What do muscarinic M5 receptors do?

• A. No effect
• B. Analgesia
• C. Inhibit dopamine release
• D. Facilitate dopamine release

Answer: (D)

The main idea is that a specific muscarinic receptor subtype on dopamine neurons enhances dopamine release in brain reward pathways. When these M5 receptors are activated, they couple to Gq proteins, triggering phospholipase C, generating IP3 and DAG, and increasing intracellular calcium. This sequence boosts the excitability of dopaminergic neurons and promotes the release of dopamine into target areas like the nucleus accumbens, which underlies reward and motivation.

This is why the correct choice is that M5 receptors facilitate dopamine release. It’s not that there is no effect or analgesia; those don’t capture the receptor’s role. And it’s not an inhibitory effect on dopamine release—M5 activation increases release, contrasting with other muscarinic subtypes that can suppress release via different signaling.