Receptors odor discrimination

Odor discrimination could involve a very large number of different odorant receptors, each specific for one or a small set of odorants 818 The information generated by hundreds of different receptor types must be organized to achieve a high level of olfactory discrimination 820 Zonal expression of olfactory receptors 821 Convergence of sensory neurons onto a few glomeruli in the olfactory bulb 821... 

In a study comparing nine moth species, Steinbrecht (1996) observed that cross-reactivity of specific antisera to one species with PBPs from another species was not correlated with taxonomic relatedness of the species, but rather with the pheromone chemistry. Given that the highly divergent specificity of pheromone receptor cells in the Noctuidae species studied appears to be mirrored by a similar diversity of PBP sequences in sensilla trichodea, Steinbrecht suggested that PBPs participate in odorant discrimination (Steinbrecht, 1996). 

How are such diverse compounds detected and recognized to elicit the associated behavior Several steps contribute to the odor coding, such as olfactory receptor interactions and processing in mushroom bodies, but also including odor interactions with OBPs. The diversity and heterogeneity of OBPs implicates these proteins in the process of odor discrimination. 

Kajiya K, Inaki K, Tanaka M, Haga T, Kataoka H, Touhara K (2001) Molecular bases of odor discrimination Reconstitution of olfactory receptors that recognize overlapping sets of odorants. J Neurosci 21 6018-6025... 

It is not always clear how pheromone signals are detected in mammals. Most vertebrates, mice for example, have a VNO in addition to the main olfactory system. The VNO has two separate families of olfactory receptors Vlr, 137 functional receptors in mice V2r, 60 functional receptors in mice. The genes for these are only distantly related to those for the main olfactory receptors, suggesting that these systems evolved independently. As a general rule, it is the VNO and not the olfactory epithelium that is responsible for detecting pheromone molecules. However, it has been demonstrated that mice whose VNO has been surgically removed can discriminate MHC-determined odor types. This finding clearly implicates the main olfactory system in the detection of pheromones. 

It is apparent that multiple recognition mechanisms are active in taste and odor recognition. There is now emerging evidence of multiple receptors and transduction mechanisms. The complex olfactory component of a flavor requires a mechanism with multiple receptors and several transduction pathways to be sufficiently sensitive ai discriminating to distinguish among closely relat materials. 

Advances in the production, immobilisation and characterisation of mammalian olfactory receptors led to the development of biosensors where isolated olfactory binding proteins were deposited on the surface of QMBs [102, 103] or were connected to nanoelectrodes [104]. Although still at the development stage, such an array-type device coated with different olfactory receptors will be a powerful and useful tool for detecting and discriminating odorants in the future. 

Vertebrates possess three primary chemosensory systems gustation ( taste ), trigeminal, and olfaction ( smell ) but only one of these, the olfactory system, mediates responses to pheromones. Chemicals that stimulate the olfactory system are known as odorants and comprise one type of biological cue (any entity that stimulates a sensory system). Bouquets of odorants that can be discriminated as specific entities are termed odors. The olfactory system contains olfactory receptor neurons (ORNs) that comprise cranial nerve I and project directly to the forebrain. ORNs are now known to express only one to a few olfactory receptor proteins ( receptors ), which means that the chemoreceptive range of each neuron can be very narrow. The olfactory system also has several subcomponents including the vomeronasal organ, which is described below. 

Importantly, each taste receptor cell expresses many different members of the T2R family. This pattern of expression stands in sharp contrast to the pattern of one receptor type per cell that characterizes the olfactory system (Figure 32.16). The difference in expression patterns accounts for the much greater specificity of our perceptions of smells compared with tastes. We are able to distinguish among subtly different odors because each odorant stimulates a unique pattern of neurons. In contrast, many tastants stimulate the same neurons. Thus, we perceive only "bitter" without the ability to discriminate cycloheximide from quinine. 

---