Systemic clonidine activates neurons of the dorsal horn, but not the locus ceruleus (A6) or the A7 area, after a formalin test: The importance of the dorsal horn in the antinociceptive effects of clonidine

Purpose: In order to clarify the principal site for the antinociceptive effects of clonidine, we investigated the nociceptive behavior and neural activity (c-fos staining) of the dorsal horn (DH), locus ceruleus (LC), and A7 area after a formalin test in normal saline- or clonidine-injected rats. Methods: Thirty-six rats were divided into 6 groups as follows: formalin test + saline (FS); formalin test + clonidine (1mg·kg^-1) (FC1); formalin test + clonidine (10mg·kg^-1) (FC10); saline (S); clonidine (1mg·kg^-1) (C1); and clonidine (10mg·kg^-1) (C10). Normal saline or clonidine was injected intraperitoneally 30min before the formalin test. In the FS, FC1, and FC10 groups, 10% formalin was injected into the left rear paw. All rats were killed 2.5 h after normal saline or clonidine injection. Sections of the lumbar spinal cord, LC, and A7 area were processed for c-fos immunohistochemistry using the avidin-biotin peroxidase complex method. To evaluate the sedative effects of clonidine, we investigated the loss of righting reflex (LORR) for 90min in 6 other rats as follows: clonidine (1mg·kg^-1) (n = 3) and clonidine (10mg·kg^-1) (n = 3). Results: The FC10 group showed fewer nociceptive behaviors and higher c-fos expression in the DH, but not in the A7 area, as well as lower c-fos expression in the LC than rats in the FS and FC1 groups (P < 0.05). The C10 group showed lower c-fos expression in the LC than that of rats in the S and C1 groups (P < 0.05). No rats exhibited LORR. Conclusion: The antinociceptive effects of clonidine might be mediated primarily by neural activity in the DH.