RU 486 reduces morphine-induced analgesia in mice, but steroid receptors are not involved

The effects of subcutaneous pretreatment with the glucocorticoid/progesterone receptor antagonist RU 486 on the antinociceptive action of peripherally administered morphine were evaluated by the hot-plate test in mice. The steroid significantly reduced the analgesic effect of the opiate. Neither dexamethasone nor progesterone modified the effects of RU 486 on morphine-induced analgesia. Therefore, the present data indicate that the modulatory effect of RU 486 on morphine-induced analgesia does not involve the binding of this drug to classical steroid receptors.