Cat taste receptors respond in a unique way to bitter compounds compared with human receptors, according to research published in the open access journal BMC Neuroscience. The study represents the first glimpse into how domestic cats perceive bitterness in food at a molecular level, and could explain why cats are sometimes such picky eaters.
The ability to detect bitter chemicals is thought to have evolved because of its utility in avoiding toxic compounds often found in plants. All cats, from pets to wild tigers, are carnivores that consume little plant material. Domestic cats, however, may still encounter bitter flavors in food and medicines.
Domestic cats have a reputation of being rather unpredictable in their dietary choices. This could be explained by their perception of bitter, which differs from that of other mammals due to variations in their repertoire of bitter receptors. It is the goal of many pharmaceutical and food manufacturers to identify compounds that either block or alter bitter perception, to create a more palatable product.
The teams at AFB International and Integral Molecular studied the behavior of two different cat bitter taste receptors in cell-based experiments, investigating their responsiveness to bitter compounds, and comparing these to the human versions of these receptors.
TAS2R38 is a bitter taste receptor in humans of which some people have 'supertaster' variants that give them an extreme sensitivity to bitter compounds, explaining some people's strong aversions to broccoli and brussels sprouts.
Compared with the human TAS2R38 receptor, the cat version was tenfold less sensitive to a key bitter compound PTC and did not respond at all to another bitter compound PROP.
Like its human counterpart, the cat bitter taste receptor Tas2r43 was activated by bitter compounds aloin (found in the aloe plant) and denatonium (used to deter children and pets from consuming chemicals such as antifreeze) but responded differently to the compounds. The cat receptor was less sensitive to aloin and more sensitive to denatonium than the human receptor. It also differed from the human taste receptor by being insensitive to saccharin, an artificial sweetener that tends to have a bitter aftertaste in humans.
Co-author Nancy Rawson from AFB International, a pet food flavor company, said: 'We confront the challenge of 'finicky cats' every day. As such, it is exciting to find an unexpected receptor response to bitter compounds that has never been described in the literature to date for any other species. These insights and future discoveries will be invaluable in formulating appealing food for cats, as well as enhancing the acceptability of their medications.'
Co-author Joseph Rucker from Integral Molecular, a biotechnology company, said: 'Feline bitter taste has not been well studied. We applied our experience in studying membrane proteins, such as taste receptors, to enable this first glimpse into how domestic cats perceive bitterness in food at a molecular level. We were surprised to see that one of the cat taste receptors responded to a more limited range of bitter compounds compared to humans, suggesting that cats may be detecting a narrower, or at least a different, repertoire of bitter-tasting compounds.'
The team also found that probenecid, a known inhibitor of human bitter taste receptors, also worked on both cat taste receptors, preventing stimulation when in the presence of PTC, aloin and denatonium.
The team says that these insights and further study could be instrumental in formulating appetizing food for household cats as well as designing masking agents to enhance the acceptability of medications.
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Rawson NE et al. A functional comparison of the domestic cat bitter receptors Tas2r38 and Tas2r43 with their human orthologs. BMC Neuroscience, Published June 3 2015. doi: 10.1186/s12868-015-0170-6