Summary: The familiar “never again” reaction after food poisoning is not just psychological — it involves a biological signal that reaches fat cells and then the brain. A new study using the fruit fly Drosophila reveals how immune detection of pathogens triggers a fat-to-brain communication that produces lasting avoidance of contaminated food, a form of conditioned taste aversion.
Researchers from the Universities of Bonn and Tohoku and University Hospital Bonn identified an unexpected pathway: immune sensors detect harmful bacteria, neurons relay that information to adipose tissue, and fat cells then release neurotransmitters that reshape brain circuits responsible for learning and smell-guided food choice. The findings were published in the journal Neuron.
Key findings
- Initial attraction to harmful odors: Unexposed flies are attracted to the odor of certain pathogenic bacteria and will initially prefer food sources contaminated with them.
- Fat as a signaling intermediary: Adipose tissue in the fly head acts as a critical middleman. When activated by neuronal signals, fat cells synthesize and release dopamine, which travels to the brain and reinforces avoidance learning.
- Starvation alters avoidance: Starved flies have fewer fat cells and produce less fat-derived dopamine, which may weaken avoidance learning and make starving animals more likely to consume risky food.
- Implications for humans: Because human fat tissue also produces bioactive molecules that influence appetite and brain function, a similar gut–fat–brain loop could contribute to eating disorders and metabolic disorders where communication between metabolism, immunity, and brain function is disrupted.
Background
Conditioned taste aversion — avoiding a food after it causes illness — is widespread across animals and has clear survival value. However, the cellular and molecular steps that connect immune sensing of pathogens in the gut to long-lasting changes in brain circuits were not well defined. To address this, the researchers used Drosophila melanogaster as a genetically tractable model to map how peripheral immune detection leads to persistent behavioral change.
Experimental approach and results
Flies were offered a choice between two otherwise identical food sources: one contaminated with the pathogenic bacterium Pseudomonas entomophila and the other with a harmless Pseudomonas strain. Naive flies preferred the contaminated food, attracted by its odor. After consuming the pathogenic bacteria and experiencing the immune response, flies learned to avoid that odor and instead chose the harmless food.
Molecularly, receptors that recognize bacterial cell wall components became activated on sensory neurons near the fly’s throat. These neurons release the neuromodulator octopamine (an invertebrate analogue of adrenaline), which travels along neuronal projections to innervate the fat body — the fly’s adipose tissue. Octopamine activates receptors in the fat body, triggering calcium signaling and causing fat cells to synthesize and release dopamine.
Dopamine released from fat cells is transported back to the brain, where it acts on Dop1R1 receptors in mushroom body output neurons — key nodes in the fly learning and memory circuit. Two-photon calcium imaging showed that ingestion of pathogens alters odor-evoked responses in these neurons, linking peripheral immune activation to central changes in sensory processing and learned avoidance.
Role of nutritional state
The study highlights how metabolic state can gate avoidance learning. Fat body involvement suggests that the amount of adipose tissue and its capacity to produce dopamine modulate how strongly animals form negative associations with contaminated food. The authors propose that when fat reserves are low, the dopamine signal from adipose tissue is reduced, lowering the threshold for risk-taking in favor of immediate energy intake. This hypothesis is under active investigation.
Broader significance
These findings identify a bidirectional fat body–brain communication axis that couples innate immune detection to long-term behavioral adaptation. Because adipose tissue in mammals and humans also releases neurotransmitters and signaling molecules that affect appetite and brain circuits, a related mechanism could contribute to human conditions in which metabolism, immune signaling, and behavior are dysregulated — for example, eating disorders and obesity. Using the fruit fly model makes it possible to dissect this complex interaction at cellular resolution and could inform future studies in mammals.
Participating institutions and funding
Researchers from the Universities of Bonn and Leipzig, Tohoku University (Japan), and University Hospital Bonn contributed to the study. Funding was provided by the German Research Foundation (DFG), the iBehave Network of North Rhine-Westphalia, and the Human Frontier Science Program Organization.
Frequently asked questions
A: Fat cells act as sensors of the body’s nutritional and immune state. In this study, fat tissue received signals from pathogen-detecting neurons and responded by producing dopamine, which communicated with brain circuits to prevent repeat ingestion of dangerous food. This places adipose tissue as an active signaling organ, not merely an energy store.
A: Possibly. The research suggests that lower fat reserves reduce the fat-derived dopamine signal that promotes avoidance, so hungry animals may be less likely to reject risky food despite prior negative experience, because the immediate need for calories outweighs the risk.
A: Yes. Immune detection of pathogens sets off a cascade of signals that ultimately alter neural activity and sensory responses in brain regions responsible for learning and smell-guided choices, producing lasting avoidance behavior.
About this research
Author: Johannes Seiler
Source: University of Bonn
Contact: Johannes Seiler, University of Bonn
Image credit: Neuroscience News
Original research: A Bidirectional Brain-Fat Body Axis for Pathogen Avoidance. Authors: Yujie Wang, Jean-François De Backer, Aurélie Muria, Ayako Abe, Kokoro Saito, Helen Holvoet, Mareike Selcho, Hiromu Tanimoto, and Ilona C. Grunwald Kadow. Neuron. DOI: 10.1016/j.neuron.2026.03.026. Open access.
Abstract (concise)
Ingesting pathogens threatens survival and selects for avoidance behaviors. This study uncovers a bidirectional communication loop between immune-sensing neurons, the fat body, and brain circuits in Drosophila. Pathogen-responsive octopaminergic neurons activate the fat body via an octopamine receptor, triggering fat-derived dopamine release. Dopamine acts on mushroom body output neurons to drive odor-guided avoidance. These results reveal an immune-to-brain signaling axis by which fat tissue participates in behavioral adaptation to infection, with potential relevance for mammalian and human physiology.