Sweet, salty, bitter, sour, and umami: the five basic tastes. It is the finely tuned array of these taste qualities which allows us to savor tasty meals or to take a sip of spoiled milk and toss it quickly down the drain. We do not think about our actions; they are innate and we are primed to know the difference. We taste for survival – bitter can alert us to poisons; sweet signals energy; umami informs us of protein content; salty helps us maintain cellular balance.
Scientists know we taste for survival but have wondered how taste messages are sent from the tongue to the brain. Charles Zuker, a neuroscientist at Columbia University, coined the “one taste, one cell” system in mammals (Chandrashekar et al. 2006), Yamolinsky et al. 2009). In short, they concluded that the taste cells in our tongue are highly specific, each cell expresses receptors that can only bind one basic taste. After the target taste molecule binds its receptor in the taste cell, a neural pathway then sends the signal to the brain. But where in the brain?
Previous brain models for taste identification suggested a broad pattern of neural recognition across taste qualities, but Zuker’s publication in Science (Chen et al. 2011, p. 1262) shook up the field of gustatory science. When artificial saliva containing different compounds was applied onto the tongues of anesthetized mice, Zuker’s team was able to see discrete clusters of nerve cells fluoresce based on the selective response to the individual basic tastes of bitter, salt, sugar and umami. Moreover, these clusters of taste-specific neurons were located in precise and spatially segregated areas of the cortex (Figure 1). Interestingly, the researchers did not find sour-selective neurons. Zuker reasoned that this may be because those neurons exist outside the region of the cortex they surveyed. So while we know for sure there is no taste map on the tongue, evidence suggests that there may be one in the brain.
Almost a decade later is where we pick this story back up. It’s not simple as just getting to the right part of the brain for taste. In 2018, Zuker’s team wanted to go beyond how we identify tastes, and learn more about how we process what we like and don’t like and where in the brain this happens. So they followed the neurons that trace the sweet and bitter pathways to the brain. Sweet being something we like from birth, also called appetitive or a predilection. And bitter something no one is born liking (yes hop heads, I know you can grow to like bitter), also called aversive. Wouldn’t you know, both went to the amygdala, aka your emotional and motivational headquarters). The amygdala is also know to play a role in both positive and negative stimuli, and the most interesting part of this work is how anatomically distinct these two taste pathways are in how they get to and where they go in the amygdala. The amygdala might just be the key to how why we gravitate to the “delicious” and why we avoid the “disgusting.” One final note, harkening us back to the role of culture and preferences, the amygdala is known to establish strong associations between stimuli (think Pavlovian association). This could help explain why we like our coffee and IPAs so much. The amygdala is likely linking and layering positive sensations and emotions to the typically bitter experience. Therefore, even though your brain still says, hey this is bitter, your behavior is attuned to the whole picture of the experience and you happy sip, imbibe and relish the experience. Have you thanked your amygdala today?