Two Neuron Populations Prolong Aggression

ABOVE: Fruit flies are helping scientists understanding the neurobiology of persistent internal states. © iStock, JossK

People tend to carry their pasts with them. An upsetting event, like a bad break-up, or an argument with a colleague, may influence behavior for days, weeks, or even months. Animals also display persistent internal states that may depend on continued activation of neural circuits. But the pathways involved and how they sustain certain states are unclear.   

Although researchers have previously identified clusters of neurons that drive fly aggression, these cells are only found in male flies.¹ In a recent reviewed preprint in eLife, scientists at the California Institute of Technology and the Janelia Research Campus delved into the neural mechanisms underlying female fly aggression and discovered that two types of neurons work together to maintain an aggressive state.²  The findings may help researchers uncover how the nervous system sustains internal states, and pave the way for a better understanding of human aggression. 

“It’s an important step on the journey to understanding this circuitry,” said David Deutsch, a neurobiologist at the University of Haifa who was not involved in the work.

Previously, Deutsch and his colleagues identified a group of neurons called pC1 that promote persistent aggression in female flies.³ When they activated the cells using optogenetics, the insects shoved and chased their neighbors, a reaction usually seen only when food is scarce or when their egg-laying sites are threatened.^4,5 By mapping the neural inputs and outputs of pC1 neurons, researchers found that these cells form a neural circuit with another type of cell called aIPg neurons. pC1 activation appears to trigger sustained activity of the circuit, suggesting a role for this pathway in persistent aggression. 

There are five subtypes of pC1 neurons that differ in morphology and connectivity. Deutsch’s study activated two types, pC1d and pC1e, and it wasn’t clear which drove hostile behavior. Another paper published by the Janelia group around the same time reported that stimulating pC1e neurons alone had no effect.⁶ In contrast, flies became momentarily hostile when pC1d cells were switched on and persistently aggressive when only aIPg cells were activated, suggesting that these two cell types regulate aggression. But the flies were housed together, so it was unclear if they were feeding off each other’s hostility.

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In the new study, the researchers solved this puzzle by separating pairs of insects with a sliding metal door. They stimulated individual neurons in one of the flies, then revealed the other insect 10 minutes later. Activating aIPg cells drove the flies to shove and head-butt their unsuspecting partners even after the delay. But stimulating pC1d cells alone wasn’t enough to change the flies’ behavior after the 10-minute interval. Instead, researchers had to stimulate pC1d and pC1e neurons together to produce sustained levels of aggression. 

The findings have prompted the group to reconsider the role of pC1e neurons. “We had kind of written it off,” said study author Catherine Schretter, a neurobiologist at the Janelia Research Campus. “But it’s actually way more exciting than we thought.”

So far, it’s unclear how the two populations of neurons work together to prolong aggression. One possibility is that pC1d neurons aren’t strong enough to activate their downstream targets without the simultaneous activity of pC1e neurons. Alternatively, pC1e neurons may release a hormone or neuropeptide that triggers long-lasting brain changes, but only when pC1d neurons are also activated. In this case, the behavior would then persist until those chemicals are cleared from the brain.

Schretter is keen to investigate these questions. First, the researchers want to determine if the pC1-aIPg circuit shows sustained activity following costimulation of pC1d and pC1e cells. If not, she said, the hunt is on for another circuit or chemical changes. 

However, to capture the full picture of fly aggression, researchers need better tools to simultaneously monitor the brain’s electrical and chemical activity, said Kenta Asahina, a neurobiologist at the Salk Institute, who was not involved in the study. “We need a more holistic way of monitoring the activity of the nervous system.”