Which neurotransmitter is released by dopaminergic neurons in the substantia nigra pars compacta (SNc) onto the striatum in the direct pathway of basal ganglia circuits?

Explanation

## Dopaminergic Innervation of the Striatum **Key Point:** The substantia nigra pars compacta (SNc) contains dopaminergic neurons that project to the striatum (caudate and putamen) and release dopamine as their primary neurotransmitter. ### Role in Direct Pathway Dopamine released onto medium spiny neurons (MSNs) in the striatum has a **facilitatory effect** on the direct pathway: - Binds to D1 receptors on direct pathway MSNs → increases excitability - Promotes movement initiation and execution - Loss of dopamine (as in Parkinson disease) impairs the direct pathway, leading to hypokinesia ### Anatomical Circuit The direct pathway sequence: 1. Striatum (D1+ MSNs) — dopamine-facilitated 2. Globus pallidus interna (GPi) — inhibited 3. Thalamus — disinhibited 4. Motor cortex — movement execution **High-Yield:** Dopamine is the **only monoamine neurotransmitter** released by SNc neurons; it is essential for normal motor control and is depleted in Parkinson disease. **Clinical Pearl:** Dopamine agonists (bromocriptine, levodopa) are used therapeutically in Parkinson disease to restore dopaminergic tone and facilitate the direct pathway. [cite:Snell's Clinical Neuroanatomy 10e Ch 9]