• Y-K Chen, J Lei, L Jin, Y-X Tan, and H-J You.
• Center for Biomedical Research on Pain (CBRP), College of Medicine, Xi'an Jiaotong University, China.
• Eur J Pain. 2013 Mar 1;17(3):336-46.

BackgroundNumerous studies focus on intramuscular (i.m.) injection of hypertonic saline-induced muscle pain and nociception. The spatio-temporal characteristics and dynamic variation of spinal neuronal activities elicited by i.m. hypertonic saline remain unknown.MethodsThe spatio-temporal variations of c-Fos expression in the lumbar spinal cord exposed to i.m. injection of 5.8% saline were investigated in male rats.ResultsAfter a unilateral i.m. 5.8% saline injection, c-Fos expression in dorsal horn of spinal L4-6 segments was significantly enhanced bilaterally (p < 0.05 and p < 0.001). These 5.8% saline-induced bilateral spinal Fos expression occurred rapidly 0.5 h following the i.m. injection, and reached the peak levels within 1 h, which declined gradually and returned to the control levels within 8 h. Compared with intact rats without i.m. insults, no significant influence of the spinalization on spinal c-Fos expression was found. However, the 5.8% saline-induced increases in Fos expression in intact and spinalized rats differed significantly. During muscle nociception, the c-Fos expression within the superficial layer (laminae I-II) and the deep layer (laminae V-VI) in spinalized rats were significantly lower and higher than that of in intact rats (p < 0.05 and p < 0.001). Fentanyl (20 μg/kg, intraperitoneal) completely attenuated the 5.8% saline intramuscularly induced increases in c-Fos expression in laminae III-VI (p < 0.001), but not laminae I-II.ConclusionsIt is suggested that spinal nociceptive neuronal activities in superficial and deep layers may differently be modulated by endogenous descending facilitation and inhibition, respectively.© 2012 European Federation of International Association for the Study of Pain Chapters.

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