Orofacial pain is associated with various pathologies such as headache, dental pain and ophthalmic pain. The trigeminal system innervates a large section of the head, including the nasal and oral cavities, the cornea and facial skin, and is responsible for the transmission of pain signals from the orofacial regions to the brain.
These investigations were undertaken to study the effect of intranasal delivery of analgesics on orofacial pain using an operant testing method in mice. Doses of either lidocaine HCl or butorphanol tartrate were administered to mice, and the analgesic effectiveness was measured using a thermal operant behavior test involving a facial heat stimulus. Two parameters were measured in the operant assay: the number of licks and the duration of facial contact. Pain response was measured at two different temperatures: 37 ºC and 49 ºC. The magnitude of analgesic response was also compared between intranasal and intraperitoneal drug administration at 49 ºC.
Mice showed a significant decrease in the number of licks and duration of facial contact for both treatment and control groups as the temperature was increased from 37 ºC to 49 ºC. A significant difference in the duration of facial contact was observed following either lidocaine or butorphanol by nasal administration. One group of animals receiving intranasal lidocaine did exhibit an increase in the duration of facial contact compared to the control. Two doses of butorphanol were tested and increases in the duration of facial contact were observed at both levels, but no significant difference was observed in the number of licks recorded.
No convincing differences were observed in the mice behaviors for intranasal or intraperitoneal dosing of lidocaine or butorphanol. This suggests that nasal administration of these two analgesics at the doses tested did not provide superior pain relief compared to systemic delivery of the agents.