Neurotransmitter and Pharmacological Data
In 1937 Charles Bradley identified a group of children whose impulsive aggressive behaviors where improved by treatment with amphetamines(3). Then and now it seems contradictory that a drug which stimulates the CNS would paradoxically resulted in decreased activity and more focused  attention. It is this very paradox which has lead scientific exploration into the etiology of AD/HD and which is at the core of our current neurochemical model of this disorder.

Stimulants (methylphenidate, dextroamphetamine and pemoline), clonidine, and tricyclic antidepressants have all been used effectively to treat children with AD/HD. The exact mechanism of their action is unknown but presumably they exert their effects through modulation of neurotransmitters. Dopamine, noradrenaline and serotonin  levels in the central nervous system are affected by these medications. This has lead to further investigation into the role of each of these neurotransmitters in the etiology of AD/HD

Multiple Neurotransmitters

No single neurotransmitter deficiency appears to explain fully the array of medications which improve symptomatology in AD/HD. The single neurotransmitter concept is refuted by the inability of specific medications known to modulate individual neurotransmitters to improve symptoms. It is possible that AD/HD is due to imbalance of multiple inter-related neurotransmitters.

Is it all in the timing??

Pharmacokinetic studies have yielded additional evidence that medications used to improve attention exert their effects by modulating the release and level of neurotransmitters in the CNS.