They then tracked the response of sensory neurons to pain evoked by pinching a paw, injecting a drug that causes inflammation, or exposing the animals to heat. In that study, they generated mice with a genetically encoded calcium sensor that fluoresces when the neurons are activated. In work published in 2016, neuroscientist Xinzhong Dong’s group at the Johns Hopkins University School of Medicine and their collaborators describe an imaging technique they developed to monitor more than 1,600 neurons in the DRG of live mice. “The most interesting thing about this paper is that-rather than looking at individual neurons and how they may or may not change after an injury-they actually look at a population level, so how the neurons work together in a group and how that might contribute to kind of pain that we have not really done a great job modeling,” says Erin Young, who studies pain at the University of Kansas Medical Center and also was not involved in the study. “The work addresses a very important clinical problem,” he adds, and “is quite a beautiful explanation for how, after peripheral nerve injury, you get one kind of spontaneous pain.” Because they’re totally unpredictable, there’s nothing a person can do to avoid them, so they cause enormous suffering and anxiety,” says Edgar Walters, who studies chronic pain at the University of Texas Health Science Center at Houston and did not participate in the work. “These paroxysms of spontaneous pain can really be debilitating.
The study authors report that this so-called cluster firing is driven by abnormal sprouting of sympathetic nerves into the DRG, which happens after injury. In a study published November 8 in Neuron , researchers link spontaneous pain in mice to coordinated firing of nonadjacent neurons in the dorsal root ganglia (DRG), which are collections of sensory neuron cell bodies just outside the spinal cord.
It’s a common issue for people who have chronic pain and, because it lacks an external stimulus, difficult to treat. Spontaneous pain arises suddenly, without a clear trigger, and can feel like shooting, stabbing, burning, or electric shocks. Green fluorescent neurons respond to pinching the hind paw, and red fluorescent neurons fire together independent of peripheral stimuli. ABOVE: Two images of the same dorsal root ganglion under different activation conditions in a live mouse are superimposed.
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