A number of studies have shown that the percentage of people who smoke is lower among individuals with Parkinson’s disease (PD) than those without. Furthermore, individuals who have ever been smokers have half the risk of developing PD compared to those who have never smoked. When results like these were initially published, they were interpreted as showing that smoking (or nicotine) somehow protected people from developing PD.
Recently, scientists have come to a different conclusion from these results. The new theory is that fewer people who go on to develop PD become or remain smokers because, for them, nicotine has less of a rewarding effect. Nicotine has a number of effects on the brain. Like all drugs that produce “rewarding” effects, nicotine increases levels of dopamine in certain areas of the brain. Behaviorally, it produces stimulating effects. Individuals with PD have a lower number of nicotine receptors in the brain, up to 50 percent lower in some areas, than those without PD. Because of this, there are fewer sites for it to bind to in the brain, so nicotine’s effects will be lesser. This may make it easier for people with PD to quit smoking.
If the reduced effect of nicotine on the brain is an early effect of PD—one that is evident long before the motor effects are diagnosed, like changes in smell, constipation, and REM sleep disorders—reduced nicotine reward and/or an easier time quitting smoking might be an early marker for PD.
A recent study published in the journal, Neurology, provides support for this idea. Dr. Beate Ritz and colleagues at the University of California, Los Angeles compared difficulty with quitting smoking as well as use of nicotine substitution products (e.g., nicotine patch, nicotine gum) in individuals with PD as compared to healthy individuals without PD. The study was designed to test several hypotheses. The authors hypothesized that among smokers, those who found it more difficult to quit smoking were less likely to develop PD than who found it less difficult to quit. Second, they predicted that smokers who used nicotine replacement products found it more difficult to quit smoking than those who did not. Finally, they assumed that those who used nicotine replacement products had a lower rate of PD than those who didn’t.
The authors conducted their study by identifying patients who had been diagnosed with PD between 1996 and 2010 based on data in the Danish National Hospital Register, and healthy individuals of matched sex and age from the Danish Central Population Registry. They interviewed and reviewed medical records from 1,808 PD patients and 1,876 individuals without PD and compared the groups on demographic information, level of education, smoking history, use of nicotine replacement products and other lifestyle habits.
The analysis showed that individuals with PD were less likely than those without to have ever smoked cigarettes (defined as smoking at least one cigarette per week for at least six months). When asked how difficult it was for former smokers to quit smoking, those who reported that it was “extremely difficult” had a 31 percent lower risk of developing PD than those who reported that quitting was “easy”. Those who reported ever having used nicotine replacement also had a lower risk of PD. Those who reported finding it “extremely difficult” to quit were nine times more likely than those who found it “was easy to quit” to have used nicotine replacements.
The results of this study confirmed the findings of earlier studies in showing that patients with PD were less likely to develop a smoking habit than those without the disease. Seventeen percent of PD patients who had ever smoked remained smokers as compared to 31 percent of controls. Because those with PD had an easier time quitting, there was a lower rate of current smokers and a lower overall amount smoked (pack/years) among that group. Overall, those who smoked and/or those who used nicotine replacement were less likely to develop PD. The findings of the study suggest that, indeed, nicotine reward is less strong in smokers who later develop PD.
The results of the study confirm that individuals should not start to smoke or to use nicotine products with the idea of delaying PD. Individuals with PD who do smoke should try to quit since smoking can cause other health problems, including cancer and stroke.