A nose for ATP
Studies have shown that fish sense ATP, the cellular unit of energy, and follow concentration gradients of the molecule released by zooplankton to track down their next meal. Testing zebrafish in the lab, neurobiologist Yoshihiro Yoshihara of the RIKEN Brain Science Institute in Japan and colleagues found that ATP appeared to activate a small number of short, pear-shape olfactory sensory neurons at the very tip of the nose.

A neuronal odor trail
When the researchers hunted for ATP receptors in the zebrafish genome, they found a novel receptor called A2c. Cell culture experiments revealed that A2c was not directly activated by ATP, however; two enzymes, tissue-nonspecific alkaline phosphatase (TNAP) and CD73, first broke down ATP to adenosine, which then bound the receptor and triggered the neurons in the nose to fire. Signals from these neurons then activated a single large cluster of nerve endings, or a glomerulus called IG2, in the olfactory bulb of the zebrafish brain.

Evolving food sense
A database search of available genomes showed that the A2c receptor is found in fish and amphibian species, but not in terrestrial reptiles, birds, and mammals. “The A2c receptor must serve a very fundamental function in all the aquatic lower vertebrates,” Yoshihara says.

Open questions
Arnaud Gaudin, a neurobiologist at Canada’s Dalhousie University who was not involved in the study, pointed out that the paper only considers the amphibian group Xenopus, which live and hunt in water even as adults. “It would be very interesting to see whether the A2c receptor would also be found in other anuran species . . . in which tadpoles have a fully aquatic olfactory phase that is lost after metamorphosis.”

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