Sweet and bitter tastes switched in breakthrough mouse study
A remarkable new study led by researchers at Columbia University has found that the brain's complex systems for processing taste can be effectively manipulated. In mice, the researchers were able to directly manipulate neurons in the brain, removing the ability to taste sweet and bitter foods, or to make sweet foods taste bitter, and vice versa.
"When our brain senses a taste it not only identifies its quality, it choreographs a wonderful symphony of neuronal signals that link that experience to its context, hedonic value, memories, emotions and the other senses, to produce a coherent response," explains the senior author of the study, Charles Zuker.
The new research builds on a greater body of work from the team that has been investigating the neuroscience of taste. Earlier studies revealed that specialized cells on the tongue send signals to a variety of locations in the brain upon encountering certain tastes. A clear connection between the taste cortex and the amygdala was also identified.
"Our earlier work revealed a clear divide between the sweet and bitter regions of the taste cortex," says Li Wang, first author on the study. "This segregation between sweet and bitter regions in both the taste cortex and amygdala meant we could independently manipulate these brain regions and monitor any resulting changes in behavior."
So, the team generated a modified mouse model with amygdala neurons that could be manipulated using light, which meant the researchers could artificially trigger the sweet or bitter pathways at will. Initially, they found that when a mouse was drinking plain water they could trigger the sweet pathway, resulting in the animal gulping down the water, but when the bitter pathway was triggered the animal stayed well clear.
The experiments also successfully altered the perceived quality of taste in the animals by triggering contradictory pathways. Sugary sweet food could be turned into an unpleasant taste by triggering the bitter pathways and vice versa, with bitter food becoming attractive when manipulating the sugar pathway.
Perhaps the most interesting part of the research was the discovery that the amygdala seems to strongly regulate an emotional response to food. When the researchers disabled the animal's amygdala connections entirely, the mice seemed to still retain an ability to recognize sweet and bitter tastes but did not display any emotional preference or aversion to either.
"It would be like taking a bite of your favorite chocolate cake but not deriving any enjoyment from doing so," says Wang. "After a few bites, you may stop eating, whereas otherwise you would have scarfed it down."
This compelling discovery highlights how complex perceptions of taste are intertwined with emotional and memory centers in the brain. More excitingly, the research shows that these different pathways can be isolated from one another so a direct taste perception can be maintained while an emotional connection can be disrupted. The revelation that these particular emotional pathways are controlled by the amygdala suggests it may be a good target to explore new treatments for eating disorders.
The next step for the team is to further investigate other brain regions that are involved in how we process taste.
"We hope our investigations will help to decipher how the brain processes sensory information and brings richness to our sensory experiences," says Wang.
The study was published in the journal Nature.
Source: Columbia Zuckerman Institute