The internal metabolic state controls behavior in Hydra through an interplay of enteric and central nervous system-like neuron populations

Hunger and satiety can have an influence on decision making, sensory processing, and motor behavior by altering the internal state of the brain. This process necessitates the integration of peripheral sensory stimuli into the central nervous system. Interestingly, even organisms without a brain, such as the Cnidaria, exhibit feeding dependent behavioral changes. The underlying mechanisms, however, remain unclear. In this study, we demonstrate that neuronal activity in two distinct neuronal populations, the ectodermal N3 neurons and the endodermal N4 neurons in Hydra, an ancestral metazoan animal with a diffuse nerve net spread throughout the body with no signs of centralization, are responsible for feeding dependent behavioral changes. Specifically, endodermal N4 neurons are essential for food intake and digestive functions, similar to the enteric nervous system, while the N3 population influences and inhibits other motor related behaviors, comparable to the central nervous system. This fascinating observation provides a new insight into the evolution and the complexity of a simple non-centralized nervous system.