The effect of morphine on the potassium evoked release of tritiated 5-hydroxytryptamine from spinal cord slices in the rat.

The effect of morphine on the potassium (40 mM) evoked release of exogenous [3H]5-HT from slices of the dorsal spinal cord of the rat was studied. The effects of in vitro applied morphine on the slices were compared to those produced by systemic morphine applied to the animals before preparation of the slices. The in vitro application of morphine (10(-6) to 10(-5) M) did not affect the release of [3H]5-HT.

The relationship between morphine analgesia and the activity of bulbo-spinal serotonergic system as studied by tolerance phenomenon.

The effect of various doses of acute morphine on both analgesia and 5-hydroxytryptamine (5-HT) synthesis in the brain and the spinal cord has been studied in rats rendered tolerant by chronic administration of the analgesic. In morphine-tolerant rats, the incorporation of tritiated-L-tryptophan (TRP) in the brain and the spinal cord was higher than in non-tolerant rats, but there was no significant difference in the synthesis rate of the newly formed 5-HT between the two groups. An acute dose of morphine (10 mg/kg) which induced a powerful analgesia and a large increase in 5-HT synthesis in non-tolerant rats, did not produce analgesia nor changes in 5-HT synthesis in tolerant rats.