Effect of dopamine uptake inhibition on brain catecholamine levels and locomotion in catechol-O-methyltransferase-disrupted mice

SERT and OCT mediate 5-HT1B receptor regulation of immobility behavior and uptake of 5-HT and HIS

5-HT clearance, commonly mediated by transporters in the uptake-1 and uptake-2 families, has been linked to 5-HT1B receptor's action on behaviors. Since no specific transporters identified yet, effects of serotonin transporter (SERT) and organic cation transporter (OCTs) on 5-HT1B-elicited immobility phenotype, and 5-HT and HIS uptake were then investigated. Intraperitoneal injections of SERT inhibitor fluoxetine (FLX) and/or OCTs inhibitor decynium (D22) were used prior to local perfusion of 5-HT1B agonist CP93129 into the ventral hippocampus to measure immobility times in the FST and TST, to measure 5-HT uptake efficiencies and HIS uptake efficiencies derived from linear regressions using the transient no-net-flux quantitative microdialysis in C57BL/6 mice. Exogenous 5-HT and HIS uptake were measured following incubation of FLX and/or D22 with CP93129 in the RBL-2H3 cells. Moreover, surface membrane levels of SERT and OCT were detected in response to CP93129. Local CP93129 prolonged immobility times, which were attenuated following pretreatment of either inhibitor. Local CP93129 lowered the slopes obtained from the lineal regressions for 5-HT and HIS (slope is reciprocal to uptake efficiency), which were then weakened following pretreatment of either inhibitor. Similar findings were obtained following CP93129 incubation, and co-incubation of CP93129 with either inhibitor in the RBL-2H3. Moreover, CP93129 dose-dependently moved SERT and OCT3 in the cytosol to the surface membrane. Both SERT and OCT are the target effectors mediating 5-HT1B regulation of immobility time and 5-HT uptake, OCT mediates 5-HT1B regulation of HIS uptake. Their underlying signal transductions need to be further explored.

A frame-shift mutation in COMTD1 is associated with impaired pheomelanin pigmentation in chicken

The biochemical pathway regulating the synthesis of yellow/red pheomelanin is less well characterized than the synthesis of black/brown eumelanin. Inhibitor of gold (IG phenotype) is a plumage colour variant in chicken that provides an opportunity to further explore this pathway since the recessive allele (IG) at this locus is associated with a defect in the production of pheomelanin. IG/IG homozygotes display a marked dilution of red pheomelanin pigmentation, whilst black pigmentation (eumelanin) is only slightly affected. Here we show that a 2-base pair insertion (frame-shift mutation) in the 5th exon of the Catechol-O-methyltransferase containing domain 1 gene (COMTD1), expected to cause a complete or partial loss-of-function of the COMTD1 enzyme, shows complete concordance with the IG phenotype within and across breeds. We show that the COMTD1 protein is localized to mitochondria in pigment cells. Knockout of Comtd1 in a mouse melanocytic cell line results in a reduction in pheomelanin metabolites and significant alterations in metabolites of glutamate/glutathione, riboflavin, and the tricarboxylic acid cycle. Furthermore, COMTD1 overexpression enhanced cellular proliferation following chemical-induced transfection, a potential inducer of oxidative stress. These observations suggest that COMTD1 plays a protective role for melanocytes against oxidative stress and that this supports their ability to produce pheomelanin.

Tolcapone Treatment for Cognitive and Behavioral Symptoms in Behavioral Variant Frontotemporal Dementia: A Placebo-Controlled Crossover Study

BACKGROUND

There are currently no disease-targeted treatments for cognitive or behavioral symptoms in patients with behavioral variant frontotemporal dementia (bvFTD).

OBJECTIVE

To determine the effect of tolcapone, a specific inhibitor of Catechol-O-Methyltransferase (COMT), in patients with bvFTD.

METHODS

In this randomized, double-blind, placebo-controlled, cross-over study at two study sites, we examined the effect of tolcapone on 28 adult outpatients with bvFTD. The primary outcome was reaction time on the N-back cognitive test. As an imaging outcome, we examined differences in the resting blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) signal intensity between subjects on placebo versus tolcapone performing the N-back test. Secondary outcomes included measures of cognitive performance and behavioral disturbance using the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS), Neuropsychiatric Inventory-Questionnaire (NPI-Q), and Clinical Global Impressions scale (CGI).

RESULTS

Tolcapone was well tolerated and no patients dropped out. The most frequent treatment-related adverse event during tolcapone treatment was elevated liver enzymes (21%). There were no significant differences between tolcapone treatment and placebo in the primary or imaging outcomes. However, there were significant differences between RBANS total scores (p < 0.01), NPI-Q total scores (p = 0.04), and CGI total scores (p = 0.035) between treatment conditions which were driven by differences between baseline and tolcapone conditions. Further, there was a trend toward significance between tolcapone and placebo on the CGI (p = 0.078).

CONCLUSIONS

Further study of COMT inhibition and related approaches with longer duration of treatment and larger sample sizes in frontotemporal lobar degeneration-spectrum disorders may be warranted.

Interactions of COMT and ALDH2 Genetic Polymorphisms on Symptoms of Parkinson's Disease

(1) Background: Monoamine neurotransmitters play essential roles in the normal functioning of our nervous system. However, the metabolism of monoamine neurotransmitters is accompanied by the production of neurotoxic metabolites, and inefficient removal of the metabolites has been suggested to cause neurodegeneration. (2) Methods: To examine the effect of reduced activity of catechol-O-methyltransferase (COMT) and aldehyde dehydrogenase 2 (ALDH2) conferred by single nucleotide polymorphisms COMT rs4680(A) and ALDH2 rs671(A) on the symptoms of patients with Parkinson’s disease (PD), a total of 114 PD patients were recruited cross-sectionally and received genotyping for rs4680 and rs671 along with MDS-UPDRS evaluation. (3) Results: We found that patients carrying rs4680(A) had more severe bradykinesia in the upper extremity and rest tremor. Besides, patients carrying rs671(A) had more difficulty maintaining personal hygiene, while patients with genotype rs671(GG) had higher scores in the item “depressed mood.” More importantly, we found the effect of rs4680 to be moderated by rs671 SNP for the symptom of “hand movements.” The detrimental impact of rs4680(A) is more pronounced in the presence of genotype rs671(GG). (4) Conclusions: This study facilitates a deeper understanding of the detrimental effect of reduced activity of COMT and ALDH2 conferred by genetic variation and provides novel insight into the interactions between enzymes metabolizing monoamine neurotransmitters in the pathogenesis of PD.

COMT Val(158) Met genotype is associated with reward learning: a replication study and meta-analysis

Identifying mechanisms through which individual differences in reward learning emerge offers an opportunity to understand both a fundamental form of adaptive responding as well as etiological pathways through which aberrant reward learning may contribute to maladaptive behaviors and psychopathology. One candidate mechanism through which individual differences in reward learning may emerge is variability in dopaminergic reinforcement signaling. A common functional polymorphism within the catechol-O-methyl transferase gene (COMT; rs4680, Val(158) Met) has been linked to reward learning, where homozygosity for the Met allele (linked to heightened prefrontal dopamine function and decreased dopamine synthesis in the midbrain) has been associated with relatively increased reward learning. Here, we used a probabilistic reward learning task to asses response bias, a behavioral form of reward learning, across three separate samples that were combined for analyses (age: 21.80 ± 3.95; n = 392; 268 female; European-American: n = 208). We replicate prior reports that COMT rs4680 Met allele homozygosity is associated with increased reward learning in European-American participants (β = 0.20, t = 2.75, P < 0.01; ΔR(2) = 0.04). Moreover, a meta-analysis of 4 studies, including the current one, confirmed the association between COMT rs4680 genotype and reward learning (95% CI -0.11 to -0.03; z = 3.2; P < 0.01). These results suggest that variability in dopamine signaling associated with COMT rs4680 influences individual differences in reward which may potentially contribute to psychopathology characterized by reward dysfunction.

A neurological perspective on the enhancement debate: Lessons learned from Parkinson's disease

Cognitive enhancement is signified by adaptive behavioural change following an intervention that targets the brain. Although much of the discussion and research into cognitive enhancement focuses on the effects of neural interventions in healthy individuals, it is useful to consider evidence from clinical populations. Diseases of the central nervous system represent the primary and richest source of evidence on the effects of brain manipulations, which are in the first instance therapeutic. Parkinson's disease (PD) is used as a model for understanding the effects of pharmacological agents that target systems with a central role in cognition. The mixed outcomes of deep brain stimulation on cognition will also be discussed. By illustrating the psychopharmacological principle of diverse and malleable neurochemical optima for different cognitive functions, and the role of individual differences, it will be argued that the entire spectrum of cognitive effects in any one individual following any given manipulation, such as the administration of a drug, often includes enhancement as well as impairment. Predicting these effects represents a complex multivariate problem, and the accuracy of this predictive effort, as well as the harm prevention it connotes, is determined by our evolving understanding of the brain and cognition. A manipulation can be said to confer cognitive enhancement; however, it is argued that using the global term cognitive enhancer to refer to such a manipulation without qualification is of limited utility.

Comparison of the structure, function and autophagic maintenance of mitochondria in nigrostriatal and tuberoinfundibular dopamine neurons

A pathological hallmark of Parkinson׳s disease (PD) is progressive degeneration of nigrostriatal dopamine (NSDA) neurons, which underlies the motor symptoms of PD. While there is severe loss of midbrain NSDA neurons, tuberoinfundibular (TI) DA neurons in the mediobasal hypothalamus (MBH) remain intact. In the present study, confocal microscopic analysis revealed that mitochondrial content and numbers of mitophagosomes were lower in NSDA neuronal cell bodies in the substantia nigra pars compacta (SNpc) compared to TIDA neuronal cell bodies in the arcuate nucleus (ARC) of C57BL/6J male mice. Mitochondrial respiration, mass, membrane potential and morphology were determined using bioenergetic, flow cytometric and transmission electron microscopic analyses of synaptosomes isolated from discrete brain regions containing axon terminals of NSDA and TIDA neurons. Maximum and spare respiratory capacities, and mitochondrial mass were lower in synaptosomal mitochondria derived from the striatum (ST) as compared with the MBH, which correlated with lower numbers of mitochondria per synaptosome in these brain regions. In contrast, there was no regional difference in mitochondrial basal, maximum or spare respirations following inhibition of Complex I activity with rotenone. These results reveal that higher numbers of viable mitochondria are correlated with more extensive autophagic mitochondrial quality maintenance in TIDA neurons as compared with NSDA neurons.

Dopaminergic denervation severity depends on COMT Val158Met polymorphism in Parkinson's disease

BACKGROUND

Catecholamine-O-methyl-tranferase (COMT) initiates dopamine degradation. Its activity is mainly determined by a single nucleotide polymorphism in the COMT gene (Val158Met, rs4680) separating high (Val/Val, COMT(HH)), intermediate (Val/Met, COMT(HL)) and low metabolizers (Met/Met, COMT(LL)). We investigated dopaminergic denervation in the striatum in PD patients according to COMT rs4680 genotype.

METHODS

Patients with idiopathic PD were assessed for motor severity (UPDRS-III rating scale in OFF-state), dopaminergic denervation using [123I]-FP-CIT SPECT imaging, and genotyped for the COMT rs4680 enzyme. [123I]-FP-CIT binding potential (BP) for each voxel was defined by the ratio of tracer-binding in the region of interest (striatum, caudate nucleus and putamen) to that in a region of non-specific activity. Genotyping was performed using TaqMan(®) SNP genotyping assay. We used a regression model to evaluate the effect of COMT genotype on the BP in the striatum and its sub-regions.

RESULTS

Genotype distribution was: 11 (27.5%) COMT(HH), 26 (65%) COMT(HL) and 3 (7.5%) COMT(LL). There were no significant differences in disease severity, treatments, or motor scores between genotypes. When adjusted to clinical severity, gender and age, low and intermediate metabolizers showed significantly higher rates of striatal denervation (COMT(HL+LL) BP = 1.32 ± 0.04) than high metabolizers (COMT(HH), BP = 1.6 ± 0.08; F(1.34) = 9.0, p = 0.005). Striatal sub-regions showed similar results. BP and UPDRS-III motor scores (r = 0.44, p = 0.04) (p < 0.001) were highly correlated. There was a gender effect, but no gender-genotype interaction.

CONCLUSIONS

Striatal denervation differs according to COMT-Val158Met polymorphism. COMT activity may play a role as a compensatory mechanism in PD motor symptoms.

Extracellular dopamine and alterations on dopamine transporter are related to reserpine toxicity in Caenorhabditis elegans

Reserpine is used as an animal model of parkinsonism. We hypothesized that the involuntary movements induced by reserpine in rodents are induced by dopaminergic toxicity caused by extracellular dopamine accumulation. The present study tested the effects of reserpine on the dopaminergic system in Caenorhabditis elegans. Reserpine was toxic to worms (decreased the survival, food intake, development and changed egg laying and defecation cycles). In addition, reserpine increased the worms' locomotor rate on food and decreased dopamine levels. Morphological evaluations of dopaminergic CEP neurons confirmed neurodegeneration characterized by decreased fluorescence intensity and the number of worms with intact CEP neurons, and increased number of shrunken somas per worm. These effects were unrelated to reserpine's effect on decreased expression of the dopamine transporter, dat-1. Interestingly, the locomotor rate on food and the neurodegenerative parameters fully recovered to basal conditions upon reserpine withdrawal. Furthermore, reserpine decreased survival in vesicular monoamine transporter and dat-1 loss-of-function mutant worms. In addition, worms pre-exposed to dopamine followed by exposure to reserpine had decreased survival. Reserpine activated gst-4, which controls a phase II detoxification enzymes downstream of nuclear factor (erythroid-derived-2)-like 2. Our findings establish that the dopamine transporter, dat-1, plays an important role in reserpine toxicity, likely by increasing extracellular dopamine concentrations.

Schizophrenia and Depression Co-Morbidity: What We have Learned from Animal Models

Patients with schizophrenia are at an increased risk for the development of depression. Overlap in the symptoms and genetic risk factors between the two disorders suggests a common etiological mechanism may underlie the presentation of comorbid depression in schizophrenia. Understanding these shared mechanisms will be important in informing the development of new treatments. Rodent models are powerful tools for understanding gene function as it relates to behavior. Examining rodent models relevant to both schizophrenia and depression reveals a number of common mechanisms. Current models which demonstrate endophenotypes of both schizophrenia and depression are reviewed here, including models of CUB and SUSHI multiple domains 1, PDZ and LIM domain 5, glutamate Delta 1 receptor, diabetic db/db mice, neuropeptide Y, disrupted in schizophrenia 1, and its interacting partners, reelin, maternal immune activation, and social isolation. Neurotransmission, brain connectivity, the immune system, the environment, and metabolism emerge as potential common mechanisms linking these models and potentially explaining comorbid depression in schizophrenia.

Mesocorticolimbic monoamine correlates of methamphetamine sensitization and motivation

Methamphetamine (MA) is a highly addictive psychomotor stimulant, with life-time prevalence rates of abuse ranging from 5–10% world-wide. Yet, a paucity of research exists regarding MA addiction vulnerability/resiliency and neurobiological mediators of the transition to addiction that might occur upon repeated low-dose MA exposure, more characteristic of early drug use. As stimulant-elicited neuroplasticity within dopamine neurons innervating the nucleus accumbens (NAC) and prefrontal cortex (PFC) is theorized as central for addiction-related behavioral anomalies, we used a multi-disciplinary research approach in mice to examine the interactions between sub-toxic MA dosing, motivation for MA and mesocorticolimbic monoamines. Biochemical studies of C57BL/6J (B6) mice revealed short- (1 day), as well as longer-term (21 days), changes in extracellular dopamine, DAT and/or D2 receptors during withdrawal from 10, once daily, 2 mg/kg MA injections. Follow-up biochemical studies conducted in mice selectively bred for high vs. low MA drinking (respectively, MAHDR vs. MALDR mice), provided novel support for anomalies in mesocorticolimbic dopamine as a correlate of genetic vulnerability to high MA intake. Finally, neuropharmacological targeting of NAC dopamine in MA-treated B6 mice demonstrated a bi-directional regulation of MA-induced place-conditioning. These results extend extant literature for MA neurotoxicity by demonstrating that even subchronic exposure to relatively low MA doses are sufficient to elicit relatively long-lasting changes in mesocorticolimbic dopamine and that drug-induced or idiopathic anomalies in mesocorticolimbic dopamine may underpin vulnerability/resiliency to MA addiction.

Catechol-O-methyltransferase Val158Met polymorphism and altered COMT gene expression in the prefrontal cortex of suicide brains

Catechol-O-methyltransferase (COMT) plays a key role in the degradation of catecholamine neurotransmitters within the brain. A functional polymorphism COMT Val158Met has been associated with psychiatric disorders including suicidal behavior. In the present study we examined whether this polymorphism was related to COMT mRNA expression in frontal cortical regions, and whether the expression of COMT differed between depressed suicide victims and psychiatric healthy controls. The Val158Met polymorphism was determined by polymerase chain reaction and restriction fragment length polymorphism (PCR-RFLP) analysis. The levels of COMT mRNA expression in the frontopolar cortex (FPC; 29 suicides vs. 27 controls) and orbital frontal cortex (OFC; 19 suicides vs. 15 controls) were significantly increased among depressed individuals that died by suicide relative to those of controls, being up-regulated by approximately 60% and 65% in the FPC and OFC, respectively. Furthermore, among individuals with the Met allele (Met/Met and Met/Val genotypes) who died by suicide COMT mRNA expression was elevated relative to that of the nondepressed Met allele carriers. However, significant differences were not detected between suicides (n=49) and controls (n=72) with respect to the Val158Met genotypic distribution and allelic frequencies. These results are consistent with the perspective that altered COMT mRNA expression in frontal cortical brain regions might contribute to suicide and/or depression, further supporting the role of dysregulation of catecholaminergic pathway genes in the pathophysiology of suicide behaviors.

The effects of pdr1, djr1.1 and pink1 loss in manganese-induced toxicity and the role of α-synuclein in C. elegans

Parkinson's disease (PD) is a neurodegenerative brain disorder characterized by selective dopaminergic (DAergic) cell loss that results in overt motor and cognitive deficits. Current treatment options exist to combat PD symptomatology, but are unable to directly target its pathogenesis due to a lack of knowledge concerning its etiology. Several genes have been linked to PD, including three genes associated with an early-onset familial form: parkin, pink1 and dj1. All three genes are implicated in regulating oxidative stress pathways. Another hallmark of PD pathophysiology is Lewy body deposition, associated with the gain-of-function genetic risk factor α-synuclein. The function of α-synuclein is poorly understood, as it shows both neurotoxic and neuroprotective activities in PD. Using the genetically tractable invertebrate Caenorhabditis elegans (C. elegans) model system, the neurotoxic or neuroprotective role of α-synuclein upon acute Mn exposure in the background of mutated pdr1, pink1 or djr1.1 was examined. The pdr1 and djr1.1 mutants showed enhanced Mn accumulation and oxidative stress that was reduced by α-synuclein. Moreover, DAergic neurodegeneration, while unchanged with Mn exposure, returned to wild-type (WT) levels for pdr1, but not djr1.1 mutants expressing α-synuclein. Taken together, this study uncovers a novel, neuroprotective role for WT human α-synuclein in attenuating Mn-induced toxicity in the background of PD-associated genes, and further supports the role of extracellular dopamine in exacerbating Mn neurotoxicity.

Sex differences in effects of dopamine D1 receptors on social withdrawal

Dopamine signaling in the nucleus accumbens (NAc) plays a critical role in the regulation of motivational states. Recent studies in male rodents show that social defeat stress increases the activity of ventral tegmental dopamine neurons projecting to the NAc, and that this increased activity is necessary for stress-induced social withdrawal. Domestic female mice are not similarly aggressive, which has hindered complementary studies in females. Using the monogamous California mouse (Peromyscus californicus), we found that social defeat increased total dopamine, DOPAC, and HVA content in the NAc in both males and females. These results are generally consistent with previous studies in Mus, and suggest defeat stress also increases NAc dopamine signaling in females. However, these results do not explain our previous observations that defeat stress induces social withdrawal in female but not male California mice. Pharmacological manipulations provided more insights. When 500 ng of the D1 agonist SKF38393 was infused in the NAc shell of females that were naïve to defeat, social interaction behavior was reduced. This same dose of SKF38393 had no effect in males, suggesting that D1 receptor activation is sufficient to induce social withdrawal in females but not males. Intra-accumbens infusion of the D1 antagonist SCH23390 increased social approach behavior in females exposed to defeat but not in females naïve to defeat. This result suggests that D1 receptors are necessary for defeat-induced social withdrawal. Overall, our results suggest that sex differences in molecular pathways that are regulated by D1 receptors contribute to sex differences in social withdrawal behavior.

Genetic vs. pharmacological inactivation of COMT influences cannabinoid-induced expression of schizophrenia-related phenotypes

Catechol-O-methyltransferase (COMT) is an important enzyme in the metabolism of dopamine and disturbance in dopamine function is proposed to be central to the pathogenesis of schizophrenia. Clinical epidemiological studies have indicated cannabis use to confer a 2-fold increase in risk for subsequent onset of psychosis, with adolescent-onset use conveying even higher risk. There is evidence that a high activity COMT polymorphism moderates the effects of adolescent exposure to cannabis on risk for adult psychosis. In this paper we compared the effect of chronic adolescent exposure to the cannabinoid WIN 55212 on sensorimotor gating, behaviours related to the negative symptoms of schizophrenia, anxiety- and stress-related behaviours, as well as ex-vivo brain dopamine and serotonin levels, in COMT KO vs. wild-type (WT) mice. Additionally, we examined the effect of pretreatment with the COMT inhibitor tolcapone on acute effects of this cannabinoid on sensorimotor gating in C57BL/6 mice. COMT KO mice were shown to be more vulnerable than WT to the disruptive effects of adolescent cannabinoid treatment on prepulse inhibition (PPI). Acute pharmacological inhibition of COMT in C57BL/6 mice also modified acute cannabinoid effects on startle reactivity, as well as PPI, indicating that chronic and acute loss of COMT can produce dissociable effects on the behavioural effects of cannabinoids. COMT KO mice also demonstrated differential effects of adolescent cannabinoid administration on sociability and anxiety-related behaviour, both confirming and extending earlier reports of COMT×cannabinoid effects on the expression of schizophrenia-related endophenotypes.

Behavioral animal models of antipsychotic drug actions

Basic research in animals represents a fruitful approach to study the neurobiological basis of brain and behavioral disturbances relevant to neuropsychiatric disease and to establish and evaluate novel pharmacological therapies for their treatment. In the context of schizophrenia, there are models employing specific experimental manipulations developed according to specific pathophysiological or etiological hypotheses. The use of selective lesions in adult animals and the acute administration of psychotomimetic agents are indispensable tools in the elucidation of the contribution of specific brain regions or neurotransmitters to the genesis of a specific symptom or collection of symptoms and enjoy some degrees of predictive validity. However, they may be inaccurate, if not inadequate, in capturing the etiological mechanisms or ontology of the disease needed for a complete understanding of the disease and may be limited in the discovery of novel compounds for the treatment of negative and cognitive symptoms of schizophrenia. Under the prevailing consensus of schizophrenia as a disease of neurodevelopmental origin, we have seen the establishment of neurodevelopmental animal models which aim to identify the etiological processes whereby the brain, following specific triggering events, develops into a "schizophrenia-like brain" over time. Many neurodevelopmental models such as the neonatal ventral hippocampus (vHPC) lesion, methylazoxymethanol (MAM), and prenatal immune activation models can mimic a broad spectrum of behavioral, cognitive, and pharmacological abnormalities directly implicated in schizophrenic disease. These models allow pharmacological screens against multiple and coexisting schizophrenia-related dysfunctions while incorporating the disease-relevant concept of abnormal brain development. The multiplicity of existing models is testimonial to the multifactorial nature of schizophrenia, and there are ample opportunities for their integration. Indeed, one ultimate goal must be to incorporate the successes of distinct models into one unitary account of the complex disorder of schizophrenia and to use such unitary approaches in the further development and evaluation of novel antipsychotic treatment strategies.

A transient inhibition and permanent lack of catechol-O-methyltransferase have minor effects on feeding pattern of female rodents

Abnormal feeding behaviours have long been linked to disruptions in brain dopaminergic activity. Dopamine is metabolized, amongst others, by catechol-O-methyltransferase (COMT). Normally, COMT only plays a subordinate role in dopamine metabolism. However, changes in COMT activity, especially in the prefrontal cortex, become more important during events that evoke dopamine release. The current study investigated the effect of acute COMT inhibition on feeding in Wistar rats and C57BL/6 mice using a selective, brain penetrating COMT inhibitor (OR-1139). Furthermore, the effect of a long-term lack of COMT on feeding behaviour was studied in COMT-deficient (COMT -/-) mice. Apart from following the gross feeding behaviour of fasted rats and mice, the first 4 hr of re-feeding were recorded with a video camera to allow a more detailed analysis of feeding microstructure. In the acute study, rats and mice received a single injection of OR-1139 (3, 10 or 30 mg/kg), just before the experiment. We found that rats and mice receiving OR-1139 had fewer very short meals but more long meals than the controls. Treated mice even ate more frequently than the controls, but other feeding parameters remained unchanged. Conversely, COMT -/- mice displayed an increased latency to initiate the first meal and spent less total time eating than wild-type mice. In conclusion, although decreased/lack of COMT activity did not robustly alter feeding behaviour of female rodents, we observed some alterations in the microstructure of feeding. However, these minor changes were highly dependent on the extent and fashion in which COMT was manipulated.

Imaging genetics for utility of risks over gains and losses

One tenet of behavioral economics is the asymmetry in how decision makers evaluate risks involving gains versus risks involving losses. Correspondingly, an increasingly important question is what neuroanatomical and neurochemical correlates underpin valuation over gains and losses. By employing an imaging genetics strategy, this paper aims at identifying the specific neurotransmitter pathways underlying these decision making processes. We find enhanced striatal activation responding to increases in the magnitude of utility for risks over gains and to increases in the magnitude of disutility for risks over losses, while increased amygdala activation correlates only with the disutility for risks over losses. Stratifying brain activation by genotype, we find that a well-characterized polymorphism in the dopamine transporter (DAT1) contributes to individual differences in striatal response for gain-oriented risks, whereas a polymorphism in the serotonin transporter (STin2) partially accounts for individual differences in amygdala responses for loss-oriented risks. Together, our results suggest the role of the amygdala and corresponding serotonergic pathway in evaluating losses. This further corroborates the hypothesis of serotonin being linked to dopamine in an "opponent partnership".

Contribution of nonprimate animal models in understanding the etiology of schizophrenia

Schizophrenia is a severe psychiatric disorder that is characterized by positive and negative symptoms and cognitive impairments. The etiology of the disorder is complex, and it is thought to follow a multifactorial threshold model of inheritance with genetic and neurodevelop mental contributions to risk. Human studies are particularly useful in capturing the richness of the phenotype, but they are often limited to the use of correlational approaches. By assessing behavioural abnormalities in both humans and rodents, nonprimate animal models of schizophrenia provide unique insight into the etiology and mechanisms of the disorder. This review discusses the phenomenology and etiology of schizophrenia and the contribution of current nonprimate animal models with an emphasis on how research with models of neuro transmitter dysregulation, environmental risk factors, neurodevelopmental disruption and genetic risk factors can complement the literature on schizophrenia in humans.

Susceptibility genes for schizophrenia: mutant models, endophenotypes and psychobiology

Schizophrenia is characterised by a multifactorial aetiology that involves genetic liability interacting with epigenetic and environmental factors to increase risk for developing the disorder. A consensus view is that the genetic component involves several common risk alleles of small effect and/or rare but penetrant copy number variations. Furthermore, there is increasing evidence for broader, overlapping genetic-phenotypic relationships in psychosis; for example, the same susceptibility genes also confer risk for bipolar disorder. Phenotypic characterisation of genetic models of candidate risk genes and/or putative pathophysiological processes implicated in schizophrenia, as well as examination of epidemiologically relevant gene × environment interactions in these models, can illuminate molecular and pathobiological mechanisms involved in schizophrenia. The present chapter outlines both the evidence from phenotypic studies in mutant mouse models related to schizophrenia and recently described mutant models addressing such gene × environment interactions. Emphasis is placed on evaluating the extent to which mutant phenotypes recapitulate the totality of the disease phenotype or model selective endophenotypes. We also discuss new developments and trends in relation to the functional genomics of psychosis which might help to inform on the construct validity of mutant models of schizophrenia and highlight methodological challenges in phenotypic evaluation that relate to such models.

Inhibitors of catechol-O-methyltransferase sensitize mice to pain

BACKGROUND AND PURPOSE

Catechol-O-methyltransferase (COMT) inhibitors are used in Parkinson's disease in which pain is an important symptom. COMT polymorphisms modulate pain and opioid analgesia in humans. In rats, COMT inhibitors have been shown to be pro-nociceptive in acute pain models, but also to attenuate allodynia and hyperalgesia in a model of diabetic neuropathy. Here, we have assessed the effects of acute and repeated administrations of COMT inhibitors on mechanical, thermal and carrageenan-induced nociception in male mice.

EXPERIMENTAL APPROACH

We used single and repeated administration of a peripherally restricted, short-acting (nitecapone) and also a centrally acting (3,5-dinitrocatechol, OR-486) COMT inhibitor. We also tested CGP 28014, an indirect inhibitor of COMT enzyme. Effects of OR-486 on thermal nociception were also studied in COMT deficient mice. Effects on spinal pathways were assessed in rats given intrathecal nitecapone.

KEY RESULTS

After single administration, both nitecapone and OR-486 reduced mechanical nociceptive thresholds and thermal nociceptive latencies (hot plate test) at 2 and 3 h, regardless of their brain penetration. These effects were still present after chronic treatment with COMT inhibitors for 5 days. Intraplantar injection of carrageenan reduced nociceptive latencies and both COMT inhibitors potentiated this reduction without modifying inflammation. CGP 28014 shortened paw flick latencies. OR-486 did not modify hot plate times in Comt gene deficient mice. Intrathecal nitecapone modified neither thermal nor mechanical nociception.

CONCLUSIONS AND IMPLICATIONS

Pro-nociceptive effects of COMT inhibitors were confirmed. The pro-nociceptive effects were primarily mediated via mechanisms acting outside the brain and spinal cord. COMT protein was required for these actions.

Effect of genetic modifications in the synaptic dopamine clearance systems on addiction-like behaviour in mice

During the last 15 years, genetically modified mouse lines have proved to be a valuable research tool. This review summarizes research that studied addiction-like behaviour in mice that had a targeted mutation in the genes of the synaptic dopamine removal systems, i.e. in the dopamine transporter (DAT), a vesicular monoamine transporter 2 (VMAT2) or two dopamine-metabolizing enzymes (monoamine oxidase, MAO, mainly MAO-A isoenzyme, and catechol-O-methyltransferase, COMT). Majority of the mice are knockouts but also some knock-in and knock down mouse lines are included. Most studies have explored DAT, and it has been shown to be the critical target in addiction to psychostimulants. Its role in the development of addiction-like behaviour to nicotine, opioids or ethanol is less clear. VMAT2 also seems to be linked to psychostimulant addiction. MAO-A and COMT have a minor role in addiction-like behaviour that is further complicated by a sexual dimorphism.

Chronic adolescent exposure to Δ-9-tetrahydrocannabinol in COMT mutant mice: impact on psychosis-related and other phenotypes

Cannabis use confers a two-fold increase in the risk for psychosis, with adolescent use conferring even greater risk. A high-low activity catechol-O-methyltransferase (COMT) polymorphism may modulate the effects of adolescent Δ-9-tetrahydrocannabinol (THC) exposure on the risk for adult psychosis. Mice with knockout of the COMT gene were treated chronically with THC (4.0 and 8.0 mg/kg over 20 days) during either adolescence (postnatal days (PDs) 32-52) or adulthood (PDs 70-90). The effects of THC exposure were then assessed in adulthood across behavioral phenotypes relevant for psychosis: exploratory activity, spatial working memory (spontaneous and delayed alternation), object recognition memory, social interaction (sociability and social novelty preference), and anxiety (elevated plus maze). Adolescent THC administration induced a larger increase in exploratory activity, greater impairment in spatial working memory, and a stronger anti-anxiety effect in COMT knockouts than in wild types, primarily among males. No such effects of selective adolescent THC administration were evident for other behaviors. Both object recognition memory and social novelty preference were disrupted by either adolescent or adult THC administration, independent of genotype. The COMT genotype exerts specific modulation of responsivity to chronic THC administration during adolescence in terms of exploratory activity, spatial working memory, and anxiety. These findings illuminate the interaction between genes and adverse environmental exposures over a particular stage of development in the expression of the psychosis phenotype.

Extracellular dopamine potentiates mn-induced oxidative stress, lifespan reduction, and dopaminergic neurodegeneration in a BLI-3-dependent manner in Caenorhabditis elegans

Parkinson's disease (PD)-mimicking drugs and pesticides, and more recently PD-associated gene mutations, have been studied in cell cultures and mammalian models to decipher the molecular basis of PD. Thus far, a dozen of genes have been identified that are responsible for inherited PD. However they only account for about 8% of PD cases, most of the cases likely involving environmental contributions. Environmental manganese (Mn) exposure represents an established risk factor for PD occurrence, and both PD and Mn-intoxicated patients display a characteristic extrapyramidal syndrome primarily involving dopaminergic (DAergic) neurodegeneration with shared common molecular mechanisms. To better understand the specificity of DAergic neurodegeneration, we studied Mn toxicity in vivo in Caenorhabditis elegans. Combining genetics and biochemical assays, we established that extracellular, and not intracellular, dopamine (DA) is responsible for Mn-induced DAergic neurodegeneration and that this process (1) requires functional DA-reuptake transporter (DAT-1) and (2) is associated with oxidative stress and lifespan reduction. Overexpression of the anti-oxidant transcription factor, SKN-1, affords protection against Mn toxicity, while the DA-dependency of Mn toxicity requires the NADPH dual-oxidase BLI-3. These results suggest that in vivo BLI-3 activity promotes the conversion of extracellular DA into toxic reactive species, which, in turn, can be taken up by DAT-1 in DAergic neurons, thus leading to oxidative stress and cell degeneration.

Quantitative role of COMT in dopamine clearance in the prefrontal cortex of freely moving mice

Catechol-O-methyltransferase (COMT) plays an active role in the metabolism of dopamine (DA) in the prefrontal cortex (PFC). Because of low levels of dopamine transporter (DAT), it is proposed that the majority of released DA is taken up by either norepinephrine transporter (NET) and subsequently metabolized by monoamine oxidize (MAO) or by uptake(2) (to glial cells and post-synaptic neurons) and metabolized by COMT. However, a comprehensive in vivo study of rating the mechanisms involved in DA clearance in the PFC has not been done. Here, we employ two types of microdialysis to study these pathways using DAT, NET and MAO blockers in conscious mice, with or without Comt gene disruption. In quantitative no-net-flux microdialysis, DA levels were increased by 60% in the PFC of COMT-knockout (ko) mice, but not in the striatum and nucleus accumbens. In conventional microdialysis studies, we showed that selective NET and MAO inhibition increased DA levels in the PFC of wild-type mice by two- to fourfold, an effect that was still doubled in COMT-ko mice. Inhibition of DAT had no effect on DA levels in either genotype. Therefore, we conclude that in the mouse, PFC COMT contributes about one half of the total DA clearance.

DRD4 and DAT1 in ADHD: Functional neurobiology to pharmacogenetics

Attention deficit/hyperactivity disorder (ADHD) is a common and potentially very impairing neuropsychiatric disorder of childhood. Statistical genetic studies of twins have shown ADHD to be highly heritable, with the combination of genes and gene by environment interactions accounting for around 80% of phenotypic variance. The initial molecular genetic studies where candidates were selected because of the efficacy of dopaminergic compounds in the treatment of ADHD were remarkably successful and provided strong evidence for the role of DRD4 and DAT1 variants in the pathogenesis of ADHD. However, the recent application of non-candidate gene strategies (eg, genome-wide association scans) has failed to identify additional genes with substantial genetic main effects, and the effects for DRD4 and DAT1 have not been replicated. This is the usual pattern observed for most other physical and mental disorders evaluated with current state-of-the-art methods. In this paper we discuss future strategies for genetic studies in ADHD, highlighting both the pitfalls and possible solutions relating to candidate gene studies, genome-wide studies, defining the phenotype, and statistical approaches.

Can the role of genetic factors in schizophrenia be enlightened by studies of candidate gene mutant mice behaviour?

Schizophrenia is one of the most severe psychiatric disorders. Despite the knowledge accumulated over years, aetiology and pathophysiology remain uncertain. Research on families and twins suggests that genetic factors are largely responsible for the disease and implies specific genes as risk factors. Genetic epidemiology indicates a complex transmission mode, compatible with a multi-locus model, with single genes accounting for specific traits rather than for the entire phenotype. To better understand every single gene contribution to schizophrenia, the use of intermediate endophenotypes has been proposed. A straight communication between preclinical and clinical researchers could facilitate research on the association between genes and endophenotypes. Many behavioural tasks are available for humans and animals to measure endophenotypes. Here, firstly, we reviewed the most promising mouse behavioural tests modelling human behavioural tasks altered in schizophrenia. Secondly, we systematically reviewed animal models availability for a selection of candidate genes, derived from linkage and association studies. Thirdly, we systematically reviewed the studies which tested mutant mice in the above behavioural tasks. Results indicate a large mutant mice availability for schizophrenia candidate genes but they have been insufficiently tested in behavioural tasks. On the other hand, multivariate and translational approach should be implemented in several behavioural domains.

Importance of membrane-bound catechol-O-methyltransferase in L-DOPA metabolism: a pharmacokinetic study in two types of Comt gene modified mice

BACKGROUND AND PURPOSE

Catechol-O-methyltransferase (COMT) metabolizes compounds containing catechol structures and has two forms: soluble (S-COMT) and membrane-bound (MB-COMT). Here we report the generation of a mouse line that expresses MB-COMT but not S-COMT. We compared the effects of deleting S-COMT only or both COMT forms on the pharmacokinetics of oral L-DOPA.

EXPERIMENTAL APPROACH

L-DOPA (10 mg kg(-1)) and carbidopa (30 mg kg(-1)) were given to mice by gastric tube, and samples were taken at various times. HPLC was used to measure L-DOPA in plasma and tissue samples, and dopamine and its metabolites in brain. Immunohistochemistry and Western blotting were used to characterize the distribution of COMT protein isoforms.

KEY RESULTS

Lack of S-COMT did not affect the levels of L-DOPA in plasma or peripheral tissues, whereas in the full COMT-knock-out mice, these levels were increased. The levels of 3-O-methyldopa were significantly decreased in the S-COMT-deficient mice. In the brain, L-DOPA levels were not significantly increased, and dopamine was increased only in females. The total COMT activity in the S-COMT-deficient mice was 22-47% of that in the wild-type mice. In peripheral tissues, female mice had lower COMT activity than the males.

CONCLUSIONS AND IMPLICATIONS

In S-COMT-deficient mice, MB-COMT in the liver and the duodenum is able to O-methylate about one-half of exogenous L-DOPA. Sexual dimorphism and activity of the two COMT isoforms seems to be tissue specific and more prominent in peripheral tissues than in the brain.

Modeling the positive symptoms of schizophrenia in genetically modified mice: pharmacology and methodology aspects

In recent years, there have been huge advances in the use of genetically modified mice to study pathophysiological mechanisms involved in schizophrenia. This has allowed rapid progress in our understanding of the role of several proposed gene mechanisms in schizophrenia, and yet this research has also revealed how much still remains unresolved. Behavioral studies in genetically modified mice are reviewed with special emphasis on modeling psychotic-like behavior. I will particularly focus on observations on locomotor hyperactivity and disruptions of prepulse inhibition (PPI). Recommendations are included to address pharmacological and methodological aspects in future studies. Mouse models of dopaminergic and glutamatergic dysfunction are then discussed, reflecting the most important and widely studied neurotransmitter systems in schizophrenia. Subsequently, psychosis-like behavior in mice with modifications in the most widely studied schizophrenia susceptibility genes is reviewed. Taken together, the available studies reveal a wealth of available data which have already provided crucial new insight and mechanistic clues which could lead to new treatments or even prevention strategies for schizophrenia.

Importance of the COMT gene for sex differences in brain function and predisposition to psychiatric disorders

As outlined elsewhere in this volume, sex differences can affect brain function and its dysfunction in psychiatric disorders. It is known that genetic factors contribute to these sex dimorphisms, but the individual genes have rarely been identified. The catechol-O-methyltransferase (COMT) gene, which encodes an enzyme that metabolises catechol compounds, including dopamine, is a leading candidate in this regard. COMT's enzyme activity, and the neurochemistry and behaviour of COMT knockout mice are both markedly sexually dimorphic. Furthermore, genetic associations between COMT and psychiatric phenotypes frequently show differences between men and women. Although many of these differences are unconfirmed or minor, some appear to be of reasonable robustness and magnitude and are reviewed in this chapter. Sexually dimorphic effects of COMT are usually attributed to transcriptional regulation by oestrogens; however, a careful examination of the literature suggests that additional mechanisms are likely to be at least as important. Here, we review the evidence for a sexually dimorphic influence of COMT upon psychiatric phenotypes and brain function, and discuss potential mechanisms by which this may occur. We conclude that despite the evidence being incomplete, there are accumulating and in places compelling data showing that COMT has markedly sexually dimorphic effects on brain function and its dysfunction in psychiatric disorders. Although oestrogenic regulation of COMT is probably partially responsible for these sex differences, other mechanisms are likely also involved. Since sex differences in the genetic architecture of brain function and psychiatric disorders are the rule not the exception, we anticipate that additional evidence will emerge for sexual dimorphisms, not only in COMT but also in many other autosomal genes.

Gonadectomy and hormone replacement exert region- and enzyme isoform-specific effects on monoamine oxidase and catechol-O-methyltransferase activity in prefrontal cortex and neostriatum of adult male rats

Sex differences and gonadal hormone influences are well known for diverse aspects of forebrain amine and indolamine neurotransmitter systems, the cognitive and affective functions they govern and their malfunction in mental illness. This study explored whether hormone regulation/dysregulation of these systems could be related to gonadal steroid effects on catechol-O-methyltransferase and monoamine oxidase which are principal enzymatic controllers of forebrain dopamine, serotonin and norepinephrine levels. Driven by male over female differences in cortical enzyme activities, by male-specific associations between monoamine oxidase and catechol-O-methyltransferase gene polymorphisms and cognitive and dysfunction in disease and by male-specific consequences of gene knockouts in mice, the question of hormone sensitivity was addressed here using a male rat model where prefrontal dopamine levels and related behaviors are also known to be affected. Specifically, quantitative O-methylation and oxidative deamination assays were used to compare the activities of catechol-O-methyltransferase's soluble and membrane-bound isoforms and of monoamine oxidase's A and B isoforms in the pregenual medial prefrontal cortex and dorsal striatum of male rats that were sham operated, gonadectomized or gonadectomized and supplemented with testosterone propionate or with estradiol for 28 days. These studies revealed significant effects of hormone replacement but not gonadectomy on the soluble but not the membrane-bound isorfom of catechol-O-methyltransferase in both striatum and cortex. A significant, cortex-specific testosterone-but not estradiol-attenuated effect (increase) of gonadectomy on monoamine oxidase's A but not B isoform was also observed. Although none of these actions suggest potential roles in the regulation/dysregulation of prefrontal dopamine, the suppressive effects of testosterone on cortical monoamine oxidase-A that were observed could have bearing on the increased incidence of cognitive deficits and symptoms of depression and anxiety that are repeatedly observed in males in conditions of hypogonadalism related to aging, other biological factors or in prostate cancer where androgen deprivation is used as a neoadjuvant treatment.

Mice mutant for genes associated with schizophrenia: common phenotype or distinct endophenotypes?

Schizophrenia is a complex neuropsychiatric disorder whose etiology involves a mixture of genetic and environmental factors. By virtue of this complexity, schizophrenia is a field of research in which a number of key technologies converge: in particular, identification of putative susceptibility genes through association studies in clinical populations leads to investigation of the behavioural roles of these genes by targeted manipulation in mice and their phenotypic characterisation ('gene-driven' approach); in a complementary manner, identification of putative pathophysiological processes and therapeutic pathways leads to investigation of behavioural phenotype in mice mutant for genes regulating such processes and pathways ('phenotype-driven' approach). As several susceptibility genes for schizophrenia and numerous genes implicated in the pathophysiology of schizophrenia have now been genetically manipulated in mice, it is timely to consider the roles of these genes in abnormal brain development and the ontogeny of putative schizophrenia-like phenotypes. The aim of this review is to outline existing knowledge from mutant studies concerning the contribution of these genes to the development of a common schizophrenia phenotype vis-à-vis discrete schizophrenia endophenotypes. Emphasis is also placed on the importance of studying gene x environment and gene x gene interactions, as well as addressing methodological issues related to genetic modelling and phenotyping strategies.

Catecholamine synthesis and metabolism in the central nervous system of mice lacking alpha-adrenoceptor subtypes

BACKGROUND AND PURPOSE

This study investigates the role of alpha(2)-adrenoceptor subtypes, alpha(2A), alpha(2B) and alpha(2C), on catecholamine synthesis and catabolism in the central nervous system of mice.

EXPERIMENTAL APPROACH

Activities of the main catecholamine synthetic and catabolic enzymes were determined in whole brains obtained from alpha(2A)-, alpha(2B)- and alpha(2C)-adrenoceptor knockout (KO) and C56Bl\7 wild-type (WT) mice.

KEY RESULTS

Although no significant differences were found in tyrosine hydroxylase activity and expression, brain tissue levels of 3,4-dihydroxyphenylalanine were threefold higher in alpha(2A)- and alpha(2C)-adrenoceptor KO mice. Brain tissue levels of dopamine and noradrenaline were significantly higher in alpha(2A) and alpha(2C)KOs compared with WT [WT: 2.8 +/- 0.5, 1.1 +/- 0.1; alpha(2A)KO: 6.9 +/- 0.7, 1.9 +/- 0.1; alpha(2B)KO: 2.3 +/- 0.2, 1.0 +/- 0.1; alpha(2C)KO: 4.6 +/- 0.8, 1.5 +/- 0.2 nmol.(g tissue)(-1), for dopamine and noradrenaline respectively]. Aromatic L-amino acid decarboxylase activity was significantly higher in alpha(2A) and alpha(2C)KO [WT: 40 +/- 1; alpha(2A): 77 +/- 2; alpha(2B): 40 +/- 1; alpha(2C): 50 +/- 1, maximum velocity (V(max)) in nmol.(mg protein)(-1).h(-1)], but no significant differences were found in dopamine beta-hydroxylase. Of the catabolic enzymes, catechol-O-methyltransferase enzyme activity was significantly higher in all three alpha(2)KO mice [WT: 2.0 +/- 0.0; alpha(2A): 2.4 +/- 0.1; alpha(2B): 2.2 +/- 0.0; alpha(2C): 2.2 +/- 0.0 nmol.(mg protein)(-1).h(-1)], but no significant differences were found in monoamine oxidase activity between all alpha(2)KOs and WT mice.

CONCLUSIONS AND IMPLICATIONS

In mouse brain, deletion of alpha(2A)- or alpha(2C)-adrenoceptors increased cerebral aromatic L-amino acid decarboxylase activity and catecholamine tissue levels. Deletion of any alpha(2)-adrenoceptor subtypes resulted in increased activity of catechol-O-methyltransferase. Higher 3,4-dihydroxyphenylalanine tissue levels in alpha(2A) and alpha(2C)KO mice could be explained by increased 3,4-dihydroxyphenylalanine transport.

Attention Deficit Hyperactivity Disorder comorbid oppositional defiant disorder and its predominately inattentive type: evidence for an association with COMT but not MAOA in a Chinese sample

Background

There are three childhood disruptive behavior disorders (DBDs), attention deficit hyperactivity disorder (ADHD), oppositional defiant disorder (ODD), and conduct disorder (CD). The most common comorbid disorder in ADHD is ODD. DSM-IV describes three ADHD subtypes: predominantly inattentive type (ADHD-IA), predominantly hyperactive-impulsive type (ADHD-HI), and combined type (ADHD-C). Prior work suggests that specific candidate genes are associated with specific subtypes of ADHD in China. Our previous association studies between ADHD and functional polymorphisms of COMT and MAOA, consistently showed the low transcriptional activity alleles were preferentially transmitted to ADHD-IA boys. Thus, the goal of the present study is to test the hypothesis that COMT Val158Met and MAOA-uVNTR jointly contribute to the ODD phenotype among Chinese ADHD boys.

Methods

171 Chinese boys between 6 and 17.5 years old (mean = 10.3, SD = 2.6) with complete COMT val158met and MAOA-uVNTR genotyping information were studied. We used logistic regression with genotypes as independent variables and the binary phenotype as the dependent variable. We used p < 0.05 as the level of nominal statistical significance. Bonferroni correction procedures were used to adjust for multiple comparisons.

Results

Our results highlight the potential etiologic role of COMT in the ADHD with comorbid ODD and its predominately inattentive type in male Chinese subjects. ADHD with comorbid ODD was associated with homozygosity of the high-activity Val allele, while the predominantly inattentive ADHD subtype was associated with the low-activity Met allele. We found no evidence of association between the MAOA-uVNTR variant and ADHD with comorbid ODD or the ADHD-IA subtype.

Conclusion

Our study of attention deficit hyperactivity disorder comorbid oppositional defiant disorder and its predominately inattentive type highlights the potential etiologic role of COMT for ADHD children in China. But we failed to observe an interaction between COMT and MAOA, which suggests that epistasis between COMT and MAOA genes does not influence the phenotype of ADHD-IA with comorbid ODD in a clinical sample of Chinese male subjects. To confirm our findings further studies with a larger number of subjects and healthy controls are needed.

Variation in dopamine genes influences responsivity of the human reward system

In humans, dopamine neurotransmission is influenced by functional polymorphisms in the dopamine transporter (DAT1) and catechol-O-methyltransferase (COMT) genes. Here, we used event-related functional magnetic resonance imaging to directly investigate the neurofunctional effects of the Val(158)Met COMT and variable number of tandem repeat DAT1 polymorphisms on distinct components of the reward system in humans. The results revealed a main effect of COMT genotype in the ventral striatum and lateral prefrontal cortex during reward anticipation (P < 0.001, uncorrected) and in the orbitofrontal cortex at the time of reward delivery (P < 0.005), met/met individuals exhibiting the highest activation. The main effect of DAT1 genotype was seen in robust blood-oxygen-level-dependent response differences in the caudate nucleus and ventral striatum during reward anticipation (P < 0.001) and in the lateral prefrontal cortex and midbrain at the time of reward delivery, with carriers of the DAT1 9-repeat allele showing the highest activity. Moreover, an interaction between the COMT and DAT1 genes was found in the ventral striatum and lateral prefrontal cortex during reward anticipation and in the lateral prefrontal and orbitofrontal cortices as well as in the midbrain at the time of reward delivery, with carriers of the DAT1 9-repeat allele and COMT met/met allele exhibiting the highest activation, presumably reflecting functional change consequent to higher synaptic dopamine availability. Taken together, these results indicate that genetically influenced variations in dopamine transmission modulate the response of brain regions involved in anticipation and reception of rewards and suggest that these responses may contribute to individual differences in reward-seeking behavior and in predisposition to neuropsychiatric disorders.

Catechol-O-methyltransferase (COMT): a gene contributing to sex differences in brain function, and to sexual dimorphism in the predisposition to psychiatric disorders

Sex differences in the genetic epidemiology and clinical features of psychiatric disorders are well recognized, but the individual genes contributing to these effects have rarely been identified. Catechol-O-methyltransferase (COMT), which metabolizes catechol compounds, notably dopamine, is a leading candidate. COMT enzyme activity, and the neurochemistry and behavior of COMT null mice, are both markedly sexually dimorphic. Genetic associations between COMT and various psychiatric phenotypes frequently show differences between men and women. Many of these differences are unconfirmed or minor, but some appear to be of reasonable robustness and magnitude; eg the functional Val(158)Met polymorphism in COMT is associated with obsessive-compulsive disorder in men, with anxiety phenotypes in women, and has a greater impact on cognitive function in boys than girls. Sex-specific effects of COMT are usually attributed to transcriptional regulation by estrogens; however, additional mechanisms are likely to be at least as important. Here we review the evidence for a sexually dimorphic influence of COMT upon psychiatric phenotypes, and discuss its potential basis. We conclude that despite the evidence being incomplete, and lacking a unifying explanation, there are accumulating and in places compelling data showing that COMT differentially impacts on brain function and dysfunction in men and women. Since sex differences in the genetic architecture of quantitative traits are the rule not the exception, we anticipate that additional evidence will emerge for sexual dimorphisms, not only in COMT but also in many other autosomal genes.

Polymorphism of dopamine D2 receptor (TaqIA, TaqIB, and-141C Ins/Del) and dopamine degradation enzyme (COMT G158A, A-278G) genes and extrapyramidal symptoms in patients with schizophrenia and bipolar disorders

The relationship is examined of the dopamine D2 receptor (DRD2) polymorphism (TaqIA, TaqIB, -141 C Ins/Del) and the catechol-O-methyltransferase (COMT) polymorphism (A-278G, G158A) to the risk of antipsychotic-induced extrapyramidal symptoms (EPS) in schizophrenia and bipolar disorders. Participants comprised 80 cases presenting with EPS (Simpson-Angus Scale score >3) and 188 controls presenting without EPS (Simpson-Angus Scale score <or=3) participated in this study. The COMT(L) allele conferred a reduction of EPS risk of 60% to heterozygotes, but the finding did not survive correction for multiple comparisons. In the bipolar subgroup, with a COMT(L) allele protection of 70%, the reduction remained significant after Bonferroni correction. The analysis of the COMT haplotypes revealed an association of the A-G haplotype with EPS risk in the overall group and the bipolar disorder subgroup, and an association of the A-A haplotype with EPS protection in the bipolar subgroup. No significant associations were found for DRD2 or COMT A-278G polymorphisms. This is the first report of an association between the COMT polymorphism and EPS susceptibility. These results are of interest in view of the increased use of antipsychotic drugs in bipolar patients in both the acute manic and the depressive phase.

A catechol-O-methyltransferase that is essential for auditory function in mice and humans

We have identified a previously unannotated catechol-O-methyltranferase (COMT), here designated COMT2, through positional cloning of a chemically induced mutation responsible for a neurobehavioral phenotype. Mice homozygous for a missense mutation in Comt2 show vestibular impairment, profound sensorineuronal deafness, and progressive degeneration of the organ of Corti. Consistent with this phenotype, COMT2 is highly expressed in sensory hair cells of the inner ear. COMT2 enzymatic activity is significantly reduced by the missense mutation, suggesting that a defect in catecholamine catabolism underlies the auditory and vestibular phenotypes. Based on the studies in mice, we have screened DNA from human families and identified a nonsense mutation in the human ortholog of the murine Comt2 gene that causes nonsyndromic deafness. Defects in catecholamine modification by COMT have been previously implicated in the development of schizophrenia. Our studies identify a previously undescribed COMT gene and indicate an unexpected role for catecholamines in the function of auditory and vestibular sense organs.

Increase in free choice oral ethanol self-administration in catechol-o-methyltransferase gene-disrupted male mice

The effect of catechol-O-methyltransferase (Comt) gene disruption on the voluntary oral consumption of water, ethanol (2.5-20%, v/v) and cocaine (0.1-0.8 mg/ml) was studied in the free-choice, two-bottle paradigm in male and female mice. Solutions containing ethanol or cocaine, or tap water were available ad libitum from drinking burettes for 4 weeks. Catechol-O-methyltransferase-deficient male mice consumed significantly more ethanol than their wild-type male littermates. In contrast, female mice did not show genotype differences in the consumption of ethanol solutions. During the cocaine experiment, male mice developed either a side preference or an aversion that obscured cocaine consumption. This pattern of drinking was not dependent on Comt genotype. In female mice, Comt genotype was not associated with cocaine consumption. In conclusion, disruption of Comt gene influenced ethanol consumption in a gender-dependent manner in mice, supporting the hypothesis that low catechol-O-methyltransferase activity is one of the predisposing factors for high alcohol consumption in males.

Phenotypic characterization of cognition and social behavior in mice with heterozygous versus homozygous deletion of catechol-O-methyltransferase

Catechol-O-methyltransferase is an important enzyme in the metabolism of dopamine and an important regulator of aspects of dopamine-dependent working memory in prefrontal cortex that are disturbed in schizophrenia. This study investigated the phenotype of mice with heterozygous deletion vs. homozygous knockout of the catechol-O-methyltransferase gene across paradigms that access processes relevant for psychotic illness. Homozygotes evidenced improved performance in spontaneous alternation, an index of immediate spatial working memory; this effect appeared more substantive in males and was reflected in performance in aspects of the Barnes maze, an index of spatial learning/memory. Heterozygotes evidenced impaired performance in object recognition, an index of recognition memory; this effect was evident for both sexes at a retention interval of 5 min but appeared more enduring in males. There were no material effects for either genotype in relation to sociability or social novelty preference. While homozygous catechol-O-methyltransferase deletion results in improvement in spatial learning/working memory with little effect on social behavior, heterozygous deletion results in impairment of recognition memory. We have reported recently, using similar methods, that mice with deletion of the schizophrenia risk gene neuregulin-1 evidence disruption to social behavior, with little effect on spatial learning/working memory. The data suggest that catechol-O-methyltransferase and neuregulin-1 may influence, respectively, primarily cognitive and social endophenotypes of the overall schizophrenia syndrome.

Computational systems analysis of dopamine metabolism

A prominent feature of Parkinson's disease (PD) is the loss of dopamine in the striatum, and many therapeutic interventions for the disease are aimed at restoring dopamine signaling. Dopamine signaling includes the synthesis, storage, release, and recycling of dopamine in the presynaptic terminal and activation of pre- and post-synaptic receptors and various downstream signaling cascades. As an aid that might facilitate our understanding of dopamine dynamics in the pathogenesis and treatment in PD, we have begun to merge currently available information and expert knowledge regarding presynaptic dopamine homeostasis into a computational model, following the guidelines of biochemical systems theory. After subjecting our model to mathematical diagnosis and analysis, we made direct comparisons between model predictions and experimental observations and found that the model exhibited a high degree of predictive capacity with respect to genetic and pharmacological changes in gene expression or function. Our results suggest potential approaches to restoring the dopamine imbalance and the associated generation of oxidative stress. While the proposed model of dopamine metabolism is preliminary, future extensions and refinements may eventually serve as an in silico platform for prescreening potential therapeutics, identifying immediate side effects, screening for biomarkers, and assessing the impact of risk factors of the disease.

The impact of catechol-O-methyltransferase and dopamine D4 receptor genotypes on neurophysiological markers of performance monitoring

Dynamic adaptations of one's behavior by means of performance monitoring are a central function of the human executive system, that underlies considerable interindividual variation. Converging evidence from electrophysiological and neuroimaging studies in both animals and humans hints at the importance of the dopaminergic system for the regulation of performance monitoring. Here, we studied the impact of two polymorphisms affecting dopaminergic functioning in the prefrontal cortex [catechol-O-methyltransferase (COMT) Val108/158Met and dopamine D4 receptor (DRD4) single-nucleotide polymorphism (SNP)-521] on neurophysiological correlates of performance monitoring. We applied a modified version of a standard flanker task with an embedded stop-signal task to tap into the different functions involved, particularly error monitoring, conflict detection and inhibitory processes. Participants homozygous for the DRD4 T allele produced an increased error-related negativity after both choice errors and failed inhibitions compared with C-homozygotes. This was associated with pronounced compensatory behavior reflected in higher post-error slowing. No group differences were seen in the incompatibility N2, suggesting distinct effects of the DRD4 polymorphism on error monitoring processes. Additionally, participants homozygous for the COMT Val allele, with a thereby diminished prefrontal dopaminergic level, revealed increased prefrontal processing related to inhibitory functions, reflected in the enhanced stop-signal-related components N2 and P3a. The results extend previous findings from mainly behavioral and neuroimaging data on the relationship between dopaminergic genes and executive functions and present possible underlying mechanisms for the previously suggested association between these dopaminergic polymorphisms and psychiatric disorders as schizophrenia or attention deficit hyperactivity disorder.

Exploratory and habituation phenotype of heterozygous and homozygous COMT knockout mice

Catechol-O-methyltransferase (COMT) inactivates dopamine in prefrontal cortex and is associated clinically with a schizophrenia endophenotype. Using an ethologically based approach, the phenotype of mice with heterozygous COMT deletion was characterised by decreased rearing with increased sifting and chewing. Heterozygous COMT deletion is associated with a distinctive phenotype. This differs from that which we have reported previously for heterozygous deletion of the schizophrenia risk gene neuregulin-1.

Functional genomics and schizophrenia: endophenotypes and mutant models

This article summarizes the rationale, methods, and results of gene discovery programs in schizophrenia research and describes functional methods of investigating potential candidate genes. It focuses next on the most prominent current candidate genes and describes (1) evidence for their association with schizophrenia and research into the function of each gene; (2) investigation of the clinical phenotypes and endophenotypes associated with each gene, at the levels of psychopathologic, neurocognitive, electrophysiologic, neuroimaging, and neuropathologic findings; and (3) research into the ethologic, cognitive, social, and psychopharmacologic phenotype of mutants with targeted deletion of each gene. It examines gene-gene and gene-environment interactions. Finally, it looks at future directions for research.

Gene-gene interaction associated with neural reward sensitivity

Reward processing depends on dopaminergic neurotransmission and is modulated by factors affecting dopamine (DA) reuptake and degradation. We used fMRI and a guessing task sensitive to reward-related activation in the prefrontal cortex and ventral striatum to study how individual variation in genes contributing to DA reuptake [DA transporter (DAT)] and degradation [catechol-o-methyltransferase (COMT)] influences reward processing. Prefrontal activity, evoked by anticipation of reward irrespective of reward probability and magnitude, was COMT genotype-dependent. Volunteers homozygous for the Met allele, associated with lower enzyme activity and presumably greater DA availability, showed larger responses compared with volunteers homozygous for the Val allele. A similar COMT effect was observed in the ventral striatum. As reported previously, the ventral striatum was also found to code gain-related expected value, i.e., the product of reward magnitude and gain probability. Individual differences in ventral striatal sensitivity for value were in part explained by an epistatic gene-gene interaction between COMT and DAT. Although most genotype combinations exhibited the expected activity increase with more likely and larger rewards, two genotype combinations (COMT Met/Met DAT 10R and COMT Val/Val 9R) were associated with blunted ventral striatal responses. In view of a consistent relationship between reduced reward sensitivity and addiction, our findings point to a potential genetic basis for vulnerability to addiction.

Estrogen deficient male mice develop compulsive behavior

BACKGROUND

Aromatase converts androgen to estrogen. Thus, the aromatase knockout (ArKO) mouse is estrogen deficient. We investigated the compulsive behaviors of these animals and the protein levels of catechol-O-methyltransferase (COMT) in frontal cortex, hypothalamus and liver.

METHODS

Grooming was analyzed during the 20-min period immediately following a water-mist spray. Running wheel activity over two consecutive nights and barbering were analyzed. COMT protein levels were measured by Western analysis.

RESULTS

Six-month old male but not female ArKO mice develop compulsive behaviors such as excessive barbering, grooming and wheel-running. Excessive activities were reversed by 3 weeks of 17beta-estradiol replacement. Interestingly, the presentation of compulsive behaviors is accompanied by concomitant decreases (p < .05) in hypothalamic COMT protein levels in male ArKO mice. These values returned to normal upon 17beta-estradiol treatment. In contrast, hepatic and frontal cortex COMT levels were not affected by the estrogen status, indicating region- and tissue-specific regulation of COMT levels by estrogen. No differences in COMT levels were detectable between female animals of both genotypes.

CONCLUSIONS

This study describes the novel observation of a possible link between estrogen, COMT and development of compulsive behaviors in male animals which may have therapeutic implications in obsessive compulsive disorder (OCD) patients.