Olfrec.com

Olfactory receptors

This domain, olfrec.com, was created to host a database with information about olfactory receptors. The database now exists, but has not reached a state in which it is presentable to the public (and probably never will).

An olfactory receptor is a receptor in the nose. It is sensitive to certain chemicals in the airstreams which passes through the nose. (I write "airstreams" because a chemical present in the inward airstream may be perceived differently from the same chemical in the outward airstream.)

Most species of mammal have 1,000 or more different olfactory receptors. The receptors differ in the protein which is present in them and which detects a chemical (known as a "ligand") or class of chemicals. Most receptor proteins detect more than one ligand, and most detectable ligands are detected by more than one receptor.

Humans have only about 800 genes coding for the characteristc proteins of olfactory receptors. Morever, fewer than half of these 800-odd genes actually code for functioning olfactory receptor proteins. This is probably a consequence of reduced selection pressure in humans; for a human, a slightly defective sense of smell is less likely to be fatal than it is for most mammalian species. The non-functioning genes are sometimes known as "pseudogenes".

All known olfactory receptor genes form a single "gene family"; that is, they all descend from a single ancestor. This is the largest known gene family.

The sense of smell

The "sense of smell" is traditionally said to be one of "the five senses", with the other four being taste, sight, hearing, and touch. This is an oversimplification. We also have a sense of balance, and can perceive heat, cold, pain, etc..

These pages will only be concerned those senses that detect chemical substances. These fall into three categories:

1. "The sense of smell". This is the detection of ligands in the nose by the olfactory receptors. These ligands are necessarily volatile. It is the most complicated and the least understood of the three categories, because
   • There are, even in humans, over 300 functioning genes for olfactory receptors,
   • Most olfactory receptor detect a range of ligands.
   • Most ligands are detected by more than one olfactory receptor.
   • A sense perception, such as "the smell of new-mown hay", is likely to involve more than one ligand and more than one olfactory receptor.
   • Our vocabulary for describing olfactory sense perception is less precise and less well established than our vocabulary for other sense perceptions.
2. "The sense of taste". This is the detection of ligands in the mouth by the sense receptors on the tongue. These ligands are water-soluble. There are five known such detectors:
   • Sweet: detects sucrose and other sugars, and other substances.
   • Sour: detects acids.
   • Bitter: detects various substances, particularly alkaloids such as quinine.
   • Salt: detects ionised salts.
   • Umami: detects glutamate ions.
3. Sometimes known as piquancy, pungency, chemesthesis or spiciness. This is the "accidental" detection of chemicals by neuroreceptors, where that is not the "purpose" for which the neuroreceptor evolved. Some examples of piquancy are
   • The detection of capsaicin (chile) by TRPV1 receptors, which are evolved to detect heat
   • The detection of menthol (peppermint) by kappa opioid receptors, which are evolved to detect cold
   • The detection of isothiocyanates (mustard, wasabi, etc.) by both TRPA1 and TRPV1 receptors, which are evolved to detect mechanical stress and heat respectively

The terminology is confusing. When we say "this tastes salty", we are indeed referring to "the sense of taste", involving sensors on the tongue. When we say "this tastes of aniseed" we are referring to the "sense of smell", involving olfactory receptors in the nose. And when we say "this tastes hot" we are describing an effect on TRPV1 neurotransmitters, present in all mucous membranes (and elsewhere, but the capsaicin is less likely to reach them elsewhere).

These pages will be mostly concerned with the sense of smell.

For further information

on olfactory receptors and their ligands, see

• HORDE, Weizmann Institute of Science
  Human receptor genes
• Sergey Zozulya, Fernando Echeverri and Trieu Nguyen
  The human olfactory receptor repertoire
  
• Applied Bioinormatics Group, University of Verona
  Complete list of receptors
  Readable version of above
  
• The Good Scents Company Information System