Abstract: Objective To explore the effects of fentanyl on expression of δ receptor and β-arrestin 1 in locus ceruleus of chronic morphinetolerant rats. Methods 40 male SD rats were randomly allocated to 5 groups with 8 cases in each group. Group NS received only normalsaline 1 ml/kg twice; Group M received morphine 10 mg/kg followed by normal saline 1 ml/kg; Groups MF₁, MF₂ and MF₃ receivedmorphine 10 mg/kg followed by fentanyl 3, 6, 12 μg/kg respectively. All animals were killed on the 9th day after measurement of painthreshold. Locus ceruleus was removed for determination of the expression of mRNA (RT-PCR) and protein (Western blotting) of δ receptorand β-arrestin 1. Results The expression of δ receptor and β-arrestin 1 was significantly decreased in Group M than in Group NS (P<0.01).There was no significant difference in the expression of δ receptor mRNA and protein in Groups MF₁, MF₂ and MF₃, and Group M (P>0.05).The expression of β-arrestin 1 mRNA and protein in Groups MF₂ and MF₃ significantly increased (P<0.05). Conclusion Fentayl with thedose of 6 and 12 μg/kg can dose-dependently increase the expression of β-arrestin 1 but δ receptor in locus ceruleus of chronic morphine tolerantrats.

Key words: morphine tolerance, fentanyl, locus ceruleus, delta-receptor, β-arrestin 1