In schizophrenia, which neurotransmitter system abnormality is most strongly implicated in the pathogenesis of positive symptoms?

Explanation

## Dopamine Hypothesis of Schizophrenia **Key Point:** The dopamine hypothesis is the most widely accepted neurochemical model of schizophrenia, particularly explaining positive symptoms through hyperactivity of mesolimbic dopamine pathways. ### The Dopamine Dysregulation Model | Dopamine Pathway | Activity Status | Clinical Correlate | |---|---|---| | **Mesolimbic** | ↑ Hyperactive | Positive symptoms (hallucinations, delusions) | | **Mesocortical** | ↓ Hypoactive | Negative symptoms (apathy, alogia, anhedonia) | | **Nigrostriatal** | Variable | Extrapyramidal side effects (from antipsychotics) | | **Tuberoinfundibular** | Blocked by antipsychotics | Hyperprolactinemia | **High-Yield:** Positive symptoms (hallucinations, delusions, disorganized speech/behavior) are primarily driven by **excessive dopamine signaling in the mesolimbic system**, which projects from the ventral tegmental area (VTA) to the nucleus accumbens and limbic structures. ### Evidence Supporting Dopamine Hypothesis 1. **Antipsychotic efficacy** — all antipsychotics block dopamine D~2~ receptors; their potency correlates with D~2~ occupancy 2. **Psychotomimetic drugs** — amphetamine and cocaine (dopamine agonists) induce psychotic symptoms 3. **PET imaging** — elevated dopamine release in striatum during acute psychosis 4. **Postmortem studies** — increased dopamine and D~2~ receptor density in mesolimbic regions **Clinical Pearl:** The **mesolimbic hyperactivity** explains why antipsychotics are highly effective for positive symptoms but less effective for negative symptoms (which require mesocortical dopamine restoration). **Mnemonic:** **"Positive = Mesolimbic"** — hyperactivity in mesolimbic dopamine → positive symptoms (hallucinations, delusions). [cite:Stahl's Essential Psychopharmacology 4e Ch 7]