Effects of opioids and opioid peptide on the release of substance P-like material induced by tooth pulp stimulation in the trigeminal nucleus caudalis of the rabbit

The superficial layer in subnucleus caudalis of the brain-stem trigeminal sensory nuclear complex (SpVc) in the rabbit was perfused with artificial cerebrospinal fluid using a push-pull perfusion cannula system. Immunoreactive substance P (iSP) and [Met 5]enkephalin (iME) released into the perfusates following electrical stimulation of the lower incisor pulp were measured. An increase in the release of iSP and iME lasting for 1 h or more was observed following electrical stimulation with 40 V. The increase in iSP release depended on the intensity of stimulation. Systemic morphine (10 mg/kg i.v.) completely inhibited the stimulus-evoked iSP release and this inhibition was antagonized by pretreatment with naloxone (5 mg/kg i.v.). The stimulus-evoked iSP release was also inhibited by local application of morphine (10 −6 M) or the opioid peptide [D-Ala 2,Met 5]enkephalinamide (10 −4 M). However, the local application of naloxone (5 × 10 −7 M) only partially antagonized the inhibitory effect of locally applied morphine and the opioid peptide. These results suggest that ther is a functional interaction between SP and enkephalin system in the superficial layer of SpVc for the regulation of dental pain transmission.