Elevation of D4 dopamine receptor mRNA in postmortem schizophrenic brain

Neurotransmission-related gene expression in the frontal pole is altered in subjects with bipolar disorder and schizophrenia

The frontal pole (Brodmann area 10, BA10) is the largest cytoarchitectonic region of the human cortex, performing complex integrative functions. BA10 undergoes intensive adolescent grey matter pruning prior to the age of onset for bipolar disorder (BP) and schizophrenia (SCHIZ), and its dysfunction is likely to underly aspects of their shared symptomology. In this study, we investigated the role of BA10 neurotransmission-related gene expression in BP and SCHIZ. We performed qPCR to measure the expression of 115 neurotransmission-related targets in control, BP, and SCHIZ postmortem samples (n = 72). We chose this method for its high sensitivity to detect low-level expression. We then strengthened our findings by performing a meta-analysis of publicly released BA10 microarray data (n = 101) and identified sources of convergence with our qPCR results. To improve interpretation, we leveraged the unusually large database of clinical metadata accompanying our samples to explore the relationship between BA10 gene expression, therapeutics, substances of abuse, and symptom profiles, and validated these findings with publicly available datasets. Using these convergent sources of evidence, we identified 20 neurotransmission-related genes that were differentially expressed in BP and SCHIZ in BA10. These results included a large diagnosis-related decrease in two important therapeutic targets with low levels of expression, HTR2B and DRD4, as well as other findings related to dopaminergic, GABAergic and astrocytic function. We also observed that therapeutics may produce a differential expression that opposes diagnosis effects. In contrast, substances of abuse showed similar effects on BA10 gene expression as BP and SCHIZ, potentially amplifying diagnosis-related dysregulation.

DNA methylome perturbations: an epigenetic basis for the emergingly heritable neurodevelopmental abnormalities associated with maternal smoking and maternal nicotine exposure†

Maternal smoking during pregnancy is associated with an ensemble of neurodevelopmental consequences in children and therefore constitutes a pressing public health concern. Adding to this burden, contemporary epidemiological and especially animal model research suggests that grandmaternal smoking is similarly associated with neurodevelopmental abnormalities in grandchildren, indicative of intergenerational transmission of the neurodevelopmental impacts of maternal smoking. Probing the mechanistic bases of neurodevelopmental anomalies in the children of maternal smokers and the intergenerational transmission thereof, emerging research intimates that epigenetic changes, namely DNA methylome perturbations, are key factors. Altogether, these findings warrant future research to fully elucidate the etiology of neurodevelopmental impairments in the children and grandchildren of maternal smokers and underscore the clear potential thereof to benefit public health by informing the development and implementation of preventative measures, prophylactics, and treatments. To this end, the present review aims to encapsulate the burgeoning evidence linking maternal smoking to intergenerational epigenetic inheritance of neurodevelopmental abnormalities, to identify the strengths and weaknesses thereof, and to highlight areas of emphasis for future human and animal model research therein.

Methamphetamine-induced psychosis is associated with DNA hypomethylation and increased expression of AKT1 and key dopaminergic genes

Methamphetamine, one of the most frequently used illicit drugs worldwide, can induce psychosis in a large fraction of abusers and it is becoming a major problem for the health care institutions. There is some evidence that genetic and epigenetic factors may play roles in methamphetamine psychosis. In this study, we examined methamphetamine-induced epigenetic and expression changes of several key genes involved in psychosis. RNA and DNA extracted from the saliva samples of patients with methamphetamine dependency with and without psychosis as well as control subjects (each group 25) were analyzed for expression and promoter DNA methylation status of DRD1, DRD2, DRD3, DRD4, MB-COMT, GAD1, and AKT1 using qRT-PCR and q-MSP, respectively. We found statistically significant DNA hypomethylation of the promoter regions of DRD3 (P = 0.032), DRD4 (P = 0.05), MB-COMT (P = 0.009), and AKT1 (P = 0.0008) associated with increased expression of the corresponding genes in patients with methamphetamine psychosis (P = 0.022, P = 0.034, P = 0.035, P = 0.038, respectively), and to a lesser degree in some of the candidate genes in non-psychotic patients versus the control subjects. In general, methamphetamine dependency is associated with reduced DNA methylation and corresponding increase in expression of several key genes involved in the pathogenesis of psychotic disorders. While these epigenetic changes can be useful diagnostic biomarkers for psychosis in methamphetamine abusers, it is also consistent with the use of methyl rich diet for prevention or suppression of psychosis in these patients. However, this needs to be confirmed in future studies. © 2016 Wiley Periodicals, Inc.

Expression of a Novel D4 Dopamine Receptor in the Lamprey Brain. Evolutionary Considerations about Dopamine Receptors

Numerous data reported in lampreys, which belong to the phylogenetically oldest branch of vertebrates, show that the dopaminergic system was already well developed at the dawn of vertebrate evolution. The expression of dopamine in the lamprey brain is well conserved when compared to other vertebrates, and this is also true for the D2 receptor. Additionally, the key role of dopamine in the striatum, modulating the excitability in the direct and indirect pathways through the D1 and D2 receptors, has also been recently reported in these animals. The moment of divergence regarding the two whole genome duplications occurred in vertebrates suggests that additional receptors, apart from the D1 and D2 previously reported, could be present in lampreys. We used in situ hybridization to characterize the expression of a novel dopamine receptor, which we have identified as a D4 receptor according to the phylogenetic analysis. The D4 receptor shows in the sea lamprey a more restricted expression pattern than the D2 subtype, as reported in mammals. Its main expression areas are the striatum, lateral and ventral pallial sectors, several hypothalamic regions, habenula, and mesencephalic and rhombencephalic motoneurons. Some expression areas are well conserved through vertebrate evolution, as is the case of the striatum or the habenula, but the controversies regarding the D4 receptor expression in other vertebrates hampers for a complete comparison, especially in rhombencephalic regions. Our results further support that the dopaminergic system in vertebrates is well conserved and suggest that at least some functions of the D4 receptor were already present before the divergence of lampreys.

Association study of dopamine receptor genes polymorphism with cognitive functions in bipolar I disorder patients

OBJECTIVE

To determine the correlation among the polymorphisms of dopamine receptor genes, cognitive function of Bipolar disorder (BD) patients, and BD.

METHODS

Twenty-three Single Nucleotide Polymorphisms (SNPs) of dopamine receptor genes were genotyped using Illumina GoldenGate genotyping assay in 375 patients with bipolar I disorder (BD-I) (patients group) and 475 healthy controls (control group). Cognitive function tests were performed in 158 patients who were clinically stable and 307 healthy controls who were matched with the patients in age, sex, and education.

RESULTS

The allele frequencies of rs3758653 in the promoter region of the DRD4 gene were significantly different between patients group and control group (χ(2)=9.386, Corrected P=0.046). This significant difference was also observed between BD-I patients with psychotic symptoms and healthy controls (χ(2)=9.27, Corrected P=0.049). Patients with BD-I performed significantly worse than healthy controls in all cognitive domains (p<0.01) except TMTA errors and illegal time. Significant interactions between polymorphisms of rs5326 in DRD1 gene and phenotype (affected or unaffected with BD-I) were found in non-perseverative errors (β=3.20 and Corrected P=0.0034) on the Wisconsin Card Sorting Test (WCST). The allele of this SNP denoted the positive effect on the WCST non-perseverative errors in BD-I patients group (β=2.80 and Corrected P=0.017). The genotypic association analyses also supported the findings (F=4.24 and P=0.007), but this effect was not found in controls.

LIMITATIONS

The sample size was relatively small and the SNP coverage was limited, making it very important to be cautious when drawing a conclusion.

CONCLUSIONS

DRD4 gene may play an important role in psychotic symptomatology rather than in unique diagnosis, BD, for example. A genetic association exists between DRD1 gene and impaired cognition in BD.

Leukocyte Gene Expression in Patients with Medication Refractory Depression before and after Treatment with ECT or Isoflurane Anesthesia: A Pilot Study

Objective. To evaluate leukocyte gene expression for 9 selected genes (mRNAs) as biological markers in patients with medication refractory depression before and after treatment with ECT or isoflurane anesthesia (ISO). Methods. In a substudy of a nonrandomized open-label trial comparing effects of ECT to ISO therapy, blood samples were obtained before and after treatment from 22 patients with refractory depression, and leukocyte mRNA was assessed by quantitative PCR. Patients' mRNAs were also compared to 17 healthy controls. Results. Relative to controls, patients before treatment showed significantly higher IL10 and DBI and lower ADRA2A and ASIC3 mRNA (P < 0.025). Both ECT and ISO induced significant decreases after treatment in 4 genes: IL10, NR3C1, DRD4, and Sult1A1. After treatment, patients' DBI, ASIC3, and ADRA2A mRNA remained dysregulated. Conclusion. Significant differences from controls and/or significant changes after ECT or ISO treatment were observed for 7 of the 9 mRNAs studied. Decreased expression of 4 genes after effective treatment with either ECT or ISO suggests possible overlap of underlying mechanisms. Three genes showing dysregulation before and after treatment may be trait-like biomarkers of medication refractory depression. Gene expression for these patients has the potential to facilitate diagnosis, clarify pathophysiology, and identify potential biomarkers for treatment effects.

A genetic association study of DNA methylation levels in the DRD4 gene region finds associations with nearby SNPs

BACKGROUND

Dopamine receptor D4(DRD4) polymorphisms have been associated with a number of psychiatric disorders, but little is known about the mechanism of these associations. DNA methylation is linked to the regulation of gene expression and plays a vital role in normal cellular function, with abnormal DNA methylation patterns implicated in a range of disorders. Recent evidence suggests DNA methylation can be influenced by cis-acting DNA sequence variation, that is, DNA sequence variation located nearby on the same chromosome.

METHODS

To investigate the potential influence of cis-acting genetic elements within DRD4, we analysed DRD4 promoter DNA methylation levels in the transformed lymphoblastoid cell-line DNA of 89 individuals (from 30 family-trios). Five SNPs located +/- 10kb of the promoter region were interrogated for associations with DNA methylation levels.

RESULTS

Four significant SNP associations were found with DNA methylation (rs3758653, rs752306, rs11246228 and rs936465). The associations of rs3758653 and rs936465 with DNA methylation were tested and nominally replicated (p-value < 0.05) in post-mortem brain tissue from an independent sample (N = 18). Interestingly, the DNA methylation patterns observed in post-mortem brain tissue were similar to those observed in transformed lymphoblastoid cell line DNA.

CONCLUSIONS

The link reported between DNA sequence and DNA methylation offers a possible functional role to seemingly non-functional SNP associations. DRD4 has been implicated in several psychiatric disease phenotypes and our results shed light upon the possible mode of action of SNP associations in this region.

Dopamine D4 and D5 receptor gene variant effects on clozapine response in schizophrenia: replication and exploration

OBJECTIVES

This study aimed to: 1) replicate previously reported associations between dopamine D4 receptor gene (DRD4) polymorphisms and antipsychotic (AP) response in a clozapine (CLZ) response sample; and 2) explore possible associations of polymorphisms across dopamine D5 receptor gene (DRD5) as well as other DRD4 regions.

METHODS

DRD4 exon III 48-bp, intron I (G)(n), and 120-bp repeat polymorphisms, and three DRD4 single nucleotide polymorphisms (SNPs); and DRD5 (CA/CT/GT)(n) microsatellite and four DRD5 SNPs were assessed using standard genotyping and statistical procedures.

RESULTS

We report evidence, which does not survive correction for multiple testing, supporting previous DRD4 findings. Findings of interest include the 120-bp 1-copy allele, intron I (G)(n) 142-bp/140-bp genotype, and exon III 4R allele with CLZ response. All DRD5 tests were negative.

CONCLUSIONS

Overall, these results suggest a possible minor contribution of DRD4 variants, but not DRD5 variants, towards the AP/CLZ response phenotype.

Genetic polymorphisms in dopamine- and serotonin-related genes and treatment responses to risperidone and perospirone

We investigated the possible association between genetic polymorphisms in the dopamine receptor and serotonin transporter genes and the responses of schizophrenic patients treated with either risperidone or perospirone. The subjects comprised 27 patients with schizophrenia who were clinically evaluated both before and after treatment. The genotyping of the polymorphisms of the dopamine D2 receptor gene (DRD2) (rs1801028 and rs6277), the dopamine D4 receptor gene (DRD4) (120-bp tandem repeats and rs1800955), and serotonin transporter gene (5HTT)(variable number of tandem repeats; VNTR) were performed using the real-time polymerase chain reaction and sequencing. In DRD2 and 5HTT-VNTR, there were no significant correlations between clinical response and polymorphism in the case of risperidone, and for perospirone treatment it was impossible to analyze the clinical evaluation due to the absence of genotype information. On the other hand, in DRD4 there were significant correlations in the two-factor interaction effect on the Positive and Negative Syndrome Scale (PANSS) between the two drugs [120-bp tandem repeat, p=0.003; rs1800955, p=0.043]. Although the small sample represents a serious limitation, these results suggest that variants in DRD4 are a predictor of whether treatment will be more effective with risperidone or with perospirone in individual patients.

Dopamine D1 versus D4 receptors differentially modulate the encoding of salient versus nonsalient emotional information in the medial prefrontal cortex

Dopamine (DA) transmission plays a critical role in the processing of emotionally salient information and in associative learning and memory processes. Within the mammalian brain, neurons within the medial prefrontal cortex (mPFC) are involved critically in the encoding, expression, and extinction of emotionally salient learned information. Within the mPFC, dopaminergic transmission is involved importantly in controlling attentional and motivational processes, particularly within the context of emotionally salient sensory information. Considerable evidence suggests differential roles for DA D(1)-like versus D(2)-like receptors, including the D(4) receptor subtype, in the regulation of neuronal activity and emotional processing within the mPFC. Using an olfactory fear-conditioning assay in rats, we compared the roles of DA D(1) versus D(4) receptor activation during the encoding and recall phases of emotional learning and memory. We report that specific activation of DA D(4) receptors within the mPFC strongly potentiates the salience of normally nonsalient emotional associative fear memories and blocks the encoding of suprathreshold conditioned fear associations. However, D(4) receptor activation has no effect on the recall of previously learned emotionally salient conditioned memories. In contrast, intra-mPFC D(1) receptor activation failed to increase the emotional salience of subthreshold fear stimuli but completely blocked the expression of previously learned emotionally relevant information, demonstrating that DA D(4) versus D(1) subtype receptor transmission within the mPFC plays distinct functional roles in the processing of emotionally salient versus nonsalient associative information and differentially modulates the encoding versus recall phases of emotional memory within the mPFC.

Variable number of tandem repeat polymorphisms of DRD4: re-evaluation of selection hypothesis and analysis of association with schizophrenia

Associations have been reported between the variable number of tandem repeat (VNTR) polymorphisms in the exon 3 of dopamine D4 receptor gene gene and multiple psychiatric illnesses/traits. We examined the distribution of VNTR alleles of different length in a Japanese cohort and found that, as reported earlier, the size of allele '7R' was much rarer (0.5%) in Japanese than in Caucasian populations (approximately 20%). This presents a challenge to an earlier proposed hypothesis that positive selection favoring the allele 7R has contributed to its high frequency. To further address the issue of selection, we carried out sequencing of the VNTR region not only from human but also from chimpanzee samples, and made inference on the ancestral repeat motif and haplotype by use of a phylogenetic analysis program. The most common 4R variant was considered to be the ancestral haplotype as earlier proposed. However, in a gene tree of VNTR constructed on the basis of this inferred ancestral haplotype, the allele 7R had five descendent haplotypes in relatively long lineage, where genetic drift can have major influence. We also tested this length polymorphism for association with schizophrenia, studying two Japanese sample sets (one with 570 cases and 570 controls, and the other with 124 pedigrees). No evidence of association between the allele 7R and schizophrenia was found in any of the two data sets. Collectively, this study suggests that the VNTR variation does not have an effect large enough to cause either selection or a detectable association with schizophrenia in a study of samples of moderate size.

Correlates of response to Olanzapine in a North Indian Schizophrenia sample

Olanzapine is widely used for the treatment of schizophrenia and is considered a first line medication in India. Along with other factors, the variation in response and side effects to this agent may be accounted for by genetic differences among patients. Olanzapine was administered for 6 weeks to Indian subjects with schizophrenia or schizoaffective disorder (DSM-IV, n=130), as part of an open label study. Intent-to-treat analysis was performed, and 10 polymorphic markers from seven genes (dopamine D1, D2, D3 and D4 receptors, serotonin 2A receptor and the drug-metabolizing enzymes (CYP1A2 and CYP2D6)), together with demographic and clinical variables, were analyzed as potential predictors of response. Olanzapine was efficacious, but significant weight gain was noted. Baseline weight and a 120 bp deletion polymorphism at the dopamine receptor D4 (DRD4) gene were associated with changes in symptom scores. Predictable covariates of treatment response were also noted. These results merit replicate studies.

Association analysis of exon III and exon I polymorphisms of the dopamine D4 receptor locus in Mexican psychotic patients

In this study, we investigated whether polymorphisms of the dopamine D4 receptor (DRD4) gene were associated with psychotic symptomatology rather than with a unique diagnosis such as schizophrenia. A number of association studies between the DRD4 gene 48 bp-VNTR polymorphism at exon 3 and psychotic disorders have been reported, but the results have been controversial. Both 48 bp-VNTR and the 12 bp-VNTR (at exon 1) polymorphisms of this gene were analyzed in a group of 149 unrelated Mexican subjects with a diagnosis of schizophrenia, schizoaffective disorder, schizophreniform disorder, major depression and bipolar disorder, both with psychotic symptoms, brief psychotic disorder, delusional disorder and non-specific psychotic disorder, and in 169 individuals free of psychiatric illnesses. There were no differences in allele or genotype frequencies between groups for the 12 bp-VNTR polymorphisms. However, a significant excess of "rare" alleles (3-, 5-, 6- and 8-48 bp repeats alleles) was found in the group of psychotics. Moreover, haplotypes 3-A1, 5-A1, 6-A1 and 8-A1 were significantly more frequently associated with cases. This positive association supports a role of this molecule as a genetic risk factor in psychotic disorders.

Genetic structure of the dopamine receptor D4 gene (DRD4) and lack of association with schizophrenia in Japanese patients

In order to investigate the contribution of genetic variation in the human dopamine receptor D4 gene (DRD4) to the risk of developing schizophrenia, we carried out a genetic analysis of 27 polymorphisms in 216 schizophrenic patients and 243 healthy controls from the Kyushu region of Japan. Twenty-two single nucleotide polymorphisms (SNPs) and five insertion/deletion polymorphisms were analyzed in this study, including four novel SNPs and a novel mononucleotide repeat. Linkage disequilibrium (LD) and haplotype analyses reveal weak LD across the DRD4 gene. In univariate analysis female individuals with allele -521C had a higher risk for schizophrenia. However, this finding was not significant after correction for multiple hypothesis testing. No other polymorphisms or haplotypes differed between schizophrenic patients and controls. Likewise, multivariate analyses did not reveal any statistically significant associations.

Association and synergistic interaction between promoter variants of the DRD4 gene in Japanese schizophrenics

Recent association studies suggest that polymorphisms in the promoter and exon 1 upstream region of the dopamine D4 receptor (DRD4) gene play a functional role in the development of common psychiatric illnesses, although there are also conflicting results. In this study, we re-sequenced this region to identify all genomic variants, and tested them for association with schizophrenia. A total of 570 Japanese schizophrenic cases with matched controls were studied by genotyping all identified/validated common polymorphisms (−1106T>C, −906T>C, −809G>A, −616G>C, −521T>C, −376C>T, −291C>T and 12-bp repeat) and a known microsatellite (120-bp tandem duplication) in the upstream region. A single nucleotide polymorphism (SNP) −809G>A in the promoter region was found to be significantly associated with disease (P=0.018 and 0.032 for allelic and genotypic comparisons, respectively), although not surviving after Bonferroni correction. Logistic regression analysis showed that a combination of the four polymorphisms, −809G>A, −616G>C, −291C>T and the 12-bp repeat, conferred a susceptibility to schizophrenia. These results suggest that the upstream variants have a primary functional effect in the etiology of schizophrenia in the Japanese population.

No direct effect of the -521 C/T polymorphism in the human dopamine D4 receptor gene promoter on transcriptional activity

Background

The human dopamine D4 receptor (DRD4) gene has been studied extensively as a candidate gene for certain psychological traits and several behavioural and psychiatric disorders. Both the 5' regulatory region and the coding sequence contain a number of polymorphisms. The promoter variants have received particular attention in the past few years due to their possible role in the regulation of gene transcription. Previously, the -521C/T SNP was shown to influence promoter activity. The aim of this study is to perform an in-depth analysis of this effect in the context of various neural cell lines.

Results

Endogenous mRNA expression of the DRD4 gene was demonstrated in two neuroblastoma (SK-N-F1, IMR32) and one retinoblastoma cell line (Y79) by RT-PCR. In addition, very low DRD4 mRNA levels were also detected in HeLa cells. The transcriptional activity of a series of 5' promoter deletion mutants was determined by transient transfection of luciferase reporter constructs. The activity profile of these promoter fragments was similar in each of the cell lines tested. The highest luciferase reporter activity was obtained with a construct containing promoter sequences between nucleotides -668 to -389, while a putative silencer region was localised spanning from nucleotide -1571 to -800. Surprisingly, the -521 C/T polymorphism had no significant effect on transcriptional activity of the reporter construct with the highest activity (-668 to -389) in any of the three cell lines tested.

Conclusion

Our results do not confirm previous data assigning different transcriptional activities to the -521 C/T alleles of the human DRD4 promoter. Furthermore, these findings highlight the need for further characterization of the 5' regulatory region of the DRD4 gene and identification of additional functional promoter polymorphic sites, especially in the context of haplotype.

A subpopulation of neurons in the medial prefrontal cortex encodes emotional learning with burst and frequency codes through a dopamine D4 receptor-dependent basolateral amygdala input

The basolateral nucleus of the amygdala (BLA) and medial prefrontal cortex (mPFC) are involved importantly in the processing and encoding of emotionally salient learned associations. Here, we examined the possible role of the mPFC in the acquisition and encoding of emotional associative learning at the behavioral and single-neuron level. A subpopulation of neurons in the mPFC that received monosynaptic and orthodromic inputs from the BLA demonstrated strong associative responding to odors paired previously with footshock by increasing spontaneous activity and bursting activity. This occurred specifically in response to postconditioning presentations of the footshock-paired odors but not to odors presented in the absence of footshock. In contrast, mPFC neurons that failed to respond to BLA stimulation showed no associative responding. Systemic or intra-mPFC blockade of dopamine (DA) D4 receptors prevented this emotional associative learning in neurons of the mPFC and blocked the expression of olfactory conditioned fear. These results demonstrate that individual neurons in the mPFC that receive a functional input from the BLA actively encode emotional learning and that this process depends on DA D4 receptor stimulation in the mPFC.

Major affective disorders and schizophrenia: a common molecular signature?

Psychiatric disorders, including affective disorders (AD) and schizophrenia (SZ) are among the most common disabling brain diseases in Western populations and result in high costs in terms of morbidity as well as mortality. Although their etiology and pathophysiology is largely unknown, family-, twin-, and adoption studies argue for a strong genetic determination of these disorders. These studies indicate that there is between 40 and 85% heritability for these disorders but point also to the importance of environmental factors. Therefore, any research strategy aiming at the identification of genes involved in the development of AD and SZ should account for the complex nature (multifactorial) of these disorders. During the last decade, molecular genetic studies have contributed a great deal to the identification of genetic factors involved in complex disorders. Here we provide a comprehensive review of the most promising genes for AD and SZ, and the methods and approaches that were used for their identification. Also, we discuss the current knowledge and hypotheses that have been formulated regarding the effect of variations on protein functioning as well as recent observations that point to common molecular mechanisms.

Genetic variation in G-protein-coupled receptors – consequences for G-protein-coupled receptors as drug targets

G-protein-coupled receptors (GPCRs), including 'orphan' GPCRs whose natural ligands are unknown, comprise the largest membrane receptor superfamily and are the most commonly used therapeutic targets. GPCR genetic loci harbour numerous variants, such as DNA insertions or deletions and single nucleotide polymorphisms that alter GPCR expression and function, thereby contributing to inter-individual differences in disease susceptibility/progression and drug responses. In this article, the authors review examples of GPCR genetic variants that influence transcription, translation, receptor folding and expression on cell surface (by affecting receptor trafficking, dimerisation, desensitisation/downregulation), or perturb receptor function (by altering ligand binding, G-protein coupling and receptor constitutive activity). In spite of such effects, assessment for genetic variants is not currently applied to the drug development and approval process or in the clinical use of GPCR drugs. Further insights will, the authors believe, alter drug discovery/development, therapeutics and likely provide new GPCR drug targets.

Aberrant regulation of NMDA receptors by dopamine D4 signaling in rats after phencyclidine exposure

Dopaminergic dysfunction in the prefrontal cortex (PFC) has been implicated in the pathophysiology of schizophrenia. On the other hand, administration of the NMDAR antagonist phencyclidine (PCP) impairs PFC functions and induces a broad range of schizophrenic-like symptoms, thus has been widely used as an animal model for schizophrenia. This study sought to determine the mechanism by which PCP may alter the dopaminergic functions in PFC. In control rats, activation of dopamine D4 receptors produced a significant suppression of NMDA receptor transmission in PFC pyramidal neurons, which was dependent on the inhibition of active CaMKII. However, in PCP-treated rats, the D4 modulation of NMDA receptors was significantly impaired, with the concomitant loss of D4 regulation of CaMKII activity. In contrast, the D4 modulation of voltage-dependent Ca2+ channels was intact following PCP administration. Furthermore, treatment with the antipsychotic drug clozapine restored the D4 regulation of NMDA receptors in PCP-treated rats. These findings suggest that the selective disruption of the interaction between D4 and NMDA receptors in the PCP model, which is attributable to the impaired D4-mediated downstream signaling, may contribute to the aberrant PFC neuronal activity in schizophrenia.

Differential effects of DRD4 and DAT1 genotype on fronto-striatal gray matter volumes in a sample of subjects with attention deficit hyperactivity disorder, their unaffected siblings, and controls

Genetic influences on behavior are complex and, as such, the effect of any single gene is likely to be modest. Neuroimaging measures may serve as a biological intermediate phenotype to investigate the effect of genes on human behavior. In particular, it is possible to constrain investigations by prior knowledge of gene characteristics and by including samples of subjects where the distribution of phenotypic variance is both wide and under heritable influences. Here, we use this approach to show a dissociation between the effects of two dopamine genes that are differentially expressed in the brain. We show that the DAT1 gene, a gene expressed predominantly in the basal ganglia, preferentially influences caudate volume, whereas the DRD4 gene, a gene expressed predominantly in the prefrontal cortex, preferentially influences prefrontal gray matter volume in a sample of subjects including subjects with ADHD, their unaffected siblings, and healthy controls. This demonstrates that, by constraining our investigations by prior knowledge of gene expression, including samples in which the distribution of phenotypic variance is wide and under heritable influences, and by using intermediate phenotypes, such as neuroimaging, we may begin to map out the pathways by which genes influence behavior.

The polymorphic nature of the human dopamine D4 receptor gene: a comparative analysis of known variants and a novel 27 bp deletion in the promoter region

BACKGROUND

The human dopamine D4 receptor (DRD4) is a candidate gene of great interest in molecular studies of human personality and psychiatric disorders. This gene is unique in having an exceptionally high amount of polymorphic sites both in the coding and in the promoter region.

RESULTS

We report the identification of a new 27 bp deletion starting 524 bp upstream of the initiation codon (27 bp del) of the dopamine D4 receptor (DRD4) gene, in the close vicinity of the -521C>T SNP. The presence of the 27 bp deletion leads to the misgenotyping of the -616C>G SNP by the Sau96 I RFLP method, thus the genotype determination of the mutation is of additional importance. The frequency of this novel sequence variation is considerably low (allele frequency is = 0.16%), as no homozygotes, and only 3 heterozygote carriers were found in a healthy, unrelated Caucasian sample (N = 955).

CONCLUSION

Remarkably, the deleted region contains consensus sequences of binding sites for several known transcription factors, suggesting that the different alleles may affect the transcriptional regulation of the gene. A comparison of methods and results for the allelic variations of the DRD4 gene in various ethnic groups is also discussed, which has a high impact in psychiatric genetic studies.

Dopamine D4 receptors: beyond schizophrenia

Dopamine D4 receptors mediate a wide range of neuronal signal transduction cascades. Malfunctions of these mechanisms may contribute to the pathophysiology of neuropsychiatric disorders, and their modification underlies the actions of many psychotropic drugs. Postmortem neuropathological and genetic studies provide inconclusive associations between D4 receptors and schizophrenia. Clinical trials of partially selective lead D4 antagonists have proved them to be ineffective against psychotic symptoms in patients diagnosed with schizophrenia. However, associations are emerging between D4 receptors and other neuropsychiatric disorders, including attention-deficit hyperactivity disorder as well as specific personality traits such as novelty seeking. Preclinical studies indicate that D4 receptors play a pivotal role in the cellular mechanisms of hyperactivity, impulsivity, and working memory. Accordingly, D4 receptors have broader implications for human illnesses than has been suggested by early focus on psychotic illness as a clinical target, and selective D4 agents may yield clinically useful drugs for several neuropsychiatric disorders that require improved treatments.

DRD4 exon 3 variants are not associated with symptomatology of major psychoses in a German population

We previously reported an association of DRD4 exon 3 long alleles with delusional symptomatology, independently from psychiatric diagnoses [Am. J. Med. Genet. 105 (2001) 283; Psychiatry Res. 80 (1998) 129]. The aim of this investigation was to replicate these results in an independent sample from Germany. We studied 394 subjects, affected by bipolar disorder (n = 32), schizoaffective disorder (n = 45), and schizophrenia (n = 317). All affected subjects were evaluated using the Operational Criteria for Psychotic Illness (OPCRIT) checklist. DRD4 variants were not associated with symptomatology of major psychosis. Our present results, obtained in an independent German sample, did not confirm the association between DRD4 variants and delusional symptomatology. However it should be considered that the original sample included a much higher rate of mood disorders and this could partially explain the discrepancy.

Association analysis of polymorphisms in the upstream region of the human dopamine D4 receptor gene in schizophrenia

Several studies have suggested that the transcriptional activity of the DRD4 gene may exert an important role in susceptibility to schizophrenia. To address this issue, we studied the association of schizophrenia and polymorphisms including -616C>G, -603T>del, -602G>del, 600G>C, -521C>T, -376C>T and a 120 bp tandem duplication polymorphism (120 bp repeat) in 1.2 kb upstream from the initiation codon in the promoter region of the DRD4 gene with 210 schizophrenic cases and 206 healthy controls. The results showed a significant excess of allele L of the 120 bp repeat in the schizophrenic patients compared to the controls (X(2)=8.585, df=1, P=0.003, OR=1.546, 95% CI=1.154-2.070). No significant difference was detected in the frequencies of genotype and allele of six other polymorphisms between the two groups. However, haplotypic distribution of 120 bp repeat, -616C>G, -602G>del, -521C>T and -376C>T was significantly different between case and control groups (P=0.005). This might cause the alteration of the transcriptional regulation of the DRD4 gene, as the consensus sequences of binding sites for several known transcription factors are involved in this region.

Understanding the pathology of schizophrenia: recent advances from the study of the molecular architecture of postmortem CNS tissue

The use of central nervous system (CNS) tissue obtained postmortem has long underpinned efforts to understand the neurobiology of schizophrenia, but the ability to use such tissue in conjunction with a wide variety of methodologies has seen a renaissance of interest in this area of research. Recent findings have shown changes in markers in a number of neurotransmitter systems in the brains of subjects with schizophrenia which include the dopaminergic, serotonergic, cholinergic, glutamatergic, and GABAergic systems of the CNS. Many of these changes also appear to be regionally specific, and abnormalities in non-neurotransmitter specific pathways have been found in schizophrenia. Changes in the neurotransmitter release pathways in schizophrenia may be important in the pathology of the illness, and recent findings suggest that abnormalities in the Wnt pathway, which controls transcription selectivity in cells, may be involved. Studies using CNS material obtained postmortem clearly show that the pathology of schizophrenia is complex while the polygenetic nature of the illness may be adding to this complexity.

The discovery of PD 89211 and related compounds: selective dopamine D4 receptor antagonists

The dopamine (DA) D2 family of receptors consists of the D2, D3, and D4 receptors. The DA D4 receptor is of interest as a target for drugs to treat schizophrenia based upon its high affinity for the atypical antipsychotic clozapine and its localization to the limbic and cortical regions of the brain. As part of a program to identify novel DA D4 receptor antagonists, a high-volume screen using the Parke-Davis compound library was initiated. This led to the discovery of PD 89211 (benzenemethanol, 2-chloro-4-[4-[(1H-benzimidazol-2-yl)methyl]-1-piperzinyl]) that displaced [3H]spiperone binding to hD4.2 with an affinity (Ki) of 3.7 nM. PD 89211 exhibited high selectivity for the DA D4.2 receptor (> 800-fold) as compared to other hDA receptor subtypes, rat brain serotonin, and adrenergic receptors. In vitro, PD 89211 had D4 receptor antagonist activity reversing quinpirole-induced [3H]thymidine uptake in CHOpro5 cells (IC50 = 2.1 nM). Limited structure-activity relationship (SAR) studies indicated that compounds with a 4-chloro-, 4-methyl-, and 3-chloro- substituents on the phenyl ring retained high affinity for D4 receptors, while those with a 4-methoxy- and no substituent had less affinity. While all clinically effective antipsychotics increase DA synthesis (DOPA accumulation) in rodents, PD 89211 did not increase DA synthesis in the DA-enriched striatum, indicating no effect on DA turnover and low propensity for exhibiting motor side effects. However, it did increase catecholamine synthesis in rat hippocampus, as did clozapine. Moreover, PD 89211 selectivity increased catecholamine synthesis in the hippocampus of wild type but not in mice lacking D4 receptors, suggesting that one function of D4 receptors may be to modulate DA/norepinephrine (NE) turnover in this brain area known to possess D4 receptors. The discovery of compounds like PD 89211 provides a tool to help in understanding the function of DA D4 receptors in the CNS.

Application of cDNA microarrays to examine gene expression differences in schizophrenia

Using cDNA microarrays we have investigated gene expression patterns in brain regions of patients with schizophrenia. A cDNA neuroarray, comprised of genes related to brain function, was used to screen pools of samples from the cerebellum and prefrontal cortex from a matched set of subjects, and middle temporal gyrus, from a separate subject cohort. Samples of cerebellum and prefrontal cortex from neuroleptic naive patients were also included. Genes that passed a 3% reproducibility criterion for differential expression in independent experiments included 21 genes for drug-treated patients and 5 genes for drug-naive patients. Of these 26 genes, 10 genes were increased and 16 were decreased. Many of the differentially expressed genes were related to synaptic signaling and proteolytic functions. A smaller number of these genes were also differentially expressed in the middle temporal gyrus. The five genes that were differentially expressed in two brain regions from separate cohorts are: tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein, eta polypeptide; sialyltransferase; proteasome subunit, alpha type 1; ubiquitin carboxyl-terminal esterase L1; and solute carrier family 10, member 1. Identification of patterns of changes in gene expression may lead to a better understanding of the pathophysiology of schizophrenia disorders.

Prodynorphin and kappa opioid receptor mRNA expression in the cingulate and prefrontal cortices of subjects diagnosed with schizophrenia or affective disorders

The present study examined the prodynorphin and kappa opioid receptor mRNA expression levels in the anterior cingulate and dorsolateral prefrontal cortices of subjects diagnosed with schizophrenia, bipolar disorder, or major depression as compared with normal controls without a psychiatric diagnosis. Multivariate analyses failed to reveal any differences in the mRNA expression levels between the four diagnostic groups, though a group trend (non-significant) was evident for the expression of the kappa opioid receptor and prodynorphin mRNAs in the prefrontal cortex. The mRNA expression levels were not associated with lifetime history of antipsychotic treatment or with suicide as a cause of death. The results, however, suggested an influence of certain drugs of abuse on the prodynorphin cortical mRNA expression. Prodynorphin mRNA expression levels were found to be elevated in individuals with a history of marihuana or stimulant use, but not alcohol. Overall, our data do not provide strong evidence for impaired prodynorphin or kappa opioid receptor mRNA levels in the dorsolateral prefrontal or cingulate cortices of schizophrenic, bipolar disorder, or major depressed subjects.

Dopamine D4 receptors and development of newer antipsychotic drugs

The last ten years have witnessed the generation of a large amount of information on the neurobiology of dopamine receptors. Molecular biology and pharmacology studies have revealed existence of at least five dopamine receptor subtypes, namely D1, D2, D3, D4 and D5. The discovery of D4 receptors and the putative affinity of clozapine for D4 receptors have kindled development of selective D4 receptor antagonists for the treatment of schizophrenia. Studies on expression of D4 receptor proteins have shown selective localisation of D4 receptors in mesolimbic/mesocortical areas which could probably explain the lack of motor side effects with atypical antipsychotics like clozapine and olanzapine. However, neuropathological and genetic studies on the role of D4 receptors in the pathophysiology of schizophrenia and preliminary clinical studies with selective D4 receptor antagonists have been disappointing. There have been, however, complimentary findings between selective D4 receptor antagonism and genetic approaches such as antisense treatment or gene targeting. The therapeutic potential of D4 receptors as a target for developing antipsychotics will be known only when selective D4 receptor-antagonists with varying D2/D4 and D4/5-HT2A ratios are developed and tested in psychiatric patients.

Cytokine and growth factor involvement in schizophrenia--support for the developmental model

Medical treatment with various cytokines can provoke psychiatric symptoms. Conversely, psychiatric patients can display abnormalities in cytokine and neurotrophic factor expression. Such observations have pointed to the potential contribution of cytokines and growth factors to schizophrenic pathology and/or etiology. The cellular targets of the relevant factors and the nature of their actions remain to be explored in mental illness, however. Recent physiological studies demonstrate that cytokines and neurotrophic factors can markedly influence synaptic transmission and plasticity upon acute or chronic application. Moreover, many of the molecular alterations observed in the schizophrenic brain are consistent with abnormalities in cytokine and neurotrophic factor regulation of these molecules. In this review, we summarize these molecular pathology findings for schizophrenia and highlight the neurodevelopmental activities of cytokines and neurotrophic factors that may contribute to the etiology or pathology of this illness.

The dopamine D(4) receptor: one decade of research

Dopamine is an important neurotransmitter involved in motor control, endocrine function, reward, cognition and emotion. Dopamine receptors belong to the superfamily of G protein-coupled receptors and play a crucial role in mediating the diverse effects of dopamine in the central nervous system (CNS). The dopaminergic system is implicated in disorders such as Parkinson's disease and addiction, and is the major target for antipsychotic medication in the treatment of schizophrenia. Molecular cloning studies a decade ago revealed the existence of five different dopamine receptor subtypes in mammalian species. While the presence of the abundantly expressed dopamine D(1) and D(2) receptors was predicted from biochemical and pharmacological work, the cloning of the less abundant dopamine D(3), D(4) and D(5) receptors was not anticipated. The identification of these novel dopamine receptor family members posed a challenge with respect to determining their precise physiological roles and identifying their potential as therapeutic targets for dopamine-related disorders. This review is focused on the accomplishments of one decade of research on the dopamine D(4) receptor. New insights into the biochemistry of the dopamine D(4) receptor include the discovery that this G protein-coupled receptor can directly interact with SH3 domains. At the physiological level, converging evidence from transgenic mouse work and human genetic studies suggests that this receptor has a role in exploratory behavior and as a genetic susceptibility factor for attention deficit hyperactivity disorder.

[(3)H]PNU-101958, a D(4) dopamine receptor probe, accumulates in prefrontal cortex and hippocampus of non-human primate brain

The D(4) dopamine receptor has been investigated for its potential role in neuropsychiatric disorders, "novelty-seeking" behaviors, and effects produced by some psychostimulants. An accurate map of D(4) distribution and density in brain is essential to clarify the role of this receptor subtype in normal brain function and in neuropsychiatric disorders. We investigated the autoradiographic distribution of D(4) receptors in non-human primate (Macaca mulatta) brain (N = 3) with the novel D(4) receptor probe [(3)H]PNU-101958. Quantification of [(3)H]PNU-101958 binding sites in 77 brain regions revealed dense levels of D(4) receptors in several cortical areas, especially in prefrontal cortex, uncus, hypothalamic median eminence, hippocampal formation, and distinct thalamic nuclei, but were significantly lower in striatum. The results correspond well with previous reports of brain distribution of D(4) receptors using other radiolabeled probes, and of D(4) mRNA localization (with some exceptions). Overall, this study reveals that [(3)H]PNU-101958 binding sites in non-human primate brain appear to reflect D(4) dopamine receptor distribution. The significance of a dense localization of D(4) receptors in prefrontal cortex and hippocampus, and broad distribution in other brain areas, allows for investigation of the relationship of these receptors to specific neuropsychiatric disorders and effects produced by psychostimulants.

Signal transmission, rather than reception, is the underlying neurochemical abnormality in schizophrenia

OBJECTIVE

This review aims to summarise the outcome of studies on changes in the molecular architecture of the brain of subjects with schizophrenia and formulate a hypothesis on mechanisms involved in the pathology of the illness.

METHOD

The outcomes from key studies using neuroimaging techniques and tissue obtained post-mortem that have been directed toward identifying abnormalities in the molecular architecture of the brain in subjects with schizophrenia were summarised. Using the results from these studies hypotheses were formulated on the underlying pathological process that precipitate schizophrenia.

RESULTS

Studies using neuroimaging techniques or tissue obtained post-mortem have revealed changes in the dopaminergic, serotoninergic, glutamatergic, GABAergic and cholinergic systems of the brain in schizophrenia. Some of these studies have identified abnormalities in presynaptic proteins or functioning that may be central to the pathology of schizophrenia.

CONCLUSIONS

There appears to be diverse changes in the molecular cytoarchitecture of the brains from subjects with schizophrenia. It could be that it is by affecting these multiple systems that the atypical antipsychotic drugs produce their improved clinical outcomes. Abnormal functioning of presynaptic processes could be central to the pathology of schizophrenia. If the 'presynaptic' hypothesis is proven, future antipsychotic drug design should be directed away from post-synaptic receptor antagonism toward the modulating the functions of presynaptic neurones.

Decreased density of tyrosine hydroxylase-immunoreactive axons in the entorhinal cortex of schizophrenic subjects

BACKGROUND

We recently reported a laminar-specific reduction in the density of tyrosine hydroxylase (TH)-immunoreactive axons in the prefrontal cortex of subjects with schizophrenia. In this report, we extend these investigations to the entorhinal cortex (ERC), another candidate site of dysfunction in this disorder.

METHODS

Using immunocytochemical techniques and blind quantitative analyses, we determined the density of TH-immunoreactive axons in the rostral subdivision of the ERC from seven matched pairs of schizophrenic and control subjects.

RESULTS

The relative density of TH-labeled axons was significantly decreased by over 60% in layers 3 and 6, but not in layer 1, of the ERC in schizophrenic subjects. In contrast, in the prefrontal cortex of the same subjects, labeled axon density was significantly decreased by 62% only in layer 6. Furthermore, the length of TH-labeled axons did not differ between six matched pairs of nonschizophrenic psychiatric and control subjects in any layer of the ERC. Finally, the density of TH-labeled axons in the ERC of cynomolgus monkeys chronically treated with haloperidol was not reduced relative to control animals.

CONCLUSIONS

These findings reveal regional- and laminar-specific alterations in TH-immunoreactive axons that appear to be specific to the pathophysiology of schizophrenia.

Dopamine D4 receptors

Initial investigations on dopamine D4 receptors generated much interest in the role of this receptor in schizophrenia and other aspects of human behavior, as well as new opportunities for novel therapeutics. However, attempts to treat patients suffering from schizophrenia with dopamine D4 agents have failed to yield satisfactory results so far. An examination of the dopamine D4 literature shows that contrasting and conflicting data seemed to be the norm in this field of research. This paper reviews the literature on the dopamine D4 receptor and discusses many of the associated methodological problems that might have contributed to the paradoxical findings.

Identification of a polymorphism in the promoter region of DRD4 associated with the human novelty seeking personality trait

Polymorphism in the human dopamine D4 receptor gene (DRD4) exon III has been associated in some but not all studies of the human personality trait of Novelty Seeking. We searched for polymorphisms in the 5' region of DRD4 and identified six polymorphisms as follows: -1217G Ins/Del, -809G/A, -616C/G, -603T Ins/Del, -602(G)8-9, and -521C/T. Associations between these polymorphisms and personality traits measured by the Temperament and Character Inventory (TCI) were investigated in 86 healthy Japanese volunteers. The -521C/T polymorphism was significantly associated with Novelty Seeking (P = 0.0001). Subjects with the C/C genotype exhibited the highest Novelty Seeking scores and those with the T/T genotype exhibited the lowest. A transient expression method revealed that the T variant of the C-521T polymorphism reduces transcriptional efficiency. The present study suggests a contribution of dopamine D4 receptor availability to individual differences in Novelty Seeking behavior. Molecular Psychiatry (2000) 5, 64-69.

A selective dopamine D4 receptor antagonist, NRA0160: a preclinical neuropharmacological profile

NRA0160, 5 - [2- ( 4- ( 3 - fluorobenzylidene) piperidin-1-yl) ethyl] - 4 -(4-fluorophenyl) thiazole-2-carboxamide, has a high affinity for human cloned dopamine D4.2, D4.4 and D4.7 receptors, with Ki values of 0.5, 0.9 and 2.7 nM, respectively. NRA0160 is over 20,000fold more potent at the dopamine D4.2 receptor compared with the human cloned dopamine D2L receptor. NRA0160 has negligible affinity for the human cloned dopamine D3 receptor (Ki=39 nM), rat serotonin (5-HT)2A receptors (Ki=180 nM) and rat alpha1 adrenoceptor (Ki=237 nM). NRA0160 and clozapine antagonized locomotor hyperactivity induced by methamphetamine (MAP) in mice. NRA0160 and clozapine antagonized MAP-induced stereotyped behavior in mice, although their effects did not exceed 50% inhibition, even at the highest dose given. NRA0160 and clozapine significantly induced catalepsy in rats, although their effects did not exceed 50% induction even at the highest dose given. NRA0160 and clozapine significantly reversed the disruption of prepulse inhibition (PPI) in rats produced by apomorphine. NRA0160 and clozapine significantly shortened the phencyclidine (PCP)-induced prolonged swimming latency in rats in a water maze task. These findings suggest that NRA0160 may have unique antipsychotic activities without the liability of motor side effects typical of classical antipsychotics.

1-(3-Cyanobenzylpiperidin-4-yl)-5-methyl-4-phenyl-1, 3-dihydroimidazol-2-one: a selective high-affinity antagonist for the human dopamine D(4) receptor with excellent selectivity over ion channels

After the requirement of pseudocycle formation in the ureas 3 and 7 for hD(4) binding and selectivity was confirmed, structural hybridization with the known hD(4) ligand 2 led to the design and identification of the lead 4-(2-oxo-1, 3-dihydroimidazol-2-yl)piperidine 8. Optimization studies were carried out on 8 with the aim of achieving 1000-fold selectivity for hD(4) over all other receptors while retaining the good pharmacokinetic properties of the lead. After initial preparation of 8 as a minor component in a low-yielding reaction, a novel and regioselective "four-step/one-pot" procedure was developed which proved to be applicable to rapid investigation of the SAR of the 1, 3-dihydroimidazol-2-one ring. Various changes to substituents attached to the 3-, 4-, or 5-position of the 1, 3-dihydroimidazol-2-one core of 8 did not significantly improve selectivity for hD(4) over hD(2) and hD(3). Greater selectivity (>1000-fold) was ultimately achieved by meta substitution of the benzyl group of 8 with various substituents. Compounds 28, 31, and 32 all possess the required selectivity for hD(4) over the other dopamine subtypes, but only 32 has >1000-fold selectivity over all the key counterscreens we tested against. Compound 32 is an antagonist at hD(4) and has a good pharmacokinetic profile in the rat, with excellent estimated in vivo receptor occupancy, thus making it a potentially useful pharmacological tool to investigate the role of the D(4) receptor.

A genetic polymorphism in the promoter region of DRD4 associated with expression and schizophrenia

The human dopamine D4 receptor gene (DRD4) is an important candidate gene for schizophrenia. We identified a novel -521C>T polymorphism in the 5'-promoter region of DRD4. A transient expression method revealed that the T allele of this polymorphism reduces the transcriptional efficiency by 40% compared with the C allele. This polymorphism is of interest because of reported elevation of D4-like sites and DRD4 mRNA in the postmortem schizophrenic brain. The C allele frequency was significantly higher in 252 Japanese schizophrenics (0.48) than in 269 Japanese controls (0.41) (p = 0. 02) [odds ratio = 1.35 (95% confidence interval 1.05 - 1.72)]. Although the association is weak and should be considered tentative until other studies replicate it, this polymorphism provides a tool with the potential to examine whether DRD4 is related to susceptibility to and neuroleptic response in schizophrenia.

4-N-linked-heterocyclic piperidine derivatives with high affinity and selectivity for human dopamine D4 receptors

The syntheses of a number of different N-linked heterocyclic pyrazole replacements based on the structure 1 are described (compounds 3-12) as hD4 ligands. After further optimisation the best compound identified was 13 which has high affinity for hD4 (5.2 nM) and >300-fold selectivity for hD4 receptors over hD2 and hD3 receptors.

Dopamine D2 and D4 receptor ligands: relation to antipsychotic action

Since the discovery that the antipsychotic action of phenothiazines was mediated by dopamine D2 receptors, the dopamine system has been scrutinized for schizophrenia related abnormalities. The focus has been to create neuroleptics with improved antipsychotic profiles and reduced side effects. With the identification of multiple dopamine receptor subtypes, the hypotheses regarding the role of dopamine in schizophrenia and antipsychotic action of neuroleptics have been refined. Even after the molecular identification of newer dopamine D2-like receptor subtypes (D3 and D4), the dopamine D2 receptor is still considered the predominant site for antipsychotic action. However, there has been much debate concerning the modulatory role of other dopamine receptor sites in the mechanism of action of antipsychotic drugs. Specifically, the dopamine D4 receptor has received much attention in this regard, since the atypical antipsychotic agent, clozapine, preferentially blocks this receptor subtype as compared with dopamine D2 and D3 receptors. In this review we will highlight some of the observations and arguments regarding the involvement of the dopamine D2 and D4 receptor sites in the therapeutic efficacy of antipsychotic medication.