Botulinum Toxin Type A Reduces Hyperalgesia and TRPV1 Expression in Rats with Neuropathic Pain

Objective. We aim to determine the effects of botulinum toxin type A (BTX-A) on the thresholds of pain and the expression of transient receptor potential vanilloid type 1 (TRPV1) in the dorsal root ganglion (DRG) in rats with neuropathic pain induced by selective ventral root transection (VRT). Methods. Neuropathic pain was induced by transection of the lumbar 5 ventral root in male Sprague-Dawley rats. BTX-A or saline was administered to the plantar surface by subcutaneous injection. SB366791 (an inhibitor of TRPV1) was administered intraperitoneally. Behavioral tests were conducted preoperatively and at predefined postoperative days. The expression of TRPV1 was detected and quantified by immunohistochemistry and Western blotting at postoperative days 3, 7, 14, and 21. Results. TRPV1 expression increased significantly in the L4 approximate to 5 dorsal root ganglia 7 days after L5 VRT compared with the sham-operated control (P<0.05). This increase persisted for at least 21 days. The thresholds of foot withdrawal to mechanical and thermal stimulation decreased significantly as well. Subcutaneous injection of BTX-A significantly and dose-dependently reduced the expression of TRPV1 (P<0.05) and partially reversed the pain thresholds. Conclusion. Upregulation of TRPV1 expression in the DRG is an important mechanism of neuropathic pain induced by the VRT. The analgesic effect of BTX-A is most likely mediated through reduction of TRPV1 expression in the nociceptors.