The Nociceptin Receptor (NOP) Agonist AT-312 Blocks Acquisition of Morphine- and Cocaine-Induced Conditioned Place Preference in Mice

Abstract

Treatment of drug addiction remains an unmet medical need due to the dearth of approved pharmacotherapies. There are no approved treatments for cocaine addiction, whereas the current opioid crisis has revealed the stark reality of the limited options to treat prescription and illicit opioid abuse. Preclinical studies in rodents and nonhuman primates have shown that orphanin FQ/nociceptin (N/OFQ), the endogenous ligand for the nociceptin opioid receptor (NOP) reduces the rewarding effects of several abused substances, including opioids, psychostimulants and alcohol. A few nonpeptide small-molecule NOP agonists have also shown efficacy in attenuating the rewarding effects of various abused drugs. We previously demonstrated that a high affinity small-molecule NOP agonist AT-312 selectively reduced the rewarding effects of ethanol in the conditioned place preference paradigm in mice. In the present study, we examined if AT-312 (3 mg/kg, i.p. or s.c. respectively), would alter the rewarding action of morphine (7.5 mg/kg, s.c.) or cocaine (15 mg/kg, i.p.). The effect of AT-312 on morphine- and cocaine-induced motor stimulation was also assessed on the conditioning days. The role of the NOP receptor in the effects of AT-312 was further confirmed by conducting the place conditioning experiments in NOP knockout mice and compared to their wild-type controls. Our results showed that AT-312 significantly reduced the acquisition of morphine and cocaine CPP in wild-type mice but not in mice lacking NOP receptors. AT-312 also suppressed morphine-induced and completely abolished cocaine-induced motor stimulation in NOP wild-type mice, but not in NOP knockout mice. These results show that small-molecule NOP receptor agonists have promising efficacy for attenuating the rewarding effects of morphine and cocaine, and may have potential as pharmacotherapy for opioid and psychostimulant addiction or for treating polydrug addiction.

Keywords: AT-312; NOP agonist; NOP receptor knockout; addiction pharmacotherapy; cocaine; conditioned place preference; morphine; polydrug addiction.

Figure 1

Effects of AT-312 on morphine-induced CPP in mice lacking NOP and their wild-type littermates/controls. Mice (n = 6 mice per treatment for each genotype) were tested for baseline place preference on day 1, conditioned with morphine (7.5 mg/kg, s.c.) in the presence and absence of AT-312 (3 mg/kg, i.p.) on days 2–4 and then tested for CPP on day 5. Vehicle or AT-312 was given 5 min before morphine on each conditioning day to wild-type (Upper) and knockout (Lower) mice. *P < 0.05; **P < 0.01; ****P < 0.0001, a significant increase in the amount of time that mice spent in the drug-paired chamber (DPCh) compared to vehicle-paired chamber (VPCh) on this day.

Figure 2

Effects of AT-312 on morphine-induced motor stimulation during the conditioning days in the CPP chambers in mice lacking NOP and their wild-type littermates/controls. On each conditioning day, wild-type (Upper) and NOP knockout (Lower) mice were treated with vehicle or AT-312 (3 mg/kg; i.p.; n = 6 mice per treatment for each genotype) 5 min before morphine (7.5 mg/kg, s.c.) and distance traveled by mice in the morphine-paired chamber (DPCh) was recorded for 60 min each day. Distance traveled by the mice in the saline-paired chamber (VPCh) was also recorded, in which mice were injected with vehicle and 5 min later with saline. *P < 0.05, **P < 0.01 and ****P < 0.0001, a significant increase in distance traveled by mice in the DPCh vs. their respective VPCh on this day.

Figure 3

Effect of AT-312 on cocaine-induced CPP in wild-type (Upper; n = 13 mice per treatment) or knockout (Lower; n = 10 mice per treatment) mice. Data are mean (±SEM) of the amount of time that mice spent in the vehicle-paired chamber (VPCh) and drug-paired chamber (DPCh) on the preconditioning (D1) and postconditioning (D5) test days. **P < 0.01; ****P < 0.0001, a significant increase in the amount of time that mice spent in the DPCh vs. its respective VPCh on D5.

Figure 4

Effects of AT-312 on cocaine-induced motor stimulation during conditioning in wild-type and knockout mice. Motor activity was measured in a subset of animals (n = 6 mice per genotype for each treatment) and the experiment was repeated three times (n = 2 mice of each genotype per treatment for each cohort). On each conditioning day, wild-type (Upper) and NOP knockout (Lower) mice were treated with vehicle or AT-312 (3 mg/kg) 5 min before cocaine (15 mg/kg, i.p.) and distance traveled by mice in the drug-paired chamber (DPCh) was recorded for 30 min each day. The distance traveled by mice in the saline-paired chamber (VPCh) was also recorded, in which mice were injected with vehicle and 5 min later with saline. *P < 0.05, **P < 0.01 and ****P < 0.0001, a significant increase in distance traveled by mice in the DPCh vs. their respective VPCh on this day. ⁺⁺P < 0.01 and ⁺⁺⁺⁺P < 0.0001 vs. DPCh in mice treated with AT-312 followed by cocaine on this day.