On the other hand, epidemiological studies have revealed an inverse correlation between smoking and the incidences of PD and AD (Picciotto and Zoli 2008; Quik 2004). Nicotine is believed to be the chemical responsible for such beneficial effects because it can stimulate the dopaminergic system and be neuroprotective in such disorders. Several mechanisms have been suggested to explain the relationship, and the most common one is that nicotine acts as an antioxidant (Obata et al. 2002; Soto-Otero et al. 2002). Also, nicotine interacts directly with the electron transport system. For example, nicotine may complete with NADH and inhibit mitochondrial NADH-ubiquinone reductase activity (Cormier et al. 2001, 2003) or regulate the electron leak at the site of complex I (Xie et al. 2005), with a consequent reduction in the amounts of ROS.
