Antiparkinsonian drug activating D2 receptors directly
**Core Concept**
Parkinson's disease is primarily treated with levodopa, a precursor to dopamine, which is depleted in the disease. However, direct activation of dopamine receptors is an alternative approach to manage symptoms. Dopamine receptors are divided into two subtypes: D1-like (including D1 and D5 receptors) and D2-like (including D2, D3, and D4 receptors).
**Why the Correct Answer is Right**
Pramipexole is a dopamine agonist that directly activates D2 receptors, mimicking the action of dopamine in the brain. This activation leads to improved motor function and reduced symptoms of Parkinson's disease. Pramipexole has high affinity for D2 receptors and can penetrate the blood-brain barrier, allowing it to exert its effects directly on the dopamine receptors in the brain.
**Why Each Wrong Option is Incorrect**
**Option B:** Entacapone is a catechol-O-methyltransferase (COMT) inhibitor, which increases the availability of levodopa in the brain by preventing its breakdown. It does not directly activate dopamine receptors.
**Option C:** Benserazide is a peripheral decarboxylase inhibitor, which prevents levodopa from being converted into dopamine outside the brain, allowing more levodopa to cross the blood-brain barrier. It does not directly activate dopamine receptors.
**Option D:** Selegiline is a monoamine oxidase B (MAO-B) inhibitor, which prevents the breakdown of dopamine in the brain, thereby increasing its availability. While it does affect dopamine levels, it does not directly activate dopamine receptors.
**Clinical Pearl / High-Yield Fact**
Pramipexole and other dopamine agonists can cause impulse control disorders, such as compulsive shopping or gambling, due to their direct activation of dopamine receptors in the brain.
✓ Correct Answer: A. Pramipexole