RESUMEN
RATIONALE: Agonists of the kappa opioid receptor (KOR) have been shown to block the rewarding effects of drugs of abuse, but with negative side effects. The antipruritic drug nalfurafine, approved in Japan in 2009, is a potent, selective KOR agonist that does not cause significant side effects in humans. Nalfurafine has not been extensively tested for its effect on drug reward and reinforcement in preclinical models. OBJECTIVES: The goal of this study was to compare the effects of nalfurafine and a reference KOR agonist for a variety of KOR-mediated endpoints in male C57BL6 mice. Specifically, we aimed to evaluate the "therapeutic window"-doses of agonists lower than those eliciting negative side effects, while still effective for desired therapeutic effects. METHODS: In this study, several low doses of nalfurafine and U50,488 were tested for serum prolactin release, rotarod-mediated sedation, and place-conditioning in male C57BL6 mice. These agonists were also tested for effects on intravenous cocaine self-administration, both on an FR1 schedule and on a progressive ratio schedule for 0.5 mg/kg/infusion cocaine. RESULTS: Serum prolactin levels increased following doses of both nalfurafine (3 µg/kg and 10 µg/kg) and U50,488 (3 mg/kg). These doses did not cause sedation in the rotarod assay or aversion in a place-conditioning assay, but blocked conditioned place preference for cocaine. Immediate pretreatment of mice with 10 µg/kg nalfurafine and 3 mg/kg U50,488, however, potentiated cocaine self-administration. Further 10 µg/kg nalfurafine was also observed to potentiate cocaine-seeking behavior as demonstrated by increased progressive ratio break point. CONCLUSIONS: Both nalfurafine and U50,488 showed a separation of negative side effects and the modulation of cocaine reward, suggesting this effect of KOR agonists at low doses may be characteristic of the KOR system in general. At higher doses, nalfurafine had similar effects to traditional KOR agonists like U50,488, indicating that its relative potency, rather than differences in KOR signaling, may be responsible for its unique effects in humans.