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Normally Solitary Desert Locust Transformed to Gregariousness by Activation of Specific Serotonin-Producing Nerve Cells

A team of biologists has identified a set of nerve cells in desert locusts that bring about ‘gang-like’ gregarious behavior when the insect are forced into a crowd. Dr. Swidbert Ott from the University of Leicester’s Department of Biology, working with Dr. Steve Rogers at the University of Sydney, Australia, has published a study, online on December 17, 2014 in an open-access article in The Royal Society Proceedings B, that reveals how newly identified nerve cells in locusts produce the neurochemical serotonin to initiate changes in their behavior and lifestyle. The findings demonstrate the importance of individual history for understanding how brain chemicals control behavior, which may apply more broadly to humans also. Locusts are normally shy, solitary animals that actively avoid the company of other locusts. But when they are forced into contact with other locusts, they undergo a radical change in behavior – they enter a “bolder” gregarious state in which they are attracted to the company of other locusts. This is the critical first step towards the formation of the notorious locust swarms. Dr. Ott said: “Locusts only have a small number of nerve cells that can synthesise serotonin. Now we have found that of these, a very select few respond specifically when a locust is first forced to be with other locusts. Within an hour, they produce more serotonin. It is these few cells that we think are responsible for the transformation of a loner into a gang member. In the long run, however, many of the other serotonin-cells also change, albeit towards making less serotonin.”

When a locust is first forced into contact with other locusts, a specific set of nerve cells that produce the neurochemical serotonin is responsible for reconfiguring its behavior so that the previously solitary locust becomes a member of the gang, which is known as “gregarious” behaviour. An entirely different set of its serotonin-producing nerve cells is then affected by life in the group in the long run.

Dr. Ott added: “The key to our success was to look in locusts that have just become gregarious and that had never met another locust until an hour earlier. If we had looked only in solitary locusts and in locusts that had a life-long history of living in crowds, we would have missed the nerve cells that are the key players in the transformation.

“There is an important lesson here for understanding the mechanisms that drive changes in social behavior in general, both in locusts and in humans. We have shown how important it is to look at what happens when a new social behavior is first set up, not just at the long-term outcome.

“Research in insects can give us deep insights into how brains work in general, including our own.”

Studies have previously shown that the change from solitary to gregarious behavior is caused by serotonin.

The new study has identified the individual serotonin-producing nerve cells that are responsible for the switch from solitary to gregarious behavior.

The scientists used a fluorescent stain that reveals the serotonin-producing nerve cells under the microscope. This allowed them to measure the amount of serotonin in individual nerve cells — the brighter a nerve cell lights up, the more serotonin it contains. The newly identified cells were much brighter in locusts that had just been forcibly crowded together with other locusts. Moreover, the same cells were also brighter in locusts that had their hind legs tickled by the researchers for an hour — which is sufficient to make the locusts behave gregariously.

Serotonin has important roles in the brains of all animals that include the regulation of moods and social interactions.

In humans, there are strong links between changes in serotonin and mental disorders such as depression and anxiety.

[Press release] [Royal Society Proceedings B article]