Scientists find how zebrafish "smell" carbon dioxide and evade

WASHINGTON, Jan. 30 (Xinhua) -- A Japanese study discovered a neuronal pathway that makes fish avoid the unpleasant carbon dioxide.

The study, published in Cell Reports on Tuesday, shows that larval zebrafish, known to have a fast response when touched on the head, are found to have an avoidance response to carbon dioxide after around four-five seconds.

There is an unknown sensation-response pathway in the brain that is different from one when they are touched, according to researchers at the RIKEN Brain Science Institute in Japan.

To identify the responsible pathway, researchers used transgenic zebrafish made for calcium imaging, a technique visualizing brain activity by genetically expressing fluorescent protein sensitive to calcium, a key molecule involved in the transmission of neuronal signals.

The team was able to see a series of responses to carbon dioxide in the brain, the earliest being in the olfactory bulb, the part of the brain that processes smell in mammals.

A few seconds later, they saw responses in trigeminal sensory neurons, the nerve that carries touch and pain sensations from the face. The final response was from the habenula, a part of the brain known to be involved in learning associations with unpleasant experiences.

In order to determine which of these three systems was necessary for the response to carbon dioxide, the team used a laser to remove each one separately. They found that only damage to the trigeminal pathway and to the nose affected the response to carbon dioxide.

This was somewhat surprising because damaging the olfactory pathway itself did not change the avoidance behavior. "This meant that a non-olfactory component in the nose is critical for avoiding carbon dioxide," said its lead author Tetsuya Koide.

The team next wanted to determine how carbon dioxide was sensed in the nose. Calcium imaging of the zebrafish nose revealed a cluster of cells that responded to carbon dioxide.

Tests indicated that these cells were part of the terminal nerve and their removal blocked the avoidance response to carbon dioxide. Thus, the zebrafish nose contains terminal nerve chemo-sensors that are unrelated to smell and that can control behavioral responses to noxious chemicals.

"As humans and other vertebrates also possess the terminal nerve system," said Koide, "we next hope to further characterize its chemosensory functions across different species, including humans." Enditem