Tooth pulp stimulation advances both medullary off-cell pause and tail flick

It has been postulated that the so-called off-cells of nucleus raphe magnus and adjacent structures in the rat are the output elements of a system which inhibits nociceptive transmission at the spinal cord. Off-cells stop firing about 0.4 s before the tail flick reflex (TF) elicited by the application of noxious heat to the tail. When continuous off-cell activity is induced by either morphine injection or periaqueductal gray stimulation, the TF is delayed. The present results show that electrical stimulation of the tooth pulp (TP) causes the off-cells to stop firing. Furthermore, when TP is stimulated during tail heating and before the expected time for TF, off-cells stop firing earlier and the TF occurs also earlier. This supports the notion that off-cells inhibit nociceptive transmission.