Abstract
Environmental cues that have been associated with drug-taking can evoke drug-craving and drug-seeking and drive relapse. Using the New Response Acquisition procedure, we evaluated the extent to which activation of delta opioid receptors (DORs) changes responding for cocaine-associated stimuli. We hypothesized that activation of DORs, either directly via agonists or indirectly via protected concentrations of endogenous enkephalin peptides, would increase the conditioned reinforcing effects of cues. First, animals undergo Pavlovian conditioning during which rats received 5 infusions of cocaine (0.32 mg/kg/inf) and either paired or unpaired presentations of a stimulus (light + tone) per day for 10 days. Next, nosepokes were added to the operant chamber and rats were allowed to respond for presentations of cocaine-associated stimuli (acquisition). Consistent with previous findings, animals assigned to paired Pavlovian conditioning emitted more responses for cue presentations than animals assigned to the unpaired control. Interestingly, acute administration of SNC80 (DOR agonist; 3.2 mg/kg s.c.) on acquisition session 4 led to robust increases in responding for the cocaine-paired cues in the paired, but also increased responding for cues in rats assigned to unpaired and saline control groups. Further, the enkephalinase inhibitor RB101 (10 mg/kg intravenous), which maintains extracellular concentrations of enkephalins, increased active responding in a DOR-dependent manner. These data suggest that activation at DORs influences behaviors maintained by cues and sheds light on the neurobiology underlying the conditioned reinforcing effects of drug-associated stimuli. SIGNIFICANCE STATEMENT: We used a more rigorous test of conditioned reinforcement to show that activation of delta opioid receptors increases the reinforcing effects of cocaine-paired cues, depending on conditioning history, which implicates the delta opioid receptor system as a target to reduce relapse.