Olfactory receptor cells

Researchers at the MoneU Center (Philadelphia, Pennsylvania) are using a variety of electrophysical and biochemical techniques to characterize the ionic currents produced in taste and olfactory receptor cells by chemical stimuli. These studies are concerned with the identification and pharmacology of the active ion channels and mode of production. One of the techniques employed by the MoneU researchers is that of "patch clamp." This method aUows for the study of the electrical properties of smaU patches of the ceU membrane. The program at MoneU has determined that odors stimulate intraceUular enzymes to produce cycUc adenosine 3, 5 -monophosphate (cAMP). This production of cAMP promotes opening of the ion channel, aUowing cations to enter and excite the ceU. MoneU s future studies wiU focus on the connection of cAMP, and the production of the electrical response to the brain. The patch clamp technique also may be a method to study the specificity of receptor ceUs to different odors, as weU as the adaptation to prolonged stimulation (3). 

Chuah M. and Zheng D. (1987). Olfactory marker protein is present in olfactory receptor cells of human fetuses. Neuroscience 23, 363-370. 

Constanzo R. (1991). Regeneration of olfactory receptor cells. In Regeneration of Vertebrate Sensory Cells (Bock G.R., ed.). Wiley, New York, pp. 233-248. 

Farbman A.I. and Squnto L.M. (1985). Early development of olfactory receptor cell axons. Brain Res 351, 205-213. 

Hatt H Gisselmann G., Wetzel C., et al. (1999). Cloning, functional expression and characterization of a human olfactory receptor. Cell Mol Biol 45, 285-291. 

Morita Y. and Finger T. (1998). Differential projections of ciliated and microvillous olfactory receptor cells in the catfish, Ictalurus punctatus. J Comp Neurol 398, 539-550. 

Schild D. and Restrepo D. (1998). Transduction mechanisms in vertebrate olfactory receptor cells. Physiol Revs 78, 429-466. 

Astic, L., Saucier, D. and Holley, A. Topographical relationships between olfactory receptor cells and glomerular foci in the rat olfactory bulb. Brain Res. 424 144—152, 1987. 

Largely, the insect detectors for pheromones and other semiochemicals are arrays of hair-like sensilla distributed over the surface of the antennae and palps. In some species, such as scarab beetles [3, 4] and the honeybee [5], semiochemicals are received by olfactory plates. The more ubiquitous hair-like sensilla typically consist of hollow cuticular hairs (10-400 pm long, 1-5 pm thick) innervated by one or several olfactory receptor cells (neurons) and three auxiliary cells [6]. 

The dendrites on the receptor cells have swellings, the olfactory knobs. These knobs have smooth vesicles that may be open to the mucus covering the epithelium. From each knob extend 1 to 150 cilia, floating in the mucus. The precise number of cilia varies with the species. The rabbit, for example, has 10-12 cilia on each knob. The surrounding mucus consists of mucopolysaccharides, lipids, and phosphatides. The axons of the olfactory receptor cells extend through the cribriform plate and terminate in the olfactory bulb (Fig. 5.1). 

The long-held dichotomy of macrosmatic and microsmatic vertebrates is no longer tenable. Neuroanatomists had assumed that taxa with relatively few olfactory receptor cells and small olfactory bulbs would also be inferior in olfactoiy performance (threshold and number of compounds detected) to those with more receptors and larger bulbs. However, we now know from single-cell recordings that a particular receptor cell type can respond to a wide range of odor compounds that share certain features. Keverne (1983) pointed out that the olfactory bulbs act as a filter, while more complex pattern analysis takes place in the neocortex. The more evolved the neocortex, as in primates, the... 

Differential sensory sensitivity. The insect s perception of plant odours differs essentially from their discrimination of non-volatile taste substances, as phytophagous insects may already perceive the odour at some distance from the plant. In adult phytophagous insects the antennae bear a large number of olfactory sensilla in order to detect the minute concentrations of the leaf odour components in the air downwind from a plant. The overall sensitivity of the antennal olfactory receptor system can be measured by making use of the electroantennogram technique (17). An electroantennogram (EAG) is the change in potential between the tip of an antenna and its base, in response to stimulation by an odour component. Such an EAG reflects the receptor potentials of the olfactory receptor cell population in the antenna. 

Figure 7. Intensity of neural activity in 23 olfactory receptor cells in the antenna of the Colorado beetle, in response to tTins-2-hexen-I-ol (t-2-H-I-ol), cis-3-hexen-I-ol (c-3-H-I-ol), and Uins-2-hexenal (t-2-H-al). The increase of neural activity in response to individual green odor components is visualized in the areas of circles...

When the antennae are completely ablated, the male cockroach still responds to the sex pheromone (Roth and Willis, 1952). This suggests that some pheromone-responsive cells are also located elsewhere, such as on the mouthparts. Olfactory receptor cells on the palps of P. americana project to the lobus glomeratus within the posterior region of the ventral deutocerebrum (Boeckh and Ernst, 1987). However, it is not known whether pheromone-responsive cells on the palps project to the more anterior macroglomerulus. 

On the specificities of antennal olfactory receptor cells of Periplaneta americana. Chemical Senses 8 375-395. 

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

Derby, C. D., Carr, W. E. S., and Ache, B. W., Purinergic olfactory receptor cells of crustaceans response characteristics and similarities to internal purinergic cells of vertebrates, /. Comp. Physiol. A, 155, 341, 1984. 

Boekhoff I., Michel W. C., Breer H. and Ache B. W. (1994). Single odors differentially stimulate dual second messenger pathways in lobster olfactory receptor cells. J. Neurosci. 14, 3304-3309. 

Shields V. D. C. and Hildebrand J. G. (2001) Responses of a population of antennal olfactory receptor cells in the female moth Manduca sexta to plant-associated volatile organic compounds. J. Comp. Physiol. A 186, 1135-1151. 

Pophof B. (2002) Moth pheromone binding proteins contribute to the excitation of olfactory receptor cells. Naturwissenschaften 89, 515-518. 

Gold G. H. and Lowe G. (1993) Nonlinear amplification by calcium-dependent chloride channels in olfactory receptor cells. Nature 366, 283-286. 

Kurahashi T. and Shibuya T. (1989) Ca(2+)-dependent adaptive properties in solitary olfactory receptor cell of the newt. Brain Res. 515, 262-268. 

Kurahashi T. and Yau K.W. (1993) Co-existence of cationic and chloride components in odorant-induced current of vertebrate olfactory receptor cells. Nature 363, 71-75. 

Lowe G. and Gold G. H. (1991) The spatial distribution of odorant sensitivity and odorant-induced currents in salamander olfactory receptor cells. J. Physiol. (Lond.) 442, 147-168. 

Menco B. P. Cunningham A. M., Qasba P., Levy N. and Reed R. R. (1997) Putative odour receptors localize in cilia of olfactory receptor cells in rat and mouse a freeze-substitution ultrastructural. J. Neuxocytol. 26, 691-706. 

Sklar P. B., Anholt R. H. and Snyder S. H. (1986) The odorant sensitive adenylate cyclase of olfactory receptor cells differential stimulation by distinct classes of odorants. J. Biol. Chem. 261, 15538-15543. 

Firestein S., Picco C. and Menini A. (1993) The relationship between stimulus and response in olfactory receptor cells of the tiger salamander. J. Physiol. (London) 468, 1-10. 

In humans the olfactory receptor cells lie in the mucous membrane at the top of the air passages on either side of the nasal septum. They occupy a total area of about 2 cm, which is small compared with most other mammals. Evidence from both anatomy and embryology shows that the development of the olfactory tissue is closely linked to that of the pituitary gland which lies at the base of the brain. Among other functions the pituitary plays a key role in the coordination of sexual activity and reproduction. This ancient association between the sense of smell and the reproductive process is one that has important implications for work of the perfumer. 

Figure 5.9 The human olfactory system. (A) Section through the nose. (B) Section through the cribriform plate. (C) The olfactory pathway to the cerebrum (forebrain). This shows the pathway of olfactory sensation. Nasal stimulation begins at the cilia of the olfactory receptor cells located at the ends of the olfactory nerves. The olfactory nerves then carry the impulse to the cerebrum, resulting in the sense of smell.

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