Concerning the neurotransmitters and receptor pharmacology within basal ganglia circuits, all of the following are true EXCEPT:

Explanation

## Basal Ganglia Neurotransmitter Systems ### GABAergic Inhibition **Key Point:** GABA is the primary inhibitory neurotransmitter throughout the basal ganglia. The globus pallidus internus (GPi) is tonically active and releases GABA onto thalamic neurons, keeping them suppressed at rest. ### Glutamatergic Excitation from STN **High-Yield:** The subthalamic nucleus (STN) is the only **excitatory** nucleus in the basal ganglia circuit. It uses glutamate to drive neurons in the GPi and GPe, making it a critical hub for movement control. ### Dopamine Receptor Signaling **Mnemonic: D1-cAMP-UP, D2-cAMP-DOWN** | Receptor | G-Protein | Second Messenger | Striatal Pathway | Effect | |----------|-----------|------------------|------------------|--------| | D1 | G~s~/G~olf~ | ↑ cAMP | Direct pathway | Facilitates movement | | D2 | G~i~/G~o~ | ↓ cAMP | Indirect pathway | Suppresses movement | ### Why Option 3 Is Incorrect **Key Point:** Acetylcholine from striatal cholinergic interneurons **inhibits** the indirect pathway (via M4 muscarinic receptors on D2 neurons) and **facilitates** the direct pathway (via M1 receptors on D1 neurons). **Clinical Pearl:** This is why anticholinergic drugs (e.g., benztropine) improve symptoms in Parkinson's disease — they reduce acetylcholine's suppression of the direct pathway. Option 3 incorrectly states that acetylcholine enhances the indirect pathway, which would **suppress** movement — the opposite of what actually occurs. ### Acetylcholine's True Role ```mermaid flowchart TD A[Acetylcholine from striatal interneurons]:::outcome --> B{Which D2 neurons?}:::decision B -->|Indirect pathway neurons| C[M4 receptor activation]:::action C --> D[Inhibition of indirect pathway]:::action D --> E[Reduced thalamic inhibition]:::action E --> F[Facilitation of movement]:::outcome B -->|Direct pathway neurons| G[M1 receptor activation]:::action G --> H[Enhancement of direct pathway]:::action H --> F ``` **Clinical Correlation:** In Parkinson's disease, the loss of dopamine creates a relative **excess** of cholinergic tone, leading to rigidity and bradykinesia. Anticholinergic agents restore balance by reducing acetylcholine's inhibitory effect on the direct pathway.