Decreased serotonin S2 and increased dopamine D2 receptors in chronic schizophrenics

The crosstalk between 5-HT2AR and mGluR2 in schizophrenia

Schizophrenia is a severe brain disorder that usually produces a lifetime of disability. First generation or typical antipsychotics such as haloperidol and second generation or atypical antipsychotics such as clozapine and risperidone remain the current standard for schizophrenia treatment. In some patients with schizophrenia, antipsychotics produce complete remission of positive symptoms, such as hallucinations and delusions. However, antipsychotic drugs are ineffective against cognitive deficits and indeed treated schizophrenia patients have small improvements or even deterioration in several cognitive domains. This underlines the need for novel and more efficient therapeutic targets for schizophrenia treatment. Serotonin and glutamate have been identified as key parts of two neurotransmitter systems involved in fundamental brain processes. Serotonin (or 5-hydroxytryptamine) 5-HT2A receptor (5-HT2AR) and metabotropic glutamate 2 receptor (mGluR2) are G protein-coupled receptors (GPCRs) that interact at epigenetic and functional levels. These two receptors can form GPCR heteromeric complexes through which their pharmacology, function and trafficking becomes affected. Here we review past and current research on the 5-HT2AR-mGluR2 heterocomplex and its potential implication in schizophrenia and antipsychotic drug action. This article is part of the Special Issue on "The receptor-receptor interaction as a new target for therapy".

Lower levels of cortical [3H]pirenzepine binding to postmortem tissue defines a sub-group of older people with schizophrenia with less severe cognitive deficits

Multiple lines of evidence argue for lower levels of cortical muscarinic M1 receptors (CHRM1) in people with schizophrenia which is possibly due to a sub-group within the disorder who have a marked loss of CHRM1 (muscarinic receptor deficit sub-group (MRDS)). In this study we sought to determine if the lower levels of CHRM1 was apparent in older people with schizophrenia and whether the loss of CHRM1 was associated with symptom severity by measuring levels of cortical [³H]pirenzepine binding to CHRM1 from 56 people with schizophrenia and 43 controls. Compared to controls (173 ± 6.3 fmol / mg protein), there were lower levels of cortical [³H]pirenzepine binding in the people with schizophrenia (mean ± SEM: 153 ± 6.0 fmol / mg protein; p = 0.02; Cohen's d = - 0.46). [³H]pirenzepine binding in the people with schizophrenia, but not controls, was not normally distributed and best fitted a two-population solution. The nadir of binding separating the two groups of people with schizophrenia was 121 fmol / mg protein and levels of [³H]pirenzepine binding below this value had a 90.7 % specificity for the disorder. Compared to controls, the score from the Clinical Dementia Rating Scale (CDR) did not differ significantly in MRDS but were significantly higher in the sub-group with normal radioligand binding. Positive and Negative Syndrome Scale scores did not differ between the two sub-groups with schizophrenia. Our current study replicates and earlier finding showing a MRDS within schizophrenia and, for the first time, suggest this sub-group have less severe cognitive deficits others with schizophrenia.

Increased amygdala and decreased hippocampus volume after schedule-induced polydipsia in high drinker compulsive rats

Fronto-limbic structures and serotonin 2A receptors (5-HT_2A) have been implicated in the pathophysiology and treatment of compulsive spectrum disorders. Schedule-Induced Polydipsia (SIP), characterized by the development of excessive drinking under intermittent food reinforcement schedules, is a valid preclinical model for studying the compulsive phenotype. In the present study, we explored the individual differences and effect of SIP in brain volume and 5-HT_2A receptor binding in fronto-limbic structures in rats selected according to their compulsive drinking behavior. Rats were divided into high (HD) and low drinkers (LD) by SIP (20 sessions); later, we analyzed the brains of HD and LD selected rats, in two different conditions: non-re-exposure (NRE) or re-exposure to SIP (RE), with four groups: LD-NRE, LD-RE, HD-NRE and HD-RE. Histological analyses were carried out for volumetric (stereology) and receptor binding (autoradiography) in the prelimbic and infralimbic cortex, dorsal hippocampus and basolateral amygdala. After SIP re-exposure, HD-RE showed an increased basolateral amygdala and a reduced hippocampus volume compared to HD-NRE rats, and also compared to LD-RE rats. No differences were found between HD and LD in NRE condition. Moreover, HD rats exhibit a lower 5-HT_2A receptor binding in the basolateral amygdala, independently of SIP re-exposure, compared to LD rats. However, LD-RE showed a decreased 5-HT_2A receptor binding in basolateral amygdala compared to LD-NRE. No differences were found in the remaining structures. These findings suggest that SIP might be differentially impacting HD and LD brains, pointing towards a possible explanation of how the latent vulnerability to compulsivity is triggered.

From Schizophrenia Genetics to Disease Biology: Harnessing New Concepts and Technologies

Schizophrenia (SZ) is a severe mental disorder afflicting around 1% of the population. It is highly heritable but with complex genetics. Recent research has unraveled a plethora of risk loci for SZ. Accordingly, our conceptual understanding of SZ genetics has been rapidly evolving, from oligogenic models towards polygenic or even omnigenic models. A pressing challenge to the field, however, is the translation of the many genetic findings of SZ into disease biology insights leading to more effective treatments. Bridging this gap requires the integration of genetic findings and functional genomics using appropriate cellular models. Harnessing new technologies, such as the development of human induced pluripotent stem cells (hiPSC) and the CRISPR/Cas-based genome/epigenome editing approach are expected to change our understanding of SZ disease biology to a fundamentally higher level. Here, we discuss some new developments.

Transcriptomic signatures of schizophrenia revealed by dopamine perturbation in an ex vivo model

The dopaminergic hypothesis of schizophrenia (SZ) postulates that dopaminergic over activity causes psychosis, a central feature of SZ, based on the observation that blocking dopamine (DA) improves psychotic symptoms. DA is known to have both receptor- and non-receptor-mediated effects, including oxidative mechanisms that lead to apoptosis. The role of DA-mediated oxidative processes in SZ has been little studied. Here, we have used a cell perturbation approach and measured transcriptomic profiles by RNAseq to study the effect of DA exposure on transcription in B-cell transformed lymphoblastoid cell lines (LCLs) from 514 SZ cases and 690 controls. We found that DA had widespread effects on both cell growth and gene expression in LCLs. Overall, 1455 genes showed statistically significant differential DA response in SZ cases and controls. This set of differentially expressed genes is enriched for brain expression and for functions related to immune processes and apoptosis, suggesting that DA may play a role in SZ pathogenesis through modulating those systems. Moreover, we observed a non-significant enrichment of genes near genome-wide significant SZ loci and with genes spanned by SZ-associated copy number variants (CNVs), which suggests convergent pathogenic mechanisms detected by both genetic association and gene expression. The study suggests a novel role of DA in the biological processes of immune and apoptosis that may be relevant to SZ pathogenesis. Furthermore, our results show the utility of pathophysiologically relevant perturbation experiments to investigate the biology of complex mental disorders.

Understanding the pathophysiology of schizophrenia: Contributions from the Melbourne Psychiatric Brain Bank

The Melbourne Psychiatric Brain Bank came into existence 25years ago. This review focusses on lines of research that have used tissue from the Brain Bank over periods of time. Hence there is a discussion on the significance of changes in levels of serotonin 2A receptors in the cortex of patients with schizophrenia and the relevance of such changes with regards to the pathophysiology of the disorder. The extensive contribution made by studies using tissue from the Melbourne Psychiatric Brain Bank to understanding the role of muscarinic receptors in the pathophysiology and treatment of schizophrenia is summarised. Finally, findings using brain bank tissue and "omics" technologies are reviewed. In each case, findings using tissue from the Melbourne Psychiatric Brain Bank is placed in context with research carried out on human postmortem CNS in schizophrenia and with findings in other lines of research that can help explain the causes or consequences of changes in CNS molecular cytoarchitecture. This timely review of data from the Melbourne Psychiatric Brain Bank reinforces the challenges faced in trying to increase our understanding of the molecular pathophysiology of schizophrenia. Continuing to increase our understanding of the disorder is important as a precursor to identifying new drug targets that can be exploited to improve the treatment of a disorder where treatment resistance remains a significant problem (Millan et al., 2016).

Using human brain imaging studies as a guide toward animal models of schizophrenia

Schizophrenia is a heterogeneous and poorly understood mental disorder that is presently defined solely by its behavioral symptoms. Advances in genetic, epidemiological and brain imaging techniques in the past half century, however, have significantly advanced our understanding of the underlying biology of the disorder. In spite of these advances clinical research remains limited in its power to establish the causal relationships that link etiology with pathophysiology and symptoms. In this context, animal models provide an important tool for causally testing hypotheses about biological processes postulated to be disrupted in the disorder. While animal models can exploit a variety of entry points toward the study of schizophrenia, here we describe an approach that seeks to closely approximate functional alterations observed with brain imaging techniques in patients. By modeling these intermediate pathophysiological alterations in animals, this approach offers an opportunity to (1) tightly link a single functional brain abnormality with its behavioral consequences, and (2) to determine whether a single pathophysiology can causally produce alterations in other brain areas that have been described in patients. In this review we first summarize a selection of well-replicated biological abnormalities described in the schizophrenia literature. We then provide examples of animal models that were studied in the context of patient imaging findings describing enhanced striatal dopamine D2 receptor function, alterations in thalamo-prefrontal circuit function, and metabolic hyperfunction of the hippocampus. Lastly, we discuss the implications of findings from these animal models for our present understanding of schizophrenia, and consider key unanswered questions for future research in animal models and human patients.

Htr2a Expression Responds Rapidly to Environmental Stimuli in an Egr3-Dependent Manner

Pharmacologic and genetic findings have implicated the serotonin 2A receptor (5-HT2AR) in the etiology of schizophrenia. Recent studies have shown reduced 5-HT2AR levels in schizophrenia patients, yet the cause of this difference is unknown. Environmental factors, such as stress, also influence schizophrenia risk, yet little is known about how environment may affect this receptor. To determine if acute stress alters 5-HT2AR expression, we examined the effect of sleep deprivation on cortical Htr2a mRNA in mice. We found that 6 h of sleep deprivation induces a twofold increase in Htr2a mRNA, a more rapid effect than has been previously reported. This effect requires the immediate early gene early growth response 3 (Egr3), as sleep deprivation failed to induce Htr2a expression in Egr3-/- mice. These findings provide a functional link between two schizophrenia candidate genes and an explanation of how environment may influence a genetic predisposition for schizophrenia.

Reelin influences the expression and function of dopamine D2 and serotonin 5-HT2A receptors: a comparative study

Reelin is an extracellular matrix protein that plays a critical role in neuronal guidance during brain neurodevelopment and in synaptic plasticity in adults and has been associated with schizophrenia. Reelin mRNA and protein levels are reduced in various structures of post-mortem schizophrenic brains, in a similar way to those found in heterozygous reeler mice (HRM). Reelin is involved in protein expression in dendritic spines that are the major location where synaptic connections are established. Thus, we hypothesized that a genetic deficit in reelin would affect the expression and function of dopamine D2 and serotonin 5-HT2A receptors that are associated with the action of current antipsychotic drugs. In this study, D2 and 5-HT2A receptor expression and function were quantitated by using radioligand binding studies in the frontal cortex and striatum of HRM and wild-type mice (WTM). We observed increased expression (p<0.05) in striatum membranes and decreased expression (p<0.05) in frontal cortex membranes for both dopamine D2 and serotonin 5-HT2A receptors from HRM compared to WTM. Our results show parallel alterations of D2 and 5-HT2A receptors that are compatible with a possible hetero-oligomeric nature of these receptors. These changes are similar to changes described in schizophrenic patients and provide further support for the suitability of using HRM as a model for studying this disease and the effects of antipsychotic drugs.

Dopaminergic activity and behaviour in SOCS2 transgenic mice: Revealing a potential drug target for schizophrenia

Alterations in immune function have been implicated in the aetiopathogenesis of schizophrenia. Specifically, the induction of inflammatory cytokines, which are important immunological factors in infection or inflammation, may be critical factors altering the normal course of brain development and increasing schizophrenia risk. Suppressor of cytokine signalling 2 (SOCS2) can negatively regulate the signalling of cytokines. The present study aimed to determine the behavioural phenotype of transgenic mice over-expressing SOCS2 (SOCS2 Tg) in paradigms of relevance to schizophrenia. Both male and female SOCS2 Tg mice displayed reduced locomotor hyperactivity after the administration of the dopamine releaser, amphetamine, compared to wildtype controls (WT). However, only male SOCS2 Tg mice showed enhanced prepulse inhibition compared to WT. Dopamine D2 receptors mRNA expression was reduced and dopamine transporter mRNA expression was increased in the nucleus accumbens of female, but not male, SOCS2 Tg mice, compared to WT. The role of hyperdopaminergia has long been implicated in the aetiology of schizophrenia. This study shows that over-expression of SOCS2 reduces the psychostimulant effects of amphetamine, enhances PPI, and alters mesolimbic dopaminergic activity. SOCS2 may provide a novel target in the development of treatments for schizophrenia.

Alterations in the serotonin system in schizophrenia: a systematic review and meta-analysis of postmortem and molecular imaging studies

Serotonergic dysfunction is thought to contribute to the pathophysiology of schizophrenia but the evidence has not been systematically synthesised before. We therefore systematically reviewed postmortem and in vivo molecular imaging studies of serotonin function in schizophrenia. We identified fifty relevant studies investigating eight different serotonin receptor systems in a total of 684 patients and 675 controls. Meta-analysis of postmortem studies found an elevation in prefrontal 5-HT1A receptors with a moderate to large effect size (N=8, 85 patients and 94 controls, SMD=0.60; CI: 0.17-1.03; p=0.007) and a reduction with a large effect size in prefrontal 5-HT2A receptors (N=8, 168 patients and 163 controls, SMD=-0.73; CI: -1.33, -0.12; p=0.019) in schizophrenia vs healthy controls. The evidence for alterations in serotonin transporter availability or other serotonin receptors (5-HT1B; 5-HT1D; 5-HT3; 5-HT4; 5-HT7) is limited. There are fewer studies investigating 5-HT receptors in schizophrenia with neuroimaging. Findings indicated possible 5-HT alterations at psychosis onset, although due to the limited number it was not possible to combine studies in a meta-analysis. Further in vivo studies, particularly in drug naive patients using radiotracers that can index high affinity states, will help determine if the postmortem findings are primary or secondary to other factors.

MicroRNA-9 and microRNA-326 regulate human dopamine D2 receptor expression, and the microRNA-mediated expression regulation is altered by a genetic variant

The human dopamine receptor D2 (DRD2) has been implicated in the pathophysiology of schizophrenia and other neuropsychiatric disorders. Most antipsychotic drugs influence dopaminergic transmission through blocking dopamine receptors, primarily DRD2. We report here the post-transcriptional regulation of DRD2 expression by two brain-expressed microRNAs (miRs), miR-326 and miR-9, in an ex vivo mode, and show the relevance of miR-mediated DRD2 expression regulation in human dopaminergic neurons and in developing human brains. Both miRs targeted the 3'-UTR (untranslated region) of DRD2 in NT2 (neuron-committed teratocarcinoma, which endogenously expresses DRD2) and CHO (Chinese hamster ovary) cell lines, decreasing luciferase activity measured by a luciferase reporter gene assay. miR-326 overexpression reduced DRD2 mRNA and DRD2 receptor synthesis. Both antisense miR-326 and antisense miR-9 increased DRD2 protein abundance, suggesting an endogenous repression of DRD2 expression by both miRs. Furthermore, a genetic variant (rs1130354) within the DRD2 3'-UTR miR-targeting site interferes with miR-326-mediated repression of DRD2 expression. Finally, co-expression analysis identified an inverse correlation of DRD2 expression with both miR-326 and miR-9 in differentiating dopaminergic neurons derived from human induced pluripotent stem cells (iPSCs) and in developing human brain regions implicated in schizophrenia. Our study provides empirical evidence suggesting that miR-326 and miR-9 may regulate dopaminergic signaling, and miR-326 and miR-9 may be considered as potential drug targets for the treatment of disorders involving abnormal DRD2 function, such as schizophrenia.

What can we learn about schizophrenia from studying the human model, drug-induced psychosis?

When drug-induced psychoses were first identified in the mid-20th century, schizophrenia was considered a discrete disease with a likely genetic cause. Consequently, drug-induced psychoses were not considered central to understanding schizophrenia as they were thought to be phenocopies rather than examples of the illness secondary to a particular known cause. However, now that we know that schizophrenia is a clinical syndrome with multiple component causes, then it is clear that the drug-induced psychoses have much to teach us. This article shows how the major neuropharmacological theories of schizophrenia have their origins in studies of the effects of drugs of abuse. Research into the effects of LSD initiated the serotonergic model; amphetamines the dopamine hypothesis, PCP and ketamine the glutamatergic hypothesis, while most recently the effects of cannabis have provoked interest in the role of endocannabinoids in schizophrenia. None of these models account for the complete picture of schizophrenia; rather the various drug models mimic different aspects of the illness. Determining the different molecular effects of those drugs whose pharmacological effects do and do not mimic the various aspects of schizophrenia has much to teach us concerning the pathogenesis of the illness.

Dysregulated 5-HT(2A) receptor binding in postmortem frontal cortex of schizophrenic subjects

Previous postmortem and neuroimaging studies have repeatedly suggested alterations in serotonin 5-HT(2A) receptor (5-HT(2A)R) binding associated with the pathophysiology of schizophrenia. These studies were performed with ligands, such as ketanserin, altanserin and LSD, that may bind with high-affinity to different structural or functional conformations of the 5-HT(2A)R. Interpretation of results may also be confounded by chronic antipsychotic treatment and suicidal behavior in the schizophrenia group. We quantified 5-HT(2A)R density by radioligand binding assays in postmortem prefrontal cortex of antipsychotic-free (n=29) and antipsychotic-treated (n=16) schizophrenics, suicide victims with other psychiatric diagnoses (n=13), and individually matched controls. [³H]Ketanserin binding, and its displacement by altanserin or the LSD-like agonist DOI, was assayed. Results indicate that the number of [³H]ketanserin binding sites to the 5-HT(2A)R was increased in antipsychotic-free (128 ± 11%), but not in antipsychotic-treated (92 ± 12%), schizophrenic subjects. In suicide victims, [³H]ketanserin binding did not differ as compared to controls. Aging correlated negatively with [³H]ketanserin binding in schizophrenia, suicide victims and controls. The fraction of high-affinity sites of DOI displacing [³H]ketanserin binding to the 5-HT(2A)R was increased in antipsychotic-free schizophrenic subjects. Functional uncoupling of heterotrimeric G proteins led to increased fraction of high-affinity sites of altanserin displacing [³H]ketanserin binding to the 5-HT(2A)R in schizophrenic subjects, but not in controls. Together, these results suggest that the active conformation of the 5-HT(2A)R is up-regulated in prefrontal cortex of antipsychotic-free schizophrenic subjects, and may provide a pharmacological explanation for discordant findings previously obtained.

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.

Chronic fluvoxamine treatment changes 5-HT(2A/2C) receptor-mediated behavior in olfactory bulbectomized mice

AIMS

Olfactory bulbectomy (OBX) in rodents represents a valuable experimental model of depression. This study was designed to shed further light on the impact of putative serotonergic neuronal degeneration in OBX mice and to assess the effect of a widely used antidepressant on serotonergic related behavioral changes induced by OBX.

MAIN METHODS

Adult male ddY mice were subject to bilateral OBX or sham surgery. The serotonin (5-HT)(2A/2C) receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) enhanced a head-twitch response (HTR) in OBX mice. Effects of 5-HT(2A), 5-HT(2C) antagonists and fluvoxamine were observed in OBX mice following DOI administration.

KEY FINDINGS

The HTR elicited by the administration of DOI (0.5 mg/kg and 1 mg/kg, i.p.) was increased about twofold in OBX mice when compared with controls on the 14th day after the surgery. The injection of ketanserin (0.025 mg/kg, i.p.), a 5-HT(2A) receptor antagonist, inhibited the enhancement of the DOI-induced HTR after OBX. Likewise, the administration of SB 242084 (1 mg/kg, s.c.), a 5-HT(2C) receptor antagonist, also inhibited the DOI-induced HTR in OBX mice. Chronic but not acute treatment with the antidepressant fluvoxamine, a selective serotonin reuptake inhibitor (SSRI), suppressed the enhancement of DOI-induced HTR after OBX.

SIGNIFICANCE

These findings indicate that OBX, and the subsequent degeneration of neurons projecting from the olfactory bulb, caused a supersensitivity of 5-HT(2A/2C) receptors which may be involved in symptoms of depression.

Increased prefrontal cortical D₁ receptors in drug naive patients with schizophrenia: a PET study with [¹¹C]NNC112

D₁ receptors are the main mediators of dopamine transmission in the cortex and subserve cognitive functions that are affected in patients with schizophrenia. Prior imaging studies have suggested abnormalities in the expression of these receptors in schizophrenia, but no conclusive picture has emerged yet. One source of discrepancy may have been prior antipsychotic exposure. We used positron emission tomography (PET) and a D1 radiotracer, [¹¹C]NNC112, in drug naïve (DN, n = 12) and drug free (DF, n = 13) patients with schizophrenia and 40 healthy control subjects (HC, n = 40 total, n = 24 per comparison group) matched for age, gender, ethnicity, parental socioeconomic status and cigarette smoking. We measured the binding potential BPP, corrected for partial volume effects. The outcome measure was obtained in cortical and striatal subregions outlined on coregistered individual MRIs. Partial volume effect corrected BPP measures were significantly higher in DN vs controls in cortical regions. No such increases were found in the DF versus controls comparison. Furthermore, in the DF group, DF interval correlated positively with cortical BPP. We conclude that upregulation of D1 receptors in schizophrenia is related to the illness itself and may be corrected and normalized by chronic antipsychotic treatment.

Serotonin 2A receptor antagonists for treatment of schizophrenia

INTRODUCTION

All approved antipsychotic drugs share an affinity for the dopamine 2 (D(2)) receptor; however, these drugs only partially ameliorate the symptoms of schizophrenia. It is, therefore, of paramount importance to identify new treatment strategies for schizophrenia.

AREAS COVERED

Preclinical, clinical and post-mortem studies of the serotonin 5-HT(2A) system in schizophrenia are reviewed. The implications of a combined D(2) and 5-HT(2A) receptor blockade, which is obtained by several current antipsychotic drugs, are discussed, and the rationale for the development of more selective 5-HT(2A) receptor antagonists is evaluated. Moreover, the investigational pipeline of major pharmaceutical companies is examined and an Internet search conducted to identify other pharmaceutical companies investigating 5-HT(2A) receptor antagonists for the treatment of schizophrenia.

EXPERT OPINION

5-HT(2A) receptor antagonists appear to assume an intermediate position by being marginally superior to placebo but inferior to conventional antipsychotic drugs. Three previous 5-HT(2A) receptor antagonists have been discontinued after Phase II or III trials, and available Phase IIa data on the remaining 5-HT(2A) receptor antagonist will need substantial additional validation to be approved as a new treatment strategy against schizophrenia.

Drug-induced psychosis: how to avoid star gazing in schizophrenia research by looking at more obvious sources of light

The prevalent view today is that schizophrenia is a syndrome rather than a specific disease. Liability to schizophrenia is highly heritable. It appears that multiple genetic and environmental factors operate together to push individuals over a threshold into expressing the characteristic clinical picture. One environmental factor which has been curiously neglected is the evidence that certain drugs can induce schizophrenia-like psychosis. In the last 60 years, improved understanding of the relationship between drug abuse and psychosis has contributed substantially to our modern view of the disorder suggesting that liability to psychosis in general, and to schizophrenia in particular, is distributed trough the general population in a similar continuous way to liability to medical disorders such as hypertension and diabetes. In this review we examine the main hypotheses resulting from the link observed between the most common psychotomimetic drugs (lysergic acid diethylamide, amphetamines, cannabis, phencyclidine) and schizophrenia.

Asenapine induces differential regional effects on serotonin receptor subtypes

Asenapine, a novel psychopharmacologic agent being developed for the treatment of schizophrenia and bipolar disorder, has high affinity for a wide range of receptors, including the serotonergic receptors 5-HT(1A), 5-HT(1B), 5-HT(2A), 5-HT( 2B), 5-HT(2C), 5-HT(5A), 5-HT(6) and 5-HT( 7). We examined the long-term effects in rat brain of multiple doses of asenapine on representative serotonin receptor subtypes: 5-HT(1A), 5-HT(2A) and 5-HT(2C). Rats were given asenapine (0.03, 0.1 or 0.3 mg/kg) subcutaneously twice daily or vehicle for 4 weeks. Brain sections were collected from the medial prefrontal cortex (mPFC), dorsolateral frontal cortex (DFC), caudate putamen, nucleus accumbens, hippocampal CA( 1) and CA(3) regions, and entorhinal cortex and processed for in-vitro receptor autoradiography. Asenapine 0.1 and 0.3 mg/kg significantly increased 5-HT(1A) binding in mPFC (by 24% and 33%, respectively), DFC (27%, 31%) and hippocampal CA(1) region (23%, 25%) (all P < 0.05). All three asenapine doses (0.03, 0.1 and 0.3 mg/kg) significantly decreased 5-HT(2A) binding by a similar degree in mPFC (40%, 44%, 47%, respectively) and DFC (45%, 51%, 52%) (all P < 0.05), but did not alter 5-HT(2A) binding in the other brain regions studied. In contrast to the effects on 5-HT(1A) and 5-HT(2A) receptors, asenapine did not alter 5-HT(2C) binding in any brain region examined at the doses tested. Our results indicate that repeated administration of asenapine produces regional-specific effects on 5-HT(1A) and 5-HT(2A) receptors in rat forebrain regions, which may contribute to the distinctive psychopharmacologic profile of asenapine.

Neurobiology of dopamine in schizophrenia

This chapter is an update on the dopamine (DA) imbalance in schizophrenia, including the evidence for subcortical hyperstimulation of D2 receptors underlying positive symptoms and cortical hypodopaminergia-mediating cognitive disturbances and negative symptoms. After a brief review of the anatomical neurocircuitry of this transmitter system as a background, we summarize the evidence for dopaminergic alterations deriving from pharmacological, postmortem, and imaging studies. This evidence supports a prominent role for D2 antagonism in the treatment of positive symptoms of schizophrenia and strongly suggests the need for alternative approaches to address the more challenging problem of negative symptoms and cognitive disturbances.

The relationship of pineal calcification and melatonin secretion to the pathophysiology of tardive dyskinesia and tourette's syndrome

Despite current intensive research, the pathophysiology of tardive dyskinesia (TD), a serious neurological side effect of neuroleptic treatment, is poorly understood. Prompted by the observation of an increased incidence and severity of abnormal perioral movements in neuroleptic-treated pinealectomized, as compared to intact rats, we suggested that the pineal gland exerts a protective effect which mitigates against the development of TD and, by inference, that reduced melatonin secretion may be related to the pathophysiology of TD. To investigate this proposition further, we studied the association of TD with pineal calcification (PC) on CT scan in chronic schizophrenic patients. Our findings revealed a significant association between TD and PC and suggest, furthermore, that PC may be a neuroradiological marker of TD. Since PC may reflect diminished secretory activity of the gland, these findings support the hypothesis that the pathophysiology of TD is linked to disturbances of melatonin secretion. The clinical and therapeutic implications of these novel findings are discussed. In the following communication, in which we introduce the hypothesis that disturbances of 5-HT and melatonin secretion are related to the pathophysiology of TD. Subsequently, we present a series of studies which relate to the association of TD with PC. We conclude by presenting the hypothesis that disturbances in melatonin secretion may also be relevant to the pathophysiology of Tourette's syndrome.

Dopamine and psychosis: theory, pathomechanisms and intermediate phenotypes

Schizophrenia is a chronic, severe, and disabling brain disorder arising from the adverse interaction of predisposing risk genes and environmental factors. The psychopathology is characterized by a wide array of disturbing cognitive, emotional, and behavioral symptoms that interfere with the individual's capacity to function in society. Contemporary pathophysiological models assume that psychotic symptoms are triggered by a dysregulation of dopaminergic activity in the brain, a theory that is tightly linked to the serendipitous discovery of the first effective antipsychotic agents in the early 1950s. In recent years, the availability of modern neuroimaging techniques has significantly expanded our understanding of the key mediator circuits that bridge the gap between genetic susceptibility and clinical phenotype. This paper discusses the pathophysiological concepts, molecular mechanisms and neuroimaging evidence that link psychosis to disturbances in dopamine neurotransmission.

D2-receptor upregulation is dependent upon temporal course of D2-occupancy: a longitudinal [11C]-raclopride PET study in cats

Long-term occupancy of dopamine D(2)-receptors, as achieved by chronic treatment with antipsychotics, leads to D(2)-receptor upregulation, and this upregulation is thought to be responsible for loss of efficacy and development of tardive dyskinesia. However, little is known about the parameters of D(2)-receptor blockade (duration and percentage of blockade) that lead to upregulation. In this study, we investigated the effects of different degrees (60 vs >80%) and durations (a transient peak vs 24 h/day) of D(2)-receptor blockade on inducing this upregulation. These different patterns of D(2)-receptor occupancy kinetics were produced in cats using bolus vs constant infusion of haloperidol for 4 weeks. D(2)-receptors were measured using positron emission tomography and Scatchard analyses of [(11)C]raclopride binding, before and after withdrawal of treatment. Continuously high (80% for 24 h/day) D(2)-receptor blockade led to a robust upregulation of striatal D(2)-receptors that was maximal at 1-week withdrawal (35+/-5%) and still detectable at 2-week withdrawal (20+/-3%). This pattern of D(2)-receptor blockade also induced behavioral tolerance to the effect of haloperidol on spontaneous locomotor activity. Continuously moderate (60% for 24 h/day) or transiently high (80% for a few hours/day) D(2)-receptor blockade did not produce any of these effects. The long-term effect of haloperidol on D(2)-receptor density and behavioral tolerance thus appears to be dependent not only on a critical threshold of D(2)-receptor blockade but also on the daily duration of D(2)-receptors blockade. This suggests that as far as antipsychotics are concerned, not only dose but disbursment throughout the day have an impact on eventual pharmacodynamic and behavioral outcomes.

Developmental changes in stress adaptation in relation to psychopathology

As modern neuroscience seeks to understand the neural bases for mental illness, it is becoming increasingly important to define how and when complex neural circuits may be altered in individuals who carry the genetic vulnerability for psychopathology. One factor that could potentially play a contributory role in mental illness is the stress response. A variety of studies suggest that stress can alter the activity of several key cortical neurotransmitters, including glutamate, γ-aminobutyric acid, dopamine, and serotonin. Specifically, exposure to neurotoxic levels of adrenal steroid hormone, particularly if this occurs early in life, could potentially induce permanent changes of these transmitter systems in corticolimbic regions, such as the hippocampal formation and cingulate gyrus, that have a high density of glucocorticoid receptors. Overall, exposure to severe stress during the perinatal period could potentially induce alterations in the circuitry of the anterior cingulate cortex and hippocampal formation and interfere with the normal mechanisms underlying attention and learning.

The biochemical womb of schizophrenia: A review

The conclusive identification of specific etiological factors or pathogenic processes in the illness of schizophrenia has remained elusive despite great technological progress. The convergence of state-of-art scientific studies in molecular genetics, molecular neuropathophysiology, in vivo brain imaging and psychopharmacology, however, indicates that we may be coming much closer to understanding the genesis of schizophrenia. In near future, the diagnosis and assessment of schizophrenia using biochemical markers may become a "dream come true" for the medical community as well as for the general population. An understanding of the biochemistry/ visa vis pathophysiology of schizophrenia is essential to the discovery of preventive measures and therapeutic intervention.

Evidence for altered post-receptor modulation of the serotonin 2a receptor in schizophrenia

We have shown a decrease in cortical serotonin(2A) receptors using tissue sections, but not with washed membranes, from the same cohort of subjects. These discrepant findings led us to determine if we could obtain similar results using samples from the same tissue block. Our studies used single-point saturation analyses to estimate the total number of [(3)H]ketanserin binding sites in tissue sections, crude homogenate, membrane-enriched and cytosol-enriched tissue samples from Brodmann's area 9. There were significant decreases in the levels of [(3)H]ketanserin binding using tissue sections (mean+/-SD: 38+/-16 vs. 56+/-16 fmol/mg ETE; p=0.008) and crude tissue homogenates (131+/-53 vs. 168+/-38 fmol/mg protein; p<0.05) from subjects with schizophrenia compared to that in controls. By contrast, there was no significant difference in radioligand binding to the membrane-enriched (155+/-95 vs. 145+/-48 fmol/mg protein; p=0.72) or cytosol-enriched (8.6+/-14 vs. 7.5+/-10 mol/mg protein; p=0.85) tissue fraction. Significantly, adding 10(-5) M risperidone or chlorpromazine, as surrogates for residual antipsychotic drugs in the CNS, to crude homogenate from control subjects did not alter [(3)H]ketanserin binding. Our data therefore is consistent with the hypothesis that apparent decreases in serotonin(2A) receptors in schizophrenia are due to altered levels of a regulatory factor(s) that modulates the binding of ligands to the serotonin(2A) receptor and that separating the membrane and cytosol removes this regulatory control.

Cortical and subcortical 5-HT2A receptor binding in neuroleptic-naive first-episode schizophrenic patients

The serotonin 5-HT(2A) receptor is suspected to be involved in a number of psychiatric disorders, including schizophrenia. In particular, atypical antipsychotics have antagonistic effects on the 5-HT(2A) receptors, supporting a specific role of the 5-HT(2A) receptor in the pathophysiology of this disease. The aim of this study is to investigate cortical and subcortical 5-HT(2A) binding in neuroleptic-naive schizophrenic patients. Fifteen neuroleptic-naive patients diagnosed with schizophrenia (age 27.5+/-4.5 years), 11 men and 4 women, and 15 healthy control subjects matched for age (28.5+/-5.7 years) and gender underwent a 40 min positron emission tomography (PET) study using the 5-HT(2A) antagonist, [(18)F]altanserin, as a radioligand. PET images were co-registered to 3 T magnetic resonance images (MRIs) for each individual subject, and ROIs were applied automatically onto the individual MRIs and PET images. The cerebellum was used as a reference region. The binding potential of specific tracer binding (BP(p)) was used as the outcome measure. No significant difference was seen in cortical receptor distribution between patients and controls. An increase in 5-HT(2A) receptor binding in the caudate nucleus was detected in the group of schizophrenic patients (0.7+/-0.1) when compared to the healthy controls (0.5+/-0.3) (p=0.02). Our results confirm other in vivo findings of no difference in cortical 5-HT(2A) receptor binding between first-episode antipsychotic-naive schizophrenic patients and age- and gender-matched healthy control subjects. However, a preliminary finding of increased 5-HT(2A) binding in the caudate nucleus requires further investigation to explore the relation of subcortical and cortical 5-HT(2A) receptor binding.

Phospholipase C-beta1 knockout mice exhibit endophenotypes modeling schizophrenia which are rescued by environmental enrichment and clozapine administration

Phospholipase C-beta1 (PLC-beta1) is a rate-limiting enzyme implicated in postnatal-cortical development and neuronal plasticity. PLC-beta1 transduces intracellular signals from specific muscarinic, glutamate and serotonin receptors, all of which have been implicated in the pathogenesis of schizophrenia. Here, we present data to show that PLC-beta1 knockout mice display locomotor hyperactivity, sensorimotor gating deficits as well as cognitive impairment. These changes in behavior are regarded as endophenotypes homologous to schizophrenia-like symptoms in rodents. Importantly, the locomotor hyperactivity and sensorimotor gating deficits in PLC-beta1 knockout mice are subject to beneficial modulation by environmental enrichment. Furthermore, clozapine but not haloperidol (atypical and typical antipsychotics, respectively) rescues the sensorimotor gating deficit in these animals, suggesting selective predictive validity. We also demonstrate a relationship between the beneficial effects of environmental enrichment and levels of M1/M4 muscarinic acetylcholine receptor binding in the neocortex and hippocampus. Thus we have demonstrated a novel mouse model, displaying disruption of multiple postsynaptic signals implicated in the pathogenesis of schizophrenia, a relevant behavioral phenotype and associated gene-environment interactions.

Serotonin depletion in the dorsal and ventral hippocampus: effects on locomotor hyperactivity, prepulse inhibition and learning and memory

We present an overview of our studies on the differential role of serotonergic projections from the median raphe nucleus (MRN) and dorsal raphe nucleus (DRN) in behavioural animal models with relevance to schizophrenia. Stereotaxic microinjection of the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) into the MRN or one of its main projections regions, the dorsal hippocampus, induced a marked enhancement of phencyclidine-induced locomotor hyperactivity and a disruption of prepulse inhibition (PPI) in rats. There was no enhancement of locomotor hyperactivity induced by amphetamine or MK-801 or after 5,7-DHT lesions of the DRN or ventral hippocampus. Rats with dorsal hippocampus lesions did not show significant changes in the Y-maze test for short-term spatial memory, the Morris water maze for long-term spatial memory, or in the T-maze delayed alternation test for working memory. These chronic lesion studies suggest a modulatory influence of serotonergic projections from the MRN to the dorsal hippocampus on phencyclidine effects and prepulse inhibition, but not on different forms of learning and memory. The results provide new insight into the role of serotonin in the dorsal hippocampus in aspects of schizophrenia.

Association of RGS2 gene polymorphisms with suicide and increased RGS2 immunoreactivity in the postmortem brain of suicide victims

Regulators of G-protein signaling are a family of proteins that negatively regulate the intracellular signaling of G protein-coupled receptors, such as the serotonin receptor. Recent studies have suggested that one of these proteins, the regulator of G-protein signaling 2 (RGS2), plays an important part in anxiety and/or aggressive behavior. To explore the involvement of the RGS2 gene in the vulnerability to suicide, we screened Japanese suicide victims for sequence variations in the RGS2 gene and carried out an association study of RGS2 gene polymorphisms with suicide victims. In the eight identified polymorphisms that were identified by mutation screening, we genotyped four common single-nucleotide polymorphisms (SNPs) in the RGS2 gene, and found significant differences in the distribution of the SNP3 (C+2971G, rs4606) genotypes and alleles of the SNP2 (C-395G, rs2746072) and the SNP3 between completed suicides and the controls. The distribution of the haplotype was also significantly different between the two groups (global p<0.0001). Furthermore, RGS2 immunoreactivity significantly increased in the amygdala and the prefrontal cortex (Brodmann area 9 (BA9)) of the postmortem brain of the suicide subjects. These findings suggest that RGS2 is genetically involved in the biological susceptibility to suicide in the Japanese population.

Altered Excitatory-Inhibitory Balance in the NMDA-Hypofunction Model of Schizophrenia

Schizophrenia is a common psychiatric disorder of high incidence, affecting approximately 1% of the world population. The essential neurotransmitter pathology of schizophrenia remains poorly defined, despite huge advances over the past half-century in identifying neurochemical and pathological abnormalities in the disease. The dopamine/serotonin hypothesis has originally provided much of the momentum for neurochemical research in schizophrenia. In recent years, the attention has, however, shifted to the glutamate system, the major excitatory neurotransmitter in the CNS and towards a concept of functional imbalance between excitatory and inhibitory transmission at the network level in various brain regions in schizophrenia. The evidence indicating a central role for the NMDA-receptor subtype in the aetiology of schizophrenia has led to the NMDA-hypofunction model of this disease and the use of phencyclidines as a means to induce the NMDA-hypofunction state in animal models. The purpose of this review is to discuss recent findings highlighting the importance of the NMDA-hypofunction model of schizophrenia, both from a clinical perspective, as well as in opening a line of research, which enables electrophysiological studies at the cellular and network level in vitro. In particular, changes in excitation–inhibition (E/I) balance in the NMDA-hypofunction model of the disease and the resulting changes in network behaviours, particularly in gamma frequency oscillatory activity, will be discussed.

Neuroreceptor imaging studies in schizophrenia

The ability of SPECT and PET to image specific biomolecules in the living brain provides a unique tool for clinical researchers. It is therefore not surprising that the use of neuroreceptor-imaging techniques has become more widespread over the past decade. This article reviews the application of these techniques to the study of schizophrenia. The design of neuroreceptor-imaging studies performed in the field of schizophrenia research can be broadly divided into two categories: (1) studies of pathophysiology and (2) studies of pharmacology. The former examines neuroreceptor and neurotransmitter parameters in individuals with schizophrenia compared to control subjects in order to provide a better understanding of the disease process. Studies of pharmacology seek to elucidate the mechanism of action for the treatments utilized in schizophrenia. This review will consider both studies of pathophysiology and pharmacology, with a discussion of the application of these techniques to drug development.

Effects of psilocybin on time perception and temporal control of behaviour in humans

Hallucinogenic psilocybin is known to alter the subjective experience of time. However, there is no study that systematically investigated objective measures of time perception under psilocybin. Therefore, we studied dose-dependent effects of the serotonin (5-HT)2A/1A receptor agonist psilocybin (4-phosphoryloxy-N, N-dimethyltryptamine) on temporal processing, employing tasks of temporal reproduction, sensorimotor synchronization and tapping tempo. To control for cognitive and subjective changes, we assessed spatial working memory and conscious experience. Twelve healthy human volunteers were tested under placebo, medium (115 microg/kg), and high (250 microg/kg) dose conditions, in a double-blind experimental design. Psilocybin was found to significantly impair subjects' ability to (1) reproduce interval durations longer than 2.5 sec, (2) to synchronize to inter-beat intervals longer than 2 sec and (3) caused subjects to be slower in their preferred tapping rate. These objective effects on timing performance were accompanied by working-memory deficits and subjective changes in conscious state, namely increased reports of 'depersonalization' and 'derealization' phenomena including disturbances in subjective 'time sense.' Our study is the first to systematically assess the impact of psilocybin on timing performance on standardized measures of temporal processing. Results indicate that the serotonin system is selectively involved in duration processing of intervals longer than 2 to 3 seconds and in the voluntary control of the speed of movement. We speculate that psilocybin's selective disruption of longer intervals is likely to be a product of interactions with cognitive dimensions of temporal processing -presumably via 5-HT2A receptor stimulation.

Alterations of serotonin transmission in schizophrenia

A role for serotonin alterations in the pathophysiology of schizophrenia has long been suspected because of the psychotogenic effects of serotonergic agonists and the therapeutic effects of 5-HT(2) antagonism. This chapter is a review of the evidence derived from pharmacological studies, postmortem, and imaging studies that have assessed the role of serotonin transmission in schizophrenia. While a clear picture of specific serotonergic alterations in schizophrenia has not emerged despite much research, this review reinforces a modulatory role of serotonergic agents on dopamine transmission in schizophrenia, which may contribute to the therapeutic effects of atypical antipsychotics.

Fatty acids differentially affect serotonin receptor and transporter binding in the rat brain

The aim of this study was to examine the influence of different fat diets on serotonin receptor and transporter binding. Male Sprague-Dawley rats were fed a diet of either high saturated fat, omega-6 polyunsaturated fatty acid, omega-3 polyunsaturated fatty acid or low fat (control) for eight weeks. Using Beta-Imager quantification techniques, [(3)H]ketanserin, [(3)H]mesulergine and [(3)H]paroxetine binding to serotonin (5-HT)(2A), 5-HT(2C) receptors and 5-HT transporters (5-HTT) was measured throughout the brain in all four groups. All three high fatty acid diets influenced serotonin receptor binding, however the most pronounced effects were that compared with the low fat control group, i) 5-HT(2A) receptor binding was increased in the caudate putamen, but reduced in the mammillary nucleus in high saturated fat and high omega-6 polyunsaturated fatty acid diet groups; ii) 5-HT(2C) receptor binding was reduced in the mamillary nucleus of saturated fat group and reduced in prefrontal cortex of the omega-6 polyunsaturated fatty acid and omega-3 polyunsaturated fatty acid groups; and iii) 5-HTT binding was reduced in the hippocampus in the omega-6 polyunsaturated fatty acid group. Overall, the omega-6 polyunsaturated fatty acid diet exerted the most influence on serotonin receptor and transporter binding. These results may be of importance in relation to neuropsychiatric diseases such as schizophrenia, where associations between altered fatty acid levels and the serotonergic system have been made.

Membrane phospholipid composition, alterations in neurotransmitter systems and schizophrenia

This review addresses the relationship between modifications in membrane phospholipid composition (MPC) and alterations in dopaminergic, serotonergic and cholinergic neurotransmitter systems in schizophrenia. The main evidence in support of the MPC hypothesis of schizophrenia comes from post-mortem and platelet studies, which show that in schizophrenia, certain omega-3 and omega-6 polyunsaturated fatty acid (PUFA) levels are reduced. Furthermore, examination of several biochemical markers suggests abnormal fatty acid metabolism may be present in schizophrenia. Dietary manipulation of MPC with polyunsaturated fatty acid diets has been shown to affect densities of dopamine, serotonin and muscarinic receptors in rats. Also, supplementation with omega-3 fatty acids has been shown to improve mental health rating scores, and there is evidence that the mechanism behind this involves the serotonin receptor complex. This suggests that a tight relationship exists between essential fatty acid status and normal neurotransmission, and that altered PUFA levels may contribute to the abnormalities in neurotransmission seen in schizophrenia.

The effect of citalopram adjunctive treatment added to atypical antipsychotic medications for cognitive performance in patients with schizophrenia

Cognitive enhancement in patients with schizophrenia is a major treatment priority. Because serotonergic approaches have been suggested as a possible mechanism to enhance cognition and many patients with schizophrenia are treated with selective serotonin reuptake inhibitor antidepressants, we evaluated a serotonin reuptake inhibitor, citalopram, as adjunctive therapy to atypical antipsychotic treatment for its cognitive enhancing effects in schizophrenic patients. Nineteen schizophrenic patients were treated in a randomized, placebo-controlled, crossover-designed 24-week study. In phase 1, subjects were randomized equally to 40 mg of citalopram or placebo and were evaluated prior to initiation of pharmacotherapy and at the end of phase 1 (after 12 weeks of treatment with double-blind agent). At the beginning of phase 2, subjects were crossed over to the other treatment and subsequently assessed after 12 weeks of treatment for symptom severity and cognitive performance. There were no statistically significant differences between citalopram 40 mg/d and placebo treatment on any clinical or cognitive measures. These results indicate that citalopram adjunctive treatment to atypical antipsychotics produces no significant cognitive improvement in patients with schizophrenia. Because the subjects in this study were all treated with atypical antipsychotics, it is possible that the pharmacologic profiles of atypical antipsychotic medications at serotonin receptors may have complicated the effects of citalopram augmentation. Further research on alternative serotonergic approaches to cognitive enhancement in schizophrenia is warranted.

5-HT2A and muscarinic receptors in schizophrenia: a postmortem study

Although evidence suggests that 5-HT(2A) and muscarinic M1/M4 receptors are implicated in the pathology of schizophrenia, the results are not conclusive. In the present study we tested the hypothesis that binding of 5-HT(2A) and M1/M4 receptors is altered in the postmortem brain of schizophrenia subjects. Quantitative autoradiography was employed to measure [(3)H]ketanserin binding to 5-HT(2A) receptors and [(3)H]pirenzepine binding to both M1 and M4 receptors in Brodmann's area 9 (BA9), caudate/putamen, and the hippocampal formation from six schizophrenic and six control subjects. A significant reduction in the density of 5HT(2A) receptors in BA 9 of schizophrenic subjects was observed (p=0.036). No significant difference was observed in the density of 5HT(2A) receptors in the hippocampus or caudate/putamen between the two groups. No significant changes in the density of M1/M4 receptors was observed in these three regions between the two groups. These findings support a possible involvement of the serotonergic system in the pathology of schizophrenia.

GABA Targets for the Treatment of Cognitive Dysfunction in Schizophrenia

Cognitive deficits, including impairments in working memory that have been linked to the prefrontal cortex, are among the most debilitating and difficult to treat features of schizophrenia. Consequently, the identification of potential targets informed by the pathophysiology of the illness is needed to develop novel pharmacological approaches for ameliorating these deficits. Postmortem studies of the prefrontal cortex in schizophrenia subjects have revealed disturbances restricted to a subpopulation of inhibitory neurons that includes chandelier neurons, whose axon terminals synapse on the axon initial segment of pyramidal neurons. Chandelier neurons play an important role in synchronizing pyramidal neuron activity and appear to be a critical component of the prefrontal cortical circuitry that subserves working memory function. Therefore, in this paper we review evidence suggesting that drugs which selectively enhance chandelier neuron-mediated inhibition of prefrontal pyramidal neurons may improve working memory dysfunction in schizophrenia. Potential novel targets for such agents include GABA(A) receptors that contain the α(2) subunit. In addition, we discuss potential complementary mechanisms for enhancing inhibitory input to pyramidal cell bodies, including drugs with activity at the CB1 receptor of the endocannabinoid system. The development of pathophysiologically-based treatments that selectively remediate disturbances in specific neural circuits underlying working memory may provide an effective approach to improving cognitive deficits in schizophrenia.

Measurement of dopamine D2-like receptors in postmortem CNS and pituitary: differential regional changes in schizophrenia

In situ radioligand binding with autoradiography and anti-human dopamine D(2) receptor antibodies with Western blots have been used to measure the density of dopamine D(2)-like receptors in the caudate-putamen and pituitary from schizophrenic subjects who did or did not have residual antipsychotic drugs in their tissue at death. There was a significant decrease in the Ki for haloperidol displaceable [(125)I]iodosulpride binding in the pituitary (p < 0.01) and caudate-putamen (p < 0.05) from subjects with schizophrenia with residual drugs in their tissue. There was a significant decrease in the density of [(125)I]iodosulpride in the pituitary (p < 0.001) and a strong trend to a decrease in binding in the caudate-putamen (p = 0.055) from subjects with schizophrenia. By contrast, [(3)H]spiperone binding was decreased in the caudate-putamen (p < 0.05) with a trend to decreased binding in the pituitary (p = 0.07) from subjects with schizophrenia. There was no difference in the density of dopamine D(2) receptors in the caudate-putamen from subjects with schizophrenia (p = 0.31). All the findings on receptor densities were independent of drug status. [(125)I]iodosulpride binds to the dopamine D(2&3) receptors. We have shown that there is no change in the dopamine D(2) receptor in the caudate-putamen from subjects with schizophrenia and therefore, these data would be consistent with there being a decrease in the dopamine D(3) in the caudate-putamen from subjects with schizophrenia. Since dopamine D(3) receptors are absent or present at low concentrations in the pituitary, our data would suggest the dopamine D(2) receptor is decreased in that tissue from schizophrenic subjects.

Serotonin 5-HT1A, 5-HT1B, and 5-HT2A receptor mRNA expression in subjects with major depression, bipolar disorder, and schizophrenia

BACKGROUND

Alterations of serotonin neurotransmission are implicated in both mood disorders and schizophrenia. Specific serotonin-receptor-based abnormalities in these psychiatric illnesses have been intensively studied; however, it has been difficult to draw any conclusions because of a lack of consensus. These inconsistencies have most likely arisen from the unavailability of selective ligands.

METHODS

Our study used in situ hybridization to quantify 5-HT(1A), 5-HT(1B), and 5-HT(2A) mRNA levels in the hippocampus (HC) and 5-HT(1A) and 5-HT(2A) mRNA levels in the dorsolateral prefrontal cortex (DLPFC) of subjects with a history of major depression disorder (MDD), bipolar disorder (BPD), schizophrenia, and a normal comparison group (15 subjects per group).

RESULTS

In the DLPFC, there is a significant decrease in 5-HT(1A) mRNA of subjects with MDD and in 5-HT(2A) mRNA of subjects with BPD. Subjects with MDD have a significant decrease in 5-HT(1A) mRNA in the HC; subjects with BPD and schizophrenia had increased 5-HT(1B) mRNA levels and a significant decrease in 5-HT(2A) mRNA levels in the hippocampal formation.

CONCLUSIONS

Alterations in 5-HT(1A,) 5-HT(1B), and 5-HT(2A) mRNA levels in the brains of subjects with both mood disorders and schizophrenia add further support for hypothesis of dysregulation of the serotonergic system in these psychiatric disorders.

Tryptophan hydroxylase immunoreactivity is altered by the genetic variation in postmortem brain samples of both suicide victims and controls

Several lines of evidence suggest that a partly genetically controlled serotonergic dysfunction is involved in the biological pathogenesis of suicide. In this study, we measured tryptophan hydroxylase (TPH) immunoreactivity as a pre-synaptic marker, and serotonin receptor 2A (5HT2A receptor) density as a post-synaptic marker in the serotonergic system in 10 postmortem brains of suicide victims. We also examined whether TPH gene polymorphisms (A218C and A-6526G polymorphisms) could affect TPH immunoreactivity and 5HT2A receptor gene polymorphism (A-1438G polymorphism) could affect 5HT2A receptor density in 28 postmortem brain samples. No significant differences were found in TPH immunoreactivity or 5HT2A receptor density between suicide victims and controls. The AA genotype of the A218C polymorphism of the TPH gene showed higher TPH immunoreactivity along with lower 5HT2A receptor density than did any other genotypes in the postmortem brains of both suicide victims and controls. Our findings suggest that the A218C polymorphism of the TPH gene can be expected to provide new insights not only for neurobiological studies of suicide, but also for research into the behavioral characteristics that may be associated with serotonergic dysfunction.

Enhancement of 5-hydroxytryptamine-induced head-twitch response after olfactory bulbectomy

5-Hydroxytryptamine(2A) receptor agonists evoke the head-twitch response in mice. The head-twitch response in olfactory bulbectomized mice elicited by the administration of 5-hydroxytryptamine (40 microgram/mouse, i.c.v.) was increased about threefold as compared with controls on the 14th day after the operation. The injection of ketanserin (1 mg/kg, i.p.), a 5-hydroxytryptamine(2A) receptor antagonist, inhibited this enhancement of 5-hydroxytryptamine-induced head-twitch response after olfactory bulbectomized. On the 14th day, the number of head-twitch response induced by 5-hydroxytryptophan (40, 80 and 160 mg/kg, i.p.), a precursor of 5-hydroxytryptamine, did not differ between olfactory bulbectomized and control mice. Monoamine oxidase-B activity in the forebrain of olfactory bulbectomized mice was higher than that in controls while monoamine oxidase-A activities were unchanged. The 5-hydroxytryptamine uptake into synaptosomes in the forebrain homogenates of olfactory bulbectomized mice was lower than that in controls. These findings indicate that olfactory bulbectomized causes the enhancement of head-twitch response by a supersensitivity of 5-hydroxytryptamine(2A) receptors in cerebral cortex derived from degeneration of neurons projecting from the olfactory bulb.

The serotonin-2A receptor polymorphism and clinical symptoms in mood disorders, schizophrenia and alcohol dependence in Japan

In the past, there have been many epidemiological and genetic studies of mood disorders, schizophrenia, and alcohol dependence, and in this study, the human serotonin 2A receptor (5-HTR2A) polymorphism was examined in 80 patients with mood disorders, 50 patients with schizophrenia and 41 patients with alcohol dependence. 5-HTR is related to affectivity, regulation, and pharmacologic effects of antidepressant, anti-anxiety and antipsychotic medications. The polymorphism in 5-HTR2A (102T/C, -1438 A/G) was identified by the polymerase chain reaction (PCR), followed by restriction fragment length polymorphism (RFLP). The results suggest that 5-HTR2A (102T/C, -1438G/A) polymorphism might not be associated with susceptibility to schizophrenia or mood disorders, and it might not be a risk factor contributing to alcohol dependency. We found that the 102T/C polymorphism was in linkage disequilibrium with the -1438G/A polymorphism in psychosis (mood disorder, schizophrenia, and alcohol dependence) and in health controls. Further studies are needed to determine whether or not the novel serotonin receptor (5-HTR) polymorphism reflects the pathogenesis of schizophrenia, mood disorders, and alcohol dependence.

Altered platelet serotonin 5-HT2A receptor density but not second messenger inositol trisphosphate levels in drug-free schizophrenic patients

The 5-HT2A receptor binding parameters using [3H]ketanserine and its intracellular signal inositol 1,4,5 trisphosphate (IP3) concentrations were determined in platelets from schizophrenic patients so as to assess differences with respect to a control group and to a standardized antipsychotic drug treatment. Seventy-five antipsychotic-free patients with a DSM-IV diagnosis of paranoid schizophrenia were included in the study. Blood samples were collected before the onset of antipsychotic treatment (baseline values) and after 3 weeks of treatment. Antipsychotic-free schizophrenic patients showed significantly increased basal 5-HT2A densities in comparison to the control group, together with a significantly increase (23%) in the 5-HT2A binding density in those patients treated with risperidone. These changes could be attributed to an up-regulation of 5-HT2A receptors caused by previous treatment with antipsychotic drugs, which is consistent with the chronic effect of 5-HT2A antagonists to up-regulate the number of binding sites. With regard to second messenger IP3 concentrations, basal concentrations in schizophrenic patients were not significantly different from control values, nor was there any significant difference between basal vs. posttreatment values. These results are possibly related to failure of second messenger systems of 'translating' extracellular messages generated presynaptically into effective neurotransmitter signals in schizophrenic patients.

The cortical serotonin2A receptor and the pathology of schizophrenia: a likely accomplice

A large body of evidence shows that there is a change in the density of cortical serotonin2A receptors (5HT2AR) in post-mortem CNS from subjects with schizophrenia. Furthermore, some antipsychotic drugs have also been shown to cause a decrease in the density of 5HT2AR in the rat CNS. Thus, it appeared possible that changes in this receptor in human post-mortem CNS simply reflected an antipsychotic drug effect. However, a great deal of research on the 5HT2AR and schizophrenia now suggests that the changes in this receptor are complex and may be involved in both the pathology of the disorder and the effects of some antipsychotic drugs. Moreover, recent advances in basic research on the role of the 5HT2AR in the CNS add further support to the hypothesis that the receptor could be involved in the pathology of the illness. In particular, an argument will be developed that the changes in the 5HT2AR in schizophrenia are reflective of a real or perceived change in serotonergic tone and that this forms an important part of the pathology of the illness.

Decreased [(3)H]spiperone binding in the anterior cingulate cortex of schizophrenia patients: an autoradiographic study

Abnormalities in the anterior cingulate cortex have been reported in patients with schizophrenia, and have been implicated in the pathophysiology of this disorder. In the present study, we have examined antipsychotic-sensitive binding sites in the left anterior cingulate cortex of schizophrenia patients and controls. Using quantitative autoradiography and [(3)H]spiperone as a ligand, both saturation and competition experiments were performed in post-mortem brain tissue obtained from six schizophrenia and six control cases. Saturation experiments revealed that the maximum number of [(3)H]spiperone binding sites was significantly reduced by 31% in the schizophrenia group as compared to the control group (65.3+/-5.6 fmol/mg tissue versus 94.2+/-7.3 fmol/mg tissue). Increased dissociation constant was also observed in the schizophrenia group (2.2+/-0.4 nM versus 1.3+/-0.2 nM), but was not statistically significant (P=0.07). Competition experiments were performed in order to examine the pharmacological profile of [(3)H]spiperone binding, and revealed that: (i) displacement of [(3)H]spiperone binding by clozapine and mianserin was significantly reduced in the schizophrenia group as compared to the control group (-26% and -16% respectively); (ii) the order of displacement potency of the drugs tested was: haloperidol>mianserin>butaclamol approximately risperidone>clozapine>2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene. Our results suggest a reduction of antipsychotic-sensitive binding sites in the anterior cingulate cortex of patients with schizophrenia. Such abnormality could lead to an imbalance in neurotransmitter regulation in the anterior cingulate cortex which may contribute to the emergence of some symptoms of schizophrenia.