A Matter of Taste
Susan Travers, PhD and Joe Travers, PhD find motivation and taste go hand in hand
Taste may seem like a fairly simple phenomenon, but in reality it is a powerful motivator that can affect how we act and ultimately the quality of the life we live.
“We all have a taste system, and it contributes to our everyday enjoyment of life,” said Susan Travers, PhD. “It impacts what we eat and how much we eat. In turn what we eat and how much we consume has major implications for diseases such as cancer and diabetes as well as our dental state.”
Susan and her husband, Joseph Travers, PhD, are professors in the College of Dentistry’s Biosciences Division who have worked together for nearly 40 years researching taste. At Ohio State for the last 30 years, the duo has focused on how the nervous system processes orosensory signals from the mouth, how those signals interact with visceral signals sent by the gut, and how taste impacts behaviors. Their body of work includes numerous peer-reviewed papers in addition to book chapters and invited presentations.
Like everything in our nervous system, the physical process of tasting involves electrical signals. Taste receptors within clusters of cells on the tongue and palate are connected to nerves that connect to the brain stem, which is a continuation of the spinal cord. When a receptor detects a specific taste—bitter, salty, sweet, sour or umami—it sends an electrical signal to the brain stem that from there goes to other regions in the brain involved with discernment, motivation, and basic oromotor functions like chewing and swallowing.
The taste receptors in our mouth, however, aren’t the brain’s only source of input during feeding. It also receives messages from the vagus nerve, the largest cranial nerve. One that runs from the brain stem to the abdomen and sends signals to and from every major organ. When the vagus nerve senses the abdomen has reached capacity, it sends a message to the brain that we are “full” and the signals from our taste receptors become dulled. Food becomes less interesting, and we stop eating.
The gut’s “full” message isn’t necessarily the final word on appetite, though. Instead, it may just apply to a specific taste, and other taste receptors can still pipe up. After filling up on a hearty meal of savory tastes, for example, we can find a sweet dessert still sounds good. On the other hand, visceral and taste signals may collude after a gut-wrenching experience with food poisoning to create a lasting aversion to a particular food.
The implications of the Travers’ work are far-reaching, potentially extending into other areas of neuroscience.
In December 2015, the Traverses published their first paper on the implications of gastric bypass surgery on taste. “Gastric bypass is more complex than just making the stomach smaller,” Joe says. “There’s a real change in appetite and profound changes in the peripheral and central nervous systems. It impacts food preference and other behaviors.”
The study, which they co-authored with colleagues at Florida State University and University College Dublin, examined how a high-fat diet and gastric bypass surgery affect the excitability of neurons that sense taste and signal satiety. It found that a high-fat diet kept potassium channels within the satiety neurons more open than usual, and the cells subsequently became less excitable. After surgery, however, those effects reversed.
“A high-fat diet renders the satiety system less excited—it just doesn’t deliver the kick,” Joe said. “What was exciting was to see that in the group that got surgery, the excitability returned to a normal state.”
While the “how” of gastric bypass surgery’s many effects is still unknown, scientists in different fields are identifying the “what.”
“We’re examining the brain stem and what we've found is that when you're on a high-fat diet there are cellular changes in that region,” said Joe. “But our findings are only part of the story. Other researchers studying the brain are finding changes in reward circuitry. We’re talking about a long-term upregulation of the nervous system.”
In a new project that started this year, Susan is collaborating with Alan Spector, PhD, from Florida State University to study mice whose sweet and amino acid receptors have been removed. Their goal for the NIH-funded study is to determine whether or not there are other types of sensors for these tastes that have yet to be identified.
The body’s sweet receptors, which were discovered roughly a decade ago, trigger major responses from the endocrine system. “When you put a simple sugar in your mouth, you will have a release of insulin before you even swallow the stimulus,” Susan said. Scientists initially assumed those sweet receptors were the only ones, but many have come to suspect that there are additional mechanisms that detect glucose and other simple sugars. By examining neural responses to sweet stimuli in mice that are missing the known sweet receptors, Susan’s new study may help determine the presence of a different mechanism that triggers the body’s anticipatory insulin response.
The implications of the Travers’ work are far-reaching, potentially extending into other areas of neuroscience. Several years ago, with support from an NIH program for high-risk, high-reward projects, they began using an exciting new technique called optogenetics, in which light-sensitive genes are inserted into specific types of neurons. Later, in the presence of optical stimulation, only those cells respond. The technique allows the Travers to study the role of particular populations of neurons in defined brain circuits and, in turn, their effects on behavior. Ultimately, optogenetics has potential for practices such as deep brain stimulation, which uses electrical stimulation to treat conditions ranging from Parkinson’s disease to multiple sclerosis to major depression.
In Joe’s opinion, the fact that the sensory function of taste can be involved in so many aspects of health and behavior makes studying taste a lifelong interest.
“These are just a handful of qualities to it, and it’s limited to one behavior—a behavior that is so fundamental to life,” he says. “It’s just fascinating.”