A major question facing the pain field remains: Is chronic pain maintained by changes in the central nervous system (CNS) alone, or does primary afferent nerve input contribute? Now, Cheryl Stucky and colleagues at the Medical College of Wisconsin, Milwaukee, US, show that primary afferent firing was decreased, not increased, compared to controls, in a mouse model of chronic inflammatory pain. The investigators also discovered differences in neuronal activity between young and aged mice during acute and chronic pain. The report was published online February 8 in eNeuro.
Nader Ghasemlou of Queen’s University in Kingston, Ontario, Canada, who was not involved in the new research, called it “really beautiful work,” and said the experiments performed for the study represent a heroic effort. “These findings begin to lay the groundwork for understanding the differences between young and old animals—and for understanding the transition from acute to chronic pain,” which Ghasemlou called “the Holy Grail in pain research now.”
The young and the old, peripheral and central
The research team began with behavioral testing of young and aged mice. “Very few studies have been done in aged animals, yet aged humans are fraught with pain more frequently than are young people,” Stucky said. They found that pain-free older (over 77 weeks of age) mice were more sensitive to mechanical stimuli than young animals (aged seven to 20 weeks). The investigators then injected mice in the hindpaw with complete Freund’s adjuvant (CFA), which created ongoing inflammation and related pain behaviors that lasted eight weeks. Both young and aged mice displayed pronounced behavioral mechanical sensitization two days after the injection, but it was much more dramatic in young mice.
First author Andy Weyer and colleagues then used a skin-nerve preparation in which the saphenous nerve was dissected along with the skin it innervates and made electrophysiological recordings from primary afferent neurons. Two days after CFA injection, C-fiber afferents from young mice showed increased firing in response to mechanical stimulation of the skin relative to naïve animals, as expected. In contrast, C-fiber sensitization was not significant in aged mice during acute inflammation, and eight weeks after injection, C-fiber firing was indistinguishable from that of naïve mice. The authors concluded that aged mice were less malleable in their neuronal responses to both acute and chronic inflammation.
Surprisingly, in young mice with chronic inflammation, C-fiber firing was actually diminished compared to naïve mice. “We did not expect to find that; by eight weeks, when we still see prominent behavioral sensitivity, firing would actually be reduced,” Stucky told PRF. C-fiber responses to capsaicin were also diminished eight weeks after injection, as were Aδ-fiber responses to mechanical stimuli, suggesting that nociceptive afferent activity is broadly dampened in this chronic inflammation model.
What could account for the reduced sensory afferent firing? Perhaps decreased expression of ion channels or receptors, but eight weeks after CFA, the researchers saw no significant changes in transcript levels of voltage-gated sodium channels, transient receptor potential (TRP) channels, or mechanically sensitive receptors. Instead, Stucky said the findings suggest that “something else is working to inhibit the firing of these neurons, perhaps serving to reduce afferent drive to the CNS.”
There are several possible sources of such inhibition, the most likely of which, Stucky said, is the CNS itself, possibly engaging descending pain modulation pathways from the brainstem. “That will be the next step: to investigate whether a CNS mechanism serves to inhibit the enhanced drive to the CNS.” However, if the CNS was the source of inhibition, Stucky and Ghasemlou agreed, it would have to be very strong and long-lasting inhibition, considering that the inhibition observed in the current study persisted in a nerve preparation completely dissociated from the CNS.
On the other hand, Stucky said, peripheral immune cells could dampen the firing. Ghasemlou pointed out that the inhibition could also arise from the primary afferents themselves. “It could be a neurogenic response; the nerves may be secreting something—like substance P or CGRP [calcitonin gene-related peptide]—in the skin, and that in turn modulates their own activity or that of nearby cells, including immune cells in the vicinity.” (See, e.g., Chiu et al., 2012.) Skin cells, such as keratinocytes or fibroblasts, might also secrete influential factors, as could satellite glial cells in the dorsal root ganglia. “It’s very complicated, but very interesting,” Ghasemlou said.
What drives chronic pain?
The new findings raise another major question, Stucky said. “Something is carrying that chronic pain drive, and it doesn’t appear to be primary afferents.” Instead, neuronal signaling changes likely occur in the spinal cord, brainstem, and higher brain centers, she said. Stucky said her findings should not be interpreted to mean that primary afferents are not important for chronic pain. “Primary afferents are definitely important for initial enhanced drive, perhaps setting up chronic pain conditions, but they may not be required for prolonged sensitization.” That realization could guide how—and when—inflammatory pain treatments are delivered. “Peripheral analgesics may be very important right away for stopping that drive, but longer-term therapies may need to be focused on the CNS,” she said.
“There won’t be one magic bullet for chronic pain. Instead, researchers will have to target multiple pathways, probably at different times, to stop chronic pain from developing,” Ghasemlou told PRF.
Immune cells could also contribute to driving chronic pain in the periphery or the CNS. However, recent work from Ghasemlou indicated that peripheral immune cells were not required for ongoing mechanical hypersensitivity in the CFA model (Ghasemlou et al., 2015). “The immune response has a role in the CFA model, but only in the first 48 hours; then, after that, [mechanical pain] seems not to be controlled by circulating immune cells,” he said. “What other component is controlling pain is really a question mark right now in the CFA model.”
It’s all about the model
At this point, the finding that sensory afferent firing is diminished, not enhanced, with chronic inflammatory pain is limited to the CFA model, Stucky said. For example, in a model of sickle cell disease-related pain, C-fiber and A-fiber firing did increase at chronic time points (Garrison et al., 2012). Pain in different models—and different human conditions—might depend on completely different cell-signaling pathways. “Inflammatory pain is a different entity than neuropathic pain,” Stucky said, and each individual condition may be unique. “The etiology makes a big difference,” she added.
The CFA model certainly has its limitations. Animals are injected with an adjuvant—a messy concoction meant to activate the immune response—setting up inflammation that likely bears little resemblance to human pain conditions. Still, it is the only model used to study chronic inflammatory pain. “If you want to understand chronic pain, you need to use a chronic model. Acute models are fine for acute pain, but that’s not modeling chronic pain,” Stucky said.
Ghasemlou agreed that when it comes to studying chronic inflammatory pain, “this model is as good as we have now.” And the new findings pave the way for a better understanding of chronic pain. “This work has really opened my eyes to potential ideas and possible mechanisms that might be going on.”
Stephani Sutherland, PhD, is a neuroscientist, yogi, and freelance writer in Southern California.
Image credit: The Stucky lab