Nerve growth factors, or neurotrophic factors, which support survival, growth, and development of brain cells, are another type of potential therapy for Parkinson's disease. One such drug, glial cell line-derived neurotrophic factor (GDNF), has been shown to protect dopamine neurons and to promote their survival in animal models of PD.

This drug has been tested in several clinical trials for people with PD, and the drug appeared to cause regrowth of dopamine nerve fibers in one person who received the drug. However, a phase II clinical study of GDNF was halted in 2004 because the treatment did not show any clinical benefit after 6 months, and some data suggested that it might even be harmful. Other neurotrophins that may be useful for treating PD include neurotrophin-4 (NT-4), brain-derived neurotrophic factor (BDNF), and fibroblast growth factor 2 (FGF-2).

While there is currently no proof that any dietary supplements can slow PD, several clinical studies are testing whether supplementation with vitamin B12 and other substances may be helpful. A 2005 study found that dietary restriction — reducing the number of calories normally consumed – helped to increase abnormally low levels of the neurotransmitter glutamate in a mouse model for early PD. The study also suggested that dietary restriction affected dopamine activity in the brain. Another study showed that dietary restriction before the onset of PD in a mouse model helped to protect dopamine-producing neurons.

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Other studies are looking at treatments that might improve some of the secondary symptoms of PD, such as depression and swallowing disorders. One clinical trial is investigating whether a drug called quetiapine can reduce psychosis or agitation in PD patients with dementia and in dementia patients with parkinsonian symptoms. Some studies also are examining whether transcranial magnetic stimulation or a food supplement called s-adenosyl-methionine (SAM-e) can alleviate depression in people with PD, and whether levetiracetam, a drug approved to treat epilepsy, can reduce dyskinesias in Parkinson's patients without interfering with other PD drugs.