Summary: Researchers identified a signaling system in the brains of C. elegans which changes to redirect behavior when survival is at risk due to lack of food.
Source: University of Rochester
Microscopic roundworms may hold the key to understanding what is happening in the brain when the instinct of an animal changes in order to survive.
In a newly published paper in the journal Current Biology University of Rochester Medical Center researchers found that a signaling system in the brain changes to redirect the behavior of an animal when their survival is at risk because there is not enough food.
The experiments were conducted in C. elegans – a microscopic roundworm that has been used by scientists for decades to understand the basic organization and function of the central nervous system and how it impacts behavior.
Researchers found C. elegans hermaphrodites (the equivalent of females in this species) produce a pheromone that allows worms to monitor how crowded their environment is and how much food there is to go around. When food becomes scarce the aversion circuit is trigged in the animal and it becomes repelled by the pheromone.
“The key thing we identified is a molecular mechanism whereby an instinctive response can be suppressed under particular environmental conditions, namely, abundant food,” said Douglas Portman, Ph.D., lead author of the study and professor of Biomedical Genetics. “Adaptively it makes sense that an animal’s instinctive response would have this kind of flexibility.”
This underlying repulsive mechanism to the pheromone is present in both hermaphrodites and males, but researchers found that in males, the mechanism is overridden by another circuit that causes males to be attracted to the pheromone. A subtlety that could provide an understanding of how the neural circuits work that cause this change in behavior.
Understanding how basic decision-making mechanisms work gives insight into the inner workings of a more complex brain. “These findings lend important insight into the mechanisms by which animals detect and integrate multiple sensory cues to make adaptive behavioral decisions. Understanding how things like this work at the molecular level, provides a framework for understanding how much more complex brains work, and how genetic and environmental insults can ‘break’ things and lead to behavioral and psychiatric disorders.”
Postdoctoral fellow Jintao Luo, Ph.D., with the University of Rochester, co-authored this study.
Funding: The research was supported with funding from the National Institute of General Medical Sciences.
To make adaptive feeding and foraging decisions, animals must integrate diverse sensory streams with multiple dimensions of internal state. In C. elegans, foraging and dispersal behaviors are influenced by food abundance, population density, and biological sex, but the neural and genetic mechanisms that integrate these signals are poorly understood.
Here, by systematically varying food abundance, we find that chronic avoidance of the population-density pheromone ascr#3 is modulated by food thickness, such that hermaphrodites avoid ascr#3 only when food is scarce.
Integrating food and pheromone signals requires the conserved neuropeptide receptor PDFR-1, as pdfr-1 mutant hermaphrodites display strong ascr#3 avoidance, even when food is abundant. Conversely, increasing PDFR-1 signaling inhibits ascr#3 aversion when food is sparse, indicating that this signal encodes information about food abundance.
In both wild-type and pdfr-1 hermaphrodites, chronic ascr#3 avoidance requires the ASI sensory neurons. In contrast, PDFR-1 acts in interneurons, suggesting that it modulates processing of the ascr#3 signal. Although a sex-shared mechanism mediates ascr#3 avoidance, food thickness modulates this behavior only in hermaphrodites, indicating that PDFR-1 signaling has distinct functions in the two sexes.
Supporting the idea that this mechanism modulates foraging behavior, ascr#3 promotes ASI-dependent dispersal of hermaphrodites from food, an effect that is markedly enhanced when food is scarce.
Together, these findings identify a neurogenetic mechanism that sex-specifically integrates population and food abundance, two important dimensions of environmental quality, to optimize foraging decisions.
Further, they suggest that modulation of attention to sensory signals could be an ancient, conserved function of pdfr-1.