Chronic use of opioid pain medication
causes molecular and genetic changes that result in pain
A brief update
American Pain Society May 2009 Symposia: Anti-analgesic Effects of Mu-opioids: Molecular and Genetic Mechanisms
The clinical benefits of opioid analgesics have not been fully realized due to substantial side effects, which include tolerance, dependence and opioid-induced hyperalgesia. Although the precise molecular mechanism of these phenomenon is not understood yet, it is generally thought to result from cellular excitatory effects of mu-opioids which contrast the major inhibitory effects.
Mark Hutchinson, PhD, discussed the new discovery that every clinically relevant class of opioid analgesics non-stereoselectively activates glial cells through TRL4 receptor. Activation of this receptor, primarily expressed by microglia, leads to the release of proinflammatory mediators that counter-regulate acute opioid analgesia.
How can opioid-induced glial activation oppose & augment different aspects of opioid action?
Opioid analgesia is opposed by opioid-induced spinal glial activation since increased neuronal excitability leads to elevated nociception. Increased brain opioid-induced glial activation also leads to increased neuronal excitability & within reward & dependence centers this is believed to increase opioid reward & dependence. Therefore analgesia is decreased & reward/dependence is increased.
Counteracting hyperalgesia with naltrexone and dextromethorphan
In summary, Dr. Hutchinson describes the TRL4 receptor where opioids act to induce activation of microglia, releasing proinflammatory mediators that counteract analgesia and produce more pain.
Naltrexone, a mu opioid antagonist, has profound anti-inflammatory effects centrally on the microglia to produce analgesia. This mechanism of action of low dose naltrexone is discussed here.
Dextromethorphan acts centrally on microglia by the same mechanism, producing analgesia. Both naltrexone and dextromethorphan are classified as morphinans, morphine-like.·
More is less: increasing the dose causes pain.
A steep road to climb, much less to understand.
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