Abstract
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 (1 mg·kg−1) (FC1); formalin test + clonidine (10 mg·kg−1) (FC10); saline (S); clonidine (1 mg·kg−1) (C1); and clonidine (10 mg·kg−1) (C10). Normal saline or clonidine was injected intraperitoneally 30 min 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 90 min in 6 other rats as follows: clonidine (1 mg·kg−1) (n = 3) and clonidine (10 mg·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.
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Fukuda, T., Furukawa, H., Hisano, S. et al. 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. J Anesth 20, 279–283 (2006). https://doi.org/10.1007/s00540-006-0426-5
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DOI: https://doi.org/10.1007/s00540-006-0426-5