Central action of narcotic analgesics. V. Participation of serotonin in the mechanism of action of narcotic analgesics.

The influence of serotonergic system on the changes in locomotor activity of mice and rats brought about by morphine, fentanyl, codeine and pentazocine and on morphine induced catalepsy in rats was studied. p-Chlorophenylalanine (pCPA) did not affect the behavioral changes produced in mice by morphine, fentanyl, codeine and pentazocine but reduced the behavioral depression produced by these drugs in rats. 5-Hydroxytryptophan (5-HTP) but not tryptophan (TP) reversed the action of pCPA on the effect of morphine and fentanyl. After reserpine the depression produced in rats by morphine and fentanyl was more pronounced. TP did not change the depression produced by combination of reserpine and morphine but counteracted the depression observed after combination of reserpine and fentanyl. In mice reserpine protected against hypermotility produced by morphine or fentanyl and TP potentiated the depression produced by the combination of reserpine and morphine or reserpine and fentanyl. Serotonin precursors, 5-HTP and TP evidently potentiated the morphine induced catalepsy. pCPA counteracted only the enhancement of the catalepsy observed after TP administration. Naloxone abolished the catalepsy after combined treatment with morphine and TP. Similarly but weaker acted cyproheptadine. The results suggest that the serotonin system plays a role in the effects of morphine and fentanyl on rat locomotor activity. An increase in the cerebral serotonin level increases the morphine catalepsy in rats.