Whether you are drawn or repelled by the smell of rotting flesh depends upon your perspective as a species. One whiff and we instantly recoil, avoiding the hazard of bacterial contamination. For animals such as rats, goldfish and some insects, the scent is a draw, a sign of food or a place to lay eggs.
The smell of carrion is carried by two small molecules: the evocatively named “putrescine” and “cadaverine”. These molecules are produced when microbes break down organic tissue, and decarboxylate amino acids.
Geneticists and cell biologists from the University of Cologne in Germany and Harvard University have found that zebrafish, a vertebrate model animal, also find these scents disgusting. As they report in PNAS Early Edition, a particular olfactory receptor, TAAR13c, is particularly tuned to cadaverine. TAAR13c triggers sensory neurons at cadaverine concentrations that lead to powerful, automatic avoidance. Though these molecules are known to automatically trigger behavioral responses, how they operate is a relative mystery that may be solved with the assistance of this newly identified receptor.
The receptor Ashiq Hussain and colleages believe is primarily responsible, TAAR13c, binds to only a few molecules, preferring medium-length, odd-chained diamines. Although a marked tuning to a chain length is common in scent receptors, TAAR13c is unusual for preferring odd-chained diamines,write the authors.
“Odd- and even-chained diamines have significant differences in the relative orientation and positioning of the two amino groups,” they say. “Interestingly, odd-chained and even-chained diamines did elicit comparable averse behaviors, which suggests the presences of additional zebrafish olfactory receptors activated by even-chained diamines, consistent with data from cross-adaptation studies.”
To measure avoidance, researchers video recorded the movements of zebrafish in a tank 5 minutes before and after the addition of the nasty odor (non-scented tank water was used in control trials). A motion tracker measured how far and how quick the fish swam away from the application site of the scent.
After an hour of exposure to either food odor, nothing or one of the diamines, fish were quickly killed and the olfactory epithelium was removed. It was then tested for the presence of early gene products that signal neuron responsiveness.
Using rotten “fish extracts” the researchers measured the responsiveness of TAAR13c receptions in functional assays (high performance liquid chromatography analysis).
This receptor developed during evolution of ray-finned fishes. Orthologs aren’t found in rats and humans, which also detect cadaverine.
“Thus, cadaverine-activated olfactory receptors in mammals may present a case of convergent evolution,” write the authors, “either within the vertebrate TAAR family or between different olfactory families.”
This association of odors, associated receptors and innate behaviors will, the authors hope, lay a foundation for studying the neural circuitry of odor repulsion and attraction.