Itchy? You've Got Some Nerve
Researchers studying itchy mice determine that separate neurons deliver sensations for itchiness and pain.
Think that itch of yours is a pain in your neck? Think again. According to a team of six researchers from Washington University in St. Louis and Peking University in Beijing, the two sensations — itching and pain that is — are completely independent of each other.
For years, scientists considered itch and pain to be two related sensations, with itch being lesser of the two evils. However, this study — published online in the journal Science — found that the feeling of "itch" is controlled by its own specific neuron in the nervous system.
"As humans, when we feel itch, we know it's not pain, and when we feel pain we know it's not itch," said co-author Zhou-Feng Chen. "[And] when you deal with the five senses, we know that senses like taste, even the difference between sweet and bitter, have their own labeled line of cells [or set of neurons that specifically transmit a particular sense to the brain] … but past studies have failed to show neurons specific to each sensation of pain and itch."
From a 2007 study, the scientists knew a gene called "Gastrin-releasing peptide receptor," or GRPR, was responsible for the itch sensation we feel, but they were unsure as to whether the neurons expressing the GRPR gene communicated other sensations to the brain beside itch (in this case pain).
To test whether these GRPR+ neurons were itch-specific, the scientists actually destroyed the neurons in a group of mice by injecting their spinal cords with a toxin called "bombesin-saporin."
"By using mice you can manipulate them on the gene level," Chen said, "and perform pain and itch tests that you could not perform on humans, especially because our responses to pain and itch are so subjective."
The team subjected the mice to plethora of itch-inducing chemicals, and found those with no GRPR+ neurons had substantially reduced scratching behavior, ranging between 71 and 88 percent less than normal mice.
Then, knowing that knocking out GRPR neurons essentially knocked out the perception of itch, the researchers determined whether feeling of pain was similarly affected.
They found behaviors like "paw withdrawal," licking, and flinching remained statistically the same throughout the chemical, thermal, and mechanical pain tests the mice underwent with no distinguishable difference between the control mice and those missing GRPR+ neurons.
One day, the scientists hope that their results could have applications for human health, especially in the treatment of people with chronic itch disease like eczema or psoriasis.
"The clinical applications are huge," said Chen. "This could lead to anti-itch therapy that blocks chronic itch without affecting sensations like pain or temperature sensation."
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