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Mechanisms of fear

Here’s a nice article By Amanda Schaffer (NY Times, May 28, 2012) that summarizes new research about how fish detect the presence of predators.  It’s really quite cool, and a little bizarre.  Keep in mind the concept of umwelt, and imagine what it might be like if we sensed our world in a similar manner.

When one fish is injured, others nearby may dart, freeze, huddle, swim to the bottom or leap from the water. The other fish know that their school mate has been harmed. But how?  In the 1930s, Karl von Frisch, the famous ethologist, noted this behavior in minnows. He theorized that injured fish release a substance that is transmitted by smell and causes alarm. But Dr. von Frisch never identified the chemical composition of the signal. He just called it schreckstoff, or “scary stuff.”

Schreckstoff is a long-standing biological mystery, but now researchers may have solved a piece of it. In a study published in February in Current Biology, Suresh Jesuthasan, a neuroscientist at the Biomedical Sciences Institutes in Singapore, and his colleagues isolated sugar molecules called chondroitins from the outer mucus of zebra fish.

They found that when these molecules are broken into fragments, as they might be when the fish’s skin is injured, and added to water, they prompt alarm behavior in other fish. At low concentrations, the fish were “mildly perturbed,” Dr. Jesuthasan said. At high concentrations, they stopped darting altogether and froze in place for an hour or longer.  He and his colleagues also showed that neurons in the olfactory bulb of these fish were activated when exposed to the sugar fragments. In a sense, the fish seemed to “smell” the injury.

The work could have broad implications for understanding fear and panic in other animals, and perhaps in humans, said Lisa Stowers, a neuroscientist at the Scripps Research Institute who was not involved in the research. Researchers have long struggled for better ways to help patients who are chronically prone to panic or anxiety.

Fear can be a useful tool for an individual animal. But it’s even more useful for one animal to be able to communicate its alarm — quickly — to others of its kind. Many lower animals seem to rely on smell to accomplish this, but surprisingly little is known of the substances used, or how they are produced or perceived.

The best-known alarm signals are used by bees and ants. The European honeybee releases a mixture of compounds after a sting. A major component is a molecule called isopentyl acetate, which rouses alarm in other honeybees. “Carpenter ants release compounds called formic acid and n-undecane to signal danger to their fellows,” Dr. Jesuthasan said. “Ants that sense these chemicals stop moving, swing their antennae and then begin moving quickly. If an enemy is spotted, they become aggressive. The exact response depends on the ratio of the chemicals.”
Sea urchins release substances when their bodies are crushed that cause other sea urchins to flee. Similar responses have been shown in marine snails, tunicates and tadpoles. But the chemical nature of the signals is not known, Dr. Jesuthasan added.

In a 2008 paper published in Science, Marie-Christine Broillet, a neuroscientist at the University of Lausanne in Switzerland, identified the system responsible for picking up on alarm signals in mice: a few hundred neurons, called the Grueneberg ganglion, in the tip of the nose. But Dr. Broillet did not identify the signaling molecules — “a major scientific challenge,” she said.

Chondroitin fragments may strike fear into the hearts of zebra fish, and perhaps even other fish, but they may mean nothing to other animals. “Fear pheromones tend to be species-specific,” said Ajay Mathuru, a neuroscientist in Dr. Jesuthasan’s lab. Animals need to “send warning signals to their friends,” rather than “tip off the enemy,” he added, though examples do exist of predators picking up on the panicked cues of prey.  Read the rest here

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