Many smokers express the desire to stop, and although many receive extensive treatments, fewer than half succeed in stopping long term. It has long been known that nicotine is highly addictive, but only recently have the specific mechanisms causing addiction been identified. Researchers at the University of California, Los Angeles, led by Dr. Arthur L. Brody, used brain imaging to study the brain-receptor nicotine occupation level needed to avoid symptoms of withdrawal. The results were published in the August issue of Archives of General Psychiatry. They conducted tests after subjects had refrained from tobacco use for two days, taking PET images using a radiotracer. The number of cigarettes the patients consumed varied. Nicotine levels and withdrawal symptoms were monitored throughout the study. In subjects who smoked during the study, nicotine replaced the radiotracer in amounts dependent on the level consumed. The brain scans specifically showed that the level of nicotine binding was highest in the thalamus, the brain stem and the cerebellum. The images showed that typical daily smokers must maintain 88 to 95 percent brain receptor occupancy to avoid cravings. Two and a half hours after some trial subjects were given one cigarette, their receptor occupancy was still at 90 percent, but they felt a presmoking craving. Subjects given one or more cigarettes felt relief of cravings, but those given less than one cigarette during their scans did not have a complete abatement of their cravings. The study’s findings were consistent with the idea that smokers desensitize their nicotine receptors and continue to smoke to avoid having receptors that are free of nicotine.