INTRATHECAL MORPHINE-3-GLUCURONIDE DOES NOT ANTAGONIZE SPINAL ANTINOCICEPTION BY MORPHINE-6-GLUCURONIDE IN RATS

Morphine or morphine-6-glucuronide either alone or in combination with morphine-3-glucuronide was administered intrathecally to rats. Antino ciceptive effects were evaluated with the tail flick and the hot plate tests. Motor function was tested using the rotarod test. Estimated ED (50) from the dose-response curves for morphine and morphine-6-glucuro nide showed about a 30 times more potent antinociceptive effect of mor phine-6-glucuronide compared with morphine. Morphine-3-glucuronide had no antinociceptive effect. Simultaneous administration of morphine-3- glucuronide 5.0 mu g did not show any significant effect on antinocice ption induced by morphine 1.0 mu g or morphine-6-glucuronide 0.05 mu g .