When positron emission tomography (PET) using a specific ligand for alpha 4 beta 2* (*other subunits may be present) nicotinic acetylcholine receptors was used to measure receptor occupancy, it was found that just 1 puff on a cigarette produced 50% occupancy and smoking a full cigarette more than 88% receptor occupancy. Desensitized nicotinic acetylcholine receptors have a higher affinity for acetylcholine than those in the state than can be activated. Nicotine upreguates nicotine binding sites and during nicotine withdrawal in humans, there is an increase in beta 2 nicotinic acetylcholine receptors. Since smokers maintain high nicotine blood levels throughout the day, this may be an attempt to maintain high occupancy and desensitization.