Morphine selectively suppresses the slow response of tooth pulp-driven neurons in first somatosensory cortex (SI) of the cat.

Tooth pulp-driven (TPD) neurons are found in the oral area of the first somatosensory cortex (SI) and they have been classified into fast (F-) type, slow (S-) type and fast type accompanied by afterdischarges (Fa). Effects of morphine on single unit responses of the F- and S-type neurons to pulp stimulation were examined by recording the discharges from a single neuron using a microelectrode. Intraperitoneal administration of morphine at 1.5 mg/kg markedly suppressed the response of S-type TPD neurons that fire with a long latency. This effect was completely reversed by naloxone at 0.2 mg/kg. On the other hand, the discharges of F-type neurons that fire with a short latency were not affected by application of morphine at 3 mg/kg. These findings suggest that S-type neurons in SI are concerned with pain perception.