MORPHINE DIFFERENTIALLY MODULATES NOCICEPTIVE INPUT IN THE SUPERFICIAL VERSUS THE DEEPER DORSAL HORN OF THE MEDULLA (TRIGEMINAL NUCLEUS CAUDALIS) IN THE RAT

Morphine enhanced the noxious thermal stimulus-evoked responses in 4/1 3 (31%) selectively nocireceptive neurons in the superficial dorsal ho rn, inhibited the responses in 4/13 (31%) neurons and produced a bipha sic effect in 2/13 (17%) neurons. Naloxone antagonized these effects i n 7/7 neurons. In contrast, morphine produced a naloxone reversible re duction in the nociceptive responses of 4/4 (100%) multireceptive neur ons in the deeper dorsal horn of the medulla. The data are interpreted to indicate that opiates may differentially modulate nociceptive inpu t in the superficial versus the deeper dorsal horn.