Have you ever needed to hurry to the toilet during times of stress or after eating a spicy meal? This may be because taste buds lining your intestine can sense inflammatory chemicals and warn your brain to move things along. We know little about these taste buds, known as enterochromaffin cells. They first provoked curiosity when it was discovered that they produce 90 per cent of the body’s serotonin, a chemical mostly known for regulating mood, appetite and sleep in the brain. To find out why gut cells are releasing such large amounts of a brain chemical, David Julius at the University of California, San Francisco and his colleagues have been studying these cells in mini-intestines, grown from mouse cells in the lab. They have discovered that enterochromaffin cells have receptors for sensing dietary irritants, stress hormones and bacterial byproducts. When exposed to these substances, the cells pump out serotonin molecules, which activate intestinal nerve endings that connect back to the brain. The brain responds by speeding up bowel movements, or – if the situation is really bad – inducing diarrhoea or vomiting. “It might also give you a general sense of discomfort as a way of letting you know you’ve got some kind of inflammatory episode going on in there,” says Julius. Wasabi woes The team found that the dietary irritant that activated this response most strongly was allyl isothiocyanate – a sulphur-containing compound found in wasabi, horseradish, cabbage and broccoli. These foods are known to affect gut movements and cause abdominal grumblings when eaten in excess. Stress hormones – including adrenaline and noradrenaline – also had an activating effect. Levels of these chemicals in the gut rise in response to local inflammation there, but may also increase in response to general stress, says Julius. More research is needed, but this may be part of the link between stress and digestive issues, he says. The other major activator of the cells was a bacterial byproduct known as isovalerate. This substance is thought to be a marker of bacterial imbalances in the gut, and has been linked to abdominal pain. Irritable bowels All the substances that stimulated enterochromaffin cells have been implicated in irritable bowel syndrome (IBS) – a condition involving alternating episodes of constipation and diarrhoea. “We’re now looking into whether these cells might be hypersensitive in people with IBS,” says Julius. Already, there is evidence that antidepressants that boost serotonin levels – selective serotonin reuptake inhibitors (SSRIs) – help relieve constipation for some people with IBS, says Waliul Khan at McMaster University in Ontario, Canada. “This is consistent with increased serotonin causing increased gut motility,” he says. Drugs that lower serotonin levels, like the anti-nausea drug ondansetron, are sometimes used to reduce gut motility and relieve diarrhoea in people with IBS, says Julius. “But this drug acts on serotonin receptors all around the body, so it’s not very targeted,” he says. His team’s findings may lead to more targeted treatments for IBS and other gastrointestinal disorders, he says. Source
