A region of 35 kb containing the trace amine associate receptor 6 (TAAR6) gene is associated with schizophrenia in the Irish study of high-density schizophrenia families

Public Transcriptomic Data Meta-Analysis Demonstrates TAAR6 Expression in the Mental Disorder-Related Brain Areas in Human and Mouse Brain

G protein-coupled trace amine-associated receptors (TAAR) recognize different classes of amine compounds, including trace amines or other exogenous and endogenous molecules. Yet, most members of the TAAR family (TAAR2-TAAR9) are considered olfactory receptors involved in sensing innate odors. In this study, TAAR6 mRNA expression was evaluated in the brain transcriptomic datasets available in the GEO, Allen Brain Atlas, and GTEx databases. Transcriptomic data analysis demonstrated ubiquitous weak TAAR6 mRNA expression in the brain, especially in the prefrontal cortex and nucleus accumbens. RNA sequencing of isolated cells from the nucleus accumbens showed that the expression of TAAR6 in some cell populations may be more pronounced than in whole-tissue samples. Curiously, in D1 and D2 medium spiny neurons of the nucleus accumbens, TAAR6 expression was co-regulated with genes involved in G protein-coupled receptor signaling. However, in cholinergic interneurons of the nucleus accumbens, TAAR6 expression was not associated with the activation of any specific biological process. Finally, TAAR6 expression in the mouse prefrontal cortex was validated experimentally by RT-PCR analysis. These data demonstrated that TAAR6 is expressed at low levels in the human and mouse brain, particularly in limbic structures involved in the pathogenesis of mental disorders, and thus might represent a new pharmacotherapeutic target.

TAAR6variation effect on clinic presentation and outcome in a sample of schizophrenic in-patients: An open label study

We recently reported an association betweenTAAR6(trace amine associated receptor 6 gene) variations and schizophrenia (SZ). We now report an association of a set ofTAAR6variations and clinical presentation and outcome in a sample of 240 SZ Korean patients. Patients were selected by a Structured Clinical Interview, DSM-IV Axis I disorders – Clinical Version (SCID-CV). Other psychiatric or neurologic disorders, as well as medical diseases, were exclusion criteria. To assess symptom severity, patients were administered the CGI scale and the PANSS at baseline and at the moment of discharge, 1 month later on average.TAAR6variations rs6903874, rs7452939, rs8192625 and rs4305745 were investigated; rs6903874, rs7452939 and rs8192625 entered the statistical investigation after LD analysis. Rs8192625 G/G homozygosis was found to be significantly associated both with a worse clinical presentation at PANSS total and positive scores and with a shorter period of illness before hospitalization. No haplotype significant findings were found. The present study stands for a role of theTAAR6in the clinical presentation of SZ. Moreover, our results show that this genetic effect may be counteracted by a correct treatment. Haplotype analysis was not informative in our sample, probably also because of the incomplete SNPs' coverage of the gene we performed. Further studies in this direction are warranted.

Molecular Variants in Human Trace Amine-Associated Receptors and Their Implications in Mental and Metabolic Disorders

We provide a comprehensive review of the available evidence on the pathophysiological implications of genetic variants in the human trace amine-associated receptor (TAAR) superfamily. Genes coding for trace amine-associated receptors (taars) represent a multigene family of G-protein-coupled receptors, clustered to a small genomic region of 108 kb located in chromosome 6q23, which has been consistently identified by linkage analyses as a susceptibility locus for schizophrenia and affective disorders. Most TAARs are expressed in brain areas involved in emotions, reward and cognition. TAARs are activated by endogenous trace amines and thyronamines, and evidence for a modulatory action on other monaminergic systems has been reported. Therefore, linkage analyses were followed by fine mapping association studies in schizophrenia and affective disorders. However, none of these reports has received sufficient universal replication, so their status remains uncertain. Single nucleotide polymorphisms in taars have emerged as susceptibility loci from genome-wide association studies investigating migraine and brain development, but none of the detected variants reached the threshold for genome-wide significance. In the last decade, technological advances enabled single-gene or whole-exome sequencing, thus allowing the detection of rare genetic variants, which may have a greater impact on the risk of complex disorders. Using these approaches, several taars (especially taar1) variants have been detected in patients with mental and metabolic disorders, and in some cases, defective receptor function has been demonstrated in vitro. Finally, with the use of transcriptomic and peptidomic techniques, dysregulations of TAARs (especially TAAR6) have been identified in brain disorders characterized by cognitive impairment.

Accelerated pseudogenization of trace amine-associated receptor genes in primates

Trace amines (TAs) in the mammalian brain have been investigated for four decades. Trace amine‐associated receptors (TAARs) were discovered during the search for receptors activated by TAs. TAARs are considered a second class of vertebrate olfactory receptors and successfully proliferated in conjunction with adaptation to living on the ground to detect carnivore odors. Thus, therian mammals have a high number of TAAR genes due to rapid species‐specific gene duplications. In primate lineages, however, their genomes have significantly smaller numbers of TAAR genes than do other mammals. To elucidate the evolutionary force driving these patterns, exhaustive data mining of TAAR genes was performed for 13 primate genomes (covering all four infraorders) and two nonprimate euarchontan genomes. This study identified a large number of pseudogenes in many of these primate genomes and thus investigated the pseudogenization event process for the TAAR repertoires. The degeneration of TAARs is likely associated with arboreal inhabitants reducing their exposure to carnivores, and this was accelerated by the change in the nose shape of haplorhines after their divergence from strepsirrhines. Arboreal life may have decreased the reliance on the chemosensing of predators, suggestive of leading to the depauperation of TAAR subfamilies. The evolutionary deterioration of TAARs in primates has been reestablished in recently derived primates due to high selection pressure and probably functional diversity.

Trace Amines and Their Receptors

Trace amines are endogenous compounds classically regarded as comprising β-phenylethyalmine, p-tyramine, tryptamine, p-octopamine, and some of their metabolites. They are also abundant in common foodstuffs and can be produced and degraded by the constitutive microbiota. The ability to use trace amines has arisen at least twice during evolution, with distinct receptor families present in invertebrates and vertebrates. The term "trace amine" was coined to reflect the low tissue levels in mammals; however, invertebrates have relatively high levels where they function like mammalian adrenergic systems, involved in "fight-or-flight" responses. Vertebrates express a family of receptors termed trace amine-associated receptors (TAARs). Humans possess six functional isoforms (TAAR1, TAAR2, TAAR5, TAAR6, TAAR8, and TAAR9), whereas some fish species express over 100. With the exception of TAAR1, TAARs are expressed in olfactory epithelium neurons, where they detect diverse ethological signals including predators, spoiled food, migratory cues, and pheromones. Outside the olfactory system, TAAR1 is the most thoroughly studied and has both central and peripheral roles. In the brain, TAAR1 acts as a rheostat of dopaminergic, glutamatergic, and serotonergic neurotransmission and has been identified as a novel therapeutic target for schizophrenia, depression, and addiction. In the periphery, TAAR1 regulates nutrient-induced hormone secretion, suggesting its potential as a novel therapeutic target for diabetes and obesity. TAAR1 may also regulate immune responses by regulating leukocyte differentiation and activation. This article provides a comprehensive review of the current state of knowledge of the evolution, physiologic functions, pharmacology, molecular mechanisms, and therapeutic potential of trace amines and their receptors in vertebrates and invertebrates.

Association between TAAR6 polymorphisms and airway responsiveness to inhaled corticosteroids in asthmatic patients

BACKGROUND

Genetic polymorphisms may be responsible for the wide variation in response to inhaled corticosteroids in asthmatic patients. We had previously reported that one polymorphism rs7772821, located on the 3'-UTR of trace amine-associated receptor 6 (TAAR6), is significantly associated with percentile changes in the forced expiratory volume in 1 s (%ΔFEV1) after inhaled corticosteroid treatment in asthmatics using a genome-wide association study. The aim of the present study was to validate the association between 15 single-nucleotide polymorphisms (SNPs) on the TAAR6 and airway responsiveness to inhaled corticosteroids in the asthmatics.

METHODS

The %ΔFEV1 induced by 4 weeks' treatment with inhaled fluticasone propionate (1000 μg daily) was measured in 246 asthmatics. The 15 SNPs of TAAR6 were genotyped using a TaqMan assay. An association analysis between %ΔFEV1 and TAAR6 polymorphisms was carried out using a linear regression model controlling for age, sex, smoking status, presence of atopy, and baseline FEV1 as covariates.

RESULTS

Among the 15 SNPs and seven haplotypes of TAAR6, rs7772821 (T>G) on the 3'-UTR showed the strongest correlation with inhaled corticosteroid-induced %ΔFEV1 (Pcorr=0.002 in the codominant model, Pcorr=0.03 in the dominant model, Pcorr=0.01 in the recessive model). The %ΔFEV1 of the rs7772821T>G minor homozygotes (60.77%) was higher than that of patients harboring either the rs7772821 T/G or T/T genotypes (21.32 and 31.60%, respectively).

CONCLUSION

The TAAR6 rs7772821 polymorphism may be one of the important genetic factors for predicting the response to treatment with inhaled corticosteroids in asthmatics.

Possible role of rare variants in Trace amine associated receptor 1 in schizophrenia

Schizophrenia (SZ) is a chronic mental illness with behavioral abnormalities. Recent common variant based genome wide association studies and rare variant detection using next generation sequencing approaches have identified numerous variants that confer risk for SZ, but etiology remains unclear propelling continuing investigations. Using whole exome sequencing, we identified a rare heterozygous variant (c.545G>T; p.Cys182Phe) in Trace amine associated receptor 1 gene (TAAR1 6q23.2) in three affected members in a small SZ family. The variant predicted to be damaging by 15 prediction tools, causes breakage of a conserved disulfide bond in this G-protein-coupled receptor. On screening this intronless gene for additional variant(s) in ~800 sporadic SZ patients, we identified six rare protein altering variants (MAF<0.001) namely p.Ser47Cys, p.Phe51Leu, p.Tyr294Ter, p.Leu295Ser in four unrelated north Indian cases (n=475); p.Ala109Thr and p.Val250Ala in two independent Caucasian/African-American patients (n=310). Five of these variants were also predicted to be damaging. Besides, a rare synonymous variant was observed in SZ patients. These rare variants were absent in north Indian healthy controls (n=410) but significantly enriched in patients (p=0.036). Conversely, three common coding SNPs (rs8192621, rs8192620 and rs8192619) and a promoter SNP (rs60266355) tested for association with SZ in the north Indian cohort were not significant (P>0.05). TAAR1 is a modulator of monoaminergic pathways and interacts with AKT signaling pathways. Substantial animal model based pharmacological and functional data implying its relevance in SZ are also available. However, this is the first report suggestive of the likely contribution of rare variants in this gene to SZ.

The emerging roles of human trace amines and human trace amine-associated receptors (hTAARs) in central nervous system

Human trace amines (TAs) are endogenous compounds, previously almost ignored in human pathology for many reasons (difficulty of their measurement in biological fluids, unknown receptors for elusive amines), are now considered to play a significant role in synaptic transmission within the central nervous system (CNS) acting as neuromodulators. The recent discovery of a novel family of G-protein-coupled receptors (GPCRs) that includes individual members that are highly specific for TAs indicates a potential role for TAs as vertebrate neurotransmitters or neuromodulators, although the majority of these GPCRs so far have not been demonstrated to be activated by TAs. Human trace amine receptors (including TAAR1 TAAR2 TAAR5 TAAR6 TAAR8 TAAR9) are expressed in the brain and play significant physiological and neuropathological roles by activation of trace amines. We herein discuss the recent findings that provide insights into the functional roles of human trace amines (including P-Octopamine, β phenylethylamine, Tryptamine, Tyramine, Synephrine, 3-Iodothyronamine, 3-Methoxytyramine, N-Methyltyramine, N-Methylphenethylamine) in brain. Furthermore, we discuss the known functions of human trace amine receptors in brain.

Transcriptome study of differential expression in schizophrenia

Schizophrenia genome-wide association studies (GWAS) have identified common SNPs, rare copy number variants (CNVs) and a large polygenic contribution to illness risk, but biological mechanisms remain unclear. Bioinformatic analyses of significantly associated genetic variants point to a large role for regulatory variants. To identify gene expression abnormalities in schizophrenia, we generated whole-genome gene expression profiles using microarrays on lymphoblastoid cell lines (LCLs) from 413 cases and 446 controls. Regression analysis identified 95 transcripts differentially expressed by affection status at a genome-wide false discovery rate (FDR) of 0.05, while simultaneously controlling for confounding effects. These transcripts represented 89 genes with functions such as neurotransmission, gene regulation, cell cycle progression, differentiation, apoptosis, microRNA (miRNA) processing and immunity. This functional diversity is consistent with schizophrenia's likely significant pathophysiological heterogeneity. The overall enrichment of immune-related genes among those differentially expressed by affection status is consistent with hypothesized immune contributions to schizophrenia risk. The observed differential expression of extended major histocompatibility complex (xMHC) region histones (HIST1H2BD, HIST1H2BC, HIST1H2BH, HIST1H2BG and HIST1H4K) converges with the genetic evidence from GWAS, which find the xMHC to be the most significant susceptibility locus. Among the differentially expressed immune-related genes, B3GNT2 is implicated in autoimmune disorders previously tied to schizophrenia risk (rheumatoid arthritis and Graves' disease), and DICER1 is pivotal in miRNA processing potentially linking to miRNA alterations in schizophrenia (e.g. MIR137, the second strongest GWAS finding). Our analysis provides novel candidate genes for further study to assess their potential contribution to schizophrenia.

Parent-of-origin effects of FAS and PDLIM1 in attention-deficit/hyperactivity disorder

BACKGROUND

Previous studies have suggested that there may be a parent-of-origin effect for attention-deficit/hyperactivity disorder(ADHD) candidate genes. The objective of the present study was to investigate parent-of-origin effects using a genome-wide association analysis of the International Multicentre ADHD Genetics (IMAGE) study sample.

METHODS

Family-based association analysis for ADHD using 846 ADHD probands and their parents was performed using the PLINK program, and parent-of-origin effects were studied using a Z score for the difference in paternal versus maternal odds ratios.

RESULTS

We identified 44 single nucleotide polymorphisms (SNPs) showing parent-of-origin effects at a significance level of p < 0.001. The most significant SNP, rs7614907, is at position 3q13.33 in the CDGAP gene (p = 0.000064 for parent-of-origin effect). Furthermore, 2 genes (FAS and PDLIM1) showed moderate parent-of-origin effects (p = 0.00086 for rs9658691 and p = 0.00077 for rs11188249) and strong maternal transmission (p = 0.000059 for rs9658691 and p = 0.0000068 for rs11188249). In addition, ZNF775 showed a moderate parent-of-origin effect (p = 0.00036 for rs7790549) and strong paternal transmission (p = 0.000041 for rs7790549).

LIMITATIONS

We only had 1 sample available for analysis.

CONCLUSION

These results suggest several genes or regions with moderate parent-of-origin effects, and these findings will serve as a resource for replication in other populations to elucidate the potential role of these genetic variants in ADHD.

Behavioral effects of clozapine: involvement of trace amine pathways in C. elegans and M. musculus

Clozapine is an antipsychotic medication with superior efficacy in treatment refractory schizophrenia. The molecular basis of clozapine's therapeutic profile is not well understood. We studied behavioral effects of clozapine in Caenorhabditis elegans to identify novel pathways that modulate clozapine's biological effects. Clozapine stimulated egg laying in C. elegans in a dose-dependent manner. This effect was clozapine-specific, as it was not observed with exposure to a typical antipsychotic, haloperidol or an atypical antipsychotic, olanzapine. A candidate gene screen of biogenic amine neurotransmitter systems identified signaling pathways that mediate this clozapine-specific effect on egg laying. Specifically, we found that clozapine-induced increase in egg laying requires tyramine biosynthesis. To test the implications of this finding across species, we explored whether trace amine systems modulate clozapine's behavioral effects in mammals by studying trace amine-associated receptor 1 (TAAR1) knockout mice. Clozapine increased prepulse inhibition (PPI) in wild-type mice. This increase in PPI was abrogated in TAAR1 knockout mice, implicating TAAR1 in clozapine-induced PPI enhancement. In transfected mammalian cell lines, we found no TAAR activation by antipsychotics, suggesting that modulation of trace amine signaling in mice does not occur directly at the receptor itself. In summary, we report a heretofore-unknown role for trace amine systems in clozapine-mediated effects across two species: C. elegans and mice.

TAAR6 variations possibly associated with antidepressant response and suicidal behavior

Trace amines are putative regulatory elements in the brain whose activity may be relevant to the pathophysiology of depressive episodes. TAAR6 is an orphan receptor probably associated with trace amines. Its genetic variations have been associated with bipolar and schizophrenic disorders. In this study we investigated for the first time the possible association between a set of TAAR6 genetic variations (rs7452939; rs4305745; rs6903874; rs6937506; and rs8192625) with clinical features of depression including antidepressant treatment response in a sample of 187 depressive patients all of Korean origins. rs6903874 T/T carriers had a statistically significant better improvement, and rs6937506 C/C genotype was found to be more frequent in patients without a history of suicide attempt (incomplete or unsuccessful suicide). Haplotype analyses confirmed the association with suicide attempt behavior being haplotype G-T at SNPs rs7452939 and rs6937506 at risk of suicide. These results suggest a possible role of TAAR6 in antidepressant response and suicide behavior in patients with depressive disorder. Heterogeneity of treatment, possible stratification bias not controlled by the statistical analyses, and the risk of false-positive finding mandate further analysis in this direction.

Functional evolution of the trace amine associated receptors in mammals and the loss of TAAR1 in dogs

Background

The trace amine associated receptor family is a diverse array of GPCRs that arose before the first vertebrates walked on land. Trace amine associated receptor 1 (TAAR1) is a wide spectrum aminergic receptor that acts as a modulator in brain monoaminergic systems. Other trace amine associated receptors appear to relate to environmental perception and show a birth-and-death pattern in mammals similar to olfactory receptors.

Results

Across mammals, avians, and amphibians, the TAAR1 gene is intact and appears to be under strong purifying selection based on rates of amino acid fixation compared to neutral mutations. We have found that in dogs it has become a pseudogene. Our analyses using a comparative genetics approach revealed that the pseudogenization event predated the emergence of the Canini tribe rather than being coincident with canine domestication. By assessing the effects of the TAAR1 agonist β-phenylethylamine on [³H]dopamine uptake in canine striatal synaptosomes and comparing the degree and pattern of uptake inhibition to that seen in other mammals, including TAAR1 knockout mice, wild type mice and rhesus monkey, we found that the TAAR1 pseudogenization event resulted in an uncompensated loss of function.

Conclusion

The gene family has seen expansions among certain mammals, notably rodents, and reductions in others, including primates. By placing the trace amine associated receptors in an evolutionary context we can better understand their function and their potential associations with behavior and neurological disease.

Tailoring the definition of the clinical schizophrenia phenotype in linkage studies

The delineation of schizophrenia-related symptomatology is critical to informative clinical phenotyping in linkage studies. A minority of first-degree relatives of schizophrenia and schizoaffective probands (RelSZSA) qualifies for a clinical diagnosis in the schizophrenia spectrum. Schizotypal personality disorder (SPD) is a key component of this spectrum, largely because of its relatively specific familial aggregation in relatives. The criteria for SPD were not developed for the purpose of identifying RelSZSA, however, and SPD is not a homogeneous clinical disorder, potentially introducing false positives and false negatives into affectedness classifications. In this study we used logistic regression (LR) to identify the combination of clinical signs and symptoms that maximized the discrimination between nonpsychotic first-degree RelSZSA (n=241) and controls (n=161). Three variables contributed significantly to optimizing this distinction: no close friends or confidants other than family members, social isolation and irritability. The combination of deviant LR scores and schizophrenia-spectrum psychotic disorders had greater sensitivity for identifying RelSZSA, 23.7%, than SPD and schizophrenia-spectrum psychotic disorders, 16%. Importantly, the diagnosis of SPD and deviant LR scores were not significantly correlated. Most individuals with deviant LR scores did not meet criteria for a diagnosis of SPD and only a minority of those who were diagnosed with SPD had deviant LR scores. Since misclassification of gene carriers as non-gene carriers in linkage analyses increases the risk of false negatives, it may be advantageous to tailor the definition of the clinical phenotype to those aspects of social-interpersonal dysfunction that optimize the discrimination of RelSZSA from controls.

Influence of TAAR6 polymorphisms on response to aripiprazole

BACKGROUND

There is some evidence suggesting a role of TAAR6 in schizophrenia. The aim of the present study is to investigate possible influences of a panel of markers in TAAR6 (rs8192625, rs4305745, rs4305746, rs6903874, rs6937506) on clinical outcomes and side effects in a sample of Korean schizophrenic aripiprazole treated patients.

METHODS

Efficacy was assessed at baseline and weeks 1, 2, 4, 6, 8 using CGI-S, CGI-I, BPRS and SANS. Side effects were evaluated through SAS, BAS and AIMS. Multivariate analysis of covariance (MANCOVA) was used to test possible influences of single SNPs on clinical and safety scores. Tests for associations using multi-marker haplotypes were performed using the statistics environment "R".

RESULTS

A significant time per genotype interaction was found between rs4305746 in repeated measures of ANOVA on BPRS scores (F=2.45, df=10,365, p=0.008). In particular G/A and A/A genotype patients were more likely to improve over time as compared to carriers of the G/G genotype. Permutation analysis confirmed a significant effect of rs4305746 on course of BPRS scores over time (p=0.007). Haplotype analysis did not reveal any significant association with clinical and safety scores at any time.

CONCLUSION

A possible association could exist between some genotypes in TAAR6 and response to aripiprazole. However, several limitations characterize the present work, such as small sample size, the finding related to a single scale and the possibility of false positive findings, thus further investigation is required.

The trace amine associated receptor (TAAR6) gene is not associated with schizophrenia in the Irish Case-Control Study of Schizophrenia (ICCSS) sample

To replicate previous association between TAAR6 and schizophrenia, including our own finding in the Irish Study of High Density Schizophrenia Families (ISHDSF) sample, we genotyped 12 single nucleotide polymorphisms (SNPs) in the Irish Case-Control Study of Schizophrenia (ICCSS) sample. Only rs9389020 provided nominal evidence for association (p<0.0228), which did not withstand the permutation testing (p<0.2196). The combined odds ratio from ISHDSF and ICCSS samples [OR (95%CI)=1.0564 (1.0078-1.1074); p=0.02], while nominally significant, did not survive correction for multiple testing. Here we demonstrate that TAAR6 is not associated with schizophrenia in the ICCSS sample.

Cloning, expression, and functional analysis of rhesus monkey trace amine-associated receptor 6: evidence for lack of monoaminergic association

Several recent studies report an association between trace amine-associated receptor 6 (TAAR6) and susceptibility to schizophrenia and bipolar affective disorder in humans. However, endogenous TAAR6 agonists and the receptor signaling profile and brain distribution remain unclear. Here, we clone TAAR6 from the rhesus monkey and use transfected cells to investigate whether this receptor interacts with brain monoamines and a psychostimulant drug to trigger cAMP signaling or extracellular signal-regulated kinase (ERK) phosphorylation, while investigating its expression profile in the rhesus monkey brain. Unlike TAAR1, rhesus monkey TAAR6 did not alter cAMP levels in response to 10 microM of monoamines (dopamine, norepinephrine, serotonin, beta-phenylethylamine (beta-PEA), octopamine, tryptamine, and tyramine) or methamphetamine in stably transfected cells in vitro. Real-time cell electronic sensing analysis indicated that the receptor did not alter cell impedance or change the effect of forskolin on cell impedance at exposure to 20 microM of each monoamine, suggesting a lack of either Gs or Gi-linked signaling. Whereas kappa opioid receptor activation led to ERK phosphorylation at exposure to 1 microM U69593, rhesus monkey TAAR6 had no such effect at exposure to 10 microM of monoamines or methamphetamine. Membrane and cell surface localization of TAAR6 was confirmed by immunocytochemistry, biotinylation, and Western blot testing with a TAAR6 antibody in the transfected cells. Real-time reverse transcriptase-polymerase chain reaction amplification showed that TAAR6 mRNA was undetectable in selected rhesus monkey brain regions. Together, the data reveal that TAAR6 is unresponsive to brain monoamines and is not expressed in rhesus monkey brain monoaminergic nuclei, suggesting TAAR6 lacks direct association with brain monoaminergic neuronal function.

Endogenous hallucinogens as ligands of the trace amine receptors: a possible role in sensory perception

While the endogenous hallucinogens, N,N-dimethyltryptamine, 5-hydroxy-N,N-dimethyl-tryptamine and 5-methoxy-N,N-dimethyltryptamine, have been acknowledged as naturally occurring components of the mammalian body for decades, their biological function remains as elusive now as it was at the time of their discovery. The recent discovery of the trace amine associated receptors and the activity of DMT and other hallucinogenic compounds at these receptor sites leads to the hypothesis that the endogenous hallucinogens act as neurotransmitters of a subclass of these trace amine receptors. Additionally, while activity at the serotonin 5-HT2A receptor has been proposed as being responsible for the hallucinogenic affects of administered hallucinogens, in their natural setting the 5-HT2A receptor may not interact with the endogenous hallucinogens at all. Additionally 5-HT2A agonist activity is unable to account for the visual altering effects of many of the administered hallucinogens; these effects may be mediated by one of the endogenous hallucinogen trace amine receptors rather than the serotonin 5-HT2A receptor. Therefore, activity at the trace amine receptors, in addition to serotonin receptors, may play a large role in the sensory altering effects of administered hallucinogens and the trace amine receptors along with their endogenous hallucinogen ligands may serve an endogenous role in mediating sensory perception in the mammalian central nervous system. Thus the theory proposed states that these compounds act as true endogenous hallucinogenic transmitters acting in regions of the central nervous system involved in sensory perception.

Genetics of clinical features and subtypes of schizophrenia: a review of the recent literature

Since its earliest descriptions, schizophrenia has been thought to be clinically heterogeneous. Symptomatic features and subtypes tend to aggregate in families, suggesting that genetic factors contribute to individual differences in illness presentation. Over the past 5 years, evidence from genetic linkage and association studies has mounted to suggest that some susceptibility genes are etiologic factors for more or less specific illness subtypes. Furthermore, modifier genes may affect clinical features dimensionally only after a given patient is already affected with the illness. In this paper, we review recent findings supporting the existence of such "modifier" genes. To date, DTNBP1 has provided the greatest evidence of illness modification, as associations with negative and cognitive symptoms and worse outcome have been published in independent samples. Future directions include using whole-genome association studies to search for genetic modifiers of schizophrenia.