Ever wondered why whenever you scratch an itch, it makes you want to scratch it more, without relieving the itch? Why does it even feel so good that you just don't want to stop, until your skin breaks? Researchers from the Washington University School of Medicine in St. Louis are shedding light on this enigma as they found a link between scratching and serotonin release--the culprit of chronic itching.
The logic for scratching comes from the idea that scratching an itchy part of the body creates a mild amount of pain that makes one's attention directed on the pain, and away from the itch.
Researchers say that the diversion is temporary though.
The new study indicates that scratching causes the brain to release serotonin--the "happy" neurotransmitter, which intensifies the itch sensation.
Prof Zhou-Feng Chen, director of Washington University's Center for the Study of Itch, explained that the spread of serotonin to the spinal cord after being triggered by the pain caused from scratching could actually heighten the skin's itch.
"The problem is that when the brain gets those pain signals, it responds by producing the neurotransmitter serotonin to help control that pain. But as serotonin spreads from the brain into the spinal cord, we found the chemical can 'jump the tracks,' moving from pain-sensing neurons to nerve cells that influence itch intensity," Chen said.
"Scientists uncovered serotonin's role in controlling pain decades ago, but this is the first time the release of the chemical messenger from the brain has been linked to itch."
The researchers used mice as the specimen for their study. They bred a strain of genetically engineered mice that lacked serotonin. When injected with itch-inducing chemicals, these mice didn't scratch as much as mice with the ability to produce serotonin. But as soon as the genetically engineered mice were injected with serotonin, the mice were observed to scratch more.
"So this fits very well with the idea that itch and pain signals are transmitted through different but related pathways. Scratching can relieve itch by creating minor pain" Chen says. "But when the body responds to pain signals, that response actually can make itching worse."
The researchers warned against interfering with serotonin in the relief of itch. Serotonin is known to be vital in growth, aging, bone metabolism, and improving mood, especially among depressed people. Blocking serotonin will have far-reaching consequences throughout the body, which is why controlling its release was not advised by the researchers.
What they suggested instead was the possible interference with the communication between serotonin and nerve cells in the spinal cord that specifically transmit itch. Those cells, known as GRPR neurons, relay itch signals from the skin to the brain. They also discovered that the receptor known as 5HT1A was the key to activating the itch-specific GRPR neurons in the spinal cord.
"We always have wondered why this vicious itch-pain cycle occurs. Our findings suggest that the events happen in this order. First, you scratch, and that causes a sensation of pain. Then you make more serotonin to control the pain. But serotonin does more than only inhibit pain. Our new finding shows that it also makes itch worse by activating GRPR neurons through 5HT1A receptors."
The findings of the study are published online in the journal Neuron.
