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Does Morphine Increase Pain?

morphine pain

The opium poppy has been used for pain relief for hundreds of years. Opium is produced by the milky fluid that seeps from the unripe poppy seed when it is scraped off and air dried. The seedpod is cut with a blade which enables the gum to ooze out.  The gum is harvested with a curved spatula and then left to dry out in open wooden boxes.  The dried opium resin is then placed in bags to be sold. Opium mimics the body’s endorphins which help to alleviate pain.  Scientists have produced opioid drugs called morphine, fentanyl, methadone and oxycodone from the opium poppy.  These help to relieve moderate to severe pain.  However new research which has been published in the Proceedings of the National Academy of Science USA has found that rats who had received morphine, actually had an increase in their pain.  Assistant Professor Peter Grace and his colleagues in the University of Colorado conducted the research. Scientists had previously thought that opioids could only cause pain sensitivity (opioid induced hyperalgesia) while they were still in the body, but these new findings suggest that it can increase pain for months after the morphine has left the body.

In the experiment, scientists induced the neuropathic pain in the rats by constricting their sciatic nerve in their thighs.  This would have felt like sciatica.  Their pain management was assessed by poking their back paw with a plastic filament.  This would not usually be painful for the rats.  After 10 days the neuropathic pain was fully established and the rats were given either morphine or salt water intravenously for 5 days.  The rats who had been given the saline solution experienced pain for another 4 weeks, whereas the rats who had received the morphine suffered pain for 10 weeks.

In a different experiment, rats that had no neuropathic pain underwent surgery and received the same 5 -day intravenous morphine treatment as the rats who had sciatica.  The pain only lasted for a day, which means that chronic pain can’t be explained by morphine addiction or withdrawal, but that there is a link between morphine and the biological mechanisms underlying neuropathic pain.  Nerves are made up by about 10 percent of the brain and spinal cord, and 90 percent are glial cells. Glial cells help to provide nutrition for the nerves and eliminate metabolic waste.  They recognize chemical signals from the nerves and release chemical immune signals that communicate between nerves.  When the body is in pain the glial increase the sensations in the spinal cord pathways, which then interprets touch as being felt as pain.

Morphine also causes a chemical signal for glial cells.  When a person receives morphine for neuropathic pain, the glial cells release too many immune signals and this causes the pain to increase and for a longer period of time.  If the abnormal function of spinal glial was repressed with medication during the morphine treatment, the pain wouldn’t be endured for so long. Research also suggests that suppressing glial may increase the tolerance of morphine, which can lead to addiction, because a higher dosage is needed to achieve the desired effect of pain relief.


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