Suppression of transmission of nociceptive impulses by morphine: selective effects of morphine administered in the region of the substantia gelatinosa.

1 In spinal cats anaesthetized with alpha-chloralose, a study was made of the effects of morphine and naloxone, administered electrophoretically from micropipettes, on the responses of dorsal horn neurones to noxious (raising of skin temperature above 45 degrees C) and innocuous (deflection of hairs) peripheral stimuli.2 Administered near cell bodies, morphine reduced the nociceptive responses of only 2 of 37 cells. Excitation occurred more commonly than depression and abnormalities in action potentials were commonly observed following ejection of morphine. None of these effects of morphine was antagonized by electrophoretically applied naloxone.3 Administered in the substantia gelatinosa from one micropipette while recording responses of deeper neurones with a second micropipette, morphine reduced the nociceptive responses of 15 of 19 neurones. Firing in response to deflection of hairs was not reduced by morphine. Depression of nociceptive responses by morphine was long lasting (>20 minutes). Naloxone ejected into the substantia gelatinosa or given intravenously in doses as low as 0.1 mg/kg antagonized the effects of morphine. The effectiveness of this dose of intravenous naloxone suggests that the concentrations of morphine in the substantia gelatinosa which reduced nociceptive responses were not unlike those present after analgesic doses of systemic morphine. Naloxone alone, and excitant and depressant amino acids ejected into the substantia gelatinosa had little effect on cell firing.4 Both the selective action of morphine on nociceptive responses and the reversal of this action by intravenous naloxone suggest that the opiate receptor present in the substantia gelatinosa is relevant to analgesia produced by opiates given systemically.