Do you eat or need to be around people when you’re stressed?

As this ScienCentral News video reports, scientists are studying the nerve cells of tiny worms to find out why.

Anxious Worms Stick Together

Humans are social animals and our social interactions and social behaviors are some of the most important and interesting features of our lives. Scientists have been trying to understand how genes and nerves can affect our behavior. Since humans are very complex animals, researchers have often turned to one of the simpler animals—like the C. elegans, a nematode, a tiny worm, barely bigger than a comma on this page.

Cori Bargmann, professor at the University of California San Francisco and an investigator of the Howard Hughes Medical Foundation says, “We know that some aspect of behavior is influenced by genetics and by genetic differences between individuals. But we don’t know exactly how the genes affect behavior, and what kinds of genes are involved.” She says the reason scientists study C. elegans is because they know all of their genes and all of their nerve cells. Therefore they can study the worm’s brain at a very high level of detail.

Bargmann, Mario de Bono, and others saw that in the wild these worms exhibited two different kinds of feeding behaviors: Some were social and others solitary. In an earlier study they found that the worms were genetically different. Now they wanted to find out what was making the “social” worms aggregate when feeding. They associated this behavioral difference to a natural variation at a single amino acid residue of a neuropeptide receptor called NPR-1. Neuropeptide receptor is a brain chemical that nerve cells use to communicate with each other. Worms lacking npr-1 strongly aggregated to feed, indicating that the presence of npr-1 represses this behavior and made them solitary.

In a study reported in the journal Nature, Bargmann and colleagues say that in the social worms they identified a set of genes that are switched on in the nerve cells that are known to sense stressful situations. The nerve cells, called nociceptive neurons, seemed to be sending repulsive or distress signals when they encountered food, and that was promoting the social feeding behavior.

“And so we tried to understand what it was about the food that made them prefer to aggregate rather than be alone”, says Bargmann. “We learned that the animals aggregate in response to conditions that they may find frightening or stressful, and that the food that we feed them actually alarms some of the animal and this is what causes them to gather into feeding groups.”

Further investigation revealed that the worm’s food source, which is E-coli and many soil bacteria, can kill C. elegans under certain conditions in the wild.

Bargmann points out that there could be evolutionary advantages for showing either social or solitary behaviors, depending on the circumstances, and that’s probably why both of those different behaviors are present. From an evolutionary perspective, social behavior in animals is often influenced by environmental conditions. Animals may gather in the presence of a predator, to protect each other from toxins, to mate, raise young, or to eat food that may be difficult to consume.

But while there are advantages to aggregation, there are also disadvantages. By coming together in a group they may be more conspicuous to a predator, they may get less food to eat (since they have to share), or they may be more susceptible to infectious diseases.

So, Bargmann says, “We think that these behaviors are regulated in the animal so that different aspects of the behavior will be present depending on whether it makes more sense for the animal to go it alone…or whether it makes more sense for them to go it together. So perhaps to our surprise, the feeding groups [of worms] are not just a positive response to each other, it’s a positive response that is encouraged by being a little frightened of the environment in which they find themselves.”

Although the researchers do not want to draw any immediate inferences to human behavior, they say that a similar gene in mammals is known to influence feeding behavior.

“The neuropeptide Y is one of the strongest feeding stimulants known in mammals, including ourselves,” says Bargmann, adding that it also “regulates anxiety levels, sensitivity to stressful signals.”

So, is it possible that the differences in human behavior between individuals may be at the molecular level, just as in the worms? Bargmann says it’s too soon to tell. Researchers are just starting to tease apart the kinds of pathways that act in those sorts of human behaviors.

Bargmann's work was supported by Wellcome Trust, Howard Hughes Medical Institute, and the Medical Research Council of Great Britain.