February 15, 2013
Researchers Discover How We Taste And Avoid High Salt Concentrations
Brett Smith for redOrbit.com - Your Universe Online
Salt is a necessity for life, yet too much of it can have detrimental effects, such as hypertension or kidney failure in humans.
According to the team´s report in the journal Nature, their findings could be used to help control the diet of individuals who eat too much salt, leading to the reduction of sodium´s negative effects on the body.
Salt is a unique taste for the human tongue because our response to it is highly dependent on the concentration. In contrast, sweet and umami tend to invite an attraction response, while bitter and sour tend to invite a repulsive response – all independent of concentration.
"Salt taste in mammals can trigger two opposing behaviors," said study co-author Charles Zuker, a professor in the Department of Biochemistry & Molecular Biophysics and in the Department of Neuroscience at Columbia University. "Mammals are attracted to low concentrations of salt; they will choose a salty solution over a salt-free one. But they will reject highly concentrated salt solutions, even when salt-deprived."
Previous research from Zuker and his collaborator Nicholas Ryba from the National Institute of Dental and Craniofacial Research has uncovered the biochemistry on tongue that is responsible for detecting sweet, sour, bitter, and umami tastes, as well as low concentrations of salt.
"But we didn't understand what was behind the aversion to high concentrations of salt," said co-author Yuki Oka, a postdoctoral fellow in Zuker's Columbia laboratory.
Expecting high-salt receptors to reside in specialized cells, the researchers were surprised to discover that this is not the case.
"Over the years our studies have shown that each taste quality–sweet, bitter, sour, umami, and low-salt–is mediated by different cells," Ryba said. "So we thought there must be different taste receptor cells for high-salt. But unexpectedly, Dr. Oka found high salt is mediated by cells we already knew."
In their research, the team found that high concentrations of salt actually trigger the bitter- and sour-sensing pathways on mammals´ tongues. The biochemists confirmed these findings by silencing the bitter or the sour pathways, resulting in a reduction in the aversion to high-salt solutions. When both of these repulsive receptors were silenced, the mammals´ distaste to high-salt solutions completely vanished. In fact, some of the test subjects even showed an unrestrained and deadly attraction to seawater-type solutions.
Because ingesting seawater can lead to dehydration, kidney failure, and death, the two aversion pathways form a robust safeguard against salt toxicity, Oda said.
While the study´s findings have dietary implications, the researchers' stated objective is to understand how we perceive the sensory world. With a better understanding of taste receptors, the researchers are now using brain imaging to map the neurons that receive information from the tongue.
The latest findings have come as a surprise to researchers as they have indicated discrete hotspots within the brain for four of the five tastes: sweet, bitter, umami, and salty.