Pain 101: How the Immune System Influences Chronic Pain

Microglia emerge as important players in the communication between immune cells and nerve cells. Image: A stylized depiction of a microglial cell. Credit: Iryna Timonina/123RF Stock Photo.

Editor’s Note: The third North American Pain School (NAPS) took place June 24-28, 2018, in Montebello, Quebec, Canada. This educational initiative brought together leading experts in pain research and management to provide 30 traineespart of the up-and-coming generation of pain researcherswith scientific education, professional development and networking experiences.

Six of the trainees were also selected to provide first-hand reporting from the event, including summaries of talks presented at the meeting. Here, Alexander Tuttle, who is doing post-PhD research at the University of North Carolina at Chapel Hill, summarizes a talk delivered by Michael Salter, a pain researcher at The Hospital for Sick Children in Toronto. Salter discussed how cells called microglia allow the immune system and nervous system to “talk” to each other, which can result in chronic pain.

Historically, in the search for new ways to treat chronic pain, scientists have focused on the nervous system. However, after decades of work and several high-profile failures of potential new painkillers, researchers began to realize that the workings of the nervous system alone could not fully explain how chronic pain starts, and why it persists.

More and more, scientists are realizing that different systems in the body interact with one another to cause and perpetuate pain. One such interaction occurs between the nervous system and the immune system.

In a talk at the 2018 North American Pain School, Michael Salter, a pain researcher at The Hospital for Sick Children in Toronto, Canada, shared his group’s efforts to understand how these two systems interact. His findings from studies in mice show that microglia, known as the immune cells of the central nervous system (brain and spinal cord), contribute to chronic pain.

To understand how, it’s first necessary to understand the basics of the pain system. Nerve cells (neurons) called nociceptors relay information about potential threats to the body—high heat or dangerous chemicals, for instance—into the spinal cord. When neurons in the spinal cord receive this information (a process known as nociceptive signaling), which is transmitted in the form of electrical signals, they can turn the signal up or down, as well as control whether to send the signal to the brain. Salter showed that during the development of chronic pain after nerve injury, microglia in the spinal cord can amplify the signal relayed by the nociceptors. Once the signal reaches the brain, it can culminate in an experience of pain.

Not just glue
Until very recently, glial cells (which include microglia and another population of cells called astrocytes) in the brain and spinal cord were thought to merely provide structure to the nervous system. Indeed, the Greek word glia means “glue.” However, recent discoveries like Salter’s reveal that glia are much more than the glue that holds the nervous system together.

Salter began his talk by noting that microglia make up 10% of the cells in the central nervous system. These cells are constantly moving around and carry out many functions. Microglia protect neurons from foreign bacteria and viruses, make neurons more or less receptive to signals coming from other neurons by reshaping the physical connections between neurons, and even tell neurons when it’s time for them to die.

In these ways, microglia help maintain a healthy nervous system. However, microglia also appear to contribute to neurological disorders, including autism spectrum disorders, schizophrenia, Alzheimer’s disease, and, as Salter’s group discovered, certain types of chronic pain.

Microglia contribute to pain
To understand how microglia change electrical signaling in the spinal cord following nerve injury, Salter’s group modeled chronic pain in mice. Specifically, the researchers would injure a large nerve in a mouse’s leg and then look at changes to microglia in the spinal cord.

In normal animals, the microglia resembled the shape of octopuses, with multiple projections constantly “feeling” the surrounding environment for foreign threats. However, following nerve injury, the microglia were activated by signals coming in from the nociceptors. They underwent a Jekyll and Hyde-like transformation by retracting their projections and increasing in size until they looked more like fat, angry rainclouds.

These activated microglia started communicating differently with neurons, releasing a chemical called brain-derived neurotrophic factor (or BDNF) that caused spinal neurons to more easily transmit electrical signals to the brain. The BDNF caused massive changes to spinal neurons next to the microglia. This included quieting neurons that normally block the transmission of nociceptive signals to the brain. In short, after sensing BDNF made by activated microglia, adjacent spinal neurons more readily sent those signals to the brain.

The researchers also saw changes in how mice responded to light pressure. Nerve-injured mice with activated microglia in the spinal cord suddenly withdrew their paws following a gentle probe with a small fiber, a common experimental test used by pain researchers. And, by using drugs to block a specific molecule on the surface of the microglia, Salter’s lab found that this sensitivity associated with nerve injury went away; now, injured mice were no different in their response to a paw poke than uninjured animals. The pain sensitivity had disappeared.

Salter likened the changes they found to driving with one foot on the brake and the other on the gas pedal at the same time. After receiving signals from activated microglia, the spinal cord takes its foot off the brake and greatly increases the signal coming in from the nociceptors. After years of study, Salter’s group was confident that this analogy explained pain in all rodents. However, in following up on this initial finding, researchers discovered a big twist to the story.

The case for microglia machismo?
Pain researchers know that women are more likely to experience chronic pain at some point in their lives than men are. Although there are many credible reasons for these sex differences in pain, several recent studies suggest that differences in the female versus male immune system can contribute to the development of chronic pain.

One way to discover sex differences is to study both male and female animals in the same set of experiments. In attempting to recreate Salter’s discovery, Robert Sorge, at the time a postdoctoral researcher in Jeffrey Mogil’s lab at McGill University in Montreal, Canada, blocked the activity of microglia in both male and female mice, to see if microglia contributed to pain after nerve injury in both sexes. Sorge, now an associate professor at the University of Alabama at Birmingham, found that male mice whose microglia had been quieted by specific drugs did not show subsequent hypersensitivity to a paw poke. However, female mice continued to show such hypersensitivity when their microglia were deactivated. It seemed, then, that microglia contributed to pain sensitivity in males, but not females.

Why was this so? Working together, Salter’s lab and Mogil’s lab showed that the sex differences could not be explained by differences in the number of activated microglia in the spinal cord; both male and female mice showed the same number of activated cells after nerve injury.

Instead, it appeared that the way microglia communicated with spinal neurons differed in male versus female mice. While Salter’s original observations in male mice showed that BDNF made by microglia in the spinal cord was necessary to boost transmission of signals coming from the nociceptors up to the brain, neurons from females responded to nerve injury even in the absence of BDNF. In fact, Salter’s group removed microglia entirely (using a toxin that kills the cells) from the spinal cord in female mice, but these animals still showed pain hypersensitivity.

Microglia and pain “memory”
Salter ended his talk by discussing the possible role that microglia play in changing pain perception later in life, in animals that experience injury during early life. There is mounting evidence that babies who experience pain in the first few days after birth, because of necessary medical procedures, show differences in processing of nociceptive signals as adults.

To better understand how pain is processed differently in the spinal cord after early life trauma, Salter’s group modeled the effects of neonatal surgery using newborn mice. Specifically, the researchers made a small incision in the paw of the newborn animals and tested their responses to pain after they grew up.

Salter saw that these mice showed exaggerated sensitivity following a second injury to the same paw as adults. And, Salter’s group saw significantly more activated microglia in the spinal cord of these mice than in animals that did not receive the paw incision.

Finally, the scientists found the same differences between the sexes that they saw previously in their earlier experiments in adult mice. Specifically, when the team used a drug to quiet the activity of microglia in the spinal cord, adult male mice that received surgery as newborns no longer showed an exaggerated pain response following re-injury of the same paw later on. Female mice, on the other hand, continued to show heightened pain sensitivity even if their microglia were quieted.

This set of findings was important for three reasons. First, it provided a potential explanation for how early life pain can affect subsequent pain in adulthood.

Second, it reinforced the emerging pattern where, as microglia respond to an injury, pain sensitivity becomes more exaggerated in male, but not female mice.

Third, it offered additional evidence that it is critical to provide newborns with adequate pain relief during early life medical procedures, especially in cases of premature birth. As rates of pre-term births increase in the US and Canada, there is overwhelming evidence that repeated painful procedures can alter pain sensitivity, just like Salter observed in his mice. By understanding what’s going on in the nervous system, scientists can show why it is so important for healthcare providers to minimize the pain that early life medical interventions might cause, in order to decrease the likelihood of pain later in life.

It’s complicated
Still, while Salter’s research is convincing, there is an ongoing need for scientists to model the interaction between the immune and nervous system using other animal species. Initial studies using rats, for example, failed to show the same difference between the sexes in the way microglia influence a mouse’s response to pain.

By the same token, there is a need for researchers who study patients to learn how microglia affect the human response to pain. So far, not a single human study has been run to determine whether there are differences between men and women in the way microglia impact pain processing in the spinal cord.

It is increasingly clear that pain is complicated. While neuroscientists like Salter had previously restricted their efforts to studying the nervous system in isolation, there is an increasingly pressing need to understand how complex systems interact with one another in the body to influence pain in people. The good news is that, study by study, the complicated picture of human pain is coming into better focus.

Salter says that his perspective on pain has changed over the course of his research. “For me, chronic pain is a disease like cancer; it’s not a single disease. Even if you’re talking about one type of cancer, the changes that occur are different across different individuals.” However, by modeling different aspects of the human pain condition in animals, Salter and others are increasing our understanding of how individual differences can impact the experience of pain.

Alexander Tuttle is a postdoctoral fellow at the University of North Carolina at Chapel Hill.