Genetic Variation of the Mu Opioid Receptor (OPRM1) and Dopamine D2 Receptor (DRD2) is Related to Smoking Differences in Patients with Schizophrenia but not Bipolar Disorder

It is not known why mentally ill persons smoke excessively. Inasmuch as endogenous opioid and dopaminergic systems are involved in smoking reinforcement, it is important to study mu opioid receptor (OPRM1) A118G (rs1799971), dopamine D2 receptor (DRD2) Taq1A (rs1800497) genotypes, and sex differences among patients with schizophrenia or bipolar disorder. Smokers and nonsmokers with schizophrenia (n=177) and bipolar disorder (n=113) were recruited and genotyped. They were classified into three groups: current smoker, former smoker, and never smoker by tobacco smoking status self-report. The number of cigarettes smoked per day was used as the major tobacco smoking parameter. In patients with schizophrenia, tobacco smoking prevalence was greater in males than in females as expected, but women had greater daily cigarette consumption (p<0.01). Subjects with schizophrenia who had the OPRM1 *G genotype smoked more cigarettes per day than the AA allele carriers with schizophrenia (p<0.05). DRD2 Taq1A genotype differences had no effect on the number of cigarettes smoked per day. However, female smokers with schizophrenia who were GG homozygous of the DRD2 receptor smoked more than the *A male smokers with schizophrenia (p<0.05). In bipolar patients, there were no OPRM1 and DRD2 Taq1A genotype differences in smoking status. There also were no sex differences for smoking behavior among the bipolar patients. The results of this study indicate that single nucleotide polymorphism (SNP) of the less functional mu opioid receptor increases tobacco smoking in patients with schizophrenia. Alteration of DRD2 receptor function also increased smoking behavior in females with schizophrenia.