________ stimulates dopamine receptors in the brain.

• A. Selegiline
• B. Rasagiline
• C. Levodopa
• D. Bromocriptine

Answer: (A)

Dopaminergic transmission in the brain can be increased either by directly activating dopamine receptors or by boosting the amount of dopamine available to stimulate those receptors. Selegiline works by inhibiting monoamine oxidase-B (MAO-B), the enzyme that breaks down dopamine in the brain. When MAO-B is blocked, dopamine isn’t degraded as quickly, so its levels rise and more dopamine can bind to and stimulate dopamine receptors. This indirect rise in receptor stimulation is why selegiline can be described as stimulating dopamine receptors in the brain, especially as an adjunct in conditions like Parkinson disease where dopaminergic signaling is reduced.

Rasagiline is another MAO-B inhibitor with a similar effect, but the question’s correct choice emphasizes the MAO-B–inhibition mechanism. Levodopa increases dopamine by providing more substrate for synthesis, and bromocriptine directly activates dopamine receptors as an agonist. The key idea is that increasing endogenous dopamine availability via MAO-B inhibition leads to greater receptor stimulation, which is the mechanism at play with selegiline.