Olfactory Receptors and Transduction

Like all sensory stimuli, odors carry information. Odors can tell you whether your food is fresh or spoiled; can warn of dangers like fire, predators, or infection; and may even provide information about the suitability of a mate. Though some odors may elicit the same stereotyped behaviors or [Page 700]hormonal changes in every individual, the meaning of most odors is heavily dependent on our previous experiences, our motivations and preferences, and the context in which they are encountered. However, no matter what their meaning might be, odors can only be perceived if they are detected. Odors are complex mixtures of chemical stimuli. Each component of a particular odor mixture is termed an odorant. Humans and other mammals use a large repertoire of receptor proteins on the surface of sensory neurons in the nose to recognize distinct odorants and other chemosensory stimuli. Once activated by their interactions with these stimuli, the receptors trigger a biochemical cascade within the cell to transduce, or change, the chemical signal into a neural signal that can be recognized by the brain.

Figure 1 The Olfactory Transduction Cascade in Canonical Olfactory Sensory Neurons (OSNs)

The mammalian nose employs many hundreds of different chemosensory receptors—approxi-mately 350 in humans and close to 1,500 in mice—as well as several distinct biochemical mechanisms to detect and transduce olfactory signals. The large array of receptors increases the number and type of chemicals that can be detected by the olfactory system; the need for several different transduction mechanisms is less clear. Recognition of this receptor diversity has contributed to our realization that the olfactory system is organized into several subsystems, each using a different combination of receptors and transduction mechanisms to recognize and encode olfactory stimuli. Thus, each subsystem is selective for those stimuli that can activate the receptors it expresses.

Why olfactory transduction is so complex remains unclear. However, a complex sensory world may require a complex sensory system. The large variety of chemosensory receptors can expand the universe of potential olfactory stimuli, not only to chemically diverse volatile molecules but to peptides and proteins as well. Distinct, multistep transduction cascades offer numerous opportunities for signal amplification, temporal control, and contextual modulation (e.g., by adjusting olfactory sensitivity in the context of metabolic needs or mating status). Finally, the coexpression of certain receptors, signaling molecules and ion channels, may reflect their own functional limitations such that certain types of receptors can only work efficiently with certain signaling molecules. This entry discusses receptors in the main and accessory olfactory systems.