Final answer:
Sinemet treats Parkinson's disease by increasing brain dopamine levels through the precursor levodopa, which crosses the blood-brain barrier, and carbidopa, which prevents peripheral conversion.
Step-by-step explanation:
The mechanism of action of Sinemet, which is used in the treatment of Parkinson's disease, involves an increase in dopamine levels in the brain. Sinemet is a combination of levodopa and carbidopa. Levodopa is a precursor to dopamine that can cross the blood-brain barrier, where it is converted to dopamine by the remaining dopaminergic neurons, particularly in the substantia nigra pars compacta.
Carbidopa is included to prevent levodopa from being converted into dopamine in the periphery, which would not be able to cross the blood-brain barrier and would result in side effects. By increasing levels of dopamine in the brain, Sinemet helps to ameliorate the symptoms of Parkinson's caused by low levels of this neurotransmitter due to neuronal degeneration.