Effect of RU-38486 on dexamethasone reversal of morphine-induced constipation in mice.

1. Morphine and dexamethasone significantly reduce gastrointestinal transit in mice. The degree of reduction was greater for morphine. 2. Dexamethasone pretreatment was found, however, to antagonize morphine-induced constipation. 3. Cycloheximide does not modify the dexamethasone effects. 4. RU-38486 reverses both the inhibitory action of dexamethasone on gastrointestinal transit and its reducing effect on morphine-induced constipation. 5. These results suggest that dexamethasone might act through binding to receptors not linked to DNA-responsive elements.