Objective:To investigate the analgesic effect of neurotropin (NTP) on bone cancer pain (BCP) and its preliminary mechanisms in rats.Methods:(1) According to the random number table method, 72 Sprague-Dawley (SD) female rats were randomly and equally divided into 6 groups: normal control group, sham-operated group, BCP model group, BCP+low-dose NTP group, BCP+medium-dose NTP group, and BCP+high-dose NTP group ( n=12). The SHZ-88 breast cancer cells were inoculated into the tibias of rats in the latter 4 groups to establish BCP models. After 15-21 d of modeling, the rats of the latter 3 groups were intraperitoneally administered with 1.2, 2.4 and 3.6 unit NTP, respectively, once per d for 7 consecutive d. The mechanical withdrawal threshold (MWT) changes were measured in each group before BCP and 5, 7, 10, 14, 17, and 21 d after BCP. The number of 5-hydroxytryptamine 7 (5-HT7) positive cells and 5-HT7 protein expression in the ventrolateral periaqueductal gray (vlPAG) were detected by immunohistochemistry and Western blotting, respectively. (2) Twenty-four rats 21 d after BCP modeling were randomly divided into BCP group, BCP+high-dose NTP group (BCP rats with intraperitoneal injection of 3.6 unit NTP), BCP+NTP+SB-269970 group (BCP rats with pretreatment of specific 5-HT7 receptor antagonist SB-269970 in the vlPAG for 30 min, and then, with intraperitoneal injection of 3.6 unit NTP, n=8). The MWT changes were measured in each group before NTP and 15, 30, 45, 60 min after NTP. Results:(1) Seventeen and 21 d after modeling, the MWT values of the modeled hind limb of rats in BCP+low-dose NTP group, BCP+medium-dose NTP group, and BCP+high-dose NTP group were significantly higher than those in BCP group; those in the BCP+high-dose NTP group were significantly higher than those in BCP+low-dose NTP group and BCP+medium-dose NTP group; those in BCP+medium-dose NTP group were statistically higher than those in the BCP+low-dose NTP group ( P<0.05). Twenty-one d after modeling, the number of 5-HT7 receptor positive cells and protein expression in the vlPAG of rats in the BCP+low-dose NTP group, BCP+medium-dose NTP group, and BCP+high-dose NTP group were significantly larger/higher than those in BCP group; those in the BCP+high-dose NTP group were significantly larger/higher than those in BCP+low-dose NTP group and BCP+medium-dose NTP group; those in BCP+medium-dose NTP group were statistically larger/higher than those in the BCP+low-dose NTP group ( P<0.05). (2) At 15, 30, 45 and 60 min after injection of NTP, the MWT values of the modeled hind limb of rats in the BCP+high-dose NTP group and BCP+NTP+SB-269970 group increased gradually, enjoying statistical significance as compared with those in the BCP group at the same time point ( P<0.05); however, the MWT values of the BCP+NTP+SB-269970 group were significantly lower as compared with those in the BCP+high-dose NTP group at the same time point ( P<0.05). Conclusion:The activation of 5-HT7 receptor in the vlPAG is involved in the analgesic effect of NTP on BCP in rats.