Genetics of childhood disorders: XXIII. ADHD, Part 7: The serotonin system.

253 A central premise in the “catecholamine hypothesis” of attention-deficit/hyperactivity disorder (ADHD) is that dopamine (DA) dysfunction leads to clinical symptoms. The hypothesis arises, in part, from the clinical efficacy of methylphenidate, as well as evidence from brain imaging studies that suggest reduced activity in frontal-striatal regions. The theory, however, overlooks the phenotypic complexity of the disorder and the possible interactions between the dopamine and serotonin (5-HT) neurotransmitter systems. ADHD is a heterogeneous disorder manifesting itself in various behavioral dimensions including inattention, hyperactivity, and impulsivity often co-occurring with other child behavioral disorders including comorbid oppositional defiant disorder and conduct disorder. It is likely that different neurotransmitter systems and the relative balance between them have varying degrees of influence over these behavioral dimensions. Variation in genes involved in these neurotransmitter systems are likely to mediate this delicate balance and have an effect on the function of these Genetics of Childhood Disorders: XXIII. ADHD, Part 7: The Serotonin System