Objective To investigate the relationship between autophagy in spinal dorsal horn and development of morphine tolerance in rats.Methods Twenty-four healthy male Sprague-Dawley rats,in which intrathecal (IT) catheters were successfully placed,were randomly divided into 3 groups (n =8 each):control group (group C),morphine tolerance group (group M) and morphine + rapamycin as a reinforcing agent for autophagy group (group MR).Morphine tolerance was induced with IT morphine 20 μg twice a day for 7 consecutive days.While the equal volume of normal saline was given in group C.In addition,rapamycin 2.3μg was injected intrathecally at the second injection of morphine on 3rd day lasting for 3 consecutive days in group MR.Mechanical withdrawal threshold (MWT) to yon Frey filament stimulation was measured before IT injection and 30 min after the second IT injection on 1st,3rd,5th and 7th days.The rats were sacrificed 1 h after the last MWT measurement and the L4-6 segment of the spinal cord was removed for determination of the total mammalian target of rapamycin (mTOR) and phosphorylated mTOR(p-mTOR) and autophagy marker protein LC3 Ⅱ expression in spinal dorsal horn by Western blot.The percentage of p-mTOR expression in total mTOR expression was considered as reflection of the activity.Results MWT was gradually decreased with the prolongation of time of IT injection (P ＜ 0.05).Compared with group C,MWT was significantly increased during IT injection,mTOR activity was decreased and LC3 Ⅱ expression was up-regulated in groups M and MR (P ＜ 0.05).Compared with group M,MWT was significandy increased on 3rd,5th and 7th days after IT injection,mTOR activity was decreased and LC3 Ⅱ expression was up-regulated in group MR (P ＜ 0.05).Conclusion Increased autophagy in spinal dorsal horn is the regulatory mechanism of the body during the development of morphine tolerance in rats and can delay the development of morphine tolerance.