[Role of cannabinoid 2 receptor in the development of bone cancer pain]

To explore the effects of cannabinoid 2 receptor (CB2) in the development of bone cancer pain in mice.

A total of 84 mice (C3H/HeJ) were randomly divided into 4 groups:tumor group (Group T, n = 30), medication administration group (Group J, n = 12), vehicle group (Group D, n = 12) and sham group (Group S, n = 30). And 2 × 10(5) osteolytic NCTC2472 cells in α-MEM were injected into medullary cavity of right distal femur to induce bone cancer pain in a murine model while sham mice received an injection of only α-MEM. All mice were tested for pain-related behaviors at pre-inoculation and at Days 5, 7, 10 and 14 post-inoculation. The tests included paw withdrawal mechanical threshold (PWMT) and paw withdrawal thermal latency (PWTL). Group J and Group D were injected intrathecally with 2 µg JWH015 dissolved in 4% DMSO and only 4% DMSO respectively in a 5 µl. volume. Pain behavior tests were performed before and at 1, 6, 24, 48 and 72 h after an intrathecal injection. Lumbar intumescentia of mice in each group were harvested to examine the expression level of CB2 by Western blot after pain behavior tests at Days 5, 7, 10 and 14 post-inoculation and 12 h after an intrathecal injection.

(1) Pain behavior tests:Mechanical allodynia appeared at Day 7 post-inoculation. The value of PWMT was (1.27 ± 0.28) g (P < 0.05) and it declined gradually to (0.53 ± 0.20) g at Day 14. The threshold of mechanical hyperalgesia increased to (1.00 ± 0.20) g at 6 h after an intrathecal injection of JWH015, peaked at (1.40 ± 0.39) g at 12 h, became alleviated after 48 h and recovered to the pre-dosing levels at 72 h. Thermal hyperalgesia appeared at Day 10 post-inoculation. The value of PWTL was (16.9 ± 0.4) s (P < 0.05) at Day 10 and declined to (11.5 ± 0.7) s at Day 14 post-inoculation. The threshold of thermal hyperalgesia increased to (15.7 ± 1.9) g at 6 h after an intrathecal injection of JWH015, peaked at (18.6 ± 2.3) g at 12 h, became alleviated after 48 h and recovered to the pre-dosing levels at 72 h. (2) Western blot: From Day 5 post-inoculation, the ratio of CB2/β-actin increased gradually. Compared with the ratio of 0.190 ± 0.010 at Day 5 post-inoculation, the ratio of CB2/β-actin increased to 0.660 ± 0.010 at Day 14 post-inoculation (P < 0.05); compared with the ratio of 0.903 ± 0.006 in group D at 12 h after an intrathecal injection of JWH015, the ratio of CB2/β-actin 0.510 ± 0.010 significantly decreased (P < 0.05).

The cannabinoid 2 receptor plays an important role in the formation of bone cancer pain.