Summary: New research demonstrates that a single olfactory neuron in fruit flies can drive two distinct behaviors in response to the same odor. When flies detect the smell of rotting fruit, one downstream neural pathway guides them toward the source, while a separate pathway controls changes in walking speed.
These findings challenge the long-standing assumption that individual neurons have only a single function, and they offer insight into how compact neural circuits can encode multiple, behaviorally relevant commands.
Key facts:
- A single olfactory projection neuron can generate divergent signals that separately influence direction and speed.
- Two different third-order neurons respond differently to the same input: one produces sustained activity that supports ongoing movement, the other produces a transient response that drives rapid changes in pace.
- The study highlights how presynaptic specialization and synaptic dynamics create parallel information streams within a small neural circuit.
Source: Yale
A single neuron can prompt fruit flies to walk toward the scent of rotting fruit and to increase their speed, according to new research from Yale scientists.
Historically, neuroscientists often thought individual neurons performed single, well-defined roles. More recent work across species has revealed that many neurons are multifunctional. The new study, published in Current Biology, adds fruit flies to that list by showing how one olfactory neuron simultaneously drives two distinct behavioral outputs.
The team focused on the fruit fly olfactory system, a model circuit that is far simpler than the human brain yet sophisticated enough to reveal fundamental principles of neural computation. Fruit fly brains contain roughly 140,000 neurons, and researchers have mapped the first two stages of olfactory processing in detail. What happens beyond those stages, at the level of third-order neurons, has been less clear.
Graduate student Hyong Kim observed that neurons in the third layer responded in strikingly different ways to the same odor, which suggested a change in how signals are transformed between the second and third layers. Senior author James Jeanne, PhD, and colleagues designed experiments to trace the electrical activity underlying those differences.
Mapping neurons in the fruit fly brain
Sensory information is converted into action through chains of connected neurons, beginning with receptor cells and passing through intermediate neurons until motor outputs are produced. In the fly olfactory system, receptor neurons detect odors and relay signals to glomerular projection neurons (PNs), which in turn connect to a diverse set of lateral horn neurons (LHNs).
The researchers examined how a single PN that responds to ethyl acetate—a volatile associated with rotting fruit—affects two different third-order neurons, which they labeled LHN1 and LHN2. Although the PN generates a consistent response to ethyl acetate, the two LHNs showed markedly different electrical patterns.
One neuron, two behaviors
When the odor was presented, LHN1 produced sustained electrical activity throughout the odor pulse, while LHN2 produced a brief, transient spike followed by a rapid decline. To determine whether these dynamics influence behavior, the team used genetic tools to selectively silence either LHN1 or LHN2 and tested flies in a wind tunnel that simulated the scent of rotting fruit.
Normal flies walk toward the odor source and increase their speed as the odor concentration intensifies. Flies lacking functional LHN2 still moved toward the source, but they did not accelerate as the odor grew stronger, indicating that LHN2 is necessary for odor-driven increases in walking speed. Flies with LHN1 disabled still oriented toward the odor but failed to sustain movement when odor concentration decreased, suggesting LHN1 supports continuous locomotion during fluctuating odor signals.
These results show that a single upstream PN can route the same sensory information into two parallel pathways with different synaptic and cellular properties, producing separate behavioral effects—directional guidance and speed modulation—without requiring multiple distinct receptor types.
The authors propose that presynaptic specializations and distinct synaptic dynamics are efficient ways to generate parallel information streams. In this case, differences in short-term synaptic depression and facilitation, along with postsynaptic mechanisms such as Na+/K+ ATPase–dependent gain control and spike-threshold nonlinearities, create divergent response patterns in the target LHNs.
Jeanne and colleagues plan to examine whether fine-scale anatomical features visible with electron microscopy predict these different functional dynamics, with the long-term goal of linking circuit structure to circuit function more directly.
Funding: The research reported here was supported by the National Institutes of Health (awards R01DC018570, R01NS116584, and RF1NS132840) and Yale University. Additional support came from the Smith Family Foundation, the Klingenstein-Simons Fellowship Award in Neuroscience, and the Kavli Institute for Neuroscience at Yale University. The content remains the responsibility of the authors and does not necessarily reflect the official views of funding agencies.
About this olfaction and neuroscience research news
Author: Freda Kreier
Source: Yale
Contact: Freda Kreier – Yale
Image: The image is credited to Neuroscience News
Original Research: Open access.
“Divergent synaptic dynamics originate parallel pathways for computation and behavior in an olfactory circuit” by James Jeanne et al. Current Biology
Abstract
Divergent synaptic dynamics originate parallel pathways for computation and behavior in an olfactory circuit
Central circuits create parallel processing streams through divergent connectivity, enabling diverse sensory processing and behavior. Linking synaptic and cellular mechanisms to circuit-level computation has been difficult. This study examines how glomerular projection neurons (PNs) in Drosophila diverge onto multiple lateral horn neurons (LHNs) to produce parallel pathways.
Comparing the effects of a single PN on two LHN types, the authors found that one LHN produces sustained responses that adapt divisively, while the other produces transient responses that adapt subtractively. These differences arise from distinct presynaptic dynamics: synapses onto sustained LHNs recover quickly from depression and support ongoing transmission, while synapses onto transient LHNs recover more slowly but facilitate when PN firing increases.
Postsynaptic mechanisms contribute as well: slow cellular gain control mediated by the Na+/K+ ATPase underlies divisive adaptation in sustained LHNs, whereas spike-threshold nonlinearities create subtractive adaptation in transient LHNs. Manipulations that disrupt facilitation reduce the initial transient responses, and behavioral tests show that transient LHNs make brief contributions to odor attraction in walking flies, while sustained LHNs contribute more continuously.
These findings indicate that subcellular presynaptic specialization provides a compact and efficient way to generate parallel information streams for specialized computation and behavior.