How do acute painful bouts of inflammation progress to chronic, intractable pain? Identifying the molecular basis of that transition—and how to stop the process—is critical to understanding, and treating, many chronic pain conditions.
A new study from Erica Schwartz, Gerald Gebhart, and colleagues at the University of Pittsburgh School of Medicine, Pennsylvania, US, shows that in a mouse model of recurrent pancreatic inflammation, the transition from acute inflammation to chronic organ damage and pain involves a switch from transient neuroinflammation driven by TRPV1 and TRPA1 channels on sensory neurons to a later, lasting pain state that is impervious to treatment with TRP inhibitors.
The work suggests that in mice subjected to repeated inflammatory insults "there is a transition point" in the molecular mechanisms driving inflammation and pain, Gebhart said. "There are some players that are involved in the beginning … but not after some critical point."
The results raise the possibility that, in people, early treatment with inhibitors of the TRPV1 and TRPA1 channels might help to block the development of chronic pancreatitis. Later treatment, however, may have little effect.
While the study focused on pancreatitis, the results may bear on other painful inflammatory conditions such as inflammatory bowel disease or arthritis, said Rodger Liddle, a gastroenterologist at Duke University Medical Center, Durham, North Carolina, US, who was not involved in the study. "It is important from the standpoint of understanding how chronic pain develops—how does that occur? The paper provides some insights into that mechanism and the timing of some of the events that lead to chronic pain."
The study appeared in the March 27 issue of the Journal of Neuroscience.
A mysterious transition
Pancreatitis arises when digestive enzymes destined for the small intestine prematurely activate in the pancreas, leading to inflammation and pain. In many cases, the condition is transient. But some patients go on to develop a chronic form of the disease, which can be extremely painful. "There are people who have chronic, debilitating pain that is almost uncontrollable. It can be devastating," Liddle said.
Studies in animal models have indicated that changes in primary sensory neurons play a major role in pancreatitis pain. In the initial stages, inflammatory mediators in the pancreas ramp up nociceptor activity. Then the neurons exacerbate the situation by releasing their own inflammatory neuropeptides into the pancreas. Liddle and others have shown that TRP channels contribute to this process, called neurogenic inflammation (Nathan et al., 2001). Recently, Schwartz and colleagues showed that treatment of mice with antagonists of TRPV1 and TRPA1 could reverse inflammation and pain in a mouse model of acute pancreatitis induced by injection of the peptide caerulein (Schwartz et al., 2011).
In the new study, the investigators set out to model repeated bouts of pancreatic inflammation, to ask how that might lead to chronic pancreatitis, and chronic pain. Schwartz said, "We wanted to see why pain, inflammation, and all the parts of the disease progress"—to a point where, in patients, the disease becomes increasingly difficult to treat.
To simulate recurrent acute pancreatitis in mice, Schwartz and colleagues administered caerulein repeatedly, twice a week for 10 weeks. For the first two weeks, immune cell infiltration was only modestly elevated (when measured 72 hours after caerulein injection). But following three weeks of injections, persistent inflammation and organ damage increased dramatically. Pancreatic tissue showed extensive inflammatory infiltrate, atrophy, necrosis, and increased sprouting of sensory nerve fibers—changes similar to those seen in samples from people with chronic pancreatitis. In pancreatic sensory neurons, the investigators saw a sustained increase in expression of TRPV1 and TRPA1, and enhanced excitability. Alongside these changes, the animals showed increased pain behaviors (measured by changes in spontaneous activity and rearing), which reached a plateau at about three weeks.
To see whether TRPV1 and TRPA1 mediated the changes induced by repeated inflammatory insult, the researchers treated the animals weekly with TRPV1 and TRPA1 antagonists (A-889425 and A-967079, both from Abbott Laboratories). They found that inhibiting the channels could reduce inflammation, organ damage, hyperinnervation, channel expression, neuronal excitability, and pain behaviors. But the timing mattered. TRP-blocking treatments worked if they were started before the third week of caerulein challenge; if treatments were started later, they had little effect.
The results indicated that TRP-driven neurogenic inflammation played a major role in the initial pain and in the onset of chronic painful pancreatic damage. That aligns with previous work showing that TRPV1 inhibition reduced pain in rats with chronic pancreatitis simulated using a single administration of the chemical agent trinitrobenzene sulfonic acid (TNBS) (Xu et al., 2007), and a new study showing that TRPA1 knockout reduced pain and inflammation in a similar model in mice (Cattaruzza et al., 2013).
But the study from Schwartz and colleagues, where persistent pain was brought on by recurrent insults, indicated that after a critical TRP-dependent period—three weeks, in this model—chronic pain was maintained by some other molecular mechanism.
Searching for downstream mediators
The current work was all done in a mouse model, but if the results are confirmed in people, it suggests that early treatment with TRP inhibitors might prevent the onset of chronic pancreatitis. For people known to be at high risk, such as individuals with inherited genetic variants that predispose them to chronic pancreatitis, such a preventive therapy could be useful, Liddle said. But both he and Schwartz said that, based on the results in mice, TRP inhibitors might not help patients whose pancreatitis has already become chronic. "For most people we see in the GI clinic who have chronic pancreatitis, they're not going to benefit," Liddle said.
Schwartz is now looking for the molecular changes that sustain chronic pancreatitis in the mouse model, and that might present targets for treatment of later-stage disease. In the current paper, she showed that repeated caerulein challenge increased activation of MAP kinase (MAPK) signaling in nociceptive sensory neurons, indicated by increased phosphorylated ERK (pERK) immunoreactivity. And in preliminary experiments, she said, she has found that ERK inhibition can block pain at later timepoints. She is also looking at downstream targets of MAPK signaling, such as voltage-gated ion channels, to see whether they are upregulated at later stages.
Moving forward, Schwartz said she would like to look at the basis for acute-to-chronic transitions in other visceral diseases such as colitis or cystitis. But a major barrier is a lack of models. Generally, she said, laboratory models of visceral pain involve a single, highly damaging insult, rather than multiple episodes that over time lead to chronic pain. To catch the transition to chronic pain, "You have to come up with a recurring acute inflammatory model that progresses into chronic inflammation." Currently, she said, "There's just not a lot out there."
Further, Gebhart said, researchers may want to examine repetitive inflammation in other tissues—including skin, muscles, and joints—where acute insults seem to prime the system for chronic pain, and where animal models involving a single insult already exist. "If you repeat the stimulus or the inflammation, you may find that the players change." Identifying those late-stage mediators, he said, "could have important therapeutic implications."