Objective: To provide an update on the "catecholamine hypothesis" of attention-deficit hyperactivity disorder (ADHD).
Method: Recent work examining the measurement of the norepinephrine, epinephrine, and dopamine systems in ADHD and normal subjects is reviewed and discussed in the context of recent neuroimaging and animal studies.
Results: While data clearly indicate a role for all three of the above neurotransmitters in ADHD, a hypothesis suggesting "too much" or "too little" of a single neurotransmitter will no longer suffice. The central norepinephrine system may be dysregulated in ADHD, such that this system does not efficiently "prime" the cortical posterior attention system to external stimuli. Effective mental processing of information involves an anterior "executive" attention system which may depend on dopaminergic input. The peripheral epinephrine system may be a critical factor in the response of individuals with ADHD to stimulant medication.
Conclusion: A multistage hypothesis is presented which emphasizes the interaction of norepinephrine, epinephrine, and dopamine in modulation of attention and impulse control.