Efeitos da toxina botulínica A em masseteres de ratos em modelo experimental de dor inflamatória

Abstract

Temporomandibular dysfunction (TMD) comprises a set of disorders involving the masticatory muscles, the temporomandibular joint and associated structures. One of the most frequent symptoms reported by patients is pain in the chewing muscles. Among the proposed therapies, the botulinum toxin (TNBo/A) is highlighted. Although there is some evidence about its antinociceptive effect, little is known about its action and its undesirable effects when applied to a chewing muscle. This study evaluated the effects of botulinum toxin on nociception, repair and inflammatory process in rat masseter muscle in an experimental model of inflammatory pain. Carrageenan, an inflammatory drug, was injected into the masseter muscle of rats treated with TNBo/A, or by the non-steroidal anti- inflammatory drug ibuprofen. Masseter infiltrates with saline were used as controls. The nociceptive threshold measurement was performed with a Von Frey digital analgesometer before and after carrageenan, in a period equivalent to 5 (five) hours, and 1 (one), 3 (three) and 7 (seven) days from its administration. Eight days after carrageenan administration, the animals were euthanized, and their masseters were sent for processing and histological analysis. Compared to the group treated only with carrageenan, the specimens treated with carrageenan and TNBo/A showed an increase in the percentile variation of the nociceptive threshold from baseline over the periods of 1 (one) day (p <0.001), 3 (three) days (p <0.001) and 7 (seven) days (p <0.05). The effect of TNBo/A on nociception was similar to that observed in the group treated with carrageenan and ibuprofen, with no statistically significant difference between the groups (p>0.05). Histological evaluation showed focal areas of tissue destruction and a chronic inflammatory infiltrate with some lymphocytes, many macrophages and rare mast cells at the carrageenan infiltration site in all experimental groups, being slightly more extensive in the TNBo/A treated group. These findings revealed that TNBo/A has an antinociceptive effect in the masseter muscle on painful conditions, but it may constitute an additional element of tissue damage if an inflammatory environment is present in the muscle tissue, thus not constituting the first option for the treatment of myofascial pain.