Fentanyl and tooth pulp evoked responses in the spinal trigeminal nucleus caudalis region

Electrical activities evoked by tooth pulp stimulation were recorded in the subnucleus caudalis region of the spinal trigeminal nucleus, and the action of fentanyl, a short-acting narcotic, on these activities was investigated in the alpha-cholinergic anesthetized cat. Fentanyl (20 to 40 mug/kg i.v.) depressed the first peak and potentiated the second one of negative potentials evoked by pulp stimulation in the border area betaeen the nucleus proprius (Pr) and the lateral reticular formation (LRF). Neurons, whose responses to pulp stimulation were depressed by fentanyl, were also predominantly localized in this region. Pulp-induced monophasic negative potential and spike discharges in the more ventro-medial portion of the LRF were not affected by fentanyl. The effect of fentanyl on cells in the marginal zone varied from unit to unit. The selective action of fentanyl on neurons in the border area between the Pr and the LRF may partially explain the analgesic action of fentanyl.