The text lists three treatments for Parkinson's disease: l-dopa administration, deep brain stimulation, and cell-replacement therapy. Choose one of these and discuss the theory behind how it works and the limitations of its success and effectiveness.

l-dopa is the precursor to dopamine. Dopamine does not cross the blood-brain barrier so patients are given l-dopa, which does cross the blood brain barrier. Since Parkinson's disease appears to be a loss of dopaminergic input to the striatum, increasing dopamine levels should help compensate for the dopaminergic loss. This treatment has been fairly successful in treating the disease. One of the side effects is levodopa-induced dyskinesia. In addition, the effects of l-dopa decrease over time so it is only effective for a few years.
Deep brain stimulation: Deep brain stimulation is used for many diseases. The theory behind it for Parkinson's is that the basal ganglia circuitry is not functioning correctly and so areas of the brain are stimulated to compensate for the increased activity in GPi/SNr. Although it is not known how DBS works, it is thought to inhibit the output of the STN, which would reduce the excitation to the GPi/SNr and decrease the inhibition of the brainstem circuits. DBS has been quite successful, however it does require invasive brain surgery to implant electrodes.
Cell-replacement therapy: The idea behind cell-replacement therapy is to replace dying dopaminergic neurons. To do this embryonic stem cells are implanted into the striatum and there they release dopamine. There are many limitations to this therapy; the stem cells must survive and must release dopamine. In addition, a large number of dopaminergic neurons are required to be effective and there is often contamination from other cell types. This therapy also requires patients to be put on immunosuppressants to reduce the likelihood of rejection of the cells.