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

[wizardforce]

the two sensations could have evolved at differing times. pain would be most useful to prevent damage and the abiltiy to sense an itch is useful for correcting problems such as dry skin, certain chemical exposure etc. pain is associated with injury perhaps cells that sense an itch don't work the same way [no one has lopped off an arm and felt an itch for it after all...]

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

[caerwyn]

You're completely right.

Evolution doesn't really incentivize anything. It provides disincentives for exactly one thing: structures and behaviors that result in a higher likelihood of death before reproduction.

Evolution doesn't give a shit what your quality of life is like- unless it prevents you from reproducing. It doesn't give a shit what you do- after you've produce offspring. This is why male and female end-of-fertility times are correlated, and why that's also highly correlated with degradation of health. We haven't evolved to be immortal- we've evolved to a), create offspring and b) survive long enough to teach them. The same holds true for other species.

There are quite a number of mutations that do not affect our reproductive ability. The fact that evolution doesn't prefer one over the other is not only a good thing- it's an essential thing; that genetic variability is what improves our odds of responding effectively to new and dangerous conditions. Neutral mutations are *very* common- and not only that, they're *essential*.

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.

Uh

[LeadLine]

So we know how we could (possibly) get rid of itches, but is there any research showing what purpose itching has?

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.

Two Channel Interpretations

[DynaSoar]

A nice article and summary. Not entirely new nor inclusive of present theory unfortunately.

Pain is handled by two channels: nocioception, the sensation itself, and the perceptual distress component. This can easily be seen in the actions of the agents affecting each. Sensation is blocked by anesthesia. Interpretation of the pain signal is altered by analgseia -- you may still feel a sensation but you don't care, or at lest you're not so bothered by it. There are different neural pathways and processes to handle these.

It is likely that itching relates to pain in this fashion. The sensation of pressure or stretching of the skin in certain places would be common to all as their are receptors in the skin for these. A parallel pathway governing perceptual interpretation of that sensation, possibly the same one as for pain, would also exist. The resulting interpretation based on personal experience and/or genetically determined wiring would cause different interpretations of the same experience on different individuals, the same individual under different conditions, or (as is common) different locations on the same individual.

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

that article actually changed my world

[circletimessquare]

i have tinnitus

which, really, is "phantom hearing"

i hear a steady tone all day, and its not in my ear, its in my brain. i've grown used to it, accepting the fact i'll have this my whole life, and so it doesn't really bother me that much anymore, i go whole weeks now barely paying it any attention, and your mind just edits it out of daily life

but in the article they talk about the guy with the phantom limb pain... and they do a little trick with him standing perpendicular to mirror, and the brain sees two arms, and... voila, it resets the brain, no more phantom limb pain

amazing! wtf!?

i'm suddenly filled with hope for my tinnitus (false hope?)

if only there were such a neurological sleight of hand like those mirrors in that article, like that, but for the ear instead

i've been thinking about it... a movie of loud noise events without a soundtrack?

i don't know, some way like those mirrors to draw attention to the brain that, in fact, there is no tone playing, you can turn that circuit off now, thank you very much

but tinnitus is not uncommon, so me, or anyone else, who figures out the neurological sleight of hand like those mirrors and the phantom limb pain... to trick the brain into noticing: oh yeah, there is no tone playing, turn that off

the person who does that for tinnitus will win a nobel in medicine and become a millionaire, i promise