People who have chronic pain are more likely to experience mood disorders, but it's not clear how this happens. Now a study in mice has found that chronic pain can induce genetic changes in brain regions that are linked to depression and anxiety, a finding that may lead to new treatments for pain.
"At least 40 per cent of patients who suffer from severe forms of chronic pain also develop depression at some point, along with other cognitive problems," says Venetia Zachariou of the Icahn School of Medicine at Mount Sinai in New York.
To see if there might be a genetic link between these conditions, Zachariou and her team studied mice with damage to their peripheral nervous system. These mice show symptoms similar to chronic pain in people – they become hypersensitive to harmless touch, and avoid other situations that might also cause them pain.
Until now, pain behaviour in mice had only been studied for at most a week at a time, says Zachariou, whose team monitored their mice for 10 weeks. "At the beginning, we saw only sensory deficits and pain-like symptoms. But several weeks later, the animals developed anxiety and depression-like behaviours."
The team then examined gene activity in three regions in the mouse brains we know are associated with depression and anxiety. Analysing the nucleus accumbens, medial prefrontal cortex, and periaqueductal gray, they found nearly 40 genes where activity was significantly higher or lower than in mice without the nervous system damage.