Which neurotransmitter system dysfunction is most consistently implicated in the pathophysiology of ADHD?

Explanation

## Neurobiological Basis of ADHD **Key Point:** ADHD is fundamentally a disorder of catecholaminergic (dopamine and norepinephrine) dysfunction, particularly affecting the prefrontal cortex and striatum. ### Dopamine and Norepinephrine Hypothesis The catecholamine hypothesis of ADHD posits: 1. **Prefrontal cortex involvement** — Hypoactivity of dopaminergic and noradrenergic pathways impairs executive function, working memory, and impulse control. 2. **Striatal dysfunction** — Reduced dopamine signaling in the dorsal striatum contributes to poor reward processing and motivation. 3. **Mesocortical and mesolimbic circuits** — Both are critical for attention regulation and behavioral inhibition. ### Evidence Base - **Neuroimaging studies** show reduced activity in the prefrontal cortex and anterior cingulate in individuals with ADHD. - **First-line medications** (stimulants: methylphenidate, amphetamines) work by increasing synaptic dopamine and norepinephrine. - **Non-stimulant alternatives** (atomoxetine, guanfacine, clonidine) selectively enhance noradrenergic transmission. **High-Yield:** The catecholamine hypothesis explains why dopamine agonists and norepinephrine reuptake inhibitors are effective, whereas serotonergic agents (SSRIs) alone are NOT effective for core ADHD symptoms. **Clinical Pearl:** While serotonin dysfunction may contribute to comorbid depression or anxiety in ADHD, it is NOT the primary mechanism of inattention, hyperactivity, or impulsivity. ### Why Other Systems Are Secondary | System | Role in ADHD | Evidence | |--------|-------------|----------| | Dopamine/Norepinephrine | PRIMARY | Stimulants effective; neuroimaging abnormalities | | Serotonin | Comorbid mood/anxiety | SSRIs do not improve core ADHD symptoms | | Acetylcholine | Not implicated | No cholinergic agents in ADHD treatment | | GABA | Possible in anxiety comorbidity | Not primary mechanism | [cite:Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5); American Psychiatric Association]