Inferior alveolar nerve transection enhanced formalin-induced nocifensive responses in the upper lip: systemic buprenorphine had more antinociceptive efficacy over morphine.

This study was designed to investigate the efficacy of a partial μ-opioid agonist, buprenorphine, against the formalin-induced hyperalgesia in the upper lip in chronically inferior alveolar nerve (IAN)-transected rats. Subcutaneous injection of diluted formalin into the upper lip in the IAN-transected rats showed an increased number of pain-related behavior (PRB; face-rubbing behavior) in every phase up to 45 min (p < 0.01) compared with that in the nontransected sham control rats. The numbers of c-Fos-immunoreactive (IR) cells in the superficial layers of the trigeminal nucleus caudalis (VcI/II) at the rostral (0-0.7 mm caudal to the obex) and middle levels (1.4-2.2 mm caudal to the obex) 2 h after the formalin injection in the IAN-transected rats were significantly increased compared with those in the control rats. The PRB in phases 1 and 2 (0-15 and 15-30 min after formalin injection) in rats with preadministration of morphine (3 mg/kg i.p.) or buprenorphine (100 µg/kg i.p.) was significantly (p < 0.05) smaller than those in the control rats. There was no significant difference in the efficacy between morphine and buprenorphine at these doses. The antinociceptive efficacy in phase 2 of buprenorphine (100 µg/kg) was higher (p < 0.05) than that of morphine (3 mg/kg) in the IAN-transected rats. The number of c-Fos-IR cells in the VcI/II at every level (0-3.6 mm caudal to the obex) after formalin injection was significantly decreased (p < 0.01) with preadministration of morphine (3 mg/kg) or buprenorphine (100 µg/kg) in the control rats. In the IAN-transected rats, the number of c-Fos-IR cells in the caudal VcI/II (2.2-3.6 mm caudal to the obex) after formalin injection was significantly decreased (p < 0.01) with preadministration of buprenorphine (100 µg/kg) but not so much (2.2-2.9 mm caudal to the obex, p < 0.05; 2.9-3.6 mm caudal to the obex, p > 0.05) with preadministration of morphine (3 mg/kg). These results indicate that IAN transection enhanced formalin-induced nocifensive responses in the upper lip, the dermatome of the intact nerve neighboring the IAN. Systemic preadministration of buprenorphine had more antinociceptive effects on the formalin-induced nocifensive behavior in the upper lip compared with morphine in the IAN-transected rats.