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.
Attention-deficit/hyperactivity disorder (ADHD) is a highly heritable developmental disorder characterized by behavioral symptoms of impulsivity, hyperactivity and/or inattention (Biederman, 2005). The prevalence of ADHD ranges from 4% to 10% (Faraone et al., 2005). Affected children have difficulties in educational, academic, and social functioning and in skill development, and 40% to 60% of children with ADHD are prone to show clinically significant symptoms in adulthood (Biederman, 2005). ADHD is a multifactorial and heterogeneous disorder. The etiology of ADHD involves the complex interplay of different demographic, psychosocial, psychiatric, cognitive, genetic and environmental factors (Biederman, 2005 and Faraone and Khan, 2006). The neurobiology of ADHD includes dysfunction of dopaminergic, noradrenergic and serotonergic systems (Comings, 2001, Biederman, 2005, Faraone and Khan, 2006 and Chamberlain et al., 2007), and these neurotransmitters regulate forms of behavior that are disturbed in ADHD (Chamberlain et al., 2007).
Monoamine oxidase (MAO) is an enzyme that catalyzes oxidative deamination of monoamines such as dopamine, serotonin and noradrenaline (Oreland, 2004). The MAO-B type is expressed in platelets, astrocytes and serotonergic neurons, catalyzes beta-phenylethylamine, benzylamine, dopamine, tyramine, and tryptamine, and is inactivated by deprenyl (Oreland, 2004). The activity of platelet MAO has been proposed to be a peripheral marker for a variety of personality traits and vulnerability to psychiatric disorders (Schalling et al., 1987, Irving et al., 1989, Oreland et al., 1995, Kirk et al., 2001, Oreland, 2004, Ruchkin et al., 2005 and Paaver et al., 2006). Lower platelet MAO activity, found in behaviors such as sensation and novelty seeking, high risk taking, aggression and impulsiveness (Reist et al., 1990, Oreland, 2004, Paaver et al., 2006 and Ruchkin et al., 2005), suggested an association between 5-HT function and various pathological behaviors (Reist et al., 1990 and Oreland, 2004), and supported the proposal that platelet MAO is a genetic marker for the capacity of central serotonergic activity (Oreland 2004). Platelet MAO activity is affected by smoking (Eensoo et al., 2007), gender (Roth et al., 1976 and Malmberg et al., 2008), ethnicity and race (Sobell et al., 1997), and medication such as haloperidol (Meszaros et al., 1998), clozapine (Ertugrul et al., 2007), antidepressants (Pivac et al., 2003) or lamotrigine (Muck-Seler et al., 2008). Since early studies, obtained on small samples, showed that children with ADHD have low platelet MAO activity (Shekim et al., 1982 and Shekim et al., 1986), and platelet MAO activity is under the influence of different factors, the aim of the present study was to evaluate platelet MAO activity in ethnically uniform Caucasian boys and girls of the Croatian origin with ADHD, subdivided into groups with predominantly hyperactive, predominantly inattentive, and combined subtype of ADHD, and in healthy control boys and girls, controlled for the effect of smoking, medication and ethnicity. The hypothesis of the study was that platelet MAO activity would differ between children with subtypes of ADHD and control children.
