ON THE NEUROBIOLOGY OF DRUG-ADDICTION DIS EASES

The positive rewarding effect of addictive drugs as well as learning a re actually considered to represent the main factors underlying the de velopment of drug dependence. The rewarding effects of drugs are prima rily mediated by dopamine and by mu-opiat receptors. Drugs like amphet amine or cocaine, that mediate their effects by enhancing dopamine-act ivity have psychomotor stimulant actions and enhance the incentive qua lity 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). Assumin g 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 salienc e to mechanisms involved in drug addiction as well as to stimuli assoc iated with addiction. Sensitization appears to be an irreversible proc ess (point of no return).