Study reveals that the tongue reacts to ammonium chloride via the same protein receptor responsible for signaling sour taste.
In the early 1900s, umami was first proposed as a basic taste—alongside sweet, sour, salty, and bitter—by Japanese scientist Kikunae Ikeda. Approximately eight decades later, this concept gained official recognition within the scientific community.
Recent findings suggest the existence of a sixth basic taste. Research led by a team at the USC Dornsife College of Letters, Arts, and Sciences provides supporting evidence.
Familiarity with a particular taste is common in Scandinavian countries, where it has been appreciated for generations. According to Liman, professor of biological sciences, salt licorice—a popular candy since at least the early 20th century—includes salmiak salt, or ammonium chloride, as a key ingredient.
For decades, strong reactions of the tongue to ammonium chloride have been acknowledged by scientists. However, the precise tongue receptors involved in this response remained unidentified, despite significant research efforts.
A potential explanation has been explored by Liman and the research team. In recent years, the protein responsible for detecting sour taste was identified. Known as OTOP1, this protein resides in cell membranes and facilitates the movement of hydrogen ions into cells.
Hydrogen ions, the defining components of acids, are responsible for the sour taste experienced with certain foods. This explains the tartness in lemonade, rich in citric and ascorbic acids, or in vinegar, containing acetic acid. These hydrogen ions move into taste receptor cells via the OTOP1 channel, signaling the sour sensation.
Due to the ability of ammonium chloride to influence the concentration of hydrogen ions within cells, researchers hypothesized its potential to activate OTOP1. To test this, the Otop1 gene was introduced into lab-grown human cells, prompting the production of the OTOP1 receptor protein. The cells were then exposed to acid and ammonium chloride, with responses carefully measured.
“Ammonium chloride is a really strong activator of the OTOP1 channel. It activates as well or even better than acids.”
Emily Liman – USC Dornsife neuroscientist
This occurs because ammonium chloride releases small amounts of ammonia, which raises intracellular pH by reducing hydrogen ions, creating a pH gradient that drives proton influx through OTOP1.
To validate these findings beyond laboratory settings, electrical conductivity techniques were applied. These simulated nerve signaling by measuring electrical responses in taste bud cells from normal mice and genetically engineered mice lacking OTOP1. When exposed to ammonium chloride, taste cells’ ability to generate action potentials was assessed, providing further evidence of the role of OTOP1 in detecting this compound.
Taste bud cells from wildtype mice exhibited a significant increase in action potentials when exposed to ammonium chloride. In contrast, taste bud cells from mice lacking OTOP1 showed no response to the compound. This finding supported the hypothesis that OTOP1 detects ammonium chloride and generates electrical signals in taste bud cells.
Similar results were observed when signals were recorded from the nerves innervating the taste cells. Courtney Wilson, a member of the research team, observed nerve responses to ammonium chloride in normal mice but no response in mice without OTOP1.
Behavioral experiments were also conducted to examine how mice reacted when offered plain water versus water containing ammonium chloride. For these tests, bitter taste receptors contributing to ammonium chloride perception were disabled. Mice with functional OTOP1 proteins found the solution unappealing and avoided it, while mice lacking OTOP1 proteins showed no aversion, even at high concentrations of ammonium chloride.
“This was really the clincher. It demonstrates that the OTOP1 channel is crucial for the behavioral response to ammonium chloride.”
Emily Liman – USC Dornsife neuroscientist
The investigation extended to other species to determine if OTOP1 channels are similarly sensitive to ammonium chloride. Variations were observed, with some species displaying greater sensitivity than others. Human OTOP1 channels were also found to respond to ammonium chloride.
The evolutionary purpose of detecting ammonium chloride remains a topic of interest. Liman suggests that this ability may have developed to help organisms avoid consuming harmful biological substances containing high levels of ammonium, serving as an important protective mechanism.
Ammonium, commonly present in waste products such as fertilizers, is known to have toxic properties. Liman explained that the evolution of taste mechanisms capable of detecting ammonium likely serves as a protective measure. For instance, chicken OTOP1 channels exhibit far greater sensitivity to ammonium than those of zebra fish, which may reflect the ecological differences between these species.
“Fish may encounter minimal ammonium in water, whereas environments like chicken coops are rich in ammonium that must be avoided.”
Emily Liman – USC Dornsife neuroscientist
These findings are part of early-stage research, and further investigations are needed to fully understand species-specific variations in ammonium sensitivity. The factors contributing to the differing responses of OTOP1 channels across species remain an area of interest.
Progress has been made in identifying a critical component of the OTOP1 channel—a specific amino acid—required for ammonium detection.
“When this amino acid is mutated, the channel’s sensitivity to ammonium decreases significantly, although it continues to respond to acid.”
Emily Liman – USC Dornsife neuroscientist
The conservation of this amino acid across species suggests that selective evolutionary pressures maintained its presence, highlighting the importance of ammonium detection for survival.
Future research aims to explore whether sensitivity to ammonium is preserved among other members of the OTOP proton family, which are expressed in various parts of the body, including the digestive tract.
As understanding deepens, the role of ammonium chloride in taste perception might eventually elevate it to the status of a sixth basic taste, expanding the traditional framework of taste classification.
https://dx.doi.org/10.1038/s41467-023-41637-4
Abstract
Ammonium (NH4+), a breakdown product of amino acids that can be toxic at high levels, is detected by taste systems of organisms ranging from C. elegans to humans and has been used for decades in vertebrate taste research. Here we report that OTOP1, a proton-selective ion channel expressed in sour (Type III) taste receptor cells (TRCs), functions as sensor for ammonium chloride (NH4Cl). Extracellular NH4Cl evoked large dose-dependent inward currents in HEK-293 cells expressing murine OTOP1 (mOTOP1), human OTOP1 and other species variants of OTOP1, that correlated with its ability to alkalinize the cell cytosol. Mutation of a conserved intracellular arginine residue (R292) in the mOTOP1 tm 6-tm 7 linker specifically decreased responses to NH4Cl relative to acid stimuli. Taste responses to NH4Cl measured from isolated Type III TRCs, or gustatory nerves were strongly attenuated or eliminated in an Otop1−/− mouse strain. Behavioral aversion of mice to NH4Cl, reduced in Skn-1a−/− mice lacking Type II TRCs, was entirely abolished in a double knockout with Otop1. These data together reveal an unexpected role for the proton channel OTOP1 in mediating a major component of the taste of NH4Cl and a previously undescribed channel activation mechanism.
