LACK OF EFFECT OF MORPHINE-3-GLUCURONIDE ON THE SPINAL ANTINOCICEPTIVE ACTIONS OF MORPHINE IN THE RAT - AN ELECTROPHYSIOLOGICAL STUDY

Previous studies have shown that morphine-3-glucuronide (M3G), a metabolite of morphine, may functionally antagonize the antinociceptive action of morphine. The interaction between morphine and M3G was therefore investigated in the halothane-anaesthetised rat. Extracellular unit recordings were made of the innocuous A-beta fibre and noxious C-fibre evoked responses of convergent dorsal horn neurones. Intrathecal M3G (5 mug, 100 mug and 500 mug) alone did not show any antinociceptive effect. There was a slight, but not statistically significant, decrease in the antinociceptive effect of 5 mug morphine in the M3G-pretreated groups. However, M3G pretreatment (5 mug, 100 mug and 500 mug) had no effect on the higher dose of morphine (50 mug) used. We conclude that M3G has, at most, a minor effect on the spinal antinociceptive effects of morphine.