BDNF and Parkinson disease: While smoking is a well-established risk factor for PUD, other studies have suggested a protective, dose-dependent inverse association between tobacco smoking and PD risk.26,27,28,29 Nicotine has been shown experimentally to prevent parkinsonism in rodents and induce brain-derived neurotrophic factor in the striatum,30 thereby appearing to be neuroprotective for dopaminergic neurons.