Occasionally I will branch out from neuroscience and into related fields such as psychology and molecular biology such as the receptors required for taste. There are five different basic types of taste: sweet, salty, bitter, sour, and umami which is sometimes debated as the fifth. Each type of taste has different sets of taste cells which respond to tastants in the mouth. From these taste receptors the signals are sent to the brain, although the functional neuronal circuitry for taste is still being discovered. For sweet, the most common sweet taste receptors are the Tas1R2 and Tas1R3 proteins which form a heterodimer that responds to sweet tastants. However, not all animals produce the Tas1R2 receptor.
Where this becomes interesting is in a further study by Jiang et al. (2012) who tested both Asian otters, known to have no functional Tas1R2 proteins, and the spectacled bear with a functional Tas1R2 receptor. As expected, when subjected to various sweet compounds the spectacled bear showed increased preference for sweet compounds such as sucrose, fructose, galactose, and lactose, shown by a preference ratio above fifty percent for the sweetened solution compared to water. Also as expected, the Asian otter showed no preference for the same sugars. However, the spectacled bear actually showed a decreased preference for the taste of the artificial sweetener aspartame.
It's interesting to see this selective avoidance to aspartame and not the other artificial sweetener saccharin in the spectacled bear. Clearly there remains to be something aversive that the bear is able to pick up upon which makes you question the continued use of it in products today, considering the alternatives available.
References:
1. Jiang, P., Josue, J., Li, X., Glaser, D., Li, W., Brand, J. G., Margolskee, R. F., Reed, D. R., Beauchamp, G. K. (2012) Major taste loss in carnivorous animals. PNAS, 109(13), 4956-4961. doi: 10.1073/pnas.1118360109
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