• E Andrew Townsend, Jennifer E Naylor, S Stevens Negus, Shelley R Edwards, Hina N Qureshi, Hunter W McLendon, Christopher R McCurdy, Coco N Kapanda, Jussara M do Carmo, Fernanda S da Silva, John E Hall, Kenneth J Sufka, and Kevin B Freeman.
• Graduate Program in Neuroscience, The University of Mississippi Medical Center, Jackson, MS, USA. s52drew@gmail.com.
• Psychopharmacology (Berl.). 2017 Sep 1; 234 (17): 2597-2605.

RationaleStrategies to reduce the misuse of mu opioid agonists are critically needed. Previous work has shown that kappa opioid agonists can diminish the abuse-related effects and augment the antinociceptive effects of mu agonists. However, use of traditional kappa agonists is limited by their dysphoric side effects.ObjectivesThe current study examined the effects of nalfurafine, a clinically available atypical kappa agonist, on the reinforcing, thermal antinociceptive, and respiratory-depressant effects of oxycodone in male rats.MethodsTo determine oxycodone/nalfurafine mixture proportions to be examined intravenously across procedures, a progressive ratio (PR) self-administration procedure compared the reinforcing effects of oxycodone (56 μg/kg/inj) available alone or as a mixture with co-administered nalfurafine (0.32, 1, or 3.2 μg/kg/inj), corresponding to oxycodone/nalfurafine proportions of 175:1, 56:1, and 18:1, respectively. Next, PR and thermal antinociception dose-effect functions were each determined for oxycodone, nalfurafine, and the same oxycodone/nalfurafine mixture proportions. Finally, the respiratory-depressant effects of equi-antinociceptive doses of oxycodone, nalfurafine, and the mixtures were compared.ResultsNalfurafine decreased the reinforcing effects of oxycodone, and the 18:1 mixture did not function as a reinforcer. Oxycodone and nalfurafine each produced dose-dependent antinociception, and the mixtures produced additive antinociception. In addition, antinociceptive doses of the 56:1 and 18:1 mixtures did not produce respiratory depression.ConclusionsThese results suggest that nalfurafine may augment the thermal antinociceptive effects while reducing the reinforcing and respiratory-depressant effects of oxycodone.

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