Effect of fentanyl, a narcotic analgesic, on two components of the jaw opening reflex.

An action of fentanyl, a short-acting narcotic, on the reflex discharge in the digastric nerve induced by the inferior alveolar nerve stimulation was investigated in alpha-chloralose anesthetized cats. In the ipsilateral digastric reflex discharge, there were an early phase induced by stimulus exciting Aalpha fibers and a late phase appearing when Adelta fibers were also stimulated. Following dorso-lateral cordotomy at the obex level, an isolation of the spinal trigeminal nucleus caudalis, a total area in the digastric reflex discharge decreased, while its first peak amplitude was little affected, indicating a disappearance of the late phase and a preservation of the early phase. Fentanyl depressed both the total discharge area and the first peak amplitude. After dorso-lateral cordotomy, the depression of the area decreased considerably, whereas that of the amplitude decreased slightly. Results indicate that fentanyl depressed both the early phase which is activated by the Aalpha fiber stimulation, not via the subnucleus caudalis and the late phase which is activated by the Adelta fiber stimulation via the sub-nucleus caudalis or its surroundings. The latter action would be related to the analgesic action of fentanyl.