On the neurobiology of drug-addiction diseases

The positive rewarding effect of addictive drugs as well as learning are actually considered to represent the main factors underlying the development of drug dependence. The rewarding effects of drugs are primarily mediated by dopamine and by mu-opiat receptors. Drugs like amphetamine or cocaine, that mediate their effects by enhancing dopamine-activity have psychomotor stimulant actions and enhance the incentive quality of stimuli emanating from the reward (preparatory phase) whereas drugs that act via mu-receptors (opioids) have psychomotor depressant actions and produce a state of well-being (consumatory phase). Assuming that addiction is the result of a pure learning process of the type of conditioned action (drug taking) does not explain sufficiently the phenomenon of drug craving. Thus additionally to learning, there seems to occur a process of sensitization that attributes incentive salience to mechanisms involved in drug addiction as well as to stimuli associated with addiction. Sensitization appears to be an irreversible process (point of no return).