The role of chemokine (C-C motif) ligand 2 in morphine tolerance to analgesia in spinal cord of rats

Abstract

Objective To investigate the role of spinal chemokine (C-C motif) ligand 2 (CCL2) in morphine tolerance to analgesia.Methods Male Sprague-Dawley (SD)rats with successful intrathecal catheter were randomly divided into ten groups by means of random number table (n=6):IgG plus morphine group (IgG+MOR),CCL2 neutralizine antibody plus morphine group (CCL2 neutralizine antibody +MOR),IgG plus saline group (IgG +NS),CCL2 neutralizine antibody plus saline group (CCL2 neutralizine antibody +NS),IgG4 plus morphine group (IgG4 +MOR),CCL2 neutralizine antibody4 plus morphine group (CCL2 neutralizine antibody4+MOR),IgG4 plus saline group (IgG4+NS),IgG8 plus morphine group (IgG8+MOR),CCL2 neutralizine antibody8 plus morphine group (CCL2 neutralizine antibody8+MOR),IgG8 plus saline group (IgG8+NS).A rat model of morphine tolerance to analgesia was induced by intrathecal injection of morphine 15 μg once daily for 7 consecutive days.The role of CCL2 in morphine antinociceptive tolerance is explored by tail flick latency (TFL) and mechanical withdrawal threshold (MWT).ELISA was used to evaluate the change in spinal CCL2 immunoreactivity.Results On days 3,5 and 7 after chronic morphine,percent of maximal possible potential effect by TFL (％MPETFL) and percent of maximal possible potential effect by MWT (％MPEMT) were significantly increased in the CCL2 neutralizing antibody plus morphine group [％MPETFL:3 d (65±6)％,5 d (47±4)％,7 d (42± 4)％.％MPEMWT:3 d (65±5)％,5 d (46±4)％,7 d (41±4)％] versus the IgG plus morphine group [％ MPETFL:3 d (55±6)％,5 d (27±4)％,7d (17±3)％.％MPEMT:3d (54±6)％,5d (27±4)％,7d (17±4)％](P＜0.05,P＜0.01),％MPEFL and ％MPEMWT were significantly up-regulated in the CCL2 neutralizine antibody4 plus morphine group [％MPETFL:5 d (46±4)％,7 d (43±4)％,％ MPEMWT:5 d (45±4)％,7 d (44±4)％]versus the IgG4 plus morphine group [％MPETFL:5 d (27±4)％,7 d (17±3)％,％MPEMT:5 d (27±4)％,7 d (17±3)％](P＜0.01).On days 9 and 11 after chronic morphine,％MPETFL and ％MPEMT were significantly up regulated in the CCL2 neutralizine antibody 8 plus morphine group [％MPETFL:9 d (26±4)％,11 d (36±4)％,％MPEMWT:9 d (26± 4)％,11 d (35±4)％] versus the IgG8 plus morphine group [％ MPETFL:9 d (16±3)％,1 1 d (15±4)％,％MPEMWT:9 d (16±3)％,11 d (14±3)％] (P＜0.05,P＜0.01).On day 7 after chronic morphine,CCL2 expression in the IgG plus morphine group compared with the IgG plus saline group was significantly up-regulated,while CCL2 expression in the CCL2 neutralizine antibody plus morphine group compared with IgG plus MOR group was significantly down-regulated.Conclusions CCL2 in the spinal cord is involved in the development and maintenance of morphine tolerance to analgesia.Inhibition of CCL2 may provide a new therapy for morphine tolerance to analgesia. Key words: Morphine antinociceptive tolerance; Hemokine (C-Cmotif) ligand 2; Spinal cord