The trace amine-associated receptor 1 agonists - non-dopaminergic antipsychotics or covert modulators of D2 receptors?

A major effort of the pharmaceutical industry has been to identify and market drug treatments that are effective in ameliorating the symptoms of psychotic illness but without the limitations of the current treatments acting at dopamine D2 receptors. These limitations include the induction of a range of adverse effects, the inadequate treatment response of a substantial proportion of people with schizophrenia, and the generally poor response to negative and cognitive features of the disease. Recently introduced drug treatments have gone some way to avoiding the first of these, with a reduced propensity for weight gain, cardiovascular risk and extrapyramidal motor effects. Despite claims of some small improvements in negative symptoms, these drugs have not demonstrated substantial increases in efficacy. Of the drugs currently in development as antipsychotic agents, several are misleadingly described as having novel 'non-dopaminergic' mechanisms that may offer improvements in addressing the limitations of adverse effects and efficacy. It will be argued, using the trace amine-associated receptor 1 agonist as an example, that several of these new drugs still act primarily through modulation of dopaminergic neurotransmission and, in not addressing the primary pathology of schizophrenia, are therefore unlikely to have the much-needed improvements in efficacy required to address the unmet need associated with resistance to current treatments.

The role of SIRT3 in mediating the cognitive deficits and neuroinflammatory changes associated with a developmental animal model of schizophrenia.

The neuroinflammatory state may contribute to the pathogenesis of many mental disorders including schizophrenia. Nicotinamide adenine dinucleotide (NAD+) is an essential cofactor for activation of proteins involved in mitochondria quality control, such as Sirtuin3 (SIRT3). Our previous study has found that NAD+ supplement could rescue early life stress (ELS)-induced neuroinflammation and down-regulation of SIRT3 in adult offspring. However, it is unclear whether SIRT3 is the key to the neuroprotective effects of NAD+ supplement in this animal model of schizophrenia. The present study used 24 h maternal separation (MS) as ELS to Wistar rat pups on the postnatal day (PND) 9. Schizophrenia-like behaviors and memory impairments were detected by behavioral tests. Microglial activation, pro-inflammatory cytokine expression, and NAD+/SIRT3 expression were detected in the prefrontal cortex and hippocampus. Meanwhile, NAM (a precursor of NAD+), and the SIRT3 activator Honokiol (HNK), and the SIRT3 inhibitor 3-TYP were used as an intervention in vivo. Our results showed that ELS could induce schizophrenia-like behaviors and M1 microglial activation, NAD+ decline, lower expression of SIRT3, and increased acetylated superoxide dismutase 2 expression at the adult stage. NAD+ supplement or HNK administration could block this process and normalize the behavioral alterations of the MS animals. 3-TYP administration in the control group and the NAM-treated MS rats caused M1 microglial activation and cognitive deficits. Our results demonstrated that SIRT3 mediated the stabilizing effect of NAD+ on normalizing M1 microglial activation and behavioral phenotypes in MS rats.

Proteomic association with age-dependent sex differences in Wisconsin Card Sorting Test performance in healthy Thai subjects.

Sex differences in cognitive function exist, but they are not stable and undergo dynamic change during the lifespan. However, our understanding of how sex-related neural information transmission evolves with age is still in its infancy. This study utilized the Wisconsin Card Sorting Test (WCST) and the label-free proteomics method with bioinformatic analysis to investigate the molecular mechanisms underlying age-related sex differences in cognitive performance in 199 healthy Thai subjects (aged 20-70 years), as well as explore the sex-dependent protein complexes for predicting cognitive aging. The results showed that males outperformed females in two of the five WCST sub-scores: %Corrects and %Errors. Sex differences in these scores were related to aging, becoming noticeable in those over 60. At the molecular level, differently expressed individual proteins and protein complexes between both sexes are associated with the potential N-methyl-D-aspartate type glutamate receptor (NMDAR)-mediated excitotoxicity, with the NMDAR complex being enriched exclusively in elderly female samples. These findings provided a preliminary indication that healthy Thai females might be more susceptible to such neurotoxicity, as evidenced by their cognitive performance. NMDAR protein complex enrichment in serum could be proposed as a potential indication for predicting cognitive aging in healthy Thai females.

Elevated Expression of Two Pore Potassium Channel THIK-1 in Alzheimer's Disease: An Inflammatory Mechanism.

INTRODUCTION: Tandem pore domain halothane-inhibited K+ channel 1 (THIK-1, coded by KCNK13) provides an upstream regulation of the activation of the NOD-like receptor pyrin domain containing 3 (NLRP3) inflammasome, which has been suggested as one of the key mechanisms of the pathological process in neurodegeneration mainly from in vitro and in vivo model systems studies. However, unequivocal evidence from neurodegenerative disorders has been lacking. OBJECTIVE: To investigate the involvement of the THIK-1/NLRP3 pathway in the pathological process of Alzheimer's disease (AD) and Parkinson's disease (PD). METHODS: This study investigated gene expression of markers in the THIK-1/NLRP3 pathway in an animal model representing AD as well as in human postmortem brains of AD and PD by quantitative real-time PCR. THIK-1 protein expression was determined using automated capillary electrophoresis immunoblotting. Furthermore, DNA methylation of KCNK13 was analysed in AD cohort by pyrosequencing. RESULTS: A substantial upregulation of KCNK13, glial activation markers, NLRP3 inflammasome components, and IL1B was observed in the animal study. Increased expression of KCNK13 support an inflammatory glial cell activation in both advanced AD and PD. The increase in KCNK13 expression was also supported by downregulation in DNA methylation of KCNK13 in AD. CONCLUSIONS: The association between THIK-1 K+ channels expression and pathology changes indicates a THIK-1-induced activation of this glial subtype in AD and PD. Therefore, specific blocks of the microglial THIK-1 K+ channels at the early stage of AD and PD may be beneficial for the patients.

Cholinergic-estrogen interaction is associated with the effect of education on attenuating cognitive sex differences in a Thai healthy population.

The development of human brain is shaped by both genetic and environmental factors. Sex differences in cognitive function have been found in humans as a result of sexual dimorphism in neural information transmission. Numerous studies have reported the positive effects of education on cognitive functions. However, little work has investigated the effect of education on attenuating cognitive sex differences and the neural mechanisms behind it based on healthy population. In this study, the Wisconsin Card Sorting Test (WCST) was employed to examine sex differences in cognitive function in 135 Thai healthy subjects, and label-free quantitative proteomic method and bioinformatic analysis were used to study sex-specific neurotransmission-related protein expression profiles. The results showed sex differences in two WCST sub-scores: percentage of Total corrects and Total errors in the primary education group (Bayes factor>100) with males performed better, while such differences eliminated in secondary and tertiary education levels. Moreover, 11 differentially expressed proteins (DEPs) between men and women (FDR<0.1) were presented in both education groups, with majority of them upregulated in females. Half of those DEPs interacted directly with nAChR3, whereas the other DEPs were indirectly connected to the cholinergic pathways through interaction with estrogen. These findings provided a preliminary indication that a cholinergic-estrogen interaction relates to, and might underpin, the effect of education on attenuating cognitive sex differences in a Thai healthy population.

The Influence of Agricultural Work and Plasma Uric Acid on Hospital Admission for Alzheimer's Disease.

BACKGROUND: Exposure to environmental neurotoxins associated with agricultural work, such as pesticides, may be a risk factor for neurodegenerative disorders such as Alzheimer's (AD) and Parkinson's (PD) diseases. There is strong evidence that such exposure is associated with the development of PD; for AD the current evidence is equivocal. Several mechanisms are proposed to mediate this environmental toxicity, one of which is oxidative stress. Uric acid (UA) is an endogenous antioxidant, low levels of which are also implicated in neurodegenerative disease. OBJECTIVE: This study aimed to determine whether agricultural work was a risk factor for AD in a population in which its association with PD was established, and whether UA was also associated with AD in this cohort. METHODS: Hospital records of subjects meeting criteria for AD (n = 128) or vascular dementia (VaD) (n = 178) after hospital admission for symptoms of dementia were studied. History of agricultural work and plasma UA were recorded and their relationship to diagnosis determined. RESULTS: In contrast to previous findings in this population in which agricultural work was strongly associated with PD, a history of agricultural work was not over-represented in hospital admission for AD versus VaD. AD was associated with a reduced level of circulating UA compared with VaD. CONCLUSION: Agricultural work as a likely proxy for exposure to pesticides appears not to be a risk factor for AD to the extent found in PD, perhaps reflecting their differences in neuronal pathology. Nevertheless, findings with UA suggests that oxidative stress may be an important factor in AD pathogenesis.

Lifetime cannabis use and childhood trauma associated with CNR1 genetic variants increase the risk of psychosis: findings from the STREAM study.

OBJECTIVES: Gene-environment interactions increase the risk of psychosis. The objective of this study was to investigate gene-gene and gene-environment interactions in psychosis, including single nucleotide variants (SNVs) of dopamine-2 receptor (D2R), N-methyl-d-aspartate receptor (NMDAR), and cannabinoid receptor type 1 (CB1R), lifetime cannabis use, and childhood trauma. METHODS: Twenty-three SNVs of genes encoding D2R (DRD2: rs1799978, rs7131056, rs6275), NMDAR (GRIN1: rs4880213, rs11146020; GRIN2A: rs1420040, rs11866328; GRIN2B: rs890, rs2098469, rs7298664), and CB1R (CNR1: rs806380, rs806379, rs1049353, rs6454674, rs1535255, rs2023239, rs12720071, rs6928499, rs806374, rs7766029, rs806378, rs10485170, rs9450898) were genotyped in 143 first-episode psychosis patients (FEPp) and 286 community-based controls by Illumina HumanCoreExome-24 BeadChip. Gene-gene and gene-environment associations were assessed using nonparametric Multifactor Dimensionality Reduction software. RESULTS: Single-locus analyses among the 23 SNVs for psychosis and gene-gene interactions were not significant (p > 0.05 for all comparisons); however, both environmental risk factors showed an association with psychosis (p < 0.001). Moreover, gene-environment interactions were significant for an SNV in CNR1 and cannabis use. The best-performing model was the combination of CNR1 rs12720071 and lifetime cannabis use (p < 0.001), suggesting an increased risk of psychosis. CONCLUSION: Our study supports the hypothesis of gene-environment interactions for psychosis involving T-allele carriers of CNR1 SNVs, childhood trauma, and cannabis use.

Dopamine partial agonists: a discrete class of antipsychotics.

Worldwide, there are now three marketed dopamine D2 partial agonists: aripiprazole, brexpiprazole and cariprazine. These three drugs share a number of properties other than their action at D2 receptors. Pharmacologically, they are 5HT2 antagonists and D3 and 5HT1A partial agonists but with little or no alpha-adrenergic, anticholinergic or antihistaminic activity. They also share a long duration of action. Clinically, D2 partial agonists are effective antipsychotics and generally have useful antimanic and antidepressant activity. They are usually well tolerated, causing akathisia and insomnia only at the start of treatment, and are non-sedating. These drugs also share a very low risk of increased prolactin and of weight gain and accompanying metabolic effects. They may also have a relatively low risk of tardive dyskinesia. There is some evidence that they are preferred by patients to dopamine antagonists. Individual dopamineD2 partial agonists have much in common and as a group they differ importantly from dopamine D2 antagonists. Dopamine D2 partial agonists should be considered a distinct class of antipsychotics.Key pointsD2 partial agonists share many pharmacological and clinical propertiesD2 partial agonists differ in several important respects from D2 antagonistsD2 partial agonists should be considered a discrete class of antipsychotics.

Lifetime cannabis use and childhood trauma increase risk of psychosis in carriers of CNR1 genetic variants: findings from the STREAM study

Objectives: Gene-environment interactions increase the risk of psychosis. The objective of this study was to investigate gene-gene and gene-environment interactions in psychosis, including single nucleotide variants (SNVs) of dopamine-2 receptor (D2R), N-methyl-d-aspartate receptor (NMDAR), and cannabinoid receptor type 1 (CB1R), lifetime cannabis use, and childhood trauma. Methods: Twenty-three SNVs of genes encoding D2R (DRD2: rs1799978, rs7131056, rs6275), NMDAR (GRIN1: rs4880213, rs11146020; GRIN2A: rs1420040, rs11866328; GRIN2B: rs890, rs2098469, rs7298664), and CB1R (CNR1: rs806380, rs806379, rs1049353, rs6454674, rs1535255, rs2023239, rs12720071, rs6928499, rs806374, rs7766029, rs806378, rs10485170, rs9450898) were genotyped in 143 first-episode psychosis patients (FEPp) and 286 community-based controls by Illumina HumanCoreExome-24 BeadChip. Gene-gene and gene-environment associations were assessed using nonparametric Multifactor Dimensionality Reduction software. Results: Single-locus analyses among the 23 SNVs for psychosis and gene-gene interactions were not significant (p > 0.05 for all comparisons); however, both environmental risk factors showed an association with psychosis (p < 0.001). Moreover, gene-environment interactions were significant for an SNV in CNR1 and cannabis use. The best-performing model was the combination of CNR1 rs12720071 and lifetime cannabis use (p < 0.001), suggesting an increased risk of psychosis. Conclusion: Our study supports the hypothesis of gene-environment interactions for psychosis involving T-allele carriers of CNR1 SNVs, childhood trauma, and cannabis use.

Nicotinamide reverses deficits in puberty-born neurons and cognitive function after maternal separation.

BACKGROUND: Early life stress (ELS) is associated with the development of schizophrenia later in life. The hippocampus develops significantly during childhood and is extremely reactive to stress. In rodent models, ELS can induce neuroinflammation, hippocampal neuronal loss, and schizophrenia-like behavior. While nicotinamide (NAM) can inhibit microglial inflammation, it is unknown whether NAM treatment during adolescence reduces hippocampal neuronal loss and abnormal behaviors induced by ELS. METHODS: Twenty-four hours of maternal separation (MS) of Wistar rat pups on post-natal day (PND)9 was used as an ELS. On PND35, animals received a single intraperitoneal injection of BrdU to label dividing neurons and were given NAM from PND35 to PND65. Behavioral testing was performed. Western blotting and immunofluorescence staining were used to detect nicotinamide adenine dinucleotide (NAD+)/Sirtuin3 (Sirt3)/superoxide dismutase 2 (SOD2) pathway-related proteins. RESULTS: Compared with controls, only MS animals in the adult stage (PND56-65) but not the adolescent stage (PND31-40) exhibited pre-pulse inhibition deficits and cognitive impairments mimicking schizophrenia symptoms. MS decreased the survival and activity of puberty-born neurons and hippocampal NAD+ and Sirt3 expression in adulthood. These observations were related to an increase in acetylated SOD2, microglial activation, and significant increases in pro-inflammatory IL-1ß, TNF-α, and IL-6 expression. All the effects of MS at PND9 were reversed by administering NAM in adolescence (PND35-65). CONCLUSIONS: MS may lead to schizophrenia-like phenotypes and persistent hippocampal abnormalities. NAM may be a safe and effective treatment in adolescence to restore normal hippocampal function and prevent or ameliorate schizophrenia-like behavior.

Subchronic PCP effects on DNA methylation and protein expression of NMDA receptor subunit genes in the prefrontal cortex and hippocampus of female rats.

BACKGROUND: N-methyl-d-aspartate receptor (NMDAR) dysfunction is implicated in schizophrenia, and NMDAR antagonists, such as phencyclidine (PCP), can induce behaviours that mimic aspects of the disorder. AIMS: We investigated DNA methylation of Grin1, Grin2a and Grin2b promoter region and NR1 and NR2 protein expression in the prefrontal cortex (PFC) and hippocampus of adult female Lister-hooded rats following subchronic PCP (scPCP) administration. We also determined whether any alterations were tissue-specific. METHODS: Rats were divided into two groups that received vehicle (0.9% saline) or 2 mg/kg PCP twice a day for 7 days (n = 10 per group). After behavioural testing (novel object recognition), to confirm a cognitive deficit, brains were dissected and NMDAR subunit DNA methylation and protein expression were analysed by pyrosequencing and ELISA. Line-1 methylation was determined as a measure of global methylation. Data were analysed using Student's t-test and Pearson correlation. RESULTS: The scPCP administration led to Grin1 and Grin2b hypermethylation and reduction in NR1 protein in both PFC and hippocampus. No significant differences were observed in Line-1 or Grin2a methylation and NR2 protein. CONCLUSIONS: The scPCP treatment resulted in increased DNA methylation at promoter sites of Grin1 and Grin2b NMDAR subunits in two brain areas implicated in schizophrenia, independent of any global change in DNA methylation, and are similar to our observations in a neurodevelopmental animal model of schizophrenia - social isolation rearing post-weaning. Moreover, these alterations may contribute to the changes in protein expression for NMDAR subunits demonstrating the potential importance of epigenetic mechanisms in schizophrenia.

The neurochemical pathology of schizophrenia: post-mortem studies from dopamine to parvalbumin.

Research in Peter Riederer's lab in Vienna in the late 1970's came from a strong tradition in post-mortem neurochemical studies, at that time a relatively niche approach in neuroscience research. He was also early to recognise the value of post-mortem brain tissue in elucidating pharmacological mechanisms of neuropsychiatric treatments. I was fortunate to have Peter Riederer as a mentor in my early post-doctoral career; his generous support and the opportunities to use post-mortem brain tissue provided an invaluable grounding on which much of my future research was based. In this paper, I shall provide a brief overview of one trajectory of my research into the neurobiology of schizophrenia that started in the Riederer lab in Vienna investigating dopamine and the D2 receptor. Subsequent research to understand findings of increased dopamine resulted in the identification of reduced GABAergic innervation, culminating in the finding of a deficit in the parvalbumin-containing subtype of GABAergic neurons. Most recent work has been studying how changes in DNA methylation of the parvalbumin gene may relate to these findings in psychotic illness and its animal models.

High dose antipsychotic polypharmacy and dopamine partial agonists - time to rethink guidelines?

Guidelines for the treatment of schizophrenia limit the use of antipsychotic agents to clinically-established maximum doses. This acknowledges both the absence of additional efficacy of dopamine D2 receptor antagonists above a receptor occupancy threshold, and the increases in side effects that can occur at higher doses. These limits restrict the dosing of combinations of antipsychotics as they do single agents; drugs sharing the major antipsychotic mechanism of D2 receptor antagonism will act additively in blocking these receptors.Several newer antipsychotic drugs, including aripiprazole and cariprazine, act as partial agonists at the D2 receptor site and avoid action at several other receptors, effects at which are responsible for some non-dopaminergic adverse effects. This pharmacology imparts different characteristics to the drugs resulting often in a more favourable side effect profile. Their partial agonism, along with high affinities for the D2 receptor, also means that these drugs given adjunctively may in part replace, rather than enhance, the D2 antagonism of other antipsychotic agents. This can result in an improvement in certain side effects without loss of antipsychotic efficacy.This article makes the case for distinguishing the D2 partial agonists from antagonists in defining maximum doses of combined treatments, which would increase the options available to the prescriber, emphasising that pharmacological mechanisms need to be understood in identifying optimal treatments for psychotic illness.

The relationship of childhood trauma and DNA methylation of NMDA receptor genes in first-episode schizophrenia.

Aim: We investigated GRIN1, GRIN2A, GRIN2B and LINE-1 DNA methylation in first-episode schizophrenia patients, their nonaffected siblings and age- and sex-matched controls testing for associations between DNA methylation and exposition to childhood trauma. Materials & methods: The Childhood Trauma Questionnaire evaluated the history of childhood trauma. Genomic DNA was bisulfite converted and pyrosequencing was employed to quantify DNA methylation. Results:GRIN2A, GRIN2B and LINE-1 DNA methylation was not associated with childhood trauma in patients, siblings and controls. Siblings with childhood trauma had hypermethylation at CpG1 of GRIN1 compared with siblings without trauma. Conclusion: Childhood trauma may influence GRIN1 methylation in subjects with liability to psychosis, but not in frank schizophrenia or controls.

Lay abstract Schizophrenia results from a combination of genetic and environmental influences. We investigated how some changes in genes can be silenced by a process named DNA methylation and may be linked to schizophrenia. For this reason, we hypothesized that childhood trauma, an environmental risk factor, would be associated with DNA methylation in schizophrenia patients compared with their unaffected siblings and controls. Our research has shown that altered blood DNA methylation of one candidate gene for psychiatric disorders may be associated with childhood trauma in the unaffected siblings of schizophrenia patients, but not in frank schizophrenia or controls. We believe that this gene plays an important role in helping identify vulnerable as well as resilient individuals to schizophrenia disorder.

Changes of BDNF exon IV DNA methylation are associated with methamphetamine dependence.

Aim: We investigated DNA methylation of BDNF in methamphetamine (METH) dependence in humans and an animal model. Materials & methods:BDNF methylation at exon IV was determined by pyrosequencing of blood DNA from METH-dependent and control subjects, and from rat brain following an escalating dose of METH or vehicle. Bdnf expression was determined in rat brain. Results:BDNF methylation was increased in human METH dependence, greatest in subjects with psychosis and in prefrontal cortex of METH-administered rats; rat hippocampus showed reduced Bdnf methylation and increased gene expression. Conclusion:BDNF methylation is abnormal in human METH dependence, especially METH-dependent psychosis, and in METH-administered rats. This may influence BDNF expression and contribute to the neurotoxic effects of METH exposure.

Lay abstract The effects of methamphetamine (METH), an addictive psychostimulant drug, on changes of DNA methylation of an important regulator of neuronal survival, BDNF, were examined in blood of METH-dependent patients and in the brain of METH-administered rats. BDNF methylation was increased in patients and in the prefrontal cortex of METH-administered rats, while rat hippocampus showed a reduction of Bdnf methylation, with an equivalent increase in gene expression. The methylation increases in humans were greatest in those with a METH-induced psychosis. Although a relationship between Bdnf methylation and its expression has not been proven, changes of BDNF DNA methylation are associated with METH dependence, especially METH-dependent psychosis, suggesting that METH neurotoxicity may relate to the effects of changes in BDNF methylation.

Early-life stress effects on BDNF DNA methylation in first-episode psychosis and in rats reared in isolation.

Stressful events during early-life are risk factors for psychiatric disorders. Brain-derived neurotrophic factor (BDNF) is implicated in psychosis pathophysiology and deficits in BDNF mRNA in animal models of psychiatric disease are reported. DNA methylation can control gene expression and may be influenced by environmental factors such as early-life stress. We investigated BDNF methylation in first-episode psychosis (FEP) patients (n = 58), their unaffected siblings (n = 29) and community-based controls (n = 59), each of whom completed the Childhood Trauma Questionnaire (CTQ); BDNF methylation was also tested in male Wistar rats housed isolated or grouped from weaning. DNA was extracted from human blood and rat brain (prefrontal cortex and hippocampus), bisulphite-converted and the methylation of equivalent sequences within BDNF exon IV determined by pyrosequencing. BDNF methylation did not differ significantly between diagnostic groups; however, individuals who had experienced trauma presented higher levels of methylation. We found association between the mean BDNF methylation and total CTQ score in FEP, as well as between individual CpG sites and subtypes of trauma. No significant correlations were found for controls or siblings with child trauma. These results were independent of age, gender, body mass index, BDNF genotype or LINE-1, a measure of global methylation, which showed no significant association with trauma. Isolation rearing resulted in increased BDNF methylation in both brain regions compared to group-housed animals, a correlate of previously reported changes in gene expression. Our results suggest that childhood maltreatment may result in increased BDNF methylation, providing a mechanism underlying the association between early-life stress and psychosis.