The chemical that puts the heat in hot chili peppers is capsaicin (kap-SAY-ih-sin). Yet scientists have known for some time that when applied to the skin, this same compound can diminish pain. Indeed, some over-the-counter pain relievers already rely on capsaicin to tackle sore muscles and joints. But how that chili chemical chilled sore nerves has remained somewhat of a mystery. Until now.
Tibor Rohacs of Rutgers New Jersey Medical School in Newark and colleagues find that capsaicin works through a cascade of biological steps. They have just outlined them in the February 10 issue of Science Signaling.
In the body, capsaicin jump starts a chemical chain reaction, they find. The first step in that chain reaction was well known. That’s where capsaicin turns on a protein — Trpv1 — that senses heat. (This protein’s activity explains why rubbing creams containing capsaicin onto the skin produces a localized burning sensation.) But the compound does quite a bit more, it turns out.
More importantly for pain control: turning on Trpv1 floods pain-sensing nerves with calcium, Rohacs’ team has just discovered. That calcium boost causes a drop in levels of two fatty substances, known as lipids. They are known as phosphoinositides (FOS-foh-in-OSS-ih-tydz). These lipids reside in the wall (membranes) of nerve cells. The lipid dip, in turn, silences two proteins. They’re called called Piezo (PEETZ-oh) 1 and 2. Their job is to monitor whether and how much a cell membrane stretches.
Those Piezo proteins are important for many biological processes. They play a role in telling blood vessels to constrict. They also play roles in sensing touch and pain. Indeed, Rohacs’ group showed, shutting down those stretch sensors blocks the signal that normally alerts the brain to pain.
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calcium A chemical element which is common in minerals of the Earth’s crust. It is also found in bone mineral and teeth, and can play a role in the movement of certain substances into and out of cells.
capsaicin The compound in spicy chili peppers that imparts a burning sensation on the tongue or skin.
lipid A type of fat.
membrane A barrier which blocks the passage (or flow through of) some materials depending on their size or other features. Membranes are an integral part of filtration systems. Many serve that function on cells or organs of a body.
nerves Long, delicate fibers that communicate across the body of an animal. An animal’s backbone contains many nerves, some of which control the movement of its legs or fins, and some of which convey sensations such as hot, cold, pain.
proteins Compounds made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. The hemoglobin in blood and the antibodies that attempt to fight infections are among the better known, stand-alone proteins.Medicines frequently work by latching onto proteins.
sensor A device that picks up information on physical or chemical conditions — such as temperature, barometric pressure, salinity, humidity, pH, light intensity or radiation — and stores or broadcasts that information. Scientists and engineers often rely on sensors to inform them of conditions that may change over time or that exist far from where a researcher can measure them directly.