The pathophysiological role of serotonin receptor systems in opioid analgesia and tolerance

Abstract

Continuous treatment with opioid analgesics, such as morphine, leads to the development of ant nociceptive tolerance in patients. Although a lot of information about antinociceptive, the pathophysiological mechanisms of tolerance to opioid analgesia are not yet completely understood. Proposed mechanisms for opioid analgesic tolerance comprise down-regulation of opioid receptors, reduction of sensitivity G-proteins, altered intracellular signalling pathway including nitric oxide, adenyl cyclase, and protein kinase C. Numerous physiological and behavioural studies have shown an interaction of the serotonergic system and opioid antinociception. The serotonin (5-HT) receptor system is a necessary component of the spinal and midbrain pain modulation circuit mediating opioid analgesia. Various types of serotonin receptors demonstrate different effects on morphine analgesia. Systemic administration of opioids rise 5-HT levels in the spinal cord dorsal horn and contribute to opioid analgesia in the normal state but reduce that in neuropathic pain via spinal 5-HT3 receptors. Spinal and supraspinal serotonergic neurons may also play a pathophysiological role in the development of morphine analgesic tolerance. Serotonin receptor subtypes show different effects on opioid tolerance. This review paper focus on the current understanding of the role of serotonin receptor systems in opioid analgesia and tolerance.