Practice question · Multiple choice
Parkinson's disease is a shortage of dopamine, yet patients are given L-DOPA rather than dopamine itself. Why prescribe the precursor?
Hints
- The deficiency is inside the brain. Ask what stands between a swallowed pill and the substantia nigra.
- One of these two molecules has a transporter across the barrier. The other does not.
Show the answer
A. Because dopamine cannot cross the blood-brain barrier and L-DOPA can
Why
The barrier decides the prescription. Dopamine is exactly what is missing and cannot get in; L-DOPA is carried across and converted on arrival. Half of neuropharmacology is this problem, the right molecule is useless if it cannot reach the tissue, so drugs are designed around the door as much as the lock.
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