How do brainless creatures control their appetites?

The hydra is a Lovecraftian-looking microorganism with a mouth surrounded by tentacles at one end, an elongated body and a foot at the other end. It has no brain or centralized nervous system. Despite the lack of any of those things, you may still feel hungry and full. How can these creatures know when they are hungry and realize when they have eaten enough?

Although they lack a brain, hydras have a nervous system. Researchers at the University of Kiel in Germany discovered that they have an endodermal (in the digestive tract) and ectodermal (in the outermost layer of the animal) neuronal population, which help them react to food stimuli. Ectodermal neurons control physiological functions such as moving towards food, while endodermal neurons are associated with feeding behaviors such as opening the mouth, which also vomits out anything indigestible.

Even such a limited nervous system is capable of performing some surprisingly complex functions. Hydras might even give us some insights into how appetite evolved and what the early evolutionary stages of the central nervous system were like.

No thanks, I’m full

Before discovering how the hydra’s nervous system controls hunger, researchers focused on what causes the strongest feelings of satiety or fullness in the animals. They were fed brine shrimp. brine shrimp, which is among its usual prey and is exposed to the antioxidant glutathione. Previous studies have suggested that glutathione triggers feeding behavior in hydras, causing them to curl their tentacles toward their mouths as if they were swallowing prey.

Hydra fed with so much Artemia When they were able to eat, they were given glutathione afterwards, while the other group was only given glutathione and no food. Hunger was measured by how quickly and how often they opened their mouths.

It turned out that the first group, which had already had its fill of shrimp, barely showed a response to glutathione eight hours after being fed. Their mouths barely opened (and slowly, if ever) because they weren’t hungry enough for even an eating trigger like glutathione to make them feel like they needed a few seconds.

It was only 14 hours after feeding that the hydra that had eaten shrimp opened its mouth wide and quickly enough to indicate hunger. However, those who were not fed and were only exposed to glutathione began to show signs of hunger just four hours after exposure. Opening their mouths was not the only behavior caused by hunger, since hungry animals also turned somersaults in the water and moved toward the light, behaviors associated with searching for food. The satiated animals stopped tumbling and clung to the wall of the tank they were in until they were hungry again.

Food in the “brain”

After observing behavioral changes in the hydra, the research team examined the neural activity behind those behaviors. They focused on two neuronal populations, the ectodermal population known as N3 and the endodermal population known as N4, both involved in hunger and satiety. While these were known to influence hydra feeding responses, exactly how they were involved was unknown until now.

Hydras have N3 neurons throughout their bodies, especially in their feet. Signals from these neurons tell the animal that it has eaten enough and is satiated. The frequency of these signals decreased as the animals became hungrier and displayed more behaviors associated with hunger. The frequency of N3 signals did not change in animals that were only exposed to glutathione and were not fed, and these hydras behaved the same as animals that had been without food for an extended period of time. Only when they were given real food did the frequency of the N3 signal increase.

“The N3 ectodermal neuronal population not only responds to satiety by increasing neuronal activity, but also controls behaviors that changed due to feeding,” the researchers said in their study, which was recently published in Cell Reports.

Although N4 neurons were only observed to communicate indirectly with the N3 population in the presence of food, they were found to influence feeding behavior by regulating the width of the hydras’ mouth and the length of time they held it open. A lower frequency of N4 signals was observed in hydra that were starved or only exposed to glutathione. A higher frequency of N4 signals was associated with the animals keeping their mouths closed.

So what can the neural activity of a tiny, brainless creature tell us about the evolution of our own complex brains?

Researchers believe that the hydra’s simple nervous system may parallel the much more complex central and enteric (in the gut) nervous systems we have. While N3 and N4 operate independently, there is still some interaction between them. The team also suggests that the way N4 regulates hydra feeding behavior is similar to the way the mammalian digestive tract is regulated.

“A similar architecture of neural circuits controlling appetite and satiety can also be found in mice, where enteric neurons, together with the central nervous system, control mouth opening,” they said in the same study.

Maybe, in a way, we really do think with our guts.

Cell Reports, 2024. DOI: 10.1016/j.celrep.2024.114210

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