Making salt taste sweet

Through the activation of different receptors in our tastebuds, humans are able to perceive five basic taste sensations: sweet, umami, bitter, salty and sour. In the case for table salt, the concentration of it is an important factor in determining the taste. The preferable concentration is 100 mM, which is perceived as salty. Higher concentrations over 500 mM may be perceived as bitter or sour and very low concentrations, below 10 mM, are perceived as sweet. Some scientific studies have proposed the presence of multiple salt detection pathways in the tastebuds, but this is not fully understood.

In common salt (NaCI), salt taste sensation is driven by the sodium ion Na⁺. The anion (chloride ion CI^-) can also be detected through unique molecular mechanisms and participate in the taste sensation. Scientists from Okayama University in Japan have conducted a study to investigate this chloride ion detection mechanism by using structured biology methods and mice models.

The scientists had previously analysed the structure of a taste receptor from the Japanese rice fish (medaka fish) which is similar to the human sweet taste receptor and compatible for structural analysis as a part of it could bind to a chloride ion (CI^-).

Prof. Atsuko Yamashita said the previous research found that the T1r3LBD receptor from medaka fish could bind to CI^-. This led to the scientists discovering that this binding can induce a conformational change in the receptor.

This change in structure was found to be similar to that induced by other taste substances, suggesting that CI^- can activate the sweet receptors. Since change in shape often indicates receptor activation, the scientists further explored the chloride ion activation of the sweet taste receptors, which respond to sugars, in this study.

“We wanted to further investigate this phenomenon using better established animal models. Since the Cl^--binding site in T1r3 was conserved across various species, we decided to use taste nerve recordings from mice to explore the physiological significance of Cl^-,” Yamashita said.

The scientists performed electrophysiological assays on mice to provide evidence of this, where they demonstrated the activation of neurons involved in signalling of sweet taste when small amounts of chloride were placed on the mice’s tongues. This showed that low concentrations of CI^- could potentially produce a ‘light’ sweet taste sensation via that T1r in the tastebuds.

“The Cl^--induced taste is similar to that induced by canonical taste substances for T1rs, such as amino acids or sugars, though its efficacy is slightly lower,” Yamashita said.

When offered a choice between dilute chloride solution and plain water, the mice demonstrated a preference for the solution and recognised the taste of it. To induce a sweet response, the concentration of sodium chloride was found to be very low — less than 10 mM — and the sweet sensation could be suppressed by the external application of sweet taste inhibitors containing gurmarin.

These findings support the hypothesis that mice identify chloride as sweet via the action of specific receptors and neurons. They also show that dilute table salt provides a taste stimulus due to the presence of the Cl^- ions.

Table salt is important in maintaining homeostasis or internal body equilibrium. This is regulated by the optimum intake and excretion of sodium. This study shows that the former process uses the counter Cl^- ion to regulate the molecular functions of the receptors involved.

The results of this study can pave the way for a more nuanced understanding of taste perception.

Image credit: Atsuko Yamashita from Okayama University