Evidence of serotonin-dopamine involvement in the inhibition of d-amphetamine stereotypy and appearance of stereotyped movements found respectively after acute and long-term treatment with morphine and methadone in rats

P-chlorophenylalanine, an inhibitor of serotonin synthesis, was found to completely prevent the inhibitory effect of morphine and methadone on the stereotypy caused by d-amphetamine and methyl-phenidate in rats. d-Fenfluramine and m-chlorophenylpiperazine, two drugs supposed to increase serotonin transmission, and halo-peridol, an antagonist of dopamine at central receptors, blocked the stereotyped movements induced by repeated treatment with morphine and methadone. The results suggest that a) brain serotonin mediates the effect of morphine and methadone on amphetamine and methylphenidate stereotypy b) serotonin and dopamine are involved in the stereotyped movements caused by long-term treatment with these narcotics in the rat.