Chili peppers help to unravel the mechanism of pain

Feb 24, 2009
Chili peppers

Capsaicin, the active ingredient in chili peppers, is most often experienced as an irritant, but it may also be used to reduce pain. A new work published by Drs. Feng Qin and Jing Yao in this week's PLoS Biology uses capsaicin to uncover novel insight into how pain-receptor systems can adapt to painful stimuli. Sensory systems are well known to adapt to prevailing stimuli. For example, adaptation happens when your eyes adjust from a dark movie theater during a matinee to the bright sunlight outside. Whether pain receptors truly adapt or rescale their responses (versus simply desensitizing) has been an open question.

Capsaicin acts by binding to a receptor in the cell wall of nerve endings and triggering an influx of calcium ions into the neuron. Eventually, the nervous system interprets this cascade of events as pain or heat, depending on which nerves are stimulated. Scientists had previously linked the pain-relieving effects of capsaicin to a lipid called PIP2, found in cell membranes. When capsaicin is applied to the skin it induces a strong depletion of PIP2 in the cell membrane.

"The receptor acts like a gate to the neurons," said Qin. "When stimulated it opens, letting outside calcium enter the cells until the receptor shuts down, a process called desensitization. The analgesic action of capsaicin is believed to involve this desensitization process. However, how the entry of calcium leads to the loss of sensitivity of the neurons was not clear."

Capsaicin creams are commonly sold over the counter as effective treatment for a variety of pain syndromes, from minor muscle or joint aches to those that are very difficult to treat, such as arthritis and neuropathic pain.

By combining electrical and optical measurements, the authors now have been able to link directly the depletion of PIP2 and the desensitization of the receptor. The authors also showed that the receptor is fully functional after desensitization - i.e. although you stop feeling pain - are desensitized - if another event occurs that would normally trigger a 'pain' response - such as an increased concentration of capsaicin - the desensitization does not affect that feeling. "What changed was the responsiveness threshold," said Qin. "In other words, the receptor had not desensitized per se, but its responsiveness range was shifted. This property, called adaptation, would allow the receptor to continuously respond to varying stimuli over a large capsaicin concentration range."

The findings have implications for pain sensation mechanisms as well as clinical applications. With an adaptive response, the receptors are essentially autoregulated without a fixed threshold, thus the intensity of the pain you experience is dependent on the recent history of pain.

More information: Yao J, Qin F (2009) Interaction with phosphoinositides confers adaptation onto the TRPV1pain receptor. PLoS Biol 7(2): e1000046. doi:10.1371/journal.pbio.1000046, biology.plosjournals.org/perls… journal.pbio.1000046

Source: Public Library of Science

Explore further: Japan lab cannot repeat ground-breaking cell finding: reports

add to favorites email to friend print save as pdf

Related Stories

New study sheds light on painkilling system in brain

Aug 24, 2010

Repeatedly boosting brain levels of one natural painkiller soon shuts down the brain cell receptors that respond to it, so that the painkilling effect is lost, according to a surprising new study led by Scripps Research Institute ...

Naked mole-rats bear chili pepper heat

Jan 29, 2008

Pity the tiny naked mole-rat. The buck-toothed, sausage-like rodent lives by the hundreds in packed, oxygen-starved burrows some six feet under ground. It is even cold-blooded -- which, as far as we know, is unique among ...

Recommended for you

Nature offers video of 10 cutest animals of 2014

1 hour ago

(Phys.org)—The journal Nature has released a video that ventures a bit from its traditional strictly-science approach to technical journalism—it's all about the cutest animal stories of the past year ( ...

Stranded pilot whale rescued in Cape Verde

2 hours ago

The archipelago nation of Cape Verde is widely recognised as a marine biodiversity hotspot, not least because of the abundance of marine mammals found in its waters.

Protection of the mouse gut by mucus depends on microbes

2 hours ago

The quality of the colon mucus in mice depends on the composition of gut microbiota, reports a Swedish-Norwegian team of researchers from the University of Gothenburg and the Norwegian University of Life Sciences in Oslo. ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.