Botulinum toxin type A (BOTOX) has been used to treat pathological pain conditions although the mechanism is not entirely understood. Subcutaneous (s.c.) BOTOX also inhibits inflammatory pain in the rat formalin model, and the present study examined whether this could be due to a direct action on sensory neurons. BOTOX (3.5-30 U/kg) was injected s.c. into the subplantar surface of the rat hind paw followed 1-5 days later by 50 mL of 5% formalin. Using microdialysis, we found that BOTOX significantly inhibited formalin-induced glutamate release (peak inhibitions: 35%, 41%, and 45% with 3.5, 7, and 15 U/kg, respectively). BOTOX also dose dependently reduced the number of formalin-induced Fos-like immunoreactive cells in the dorsal horn of the spinal cord and significantly (15 and 30 U/kg) inhibited the excitation of wide dynamic range neurons of the dorsal horn in Phase II but not Phase I of the formalin response. These results indicate that s.c. BOTOX inhibits neurotransmitter release from primary sensory neurons in the rat formalin model. Through this mechanism, BOTOX inhibits peripheral sensitization in these models, which leads to an indirect reduction in central sensitization.
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