Effects of narcotic analgesics on serotonin metabolism in brain of rats and mice.

The effects of narcotic analgesics on the brain 5-hydroxytryptamine (5-Ht) and 5-hydroxyindoleacetic acid (5-HIAA) levels of rats and mice were investigated in relation to our preceding data on the effect of humoral modulatorents. The results suggest that morphine accelerates the release of brain 5-HT both in rats and mice, and that neither methadone nor pethidine alters the brain 5-HT and 5-HIAA levels in rats. The morphine-induced increase in brain 5-HT turnover is likely to be involved in the morphine-induced decrease in locomotor activity and hypothermia in rats. The activity-decreasing effects of methadone or pethidine, on the other hand, are mediated by mechanisms different from those which mediate the effects of morphine. In contrast, an increase in brain 5-HT turnover in mice apparently does not play an important role on activity-increasing effects of morphine but rather participates in other pharmacological effects of morphine.