The relationship between blood serotonin, norepinephrine, dopa and lipid levels and the degree of hyperactivity, impulsiveness, inattention, and aggressiveness in 35 boys with ADHD, combined type, was studied at the Ness Ziona Mental Health Center, Nes Zona, Tel Aviv University, and other centers in Israel. The high-severity ADHD subgroup, as measured by the Conners Teacher Rating Scale, showed a tendency toward a correlation with low platelet-poor plasma serotonin concentrations (P=0.08). Catecholamine and lipid levels were unrelated to severity of ADHD. 
COMMENT. A tendency toward lower plasma serotonin levels may correlate with severity of ADHD, and a disruption in serotonin function may underly the mechanism of one subtype of ADHD. A previously suggested link between low serum cholesterol levels and impulsive and aggressive behavior in ADHD children is not supported by the present study. The authors are to be commended on their attempt to document the neurobiological basis and relationship of serotonin function to the severity of symptomatology in ADHD. Further biochemical investigations of this type, including methylphenidate-induced changes, may lead to a better understanding of the pathogenesis of ADHD.
Brain serotonin in autistic children was studied at the Children’s Hospital of Michigan, Detroit . Serotonin synthesis capacity (SSC) measured in vivo by PET showed a gradual increase in 30 autistic children from 2 to 15 years, reaching to 1.5 times adult normal values. In comparison, SSC in 8 nonautistic siblings and 16 epileptic children without autism was 200% higher than in adults up to 5 years and then declined toward adult values. It seems that the developmental increase in SSC usually present in childhood is disrupted in autistic children. Previous studies in animals, cited by the authors, have shown that changes in brain serotonin during development may affect synaptic connections in the sensory cortex and alter neuronal differentiation.