Neurobiology of Addiction

Background

Drug addiction is a process defined by uncontrollable, compulsive drug seeking and use, which persists despite negative health and social consequences. Although addiction is complex, there is consensus on several established principles. First, while sustained exposure to drugs of abuse is a necessary prerequisite for drug addiction, addiction is also influenced by biological and environmental factors, as well as the developmental stage of the affected individual. Second, during the initial stages of drug use, individuals often choose to use drugs to replicate an originally pleasurable experience with the substance. However, as addiction progresses, fundamental motivational shifts take place whereby the drug is no longer consumed to provide pleasure to the consumer but rather to relieve drug craving and withdrawal-associated distress. Third, there is an accumulating amount of evidence regarding the cumulative impact of chronic drug exposure on the human brain.

Although different types of drugs have different pharmacological and pharmacokineuc properties, their habit-forming effects appear to involve a common denominator, namely, an impact on brain mechanisms involved with reward and motivation. These regions evolved to aid the organism in the search for, and motor approach of, natural rewards, such as food and sexual contact, which are essential to survival and reproduction. Evidence increasingly suggests that drug addiction is a process through which drugs of abuse "hijack" the neural circuitry of reward and motivation and produce a chronic state of drug seeking, which continues even in the face of strong negative consequences of use.

Neuroimaging Methods Used in Addiction Research

Recently the scientific community has witnessed a revolution in the field of human brain imaging. New technology, specifically positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and single photon emission computer tomography (SPECT), allow imaging of brain activity in vivo among human drug users. These advances provide a unique opportunity to understand the processes of addiction through the ability to measure changes in the function of the human brain, including the mapping of molecular kinetics and enzyme reactions over reasonably short time intervals. At the same time, spatially clear neural networks are displayed. Thus, imaging techniques can track neurochemical and neuroanatomical changes that occur due to neural plasticity and the effects of drugs.