Botulinum toxin type A inhibits hyperalgesia in the rat masseter muscle in a carrageenan model of myofascial pain.

Objective: Previous studies have shown that botulinum toxin type A (BoNT-A) attenuates nociception, but the underlying mechanisms remain unclear. Studies of experimental pain in humans have also shown conflicting results. Carrageenan is commonly used to produce short-term acute inflammation and hyperalgesia in animal models, and the effect of BoNT-A on carrageenan-induced pain in the masseter muscle has not been studied. This study evaluated the antinociceptive and anti-inflammatory effects of intramuscular injection of BoNT-A in an experimental model of inflammatory pain in the masseter muscle of rats.

Design: Carrageenan (2 %) was injected into the masseters of sixty rats pretreated with three sessions of BoNT-A (3.5 U/kg) or daily with ibuprofen (40 mg/kg) for seven days. Masseter injected with saline was used as a control. An electronic von Frey anesthesiometer determined the head withdrawal threshold before carrageenan and at 5 h, 1, 3, and 7 days following administration. The masseters were processed for paraffin embedding and H&E staining and subjected to histomorphometric analysis 1 and 8 days after carrageenan administration.

Results: Pretreatments with BoNT-A or ibuprofen significantly decreased carrageenan-induced hyperalgesia. BoNT-A did not inhibit inflammation and tissue damage induced by carrageenan.

Conclusions: These findings reveal that BoNT-A promotes antinociceptive effects in the masseter muscle during painful conditions independently of anti-inflammatory mechanisms.