Olfaction receptors

Aroma is the sensation that results when olfactory receptors in the nose are stimulated by particular chemicals in gaseous form, and is synonymous with odour and scent. Around 10,000 different olfactive receptors have been detected in humans, while a well-trained nose can differentiate up to about 1000 aroma compounds. 

Insect Olfaction Receptors, Signal Transduction, and Behavior... 

Indeed, most of the solid-state sensors developed since the 1970s are intrinsically non-selective, making them unsuitable for analytical purposes. The non-selectivity of chemical sensors were considered as one of the main problems limiting their diffusion for practical applications. Nonetheless, physiological investigations about olfaction receptors show that Nature strategy for odour recognitirm... 

According to the chemical theory of olfaction, the mechanism by which olfaction occurs is the emittance of particles by the odorous substances. These particles are conveyed to the olfactory epithelium by convection, diffusion, or both, and dkecdy or indkectly induce chemical changes in the olfactory receptors. 

Trotier D., Dpving K.B. and Rosin J.-F. (1994). Functional properties of frog vomeronasal receptor cells. In Olfaction and Taste XI (Kurihara K., et al., eds.). Springer, Berlin, pp. 188-191. 

The other major class of extracellular LBPs of mammals is the lipocalins (Flower, 1996). These are approximately 20 kDa, P-sheet-rich proteins, performing functions such as the transport of retinol in plasma or milk, the capture of odorants in olfaction, invertebrate coloration, dispersal of pheromones, and solubilizing the lipids in tears (Flower, 1996). The retinol-binding protein (RBP) of human plasma is found in association with a larger protein, transthyretin, the complex being larger than the kidney threshold and thus not excreted, although the RBP itself may dissociate from the complex to interact with cell surface receptors in the delivery of retinol (Papiz et al., 1986 Sundaram et al., 1998). 

Olfaction, once thought to be a primitive sense, is now recognized as an elaborate sensory system that deploys a large family of odorant receptors to analyse the chemical environment. Interactions between these receptors and their diverse natural binding molecules (ligands) translate the world of odors into a neural code. Humans have about 350 odorant receptors. Rodents have more than a thousand. 

In vertebrates the neurons for olfaction are located in the nose mucosa and consist of short neurons with a peripheral ending endowed with odorant receptors for a large number of molecules in the environment. Each receptor neuron only contains one odorant receptor and is connected directly with the olfactory lobe of the brain. The vertebrate olfactory system must cope with a staggering developmental problem how to connect millions of olfactory neurons expressing different odorant receptors to appropriate targets in the brain. 

Olfaction, research in, 18 383-384 Olfactory membrane, 11 567 Olfactory perceptions, 11 510-511 Olfactory receptors, 18 383-384 Olfactory response, 11 566-567 01igo(2-propenyloxy)methyloxyirane, sulfonation of, 23 720 Oligocyclic lattice host inclusion compounds, 14 177-179 Oligocyclic lattice hosts, 14 177 01igo(y-caprolactone)dimethylacrylate, in shape- memory polymer networks, 22 364... 

Here is a bit of a complication there is a lot of individual variation in the sense of human olfaction. Not everything smells the same to everyone. This holds both for the intensity of the perceived smeU as well as for its quality pleasant, floral, skunky, sweaty, or no odor at all. Andreas Keller has recently demonstrated that some significant part of this individual variation in the sense of smell derives from genetic variation in human odorant genes. Specifically, two single nucleotide polymorphisms (SNPs), leading to two amino acid substitutions in an odorant receptor, have dramatic affects on the perception of the odor of androstenone, a steroid derived from testosterone. 

Figure 12.7 Effector mechanism opening or dosing of an ion channel. The binding of the hormone opens an ion channel. The link between the receptor and the ion channel may be via a messenger molecule that causes a conformational change in one of the proteins in the channel or may be via phosphorylation of one of the proteins in the channel. An example of opening an ion channel is that of a pheromone on a sensory cell in the olfaction epithelium (see below).

Of the different sensory modalities, olfaction does not appear to feature in reports of the effects of plant hallucinogens. Since olfaction is the one sense not relayed through the thalamus, this brain area is likely to be central to the changes in consciousness described. Many target receptors implicated, e.g. muscarinic, 5-HT2, D2, D3 and opiate, are present in this thalamic nuclei the human brain. 

FIGURE 12-36 Molecular events of olfaction. These interactions occur in the cilia of olfactory receptor cells. 

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