Motivational effects of opioids: evidence on the role of endorphins in mediating reward or aversion.

It has been suggested that endogenous peptides with opiate-like effects may contribute to the mediation of reward or aversion. One line of evidence relating to these hypotheses derives from studies of the motivational effects of opioids. The ability of opioid agonists and antagonists to serve as positively reinforcing or aversive stimuli is reviewed, with results compared across several different behavioural procedures. The results for rewarding effects are consistent and independent of procedure: in self-administration, conditioned place preference and conditioned taste preference studies, opioid agonists are consistently effective whereas antagonists are inactive. Results for indices of aversive effects are more difficult to interpret because they are, to some extent, dependent on the procedure used. Neither agonists nor antagonists seem able to support operant escape/avoidance conditioning. Agonists can support taste aversion and place aversion conditioning to some extent, whereas antagonists are clearly active in both procedures. The results provide some support for the involvement of enkephalins or endorphins in reward and aversion, but there are significant gaps and contradictions in the evidence.