Effects of Kappa Opioid Receptor Agonists on Fentanyl vs. Food Choice in Male and Female Rats: Contingent vs. Non-Contingent Administration

RationaleStrategies are needed to decrease the abuse liability of mu opioid receptor (MOR) agonists. One strategy under consideration is to combine MOR agonists with kappa opioid receptor (KOR) agonists. ObjectivesThe effects of KOR-agonists (U50488, nalfurafine) on fentanyl-versus-food choice were compared under conditions where the KOR agonists were added to the self-administered fentanyl (contingent delivery) or administered as pretreatments (non-contingent delivery) in male and female rats. The effects of increasing and decreasing the magnitude of the alternative food reinforcer were also determined. MethodsRats were trained to respond under a concurrent schedule of fentanyl (0, 0.32-10 g/kg/infusion) and food reinforcement. In Experiment 1, U50488 and nalfurafine were co-administered with fentanyl as fixed-proportion mixtures (contingent administration). In Experiment 2, U50488 (1-10 mg/kg) and nalfurafine (3.2-32 g/kg) were administered as acute pretreatments (non-contingent administration). nor-BNI (32 mg/kg) was administered prior to contingent and non-contingent KOR-agonist treatment in Experiment 3. Experiment 4 evaluated the effects of increasing and decreasing the magnitude of the non-drug reinforcer. ResultsBoth U50488 and nalfurafine decreased fentanyl choice when administered contingently, demonstrating that KOR agonists punish opioid choice. Non-contingent U50488 and nalfurafine administration decreased rates of fentanyl and food self-administration without altering fentanyl choice. Both contingent and non-contingent U50488 and nalfurafine effects on fentanyl choice were attenuated by nor-BNI. Fentanyl choice was sensitive to increases and decreases in the magnitude of the non-drug reinforcer. ConclusionsThese results demonstrate that the effects of KOR agonists on fentanyl reinforcement are dependent upon the contingencies under which they are administered.