Evidence for non-receptor odor discrimination using neuroblastoma cells as a model for olfactory cells

2,4,6-trichloroanisole is a potent suppressor of olfactory signal transduction

We investigated the sensitivity of single olfactory receptor cells to 2,4,6-trichloroanisole (TCA), a compound known for causing cork taint in wines. Such off-flavors have been thought to originate from unpleasant odor qualities evoked by contaminants. However, we here show that TCA attenuates olfactory transduction by suppressing cyclic nucleotide-gated channels, without evoking odorant responses. Surprisingly, suppression was observed even at extremely low (i.e., attomolar) TCA concentrations. The high sensitivity to TCA was associated with temporal integration of the suppression effect. We confirmed that potent suppression by TCA and similar compounds was correlated with their lipophilicity, as quantified by the partition coefficient at octanol/water boundary (pH 7.4), suggesting that channel suppression is mediated by a partitioning of TCA into the lipid bilayer of plasma membranes. The rank order of suppression matched human recognition of off-flavors: TCA equivalent to 2,4,6-tribromoanisole, which is much greater than 2,4,6-trichlorophenol. Furthermore, TCA was detected in a wide variety of foods and beverages surveyed for odor losses. Our findings demonstrate a potential molecular mechanism for the reduction of flavor.

Induction of differentiation of human olfactory neuroblastoma cells into odorant-responsive cells

Olfactory neuroblastoma is a rare malignancy of the olfactory mucosa that may be derived from the olfactory epithelium. To characterize this tumor, we cultured olfactory neuroblastoma cells in the presence or absence of growth factors (transforming growth factor alpha and basic fibroblast growth factor) known to affect olfactory tissue and assessed their responsiveness to known odorants by measuring changes in intracellular calcium. Untreated cells did not respond to odorants. Basic fibroblast growth factor treatment had cytotoxic effects, and treated cells did not respond to odorants. Transforming growth factor alpha treatment resulted in the induction of odor responsiveness in these cells. Cells responded to odorants at 100 nM to 100 microM concentrations and responded with both increases and decreases in intracellular calcium. Increases in intracellular calcium were mediated by a calcium influx and were reversibly blocked by compounds known to inhibit second messenger pathways in olfactory receptor neurons. The calcium responses of the olfactory neuroblastoma cells were thus specific to the odorants and similar to those found in olfactory receptor neurons. The results support the notion that olfactory neuroblastoma cells may be of olfactory origin and thus they can be used as a model cell line to study human olfaction.

Membrane fluidity response to odorants as seen by 2H-NMR and infrared spectroscopy

Fourier transform infrared spectroscopy (FTIR) and deuterium nuclear magnetic resonance spectroscopy (2H-NMR) have been used to study the location of two odorants, beta-ionone and menthone, in a model membrane of dimyristoylphosphatidylcholine, as well as the effect of the odorants on the structure and dynamics of the phospholipids. The interaction has been investigated for two lipid-to-odorant molar ratios, 10:1 and 1:1. The two odorants were found to affect the fluidity of the membrane. More specifically, the 2H-NMR results indicate that at a lipid-to-odorant molar ratio of 10:1, both beta-ionone and menthone increase the order of the deuterons in the interfacial and headgroup regions of the lipid while the incorporation of the odorants at a lipid-to-odorant molar ratio of 1:1 decreases the order of both the lipid headgroup and acyl chains. On the other hand, the infrared results show that the incorporation of beta-ionone and menthone decreases the phase transition temperature and cooperativity of the lipid acyl chains. The results suggest that the site of incorporation of beta-ionone and menthone is very similar in DMPC membranes.

Enhancement of the turtle olfactory responses to fatty acids by treatment of olfactory epithelium with phosphatidylserine

The turtle olfactory epithelium was treated with suspensions of various lipids and their effects on the olfactory responses were examined by measuring the olfactory bulbar responses. The phosphatidylserine (PS)-treatment greatly lowered the threshold for n-valeric acid and enhanced its responses at all concentrations examined. The responses to isovaleric acid and n-butyric acid were also greatly enhanced by the PS-treatment. The responses to ten other odorants examined were a little enhanced or unchanged by the PS-treatment. The enhanced responses to the fatty acids returned to the original level about 10 h after the treatment. It was confirmed that PS was incorporated into olfactory epithelium by incubating the epithelium with PS-suspension containing [14C]PS. The treatment of the epithelium with phosphatidic acid or cardiolipin unchanged or suppressed the responses to odorants including the fatty acids. The present results suggest that lipids as well as proteins in the receptor membranes play an important role in odor reception.

Electrically controlled proliferation of human carcinoma cells cultured on the surface of an electrode

Human carcinoma cells, MKN45, were cultured on the surface of a metal-coated plastic plate electrode the potential of which was controlled. The proliferation rate and cell morphology were altered depending on the applied potential. Cell proliferation was halted in the potential range above 0.4 V vs. Ag/AgCl, although cells started to proliferate again when the applied potential was shifted from 0.4 V to 0.1 V vs. Ag/AgCl. Fluorescence probe studies indicated that the fluidity of plasma membrane decreased in association with halting of cell proliferation. These results suggest that electrical stimulation causes cells to temporarily halt proliferation, and that cell proliferation was reversibly controlled by electrode potential. The mechanism is interpreted in relation to the change of plasma membrane structure represented by membrane fluidity.

Immunological and behavioral effects of fragrance in mice

The aim of this study was to determine the effects of olfactory stimulation on immunological and behavioral states in mice. Anti-SRBC (IgM) plaque forming cell (PFC) count and spontaneous running activity (SRA) were measured to demonstrate the effects of exposure to a given fragrance. The decreased PFC count and thymic involution induced by high pressure stress in mice were recovered after exposing the stressed mice to the fragrance continuously for 4 days after the stress was given. The PFC and SRA also appeared to be maintained at normal levels by oflactory stimulation with the fragrance for 24 hrs after the given stress. The immunological suppression induced by high pressure stress was considered to be caused by the induction and activation of suppressor cells. However, exposure to the fragrance after the stress did not enhance suppressor activity. The restoration of the stress-induced immune suppression by olfactory stimulation was blocked by procain administration onto the olfactory cells.

Liposomes having high sensitivity to odorants

The conditions to increase the sensitivities of liposomes to odorants were examined. The results obtained are as follows. (1) The minimum concentration of amyl acetate to induce the membrane potential changes (threshold) in phosphatidylcholine (PC) liposomes was about 10(-4) M and addition of 10 or 20% phosphatidylserine (PS) lowered the threshold to about 10(-9) M, which was lower than the thresholds for amyl acetate in the turtle and frog olfactory systems. (2) Similar to amyl acetate, addition of PS to PC greatly lowered the threshold for beta-ionone. On the other hand, addition of PS to PC in certain ratio increased the threshold for citral, suggesting that addition of PS to PC does not always increase the responses to all odorants. (3) The membrane fluidity change of the liposomes in response to odorants occurred at similar concentration region where the membrane potential changes occurred. The presence of CaCl2 in external solution much greatly increased both the magnitude of the membrane potential changes and the membrane fluidity changes of the PC-PS liposomes in response to amyl acetate than the presence of NaCl and MgCl2. These results suggest that the membrane fluidity change is related to generation of the membrane potential change. (4) It was estimated that adsorption of less than a few molecules of amyl acetate on single liposome elicits detectable changes in the membrane potential and the membrane fluidity.

Membrane fluidity changes of liposomes in response to various odorants. Complexity of membrane composition and variety of adsorption sites for odorants

Three kinds of liposomes prepared from phosphatidylcholine (PC), azolectin, and azolectin-containing membrane proteins of the canine erythrocytes were used as models for olfactory cells. To explore properties of the adsorption sites of odorants, membrane fluidity changes in response to various odorants were measured with various fluorescence dyes which monitor the fluidity at different depths and different regions of the membranes. (a) Application of various odorants changed the membrane fluidity of azolectin liposomes. The patterns of membrane fluidity changes in response to odorants having a similar odor were similar to each other and those in response to odorants having different odors were different from each other. These results suggested that odorants having a similar odor are adsorbed on a similar site and odorants having different odors are adsorbed on different sites. (b) Such variation of the pattern was not seen in liposomes of a simple composition (PC liposome). (c) In the proteoliposomes whose composition was more complex than that of azolectin liposomes, the patterns of membrane fluidity changes varied among odorants having a similar odor. It was concluded that liposomes of complex membrane composition have the variety of adsorption sites for odorants.

Evaluation of neuron-based sensing with the neurotransmitter serotonin

Results are presented on the development of a novel biosensor which will use neurons or neuronal components as both the recognition elements and primary transducers for analyte quantitation. This concept is demonstrated and evaluated by exposing identified neurons from the visceral ganglia of the pond snail Limnea stagnalis to the model analyte serotonin. Experiments reveal a reversible, concentration-dependent increase in the rate of spontaneous action potential generation, over a concentration range of four orders of magnitude. Studies with the antagonist methysergide verify that this response is mediated through serotonin-sensitive receptors. Exposure of the neurons to serotonin causes the firing frequency to rapidly increase to a maximum and then slowly diminish to a sub-optimal level. It was found that the maximum frequency provides an indication of chemical concentration that is repeatable. Data are also presented which further advance the field of neuronal biosensing by demonstrating both the effects of cell to cell variability on response reproducibility and the effects of the desensitizing response on the operation of a neuron-based sensor in both a continuous and discontinuous mode.

Molecular physiology of olfaction

Olfactory reception is mediated by olfactory receptor cells located in the olfactory epithelium. These cells are bipolar neurons that extend a dendrite toward the nasal lumen and an axon toward the olfactory bulb in the brain. The dendrite possesses a group of apical cilia embedded in mucus. Odorant recognition and signal transduction are initiated at the membranes of these chemosensory cilia and culminate in excitation of the olfactory receptor cell. Differential activation by odorants of distinct groups of olfactory receptor cells generates patterns of neuronal activity that encode odor quality and concentration. The identities of primary odorant recognition sites at the ciliary membrane remain to be established. However, a significant body of information has become available with respect to olfactory transduction mechanisms. It is now becoming clear that olfactory transduction involves the interplay of several second messenger systems to control the responses of these exquisitely sensitive chemosensory neurons.

Odorant response of isolated olfactory receptor cells is blocked by amiloride

Olfactory receptor cells were isolated from the nasal mucosa of Rana esculenta and patch clamped. Best results were obtained with free-floating cells showing ciliary movement. 1) On-cell mode: Current records were obtained for up to 50 min. Under control conditions they showed only occasional action potentials. The odorants cineole, amyl acetate and isobutyl methoxypyrazine were applied in saline by prolonged superfusion. At 500 nanomolar they elicited periodic bursts of current transients arising from cellular action potentials. The response was rapidly, fully and reversibly blocked by 50 microM amiloride added to the odorant solution. With 10 microM amiloride, the response to odorants was only partially abolished. 2) Whole-cell mode: Following breakage of the patch, the odorant response was lost within 5 to 15 min. Prior to this, odorants evoked a series of slow transient depolarizations (0.1/sec, 45 mV peak to peak) which reached threshold and thus elicited the periodic discharge of action potentials. These slow depolarizing waves were reversibly blocked by amiloride, which stabilized the membrane voltage between -80 and -90 mV. We conclude that amiloride inhibits chemosensory transduction of olfactory receptor cells, probably by blocking inward current pathways which open in response to odorants.

Large olfactory responses of the carp after complete removal of olfactory cilia

To study the role of olfactory cilia on olfactory reception, the carp olfactory cilia were removed by modified "ethanol-calcium shock" and the bulbar responses were recorded before and after deciliation. Large olfactory responses to various amino acids were observed after complete deciliation. The relation between magnitude of olfactory response and alanine concentration before and after deciliation was essentially unchanged. The present results suggests that the olfactory cilia may not be necessary for receptor neuron function in the carp.

Olfaction by melanophores: what does it mean?

Hypotheses on general olfaction can be divided into two broad groups: those that predict the existence of olfactory-specific olfactory receptor proteins and those that do not. Recently, much attention has been paid to the discovery of an odorant-stimulated adenylate cyclase in purified olfactory cilia. This finding has, for the most part, been accepted as evidence that the former hypotheses are correct. Here we report that frog melanophores, which are nonolfactory in nature, disperse their melanosomes in response to the same types and concentrations of odorants used in the investigations of olfactory cilia and that pigment dispersion is accompanied by rises in intracellular cAMP levels. The effects show that the existence of a cAMP-based second messenger system in olfactory cilia is not in itself proof of the existence of olfactory-specific olfactory receptor proteins. Also they explain the basis of Ottoson's pioneering work of 30 years ago on the electrical responses of frog olfactory epithelium to stimulation with alcohols. The results suggest that there could be two mechanisms that are important for the detection of odorants: one based on specific receptors, the other nonspecific, but both working through activation of cAMP.

Effects of odorant mixtures on olfactory receptor cells

The general mechanisms by which chemical stimuli may influence the firing frequency of olfactory neurons were briefly described. They include specific mechanisms mediated by receptor molecules and nonspecific mechanisms involving general properties of the chemicals and of cells. It is difficult to imagine that odorant mixtures influence receptor cells by mechanisms that are fundamentally different from those by which homogeneous chemicals act. It is argued that even under the best experimental conditions the presentation of odorants usually or always involves exposing the receptor cells to more than one additional molecular species compared to the unstimulated condition. This is because odorants invariably have contaminants that may be of potency such that their contribution to the odor is large even though their contribution to the number of molecules in the stimulus stream is small. Furthermore, the partition coefficients of the major and minor components are unlikely to be identical; therefore, their relative concentrations in the aqueous environment of the receptor cells can differ greatly from that in the gas phase. Finally, metabolic transformations of odorants in the olfactory mucosa can result in the exposure of receptor cells to mixtures of odorant and metabolites, with the mixture composition varying with time. Finally, some pitfalls in analyzing the effects of odorant mixtures are discussed. At the very least, it is necessary to determine the relation of concentration to response for each odorant in the mixture in order to interpret results in terms of interactions. Even with such data caution must be used, especially in attaching significance to reductions in the apparent maximal responses to one odorant induced by the presence of the other.

Cell suspensions from porcine olfactory mucosa. Changes in membrane potential and membrane fluidity in response to various odorants

A suspension of olfactory epithelial cells was prepared from porcine olfactory mucosa and the physiological functions of the suspension were examined. The membrane potential of the cell suspension, which was monitored by measuring the fluorescence changes of rhodamine 6G, was depolarized by an increase in the K+ concentration in the external medium. Various odorants depolarized the cell suspension in a dose-dependent fashion. The magnitude of depolarization by odorants was either unchanged or slightly increased by a reduction of the concentration of Na+, Ca2+, and Cl- in the external medium, which suggests that changes in the permeabilities of specific ions are not involved in depolarization by odorants. The application of various odorants to the cell suspension induced changes in the membrane fluidity at different sites of the membrane that were monitored with various fluorescent dyes [8-anilino-1-naphthalene sulfonate, n-(9-anthroyloxy) stearic acids, 12-(9-anthroyloxy) oleic acid, and (1,6-diphenyl-1,3,5-hexatriene)], which suggests that the odorants having different odors are adsorbed on different sites in the membrane. On the basis of these results, a possible mechanism of odor discrimination is discussed.

Contribution of electrostatic and hydrophobic interactions of bitter substances with taste receptor membranes to generation of receptor potentials

The effects of changed ionic environments on the frog taste nerve responses to the bitter substances were examined. The responses to quinine and strychnine carrying a positive charge were suppressed by an increase in ionic strength of stimulating solutions. It was concluded that electrostatic interaction of these positive bitter substances with the receptor membranes greatly contributes to the adsorption of the substances on the membranes and that this interaction was suppressed by an increase in ionic strength. The responses to neutral bitter substances (caffeine and theophylline) were unchanged by an increase in salt concentration. The zeta potential of the mouse neuroblastoma (N-18 clone), which was depolarized by various bitter substances similarly to a taste cell, was measured in the presence of the bitter substances. The zeta potential was a little changed by quinine and practically unchanged by strychnine, caffeine and theophylline. The membrane fluidity of the N-18 cell monitored with 2-(9-anthroyloxy)stearic acid was changed in response to the bitter substances, while the fluidity monitored with 12-(9-anthroyloxy)stearic acid or 1,6-diphenyl-1,3,5-hexatriene was unchanged. This suggested that the bitter substances are adsorbed on the hydrophobic region near the surface and induce a conformational change at the region. The depolarization by the bitter substances seems to stem from changes in the "boundary potential" at the region near the surface within the membrane interior.

Odorant-binding protein: localization to nasal glands and secretions

An odorant-binding protein (OBP) was isolated from bovine olfactory and respiratory mucosa. We have produced polyclonal antisera to this protein and report its immunohistochemical localization to mucus-secreting glands of the olfactory and respiratory mucosa. Although OBP was originally isolated as a pyrazine binding protein, both rat and bovine OBP also bind the odorants [3H]methyldihydrojasmonate and 3,7-dimethyl-octan-1-ol as well as 2-isobutyl-3-[3H]methoxypyrazine. We detect substantial odorant-binding activity attributable to OBP in secreted rat nasal mucus and tears but not in saliva, suggesting a role for OBP in transporting or concentrating odorants.