The effect of concanavalin A on the rat electro-olfactogram. Differential inhibition of odorant response

Concanavalin A application to the olfactory epithelium reveals different sensory neuron populations for the odour pair D- and L-carvone

Carvone enantiomers (D and L optical isomers) have been shown to be discriminable by humans even though the odor qualities are quite similar. Our experiment is based on a finding (J. Steroid Biochem. Molec. Biol. 1991;39(4B):621) that Concanavalin A (ConA) applied to a frog olfactory epithelium preparation blocks cAMP transduction induced by D- but not by L-carvone. We used standard operant conditioning methods to train animals to discriminate low odor concentrations of D-carvone from clean air, to discriminate L-carvone from clean air; or to discriminate between clean air and the odors of D-carvone, L-carvone, ethyl acetate and methacrylic acid. After perfusion of the nasal cavity with ConA, rats did not respond to D-carvone above or near chance level, while the L-carvone response was not affected at the same or higher ConA doses. However, for rats trained on both enantiomers and the two other unrelated odorants, the D-carvone response remained unaffected by ConA. These results suggest to us that: (1) ConA blocks at least one chiral receptor selective for D-carvone; (2) D-carvone odor quality is modified by ConA so that it is no longer recognized by rats trained on D-carvone only, while rats trained to generalize odors still respond to D-carvone.

Selective and reversible reduction of odor sensitivity in the rat by concanavalin A

Rats were trained using an olfactometer to detect low concentrations of ethyl acetate (EA) or dimethyl disulphide vapor (DMDS). Concanavalin A (ConA) applied to the olfactory mucosa had no effect on ability of rats to detect low concentrations of EA but produced a marked deficit in detection in DMDS. DMDS detection performance returned to control levels 3 days after the ConA treatment. These results provide the first behavioral evidence for odor-specific inhibition by chemical modification of the olfactory epithelium and support the notion that ConA selectively inactivates one or more types (subclasses) of olfactory receptor proteins.

Probing olfactory receptors with sequence-specific antibodies

Molecular cloning has revealed the structure of several putative odorant receptors. Chemically synthesized peptides, that correspond to a predicted extracellular domain of the encoded proteins, were employed to generate receptor-specific antibodies. Immunohistological approaches as well as Western-blot analysis confirmed the specificity of the antipeptide sera. Furthermore, deglycosylation experiments explained the observed discrepancy between the molecular mass of odorant receptors, as determined by SDS/PAGE and Western-blot analysis of ciliary proteins (M(r) 50,000), and the predicted protein size based on the deduced primary structure from cloned receptor genes (M(r) 30,000-35,000). Receptor proteins become phosphorylated upon odorant stimulation of olfactory cilia preparations; this was demonstrated by immunoprecipitation experiments employing the sequence-directed, receptor-specific antibodies. Functional assays revealed that the receptor-specific antibodies significantly attenuate second messenger signalling elicited by inositol 1,4,5-trisphosphate-inducing odorants, whereas activation of the cAMP cascade by appropriate odorants was not affected. These observation indicate that the sequence-specific antibodies not only recognize odorant receptors, but also discriminate between receptor subtypes coupling to different second-messenger pathways.

Induction of olfactory receptor sensitivity in mice

Repeated exposure to olfactory ligands (odorants) increased peripheral olfactory sensitivity in mice. For two unrelated ligands, androstenone and isovaleric acid, induction of olfactory sensitivity was odorant-specific and occurred only in inbred strains that initially had low sensitivity to the exposure odorant. These data demonstrate stimulus-induced plasticity in a sensory receptor cell, suggesting a form of stimulus-controlled gene expression. Induction with two unrelated odorants implies that olfactory induction is a general phenomenon that may occur in a large fraction of the human population.

Towards an identification of odorant receptors

The molecular nature and diversity of receptive sites for odorous molecules is a central unanswered issue in olfaction. Based on the enormous resolving power of the olfactory system, which enables the stereospecific discrimination of numerous compounds at low concentrations specific receptor proteins have been proposed. Due to the central role of G-proteins in olfactory signalling it has been predicted that odorant receptors might be members of the superfamily of receptor proteins with seven transmembrane domains. Upon application of degenerated oligonucleotides and the PCR-technology a number of putative odorant receptors have been cloned and sequenced. In situ hybridisation studies using receptor-specific probes have been performed and olfactory neurons specifically expressing a particular receptor subtype are topologically identified in the nasal epithelium of rats. Clones of interest are now being expressed in heterologous systems in order to demonstrate functional activity of these putative receptor proteins and to match defined odorants to identified receptors.

Lectins bind differentially to cilia and microvilli of major and minor cell populations in olfactory and nasal respiratory epithelia

Binding of colloidal gold-conjugated lectins was studied in cilia and microvilli of rat olfactory and respiratory epithelia. This was done in sections of rapidly frozen, freeze-substituted specimens embedded in Lowicryl K11M or, for wheat germ agglutinin (WGA) alone, in deep-etched replicas. Olfactory dendritic endings and cilia labeled with WGA and faintly with soybean agglutinin (SBA); olfactory supporting cell microvilli bound only Dolichos biflorus agglutinin (DBA). Microvilli of an infrequent cell bound peanut agglutinin (PNA), SBA, and WGA. These microvilli labeled more strongly with the last two lectins than the olfactory cilia. Respiratory cilia bound WGA and, somewhat more weakly, PNA; microvilli of ciliated respiratory cells bound all four lectins. Visualization of specific labeling improved after preincubation of sections with neuraminidase, except for DBA where lectin binding was abolished. PNA labeling was seen only after neuraminidase preincubation. The densities of membrane surface particles that labeled with WGA corresponded with those of fracture plane particles in a quantitative freeze-fracture, deep-etch analysis. Therefore, a considerable fraction of the WGA-bound particles could reflect transmembrane proteins in olfactory dendritic endings and cilia and in respiratory cilia. The possible nature of these particles is discussed.

Molecular reaction cascades in olfactory signal transduction

Odorant induced second messenger signals in ciliary preparations from rat olfactory epithelia were monitored in the subsecond time range using a rapid kinetic methodology. Application of micromolar concentrations of odorants induced a rapid and transient elevation of second messenger concentrations. The odorous compounds analyzed induced in a mutually exclusive way the formation of either cyclic adenosine monophosphate or inositol-triphosphate. The activating effects of odorants on intracellular signalling cascades appear to be mediated via different G-proteins. Thus, at least two different second messenger pathways appear to be involved in olfactory signal transduction. Selective inhibition of odor-induced second messenger responses by certain lectins indicate that glycoproteins appear to be involved in the perception or transduction of olfactory signals. In the presence of protein kinase inhibitors the odorant-induced second messenger response is no longer transient but persistent over a longer time period, suggesting that termination of the signal is realized via feedback phosphorylation of functional elements in the reaction cascade.

Concanavalin A reveals olfactory receptors which discriminate between alkane odorants on the basis of size

For certain odorants, the amplitude of the rat electro-olfactogram is reduced if the olfactory epithelium is treated with the lectin concanavalin A. When normal and cycloalkanes of one to ten carbon atoms are used as odorants at equimolar concentration, the maximum reduction in amplitude is found to correlate with the size of the stimulus molecule. This observation is consistent with the notion that concanavalin A disables an olfactory receptor molecule which normally responds to the alkyl moiety of odorants in a particular size range. That moiety may thus represent a 'primary' quality-determining component in odour discrimination.

A comparative study of 2-deoxyglucose patterns of glomerular activation in the olfactory bulbs of C57 BL/6J and AKR/J mice

We have studied the patterns of glomerular activation evoked in the olfactory bulbs of C57BL/6J and AKR/J mice by olfactory stimulations with amyl acetate or isovaleric acid. Patterns of glomerular activation were obtained with the 2-deoxyglucose method and subsequently compared using a computer-assisted image analysis. The mice of both inbred strains stimulated with amyl acetate or isovaleric acid were characterized by areas of high 2-deoxyglucose uptake in their glomerular layer. Statistical comparisons of the patterns demonstrated that they were odor-specific. Although C57BL/6J mice have been found to be specifically anosmic to isovaleric acid, our findings indicate that their olfactory system is topographically activated by olfactory stimulations with this odorant. However, patterns of glomerular activity evoked by isovaleric acid stimulations are not similar in the two studied strains.

Spatial variation in response to odorants on the rat olfactory epithelium

We have measured the electro-olfactogram produced by four odorants, nicotine, i-pentyl acetate, i-pentanoic acid and cineole from twelve positions on an in vitro preparation of rat olfactory tissue. Each odorant shows a different pattern of response over the twelve positions which can be explained by differences in olfactory receptor populations between regions of the rat olfactory epithelium. The result for nicotine is further evidence that there are olfactory receptors which are stimulated by nicotine when it is presented as a vapour.

The effect of concanavalin A on the rat electro-olfactogram at various odorant concentrations

We have studied the effect of concanavalin A (Con A) on the rat electro-olfactogram response to several odorants. Each odorant was applied over a range of concentrations. For hydrophobic odorants whose response was affected by Con A, the diminution in response was maximal at odorant concentrations of about 1 microM in the olfactory mucus. The (odour) concentration-dependence of the change is compatible with the idea that Con A inactivates one or more types of olfactory receptor that normally bind odorants with dissociation constants of the order of 100 nM. With hydrophilic odorants we had to apply concentrations very much higher than this to elicit any response from the system. At these high concentrations we could observe Con A-induced diminutions in response.