Fowler and colleagues have performed a series of important studies demonstrating decreases in MAO A and B activity in cigarette smokers using the PET tracers [11C]clorgyline (Fowler et al. 1996b) and [11C]L-deprenyl-D2 (Fowler et al. 1996a, 1998b), respectively. When compared to former smokers and nonsmokers, average reductions for current smokers are 30 and 40% for MAO A and B (Fowler et al. 2003a). These reductions were the result of chronic smoking behavior rather than a single administration of intravenous nicotine (Fowler et al. 1998a) or smoking a single cigarette (Fowler et al. 1999, 2000, 2005), and are less than those seen with antidepressant MAO inhibitors (Fowler et al. 1994, 1996b). MAO A levels were found to be reduced up to 50% in peripheral organs (heart, lungs, and kidneys) in smokers when compared to nonsmokers. Additionally, a human postmortem study of chronic smokers demonstrated a modest reduction in MAO A binding that did not reach statistical significance (Klimek et al. 2001). Peripheral MAO B is also reduced in cigarette smokers (Fowler et al. 2003b).
