Scientist have long known that the motivation to feed – structured by hunger and feelings of fullness – is regulated by hormones and small proteins called neuropeptides, which are found in a diversity of organisms, including fruit flies, mice, and humans.
Since such a widespread occurrence suggests a common evolutionary origin, a team of researchers from the University of Tokyo has investigated how they function in simpler animals such as jellyfish and fruit flies.
Jellyfish shared a common ancestor with mammals more than 600 million years ago. Yet, they are simpler organisms, possessing diffused nervous systems called nerve nets instead of localized structures such as the brain or ganglia found in mammals.
Nonetheless, they posses a rich behavioral repertoire, including complex foraging strategies, mating rituals, and even learning capacities for hunger regulation, among many other things. Despite their complexity and importance, their feeding behavior has not yet been comprehensively studied.
The researchers focused on Cladonema, a small species of jellyfish with branched tentacles that regulate how much they eat based on how hungry they are. “First, to understand mechanisms underlying feeding regulation, we compared the gene expression profiles in hungry and fed jellyfish,” said senior author Hiromu Tanimoto, a professor of Neurobiology at Tokyo. “The feeding state changed the expression levels of many genes, including some that encode neuropeptides. By synthesizing and testing these neuropeptides, we found five that reduced feeding in hungry jellyfish.”
The experts found that one such neuropeptide – called GLWamide – inhibited tentacle shortening, which is a crucial step for transferring captured prey to the mouth, suggesting that it acts as a satiety signal indicating that the body has had enough food.
To clarify whether similar neuropeptides are found in other species, the researchers analyzed fruit flies, and discovered that their feeding patterns and hunger regulation are ruled by the neuropeptide myoinhibitory peptide (MIP), with fruit flies lacking this peptide tending to eat more and eventually becoming obese. Since MIP and GLWamide share structural similarities, they are likely related through evolution.
“Since the functions of GLWamide and MIP have been conserved despite 600 million years of divergence, this led us to ponder whether it was possible to exchange the two,” said lead author Vladimiros Thoma, an assistant professor of Neuroethology at Tohoku University in Japan. “And we did exactly that, first giving MIP to jellyfish and then expressing GLWamide in flies that had no MIP.”
The results were striking: MIP inhibited jellyfish feeding behavior, just like GLWamide did, while GLWamide in fruit flies stopped their abnormal over-eating behavior, suggesting that the GLWamide/MIP system in jellyfish and insects has been functionally conserved during evolution, and highlighting the deep evolutionary origins of a satiety signal in a variety of organisms.
“We hope that our comparative approach will inspire focused investigation of the role of molecules, neurons, and circuits in regulating behavior within a wider evolutionary context,” Tanimoto concluded.
The study is published in the journal PNAS Neuroscience.
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