This chapter aims to identify, among the dopaminergic and noradrenergic molecules strongly associated to aetiopathogenesis of the disorder, potential genetic and biochemical markers linked to ADHD diagnosis and to assess whether treatments can change peripheral levels of a biomarker, to be then useful, if tested, as a response predictor. The results, based on literature research, evidenced a role of some molecules such as SLC6A3, DRD4, MHPG, MAOA, NE, SLC6A2, DBH, COMT that could represent, in

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... his order, a hypothetical signature of genetic and biochemical markers useful for ADHD diagnosis. From this hypothetical signature, the NE metabolite MHPG resulted, after a meta-analytic approach, the main molecule whose urinary levels were influenced by the d-AMP treatment. Urinary MHPG levels were decreased after stimulants administration only in the responder patients, indicating that MHPG could be a useful predictor of the response to these drugs. This, along with the well-reported correlation between decrease in MHPG and behavioral improvements after d-AMP treatment, focuses attention on MHPG as a potential mediator of stimulant drug response, in addition to a potential useful biological marker for diagnostic assessment. Future studies on specificity, sensitivity and replication of these findings are needed.