EFFECTS OF MORPHINE MICROINJECTIONS INTO THE TRIGEMINAL SENSORY COMPLEX ON THE FORMALIN TEST IN THE RAT

The aim of this study was to find out whether morphine locally applied into the different subnuclei of the spinal trigeminal nucleus could i mpair the behavioral response evoked by a tonic nociceptive stimulus. Microinjections of morphine were performed unilaterally in rats throug h a chronically implanted tube. The duration of the formalin-induced b iphasic rubbing activity was used as a measure of nociception. Morphin e, 0, 2, 7, or 14 mu g, in 0.2 mu l of saline were microinjected 10 mi n before a subcutaneous injection of formalin (1.5%) in the ipsilatera l upper lip. Morphine microinjections into subnucleus oralis (Sp5O) an d subnucleus caudalis (Sp5C) induced a significant decrease in rubbing duration of either the early or late phase of the response or both. T he early response to formalin was depressed only after morphine inject ion into Sp5O. The late response was depressed by microinjections into both Sp5O and Sp5C. No significant effect was observed in subnucleus interpolaris (Sp5I). These results are further evidence for the role p layed by Sp5C in orofacial nociception. They also give support to the involvement of the Sp5O in perioral nociceptive mechanisms of short du ration. (C) 1996 Academic Press, Inc.