of a significant correlation with different PANSS improvement subscores after four weeks of antipsychotic treatment. Our observation of an association between genetic polymorphisms of a protein of the PH domain and psychopathology data in schizophrenic patients might be indicative for an involvement of PLEKHA6 in the pathophysiology of schizophrenia and the therapy response towards antipsychotics.

Clozapine effects on adenylyl cyclase activity and serotonin type 1A receptors in human brain post-mortem

Although the pharmacological profile of the atypical antipsychotic clozapine has been extensively studied in animal models, little information is available on its effects in the human brain. In particular, much interest is focused on the understanding of clozapine activity on serotonin (5-HT) neurotransmission, particularly on 5-HT receptor of type 1A (5-HT(1A)) that seems to play a pivotal role in the control of the 5-HT system. The present work, therefore, aimed at evaluating the effects of clozapine and its major metabolite, norclozapine, on the modulation of adenylyl cyclase (AC) velocity via 5-HT(1A) receptors in human post-mortem brain regions, in particular the prefrontal cortex, hippocampus and raphe nuclei. Concomitantly, the ability of the two compounds to displace the specific binding of the 5-HT(1A) receptor agonist [³H]-8-hydroxy-(2-di-N-propylamino) tetralin ([³H]-8-OH-DPAT) was evaluated in the same brain areas. The results showed that both clozapine and norclozapine, although with a 20-fold lower affinity, displaced [³H]8-OH-DPAT binding in all of the brain regions analysed, suggesting their interaction with 5-HT(1A) receptors. At the same time, clozapine and, to a lesser extent, norclozapine were found to inhibit the forskolin (FK)-stimulated AC system, while decreasing cyclic adenosine monophosphate (cAMP) concentrations in the hippocampus only. The receptor characterisation of the clozapine effect on AC observed in the hippocampus by the use of antagonists showed a mixed profile, involving not only the 5-HT(1A) receptor but also a muscarinic (M) receptor subtype, most likely the M₄ one. These findings, while considering all the limitations due to the use of post-mortem tissues, are strongly suggestive of a region-dependent pharmacological action of clozapine in the human brain that may explain its peculiar clinical effects and open up research towards novel targets for future antipsychotic drugs.

MicroRNAs in neuronal communication

MicroRNAs (miRNAs) are short nucleotides sequences that regulate the expression of genes in different eukaryotic cell types. A tremendous amount of knowledge on miRNAs has rapidly accumulated over the last few years, revealing the growing interest in this field of research. On the other hand, clarifying the physiological regulation of gene expression in the central nervous system is important for establishing a reference for comparison to the diseased state. It is well known that the fine tuning of neuronal networks relies on intricate molecular mechanisms, such as the adjustment of the synaptic transmission. As determined by recent studies, regulation of neuronal interactions by miRNAs has critical consequences in the development, adaptation to ambient demands, and degeneration of the nervous system. In contrast, activation of synaptic receptors triggers downstream signaling cascades that generate a vast array of effects, which includes the regulation of novel genes involved in the control of the miRNA life cycle. In this review, we have examined the hot topics on miRNA gene-regulatory activities in the broad field of neuronal communication-related processes. Furthermore, in addition to indicating the newly described effect of miRNAs on the regulation of specific neurotransmitter systems, we have pointed out how these systems affect the expression, transport, and stability of miRNAs. Moreover, we discuss newly described and under-investigation mechanisms involving the intercellular transfer of miRNAs, aided by exosomes and gap junctions. Thus, in the current review, we were able to highlight recent findings related to miRNAs that indisputably contributed towards the understanding of the nervous system in health and disease.

Epigenetics, stress, and their potential impact on brain network function: a focus on the schizophrenia diatheses

The recent sociodevelopmental cognitive model of schizophrenia/psychosis is a highly influential and compelling compendium of research findings. Here, we present logical extensions to this model incorporating ideas drawn from epigenetic mediation of psychiatric disease, and the plausible effects of epigenetics on the emergence of brain network function and dysfunction in adolescence. We discuss how gene-environment interactions, effected by epigenetic mechanisms, might in particular mediate the stress response (itself heavily implicated in the emergence of schizophrenia). Next, we discuss the plausible relevance of this framework for adolescent genetic risk populations, a risk group characterized by vexing and difficult-to-explain heterogeneity. We then discuss how exploring relationships between epigenetics and brain network dysfunction (a strongly validated finding in risk populations) can enhance understanding of the relationship between stress, epigenetics, and functional neurobiology, and the relevance of this relationship for the eventual emergence of schizophrenia/psychosis. We suggest that these considerations can expand the impact of models such as the sociodevelopmental cognitive model, increasing their explanatory reach. Ultimately, integration of these lines of research may enhance efforts of early identification, intervention, and treatment in adolescents at-risk for schizophrenia.

Allosteric modulators for the treatment of schizophrenia: targeting glutamatergic networks

Schizophrenia is a highly debilitating mental disorder which afflicts approximately 1% of the global population. Cognitive and negative deficits account for the lifelong disability associated with schizophrenia, whose symptoms are not effectively addressed by current treatments. New medicines are needed to treat these aspects of the disease. Neurodevelopmental, neuropathological, genetic, and behavioral pharmacological data indicate that schizophrenia stems from a dysfunction of glutamate synaptic transmission, particularly in frontal cortical networks. A number of novel pre- and postsynaptic mechanisms affecting glutamatergic synaptic transmission have emerged as viable targets for schizophrenia. While developing orthosteric glutamatergic agents for these targets has proven extremely difficult, targeting allosteric sites of these targets has emerged as a promising alternative. From a medicinal chemistry perspective, allosteric sites provide an opportunity of finding agents with better drug-like properties and greater target specificity. Furthermore, allosteric modulators are better suited to maintaining the highly precise temporal and spatial aspects of glutamatergic synaptic transmission. Herein, we review neuropathological and genomic/genetic evidence underscoring the importance of glutamate synaptic dysfunction in the etiology of schizophrenia and make a case for allosteric targets for therapeutic intervention. We review progress in identifying allosteric modulators of AMPA receptors, NMDA receptors, and metabotropic glutamate receptors, all with the aim of restoring physiological glutamatergic synaptic transmission. Challenges remain given the complexity of schizophrenia and the difficulty in studying cognition in animals and humans. Nonetheless, important compounds have emerged from these efforts and promising preclinical and variable clinical validation has been achieved.

Reduced expression of STOP/MAP6 in mice leads to cognitive deficits

BACKGROUND

STOP/MAP6 null (KO) mice recapitulate behavioral abnormalities related to positive and negative symptoms and cognitive deficits of schizophrenia. Here, we investigated whether decreased expression of STOP/MAP6 proteins in heterozygous mice (only one allele expressed) would result in abnormal behavior related to those displayed by STOP null mice.

METHODS

Using a comprehensive test battery, we investigated the behavioral phenotype of STOP heterozygous (Het) mice compared with STOP KO and wild type (WT) mice on animals raised either in standard conditions (controls) or submitted to maternal deprivation.

RESULTS

Control Het mice displayed prominent deficits in social interaction and learning, resembling KO mice. In contrast, they exhibited short-lasting locomotor hyperreactivity to acute mild stress and no impaired locomotor response to amphetamine, much like WT mice. Additionally, perinatal stress deteriorated Het mouse phenotype by exacerbating alterations related to positive symptoms such as their locomotor reactivity to acute mild stress and psychostimulant challenge.

CONCLUSION

Results show that the dosage of susceptibility genes modulates their putative phenotypic contribution and that STOP expression has a high penetrance on cognitive abilities. Hence, STOP Het mice might be useful to investigate cognitive defects related to those observed in mental diseases and ultimately might be a valuable experimental model to evaluate preventive treatments.

Intra- and inter-frequency brain network structure in health and schizophrenia

Empirical studies over the past two decades have provided support for the hypothesis that schizophrenia is characterized by altered connectivity patterns in functional brain networks. These alterations have been proposed as genetically mediated diagnostic biomarkers and are thought to underlie altered cognitive functions such as working memory. However, the nature of this dysconnectivity remains far from understood. In this study, we perform an extensive analysis of functional connectivity patterns extracted from MEG data in 14 subjects with schizophrenia and 14 healthy controls during a 2-back working memory task. We investigate uni-, bi- and multivariate properties of sensor time series by computing wavelet entropy of and correlation between time series, and by constructing binary networks of functional connectivity both within and between classical frequency bands ([Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text]). Networks are based on the mutual information between wavelet time series, and estimated for each trial window separately, enabling us to consider both network topology and network dynamics. We observed significant decreases in time series entropy and significant increases in functional connectivity in the schizophrenia group in comparison to the healthy controls and identified an inverse relationship between these measures across both subjects and sensors that varied over frequency bands and was more pronounced in controls than in patients. The topological organization of connectivity was altered in schizophrenia specifically in high frequency [Formula: see text] and [Formula: see text] band networks as well as in the [Formula: see text]-[Formula: see text] cross-frequency networks. Network topology varied over trials to a greater extent in patients than in controls, suggesting disease-associated alterations in dynamic network properties of brain function. Our results identify signatures of aberrant neurophysiological behavior in schizophrenia across uni-, bi- and multivariate scales and lay the groundwork for further clinical studies that might lead to the discovery of new intermediate phenotypes.

Do former substance users with psychosis differ in their symptoms or function from non-substance users? A systematic meta-analysis

OBJECTIVE

To compare the symptoms and function of patients with psychosis who have ceased using substances to those who have psychosis but do not have a history of substance use.

METHOD

The databases EMBASE, MEDLINE and PsycINFO were searched for peer-reviewed publications in English reporting the characteristics of patients with psychotic illness who had stopped using substances and those who had never used substances. The searches yielded 20 articles that met the inclusion criteria. Four key outcome variables - positive symptoms, negative symptoms, depression and global function - and four other outcome measures reported in five or more studies were examined using meta-analysis.

RESULTS

Former substance-using patients were significantly younger than non-substance-using patients and were more likely to be male, but did not differ in age at onset of psychosis or in their level of education. There were no significant differences between former substance users and non-substance users in ratings of positive symptoms, negative symptoms, depression or global function. Among first-episode patients there was a trend towards former substance users having less severe depressive symptoms than non-substance users. In contrast, among non-first-episode patients, former substance users had significantly more depressive symptoms than non-substance users. In studies rated as being of higher quality, former substance users had significantly less severe positive symptoms than non-substance users.

CONCLUSION

The absence of significant differences between the two groups suggests that a history of substance use is not a poor prognostic indicator for patients who are able to stop using substances.

Mapping neurotransmitter networks with PET: an example on serotonin and opioid systems

All functions of the human brain are consequences of altered activity of specific neural pathways and neurotransmitter systems. Although the knowledge of "system level" connectivity in the brain is increasing rapidly, we lack "molecular level" information on brain networks and connectivity patterns. We introduce novel voxel-based positron emission tomography (PET) methods for studying internal neurotransmitter network structure and intercorrelations of different neurotransmitter systems in the human brain. We chose serotonin transporter and μ-opioid receptor for this analysis because of their functional interaction at the cellular level and similar regional distribution in the brain. Twenty-one healthy subjects underwent two consecutive PET scans using [(11)C]MADAM, a serotonin transporter tracer, and [(11)C]carfentanil, a μ-opioid receptor tracer. First, voxel-by-voxel "intracorrelations" (hub and seed analyses) were used to study the internal structure of opioid and serotonin systems. Second, voxel-level opioid-serotonin intercorrelations (between neurotransmitters) were computed. Regional μ-opioid receptor binding potentials were uniformly correlated throughout the brain. However, our analyses revealed nonuniformity in the serotonin transporter intracorrelations and identified a highly connected local network (midbrain-striatum-thalamus-amygdala). Regionally specific intercorrelations between the opioid and serotonin tracers were found in anteromedial thalamus, amygdala, anterior cingulate cortex, dorsolateral prefrontal cortex, and left parietal cortex, i.e., in areas relevant for several neuropsychiatric disorders, especially affective disorders. This methodology enables in vivo mapping of connectivity patterns within and between neurotransmitter systems. Quantification of functional neurotransmitter balances may be a useful approach in etiological studies of neuropsychiatric disorders and also in drug development as a biomarker-based rationale for targeted modulation of neurotransmitter networks.

Improving myelin/oligodendrocyte-related dysfunction: a new mechanism of antipsychotics in the treatment of schizophrenia?

Schizophrenia is a severe psychiatric disorder with complex clinical manifestations and its aetiological factors remain unclear. During the past decade, the oligodendrocyte-related myelin dysfunction was proposed as a hypothesis for schizophrenia, supported initially by a series of neuroimaging studies and genetic evidence. Recently, the effects of antipsychotics on myelination and oligodendroglial lineage development and their underlying molecular mechanisms were evaluated. Data from those studies suggest that the antipsychotics-resulting improvement in myelin/oligodendrocyte-related dysfunction may contribute, at least in part, to their therapeutic effect on schizophrenia. Importantly, these findings may provide the basis for a new insight into the therapeutic strategy by targeting the oligodendroglia lineage cells against schizophrenia.

The DTNBP1 (dysbindin-1) gene variant rs2619522 is associated with variation of hippocampal and prefrontal grey matter volumes in humans

DTNBP1 is one of the most established susceptibility genes for schizophrenia, and hippocampal volume reduction is one of the major neuropathological findings in this severe disorder. Consistent with these findings, the encoded protein dysbindin-1 has been shown to be diminished in glutamatergic hippocampal neurons in schizophrenic patients. The aim of this study was to investigate the effects of two single nucleotide polymorphisms of DTNBP1 on grey matter volumes in human subjects using voxel-based morphometry. Seventy-two subjects were included and genotyped with respect to two single nucleotide polymorphisms of DTNBP1 (rs2619522 and rs1018381). All participants underwent structural magnetic resonance imaging (MRI). MRI data were preprocessed and statistically analysed using standard procedures as implemented in SPM5 (Statistical Parametric Mapping), in particular the voxel-based morphometry (VBM) toolbox. We found significant effects of the DTNBP1 SNP rs2619522 bilaterally in the hippocampus as well as in the anterior middle frontal gyrus and the intraparietal cortex. Carriers of the G allele showed significantly higher grey matter volumes in these brain regions than T/T homozygotes. Compatible with previous findings on a role of dysbindin in hippocampal functions as well as in major psychoses, the present study provides first direct in vivo evidence that the DTNBP1 SNP rs2619522 is associated with variation of grey matter volumes bilaterally in the hippocampus.

Agonist high- and low-affinity states of dopamine D₂ receptors: methods of detection and clinical implications

Dopamine D(2) receptors, similar to other G-protein-coupled receptors, exist in a high- and low-affinity state for agonists. Based upon a review of the methods for detecting D(2) receptor agonist high-affinity states, we discuss alterations of such states in animal models of disease and the implications of such alterations for their labelling with positron emission tomography (PET) and single-photon emission computed tomography (SPECT) tracers. The classic approach of detecting agonist high-affinity states compares agonist competition for antagonist radioligands, in most cases using [(3)H]-spiperone as the radioligand; alternative approaches and radioligands have been proposed, but their claimed advantages have not been substantiated by other investigators. In view of the advantages and disadvantages of various techniques, we critically have reviewed reported findings on the detection of D(2) receptor agonist high-affinity states in a variety of animal models. These data are compared to the less numerous findings from human in vivo studies based on PET and SPECT tracers; they are interpreted in light of the finding that D(2) receptor agonist high-affinity states under control conditions may differ between rodent and human brain. The potential advantages of agonist ligands in studies of pathophysiology and as diagnostics are being discussed.

Beyond dopamine: glutamate as a target for future antipsychotics

The dopamine hypothesis of schizophrenia remains the primary theoretical framework for the pharmacological treatment of the disorder. Despite various lines of evidence of dopaminergic abnormalities and reasonable efficacy of current antipsychotic medication, a significant proportion of patients show suboptimal treatment responses, poor tolerability, and a subsequent lack of treatment concordance. In recent decades, intriguing evidence for the critical involvement of other neurotransmitter systems in the pathophysiology of schizophrenia has emerged, most notably of dysfunctions within the glutamate pathways. Consequently, the glutamate synapse has arisen as a promising target for urgently needed novel antipsychotic compounds—particularly in regards to debilitating negative and cognitive symptoms poorly controlled by currently available drugs. In this paper, recent findings integrating glutamatergic and dopaminergic abnormalities in schizophrenia and their implications for novel pharmacological targets are discussed. An overview of compounds in various stages of development is given: drugs enhancing NMDA receptor function as well as metabotropic glutamate receptor (mGluR) agonist and positive allosteric modulators (PAMs) are emphasised. Together with other agents more indirectly affecting glutamatergic neurotransmission, their potential future role in the pharmacotherapy of schizophrenia is critically evaluated.

The utility of (11)C-arachidonate PET to study in vivo dopaminergic neurotransmission in humans

We developed a novel method to study dopaminergic neurotransmission using positron emission tomography (PET) with [1-(11)C]arachidonic acid ([1-(11)C]AA). Previous preclinical studies have shown the utility of [1-(11)C]AA as a marker of signal transduction coupled to cytosolic phospholipase A(2) (cPLA(2)). Using [1-(11)C]AA and [(15)O]water PET, we measured regional incorporation coefficients K(*) for AA and regional cerebral blood flow (rCBF), respectively, in healthy male volunteers given the D(1)/D(2) agonist (10 or 20 μg/kg subcutaneous) apomorphine. We confirmed a robust central dopaminergic response to apomorphine by observing significant increases in the serum concentration of growth hormone. We observed significant increases, as well as decreases in K(*) and increases in rCBF in response to apomorphine. These changes remained significant after covarying for handedness and apomorphine dosage. The magnitude of increases in K(*) was lower than those in our previous animal experiments, likely reflecting the smaller dose of apomorphine used in the current human study. Changes in K(*) may reflect neuronal signaling downstream of activated D(2)-like receptors coupled to cPLA(2). Changes in rCBF are consistent with previous studies showing net functional effects of D(1)/D(2) activation. [1-(11)C]AA PET may be useful for studying disturbances of dopaminergic neurotransmission in conditions such as Parkinson's disease and schizophrenia.

Dopamine agonist radioligand binds to both D2High and D2Low receptors, explaining why alterations in D2High are not detected in human brain scans

The features of schizophrenia are consistent with increased sensitivity to endogenous dopamine. Animal models of schizophrenia reveal an increase in the in vitro proportion of striatal dopamine D2 receptors in the high-affinity state for dopamine (i.e., D2High), as measured by dopamine/[(3) H]domperidone competition. However, in vivo studies did not reveal the dopamine agonist [(11) C](+)PHNO to be elevated in amphetamine-sensitized rats. Also, no increase was found in the in vivo binding of [(11) C](+)PHNO in schizophrenia patients. This work was done to resolve the contradictory findings. It was found that the in vitro density of rat striatal D2 receptors was 18 pmol/g for [(3) H]raclopride and 12 pmol/g for [(3) H](+)PHNO; most of the latter sites disappeared in the presence of guanine nucleotide. Using 2 nM [(3) H](+)PHNO (K(d) of 0.72 nM at D2) to label D2 receptors in the striata and the human D2 clone, 10 nM to 100 nM dopamine inhibited 10-20% of the [(3) H](+)PHNO bound, representing high-affinity binding of [(3) H](+)PHNO, with the remainder inhibited above 100 nM dopamine, representing low-affinity binding of [(3) H](+)PHNO. It was found that (+)PHNO and (-)NPA dissociated from the D2 clone with half-times of 96 and 600 s, respectively. These rates are slower than the reported sub-second dissociation of the G protein from a receptor, suggesting that these two ligands still occupy the D2Low receptor after the G protein has separated. Thus, the radio-agonist label for (+)PHNO is not selective for dopamine D2High receptors, but also binds to the D2Low state of the dopamine receptor.

Proteins interacting with monoamine transporters: current state and future challenges

Plasma membrane and vesicular transporters for the biogenic amines, dopamine, norepinephrine, and serotonin, represent a group of proteins that play a crucial role in the regulation of neurotransmission. Clinically, mono amine transporters are the primary targets for the actions of many therapeutic agents used to treat mood disorders, as well as the site of action for highly addictive psychostimulants such as cocaine, amphetamine, methamphetamine, and 3,4-methylenedioxymethamphetamine. Over the past decade, the use of approaches such as yeast two-hybrid and proteomics has identified a multitude of transporter interacting proteins, suggesting that the function and regulation of these transporters are more complex than previously anticipated. With the increasing number of interacting proteins, the rules dictating transporter synthesis, assembly, targeting, trafficking, and function are beginning to be deciphered. Although many of these protein interactions have yet to be fully characterized, current knowledge is beginning to shed light on novel transporter mechanisms involved in monoamine homeostasis, the molecular actions of psychostimulants, and potential disease mechanisms. While future studies resolving the spatial and temporal resolution of these, and yet unknown, interactions will be needed, the realization that monoamine transporters do not work alone opens the path to a plethora of possible pharmacological interventions.