Neuron Path Discovery May Change Our Conception of Itching

Hugh Pickens writes "Historically, many scientists have regarded itching as just a less intense version of pain, though decades spent searching for itch-specific nerve cells have been unfruitful. Now, Nature reports that neuroscientist Zhou-Feng Chen and his colleagues at Washington University in St. Louis, Missouri have found the first behavioral evidence that there are separate circuits of nerve cells to convey itchiness and pain, and their studies suggest that itch and pain signals are transmitted along different pathways in the spinal cord. 'Most people accept that there are specific, highly specialized neurons for sensations like taste,' says Chen. 'But for pain and itch this is much more controversial.'" (Continues below.) "Two years ago, Chen's group discovered that a cell-surface protein called the gastrin-releasing peptide receptor (GRPR) is important for sensing itchiness but not pain in mice. When Chen and his colleagues destroyed GRPR-bearing neurons by means of a cell toxin, the mice reacted to painful stimuli just like normal mice, licking themselves and flinching or jumping in response to heat, highly irritant chemicals and mechanical pressure. But when the researchers injected the animals with chemicals that normally cause scratching, such as histamine, they barely responded, and the greater the number of GRPR-expressing neurons destroyed, the more subdued was the scratching response."

Comments

Interesting from an evolution POV

[WindBourne]

I have always thought that Evolution prefers the minimal amounts needed for life (greater complexity is difficult to maintain UNLESS for a reason). As such, it would be easier on life if the same neuron conveys pain and itch. Yet, Evolution chose to do something different.

I wonder what was the stimuli for that?

Re:Interesting from an evolution POV

[ZackSchil]

[no one has lopped off an arm and felt an itch for it after all...]

Clearly you've never lost a limb. Phantom limb sensations cover pressure, pain, temperature, and irritation. You definitely can feel an itch on an arm or leg that has been lopped off.

Re:Interesting from an evolution POV

[ShakaUVM]

>>I'll bet the 'itching' pathways have other uses as well. Perhaps the tickling response is there?

Tickling, I believe, is linked with touch. Your brain suppresses/mutes touches done to yourself, which is why most people can't tickle themselves. How does the brain tell? If your motion and the sensation come within a threshold of each other, it mutes the sensation. I think there's something like a 45ms threshold involved - when people moved a machine that then moved a tickling finger, if they added a delay of more than 45ms to it, suddenly people could tickle themselves.

There's a lot of interesting hacks inside the human brain.

Torture

[mr100percent]

I hope they don't use this to build the Agony Booth found in Star Trek.

I could have told them that after my dentist visit

[mrjb]

After a dentist visit with local anesthesia, I got bitten by a mosquito which caused a terrible itch on my cheek- but I couldn't feel my scratching to relief it. NOT FUN.

Re:Pondering the luck of others

[skeffstone]

This awesome TED talks talks about phantom limbs, and one way to truly remove them! http://www.ted.com/talks/lang/eng/vilayanur_ramachandran_on_your_mind.html Grab a cup of coffee, and enjoy! :)

a curse you should never wish on your worst enemy:

[circletimessquare]

the phantom itch

http://www.newyorker.com/reporting/2008/06/30/080630fa_fact_gawande?currentPage=all

same neurological basis as a phantom limb, but far more rare (blessedly so)

it is probably one of the greatest definitions of hell on earth. the itch that never, ever goes away:

M. was willing to consider such possibilities. Her life had been a mess, after all. But the antidepressant medications often prescribed for O.C.D. made no difference. And she didn't actually feel a compulsion to pull out her hair. She simply felt itchy, on the area of her scalp that was left numb from the shingles. Although she could sometimes distract herself from it--by watching television or talking with a friend--the itch did not fluctuate with her mood or level of stress. The only thing that came close to offering relief was to scratch.
"Scratching is one of the sweetest gratifications of nature, and as ready at hand as any," Montaigne wrote. "But repentance follows too annoyingly close at its heels." For M., certainly, it did: the itching was so torturous, and the area so numb, that her scratching began to go through the skin. At a later office visit, her doctor found a silver-dollar-size patch of scalp where skin had been replaced by scab. M. tried bandaging her head, wearing caps to bed. But her fingernails would always find a way to her flesh, especially while she slept.
One morning, after she was awakened by her bedside alarm, she sat up and, she recalled, "this fluid came down my face, this greenish liquid." She pressed a square of gauze to her head and went to see her doctor again. M. showed the doctor the fluid on the dressing. The doctor looked closely at the wound. She shined a light on it and in M.'s eyes. Then she walked out of the room and called an ambulance. Only in the Emergency Department at Massachusetts General Hospital, after the doctors started swarming, and one told her she needed surgery now, did M. learn what had happened. She had scratched through her skull during the night--and all the way into her brain.

...

The second theory seemed less likely. If the nerves to her scalp were dead, how would you explain the relief she got from scratching, or from the local anesthetic? Indeed, how could you explain the itch in the first place? An itch without nerve endings didn't make sense. The neurosurgeons stuck with the first theory; they offered to cut the main sensory nerve to the front of M.'s scalp and abolish the itching permanently. Oaklander, however, thought that the second theory was the right one--that this was a brain problem, not a nerve problem--and that cutting the nerve would do more harm than good. She argued with the neurosurgeons, and she advised M. not to let them do any cutting.
"But I was desperate," M. told me. She let them operate on her, slicing the supraorbital nerve above the right eye. When she woke up, a whole section of her forehead was numb--and the itching was gone. A few weeks later, however, it came back, in an even wider expanse than before. The doctors tried pain medications, more psychiatric medications, more local anesthetic. But the only thing that kept M. from tearing her skin and skull open again, the doctors found, was to put a foam football helmet on her head and bind her wrists to the bedrails at night.
She spent the next two years committed to a locked medical ward in a rehabilitation hospital--because, although she was not mentally ill, she was considered a danger to herself. Eventually, the staff worked out a solution that did not require binding her to the bedrails. Along with the football helmet, she had to wear white mitts that were secured around her wrists by surgical tape. "Every bedtime, it looked like they were dressing me up for Halloween--me and the guy next to me," she told me.
"The guy next to you?" I asked. He had had shingles on his neck, she explain