Why Facial Itch Feels Different

Researchers at North Carolina State University found that itch on the face is perceived differently from itch on the body because the signals travel through different pathways and are processed differently in trigeminal and spinal sensory systems. The study suggests this difference may help explain why facial itch responses can be lower than those in other parts of the body and could point to future molecular targets for treating facial pain or itch. 

The researchers compared itch responses in mice after histamine exposure on the cheek and the nape of the neck. They saw that the cheek produced a significantly reduced itch response compared with the neck. To determine whether anatomy might explain the difference, they next examined innervation in the face and neck and found that nerve presence alone did not account for the different responses. 

They then analyzed neuronal populations in the dorsal root ganglia, which serve the body, and the trigeminal ganglia, which serve the face and head. According to the study, these two systems differ because they operate in different sensory environments and do not appear to process itch signals in the same way. In the dorsal root ganglia, there are distinct pathways for pain and itch. Substance P is associated with pain, while B-type natriuretic peptide is associated with itch. When the signal for Substance P is received, it initiates a pain response; when the signal for BNP is received, it triggers itch.

The trigeminal ganglia showed a more complex pattern, with three possible pathways: one for Substance P expression, one for BNP expression, and one in which both are expressed. In the mice, activation of the third pathway appeared to lead to greater production of Substance P than BNP. The researchers think this overlap may push the system away from itch and toward pain-related signaling.

“This overlap in the TG and overproduction of SP seems to be ‘shunting’ the itch response aside,” explains Santosh Mishra, senior author of the study published in Communications Biology. “Our next steps will be to explore why SP is overproduced in this situation.”

The researchers hope that the work could lead to better treatments for facial itch.