[Mechanism of the modulation of pain transmission at the subnucleus caudalis of the trigeminal sensory nuclear complex in rabbits].

The modulation of dental pain transmission at the subnucleus caudalis of the trigeminal sensory nuclear complex (SpVc) was investigated in rabbits in vivo. The superficial layers of SpVc were perfused with artificial cerebrospinal fluid using a push-pull cannula system. Immunoreactive substance P (SP) released into the perfusates following electrical stimulation of the lower incisor pulp was measured. The obtained results were as follows. 1. An increase in the release of SP and [Met5]-enkephalin was observed by the electrical stimulation with 40 V. 2. The increase of SP release following electrical stimulation was inhibited by systemic administration of morphine (10 mg/kg i.v.) or local application of morphine (10(-6) M) to SpVc. The stimulus-evoked SP release was also inhibited by local application of [D-Ala2, Met5]-enkephalinamide (an analog of [Met5]-enkephalin; 10(-4) M). 3. Spontaneous release of serotonin (5-HT) into the perfusates was observed, while that of norepinephrine was not. Tooth pulp stimulation tended to increase the level of 5-HT. Systemic administration of morphine (10 mg/kg i.v.) and electrical stimulation of the nucleus raphe magnus (NRM) significantly enhanced the release of 5-HT. 4. The release of SP evoked by tooth pulp stimulation was inhibited by local application of 5-HT (10(-6)M) and electrical stimulation of NRM. These results suggest that there are two modulatory systems controlling the delivery of the ascending sensory message at the superficial layers of SpVc. One is an intrinsic mechanism associated with the segmental enkephalinergic system, the other is a descending monoaminergic system originating in NRM. It is also suggested that these two systems play an important role in producing the analgesic effect of morphine.