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Melatonin, a neurohormone that binds to two G protein-coupled receptors MT 1 and MT 2, is involved in pain regulation, but the distinct role of each receptor has yet to be defined. We characterized the nociceptive responses of mice with genetic inactivation of melatonin MT 1 (MT 1 -/- ), or MT 2 (MT 2 -/- ), or both MT 1 /MT 2 (MT 1 -/- /MT 2 -/- ) receptors in the hot plate test (HPT), and the formalin test (FT). In HPT and FT, MT 1 -/- display no differences compared to their wild-type littermates (CTL), whereas both MT 2 -/- and MT 1 -/- /MT 2 -/- mice showed a reduced thermal sensitivity and a decreased tonic nocifensive behavior during phase 2 of the FT in the light phase. The MT 2 partial agonist UCM924 induced an antinociceptive effect in MT 1 -/- but not in MT 2 -/- and MT 1 -/- /MT 2 -/- mice. Also, the competitive opioid antagonist naloxone had no effects in CTL, whereas it induced a decrease of nociceptive thresholds in MT 2 -/- mice. Our results show that the genetic inactivation of melatonin MT 2 , but not MT 1 receptors, produces a distinct effect on nociceptive threshold, suggesting that the melatonin MT 2 receptor subtype is selectively involved in the regulation of pain responses., (© 2020 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Background and Purpose: Nociceptin/orphanin FQ (N/OFQ) peptide and its cognate receptor (NOP) are widely expressed in mesolimbic brain regions where they play an important role in modulating reward and motivation. Early evidence suggested that NOP receptor activation attenuates the rewarding effects of drugs of abuse, including alcohol. However, emerging data indicate that NOP receptor blockade also effectively attenuates alcohol drinking and relapse. To advance our understanding of the role of the N/OFQ-NOP receptor system in alcohol abuse, we examined the effect of NOP receptor blockade on voluntary alcohol drinking at the neurocircuitry level., Experimental Approach: Using male and female genetically selected alcohol-preferring Marchigian Sardinian (msP) rats, we initially evaluated the effects of the selective NOP receptor antagonist LY2817412 (3, 10, and 30 mg·kg -1 , p.o.) on alcohol consumption in a two-bottle free-choice paradigm. We then microinjected LY2817412 (3 and 6 μg·μl -1 per rat) in the central nucleus of the amygdala (CeA), ventral tegmental area (VTA), and nucleus accumbens (NAc)., Key Results: Peripheral LY2817412 administration dose-dependently and selectively reduced voluntary alcohol intake in male and female msP rats. Central injections of LY2817412 markedly attenuated voluntary alcohol intake in both sexes following administration in the CeA and VTA but not in the NAc., Conclusion and Implications: The present results revealed that the CeA and VTA are neuroanatomical substrates that mediate the effects of NOP receptor antagonism on alcohol consumption. Overall, our findings support the potential of NOP receptor antagonism as a treatment strategy to attenuate alcohol use and addiction., (© 2019 The British Pharmacological Society.)