According to the World Health Organization(WHO) guidelines for patients with moderate or severe pain, morphine has been used as a “gold standard” treatment for cancer pain. However, the use of morphine for the treatment of pain was sometimes accompanied with side effects such as emesis, constipation and drowsiness.
We showed that morphine at the dose of which had no antinociceptive effect produced emetic response and gastrointestinal transit inhibition. It should be mentioned that morphine with lower doses produces severe side effects without antinociception/analgesia.
Recent clinical studies have demonstrated that when morphine is used to control pain, psychological dependence is not a major concern. We confirmed that animals with chronic pain failed to exhibit the morphine-induced rewarding effect. It should be pointed out that the endogenous κ-opioidergic system in the nucleus accumbens may be directly involved in the suppression of the morphine-induced rewarding effect under an inflammatory pain-like state. In contrast, the reduction of μ-opioid receptor function in the ventral tegmental area may contribute to the suppression of the rewarding effect induced by morphine under an neuropathic pain-like state. These findings strongly suggest that treatment of morphine with the adequate dose could be highly recommended for the relief of severe chronic pain.
