Neurotransmitter aberrations in schizophrenia: new perspectives and therapeutic implications

Phencyclidine-Like Abuse Liability and Psychosis-Like Neurocognitive Effects of Novel Arylcyclohexylamine Drugs of Abuse in Rodents

Abuse of novel arylcyclohexylamines (ACX) poses risks for toxicities, including adverse neurocognitive effects. In vivo effects of ring-substituted analogs of phencyclidine (PCP), eticyclidine (PCE), and ketamine are understudied. Adult male National Institutes of Health Swiss mice were used to assess locomotor effects of PCP and its 3-OH, 3-MeO, 3-Cl, and 4-MeO analogs, PCE and its 3-OH and 3-MeO analogs, and ketamine and its deschloro and 2F-deschloro analogs, in comparison with those of methamphetamine (METH), 3,4-methylenedioxymethamphetamine (MDMA), and two benzofuran analogs of MDMA. PCP-like interoceptive effects for all of these ACXs were determined using a food-reinforced drug discrimination procedure in adult male Sprague Dawley rats. A novel operant assay of rule-governed behavior incorporating aspects of attentional set-shifting was used to profile psychosis-like neurocognitive effects of PCP and 3-Cl-PCP in rats, in comparison with cocaine and morphine. PCP-like ACXs were more effective locomotor stimulants than the amphetamines, PCE-like ACXs were as effective as the amphetamines, and ketamine-like ACXs were less effective than the amphetamines. Addition of -Cl, -OH, or -OMe at the 3-position on the aromatic ring did not impact locomotor effectiveness, but addition of -OMe at the 4-position reduced locomotor effectiveness. Lethal effects were induced by drugs with -OH at the 3-position or -OMe at the 3- or 4-position. All novel ACXs substituted at least partially for PCP, and PCP and 3-Cl-PCP elicited dose-dependent psychosis-like neurocognitive deficits in the rule-governed behavior task not observed with cocaine or morphine. Novel ACXs exhibit substantial abuse liability and toxicities not necessarily observed with their parent drugs. SIGNIFICANCE STATEMENT: Novel arylcyclohexylamine analogs of PCP, PCE, and ketamine are appearing on the illicit market, and abuse of these drugs poses risks for toxicities, including adverse neurocognitive effects. These studies demonstrate that the novel ACXs exhibit PCP-like abuse liability in the drug discrimination assay, elicit varied locomotor stimulant and lethal effects in mice, and induce psychosis-like neurocognitive effects in rats.

In vivo mapping of protein-protein interactions of schizophrenia risk factors generates an interconnected disease network

Genetic analyses of Schizophrenia (SCZ) patients have identified thousands of risk factors. In silico protein-protein interaction (PPI) network analysis has provided strong evidence that disrupted PPI networks underlie SCZ pathogenesis. In this study, we performed in vivo PPI analysis of several SCZ risk factors in the rodent brain. Using endogenous antibody immunoprecipitations coupled to mass spectrometry (MS) analysis, we constructed a SCZ network comprising 1612 unique PPI with a 5% FDR. Over 90% of the PPI were novel, reflecting the lack of previous PPI MS studies in brain tissue. Our SCZ PPI network was enriched with known SCZ risk factors, which supports the hypothesis that an accumulation of disturbances in selected PPI networks underlies SCZ. We used Stable Isotope Labeling in Mammals (SILAM) to quantitate phencyclidine (PCP) perturbations in the SCZ network and found that PCP weakened most PPI but also led to some enhanced or new PPI. These findings demonstrate that quantitating PPI in perturbed biological states can reveal alterations to network biology.

The role of dopamine and endocannabinoid systems in prefrontal cortex development: Adolescence as a critical period

The prefrontal cortex plays a central role in the control of complex cognitive processes including action control and decision making. It also shows a specific pattern of delayed maturation related to unique behavioral changes during adolescence and allows the development of adult cognitive processes. The adolescent brain is extremely plastic and critically vulnerable to external insults. Related to this vulnerability, adolescence is also associated with the emergence of numerous neuropsychiatric disorders involving alterations of prefrontal functions. Within prefrontal microcircuits, the dopamine and the endocannabinoid systems have widespread effects on adolescent-specific ontogenetic processes. In this review, we highlight recent advances in our understanding of the maturation of the dopamine system and the endocannabinoid system in the prefrontal cortex during adolescence. We discuss how they interact with GABA and glutamate neurons to modulate prefrontal circuits and how they can be altered by different environmental events leading to long-term neurobiological and behavioral changes at adulthood. Finally, we aim to identify several future research directions to help highlight gaps in our current knowledge on the maturation of these microcircuits.

Characterizing serum amino acids in schizophrenic patients: Correlations with gut microbes

Amino acid abnormalities have been suggested to be a key pathophysiological mechanism in schizophrenia (SZ). Recently, gut microbes were found to be critically involved in mental and metabolic diseases. However, the relationship between serum amino acid levels and gut microbes in SZ is rarely studied. Here, we analyzed serum amino acid levels in 76 untreated SZ patients and 79 healthy controls (HC). Serum levels of 10 amino acids were significantly altered in patients with SZ. We further classified the cut-off values for serum arginine, leucine, glutamine, and methionine levels to distinguish SZ patients from controls. These classifiers were shown to be effective in another validation cohort (49 SZ and 48 HC). The correlation between serum amino acids and clinical symptoms and cognitive functions was also analyzed. Arginine, leucine, glutamine, and methionine levels were significantly correlated with clinical symptoms and cognitive impairments in SZ patients. By metagenome shotgun sequencing of fecal samples, we found that patients with SZ with a low level of serum amino acids have higher richness and evenness of the gut microbiota. At the genus level, the abundances of Mitsuokella and Oscillibacter are significantly abnormal. At the mOTU level, 15 mOTUs in the low-level SZ group were significantly different from the HC group. In addition, Mitsuokella multacida was correlated with glutamine and methionine, respectively. Our research revealed that alterations in serum amino acid levels are critically related to changes in gut microbiota composition in SZ patients. These findings may shed light on new strategies for the diagnosis and treatment of SZ.

Cannabis exposure during adolescence: A uniquely sensitive period for neurobiological effects

Adolescence is a crucial developmental period where neural circuits are refined and the brain is especially vulnerable to external insults. The endocannabinoid (eCB) system undergoes changes during adolescence which affect the way in which it modulates the development of other systems, in particular dopamine circuits, which show protracted development into adolescence. Given the rise of cannabis use by adolescents and young people, as well as variants containing increasingly higher concentrations of THC, it is now crucial to understand the unique effects of adolescent exposure to cannabis on the developing brain and it might shape future adult vulnerabilities to conditions such as psychosis, schizophrenia, addiction and more. Here we discuss the development of the eCB system across the lifespan, how CB1 receptors modulate dopamine release and potential neurobiological and behavioral effects of adolescent THC exposure on the developing brain such as alterations in excitatory/inhibitory balance during this developmental period.

Changes in subunit composition of NMDA receptors in animal models of schizophrenia by repeated administration of methamphetamine

The dopamine and glutamate hypotheses reflect only some of the pathophysiological changes associated with schizophrenia. We have proposed a new "comprehensive progressive pathophysiology model" based on the "dopamine to glutamate hypothesis." Repeated administration of methamphetamine (METH) at a dose of 2.5 mg/kg in rats has been used to assess dynamic changes in the pathophysiology of schizophrenia. Previous use of this model suggested N-methyl-d-aspartate receptor (NMDA-R) dysfunction, but the mechanism could only be inferred from limited, indirect observations. In the present study, we used this model to investigate changes in the expression of NMDA-R subunits. Repeated administration of METH significantly decreased the gene expression levels of glutamate ionotropic receptor NMDA type subunit (Grin) subtypes Grin1 and Grin2c in the prefrontal cortex (PFC), Grin1 and Grin2a in the hippocampus (HPC), and Grin1, Grin2b, and Grin2d in the striatum (ST).We observed a significant difference in Grin1 expression between the PFC and ST. Furthermore, repeated administration of METH significantly decreased the protein expression of GluN1 in both cytosolic and synaptosomal fractions isolated from the PFC, and significantly decreased the protein expression of GluN1 in the cytosolic fraction, but not the synaptosomal fraction from the ST. These regional differences may be due to variations in the synthesis of GluN1 or intracellular trafficking events in each area of the brain. Considering that knockdown of Grin1 in mice affects vulnerability to develop schizophrenia, these results suggest that this model reflects some of the pathophysiological changes of schizophrenia, combining both the dopamine and glutamate hypotheses.

Brain-on-a-Chip Device for Modeling Multiregional Networks

Animal models are frequently used in drug discovery because they represent a mammalian in vivo model system, they are the closest approximation to the human brain, and experimentation in humans is not ethical. Working with postmortem human brain samples is challenging and developing human in vitro systems, which mimic the in vivo human brain, has been challenging. However, the use of animal models in drug discovery for human neurological diseases is currently under scrutiny because data from animal models has come with variations due to genetic differences. Evidence from the literature suggests that techniques to reconstruct multiple neurotransmission projections, which characterize neurological disease circuits in humans, in vitro, have not been demonstrated. This paper presents a multicompartment microdevice for patterning neurospheres and specification of neural stem cell fate toward networks of multiple neuronal phenotypes. We validated our design by specification of human neural stem cells to dopaminergic and GABAergic neurons in different compartments of the device, simultaneously. The neurospheres formed unrestricted robust neuronal circuits between arrays of neurospheres in all compartments of the device. Such a device design may provide a basis for formation of multineurotransmission circuits to model functional connectivity between specific human brain regions, in vitro, using human-derived neural stem cells. This work finds relevance in neurological disease modeling and drug screening using human cell-based assays and may provide the impetus for shifting from animal-based models.

Candidate metabolic biomarkers for schizophrenia in CNS and periphery: Do any possible associations exist?

Due to the limitations of analytical techniques and the complicity of schizophrenia, nowadays it is still a challenge to diagnose and stratify schizophrenia patients accurately. Many attempts have been made to identify and validate available biomarkers for schizophrenia from CSF and/or peripheral blood in clinical studies with consideration to disease stages, antipsychotic effects and even gender differences. However, conflicting results handicap the validation and application of biomarkers for schizophrenia. In view of availability and feasibility, peripheral biomarkers have superior advantages over biomarkers in CNS. Meanwhile, schizophrenia is considered to be a devastating neuropsychiatric disease mainly taking place in CNS featured by widespread defects in multiple metabolic pathways whose dynamic interactions, until recently, have been difficult to difficult to investigate. Evidence for these alterations has been collected piecemeal, limiting the potential to inform our understanding of the interactions among relevant biochemical pathways. Taken these points together, it will be interesting to investigate possible associations of biomarkers between CNS and periphery. Numerous studies have suggested putative correlations within peripheral and CNS systems especially for dopaminergic and glutamatergic metabolic biomarkers. In addition, it has been demonstrated that blood concentrations of BDNF protein can also reflect its changes in the nervous system. In turn, BDNF also interacts with glutamatergic, dopaminergic and serotonergic systems. Therefore, this review will summarize metabolic biomarkers identified both in the CNS (brain tissues and CSF) and peripheral blood. Further, more attentions will be paid to discussing possible physical and functional associations between CNS and periphery, especially with respect to BDNF.

Carbon-Based Quantum Dots for Electrochemical Detection of Monoamine Neurotransmitters-Review

Imbalance in the levels of monoamine neurotransmitters have manifested in severe health issues. Electrochemical sensors have been designed for their determination, with good sensitivity recorded. Carbon-based quantum dots have proven to be an important component of electrochemical sensors due to their high conductivity, low cytotoxicity and opto-electronic properties. The quest for more sensitive electrodes with cheaper materials led to the development of electrochemical sensors based on carbon-based quantum dots for the detection of neurotransmitters. The importance of monoamine neurotransmitters (NTs) and the good electrocatalytic activity of carbon and graphene quantum dots (CQDs and GQDs) make the review of the efforts made in the design of such sensors for monoamine NTs of huge necessity. The differences and the similarities between these two quantum dots are highlighted prior to a discussion of their application in electrochemical sensors over the last ten years. Compared to other monoamine NTs, dopamine (DA) was the most studied with GQDs and CQD-based electrochemical sensors.

Hispidulin attenuates the social withdrawal in isolated disrupted-in-schizophrenia-1 mutant and chronic phencyclidine-treated mice

BACKGROUND AND PURPOSE

Hispidulin is a flavonoid isolated from Clerodendrum inerme that was found to inhibit intractable motor tics. Previously, we found that hispidulin attenuates hyperlocomotion and the disrupted prepulse inhibition induced by methamphetamine and N-methyl-d-aspartate (NMDA) receptor antagonists, two phenotypes of schizophrenia resembling positive symptoms. Hispidulin can inhibit COMT, a dopamine-metabolizing enzyme in the prefrontal cortex (PFC) that is important for social interaction. Here, we investigated whether hispidulin would affect social withdrawal, one of the negative symptoms of schizophrenia.

EXPERIMENTAL APPROACH

We examined whether acute administration of hispidulin would attenuate social withdrawal in two mice models, juvenile isolated disrupted-in-schizophrenia-1 mutant (mutDISC1) mice and chronic phencyclidine (PCP)-treated naïve mice.

KEY RESULTS

In chronic PCP-treated mice, hispidulin (10 mg·kg^-1 , i.p.) attenuated social withdrawal similar to that observed with dopamine D₁ receptor antagonist (SCH-23390, 0.02 mg·kg^-1 , i.p.) and was mimicked by the selective COMT inhibitor, OR-486 (10 mg·kg^-1 , i.p.). Hispidulin increased extracellular dopamine levels in the PFC of chronic PCP-treated mice. In isolated mutDISC1 mice, hispidulin also reversed social withdrawal. In both models, intra-PFC microinjection of a D₁ agonist (SKF-81297: 10 nmol/mouse/bilateral) reversed the impairment of Ser⁸⁹⁷ phosphorylation at the GluN1 subunit of NMDA receptors, suggesting the association between GluN1 Ser⁸⁹⁷ -phosphorylation and D₁ activation in the PFC exits in both models.

CONCLUSIONS AND IMPLICATIONS

Hispidulin attenuated social withdrawal by activating D₁ receptors indirectly through elevated dopamine levels in the PFC by COMT inhibition. This nature of hispidulin suggests that it a potential novel therapeutic candidate for the treatment of negative symptoms in schizophrenia.

Variants of GRM7 as risk factor and response to antipsychotic therapy in schizophrenia

Genome-wide association study (GWAS) has determined the metabotropic glutamate receptor 7 (GRM7) gene as potential locus for schizophrenia risk variants; However, the relationship between the GRM7 variants and the risk of schizophrenia is still uncertain, and there are significant individual variations in response to the antipsychotic drugs. In order to identify susceptible gene and drug-response-related markers, 2413 subjects in our research were chosen for determining drug-response-related markers in schizophrenia. The rs1516569 variant (OR = 0.95, P < 3.47 × 10^-4) was a significant risk factor, and a single-nucleotide polymorphism of GRM7 gene- rs9883258 (OR = 0.84, P = 2.18 × 10^-3) has been determined as potential biomarkers for therapeutic responses of seven commonly used antipsychotic drugs (aripiprazole, haloperidol, olanzapine, perphenazine, quetiapine, risperidone and ziprasidone) in Chinese Han population; Significant associations with treatment response for several single-nucleotide polymorphisms in every antipsychotic drugs, such as rs779746 (OR = 1.39, P = 0.03), rs480409 (OR = 0.73, P = 0.04), rs78137319 (OR = 3.09, P = 0.04), rs1154370 (OR = 1.51, P = 0.006) have been identified in our study. Hence our research elucidates that GRM7 variants play the critical role of predicting the risk of schizophrenia and antipsychotic effect of seven common drugs.

Functional roles of the glial glutamate transporter (GLAST) in emotional and cognitive abnormalities of mice after repeated phencyclidine administration

Alterations of the glutamatergic system components, including N-methyl-d-aspartate (NMDA) receptors are relevant to the pathophysiology of schizophrenia. Repeated phencyclidine (PCP) administration induces several schizophrenia-like psychobehavioral abnormalities and decreases extracellular glutamate levels, which are associated with increased levels of glial glutamate and aspartate transporter (GLAST) in the prefrontal cortex (PFC) of mice. In the present study, we investigated the functional roles of GLAST in the emotional and cognitive abnormalities in mice following repeated PCP administration by using GLAST heterozygous (+/-) mice, since GLAST mutant mice are a useful tool for elucidating the contribution of glutamate dysfunction to the pathophysiology of schizophrenia. PCP-administered GLAST wild-type (+/+) mice showed enhancement of immobility in a forced swimming test, impairments of visual recognition memory in a novel object recognition test, decrease in high potassium (K⁺)-induced extracellular glutamate release, and overexpression of GLAST and S100 proteins in the PFC, compared to saline-administered GLAST^+/+ mice. Such behavioral and neurochemical abnormalities were not observed in PCP-administered GLAST^+/- mice. In conclusion, these results clearly suggest that genetic GLAST dysfunction and glial activation play important roles in the development of emotional and cognitive abnormalities in PCP-administered GLAST^+/+ mice.

Access to the CNS: Biomarker Strategies for Dopaminergic Treatments

Despite substantial research carried out over the last decades, it remains difficult to understand the wide range of pharmacological effects of dopaminergic agents. The dopaminergic system is involved in several neurological disorders, such as Parkinson's disease and schizophrenia. This complex system features multiple pathways implicated in emotion and cognition, psychomotor functions and endocrine control through activation of G protein-coupled dopamine receptors. This review focuses on the system-wide effects of dopaminergic agents on the multiple biochemical and endocrine pathways, in particular the biomarkers (i.e., indicators of a pharmacological process) that reflect these effects. Dopaminergic treatments developed over the last decades were found to be associated with numerous biochemical pathways in the brain, including the norepinephrine and the kynurenine pathway. Additionally, they have shown to affect peripheral systems, for example the hypothalamus-pituitary-adrenal (HPA) axis. Dopaminergic agents thus have a complex and broad pharmacological profile, rendering drug development challenging. Considering the complex system-wide pharmacological profile of dopaminergic agents, this review underlines the needs for systems pharmacology studies that include: i) proteomics and metabolomics analysis; ii) longitudinal data evaluation and mathematical modeling; iii) pharmacokinetics-based interpretation of drug effects; iv) simultaneous biomarker evaluation in the brain, the cerebrospinal fluid (CSF) and plasma; and v) specific attention to condition-dependent (e.g., disease) pharmacology. Such approach is considered essential to increase our understanding of central nervous system (CNS) drug effects and substantially improve CNS drug development.

Glutamate and psychiatric disorders

Most of the excitatory neurotransmission in the central nervous system (CNS) is mediated by the endogenous excitatory amino acids (EAAs) glutamate, aspartate and homocysteine. Most of the endogenous inhibitory neurotransmission is mediated by gamma-aminobutyric acid (GABA). EAAs modulate the firing of almost all neurons in the CNS, as excitatory neurotransmission can result in both neuronal inhibition and excitation. The glutamate system is the best characterised of the EAA systems (Box 1).

VGLUT2 Trafficking Is Differentially Regulated by Adaptor Proteins AP-1 and AP-3

Release of the major excitatory neurotransmitter glutamate by synaptic vesicle exocytosis depends on glutamate loading into synaptic vesicles by vesicular glutamate transporters (VGLUTs). The two principal isoforms, VGLUT1 and 2, exhibit a complementary pattern of expression in adult brain that broadly distinguishes cortical (VGLUT1) and subcortical (VGLUT2) systems, and correlates with distinct physiological properties in synapses expressing these isoforms. Differential trafficking of VGLUT1 and 2 has been suggested to underlie their functional diversity. Increasing evidence suggests individual synaptic vesicle proteins use specific sorting signals to engage specialized biochemical mechanisms to regulate their recycling. We observed that VGLUT2 recycles differently in response to high frequency stimulation than VGLUT1. Here we further explore the trafficking of VGLUT2 using a pHluorin-based reporter, VGLUT2-pH. VGLUT2-pH exhibits slower rates of both exocytosis and endocytosis than VGLUT1-pH. VGLUT2-pH recycling is slower than VGLUT1-pH in both hippocampal neurons, which endogenously express mostly VGLUT1, and thalamic neurons, which endogenously express mostly VGLUT2, indicating that protein identity, not synaptic vesicle membrane or neuronal cell type, controls sorting. We characterize sorting signals in the C-terminal dileucine-like motif, which plays a crucial role in VGLUT2 trafficking. Disruption of this motif abolishes synaptic targeting of VGLUT2 and essentially eliminates endocytosis of the transporter. Mutational and biochemical analysis demonstrates that clathrin adaptor proteins (APs) interact with VGLUT2 at the dileucine-like motif. VGLUT2 interacts with AP-2, a well-studied adaptor protein for clathrin mediated endocytosis. In addition, VGLUT2 also interacts with the alternate adaptors, AP-1 and AP-3. VGLUT2 relies on distinct recycling mechanisms from VGLUT1. Abrogation of these differences by pharmacological and molecular inhibition reveals that these mechanisms are dependent on the adaptor proteins AP-1 and AP-3. Further, shRNA-mediated knockdown reveals differential roles for AP-1 and AP-3 in VGLUT2 recycling.

Profiles of VGF Peptides in the Rat Brain and Their Modulations after Phencyclidine Treatment

From the VGF precursor protein originate several low molecular weight peptides, whose distribution in the brain and blood circulation is not entirely known. Among the VGF peptides, those containing the N-terminus portion were altered in the cerebro-spinal fluid (CSF) and hypothalamus of schizophrenia patients. "Hence, we aimed to better investigate the involvement of the VGF peptides in schizophrenia by studying their localization in the brain regions relevant for the disease, and revealing their possible modulations in response to certain neuronal alterations occurring in schizophrenia". We produced antibodies against different VGF peptides encompassing the N-terminus, but also C-terminus-, TLQP-, GGGE- peptide sequences, and the so named NERP-3 and -4. These antibodies were used to carry out specific ELISA and immunolocalization studies while mass spectrometry (MS) analysis was also performed to recognize the intact brain VGF fragments. We used a schizophrenia rat model, in which alterations in the prepulse inhibition (PPI) of the acoustic startle response occurred after PCP treatment. In normal rats, all the VGF peptides studied were distributed in the brain areas examined including hypothalamus, prefrontal cortex, hippocampus, accumbens and amygdaloid nuclei and also in the plasma. By liquid chromatography-high resolution mass, we identified different intact VGF peptide fragments, including those encompassing the N-terminus and the NERPs. PCP treatment caused behavioral changes that closely mimic schizophrenia, estimated by us as a disruption of PPI of the acoustic startle response. The PCP treatment also induced selective changes in the VGF peptide levels within certain brain areas. Indeed, an increase in VGF C-terminus and TLQP peptides was revealed in the prefrontal cortex (p < 0.01) where they were localized within parvoalbumin and tyrosine hydroxylase (TH) containing neurons, respectively. Conversely, in the nucleus accumbens, PCP treatment produced a down-regulation in the levels of VGF C-terminus-, N-terminus- and GGGE- peptides (p < 0.01), expressed in GABAergic- (C-terminus/GGGE) and somatostatin- (N-terminus) neurons. These results confirm that VGF peptides are widely distributed in the brain and modulated in specific areas involved in schizophrenia.

Genetic and Functional Study of L-Type Amino Acid Transporter 1 in Schizophrenia

Schizophrenia involves neural catecholaminergic dysregulation. Tyrosine is the precursor of catecholamines, and its major transporter, according to studies on fibroblasts, in the brain is the L-type amino acid transporter 1 (LAT1). The present study assessed haplotype tag single-nucleotide polymorphisms (SNPs) of the SLC7A5/LAT1 gene in 315 patients with psychosis within the schizophrenia spectrum and 233 healthy controls to investigate genetic vulnerability to the disorder as well as genetic relationships to homovanillic acid (HVA) and 3-methoxy-4-hydroxyphenylglycol (MHPG), the major catecholamine metabolites in the cerebrospinal fluid (CSF). Moreover, the involvement of the different isoforms of the system L in tyrosine uptake and LAT1 tyrosine kinetics were studied in fibroblast cell lines of 10 patients with schizophrenia and 10 healthy controls. The results provide suggestive evidence of individual vulnerability to schizophrenia related to the LAT1 SNP rs9936204 genotype. A number of SNPs were nominally associated with CSF HVA and MHPG concentrations but did not survive correction for multiple testing. The LAT1 isoform was confirmed as the major tyrosine transporter in patients with schizophrenia. However, the kinetic parameters (maximal transport capacity, affinity of the binding sites, and diffusion constant of tyrosine transport through the LAT1 isoform) did not differ between patients with schizophrenia and controls. The present genetic findings call for independent replication in larger samples, while the functional study seems to exclude a role of LAT1 in the aberrant transport of tyrosine in fibroblasts of patients with schizophrenia.

In Vivo Systems Response Profiling and Multivariate Classification of CNS Active Compounds: A Structured Tool for CNS Drug Discovery

This paper describes the application of in vivo systems response profiling in CNS drug discovery by a process referred to as the Integrative Screening Process. The biological response profile, treated as an array, is used as major outcome for selection of candidate drugs. Dose-response data, including ex vivo brain monoaminergic biomarkers and behavioral descriptors, are systematically collected and analyzed by principal component analysis (PCA) and partial least-squares (PLS) regression, yielding multivariate characterization across compounds. The approach is exemplified by assessing a new class of CNS active compounds, the dopidines, compared to other monoamine modulating compounds including antipsychotics, antidepressants, and procognitive agents. Dopidines display a distinct phenotypic profile which has prompted extensive further preclinical and clinical investigations. In summary, in vivo profiles of CNS compounds are mapped, based on dose response studies in the rat. Applying a systematic and standardized work-flow, a database of in vivo systems response profiles is compiled, enabling comparisons and classification. This creates a framework for translational mapping, a crucial component in CNS drug discovery.

Dopamine, fronto-striato-thalamic circuits and risk for psychosis

A series of parallel, integrated circuits link distinct regions of prefrontal cortex with specific nuclei of the striatum and thalamus. Dysfunction of these fronto-striato-thalamic systems is thought to play a major role in the pathogenesis of psychosis. In this review, we examine evidence from human and animal investigations that dysfunction of a specific dorsal fronto-striato-thalamic circuit, linking the dorsolateral prefrontal cortex, dorsal (associative) striatum, and mediodorsal nucleus of the thalamus, is apparent across different stages of psychosis, including prior to the onset of a first episode, suggesting that it represents a candidate risk biomarker. We consider how abnormalities at distinct points in the circuit may give rise to the pattern of findings seen in patient populations, and how these changes relate to disruptions in dopamine, glutamate and GABA signaling.

Olanzapine Reversed Brain Gene Expression Changes Induced by Phencyclidine Treatment in Non-Human Primates

The NMDA receptor antagonist phencyclidine (PCP) creates schizophrenia-like symptoms in normal controls. The effect of PCP on non-human primate brain gene expression was examined and compared to changes induced by olanzapine treatment. Experimental studies of PCP and antipsychotic drugs have direct relevance to understanding the patho-physiology and treatment of schizophrenia. Genome-wide changes in prefrontal cortex gene expression revealed alterations of 146 transcripts in the PCP treatment group compared to vehicle controls. Dysregulated genes were enriched in identified classes implicated in neurological and genetic disorders, including schizophrenia genes from the Psychiatric Genomics Consortium 108 loci as well as cell death in PCP-treated primates. Canonical pathway analysis revealed a significant overrepresentation of several groups including synaptic long-term potentiation and calcium signaling. Olanzapine coadministered with PCP normalized 34% of the 146 PCP-induced probe set expression changes, and a network of 17 olanzapine-normalized genes was identified enriched in schizophrenia candidate genes containing RGS4, SYN1 and AKT as nodes. The results of this study support the use of PCP administration in non-human primates as a glutamatergic model of schizophrenia and suggest that a large number of PCP-induced expression differences can be reversed by olanzapine. The results of this study may be informative for identification of potential candidates for pharmacogenetics and biomarker research related to the treatment of schizophrenia.

Significant association between DRD3 gene body methylation and schizophrenia

The current study was the first one to reveal the contribution of DRD3 methylation to the risk of different (SCZ) subtypes. This study comprised a total of 30 paranoid (15 males and 15 females) and 29 undifferentiated (15 males and 14 females) SCZ patients and 26 age- and gender-matched controls. Our results showed a significant association of CpG2 with SCZ. A breakdown analysis by gender showed that CpG2 and CpG3 methylation were significantly higher in male patients than male controls, and that CpG5 methylation was significantly higher in female patients than female controls. A further breakdown analysis by both gender and SCZ subtype showed that CpG2 and CpG3 methylation were significantly higher in male paranoid SCZ and male undifferentiated SCZ than male controls. In contrast, CpG2 and CpG3 methylation were significantly lower in female undifferentiated SCZ than female controls. Additionally, CpG5 methylation was significantly higher in female paranoid SCZ than female controls. In conclusion, our findings supported that DRD3 gene body hypermethylation was significantly associated with the risk of SCZ. Future study is needed to clarify the mechanisms by which DRD3 gene body hypermethylation contributes to the risk of SCZ.

Is elevated norepinephrine an etiological factor in some cases of schizophrenia?

A number of hypotheses have been put forth regarding the etiology of schizophrenia, including the dopamine hypothesis, NMDA receptor hypofunction hypothesis, and others. A lesser known theory is that elevated noradrenergic signaling plays a causative role in the disease. This paper briefly re-examines the merits of this hypothesis, including as it relates to some recently published studies. Several lines of evidence are investigated, including: endogenous level studies of norepinephrine (NE); modulation of the disease by noradrenergic drugs; association of the disease with bipolar disorder and hypertension, since these latter two conditions may involve elevated NE transmission; and effects of psychological stress on the disease, since stress can produce elevated release of NE. For many of these lines of evidence, their relationship with prepulse inhibition of startle is examined. A number of these studies support the hypothesis, and several suggest that elevated NE signaling plays a particularly prominent role in the paranoid subtype of schizophrenia. If the hypothesis is correct for some persons, conventional pharmaceutical treatment options, such as use of atypical antipsychotics (which may themselves modulate noradrenergic signaling), may be improved if selective NE transmission modulating agents are added to or even substituted for these conventional drugs.

Chronic phencyclidine treatment induces long-lasting glutamatergic activation of VTA dopamine neurons

Use of phencyclidine (PCP) can mimic some aspects of schizophrenia. However, the underlying mechanism is unclear. Administration of PCP is known to activate mesolimbic dopamine pathway. In this study, we focused on ventral tegmental area (VTA) of mesolimbic dopamine pathway as target of PCP for inducing schizophrenia-like symptoms. Single VTA neuron was isolated and its neural activity was monitored by measuring cytosolic Ca(2+) concentration ([Ca(2+)]i) followed by immunocytochemical identification of dopamine neurons. Administration of glutamate increased [Ca(2+)]i in dopamine neurons from control rats, and the [Ca(2+)]i increase was inhibited in the presence of PCP. In contrast, in VTA dopamine neurons from rats chronically treated with PCP for 7 days, administration of glutamate was able to induce [Ca(2+)]i increase in the presence of PCP. Furthermore, this glutamate-induced [Ca(2+)]i increase in the presence of PCP continued even after washout of glutamate and this effect lasted as long as PCP was present. This long-lasting glutamate-induced [Ca(2+)]i increase in the presence of PCP was not observed or significantly attenuated under Ca(2+) free condition and by N-type Ca(2+) channel blocker ω-conotoxin. The results indicate that chronic treatment with PCP reverses the acute PCP effect on VTA dopamine neurons from inhibitory to stimulatory tone, and consequently induces long-lasting activation of dopamine neurons by glutamate.

Syzygium cumini is more effective in preventing the increase of erythrocytic ADA activity than phenolic compounds under hyperglycemic conditions in vitro

Syzygium cumini (S. cumini) is a plant known for its antidiabetic properties. The aim of this study was to evaluate the effect of Sc aqueous leaf extract (ASc) on adenosine deaminase (ADA) activity in erythrocytes (RBCs) exposed to high glucose concentrations (30 mM) in vitro. We also investigated the effects of the main phenolic compounds found in ASc (gallic acid, rutin, and chlorogenic acid) and the effects of insulin, caffeine, and dipyridamole, which are substances involved in the adenosine metabolism, on ADA activity in vitro. Blood samples were obtained from healthy volunteers and a suspension of RBCs was used for the determination of ADA activity. The results showed that: (1) the effect of ASc on ADA activity was more significant than the combination of phenolic compounds; (2) insulin, caffeine, or dipyridamole prevented high glucose increase of ADA activity at doses as low as 50 μU/mL, 25 μM, and 1 μM, respectively; (3) the inhibitory effect caused by ASc on erythrocyte ADA activity remained practically the same after the combination of the extract with insulin or caffeine; (4) when RBCs were exposed to ASc plus dipyridamole, this chemical attenuated the effect of ASc on ADA activity, suggesting an antagonism or a competition with ASc by the same site of action. Therefore, ASc was more effective in preventing the increase in ADA activity than phenolic compounds, suggesting that ASc may collaborate to improve endothelial dysfunction, antioxidant, anti-inflammatory, and antithrombotic properties of adenosine by affecting its metabolism. The results of this study help to provide evidence of the empirically supported benefits of the use of S. cumini in diabetes.

Relationship of resting brain hyperconnectivity and schizophrenia-like symptoms produced by the NMDA receptor antagonist ketamine in humans

N-methyl-D-aspartate glutamate receptor (NMDA-R) antagonists produce schizophrenia-like positive and negative symptoms in healthy human subjects. Preclinical research suggests that NMDA-R antagonists interfere with the function of gamma-aminobutyric acid (GABA) neurons and alter the brain oscillations. These changes have been hypothesized to contribute to psychosis. In this investigation, we evaluated the hypothesis that the NMDA-R antagonist ketamine produces alterations in cortical functional connectivity during rest that are related to symptoms. We administered ketamine to a primary sample of 22 subjects and to an additional, partially overlapping, sample of 12 subjects. Symptoms before and after the experimental session were rated with the Positive and Negative Syndrome Scale (PANSS). In the primary sample, functional connectivity was measured via functional magnetic resonance imaging almost immediately after infusion began. In the additional sample, this assessment was repeated after 45 min of continuous ketamine infusion. Global, enhanced functional connectivity was observed at both timepoints, and this hyperconnectivity was related to symptoms in a region-specific manner. This study supports the hypothesis that pathological increases in resting brain functional connectivity contribute to the emergence of positive and negative symptoms associated with schizophrenia.

Combination of neonatal PolyI:C and adolescent phencyclidine treatments is required to induce behavioral abnormalities with overexpression of GLAST in adult mice

Cumulative incidences of multiple risk factors are related to pathology of psychiatric disorders. The present study was designed to examine combinative effects of a neonatal immune challenge with adolescent abused substance treatment on the psychological behaviors and molecular expressions in the adult. C57BL/6J mice were neonatally treated, with polyriboinosinic-polyribocytidylic acid (PolyI:C: 5mg/kg) during postnatal days (PD) 2-6, then with phencyclidine (PCP: 10mg/kg) during adolescence (PD35-41). Locomotor activity was analyzed to evaluate sensitivity to PCP on PD35 and PD41. Emotional and cognitive tests were carried out on PD42-48. Neonatal PolyI:C treatment markedly enhanced sensitivity to PCP- and methamphetamine-induced hyperactivity in the adolescent. Mice treated with both neonatal PolyI:C and adolescent PCP (PolyI:C/PCP) showed social deficit and object recognition memory impairment. The expression of glutamate/aspartate transporter (GLAST) in the prefrontal cortex (PFC) was significantly increased in the (PolyI:C/PCP)-treated mice. Infusion of glutamate transporter inhibitor (DL-TBOA: 1 nmol/bilaterally) into the PFC reversed the object recognition impairment in the (PolyI:C/PCP)-treated mice. These results indicate that the combined treatment of neonatal PolyI:C with adolescent PCP leads to behavioral abnormalities, which were associated with increase of GLAST expression in the adult PFC.

Neuronal injury, but not microglia activation, is associated with ketamine-induced experimental schizophrenic model in mice

Schizophrenia is a chronic debilitating psychiatric disorder affecting as many as 1% of the population worldwide. Unfortunately, its etiology and pathophysiology are poorly defined. Previous studies have shown that neuronal injury and microglia activation were observed in the schizophrenic patients. The present study aims to evaluate the role of neurons and microglia in ketamine-induced experimental schizophrenic model to further understand its pathophysiology. Firstly, ketamine was used to simulate the behavior abnormalities associated with schizophrenia. The effects of ketamine on mouse locomotor activity, Y-maze task, novel object recognition, and forced swimming test were studied. The results showed that ketamine (25, 50, and 100mg/kg i.p.) administered acutely or repeatedly (for 7 days) can increase the locomotor number significantly. In Y-maze task, ketamine (25, 50, and 100mg/kg) impaired spontaneous alternation after both acute and repeated treatments. In novel object recognition test, acute or chronic ketamine treatment showed no significant effect on mouse exploratory preference behavior. In forced swimming test, repeated treatment of ketamine (100mg/kg) enhanced the immobility duration. Secondly, immunohistochemical method was used to study the changes of neurons and microglia. The results showed that acute treatment of ketamine (100mg/kg) had no effect on neurons in the prefrontal cortex or hippocampus (1, 3, 5, and 7 days after the treatment). In contrast, repeated treatment of ketamine caused neuronal impairment in mouse hippocampus (3rd day, 5th day and 7th day after the final administration). The results of immunohistochemistry demonstrated that microglia in the prefrontal cortex and hippocampus were not affected after acute or repeated administration of ketamine. Finally, the neuronal impairment caused by repeated administration of ketamine was further investigated from the oxidative stress aspects. The results showed that repeated administration of ketamine increased nitric oxide (NO) and nitric oxide synthase (NOS) in prefrontal cortex, hippocampus and serum, while decreased SOD in hippocampus and serum. In summary, chronic ketamine treatment to mice successfully mimics the core behavioral deficits in schizophrenia. It is demonstrated for the first time that neuronal injury was associated with the chronic ketamine-induced experimental schizophrenic model, while microglial cells may play little role in this model. Oxidative stress may contribute to the significant neuronal injury in mouse brain induced by chronic ketamine treatment.

Cholinergic connectivity: it's implications for psychiatric disorders

Acetylcholine has been implicated in both the pathophysiology and treatment of a number of psychiatric disorders, with most of the data related to its role and therapeutic potential focusing on schizophrenia. However, there is little thought given to the consequences of the documented changes in the cholinergic system and how they may affect the functioning of the brain. This review looks at the cholinergic system and its interactions with the intrinsic neurotransmitters glutamate and gamma-amino butyric acid as well as those with the projection neurotransmitters most implicated in the pathophysiologies of psychiatric disorders; dopamine and serotonin. In addition, with the recent focus on the role of factors normally associated with inflammation in the pathophysiologies of psychiatric disorders, links between the cholinergic system and these factors will also be examined. These interfaces are put into context, primarily for schizophrenia, by looking at the changes in each of these systems in the disorder and exploring, theoretically, whether the changes are interconnected with those seen in the cholinergic system. Thus, this review will provide a comprehensive overview of the connectivity between the cholinergic system and some of the major areas of research into the pathophysiologies of psychiatric disorders, resulting in a critical appraisal of the potential outcomes of a dysregulated central cholinergic system.

Group II metabotropic glutamate receptor agonist LY379268 regulates AMPA receptor trafficking in prefrontal cortical neurons

Group II metabotropic glutamate receptor (mGluR) agonists have emerged as potential treatment drugs for schizophrenia and other neurological disorders, whereas the mechanisms involved remain elusive. Here we examined the effects of LY379268 (LY37) on the expression and trafficking of the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor subunits GluA1 and GluA2 in prefrontal neurons. We show that LY37 significantly increased the surface and total expression of both GluA1 and GluA2 subunits in cultured prefrontal neurons and in vivo. This effect was mimicked by the selective mGluR2 agonist LY395756 and was blocked by mGluR2/3 antagonist LY341495. Moreover, we found that both GluA1 and GluA2 subunits were colocalized with PSD95 but not synapsin I, suggesting a postsynaptic localization. Consistently, treatment with LY37 significantly increased the amplitude, but not frequency, of miniature excitatory postsynaptic currents. Further, actinomycin-D blocked LY37's effects, suggesting a transcriptional regulation. In addition, application of glycogen synthase kinase-3beta (GSK-3β) inhibitor completely blocked LY37's effect on GluA2 surface expression, whereas GSK-3β inhibitor itself induced decreases in the surface and total protein levels of GluA1, but not GluA2 subunits. This suggests that GSK-3β differentially mediates GluA1 and GluA2 trafficking. Further, LY37 significantly increased the phosphorylation, but not total protein, of extracellular signal-regulated kinase 1/2 (ERK1/2). Neither ERK1/2 inhibitor PD98059 alone nor PD98059 combined with LY37 treatment induced changes in GluA1 or GluA2 surface expression or total protein levels. Our data thus suggest that mGluR2/3 agonist regulates postsynaptic AMPA receptors by affecting the synaptic trafficking of both GluA1 and GluA2 subunits and that the regulation is likely through ERK1/2 signaling in GluA1 and/or both ERK1/2 and GSK-3β signaling pathways in the GluA2 subunit.

Glutamatergic Approaches for the Treatment of Schizophrenia

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system and plays a key role in most aspects of normal brain function including cognition, learning and memory. Dysfunction of glutamatergic neurotransmission has been implicated in a number of neurological and psychiatric disorders with a growing body of evidence suggesting that hypofunction of glutamatergic neurotransmission via the N-methyl-d-aspartate (NMDA) receptor plays an important role in the pathophysiology of schizophrenia. It thus follows that potentiation of NMDA receptor function via pharmacological manipulation may provide therapeutic utility for the treatment of schizophrenia and a number of different approaches are currently being pursued by the pharmaceutical industry with this aim in mind. These include strategies that target the glycine/d-serine site of the NMDA receptor (glycine transporter GlyT1, d-serine transporter ASC-1 and d-amino acid oxidase (DAAO) inhibitors) together with those aimed at enhancing glutamatergic neurotransmission via modulation of AMPA receptor and metabotropic glutamate receptor function. Such efforts are now beginning to bear fruit with compounds such as the GlyT1 inhibitor RG1678 and mGlu2 agonist LY2140023 proving to have clinical meaningful effects in phase II clinical trials. While more studies are required to confirm long-term efficacy, functional outcome and safety in schizophrenic agents, these agents hold real promise for addressing unmet medical needs, in particular refractory negative and cognitive symptoms, not currently addressed by existing antipsychotic agents.

NMDA receptor antagonism: escalation of aggressive behavior in alcohol-drinking mice

RATIONALE

Memantine is a potential treatment for alcoholic patients, yet few studies investigate the effect of concurrent treatment with memantine and ethanol on aggression. We evaluated aggressive behavior following ethanol consumption and treatment with glutamatergic drugs to characterize interactions between these compounds.

OBJECTIVE

This study aimed to use rodent models of aggression to examine interactions between glutamatergic compounds and ethanol.

MATERIALS AND METHODS

Once male CFW mice reliably self-administered 1 g/kg ethanol or water, they were assessed for aggression in resident-intruder confrontations. Alternatively, aggression was evaluated following a social-instigation procedure. Animals were then injected with memantine, ketamine, neramexane, MTEP, or LY379268 before aggressive confrontations. Effects of the pharmacological manipulations on salient aggressive and non-aggressive behaviors were analyzed.

RESULTS

Moderate doses of memantine, neramexane, and MTEP interacted with ethanol to increase the frequency of attack bites while ketamine did not. The highest dose of LY379268, an mGluR(2/3) agonist, reduced both aggressive and non-aggressive behaviors after water and ethanol self-administration. Attack bites increased with social instigation and decreased with administration of high doses of MTEP and LY379268. Memantine and MTEP both reduced attack bite frequency in the instigation condition without reducing locomotor behavior.

CONCLUSIONS

Memantine and neramexane interacted with ethanol to heighten aggression. The binding characteristics of these compounds allow for 'partial trapping' by which some NMDARs are unblocked between depolarizations. We propose that this feature may contribute to the differential aggression-heightening interactions between these compounds and ethanol. MTEP also interacted with ethanol to escalate aggression, possibly through inhibition of mGluR(5) modulation of NMDARs.

Capturing the angel in "angel dust": twenty years of translational neuroscience studies of NMDA receptor antagonists in animals and humans

Here, we describe our collaborative efforts to use N-methyl-d-aspartate (NMDA) receptor antagonists as a translational tool to advance our understanding of the pathophysiology of schizophrenia and identify potential new targets for treatment of schizophrenia. We began these efforts in the late 1980s with a keen sense that, in both human and animal studies, we needed to move beyond the dopamine hypothesis of schizophrenia; if the dopamine hypothesis were correct, the existing dopamine antagonists should have cured the disease but they have not. We used NMDA receptor antagonists, not to produce schizophrenia, but as a tool to provide insights into effects of disturbances in glutamate synaptic function in schizophrenia. Our work has provided insights into potential mechanisms that may contribute to disrupted cortical function in schizophrenia and has helped identify potential treatment targets for the disorder. The translational nature of this study made the clinical testing of the first of these targets feasible. Advances in systems neuroscience approaches in animals and humans make new types of translational research possible; however, our concern is that the current obstacles facing translational research funding and academia-industry collaborations threaten the future progress in this field.

A Review of Brain-derived Neurotrophic Factor as a Candidate Biomarker in Schizophrenia

Brain-derived neurotrophic factor (BDNF), a neurotrophin known to be responsible for development, regeneration, survival and maintenance of neurons has been implicated in the pathophysiology of schizophrenia. This review seeks to complement previous reviews on biological roles of BDNF and summarizes evidence on the involvement of BDNF in the pathophysiology of schizophrenia with an emphasis on clinical relevance. The expressions of BDNF were altered in patients with schizophrenia and were found to be correlated with psychotic symptomatology. Antipsychotics appeared to have differential effects on expression of BDNF but did not restore BDNF expression of patients with schizophrenia to normal levels. In addition, evidence suggests that BDNF is involved in the major neurotransmitter systems and is associated with disruptions in brain structure, neurodevelopmental process, cognitive function, metabolic and immune systems commonly associated with schizophrenia. Besides that, BDNF has been demonstrated to be tightly regulated with estrogen which has also been previously implicated in schizophrenia. Evidence gathered in this review confirms the relevance of BDNF in the pathophysiology of schizophrenia and the potential utility of BDNF as a suitable biomarker for diagnostic and prognostic purposes for disease outcome and other co-morbidities. However, further investigations are warranted to examine the specificity of BDNF in schizophrenia compared to other neurodegenerative disorders and other neuropsychiatric illness. Longitudinal prospective studies will also be of added advantage for evaluation of prognostic utility of BDNF in schizophrenia.

Impact of catechol-o-methyltransferase polymorphisms on risperidone treatment for schizophrenia and its potential clinical significance

OBJECTIVES

The main aim was to study the effects of COMT polymorphisms on response of risperidone treatment for schizophrenia and investigate the correlation between memory function of schizophrenia patients and COMT polymorphisms.

DESIGN AND METHODS

Subjects were 83 schizophrenic patients who were antipsychotic drug free at the initiation of this study. Peripheral blood samples were obtained to identify COMT polymorphisms by using a polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method. Clinical Global Impressions (CGI), Positive and Negative Syndrome Scale (PANSS) and Wechsler Memory Scale (WMS) test were used to assess the effect of risperidone treatment.

RESULTS

The Val/Met carriers showed a significant increase in change of P300 during treatment (P=0.032). Association of Val/Met carriers performed better than other genotypes (P=0.028). The mean plasma concentration of prolactin of Val/Val carriers was significantly lower (P=0.017).

CONCLUSIONS

The COMT polymorphisms may be a potential biomarker for clinical risperidone treatment in schizophrenia.

Modulation of midbrain dopamine neurotransmission by serotonin, a versatile interaction between neurotransmitters and significance for antipsychotic drug action

Schizophrenia has been associated with a dysfunction of brain dopamine (DA). This, so called, DA hypothesis has been refined as new insights into the pathophysiology of schizophrenia have emerged. Currently, dysfunction of prefrontocortical glutamatergic and GABAergic projections and dysfunction of serotonin (5-HT) systems are also thought to play a role in the pathophysiology of schizophrenia. Refinements of the DA hypothesis have lead to the emergence of new pharmacological targets for antipsychotic drug development. It was shown that effective antipsychotic drugs with a low liability for inducing extra-pyramidal side-effects have affinities for a range of neurotransmitter receptors in addition to DA receptors, suggesting that a combination of neurotransmitter receptor affinities may be favorable for treatment outcome.This review focuses on the interaction between DA and 5-HT, as most antipsychotics display affinity for 5-HT receptors. We will discuss DA/5-HT interactions at the level of receptors and G protein-coupled potassium channels and consequences for induction of depolarization blockade with specific attention to DA neurons in the ventral tegmental area (VTA) and the substantia nigra zona compacta (SN), neurons implicated in treatment efficacy and the side-effects of schizophrenia, respectively. Moreover, it has been reported that electrophysiological interactions between DA and 5-HT show subtle, but important, differences between the SN and the VTA which could explain (in part) the effectiveness and lower propensity to induce side-effects of the newer atypical antipsychotic drugs. In that respect the functional implications of DA/5-HT interactions for schizophrenia will be discussed.

Pharmacological effects of metabotropic glutamate receptor ligands on prepulse inhibition in DBA/2J mice

Schizophrenic patients typically exhibit impairment of sensorimotor gating, which can be modeled in animals using acoustic prepulse inhibition of the startle. Both classical and atypical antipsychotics have been shown to improve prepulse inhibition in DBA/2J mice, a non-pharmacological model for impaired sensorimotor gating. The purpose of the present study was to clarify whether metabotropic glutamate receptors participate in control of sensorimotor gating. We evaluated various metabotropic glutamate receptor ligands on prepulse inhibition in DBA/2J mice. This basal level of prepulse inhibition in DBA/2J mice was increased by only the mGlu(1) receptor antagonists [2-cyclopropyl-5-[1-(2-fluoro-3-pyridinyl)-5-methyl-1H-1,2,3-triazol-4-yl]-2,3-dihydro-1H-isoindol-1-one] (CFMTI), 6-amino-N-cyclohexyl-N,3-dimethylthiazolo[3,2-alpha]benzimidazole-2-carboxamide hydrochloride (YM-298198), and (3,4-dihydro-2H-pyrano[2,3-b]quinolin-7-yl)-(cis-4-methoxycyclohexyl)-methanone (JNJ16259685). There was no effect after treatments with the mGlu(5) receptor antagonist 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP), the mGlu(2/3) receptor agonist (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate (LY379268), the mGlu(2/3) receptor antagonist (2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid (LY341495), the mGlu(7) receptor agonist N,N'-dibenzhydrylethane-1,2-diamine dihydrochloride (AMN082), the mGlu(7) receptor antagonist 6-(4-methoxyphenyl)-5-methyl-3-pyridin-4-ylisoxazonolo[4,5-c]pyridin-4(5H)-one (MMPIP), or the mGlu(8) receptor agonist (S)-3,4-dicarboxyphenylglycine (DCPG). These findings indicate that inhibition of mGlu(1) receptor selectively increases prepulse inhibition in DBA/2J mice and suggest that mGlu(1) receptor antagonists could be a novel treatment for some aspects of schizophrenia.

Acetylcholine-dopamine interactions in the pathophysiology and treatment of CNS disorders

Dopaminergic neurons in the substantia nigra pars compacta and ventral tegmental area of the midbrain form the nigrostriatal and mesocorticolimbic dopaminergic pathways that, respectively, project to dorsal and ventral striatum (including prefrontal cortex). These midbrain dopaminergic nuclei and their respective forebrain and cortical target areas are well established as serving a critical role in mediating voluntary motor control, as evidenced in Parkinson's disease, and incentive-motivated behaviors and cognitive functions, as exhibited in drug addiction and schizophrenia, respectively. Although it cannot be disputed that excitatory and inhibitory amino acid-based neurotransmitters, such as glutamate and GABA, play a vital role in modulating activity of midbrain dopaminergic neurons, recent evidence suggests that acetylcholine may be as important in regulating dopaminergic transmission. Midbrain dopaminergic cell tonic and phasic activity is closely dependent upon projections from hindbrain pedunculopontine and the laterodorsal tegmental nuclei, which comprises the only known cholinergic inputs to these neurons. In close coordination with glutamatergic and GABAergic activity, these excitatory cholinergic projections activate nicotinic and muscarinic acetylcholine receptors within the substantia nigra and ventral tegmental area to modulate dopamine transmission in the dorsal/ventral striatum and prefrontal cortex. Additionally, acetylcholine-containing interneurons in the striatum also constitute an important neural substrate to provide further cholinergic modulation of forebrain striatal dopaminergic transmission. In this review, we examine neurological and psychopathological conditions associated with dysfunctions in the interaction of acetylcholine and dopamine and conventional and new pharmacological approaches to treat these disorders.

Uncoupling the D1-N-methyl-D-aspartate (NMDA) receptor complex promotes NMDA-dependent long-term potentiation and working memory

BACKGROUND

Although dopamine D1 receptors are involved in working memory, how D1 receptors contribute to this process remains unclear. Numerous studies have shown that D1 receptors have extensive functional interaction with N-methyl-D-aspartate (NMDA) receptor. Our group previously demonstrated that D1 receptors were able to regulate NMDA receptor functions through direct protein-protein interactions involving the carboxyl terminals of D1 receptors and NMDA receptor NR1a and NR2A subunits respectively. In this study, we explored the effects of the D1-NR1 interaction on NMDA receptor-dependent long-term potentiation (LTP) and working memory by using the TAT-conjugated interfering peptide (TAT-D1-t2).

METHODS

Miniature excitatory postsynaptic currents are recorded in rat hippocampal primary cultures. Coimmunoprecipitation and calcium/calmodulin-dependent protein kinase II (CaMKII) activity are measured in hippocampal slices and hippocampal neurons under the specified experimental conditions, respectively. Working memory was assessed using a delayed match-to-place protocol in the Morris Water Maze following administration of the TAT-D1-t2 peptide.

RESULTS

Electrophysiology experiments showed that activation of D1 receptor upregulates NMDA receptor-mediated LTP in a CaMKII-dependent manner. Furthermore, D1 receptor agonist stimulation promotes the NR1-CaMKII coupling and enhances the CaMKII activity; and the D1 receptor-mediated effects can be blocked by the application of the TAT-D1-t2 peptide. Interestingly, animals injected with TAT-D1-t2 peptide exhibited significantly impaired working memory.

CONCLUSIONS

Our study showed a critical role of NMDA-D1 direct protein-protein interaction in NMDA receptor-mediated LTP and working memory and implicated the involvement of CaMKII in this process.

Incidental and intentional sequence learning in youth-onset psychosis and Attention-Deficit/Hyperactivity Disorder (ADHD)

The goal was to compare incidental and intentional spatial sequence learning in youth-onset psychosis and ADHD. The authors tested 8- to 19-year-olds with psychosis or ADHD and healthy controls on a serial reaction time (RT) task and used manual and oculomotor measures to examine learning. Participants were also administered a block in which they were explicitly instructed to learn a sequence. As in our previous studies with healthy adults and children, oculomotor anticipations and RTs showed learning effects similar to those in the manual modality. Results showed intact sequence-specific learning but fewer oculomotor anticipations in both clinical groups during incidental learning. In intentional learning, only the psychosis group showed impairments compared to controls. There were no interactions between age and diagnosis. Thus, the psychosis group showed relatively preserved incidental learning despite impairments in intentional learning. Additionally, both clinical groups showed impairments in the ability to search for, extract, and anticipate regularities (whether the regularities were there or not), but not in the ability to respond to these regularities when they were there.