How does chronic pain affect the sensation of touch?

Rob Bonin receives new research funding to examine neurobiology of pleasant touch and how response to gentle touch can be altered in chronic pain

A pat on the back, a soft touch on the arm, a warm hug – from the right person at the right time, these types of gentle touch can feel pleasant to many people. But they don’t always. Not only can the person and environment play a role in whether the touch feels pleasant, but conditions such as schizophrenia, autism and chronic pain can also change how gentle touch and other pleasant stimuli are perceived.

Bonin’s research program is focused on sensory plasticity – how the nervous system “rewires” and changes activity in nervous system disorders such as chronic pain. He recently received a five-year, $876,000 grant from the Canadian Institutes of Health Research (CIHR) to examine changes in the neurobiology of chronic pain that affect a person’s ability to feel pleasure from gentle touch.

“We’re looking for differences in the perception of gentle touch to see changes in the nervous system response that occur in chronic pain and whether that may lead to some of the behavioural changes we see in people with this condition.”

“An individual’s response to gentle touch is very plastic and modifiable. You can have a variable response from different scenarios, ranging from pleasant to aversive. We can take advantage of this wide spectrum of responses to study the neurobiology of touch perception,” says Rob Bonin, associate professor at U of T’s Leslie Dan Faculty of Pharmacy. “We’re looking for differences in the perception of gentle touch to see changes in the nervous system response that occur in chronic pain and whether that may lead to some of the behavioural changes we see in people with this condition.”

Many animal species – from mice to monkeys – use gentle touch as an important form of social communication, and, generally, humans are no exception. But some people with chronic pain, which is associated with depression and loss of pleasure (anhedonia), do not feel pleasure from gentle touch – all of which may be connected.

“Touch perception is altered in chronic pain in a way that is independent from the actual pain itself,” says Bonin. “This research may give some insight into the broad nervous system changes associated with chronic pain and perhaps even how depression and changes in social behaviour can arise in chronic pain. If individuals with chronic pain and anhedonia do not get that reward or satisfaction from social interactions, they may be less inclined seek out those interactions.”

Changes in sensory processing could contribute to poor mental health

Researchers have recently identified a subset of sensory fibres that respond to pleasant gentle touch, which has opened up a new field of research examining the nervous system and its responses to various pleasant and aversive stimuli.

In Bonin’s sensory plasticity research, his team has found a way to activate the nerves that perceive pleasant touch in animal models using blue light. Because environments with this light are pleasurable for the animals, they seek out the light, which allows the researchers to study biological and behavioural responses to gentle touch and interventions that could restore the pleasantness of this touch.

With the newest research funding, Bonin and his team will examine how mice with chronic pain respond when the gentle touch nerves are activated, and determine whether the sensation is pleasant. The team will also study the regions of the brain that are activated when the nerves are stimulated to look for differences in the brain’s responses to this stimulus in mice with and without chronic pain.

According to the Canadian Psychological Association, more than a quarter of people with chronic pain also experience significant depression or anxiety. Bonin’s new research will help to build the understanding of sensory processing in chronic pain and the potential impact on a person’s behaviour and even mental health.

“We will have a better understanding of the neurobiology of how this important form of social communication is altered in chronic pain, as well as how potentially this contributes to depression or anhedonia,” says Bonin. “It’s a new way of understanding changes in emotional processing associated with chronic pain.”