Interaction of morphine and 5-hydroxytryptamine in the raphe-hippocampus system.

In order to describe the interaction of morphine and 5-hydroxytryptamine (5-HT) in the raphe-hippocampus system we tested the influence on the antinocifensive effect of topic administrations of morphine and serotonergic substances into the dorsal hippocampus and the median raphe nucleus in rats. 5-HT administered into the dorsal hippocampus increased the morphine analgesia. Lysergic acid diethylamide injected into the raphe nucleus antagonized the morphine effect. Morphine given into the raphe nucleus was highly effective, while its injection into the striatum was ineffective. The effect of the intrahippocampal morphine was antagonized by methysergide. The results indicate the important role of the serotonergic raphe-hippocampus system in the mechanism of the morphine analgesia.