A new organ involved in the sensation of pain has been discovered by scientists, raising hopes that it could lead to the development of new painkilling drugs.
Researchers say they have discovered that the special cells that surround the pain-sensing nerve cells that extend into the outer layer of skin appear to be involved in sensing pain – a discovery that points to a new organ behind the feeling of “ouch!”.
The scientists say the finding offers new insight into pain and could help answer longstanding conundrums.
“The major question for us now is whether these cells are actually the cause for certain kinds of chronic pain disorders,” Prof Patrik Ernfors, a co-author of the research from the Karolinska Institute in Sweden, told the Guardian.
Writing in the journal Science, the researchers reveal how they examined the nature of cells in the skin that, they say, have largely been overlooked. These are a type of Schwann cell, which wrap around and engulf nerve cells and help to keep them alive.
The study has revealed these Schwann cells have an octopus-like shape. After examining tissues, the team found the body of the cells sits below the outer layer of the skin, but that the cells have long extensions that wrap around the ends of pain-sensing nerve cells that extend up into the epidermis, the outer layer of the skin.
The scientists were surprised at the findings because it has long been believed that the endings of nerve cells in the epidermis were bare or unwrapped. “In the pain field, we talk about free nerve endings that are responsible for pain sensation. But actually they are not free,” Ernfors said.
The team’s biggest finding was that these Schwann cells can sense pain. The discovery was made using optogenetics, which involved the researchers genetically modifying mice so Schwann cells in the skin of their feet produced a protein that could absorb light.
When light is shone on the cells, this protein changes, affecting the membrane and producing a shift in the cells’ electrical charge – in other words, the cells are stimulated.
When light was shone on these cells, the mice lifted their feet. They also showed behaviour including licking, shaking and guarding of their paws – signs that stimulating these Schwann cells caused pain.
As the pulses of light increased in duration, the number of nearby nerve cells firing increased, supporting the idea that these Schwann cells send a signal to the brain through the nerve cells.
To explore what else might activate these Schwann cells, the team exposed the feet of the mice to heat, cold and pinpricks and looked at their behaviour. They then compared that behaviour with the animals’ responses when light was used to slightly activate these Schwann cells, making them more sensitive, or to deactivate them.
The results showed that for all three stimuli, mice showed a stronger pain response after the cells were activated by light and, in the case of pinpricks, a weaker response after the cells were deactivated. The work has yet to be replicated in humans.
This suggests the special Schwann cells are important in sensing pain – at least for mechanical harm. With the special Schwann cells and the nerves they engulf forming a mesh-like network, the researchers say they have essentially discovered a new pain-sensing organ. “It is a two-cell receptor organ: the nerve and Schwann cell together,” Ernfors said.
Prof Peter McNaughton, an expert in sensory systems from King’s College London, said the study was interesting and radical. “If borne out by subsequent studies, this paper will be a paradigm shift showing that pain-sensitive nerve cell terminals are not in fact always directly driven by a painful stimulus but instead can be driven by associated [Schwann] cells,” he said.
He said questions remained, including how the Schwann cells transmit a signal to the pain-sensitive nerve cells.
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