Medullary substrates of descending spinal inhibition activated by intravenous administration of [D-Ala2]methionine enkephalinamide in the rat.

Two experiments were performed in pentobarbital-anesthetized rats to provide information about the locus of the descending inhibitory system(s) in the brainstem engaged by [D-Ala2]methionine enkephalinamide (DALA) activation of vagal afferents. In Expt. 1, local anesthetic block of either the medial medullary nucleus raphe magnus (NRM) or the combined NRM-lateral medullary reticular formation (MRF) region failed to affect DALA-induced inhibition of the tail-flick reflex or reflex bradycardia. The hypotensive action of DALA was marginally enhanced in rats with a combined NRM-MRF local anesthetic block. However, inhibition of the tail-flick reflex produced by electrical stimulation in the NRM-MRF was eliminated by the local anesthetic. In Expt. 2, local anesthetic blocks of regions at the level of and caudal to obex were effective in blocking DALA-induced inhibition of the tail-flick reflex. These regions included lateral sites approximately 0.4 mm rostral to obex to -2.4 mm caudal to obex, and midline sites approximately -2.4 to -3.2 mm caudal to obex. These findings suggest that brainstem areas mediating DALA-induced antinociception are lateral to the midline at the level of the obex. The spinopetal efferents mediating antinociception then course medially and ventrally, and finally course bilaterally in the dorsolateral funiculi before terminating in the spinal dorsal horn.