Chronic pain may rewire the brain to reduce motivation, a new study in mice suggests.
“Acute pain is useful because it limits or stops our behaviour to prevent further injury, promote healing and prevent a similar injury in the future,” said the study’s lead researcher, Neil Schwartz, a postdoctoral research fellow of psychiatry at Stanford University in California.
In the study, Schwartz and his colleagues gave mice a chocolate-flavoured pellet if they poked their noses into a small hole. But the task became more difficult over time, requiring dozens of nose pokes for a pellet.
The researchers divided the mice into three groups — one group of mice with injuries to their sciatic nerves (the nerves that run down the back of the leg), one group of mice with inflamed paws and a control group of uninjured mice.
In tests a week later, the injured mice poked their noses into the hole, but they gave up sooner than the uninjured mice. This finding suggests they had decreased motivation.
Tests showed that the injured mice still liked the reward and were still able to move around the chamber. But somehow, the pain decreased their motivation. Even painkillers failed to improve their performance, ‘Live Science’ reported.
The researchers also found that some of the nerve cells in the nucleus accumbens, an area of the brain associated with pain and motivation, were not firing properly.
They identified a signalling chemical called galanin as the link between changes in the brain’s circuits and the decreased motivation.
When the researchers inactivated the galanin receptor in the nucleus accumbens, the brain currents returned to normal, and the injured mice completed the test as well as the uninjured mice did.
The study was published in the journal Science.