Reduction of the antinociceptive effect of 5-hydroxytryptophan in morphine tolerant rats

The effect of 5-hydroxytryptophan on pain threshold was studied in rats both tolerant and nontolerant to the analgesic action of morphine as assessed using a procedure of electrical stimulation. The compound elevated pain threshold and exhibited an additive effect with morphine analgesia in nontolerant rats. A marked reduction of the antinociceptive action of the serotonin precursor as well as absence of the additive effect with morphine was observed in rats tolerant to the analgesic. These results are discussed in terms of the possible mechanism of action of serotonin on morphine effects.