Since the associations were found with both forms of tobacco and a dose�Cresponse relationship was found, the results supported the hypothesis of a link between Navitoclax 923564-51-6 nicotine intake and bruxism (Rintakoski et al., 2010). However, all other aforementioned epidemiological studies on bruxism and smoking have ignored the possible confounding effects of genes. Twin studies indicated that both smoking (Rose et al., 2009) and bruxism (Hublin et al., 1998) have significant genetic components. Thus, the association between the two may be due to underlying genetic effects in common, that is, pleiotropic effects of genes resulting in two different phenotypes. Familial aggregation may be due to family members sharing genes or sharing environments. The twin study on bruxism (Hublin et al.

, 1998) did not find a shared familial effect, so the association with smoking cannot be due to shared family effects in common to these two phenotypes. The discordant pair analysis suggests that the association between smoking and bruxism exists even when family background is taken into account, but a formal analysis of the contribution of genes and environment would require multivariate quantitative genetic modeling. The Finnish Twin Cohort data have several strengths. Representativeness of bruxism is adequate in the present study population and database as other studies have given similar prevalence (Partinen & Hublin, 2000). The data are also representative of the smoking behavior of Finnish population.

Lung cancer incidence is an excellent indirect measure of smoking behavior in a population and among the Finnish Twin Cohort; lung cancer incidence did not differ from that in the population (Verkasalo, Kaprio, Koskenvuo, & Pukkala, 1999), indicating that data represent well Finnish smoking population. The NAG study is focused more specifically on smoking and nicotine dependence but is based on the Finnish Twin Cohort. As in almost all surveys, the heaviest smokers were somewhat underrepresented in the NAG study (Broms et al., 2007). We had the opportunity to deepen the assessment of causality of tobacco use with respect to bruxism by using discordant twin pairs as matched cases and controls. Smoking is decreasing in western societies but it is still rather common and detrimental for several aspects. A high proportion of smokers are dependent on nicotine (Fagerstr?m & Furberg, 2008).

In the present study, we used the psychiatric diagnostic scheme DSM-IV to diagnose nicotine dependence in a relatively small subset of our twins. Brefeldin_A The association between nicotine dependence and bruxism held even after adjustment for a lifetime history of another dependence, namely alcohol dependence, as well as major depression. Nicotine dependence plays a central role in maintaining smoking, and nicotine affects the dopaminergic system.