Evidence that stress augments morphine analgesia by increasing brain tryptophan

Morphine analgesia measured by the tail withdrawal test was examined in rats that were either restrained or left free during testing. It was found that restraint potentiated morphine analgesia and decreased the latency of the peak analgesic effect. Methysergide, a serotonin antagonist, and valine, which prevents the increase in brain tryptophan induced by restraint, blocked the effect of restraint on morphine analgesia. Valine did not alter analgesia in unrestrained rats. An increase in brain tryptophan uptake induced by stress is suggested as a possible mechanism by which stress can interact with pain modulation systems.