Can Botulinum Toxin Type E Serve as a Novel Therapeutic Target for Managing Chronic Orofacial Pain?

Abstract

The existing literature offers limited experimental evidence on the role of botulinum neurotoxin type E (BoNT-E) in pain transmission. The present study investigated the antinociceptive effects of subcutaneously administered BoNT-E in chronic orofacial pain conditions. This study used orofacial formalin-induced pronociceptive behavior and complete Freund's adjuvant (CFA)-induced thermal hyperalgesia as inflammatory pain models in male Sprague Dawley rats. A neuropathic pain model was also developed by causing an injury to the inferior alveolar nerve. Subcutaneously administered BoNT-E (6, 10 units/kg) significantly reduced nociceptive behavior during the second phase of the formalin test compared to that of the vehicle treatment. These doses similarly alleviated thermal hypersensitivity in the CFA-treated rats. Moreover, BoNT-E (6, 10 units/kg) markedly attenuated mechanical allodynia in rats with an inferior alveolar nerve injury. At a dose of 10 units/kg, BoNT-E produced antinociceptive effects that became evident 8 h post-injection and persisted for 48 h. Notably, BoNT-E (10 units/kg) significantly reduced the number of c-fos-immunostained neurons in the trigeminal subnucleus caudalis of rats with an inferior alveolar nerve injury. In comparison, intraperitoneally administered gabapentin (30, 100 mg/kg) demonstrated significant mechanical anti-allodynic effects but exhibited lower analgesic efficacy than that of BoNT-E. These findings highlight the potential of BoNT-E as a therapeutic agent for chronic pain management.

Keywords: antinociception; botulinum toxin type E; complete Freund’s adjuvant; formalin; trigeminal neuropathic pain.

Figure 1

Changes in animal body weight following inferior alveolar nerve injury. BoNT-E was administered on post-operative day 3 (POD 3). The administration of BoNT-E did not significantly affect body weight compared to the vehicle treatment.

Figure 2

Changes in formalin-induced nociceptive behavior after botulinum neurotoxin type E (BoNT-E) treatment. Administration of 5% formalin resulted in biphasic nociceptive behavior. Subcutaneous pretreatment with BoNT-E (6, 10 units/kg) 8 h prior to injecting 5% formalin significantly reduced nociceptive behavior during the second phase. Each group comprised seven animals. * p < 0.05, BoNT-E-treated vs. vehicle-treated group.

Figure 3

Changes in head withdrawal latency following subcutaneously administered botulinum neurotoxin type E (BoNT-E) in rats treated with complete Freund’s adjuvant (CFA). Subcutaneous injection of CFA induced thermal hyperalgesia within 1 day after injection, which persisted for 10 days. BoNT-E treatment (6, 10 units/kg) at 3 days significantly alleviated thermal hyperalgesia. Each group comprised seven animals. * p < 0.05, BoNT-E-treated vs. vehicle-treated group.

Figure 4

Effects of subcutaneously administered botulinum neurotoxin type E (BoNT-E) on mechanical allodynia in rats with trigeminal neuropathic pain. Vehicle treatment did not alter air-puff thresholds. However, BoNT-E treatment (6, 10 units/kg) on POD 3 significantly increased the air-puff thresholds compared to those of the vehicle-treated group, with anti-allodynic effects persisting for up to 48 h following a 10 units/kg injection of BoNT-E. Each group comprised seven animals. * p < 0.05, BoNT-E-treated vs. vehicle-treated group.

Figure 5

Effects of subcutaneously administered gabapentin on mechanical allodynia in rats with trigeminal neuropathic pain. Vehicle treatment did not alter air-puff thresholds. On POD 5, gabapentin treatment (30, 100 mg/kg) significantly alleviated mechanical allodynia compared to that of the vehicle-treated group, with the anti-allodynic effects lasting up to 8 h after a 100 mg/kg injection of gabapentin. Each group comprised seven animals. * p < 0.05, gabapentin-treated vs. vehicle-treated group.

Figure 6

Effects of subcutaneously administered botulinum neurotoxin type E (BoNT-E) on c-fos expression. The c-fos immunostainings were conducted 5 days post-operation in the ipsilateral trigeminal subnucleus caudalis. (A) c-Fos-immunoreactive neurons in sham-operated rats. (B) c-Fos-immunoreactive neurons following vehicle treatment in rats with inferior alveolar nerve injury. (C) c-Fos-immunoreactive neurons following BoNT-E treatment (10 units/kg) in rats with inferior alveolar nerve injury. (D) Histograms illustrate the mean number of c-fos-immunostained neurons. Injury to the inferior alveolar nerve significantly upregulated the number of c-fos-immunoreactive neurons compared to that of the sham-operated rats, while BoNT-E treatment significantly reduced c-fos-immunopositive neurons compared to those of vehicle-treated rats. Each group comprised six animals. Scale bar: 50 μm. * p < 0.05, sham-treated vs. vehicle-treated group; ^# p < 0.05, vehicle-treated vs. BoNT-E-treated group.