The effect of prostaglandin E2 on increased naloxone potency induced by morphine pretreatment

Pretreatment with prostaglandin E2 caused an increase in naloxone potency while the antinociceptive effect of morphine remained unaffected. These effects of prostaglandin E2 were not blocked by concomitant administration of naloxone in the pretreatment. Furthermore, when prostaglandin E2 was given together with morphine as pretreatment, the resulting naloxone antagonism measured was higher than those induced by pretreatment with the individual drugs. It is suggested that the induction of increased naloxone potency is likely to be mediated through a prostaglandin-linked process.