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 0.05) 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.