Abstract
Although central sensitization is assumed to be an essential factor causing chronic low back pain (CLBP), the underlying spinal mechanisms are poorly understood. In the present study, we recorded activities of spinal dorsal horn neurons receiving input from low back structures (defined as "LB neurons") in rats, and examined the chronic sensitization effect of complete Freund's adjuvant (CFA) on these neurons. Extracellular recording was performed in the spinal segments L2 and L3 in non-treated control animals (CTR), and animals after intramuscular injection of CFA into the multifidus (MF) muscle bilaterally 6 or 12 days prior to the experiment. Mechanical withdrawal threshold following stimulation of the MF significantly decreased 6 and 12 days after CFA injection, indicating mechanical allodynia. The proportion of LB neurons 6 days after CFA was significantly higher than that in the CTR in the segments L2 and L3 (9/76 neurons in the CTR vs. 16/61 neurons in the CFA, p < 0.05, Fisher's exact test). In addition, LB neurons which had convergent inputs from different tissues and regions of the body were more frequent in the CFA group than in the CTR (6/9 neurons in the CTR vs. 16/16 neurons in the CFA, p < 0.05, Fisher's exact test). These results show that the responsiveness of LB neurons is increased during a pathologic alteration of the MF. The findings may explain the allodynia of some cases of chronic low back pain in patients. [J Physiol Sci. 2008;58 Suppl:S162]
