Effect of odorants on lipid monolayers from bovine olfactory epithelium

Polyunsaturated Fatty Acids as Potential Treatments for COVID-19-Induced Anosmia

Some individuals with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) experience anosmia, or loss of smell. Although the prevalence of anosmia has decreased with the emergence of the Omicron variant, it remains a significant concern. This review examines the potential role of polyunsaturated fatty acids (PUFAs), particularly omega-3 PUFAs, in treating COVID-19-induced anosmia by focusing on the underlying mechanisms of the condition. Omega-3 PUFAs are known for their anti-inflammatory, neuroprotective, and neurotransmission-enhancing properties, which could potentially aid in olfactory recovery. However, study findings are inconsistent. For instance, a placebo-controlled randomized clinical trial found no significant effect of omega-3 PUFA supplementation on olfactory recovery in patients with COVID-19-induced anosmia. These mixed results highlight the limitations of existing research, including small sample sizes, lack of placebo controls, short follow-up periods, and combined treatments. Therefore, more rigorous, large-scale studies are urgently needed to definitively assess the therapeutic potential of omega-3 PUFAs for olfactory dysfunction. Further research is also crucial to explore the broader role of PUFAs in managing viral infections and promoting sensory recovery.

Odor Discrimination by Lipid Membranes

Odor detection and discrimination in mammals is known to be initiated by membrane-bound G-protein-coupled receptors (GPCRs). The role that the lipid membrane may play in odor discrimination, however, is less well understood. Here, we used model membrane systems to test the hypothesis that phospholipid bilayer membranes may be capable of odor discrimination. The effect of S-carvone, R-carvone, and racemic lilial on the model membrane systems was investigated. The odorants were found to affect the fluidity of supported lipid bilayers as measured by fluorescence recovery after photobleaching (FRAP). The effect of odorants on surface-supported lipid multilayer microarrays of different dimensions was also investigated. The lipid multilayer micro- and nanostructure was highly sensitive to exposure to these odorants. Fluorescently-labeled lipid multilayer droplets of 5-micron diameter were more responsive to these odorants than ethanol controls. Arrays of lipid multilayer diffraction gratings distinguished S-carvone from R-carvone in an artificial nose assay. Our results suggest that lipid bilayer membranes may play a role in odorant discrimination and molecular recognition in general.

Simultaneously propagating voltage and pressure pulses in lipid monolayers of pork brain and synthetic lipids

Hydrated interfaces are ubiquitous in biology and appear on all length scales from ions and individual molecules to membranes and cellular networks. In vivo, they comprise a high degree of self-organization and complex entanglement, which limits their experimental accessibility by smearing out the individual phenomenology. The Langmuir technique, however, allows the examination of defined interfaces, the controllable thermodynamic state of which enables one to explore the proper state diagrams. Here we demonstrate that voltage and pressure pulses simultaneously propagate along monolayers comprised of either native pork brain or synthetic lipids. The excitation of pulses is conducted by the application of small droplets of acetic acid and monitored subsequently employing time-resolved Wilhelmy plate and Kelvin probe measurements. The isothermal state diagrams of the monolayers for both lateral pressure and surface potential are experimentally recorded, enabling us to predict dynamic voltage pulse amplitudes of 0.1-3 mV based on the assumption of static mechanoelectrical coupling. We show that the underlying physics for such propagating pulses is the same for synthetic and natural extracted (pork brain) lipids and that the measured propagation velocities and pulse amplitudes depend on the compressibility of the interface. Given the ubiquitous presence of hydrated interfaces in biology, our experimental findings seem to support a fundamentally new mechanism for the propagation of signals and communication pathways in biology (signaling), which is based neither on protein-protein or receptor-ligand interaction nor diffusion.

The sense of smell: molecular basis of odorant recognition

Most animal species rely on odorant compounds to locate food, predators, or toxins. The sense of smell is also involved in animal communication, and revealing the underlying mechanisms will therefore facilitate a deeper understanding of animal behaviour. Since the 1940s different theories have speculated on the fundamental basis of olfaction. It was assumed that odorant molecules were recognized by selective protein receptors in the nose, triggering a nervous signal processed by the brain. The discovery of these receptors in the early 1990s allowed great progress in understanding the physiological and biochemical principles of olfaction. An overview of the different mechanisms involved in the coding of odour character as well as odour intensity is presented here, focusing on the biochemical basis of odorant recognition. Despite the enormous progress achieved in recent years, details of odorant-receptor interaction at the molecular level and the mechanisms of olfactory receptor activation are poorly understood. The likely role of metal ions in odorant recognition is discussed, and also the perireceptor events involved in odorant transport and biotransformation, with a view to providing a comprehensive overview of mammalian olfaction to guide future computational structural models and the design of functional experiments. Recent studies have analysed the olfactory genome of several species, providing information about the evolution of olfaction. The role of the olfactory system in animal communication is also described.

Morituri te salutant? Olfactory signal transduction and the role of phosphoinositides

During the past 150 years, researchers have investigated the cellular, physiological, and molecular mechanisms underlying the sense of smell. Based on these efforts, a conclusive model of olfactory signal transduction in the vertebrate's nose is now available, spanning from G-protein-mediated odorant receptors to ion channels, which are linked by a cyclic adenosine 3',5'-monophosphate-mediated signal transduction cascade. Here we review some historical milestones in the chronology of olfactory research, particularly emphasising the role of cyclic nucleotides and inositol trisphosphate as alternative second messengers in olfactory cells. We will describe the functional anatomy of the nose, outline the cellular composition of the olfactory epithelium, and describe the discovery of the molecular backbone of the olfactory signal transduction cascade. We then summarize our current model, in which cyclic adenosine monophosphate is the sole excitatory second messenger in olfactory sensory neurons. Finally, a possible significance of microvillous olfactory epithelial cells and inositol trisphosphate in olfaction will be discussed.

A piezoelectric biosensor as an olfactory receptor for odour detection: electronic nose

Humans can detect and differentiate the presence of different odours even at trace levels of these odorous compounds. The odour quantification of any particular samples is normally based on conventional panel decisions. Other analytical instruments could be used to detect trace levels of odorous molecules. This study presents the results of a biological sensor system subject to different odorants. The system consists of a sensor in which the isolated olfactory receptor proteins (ORPs) from bullfrogs (Rana spp.) were coated onto the surface of a piezoelectric (PZ) electrode, similar to the mechanism of human olfaction. The PZ crystal served as a signal transducer. The results indicate rapid (about 400 s), reversible, and longterm (up to 3 months) stable responses to different volatile compounds such as n-caproic acid, isoamyl acetate, n-decyl alcohol, beta-ionone, linalool, and ethyl caporate. The sensitivity of the sensor ranges from 10(-6)-10(-7) g, fully correlated with the olfactory threshold values of human noses. An array of six sensors consisting of five fractionated ORPs and one referenced phospholipid probe is able to respond to different odorants and form a typical fingerprint for each odorant.

Characteristic response to taste stimuli of the intensities of higher harmonics in an electrochemical oscillatory system

In general, the electrochemical characteristics of solid/liquid or liquid/liquid interfaces are highly nonlinear, i.e., the capacitance changes markedly according to the applied voltage. In this paper, we propose a novel method for evaluating these nonlinear characteristics quantitatively. That is, a sinusoidal voltage source is applied to a test solution and the waveform of the output current is analyzed by Fourier transformation. It is shown theoretically that higher harmonic components in the Fourier transformation afford us useful information on nonlinear behavior. It is stressed that our technique is entirely different from the classical impedance method, i.e., nonlinear components of the impedance can be evaluated in our method, having been ignored previously in the classical impedance measurement. As an application of this method, we have studied the effect of taste compounds on the intensities of the higher harmonics, using an electrochemical cell containing an aqueous solution of sodium oleate. It has been found that the intensities of the higher harmonics exhibit characteristic changes upon the addition of taste compounds, the change being dependent upon the taste category. The characteristic response to taste compounds in the electrochemical nonlinearity is discussed in relation to the experimental trend of the dynamic isotherm for oleic acid at an air/water interface.

Characteristic effects of taste-compounds on the dynamic behavior of oleate-monolayer

Dynamic behavior of the monolayer of oleic acid on an aqueous solution was studied. The hysteresis loop of the surface, pressure (pi)-area (A) curve, was found to be characteristically dependent on the chemical stimuli with different taste-categories: salty, sweet, bitter and sour. The characteristic response of the dynamic surface behavior was discussed in relation with our recent finding, i.e., the features of the oscillation change in a different manner with the addition of various chemical species belonging to different taste categories in an excitable artificial liquid-membrane of oleic acid (Yoshikawa, et al, Langmuir, 4, 759-762 (1988).

Molecular mechanisms of olfaction

Recent studies provide initial insights into molecular mechanisms of olfaction. The identification of an odorant-sensitive adenylate cyclase which responds to most odorants, affords a second messenger system following odorant interactions with receptors. Cyclic nucleotide- and odorant-gated ion channels have been demonstrated in olfactory cilia, providing signalling systems in place of or in addition to protein phosphorylation. A unique odorant-binding protein localized to nasal mucosa binds odorants in proportion to their odoriferous potencies. Molecular cloning of the isolated protein reveals it to be a member of a family of proteins that serve as carriers for small lipophilic molecules such as retinol and cholesterol. The odorant-binding protein is localized to lateral nasal glands whose secretions are atomized into the tip of the nose where the binding protein presumably interacts with odorants in the inspired air.

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

The mouse neuroblastoma cell (N-18 clone), which is independent of an olfactory cell, was depolarized by 20 odorants examined, suggesting that specific proteins are not required for reception of odorants. The mechanism of non-receptor-mediated odor discrimination was examined using the N-18 cell. Changes in the membrane fluidity of the cell induced by adsorption of odorants were measured with various fluorescence probes, which monitor the fluidity at the different depth and in the different phase of the membrane. The profiles of the membrane fluidity changes monitored with these dyes were different from one species of odorants to another, suggesting that odorants having different odors are adsorbed at different sites in the membranes. The alteration of the lipid composition of the cell membrane brought about by exogenous application of stearic acid and cholesterol led to modification of the responses (magnitude of depolarization) to various odorants. The extent and direction (increase or decrease) of changes in the responses greatly varied among species of odorants. The following mechanism on odor discrimination was proposed. A membrane composition of each olfactory cell is postulated to be different from cell to cell. Different combinations of lipids and proteins in the membranes provide different adsorption sites for odorants. Relative amounts of the membrane potential changes in many olfactory cells in response to an odorant are characteristic of the species of the odorant. The response profiles at the cell level determine the quality of the odor.

Neuroblastoma cell as a model for a taste cell: mechanism of depolarization in response to various bitter substances

The mouse neuroblastoma cell (N-18 clone) was used as a model for a taste cell. The N-18 cell was found to be reversibly depolarized by various bitter substances. The minimum concentrations of bitter substances which induced depolarization (threshold concentration) varied greatly with the type of the substance. There was a good correlation between the threshold concentrations for various bitter substances in the N-18 cell and those in the human taste responses. The input membrane resistance was little changed during the depolarization induced by the bitter substances. Replacement of Na+ and Cl- with impermeable ions had practically no effect on the depolarization response to the bitter substances and reduction of calcium concentration from 1.8 to 0.2 mM led to a slight increase in the responses. It was suggested that the depolarization of the N-18 cell by bitter substances mainly stems from changes in the phase-boundary potential at the outer surface of the cell.

Neuroblastoma cell as model for olfactory cell: mechanism of depolarization in response to various odorants

The mouse neuroblastoma cell (N-18 clone) was used as a model for an olfactory cell. The N-18 cell was found to be depolarized reversibly by various species of odorants. The minimum concentrations of odorants which induced depolarization (threshold concentration) varied greatly with the species of odorants. There was a good correlation between the order of the threshold concentrations for various odorants in the N-18 cell and that in the frog olfactory responses. Replacement of Na+ and Cl- with impermeable ions or reduction of calcium concentration from 1.8 mM to 0.1 mM had practically no effect on the magnitude of the depolarization response to odorants. The input membrane resistance was little changed during the depolarization induced by various odorants. No reversal potential was observed when the cell was depolarized by n-amyl acetate or vanillin. It is suggested that the depolarization of N-18 cell by odorants is induced by changes in the phase-boundary potential at the outer surface of the cell.

Molecular mechanisms of olfactory reception. IV. Some biochemical characteristics of the camphor receptor from rat olfactory epithelium

Some parameters of the receptor element from the rat olfactory epithelium are evaluated; it is characterized by high affinity for camphor (KD = 1.5. x 10(-9) M). Triton X-100 has no marked effect on the binding of [3H]camphor. Neither RNAase nor phospholipase C affected [3H]camphor-binding activity. Pronase and trypsin abolished [3H]camphor binding activity by 65 and 40%, respectively. Sulfhydryl reagents decrease the binding of [3H]camphor by a factor of 5--8. The isoelectric point of the receptor solubilized with Triton X-100 is 4.8, as determined by isoelectric focusing. The molecular weight of the receptor as determined by gel electrophoresis is about 120 000. It is proposed that the camphor receptor is a membrane protein containing sulfhydryl groups and playing a key role in olfactory reception.

Cell suspensions from rat olfactory neuroepithelium: biochemical and histochemical characterization

Cell suspensions were generated from rat olfactory epithelium by digestion with collagenase and hyaluronidase followed by gentle mechanical disruption. These cell suspensions excluded nigrosin dye and synthesized RNA, protein and carnosine from radiolabeled precursors. Sustentacular cells, repiratory epithelial cells and olfactory neurons but not basal cells could be identified by phase-contrast microscopy. Sedimentation of these cell suspensions at unit gravity in discontinuous gradients of buffered bovine serum albumin resulted in partial separation of the various cell types as indicated by the distribution of several biochemical markers. Olfactory marker protein and carnosine synthetase activity were found in the upper gradient fractions, while carnosinase activity was present predominantly in the lower gradient fractions. Cellular localization of olfactory neuron marker protein and non-neuronal S-100 protein by immunoperoxidase staining of gradient-fractionated cells indicated that neuronal cells were only partially separated from non-neuronal cells by our fractionation techniques. Evaluation of gradient fractionated cells by histochemical staining for carbohydrates demonstrated that secretory Bowman's gland cells were quite efficiently separated from neurons. This study demonstrates the ease with which cell suspensions may be produced from the olfactory epithelium, and emphasizes the importance of utilizing both biochemical and histochemical approaches in studies of mixed populations of cells, particularly when the purity of the cell fractions is a consideration.

Hydrophobicity of biosurfaces as shown by chemoreceptive thresholds in Tetrahymena, Physarum and Nitella

Responses (chemotaxis and changes in membrane potential) of Tetrahymena, Physarum, and Nitella against aqueous solution of homologous series of n-alcohols, n-aldehydes and n-fatty acids were studied for clarifying the hydrophobic character of chemoreceptive membranes. Results were: (1) All organisms studied responded to homologous compounds examined when the concentration of these chemicals exceeded their respective threshold, Cth, and the response, R, were expressed approximately as R=alpha log (C/Cth) for C greater than Cth. (2) Increase of the length of hydrocarbon chain in homologues decreased Cth. Plots of log Cth against the number of carbon atoms, n, in n-alcohols, n-aldehydes and n-fatty acids showed linear relationships as represented by long Cth=-An+B. A and B are positive constants for respective functional end groups of the chemicals and biological membranes used. The above empirical equation was interpreted in terms of the partition equilibrium of methylene groups between bulk solution and membrane phase. Parameter A was shown to be a measure of hydrophobicity of the membrane, and B represented the sensitivity of chemoreception of the membrane. (3) Thresholds, Cth, for various hydrophobic reagents were compared with those of human olfactory reception, T. Plots of log T against log Cth fell on straight lines for respective organisms with different slopes which were proportional to parameter A.

Response of Nitella internodal cell to chemical stimulation. A model for olfactory receptor system

Electrical response to excitable internodal cell of Nitella was studied by applying various kinds of odorants to the cell. Changes in membrane potential and resistance during responses induced by odorants were measured intracellularly under a variety of ionic environments in the media. Results were: 1) Some odorants (coumarin, isoamylacetate, methylacetate, 1-octanol, 1-butanol, 1-propanol) produced an all-or-nothing type action potential when the concentration of odorant exceeded a certain threshold. The action potential was followed by a gradual depolarization of the potential whose amplitude depended on the odorant concentration, C. Other odorants (heptanoic acid, beta-ionon) induced gradual depolarization of the membrane potential without evoking an action potential. 2) Membrane resistance Rm changed in various ways during depolarization: some odorants led to a temporal or gradual decrease in Rm, and others caused an increase in Rm when the membrane potential was depolarized by the application of odorants. 3) Magnitude of response to odorants OR was found to be represented by the following equation: OR =(alpha + beta square root I) log (C/Cth) for C greater than or equal to Cth where alpha and beta are constants for a given odorant, I the ionic strength in the medium, and Cth the threshold concentration of the odorant. 4) Plots of olfactory threshold of human and of internodal cell of Nitella gave a straight line having slope unity. 5) Local application of odorants on the internodal cell induced impulses which transmitted from the part treated by odorants to the other portion. Physico-chemical and physiological implications of the results obtained were discussed.