New research on mice suggests the mammalian brain has a “gustotopic map” comprising a unique group of neurons that code tastes from sweet to salty. The findings show that this group of neurons responds differently and discretely as the tongue encounters specific tastes. Previous studies had suggested the brain had a more general response, with overlaps, but this study suggests the maps are unique and located in specific areas for each taste.
The study is the work of scientists at the Howard Hughes Medical Institute and National Institutes of Health (NIH) in the US and is published in the 2 September issue of Science.
Previous studies have already shown that the brain has specialized, finely tuned cells for detecting unique tastes. But this new study shows that the brain of mammals also perceives four of our basic tastes: sweet, bitter, salty and “umami” or savory, as unique maps, in distinct areas of the brain.
Study author Charles S. Zuker, a Howard Hughes Medical Institute investigator who is based at Columbia University College of Physicians and Surgeons in New York, told the press:
“This work further reveals coding in the taste system via labeled lines, and it exposes the basic logic for the brain representation of the last of the classical five senses.”
Study collaborator and co-author Nicholas J. P. Ryba, of the National Institute of Dental and Craniofacial Research in Bethesda, Maryland, said we have always been fascinated by how we perceive the sensory world:
“What is a taste, really? It’s the firing of a set of neurons in the brain, and that’s what we want to understand,” said Ryba, who with Zuker and others had already identified that brain cells carry unique taste receptors and taste receptor cells for each taste, with a coding scheme based on a “one taste, one cell class” system.
They showed then that activating these receptors made mice behave in unique ways. For example, activating the receptors for sweet and savory encouraged them, while activating the ones for bitterness and sourness discouraged them.
And, before this study, other researchers, also working with mice, had tried measuring electrical activity of small groups of neurons in response to different tastes and found that the areas that were activated seemed to blend together. This led them to conclude that neurons seemed to process tastes in a broad sense.
So, while it appeared there was a clear link between taste and “hardwired” behaviors, why would the same neurons be processing different tastes?
Lead author Xiaoke Chen, a postdoctoral associate in Zuker’s lab tested a hunch using a new method called two-photon calcium imaging. This allowed the team to find out which neurons were activated by different tastes.
When an animal is exposed to a taste, a wave of calcium flows through the activated neuron. The team used this feature to make the neurons involved literally light up when a unique taste was encountered. By injecting fluorescent dye into the brains of the mice, they could see that each unique taste lit up a map, comprising hundreds of neurons in a unique constellation, which was visible under high powered microscopes.
This was a better method of viewing large numbers of neurons at the same time. Previous methods had only allowed researchers to track a few cells at a time.
They saw for instance, when the mouse encountered a bitter taste, it lit up a map in one small, specific part of the brain. When it encountered a salty taste, this lit up a neighbouring area. Each taste thus had its own “hotspot” in the brain, and none of the areas overlapped – indeed there was space between them.
Ryba said the idea of maps has already been found in the other senses. But those maps correspond to external maps. For example sound maps of auditory neurons are arranged in order of frequency, and the arrangement of visual neurons mimics the field of vision we see through our eyes.
“However, taste offers no preexisting arrangement before reaching the brain; furthermore, the receptors for all tastes are found randomly throughout the tongue – thus the spatial organization of taste neurons into a topographic brain map is all the more surprising,” said Ryba.
The team now wants to explore how taste combines with the other senses, like smell and touch, and internal states like hunger and expectation, to produce flavors, memories of tastes, and behavioral responses to tastes.
Written by Catharine Paddock PhD