Assessment of individual differences in the rewarding and adverse effects of morphine

A number of explanations have been presented for the ability of drugs of abuse to suppress consumption of saccharin solution. The one receiving the most attention is the reward comparison hypothesis, according to which, animals avoid the consumption of saccharin in anticipation of the more rewarding drug. Thus, the taste avoidance is a function of the rewarding effects of drugs. Accordingly, one would predict that there would be a direct relationship between the rewarding and suppressive effects of drugs of abuse. The present study directly assessed this relationship by examining the ability of morphine to induce both place preference and taste avoidance in the same animal. Although both preferences and aversions were acquired in individual subjects, there was no relationship between the two. As such, the results suggest that drugs of abuse have multiple stimulus effects, e.g., rewarding and aversive, that condition place preferences and taste aversions separately.