Pain Manifestation is Less if You Are with Companions

A new animal study highlights an interesting fact that, companionship has the potential to reduce pain linked to nerve damage.

More than 20 million Americans experience the nerve pain known as peripheral neuropathy as a consequence of diabetes or other disorders as well as trauma, including spinal cord injury. But only a few reliable treatments are available.

However, social contract lowered the pain response and signs of inflammation even in animals that had experienced stress prior to the nerve injury.

For the study, the researchers paired one group of mice with a single cage-mate for one week while other mice were kept socially isolated. For three days during this week, some mice from each group were exposed to brief stress while others remained non-stressed. Later, a nerve surgery producing sensations that mimic neuropathic pain was done on one group of mice and a sham procedure that didn't involve the nerves on a control group.

Based on response to a light touch to their paws, researchers tested all groups of mice behaviorally for a week after the surgery.

They noticed that the mice that were paired with a cage-mate showed lower pain responses and fewer signs of inflammation in their nervous system after undergoing a surgery that affected their nerves. But the isolated rats differed in this from the cage rats.

'Allodynia' is the nerve related pain that the mice experienced. This is a withdrawal response to a stimulus that normally would not extract a response in this case.

Mice that had lived with a social partner, irrespective of the stress level, require a higher level of force before they showed a withdrawal response compared to isolated mice that were increasingly responsive to a lighter touch.

"If they were alone and had stress, the animals had increased inflammation and allodynia behavior," said Adam Hinzey, a graduate student in neuroscience at Ohio State University and lead author of the study. "If the mice had a social partner, both allodynia and inflammation were reduced."

"A better understanding of social interaction's beneficial effects could lead to new therapies for this type of pain," Hinzey said.

"Animals that were both stressed and isolated maintained a lower threshold -- less force was needed to elicit a paw withdrawal response. Animals that were pair housed and not stressed withstood a significantly greater amount of force applied before they showed a paw withdrawal response," Hinzey said. "Within animals that were stressed, pairing was able to increase the threshold required to see a withdrawal response."

The researchers also examined the animals' brain and spinal cord tissue for gene activation affecting production of two proteins that serve as markers for inflammation. These cytokines, called interleukin-1 beta (IL-1B) and interleukin-6 (IL-6), are typically elevated in response to both injury and stress.

Compared to animals that received a sham procedure, isolated mice with nerve damage showed higher levels of IL-1B gene expression in their brain and spinal cord tissue. There was a significant decrease in gene activity related to IL-6 production in the spinal cords of non stressed animals compared to the mice that were stressed.

"We believe that socially isolated individuals are physiologically different from socially paired individuals, and that this difference seems to be related to inflammation," said Courtney DeVries, professor of neuroscience at Ohio State and principal investigator on this work. "These data showed very nicely that the social environment is influencing not just behavior but really the physiological response to the nerve injury."