The findings could explain the explosion of painkiller addiction in the past few decades.
“We are showing for the first time that even a brief exposure to opioids can have long-term negative effects on pain,” said lead researcher Peter Grace, assistant research professor at University of Colorado Boulder in the US.
The researchers found that opioids like morphine paradoxically cause an increase in chronic pain in lab rats.
The results suggest that the recent escalation of opioid prescriptions in humans may be a contributor to chronic pain, Grace said.
The study showed that just a few days of morphine treatment caused chronic pain that went on for several months by exacerbating the release of pain signals from specific immune cells in the spinal cord.
“We found the treatment was contributing to the problem,” Grace noted.
The study, published in the journal Proceedings of the National Academy of Sciences, showed that a peripheral nerve injury in rats sends a message from damaged nerve cells to spinal cord immune cells known as glial cells, which normally act as “housekeepers” to clear out unwanted debris and microorganisms.
The first signal of pain sends glial cells into an alert mode, priming them for further action.
When the injury was treated with just five days of opioids the glial cells went into overdrive, triggering a cascade of actions, including spinal cord inflammation.
The team discovered that the pain signals from a peripheral injury combined with subsequent morphine treatment worked together to cause a glial cell signalling cascade.
The cascade produces a cell signal from a protein called interleukin-1beta (IL-1b), which increases the activity of pain-responsive nerve cells in the spinal cord and brain.
That can cause increases in pain duration lasting several months.
“The implications for people taking opioids like morphine, oxycodone and methadone are great, since we show the short-term decision to take such opioids can have devastating consequences of making pain worse and longer lasting,” Linda Watkins, professor at University of Colorado Boulder, said.
“This is a very ugly side to opioids that had not been recognised before,” Watkins noted.