As is usually the case in psychiatry, the intensity of the debate was inversely proportional to the availability of factual information. As in a good mystery, the journey from suspicion to proof, from speculation to empirical evidence, from Kahn and Cohen to Paul Wender and Alan Zametkin and Rachel Gittleman-Klein and the other current researchers, has been riddled with false leads, multiple possibilities, contradictory findings, and many gut reactions of all kinds. One of the first attempts to unite the effects of the stimulants with what we know about the brain was made by C. Kornetsky, who in 1970 proposed the Catecholamine Hypothesis of Hyperactivity. Catecholamines are a class of compounds that includes the neurotransmitters norepinephrine and dopamine. Since the stimulants affect the norepinephrine and dopamine neurotransmitter systems by increasing the amount of these neurotransmitters, Kornetsky concluded that ADD possibly was caused by an underproduction or underutilization of these neurotransmitters.
Neurons can convert dopamine into norepinephrine.
Many of the drugs that act on the catecholamines act on serotonin. Some of the drugs that act on serotonin can act on norepinephrine and dopamine. But yes, there is enough evidence that neurochemical systems are altered in people with ADD to state that the problem derives from the chemistry of the brain. Most likely, it is a dysregulation along the catecholamine-serotonin axis, a dance where one misstep by one partner creates a misstep by the other, which creates another misstep by the first.