Concerning the role of dopamine in basal ganglia function, all of the following are true EXCEPT:

Explanation

## Dopamine's Dual Action in Basal Ganglia Pathways ### The "All EXCEPT" Framework This question asks which statement is **FALSE**. Three options are true; one is false. ### Evaluating Each Option **Option A (TRUE):** Loss of dopamine in Parkinson's disease results in relative overactivity of the indirect pathway. - Dopamine normally inhibits indirect pathway neurons via D2 receptors. When dopamine is depleted, this inhibition is lost → indirect pathway becomes **overactive** → excessive inhibition of thalamus → bradykinesia and rigidity. *(Kandel, Principles of Neural Science, 6e, Ch. 41)* **Option B (TRUE):** D2 dopamine receptors on indirect pathway neurons mediate inhibition of those neurons when dopamine is present. - D2 receptors are Gi-coupled (inhibitory). Dopamine binding → decreased cAMP → hyperpolarization of striatal indirect pathway neurons → reduced output to GPe. This is well-established basal ganglia physiology. *(Stahl's Essential Psychopharmacology; Kandel 6e)* **Option C (FALSE — the EXCEPT answer):** Dopamine depletion increases the excitatory output of the subthalamic nucleus to the **substantia nigra pars reticulata**. - The STN does project to both GPi and SNr, but the primary and classically described target of increased STN excitatory output in Parkinson's disease is the **globus pallidus interna (GPi)**, not the SNr specifically. More importantly, the SNr is itself a dopaminergic output nucleus (pars reticulata), and framing STN output as directed "to the SNr" in this context is anatomically misleading and factually imprecise. The canonical circuit describes: DA depletion → GPe hypoactivity → STN disinhibition → **↑ STN → GPi excitation** → thalamic inhibition. Stating the target as SNr (rather than GPi) makes this statement inaccurate as written. *(DeLong MR, Wichmann T. Circuits and circuit disorders of the basal ganglia. Arch Neurol 2007)* **Option D (TRUE):** Dopamine released from SNc neurons preferentially enhances D1 receptor signaling in the direct pathway. - D1 receptors are concentrated on direct pathway (striatonigral) neurons. Dopamine binding to D1 (Gs-coupled) → increased cAMP → excitation of direct pathway neurons → facilitates movement. This is a well-established, textbook-correct statement. *(Kandel 6e; Harrison's Principles of Internal Medicine)* ### Basal Ganglia Circuit Summary | Pathway | Receptor | DA Effect | DA Depletion Effect | |---------|----------|-----------|---------------------| | **Direct** | D1 (Gs) | Excitation | Hypoactivity → ↓ thalamic disinhibition | | **Indirect** | D2 (Gi) | Inhibition | Hyperactivity → ↑ thalamic inhibition | | **Net (PD)** | — | Balanced | **Bradykinesia + Rigidity** | ### Why Option C Is the FALSE Statement The STN's increased excitatory output in dopamine depletion is directed primarily to the **GPi** (globus pallidus interna), not specifically to the substantia nigra pars reticulata. The SNr is an output nucleus of the basal ganglia, not the primary downstream target of STN in the classic indirect pathway model. This anatomical inaccuracy makes Option C the false statement. **High-Yield Mnemonic:** **"STN fires at GPi"** — in Parkinson's, disinhibited STN → ↑ GPi activity → ↑ thalamic inhibition → poverty of movement.