Use of PET to study addiction in humans

The process through which repeated use of some psychoactive drugs that generate addiction has elicited the curiosity of many researchers. Despite widespread research in this area, there is very little knowledge with respect to the chemical and functional changes that occur in the human brain during addiction. Addiction to drugs has been classified as either physical or psychological (Seevers, 1972). The term, physical addiction, was used to describe a condition produced by repeated drug use and characterized by physical changes that perpetuate the use of the drug. Examples of drugs which produce physical addiction include the opiates, barbiturates, and alcohol. Discontinuation of drug use triggers an abstinence syndrome with signs and symptoms that are usually opposite to those produced by the drug. In contrast, the term psychological addiction, was used to describe those drugs that led to a conditioned pattern of drug-seeking behavior secondary to the psychologically rewarding effects of the drug. For example, stimulants such as marijuana and psychedelic drugs produce this condition. Discontinuation of the use of these drugs is not associated with physical symptoms of withdrawal 282(Seevers, 1972). However, the distinction between psychological and physical addiction is becoming blurred as research uncovers chemical changes in the brain of animals chronically exposed to drugs initially thought to induce only psychological dependence (Hansen et al., 1989; Zahniser et al., 1988; Post, 1987). It is also becoming apparent that the mechanisms through which a drug produces compulsive use do not necessarily account for the processes of physical withdrawal (Snyder, 1984).