Neurobiological research on addiction: What value has it added to the concept?

The initial goal of neurobiological research on addiction was to identify the neural mechanisms involved in the mediation and expression of addictive behavior. More recently, however, it has attributed causal roles to these mechanisms, as illustrated by the definition of addiction as a brain disease caused by chronic exposure to a drug. This concept carries a number of implications that can be assessed experimentally and clinically. None of these implications is borne out by the currently available evidence. The interactions of neuronal systems involved in addiction are also involved in adaptation to experience and environmental change. Much of the neurobiological research to date has not differentiated between causes of addiction, neuronal mechanisms that are activated by them, and risk factors that contribute to individual vulnerability. It has largely ignored the important experiential and environmental influences known to affect the prevalence of addiction in different populations or different times, and it has so far directed much less attention to other forms of addiction-like behavior that do not involve drugs. These failures are not inherent in neurobiological research but require reorientation of objectives, including more emphasis on the study of mechanisms by which environment and experience, including drug experience, can determine whether genetic risk factors are expressed or remain dormant and can direct neuroadaptive mechanisms toward alternative outcomes.