Interpretive Summary: The essence of appetite: does olfactory receptor variation play a role?
By: Jackie Walling
An article published in the 2018 April Journal of Animal Science highlights research on links between the olfactory system and appetite regulation in production animals. Identifying relationships foreshadows possible developments of odorant receptor (OR) agonists and genetically selecting receptor variants to improve feed intake and related traits. Improving animal production in this manner requires understanding the structure, function, and genetic variability of ORs in vertebrate.
Odor perception was evaluated primarily in the olfactory epithelium. Structurally, ORs are G-protein coupled chemoreceptors found principally in the olfactory epithelium, but they can also be found on other organs demonstrating abilities to impact physiological processes. ORs can detect a plethora of odorants and odorants are capable of binding with different affinities to multiple ORs. This varied pattern of binding gives animals a heightened sense of smell. The binding results in a movement of ions in and out of cells creating a nerve impulse sent to the olfactory bulb where the brain relays the information. In other organs, such as the kidney, ORs impact physiological processes. Various concentrations of volatile fatty acids bind to ORs and cause renin secretion which regulates blood pressure.
Exposure of rats to limonene and linalool proved ORs located in the olfactory epithelium can be activated to influence appetite regulation. Effects of odorants are controlled by the secretion of appetite stimulant or suppressant neuropeptides and activity of gastric vagal nerves. OR51E2 is an OR in the colon found to influence appetite. Enteroendocrine L cells contain OR51E2 which coexpresses appetite-regulating gut hormone peptide YY (PYY) and sometimes glucagon-like peptide 1 (GLP-1). Together PYY and GLP-1 regulate the “ileal brake” which ultimately increases satiety. Additionally, olfactory receptor neurons were found to house many hormonal receptors responsible for appetite fluctuations by changing the frequency and amplitude of neuronal firing.
Genetic structure of ORs are variable across species correlating to the way animals obtain food. More OR genes were found in animals primarily using sense of smell than in animals relying on other senses such as sight. A main cause for variation is caused by copy number variations in DNA. These large repeating segments of DNA are expressed differently among individuals and this individuality contributes to differences in olfactory sensitivity in species. Past studies of livestock report genes saturated with ORs, taste, and pheromone receptors located near SNPs associated with feed intake and intake traits suggesting a strong association between ORs and appetite regulation.
Recognizing the genetic structure of ORs paves the way for appetite manipulation by either activating ORs directly or by use of genetic selection. OR-expressing cells can be isolated, cultured, and exposed to odorants of interest. Then the mRNA expression or protein production for known appetite regulators can be quantified. Finally, odorants can be tested in animals to see if they produce the intended effect. Another option is cloning ORs, subjecting the clones to luciferase assay, and adding the same odorant to the clones to find the OR with highest activation. Or, using one OR and multiple odorants can determine the odorant with the highest effect of stimulation.
Evaluating the links between the olfactory system and appetite regulation in animals is still in the rudimentary stages of development. Recognizing the structure, function, and genetics behind ORs has led to the beginnings of experimentation in manipulation of appetite to improve animal production. To read the full article, go to the Journal of Animal Science.
To read the full article, "The essence of appetite: does olfactory receptor variation play a role," visit the Journal of Animal Science.