Neurons That Produce Dopamine Link Physical State to Innate Preferences in Fruit Flies

Dopamine appears to shape our perception of the world by conditionally relating innate preferences for things like taste and smell, to physiological states, such as hunger and movement, according to a study of fruit flies.

The study, “Valence and State-Dependent Population Coding in Dopaminergic Neurons in the Fly Mushroom Body,” was published in the journal Current Biology.

Dopamine signaling is one of the most intensively-studied circuits in the brain. Disorders involving dopamine signaling include addiction, obesity and Parkinson’s disease, in which the neurons that produce dopamine — known as dopaminergic neurons — break down and die, resulting in the loss of motor control, as well as behavioral changes.

Understanding exactly how dopamine links cognitive behaviors to motor control, however, has remained elusive.

Researchers at at the Technical University of Munich (TUM), in Germany, hypothesized that the fruit fly Drosophila melanogaster might hold clues to this process. Although the fly’s brain structure is quite different from that of a human, D. melanogaster uses dopamine in much the same way.

The research team focused its efforts on a structure within the fly brain called the mushroom body, which houses a dense network of some 200 dopaminergic cells, organized into 15 interconnected neural compartments. These neurons are known to respond to stimuli of a so-called “innate” value, such as sweetness and heat.

Using a genetic technique that allowed them to see changes in neural activity, the team found that distinct compartments within the mushroom body responded to innate stimuli and others to physiological conditions such as movement. Meanwhile, others appeared most active in the absence of stimuli.

They observed that networks of neurons from different compartments would act in concert to support combined cognitive and motor processes, such as in relating an odor to movement.

“By doing that, the neurons can react flexibly and individually to the most important information — such as smell, taste, but also hunger or one’s own movement. This is important to reach a balanced decision, because an external sensory signal can sometimes mean something good or bad, depending on an organism’s condition,” said Ilona Grunwald Kadow, PhD, the study’s lead author, in a university press release.

Of interest, the researchers observed that the precise dopaminergic neurons of individual flies responded differently to the same stimuli. They speculated this could provide the basis for individual preferences and behavioral differences between animals. It remains unclear, however, if these distinctions arise because of differences in individual flies’ experiences, or through more random, or at least less predictable, molecular processes.

Finally, the team noted that movement activated not only the dopaminergic neurons of the mushroom body, but also other areas of the brain that have no known relationship to movement. This discovery opens doors to future research into topics like the general role that movement plays in reacting to environmental stimuli.