[Answered]: By the 1980s it was known that mesolimbic…

By the 1980s it was known that mesolimbic dopamine projections were also a chief brain mech- anism of reward, including drug reward (Wise 1985, Fibiger & Phillips 1986, Wise & Bozarth 1987). The dominant theory at the time was that the role of dopamine in reward was to mediate the hedonic pleasure, or liking, produced by drugs, foods, sex, etc. (Wise 1985). Researchers also proposed that learned drug cues could evoke dopamine release that produced pleasure as a con- ditioned response and consequently increase incentive motivation to seek drugs and cause relapse (Stewart et al. 1984). The prevalence of the dopamine= pleasure hypothesis was probably the reason that early theorists (including ourselves) did not think at first that drug-induced mesolim- bic sensitization was relevant to addiction