The influence of the new enkephalin derivative, cyclo[Nε,Nβ-carbonyl-d-Lys2,Dap5] enkephalinamide (cUENK6), on reinstatement of ethanol-induced conditioned place preference in rats

• The expression of ethanol CPP was reversed by DORs and MORs antagonist.
• The cUENK6 reinstated the ethanol-induced CPP.
• The effect of cUENK6 was attenuated by DORs and with lower extend by MORs antagonist.
• Rather DORs than MORs may be involved in the reinstatement induced by cUENK6.

The aim of the present study was to determine whether a new cyclic analog of enkephalin, cyclo[Nε,Nβ-carbonyl-d-Lys2,Dap5] enkephalinamide (cUENK6), a preferential μ-(MORs), and, to a lower extent, a δ-opioid receptor (DORs) agonist in vitro, could reinstate ethanol-induced conditioned place preference (CPP). In our work, male Wistar rats were first conditioned either with ethanol (10% w/v, 0.5 g/kg, intraperitoneally (i.p.)) or 0.9% NaCl in a biased CPP procedure. The intracerebroventricular (i.c.v.) administration of DORs antagonist (naltrindole, 2.5 and 5 nmol) or MORs antagonist (β-funaltrexamine, 5 and 10 nmol), but not the κ opioid receptor (KORs) antagonist (norbinaltorphimine, 5 and 10 nmol) was then administered and inhibited the expression of ethanol-induced CPP. After the extinction session, i.c.v. administration of cUENK6 at the dose of 0.125, 0.25 and 0.5 nmol occurred, and was found to reinstate the ethanol-induced CPP similar to that of the priming injection of ethanol. However, the reinstated effect of cUENK6 (0.25 nmol) was strongly abolished by administration of naltrindole and, to lesser extent, by β-funaltrexamine. Furthermore, the preferential MORs agonist-morphine (13 nmol, i.c.v.) and the DORs agonist-[Leu5]-enkephalin (2.7 and 5.4 nmol, i.c.v.) also reinstated the ethanol-induced CPP. cUENK6 given alone at the dose of 0.25 nmol before the testing phase had no effect in animals that received 0.9% NaCl during the conditioning phase and also did not influence their locomotor activity. These data suggest that the effects of cUENK6 did not have an impact on the results obtained in the reinstatement procedure of CPP. Overall, the data support the idea that both MORs and DORs are normally involved in the expression and reinstatement of ethanol conditioned seeking behavior — as indexed by CPP in rats.