In the June 14th issue Proceedings of the National Academy of Sciences, an article appeared that suggested opioid/narcotic medications after a nerve injury doubled the duration of pain in rats. Now, humans are not rats, and this study specifically studied the effect of morphine, but this adds one more concern with regards to the use of narcotic pain medications. Not only can they be addictive and cause increased pain, but now also prolong the duration of pain.
The study was conducted at the University of Colorado. For the study, the rats underwent surgery and a suture was placed around the sciatic nerve which then is known to cause pain down the leg afterward. Ten days after surgery, one group of rats was given morphine for five days and a second group was given saline for five days. Saline is salt water and has no effect on pain or the body. Pain was tested by poking the affected leg. The rats that received the saline showed more recovery and less sensitivity to pokes over the course of four weeks. Those that received the morphine took about eight weeks to recover, double the time to become less sensitive to pain. Female rats took even longer than male rats to start to recover.
Morphine Inhibiting Recovery
The study found a very interesting reason why the pain lasted longer. Morphine seems to increase the inflammatory response in the spinal cord. Apparently, the immune system, particularly the microglia cells in the spinal cord see morphine as a threat. These cells become more active, producing more inflammatory compounds that prolong pain. If the microglia are shut down, the duration of pain shortens.
This obviously is an experimental situation, but how it generalizes to a more broad extent is extremely interesting. The most pressing issue is that this same situation occurs in humans when we give a patient morphine. We know in many cases of spine pain, there is ongoing inflammation occurring. Post surgery we also often try to control pain with morphine. If we give morphine (or codeine that breaks down to morphine during metabolism) are we just prolonging pain? Furthermore, are we somehow stimulating microglia in the spine and maintaining inflammation every time we give morphine?
The next question that this research brings up is whether this is a property of all opioids/narcotic type painkillers. This study just involved morphine, but if it applies to other medications, it may give us a clue why these medications have limited effectiveness. It may also mean that by giving opioids to people on a long-term basis for pain, we’re merely prolonging pain. It also is an indication that there is probably another good reason not to be opioids.
The last area is the role of microglia and pain. In the past, we have thought pain only had to do with nerve cells and the transmission of impulses from sensory pain receptors to the brain. Now we are learning that there are other nervous system cells, specifically the microglia that are incredibly important. These cells seem to modulate the spinal response to inflammation and produce chemicals that can keep inflammation going.
The findings in this research seem to cause more questions about how and why we have pain and what all is going on in our bodies when we have pain. It also tells us we may be doing things all wrong when we give people opioids for pain and it may be just worsening the problems for any number of reasons. However, it may be also be giving us new insights, especially that we should be studying the microglia more and understanding its role and how we can modulate these cells activity and the compounds they produce. This also may be just one more reason why one should try to wean oneself off opioids. Pain is complex, there really are more and more questions on why we have pain, but as we ask the questions, hopefully research will lead to better answers.