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Dopamine and octopamine push a locust's odor responses in opposite directions by different routes
Washington University researchers found dopamine raised locusts' brain and feeding responses to odors while octopamine lowered them. Brain chemistry alone set how strongly the insects answered an unchanged smell, with the experimenters supplying the chemicals.
The Scientist · Science desk
What happened
- Octopamine did not change the activity of those local neurons at all, according to Raman.
- A network model combining the results with earlier serotonin data needed two neuron groups, one raising the palp-opening response and one suppressing it.
- The team proposes that octopamine and serotonin act on the excitability of projection neurons, a putative step it plans to study further.
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Why it matters
- constraint The experimenters imposed the internal states by applying each chemical, so tying the effect to hunger or fullness in a living locust needs experiments that measure natural release of these neuromodulators.
- constraint If the model holds, silencing or altering the GABAergic local neurons should change dopamine's boost and leave octopamine's damping intact, so researchers cannot use one cell type to probe both effects.
- capability Because identity survived the change in strength, downstream neurons can in principle read which odor is present separately from how hard to respond. Raman's cookie remark describes that split.
The experiment holds the stimulus still and changes the brain. The locusts smelled the same odors under each condition, from a panel that included scents a human would describe as grass, lemon or citrus, rose, almond and spicy floral [4]. Raman said nothing in the animals' environment changed. Only the way their brains processed the odors did [8]. That design removes the usual reasons an animal's appetite shifts, such as scarce food. It also means the experimenters set the internal state themselves, by exposing the brains to each chemical [3][5]. The university's account opens with a person drawn into a bakery by the smell of fresh cookies and not drawn back after eating a handful [15]. The subjects of this study were locusts [1].
I find the most useful detail to be that the odor code held while its strength moved. Raman described the network changing its output while keeping its labels. "This was a patterned increase and decrease in neural responses evoked by odorants presented that still maintained the identity of the stimulus," Raman said [6]. "The cookie still smells like a cookie, but maybe the response is stronger or lesser, depending on which neuromodulator is released" [7].
The two chemicals act at different places. The paper's title says distinct mechanisms mediate the opponency [2]. The team looked at GABAergic local neurons, inhibitory cells that modulate the overall activity of the antennal lobe, the network that processes input from the insect antenna [9]. "What we found was that octopamine did not affect the activity of local neurons at all," Raman said [10]. "But dopamine, on the other hand, suppressed one subpopulation of local neurons. It released the circuit from inhibition to get that boost in the neural network output. Octopamine did not do that." [11]
Where octopamine acts is, so far, an inference from a model. The team combined its findings with earlier research showing serotonin can raise or lower palp opening depending on whether an odor is food-related [12]. "The simplest antennal lobe network model that integrated all our results required two groups of neurons: one subgroup to increase the behavioral response and a second group to reduce or suppress the same behavioral output," Raman said [13]. In that account, octopamine and serotonin putatively change the excitability of projection neurons, and the researchers plan to study the effect further [14]. The published summary does not report how many locusts were tested or how large the shifts in neural activity and palp opening were.
Raman also connected the work to swarming. Locusts can live alone or become gregarious and form destructive swarms, a switch serotonin is considered to trigger [16]. He gave the role of other neuromodulators as his own expectation. "Gregarious locusts are much more active than a solitary one, so those changes in behavior have to be sustained, and I think different neuromodulators can play a role," Raman said [17].
What to watch
- Recordings from antennal lobe projection neurons under octopamine and serotonin, testing the excitability change the two-group model predicts.
- Experiments linking natural states such as feeding, starvation or the shift to gregarious behavior to measured dopamine or octopamine release in the locust antennal lobe.