Oades, R. D., (2007).
The Role of the serotonin system in ADHD: treatment implications.
Expert Reviews in Neurotherapeutics, 7, 1357-1374.[Request a copy] - [view article]

Introduction: Few studies have considered whether the activity of 5HT contributes to the problems experienced by people with ADHD. Relevant reports are reviewed here and some interpretations put forward.

- 1 - Measures of the transmitter (5-HT) and its metabolite (5-HIAA) in CSF, plasma or urine do not show convincing associations with the diagnosis. But, a look at the major domains of dysfunction (motor activity, inattention & impulsivity) support a more differentiated assessment.

- 2 - A notable innervation of motor regions of the brain by 5HT projections & the contribution of 5HT systems to the control of locomotion in animals would suggest a possible involvement for 5-HT in the physical activity and restlessnes typical of those with ADHD - but the few current studies do not offer any support for this idea. Nonetheless a closer look should be taken.

- 3 - In contrast, 5-HT activity does seem to be relevant to attention-related processes. For example, altered 5-HT activity is related to deficiencies in perceptual sensitivity & the appropriate designation of saliency to stimulation. In exogenous attentional processing perceptual sensitivity is disturbed by overactive 5-HT systems, while endogenous selective processes, as in switching set, are sensitive to the role of 5-HT in the 'volume-control' of information processing.

- 4 - Marked & opposite changes of 5-HT responsivity are associated with behavioral & cognitive impulsivity. It is useful to consider impulsivity, common in those with ADHD, as two separate domains: behavioral and cognitive impulsivity. These reflect relatively low and relatively high 5-HT activity, respectively. In turn these levels of activity will reflect partly the comorbidity of more or fewer externalizing symptoms, respectively.

- 5 - More and more reports describe preferential transmission of various genetic markers for 5HT receptors that are expressed in ADHD. Currently the heterogeneity of methods is still restricting attempts to define these markers & the types of ADHD in which they are expressed. Yet, there is tentative evidence for transmission of an allele associated with 5-HT1b activity more in ADHD patients with inattentive features, while there are indications that an allele associated with receptors from the 5-HT2 family (5-HTa/c) may be preferentially transmitted to those showing hyperactivity/impulsivity. It seems likely there are also genetic influences on the synthesis (affecting release into the synapse), breakdown (monoamine oxidase), and on uptake of 5-HT from the synapse and that these moderate 5-HT activity in patients with ADHD

- 6 - Relatively neglected fields of 5-HT study concern its non-transmitter, support functions in development (neurotrophic role) and rapid neural firing (astrocytic energy supply to the synapse) needed to maintain ongoing responses.

see also

Oades R. D., (2006) Function and dysfunction of monoamine interactions in children and adolescents with AD/HD. In, "Neurotransmitter interactions and cognitive function". E. D. Levin (Eds.). Birkhäuser Verlag, pp. 207-245. flyer/order form : book contents : chapter

Oades, R. D., (2005) The Role of Norepinephrine and Serotonin in ADHD. In: “Attention Deficit Hyperactivity Disorder: From Genes to Animal Models to Patients”, D. Gozal and D. L. Molfese (Eds.), Humana Press, Totawa, NJ: pp.97-130.