Now, Canadian researchers show in an animal study how morphine leads to withdrawal, and that two commonly used drugs—one for gout and another for malaria—may be repurposed to prevent withdrawal in patients.
The study reveals that Pannexin-1, a protein in cells called microglia, controls withdrawal in mice and in rats. The work also shows that the anti-gout drug and the anti-malaria drug each reduced withdrawal symptoms in rats by changing the function of Pannexin-1.
The study was published online January 30 in the journal Nature Medicine.
All eyes turn to microglia
Microglia are well known as the immune cells of the central nervous system (brain and spinal cord), and they contribute to pain and the workings of opioids. So, Nicole Burma, first author on the study at the University of Calgary, Alberta, Canada, turned her attention to these cells.
“There’s a whole body of evidence that says microglia respond to opioids,” says Tuan Trang, who led the study, “so Nicole asked what their role in withdrawal might be.”
To learn more, Burma used a common rodent model of morphine withdrawal. First, she treated rats with morphine twice a day for five days to cause physical dependence on the drug. On the fifth day, she injected the animals with naloxone, a drug that immediately blocks the effects of morphine. This sends the animals into a state of withdrawal, similar to what is seen in people.
“Some of the symptoms we see in the rats are headshakes, wet-dog shakes, teeth-chattering, increased grooming, piloerection [hair standing on end], allodynia [when normally harmless stimuli become painful], and salivation,” said Burma.
To understand the role of microglia in withdrawal, Burma used a toxin in a new set of rats to eliminate microglia in the spinal cord, before the animals were treated with morphine.
After a week of morphine treatment, the drug still relieved pain, but upon naloxone injection, withdrawal was dramatically reduced. Somehow, the absence of microglia didn’t affect morphine’s capacity to relieve pain, but significantly reduced withdrawal symptoms.
Say hello to Pannexin-1
To understand how morphine affected microglia, Burma removed the spinal cord from rats that had been treated with morphine for five days and then measured changes in the microglia within them. She found that morphine treatment increased the amount of Pannexin-1, which sits on the surface of microglia.
Not only was there more Pannexin-1 in microglia, but the activity of the protein was increased too. And, exposure to naloxone enhanced Pannexin-1 activity even further.
The team then gave a drug that interferes with Pannexin-1 to the rats one hour before causing withdrawal with naloxone. Strikingly, withdrawal behaviors were again dramatically reduced.
“This provided an approximation of how microglia might be involved in withdrawal, which then allowed us to go in with more refined [experimental] tools,” says Trang.
To understand exactly how Pannexin-1 contributes to withdrawal, Burma and colleagues switched from rats to mice, in which there are more genetic tools to do experiments. They created genetically engineered mice that were missing Pannexin-1 in microglia, and again used morphine, followed by naloxone, to induce withdrawal. Once again, morphine relieved pain, but withdrawal behaviors were greatly reduced.
Together, the results show that morphine contributes to withdrawal by affecting the function of Pannexin-1 on microglia. They also reveal that it is possible to prevent withdrawal while maintaining the pain relief that morphine provides.
From rodents to people
The findings presented Burma and colleagues with an exciting opportunity, because two drugs already used in people to prevent gout and malaria also work by inhibiting Pannexin-1.
So, the team turned to their model of morphine withdrawal and showed that both drugs reduced withdrawal in rats. The investigators are now working towards a clinical trial to test the drugs in people at a pain clinic in Calgary.
“In people who are managing long-term pain with opioids, it becomes very difficult to dissociate their chronic pain from withdrawal pain,” Burma says. “So we are hoping this therapy can improve pain management and help those wishing to stop their opioid treatment,” she added.
To read about the research in more detail, see the related Pain Research Forum news story here. —Nathan Fried
Nathan Fried is a postdoctoral fellow at the University of Pennsylvania, Philadelphia, US.