Despite lacking a brain or centralized nervous system, the hydra—a microorganism with a mouth surrounded by tentacles, an elongated body, and a foot—can still sense hunger and fullness. How do these brainless creatures know when they are hungry and when they have eaten enough?
Separate Systems for Appetite Control
Hydras possess a decentralized nervous system with two types of neurons: endodermal neurons in the digestive tract and ectodermal neurons in the outermost layer. Researchers from Kiel University in Germany found that these neurons help hydras respond to food stimuli. Ectodermal neurons control movements toward food, while endodermal neurons manage feeding behaviors, such as opening the mouth to ingest food and expelling indigestible matter.
Satiety and Feeding Behavior
Researchers explored what causes the strongest feelings of satiety in hydras by feeding them brine shrimp (Artemia salina) and exposing them to the antioxidant glutathione, known to trigger feeding behavior. Hydras fed with shrimp showed minimal response to glutathione eight hours post-feeding, indicating they were not hungry. In contrast, hydras exposed only to glutathione without actual food exhibited hunger behaviors much sooner.
Neuronal Activity Behind Hunger
The research focused on two neuronal populations: N3 neurons (ectodermal) and N4 neurons (endodermal), both involved in hunger and satiety. N3 neurons, located mainly in the foot, signal satiety and reduce their activity as hunger increases. Hydras fed with shrimp showed increased N3 signals, indicating satiety, while those only exposed to glutathione behaved as if they were starved.
N4 neurons regulate how wide hydras open their mouths and how long they keep them open. Starved hydras or those only exposed to glutathione had lower N4 signal frequencies, while higher N4 signals correlated with keeping their mouths shut.
Implications for Understanding Appetite Control
The study suggests that the hydra’s simple nervous system may parallel more complex systems in higher organisms, including humans. The interaction between N3 and N4 neurons in hydras could be similar to how enteric and central nervous systems in mammals regulate feeding behaviors. This insight offers a glimpse into the early evolutionary stages of appetite control mechanisms and the development of complex nervous systems.
By understanding the neuronal activity in hydras, researchers hope to uncover fundamental principles of how appetite and satiety are controlled, potentially providing clues about the evolution of these processes in more complex organisms.