New research has revealed how fish are alerted to potential danger -- through the smell of sugar.
A group of scientists from the Agency of Science, Technology and Research's Neuroscience Research Partnership and the Duke-National University of Singapore (Duke-NUS) Graduate Medical school have figured out the nature of "Schreckstoff" -- which causes the rest of a school of fish to take off in fear when one member gets injured. (German for alarm substance or scary stuff).
They found that one component of the alarm substance is a class of sugars — specifically a type of glycosoaminoglycan (GAG) called chondroitin sulfate.
When a fish is injured, it causes the breakdown of this sugar, which is found in abundance in the skin, and it is the fragments that cause the fear response.
Using brain imaging to see how alarm signals are processed in a fish's brain, scientists discovered that one region in the olfactory bulb -- part of the brain which first processes smell -- is activated by the sugars.
Fishes around an injured fish would pick up traces of such sugars by means of special receptor neurons called the "crypt cells", implying that there are neurons dedicated to sensing the alarm pheromone.
This region also connects to unique high centers of the brain, suggesting that there may be a special brain circuit mediating aspects of the innate fear response.
Researchers found that different concentrations of the alarm substance help to trigger different fear responses. Low concentrations trigger mild fear characterised by darting while high concentrations cause freezing.
Dr Suresh Jesuthasan, Principal Investigator from the A*STAR Neuroscience Research Partnership said, "How the brain switches from one fear state to another is an interesting question, with relevance to anxiety disorders. By using the alarm response, we have a reliable way to investigate the neural basis of differential fear responses."
The potential application of this discovery can extend to other field of biology such as developmental neuroscience, where chondroitin fragments function as signalling molecules.
"Studying the feat behaviour in a tiny fish may seem somewhat simplistic but such animal models can offer important insights to more complex human disorders," said Prof Dale Purves, program director of the Neuroscience and Behavioural Disorders Program at Duke-NUS Graduate Medical School.