Elevated platelet MAO is related to impulsivity in disruptive behavior disorders

A systematic review and meta-analysis examining the interrelationships between chemical and non-chemical stressors and inherent characteristics in children with ADHD

Children may be more vulnerable to the combined interactions of chemical and non-chemical stressors from their built, natural, and social environments when compared to adults. Attention deficit/hyperactivity disorder (ADHD) is the most commonly diagnosed childhood neurodevelopmental disorder and is considered a major public health issue, as 75% of childhood cases persist into adulthood. ADHD is characterized by developmentally inappropriate levels of hyperactivity, impulsivity, and inattention, with the neurotransmitter serotonin regulating these symptoms. Monoamine oxidase A (MAOA) aids in serotonin uptake and is often implicated in behavioral and emotional disorders, including ADHD. When children are exposed to cigarette smoke, bisphenol A (BPA), or organophosphate pesticides, MAOA activity is inhibited. Non-chemical stressors, such as traumatic childhood experiences, and lifestyle factors, complicate the relationship between genotype and exposures to chemical stressors. But the co-occurrence among outcomes between exposures to chemical stressors, non-chemical stressors, and the low activity MAOA genotype suggest that mental illness in children may be influenced by multiple interacting factors. In this systematic review, we examine the existing literature that combines exposures to chemical and non-chemical stressors (specifically childhood trauma), MAOA characteristics, and ADHD diagnosis to investigate the interrelationships present. We observe that chemical (lead [Pb], phthalates/plasticizers, persistent organic pollutants, and cigarette smoke) exposure is significantly related to ADHD in children. We also observed that existing literature examining the interaction between MAOA, exposures to chemical stressors, and traumatic experiences and their effect on ADHD outcomes is sparse. We recommend that future studies investigating childhood ADHD include chemical and non-chemical stressors and inherent characteristics to gain a holistic understanding of childhood mental health outcomes.

The effect of a functional NOS1 promoter polymorphism on impulsivity is moderated by platelet MAO activity

RATIONALE

Platelet monoamine oxidase (MAO) activity is associated with impulsivity in clinical samples. Recently, a functional promoter polymorphism of neuronal nitric oxide synthase (NOS1) termed NOS1 ex1f-VNTR was found to have an effect on impulsivity-related traits and resulting psychopathology.

OBJECTIVE

The study aims to explore the effect of both platelet MAO activity and NOS1 ex1f-VNTR genotype on impulsivity in a population-derived sample.

METHODS

This study was on a non-clinical sample of adult male subjects, previously used to investigate the effect of platelet MAO activity on impulsivity-related behaviour (Paaver et al., Psychopharmacology 186:32-40, 2006). Six hundred thirty-seven male subjects were genotyped for the NOS1 ex1f-VNTR promoter polymorphism. Impulsivity was self-reported. Effects of age and smoking, known to affect platelet MAO activity, were controlled for.

RESULTS

No main effect of either NOS1 genotype or platelet MAO activity was present. However, significant interactions were found between effects of the NOS1 genotype and platelet MAO activity on impulsivity measures. Impulsivity and in particular the aspects of adaptive impulsivity (e.g. fast decision-making and excitement-seeking behaviour) were higher in subjects with the NOS1 ex1f-VNTR short/short genotype if they belonged to the platelet MAO medium activity (interquartile) range.

CONCLUSIONS

This study supports evidence for higher impulsivity in the NOS1 short/short genotype subjects and further suggests that this is present in the subset of subjects who have close to average platelet MAO activity.

Platelet monoamine oxidase activity in children with attention-deficit/hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) is a highly heritable developmental disorder characterized by symptoms of impulsivity, hyperactivity and/or inattention, and associated with structural and biochemical abnormalities in cortical and limbic structures innervated by dopamine, noradrenalin and serotonin. The enzyme monoamine oxidase, type B (MAO-B), is expressed in platelets, and metabolizes endogenous amines. Its activity has been proposed to represent a peripheral marker of various traits and forms of psychopathology. This study evaluated platelet MAO activity with a spectrofluorimetric method in 72 boys and 12 girls with predominantly hyperactive, predominantly inattentive, and combined subtype of ADHD (DSM-IV criteria), and in 64 control children. The results showed significantly lower platelet MAO activity in children with hyperactive, inattentive, and combined subtype of ADHD than in control children. There was no significant association between platelet MAO activity and gender or age. The limitation of the study was in the small sample of girls with ADHD (N=12), and in the determination of only one peripheral marker. In line with hypotheses of lower platelet MAO activity in different types of psychopathology, children with different subtypes of ADHD had significantly lower platelet MAO-B activity than control children.

The psychobiology of undersocialized aggressive conduct disorder: A theoretical perspective

Undersocialized aggressive conduct disorder is conceptualized within the framework for personality and motivation of Jeffrey Gray. The disorder is seen as reflecting a dominance of the reward system over the behavioral inhibition system. Evidence for this conceptualizing coming from behavioral, psychophysiological, biochemical, and pharmacological studies is reviewed. Relevant findings from these studies include perseverative responding for reward, indices of inefficient noradrenergic and serotonergic functioning, and electrodermal underresponding. Additional research to test the proposed hypothesis is suggested.

The monoamine oxidase B gene exhibits significant association to ADHD

Attention deficit hyperactivity disorder (ADHD) is a common neuropsychiatric condition with strong genetic basis. Recent work in China indicated that ADHD may be linked to Xp1-2 in the Han Chinese population. The gene encoding monoamine oxidase B (MAOB), the main enzyme degrading dopamine in the human brain, is located in this region. The current study sequenced the exons and the 5' and 3' flanking regions of the MAOB gene and found four common variants including 2276C>T and 2327C>T in exon 15, rs1799836 in intron 13 and rs1040399 in 3'-UTR. We assessed the association of these variants with ADHD in 548 trios collected from 468 males and 80 females probands. TDT analysis showed that alleles of each polymorphism were preferentially transmitted to probands (rs1799836, P = 3.28E-15; rs1040399, P = 1.87E-6; 2276T>C or 2327T>C, P = 2.20E-6) and haplotype-based TDT analyses also found distorted transmission. In conclusion, this study provides the strongest evidence for the involvement of MAOB gene in the etiology of ADHD to date, at least in Han Chinese population.

Adaptive and maladaptive impulsivity, platelet monoamine oxidase (MAO) activity and risk-admitting in different types of risky drivers

RATIONALE

Platelet monoamine oxidase (MAO) activity reflects serotonergic functioning associated with impulsive behaviour, but the significance of these associations to real-life impulsive behaviour in healthy subjects is not clear.

OBJECTIVES

The present study explores impulsivity and platelet MAO activity among people with driving violations.

MATERIALS AND METHODS

We compared facets of impulsivity and platelet MAO activity in 1,004 male drivers, out of whom 203 had been caught by the police driving drunk and 292 had been caught exceeding speed limits and committing other non-alcohol-related driving violations. Subjects with speeding and other non-alcohol-related violations were further divided according to their self-reported risk-admitting of exceeding speed limits.

RESULTS

While drunk driving was associated only with maladaptive types of impulsivity, exceeding speed limits was associated with functional impulsivity and excitement seeking and, to a lesser degree, with dysfunctional impulsivity. Drunk drivers had lower platelet MAO activity. Risk-admitting high-risk drivers had higher platelet MAO activity, neuroticism-related impulsivity, dysfunctional impulsivity and excitement seeking compared to all other groups and higher functional impulsivity compared to controls. Risk-denying high-risk drivers had only higher functional impulsivity compared to controls.

CONCLUSIONS

This study demonstrates different expressions of functional and dysfunctional impulsivity in behaviour. While platelet MAO activity is lower in alcohol-related risky behaviour, non-alcohol-related self-acknowledged risky behaviour is related to higher platelet MAO activity. Thus, deviance towards lower as well as higher end of central serotonergic functioning may lead to impulsive behaviour. While self-reported impulsivity did not correlate with MAO activity, both MAO activity and impulsivity were related to risky behaviour.

Serotonin in early-onset alcoholism

This chapter examines current, common schemes to subgroup alcoholics to arrive at relatively homogeneous groups of patients to facilitate psychobiological and molecular genetic studies. Early-onset, male-limited alcoholism is commonly associated with antisocial personality disorder or antisocial behavioral traits. It is often preceded by early-onset aggressiveness, which is followed by conduct disorder. Early-onset alcoholism among men is associated with low central serotonin turnover rate. The data concerning platelet MAO activity and serotonin uptake to platelets among early-onset alcoholics are conflicting. Recent molecular genetic and brain imaging studies on early-onset alcoholics are preliminary but appear very promising.

Dopamine-beta-hydroxylase in behaviorally disturbed youth. Relationship between teacher and parent ratings

A series of neurotransmitters, metabolites, and enzymes considered relevant to emotional and behavioral disorders was studied in blood samples obtained from boys admitted to a residential setting because of disruptive or unmanageable behavior in their home environments. The youth were categorized into mainly prepubertal (under 12.0 years) and pubertal/post pubertal (12.0 years and over) groups. Behavior ratings of the younger group completed by parents revealed significant inverse correlations between several behavioral factors and dopamine-beta-hydroxylase (DBH) activity levels, notably in the aggressive behavior and antisocial domains. Teacher ratings completed after about 1 month of living in the residential setting also showed numerous significant, but positive, correlations between several behavior factors and DBH activity levels. Furthermore, teacher and parent ratings themselves often were significantly and inversely correlated. The findings related to DBH activity levels were strongest in the younger group of boys. The results are discussed in terms of the possibility that low DBH activity in boys reflects a vulnerability towards behavioral disorder, which in certain environments becomes manifested by conduct problems and antisocial behavior.

Biodevelopmental aspects of conduct disorder in boys

The association between biochemical parameters and conduct disorder (CD) was studied in 22 boys admitted to a residential center. Three-methoxy-4-hydroxyphenylglycol (MHPG) levels were significantly higher in prepubertal CD youth than in pubertal/post pubertal CD youth. Homovanillic acid (HVA) levels were significantly lower in CD youth under age 12.0 years than in non CD youth under age 12.0 years. The implications of these biodevelopmental findings in the study of CD are discussed.