Role of the opioid system in incentive downshift situations

Previous research has shown that opioid blockage enhances consummatory successive negative contrast (cSNC)—a suppression of consummatory behavior following a downshift from 32% to 4% sucrose solution. In Experiment 1, administration of the nonselective opioid receptor antagonist naloxone (2 mg/kg, ip) distorted the comparison between expected and received incentives. The results of Experiment 2 discarded the alternative that naloxone enhances cSNC by inducing a conditioned taste aversion. The results of Experiments 3a–3c provided no evidence that opioid administration after the first downshift trial modulated subsequent consummatory performance. The opioids tested included naloxone (2 mg/kg, ip), the δ-opioid receptor selective antagonist naltrindole (1 mg/kg, ip), and the δ-opioid receptor selective agonist DPDPE (24 μg/kg, ip). The selected doses have proven in earlier experiments to be effective when administered before training. Experiments 4–5 failed to uncover any effects of posttraining opioid blockage with naloxone in an appetitive extinction task (autoshaping with lever-food pairings). These results add to our previous understanding of opioid function in situations involving incentive downshifts, suggesting a role in the comparison process that triggers cSNC, but no apparent function in memory consolidation related to the downshift event.