Monoamine oxidase: associations with alcohol dependence, smoking and othermeasures of psychopathology

Background. Many reports have appeared on associations between platelet mon oamine oxidase (MAO) activity and susceptibility to psychiatric conditions; principally alcohol dependence but also conduct disorder, other drug use a nd depression. Recently, it has become apparent that MAO activity is inhibi ted by some component of cigarette smoke, and smokers have low platelet MAO activity. Since the prevalence of smoking is higher in many of the conditi ons in which MAO has been implicated, the MAO susceptibility associations m ay be partly, or entirely, false. Methods. We have measured platelet MAO in 1551 subjects, recruited from the Australian NHMRC Twin Registry, who have provided information on alcohol u se and dependence, smoking, conduct disorder, depression, attempted suicide , panic disorder and social phobia. Results. Current smoking reduced platelet MAO activity in a significant and dose-related manner, with no evidence of lower MAO in ex-smokers or in non -smoking subjects with co-twins who smoked. Alcohol use and lifetime DSM-II I-R alcohol dependence history were not associated with MAO activity when s moking was taken into account. Depression, panic disorder and social phobia showed no significant associations with platelet MAO activity. Subjects wi th a history of serious attempts at suicide had low platelet MAO activity; but although the difference from controls was as great as the reduction ass ociated with smoking it was not significant after correction for smoking ef fects. Conclusions. Although synaptic MAO activity undoubtedly plays a role in psy chopathology, the concept that platelet MAO activity is a direct genetic ma rker of vulnerability to alcohol dependence cannot be sustained.