People tend to scratch itchy parts of their body, but this actually makes the itching worse.
A group of researchers from the Washington University School of Medicine in St. Louis offers a plausible explanation why scratching actually makes an itch feel itchier.
The study, which was published in the journal Neuron on Oct. 30, involved experiments with mice.
Zhou-Feng Chen from the Center for the Study of Itch at the Washington University School of Medicine and colleagues found evidence that scratching prompts the brain to release serotonin, which then intensifies the itchy sensation.
To assess the effects of the neurotransmitter serotonin, Chen and colleagues injected a chemical, which causes skin to itch, into the mice that were genetically altered so they would not have the genes to produce serotonin. They found that these mice did not scratch as much as normal mice.
When the researchers injected serotonin into these genetically engineered mice, however, they observed that these animals scratch just like normal mice.
"We report that enhancing serotonergic tone via administration of 5-HT potentiates itch sensation, whereas mice lacking 5-HT or serotonergic neurons in the brainstem exhibit markedly reduced scratching behavior," Chen and colleagues wrote.
Scratching is supposed to relieve itching by causing minor pain in the skin, which results in the nerve cells transmitting pain signals instead of itch signals to the brain. The brain responds to this signal by releasing the serotonin that helps control the pain, and this is when the problem starts.
Chen explained that when the chemical spreads into the spinal cord from the brain, serotonin can move from pain-sensing neurons to nerve cells that worsen the itch.
Scientists have long discovered the role of serotonin in controlling pain, but the study marks the first time that the release of this neurotransmitter from the brain has been associated with itch.
Based on their study, Chen explained how scratching worsens itching. He said that the pain caused by scratching leads to the production of more serotonin. Unfortunately, serotonin does more than inhibit the pain. The serotonin receptor called 5-HT1A also activates the itch-specific GRPR neurons.
"Our new finding shows that it also makes itch worse by activating GRPR neurons through 5-HT1A receptors," Chen explained.
Serotonin also has a role in aging, growth and bone metabolism and in regulating mood. Antidepressants such as Paxil, Prozac and Zoloft likewise work by increasing an individual's serotonin levels.
