A newly devised reliable method for evaluating analgesic potencies of drugs on trigeminal pain.

A pain-producing substance, bradykinin (0.63--1.25 ng in 0.5--1.0 microliter of distilled water), was applied onto the tooth pulp of the lower incisor of unrestrained rats through a cannula fixed on the surfaces of the incisors using a microsyringe. Such a microapplication of bradykinin induced biting response and other aversive behaviors. When the microapplications were repeated at intervals of 60 min, the biting response seemed most characteristic and reproducible. Therefore, the duration of biting response was used as a measure in studying the effects of drugs on trigeminal pain. If biting duration was inhibited 90% or more after drug administration the effect was considered analgesic. Carbamazepine and phenytoin, which are clinically employed for treating trigeminal neuralgia, and morphine, a narcotic analgesic, produced dose-dependent analgesic effects in this method and the corresponding ED50 values were 13.1 intraperitoneally (ip), 75.0 ip, and 3.00 subcutaneously (sc) mg/kg, respectively. On the other hand, pentobarbital (15 mg/kg, ip), diazepam (1.0 mg/kg, ip), and aspirin (150 mg/kg, ip) were not effective in suppressing the biting response. These results indicate that this newly devised method in the rat is reliable and feasible in evaluating pain related the trigeminal system.