Antiparkinson Agents

Parkinson disease results from the death of dopamine-producing neurons in a movement-control area of the brain (the substantia nigra). Losing dopamine unbalances the movement circuits, causing the classic tremor, rigidity, bradykinesia (slowness), and postural instability. Treatment doesn’t stop the neuron loss — it replaces the missing dopamine to restore function.

Dopamine itself can’t be given (it doesn’t cross the blood-brain barrier), so the drugs use clever workarounds.

Levodopa is dopamine’s precursor — it does cross into the brain, where it is converted to dopamine. The problem: most levodopa is converted to dopamine in the body before it reaches the brain, causing nausea and dropping the amount available centrally. The fix is carbidopa, which blocks that peripheral conversion — so more levodopa reaches the brain, with less nausea and vomiting. That’s why they’re combined as carbidopa-levodopa (Sinemet).

Two supporting strategies: dopamine agonists (pramipexole, ropinirole) directly stimulate dopamine receptors, and enzyme inhibitors — COMT inhibitors (entacapone) and MAO-B inhibitors (selegiline, rasagiline) — extend dopamine’s action. Note high-protein meals compete with levodopa absorption, and (without carbidopa) vitamin B6 speeds its peripheral breakdown.

Over years, levodopa’s effect becomes less smooth: patients develop “wearing-off” (symptoms return before the next dose) and “on-off” fluctuations, and peak levels cause dyskinesias (involuntary writhing movements). Other effects include orthostatic hypotension, hallucinations/confusion, and darkened urine/sweat (harmless).

A crucial, easily-missed effect of the dopamine agonists: they can trigger impulse-control disorders — compulsive gambling, shopping, eating, or hypersexuality — and sudden "sleep attacks." Ask about these directly. And never stop dopaminergic therapy abruptly — it can cause a dangerous NMS-like syndrome (fever, rigidity).