Kristin Scott owes her interest in neuroscience to her father, a professor of philosophy. "Growing up, my dad often talked to me about logic, ethics, and the mind. This really got me interested in the underlying question of how the brain works and, ultimately, in neuroscience."
Scott's approach to studying the brain was to start by examining one aspect of brain function: how the brain receives information from the senses and then turns it into behavior. Her research focuses on the gustatory system—the networks of neurons responsible for the sense of taste—in the fruit fly Drosophila melanogaster. By mapping these relatively simple circuits in the fly, Scott hopes to reveal how animals with more complex nervous systems recognize and respond to cues in their environment.
When a fly comes across a banana, it can tell that it tastes sweet using neurons located in different parts of the body—proboscis, wings, legs, and internal mouthparts. Long projections, or axons, from these peripheral neurons extend to the fly's brain, where other neurons process taste information and translate it into particular behaviors.
The fly will do different things depending on the quality of the taste and the location of the neurons that detected it. For instance, if taste neurons in the legs are activated when the fly lands on the banana, the proboscis will extend toward the food—an acceptance behavior. But if the banana is sprinkled with a chemical that gives it a bitter taste, the proboscis retracts—a rejection behavior. Similarly, activation of taste neurons in the proboscis makes the fly either eat the food or avoid it, depending on the taste. Scott determined that, in addition to sweet and bitter, flies can also taste carbon dioxide, a taste that triggers an acceptance behavior, with the fly preferring carbonation.
In 2004, Scott determined that the fly has a "body map" in its tiny brain, which may allow it to locate the position of a taste compound. For example, when the fly tastes something sweet with its legs, it may know not only that it's a good taste but also where the taste is located, so that the animal can then reach its proboscis toward the food.
To determine what this map looks like, Scott followed the projections of peripheral neurons as they extend from the legs, wings, proboscis, or mouth into the brain. She was able to show that neurons from the fruit fly mouth travel to the most anterior part of a region in the middle of brain known as the subesophageal ganglion (SOG). On the other hand, neurons from the proboscis end up in the medial part of the SOG, and those from the legs to the most posterior parts of this brain region.
Scott and colleagues in her lab at the University of California, Berkeley also determined that different groups of peripheral neurons are responsible for detecting sweet or bitter tastes and that these neurons are hardwired to taste behaviors. Using molecular genetic methods, they turned specific peripheral neurons on or off and then observed how the flies behaved. "By inducing activation of these different cell populations, we've triggered taste acceptance or rejection behavior in the fly," Scott says. For example, by turning on sugar-sensing peripheral neurons, they could get the proboscis to extend, even if the legs had not tasted something sweet. They also found that these sweet- and bitter-detecting neurons travel to different parts of the fly brain.
"Our studies have advanced our understanding of taste recognition in the periphery and provide a strong foundation to examine taste processing in the central nervous system," Scott says. "Our goal is to address the central question of how the brain processes sensory information to orchestrate behavior. The most important thing we're trying to do now is trace neural circuits underlying different taste behaviors. We intend to compare neural pathways that transmit information about sugars to ones that transmit information about bitter compounds with the goal of identifying the anatomy, connectivity, and function of neurons. This should tell us a lot about taste information processing."
An understanding of how the fly's taste system works to distinguish between good and bad food might help scientists determine how the brain processes even more complex information. For example, one problem Scott would like to tackle is how information from different senses—touch, movement, sight, hearing, taste, and smell—is integrated in the brain to elicit a particular behavior. Another question is how taste behaviors and taste neural circuits are modified by internal states such as hunger and satiety. Answering these questions is a challenge Scott relishes.