With brain infusions, a growth factor called glial-derived neurotrophic factor (GDNF) is placed in the part of the brain where dopamine is lacking in the hope of encouraging the damaged cells to rejuvenate and produce more dopamine.
Although initial results were promising, the outcome of later studies was very mixed. Because of this and safety issues, Amgen, the pharmaceutical company involved has stopped making GDNF, despite the fact that some researchers disagree with this decision.