Hydras are Lovecraftian looking microorganisms with a mouth surrounded by tentacles on one end, a long, thin body, and legs on the other. They have no brain or central nervous system. Despite having neither, hydras can feel hungry or full. How do these creatures know when they’re hungry or when they’ve had enough?
Hydras don’t have a brain, but they do have a nervous system. Researchers at the University of Kiel in Germany discovered that hydras have populations of neurons in the endoderm (in the digestive tract) and ectoderm (the outermost layer of the animal), both of which help them respond to food cues. Ectoderm neurons control physiological functions such as heading for food, while endodermal neurons are associated with feeding behaviors such as opening the mouth and expelling indigestible matter.
Such a limited nervous system can perform surprisingly complex functions, and hydra may offer some insight into how appetite evolved and what the early evolutionary stages of the central nervous system were like.
No, thank you. I’m full.
Before figuring out how the hydra’s nervous system controls hunger, the researchers focused on what makes the animals feel most full, or satisfied. The hydra were fed a diet of brine shrimp. Artemia salina, This is one of their usual prey, exposing them to the antioxidant glutathione. Previous research It has been suggested that glutathione may induce feeding behaviour in hydra, causing them to curl their tentacles towards their mouth as if swallowing prey.
Hydra is Artemia The group that ate as much as they could was then given glutathione, while the other group was given only glutathione but no actual food. Hunger was measured by how quickly and how often they opened their mouths.
The first group, already stuffed with shrimp, showed little response to glutathione eight hours after eating the shrimp: They barely opened their mouths, and when they did, they did so slowly — because they weren’t hungry enough to feel the need for more, even from a feeding stimulant like glutathione.
It wasn’t until 14 hours after feeding that the shrimp-fed hydra opened their mouths large enough and fast enough to indicate hunger. However, hydra that were not fed and only given glutathione began to show signs of hunger just four hours after exposure. Opening their mouths was not the only behavior caused by hunger; hungry animals also performed somersaults in the water, moved towards the light, and foraged for food. Full animals stopped somersaulting and clung to the walls of the tank until they became hungry again.
Foods that are good for the brain
After observing the behavioral changes in the hydra, the team investigated the neural activity behind those behaviors. They focused on two neural populations known to be involved in hunger and satiety: an ectodermal population known as N3 and an endodermal population known as N4. It was known that these influenced the feeding response of hydra, but until now it was unclear exactly how they were involved.
Hydra have N3 neurons all over their body, but especially on their legs. Signals from these neurons tell the animal that they’ve had enough to eat and that they feel full. As the animals became hungrier and exhibited more hunger-related behaviors, the frequency of these signals decreased. The frequency of N3 signals did not change in animals that were exposed to glutathione only and not fed, and these Hydra behaved exactly like animals that had not been fed for long periods of time. Only when the animals were actually fed did the frequency of N3 signals increase.
“The ectodermal neuronal population N3 not only responds to satiety by increasing neuronal activity but also controls feeding-induced altered behavior,” the researchers wrote in their paper. studyThis was recently published in Cell Reports.
N4 neurons have been shown to indirectly communicate with the N3 population only when food is present, influencing feeding behavior by controlling how wide the Hydra’s mouth opens and how long it stays open for. Starved Hydra and Hydra exposed to glutathione only have lower frequencies of N4 signals. Higher frequencies of N4 signals are associated with animals with their mouths closed.
So what can the neural activity of tiny brainless creatures tell us about the evolution of our own complex brains?
The researchers believe that Hydra’s simple nervous system may be similar to the much more complex central and enteric nervous systems that humans have. N3 and N4 function independently, but there is some interaction between them. The team also suggests that the way N4 controls Hydra’s feeding behavior is similar to the way it controls the mammalian digestive tract.
“A similar architecture of the neural circuitry controlling appetite and satiety is also found in mice, where enteric nerves, together with the central nervous system, control mouth opening,” the researchers wrote in the same paper. study.
Maybe in some ways we really are thinking intuitively.
Cell Reports, 2024. DOI: 10.1016/j.celrep.2024.114210
