Everyone must have experienced it, originally only a little itchy, but the more you scratch it, the more itchy it becomes; Scratching the skin with nails is very comfortable, but the comfort is too short-lived because it often leads to another round of itching "attack". And the culprit of this vicious cycle is actually a substance called the "happiness hormone" - serotonin, a neurotransmitter. Scientists once thought that itching was a type of pain sensation, belonging to relatively mild pain sensations.   Although pain and itch sensations are not exactly the same, they still have a close relationship and share the same neural circuit in a certain area of the brain. And it is precisely because of the shared circuit that one party is activated while the other is inhibited - scratching causes pain sensation, and the sensitivity of itching sensation is reduced, so scratching can temporarily eliminate itching sensation. At the same time, scratching also releases serotonin, which can reduce pain sensation. In this way, scratching makes people feel very comfortable, without pain or itching. However, recent research by Chen Zhoufeng's team has shown that scratching can exacerbate the vicious cycle of "itch scratch itch". A portion of itch sensation neurons have receptors that help reduce pain sensation, while another portion of receptors can cause itch sensation. Although serotonin can only bind to the part of receptors that can reduce pain sensation, due to the proximity of the two receptors, they interact with each other. Therefore, serotonin, which binds to receptors that reduce pain sensation, indirectly affects the enhancement of itch sensation receptors, making people more itchy. Chen Zhoufeng and colleagues found that activating both groups of receptors in mice simultaneously resulted in more frequent scratching compared to activating itch receptors alone. In another experiment, the experimental mice used lacked cells that produce serotonin. As a result, compared to normal mice, this group of experimental mice scratched less frequently after skin irritation. The research results have been published in the journal Neuron.

Researchers have not yet found neurons specialized in transmitting itch sensation in the human body (itch sensation neurons exist in rhesus monkeys). However, we can responsibly remind everyone to think twice before scratching.