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Background

Nociceptin/orphanin FQ, the endogenous peptide agonist for the opioid receptor-like receptor (also known as NOP or the nociceptin receptor), has been shown to block the acquisition and expression of ethanol (EtOH)-induced conditioned place preference (CPP). Here, we report the characterization of a novel small-molecule NOP ligand AT-312 (1-(1-((cis)-4-isopropylcyclohexyl)piperidin-4-yl)-1H-indol-2-yl)methanol) in receptor binding and GTPγS functional assays in vitro. We then investigated the effect of AT-312 on the rewarding action of EtOH in mice using the CPP paradigm. Further, using mice lacking the NOP receptor and their wild-type controls, we also examined the involvement of NOP in the effect of AT-312. Motivational effects of AT-312 alone were also assessed in the CPP paradigm.

Methods

Female mice lacking NOP and/or their wild-type controls received conditioning in the presence or absence of the NOP agonist [AT-312 (1, 3, and 10 mg/kg) or the control NOP agonist SCH221510 (10 mg/kg)] followed by saline/EtOH for 3 consecutive days (twice daily) and tested for CPP in a drug-free state on the next day.

Results

Our in vitro data showed that AT-312 is a high-affinity, selective NOP full agonist with 17-fold selectivity over the mu opioid receptor and >200-fold selectivity over the kappa opioid receptor. The results of our in vivo studies showed that AT-312 reduced EtOH CPP at the lowest dose (1 mg/kg) tested but completely abolished EtOH CPP at higher doses (3 or 10 mg/kg) compared to their vehicle-treated control group. AT-312 (3 mg/kg) did not alter EtOH-induced CPP in mice lacking NOP, confirming that AT-312 reduced EtOH CPP through its action at the NOP receptor. AT-312 (3 mg/kg) did not induce reward or aversion when administered alone, showing that the novel small-molecule NOP agonist shows efficacy in blocking EtOH-induced CPP via the NOP receptor.

Conclusions

Together, these data suggest that small-molecule NOP agonists have the potential to reduce alcohol reward and may be promising as medications to treat alcohol addiction.