Effects of a newly synthesized κ-opioid receptor agonist, TRK-820, on the discriminative stimulus and rewarding effects of cocaine in rats

Rationale: We previously demonstrated that the prototypical κ-opioid receptor agonist U-50,488H did not affect the discriminative stimulus effects of cocaine, and the dose of U-50,488H which significantly induced aversive effects attenuated the rewarding effects of cocaine. Objectives: In the present study, the effects of a newly synthesized κ-opioid receptor agonist, TRK-820, on the discriminative stimulus and rewarding effects of cocaine were examined in rats. Methods: In the drug discrimination procedure, the effects of TRK-820 on the discriminative stimulus effects of cocaine were examined in rats that had been trained to discriminate between 10 mg/kg cocaine and saline. TRK-820-induced place preference or place aversion and the effects of TRK-820 on the cocaine (4 mg/kg)-induced place preference were examined using a conditioned place preference procedure in rats. Results: TRK-820 did not engender cocaine-like responding in rats trained to discriminate between 10 mg/kg cocaine and saline. In combination tests, low doses of TRK-820, which did not affect the response rate, significantly attenuated the discriminative stimulus effects of cocaine, and these effects of TRK-820 were reversed by a κ-opioid receptor antagonist, nor-BNI. In the conditioned place preference procedure, low doses of TRK-820, which did not affect the response rate in the drug discrimination, did not produce either place preference or place aversion, whereas, higher dose (80 µg/kg) of TRK-820 slightly but significantly induced a place aversion. Under these conditions, the cocaine-induced place preference was completely attenuated by low doses of TRK-820. These results may prompt further investigation of the effectiveness of the new κ-opioid receptor agonist TRK-820 as a novel pharmacotherapeutic compound for the treatment of cocaine addiction.