Which statement about muscarinic M1 receptor effects on dopamine and locomotion is true?

• A. Increase dopamine release and locomotion
• B. Decrease dopamine release and locomotion
• C. No effect on dopamine or locomotion
• D. Increase dopamine release but decrease locomotion

Answer: (B)

Muscarinic M1 receptors in the striatum are Gq-coupled and modulate the balance of activity in the basal ganglia circuits that control movement. When M1 receptors are activated, they drive signaling that tends to favor the indirect pathway, which suppresses movement. They also can reduce dopamine release from nigrostriatal terminals through local circuit interactions involving cholinergic interneurons. Since dopamine supports locomotion, a decrease in dopamine release leads to lower locomotor activity. This combination—less dopamine and less movement—fits the statement as true. Blocking M1 would have the opposite effect, often increasing dopamine release and promoting locomotion, which is why the other potential outcomes aren’t correct.