Effects of tractotomy on nociceptive reactions induced by tooth pulp stimulation in the rat

The effects of a trigeminal tractotomy on nociceptive reactions induced by electrical stimulation of an inferior incisor were studied in freely moving rats. Behavioral tests were based on the observation of three reactions: jaw opening reflex, face scratching with forepaws, and head rotation toward the stimulated side. These reactions appear successively when the stimulation intensity increases. The results were evaluated by comparing a test group with a sham-operated group. Both groups were prepared surgically in the same way but only the former had undergone tractotomy. The results were as follows: The threshold of the jaw opening reflex triggered by stimulation of the tooth pulp ipsilateral to the tractotomy was not modified; however, the threshold of the face-scratching and the head rotation reactions rose (P less than or equal to 0.05) after tractotomy. These data suggest that the rostral part of the trigeminal sensory complex is involved in oral nociception. However, the modification of the face-scratching and head rotation thresholds reveals that the subnucleus caudalis also participates in this function when a certain level of pain is reached. The thresholds of the three nociceptive reactions evoked by stimulation of the contralateral tooth pulp were significantly lower in the test group than those in the sham-operated group (P less than or equal to 0.001), which is interpreted as being indicative of an modulating effect of the subnucleus caudalis on the contralateral trigeminal sensory complex. This effect was probably revealed by the use of a sham-operated group as a control.