What is a receptor?

With all this conversation about receptors, you may well wonder: what is a receptor?

To put it simply, a receptor is a molecule that is attached to a cell's membrane, and that can grab on to specific chemicals and send a signal to the cell that it has done so.

The way the receptor lets the cell know that a chemical has bound to it is by changing shape. The shape change leads to one of at least two types of signal: the direct movement of ions into the cell, and the indirect opening or closing of ion channels.

Some receptors are ion channels as well as receptors — when these receptors change shape, the channel opens, and ions (usually sodium and/or calcium) enter the cell. The entry of sodium or calcium causes the cell to react by changing its metabolism and/or by send a message to the next cell in line.

An example of a receptor that is also an ion channel is the epithelial sodium channel or ENaC (Na is the chemical abbreviation for sodium, called natrium in Latin). Salt-sensitive taste bud cells use ENaC to allow sodium to enter the cell and start the process of notifying neighbor cells that there is sodium present.

Some receptors are G protein–coupled receptors (GPCRs). G-proteins get their name from the fact that they use a chemical, guanine triphosphate(GTP), to start reactions inside the cell. When a G protein–coupled receptor responds to its specific chemical, its shape change activates the G-protein attached to it, causing the G-protein to start a cascade of chemical reactions inside the cell. 

Odorant receptors (ORs) are G protein-coupled receptors. When an odorant attaches to its receptor, the receptor activates its associated G-protein, which in turn initiates chemical signals inside the cell. These signals cause an ion channel to open and allow sodium and calcium to enter the cell. Once this happens, the cell can send a signal to the olfactory bulb that an odorant molecule has been attached.

You can find a good description of the process of G-protein activation by odorants — and much more about olfaction — in the 2004 press release announcing the award of the Nobel Prize to Richard Axel and Linda B. Buck (below) for their work on odorant receptors: http://www.nobelprize.org/nobel_prizes/medicine/laureates/2004/press.html. 

Richard Axel

Linda Buck