Oades, R. D., The role of serotonin in attention-deficit hyperactivity disorder (ADHD).
In, "The Handbook of Behavioral Neurobiology of Serotonin", Eds., Christian Müller & Barry Jacobs, Academic Press/Elsevier, London/Amsterdam, Chapter 4.5, pp. 565-584. [Request a copy]

Background - the disorder: The symptoms of childhood attention-deficit hyperactivity disorder (ADHD) cover the domains of cognition, impulsivity and motor control, and often embody motivational/emotional features. Prevalence of the combined-type disorder may decrease with age, but the former features predominate increasingly through adolescence to adulthood.

Background - the neurotransmitter: The widespread innervation through the CNS by fibers containing serotonin (5-HT) means that, as cause or effect, it is likely
that 5-HT is involved where function in these domains is impaired. The relative success of treatment with catecholaminergic agents has overshadowed investigation of the reasons for the partial response of half of these patients. This chapter suggests that anomalous 5-HT function may provide one potential explanation

Lines of evidence:
1- The best evidence lies with the high heritability of the disorder, where several genes with small effect contribute to the availability of 5-HT and its metabolism..

2 - Extra-neuronal availability is under the control of enzymes responsible for 5-HT synthesis, and its removal depends on breakdown or reuptake. Alleles are expressed in cases with ADHD that disturb this availability; they can be over- or under-expressed, depending on phenotype (e.g., impulsivity).

3 - The effect of altered levels of 5-HT is mediated by the expression of pre- and postsynaptic receptors of the 5-HT 1 and 5-HT 2 families..

4 - Neuropsychological and neurophysiological studies show that the processes affected range from perception, through attention to motivationally guided learning
and mood.

5 - While the function of a range of brain regions may be involved, the thalamus and frontal lobes are taken as potentially influential examples. .

Discussion: Study in this field is in its infancy, and many putative interactions (e.g., between genes, gene/environment, co-morbid problems, development and motor function) require closer investigation.

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