A longitudinal descriptive study of self-reported abnormal smell and taste perception in pregnant women

Self-reported abnormal sensitivity, qualitative distortions and phantom sensations with respect to smell and taste was assessed with a longitudinal design, based on questions referring to gestational weeks 13-16 and 31-34 of pregnancy in comparison with 9-12 weeks post partum and with non-pregnant women with corresponding time durations and intervals. The results show that abnormal smell and/or taste perception was reported by 76% of the pregnant women, typically believed to be caused by their pregnancy. Increased smell sensitivity was found to be very common at the early stage of pregnancy (67% of all pregnant respondents) and occasionally accompanied by qualitative smell distortions (17%) and phantom smells (14%). The smell abnormalities were less common at the late pregnancy stage and almost absent post partum. Abnormal taste sensitivity was fairly commonly reported (26%), often described as increased bitter sensitivity and decreased salt sensitivity. These results, suggesting that abnormal smell and/or taste perception is experienced by a large majority of pregnant women, imply that further research is needed to understand to what extent these chemosensory changes may underlie food aversions and craving with implications for food intake during pregnancy.     

Chemosensory cross-modal stroop effects: congruent odors facilitate taste identification

In order to explore the cross-modal cognitive associations between smell and taste, a chemosensory analogue of the Stroop task (Stroop 1935) was developed. Fourteen participants were presented with an odorant and a tastant and asked to identify the tastant as "sweet" or "sour" by pressing 1 of 2 buttons as quickly as possible. Participants were faster to name the taste when it was presented with an odor that was congruent (e.g., strawberry/sweet) than with an incongruent odor (e.g., strawberry/sour). These results support the concept of a high level of cognitive integration between the senses of smell and taste and illustrates occasions of interference between information arising from different sensory systems.     

Orosensory considerations

The chemical senses (taste, smell, and chemical irritation) convey information from the external to the internal environment. This information influences an organism's quality of life, safety, reproductive function, and, to the present point, nutritional status. To illustrate this role, the effects of chemosensory stimulation on food choice, gastrointestinal function, and energy balance will be briefly reviewed. Each role is achieved, in part, by the chemosensory cue initiating anticipatory responses for an impending homeostatic challenge, a meal.     

Contributions of Smell and taste to overall intensity

The integration of olfaction and gustation in producing thesensation of flavor was studied using an almond extract stimuluspresented with the Two-Module Delivery System. Almond in themouth was found to influence the scaling of smell. The datasugest that this background effect is due to the diffusion ofairborne molecules from the oral cavity to the olfactory receptorsarea. At least under some stimulation conditons, almond vaporin the nose was shown to afect the scaling of taste. When scalingoverall intensity, subjects apparently added together the sensationsof smell and taste to produce the sensation of overall intensity.However, for all combinations of odorants and tastants, theestimate of overall intensity was less than the sum of the estimateof the smell and the estimate of the taste. The data furthersuggest that when estimating the overall intensity, the magnitudeassigned to the olfactory or gustatory sensations was less thanthat which have been assigned when estimating just smell ortaste.     

Marine mammal chemoreception

The presence or absence of a chemoreceptive capacity in marine mammals has drawn relatively little attention from the research community outside the Soviet Union. Toothed whales are typically labelled anosmic (lacking a sense of smell) since they do not have the peripheral olfactory structures typically associated with terrestrial mammals. Baleen whales are known to possess reduced olfactory tracts; their olfactory bulbs also may be reduced or absent. Although the neural structures that mediate taste in terrestrial mammals have been reported to be present in both groups of whales, cetaceans have been considered to have a poor sense of taste because typical mammalian taste receptors have been thought to be absent. Soviet researchers, however, recently have reported that gustatory receptors are present on some cetacean tongues and that the tongue of Tursiops truncatus appear to be well innervated. These workers also have been conducting investigations which seem to be aimed at describing a specialized gustatory capability in cetaceans. No experimental work has been reported by Western scientists. Little work has been done by either Western or Soviet researchers with regard to chemoreception among the other orders of marine mammals (Pinnipedia, Carnivora and Sirenia). Pinnipedia are typically labelled microsmatic (having a poor sense of smell); research has been restricted to histological examination of the nasal pathways, and neural anatomy. Sea otters are credited with a keen sense of smell, but no quantitative work has been reported. The chemosensory abilities of Sirenia remain unknown. The tongues of non-cetacean marine mammals have been histologically examined and found to resemble those of terrestrial mammals. No other investigations of gustation in non-cetacean marine mammals have been reported.     

“Tasting” the airway lining fluid

Specialized epithelial cells of the respiratory tract have been termed "solitary chemosensory cells" based upon the expression of components of the canonical sweet, umami and bitter taste transduction pathway, or "brush cells" based upon their characteristic morphological feature, i.e. an apical, brush-like tuft of rigid, villin containing microvilli. Cells defined by these criteria might not match one-to-one, and a generally accepted terminology is still lacking. With respect to cellular shape, ultrastructure, expression of elements of the taste transduction cascade, innervation and synapse formation, and effects evoked upon their stimulation, it appears that chemosensory/brush in the upper respiratory tract (nasal respiratory mucosa, vomeronasal duct, auditory tube), in the olfactory mucosa, in the larynx, in the lower airways (trachea, bronchi) and in the alveolar region (rat only) each represent distinct groups. Still, they have in common to monitor the chemical composition of the mucosal lining fluid. They serve as sentinels detecting bacterial colonization or the presence of other harmful components in the mucosal lining fluid, leading to the initiation of avoidance reflexes and/or local defense mechanisms which are adapted to their anatomical localization. Free nerve endings are also responsive to inhaled irritants and further work will be needed to discriminate between the contributions of such nerve endings and chemosensory cells in chemical monitoring and defense initiation. Interestingly, there is first emerging evidence that respiratory chemosensory cells may respond to more than one canonical taste quality so that they, in analogy to polymodal nociceptors, may serve as polymodal chemosensors of potentially dangerous signals.     

G-protein-coupled receptors in intestinal chemosensation

Food intake is detected by the chemical senses of taste and smell and subsequently by chemosensory cells?in the gastrointestinal tract that link the composition of ingested foods to feedback circuits controlling gut motility/secretion, appetite, and peripheral nutrient disposal. G-protein-coupled receptors responsive to?a range of nutrients and other food components have been identified, and many are localized to intestinal chemosensory cells, eliciting hormonal and neuronal signaling to the brain and periphery. This review examines the role of G-protein-coupled receptors as signaling molecules in the gut, with a particular focus on pathways relevant to appetite and glucose homeostasis.     

Zinc in taste function

Zinc has been associated with taste function in humans at several levels of organization—the taste bud, the nerves transmitting taste information, and the brain. Zinc plays specific yet varied roles at each organizational level, although many of these roles have not been clearly identified. They include participation in the structural architecture of the cell, maintenance of cell membrane integrity, and control of activity of several cytoplasmic and membrane enzymes. Early investigators noted that some patients given drugs that altered zinc metabolism or who experienced disease processes associated with abnormalities of zinc metabolism exhibited taste dysfunction. Because of these findings zinc was given to a variety of patients as treatment for taste dysfunction. Initial treatment success was observed, but was quickly tempered by more extensive studies that yielded widely variable results leading to confusion about the role of zinc in both taste function and taste treatment. Further studies revealed that taste disorders were diverse and complex with multiple underlying pathophysiologies that were little understood. Subsequent work by several investigators revealed that patients with zinc deficiency, of any etiology, exhibited taste dysfunction and that treatment of these patients with zinc usually produced improvement of clinical symptoms. These results raised the question of how to define zinc deficiency, for zinc treatment in patients without zinc deficiency was unsuccessful and these patients represent more than three-quarters of all patients with taste dysfunction. New clinical techniques for the definition of human zinc deficiency have been achieved through the use of binding and displacement of⁶⁵Zn on specific sites on erythrocyte membranes; these results offer a guide to the identification of patients (i.e., those with zinc deficiency) who may benefit from zinc treatment.     

Prenatal Alcohol Exposure Increases Postnatal Acceptability of Nicotine Odor and Taste in Adolescent Rats

Human studies indicate that alcohol exposure during gestation not only increases the chance for later alcohol abuse, but also nicotine dependence. The flavor attributes of both alcohol and nicotine can be important determinants of their initial acceptance and they both share the component chemosensory qualities of an aversive odor, bitter taste and oral irritation. There is a growing body of evidence demonstrating epigenetic chemosensory mechanisms through which fetal alcohol exposure increases adolescent alcohol acceptance, in part, by decreasing the aversion to alcohol''s bitter and oral irritation qualities, as well as its odor. Given that alcohol and nicotine have noteworthy chemosensory qualities in common, we investigated whether fetal exposure to alcohol increased the acceptability of nicotine''s odor and taste in adolescent rats. Study rats were alcohol-exposed during fetal development via the dams'' liquid diet. Control animals received ad lib access to an iso-caloric, iso-nutritive diet throughout gestation. Odorant-induced innate behavioral responses to nicotine odor (Experiment 1) or orosensory-mediated responses to nicotine solutions (Experiment 2) were obtained, using whole-body plethysmography and brief access lick tests, respectively. Compared to controls, rats exposed to fetal alcohol showed an enhanced nicotine odor response that was paralleled by increased oral acceptability of nicotine. Given the common aversive component qualities imbued in the flavor profiles of both drugs, our findings demonstrate that like postnatal alcohol avidity, fetal alcohol exposure also influences nicotine acceptance, at a minimum, by decreasing the aversion of both its smell and taste. Moreover, they highlight potential chemosensory-based mechanism(s) by which fetal alcohol exposure increases the later initial risk for nicotine use, thereby contributing to the co-morbid expression with enhanced alcohol avidity. Where common chemosensory mechanisms are at play, our results suggest broader implications related to the consequence of fetal exposure with one substance of abuse and initial acceptability of others.     

TRPs in our senses

In the last decade, studies of transient receptor potential (TRP) channels, a superfamily of cation-conducting membrane proteins, have significantly extended our knowledge about the molecular basis of sensory perception in animals. Due to their distinct activation mechanisms and biophysical properties, TRP channels are highly suited to function in receptor cells, either as receptors for environmental or endogenous stimuli or as molecular players in signal transduction cascades downstream of metabotropic receptors. As such, TRP channels play a crucial role in many mammalian senses, including touch, taste and smell. Starting with a brief survey of sensory TRP channels in invertebrate model systems, this review covers the current state of research on TRP channel function in the classical mammalian senses and summarizes how modulation of TRP channels can tune our sensations.     

Suggestions for the functional relationship between differently located taste buds and vomeronasal olfaction in several mammals

In almost all mammals a well developed, paired and blind ending vomeronasal Organ (VNO) situated within the basement of the nasal septum, communicates with the oral cavity. This contact is established by two nasopalatine ducts, which penetrate the rostral palate close to the incisors. These ducts open orally into the sulcus which moulds the palatine papilla. In several mammals taste buds were found in the epithelium of the patatine papilla located within the nasopalatine ducts or close to their oral openings. Presumably these taste buds interact with the vomeronasal olfaction. It is likely that they are leading to a chemosensory sensation comparable to the combination of normal taste and smell. As not all mammals with a functionable VNO possess taste buds in this position, an inspection of the rostral part of the tongue which touches the palatine papilla presented an interesting situation concerning the distribution of taste buds. This region of the tongue is almost completely free of taste buds in species like Tupaia glis and Didelphis marsupialis virginiana, which have taste buds in the epithelium of their palatine papilla. In Lemur catta however, where the palatine papilla is lacking taste buds, the respective tongue part is densely covered with them. In this case it appears likely that they in a way of substitution functionally are connected with vomeronasal olfaction.     

Individual differences in the chemical senses: is there a common sensitivity?

Taste, smell, and chemical irritation (so-called trigeminal sensation) combine in our daily experience to produce the supramodal sensation of flavor, are processed by partly overlapping neural mechanisms, and show functional interconnectivity in experiments. Given their collaboration in flavor formation and the well-established connections between these senses, it is plausible that polymodal detection mechanisms might contribute to individual differences in measured sensitivity. One would expect the existence of a general chemosensory sensitivity factor to result in associations among taste, smell, and trigeminal stimulation thresholds. Measures of 5 detection thresholds from all the chemical senses were assessed in the same group of young healthy subjects (n=57). An unbiased principal components analysis (PCA) yielded a 2-component solution. Component 1, on which taste thresholds loaded strongly, accounted for 29.4% of the total variance. Component 2, on which the odor and trigeminal lateralization thresholds loaded strongly, accounted for 26.9% of the total variance. A subsequent PCA restricted to a 3-component solution cleanly separated the 3 sensory modalities and accounted for 75% of the total variance. Thus, though there may be a common underlying factor that determines some individual differences in odor and trigeminal lateralization thresholds, a general chemical sensitivity that spans chemosensory modalities seems unlikely.     

Functional development of chemosensory systems in the ontogeny of fish

Regularities of the functional development of chemosensory systems in the ontogeny of fish has been studied, i.e., the olfactory system, the taste system, and the common chemical sense. The olfactory system begins to function and provides response of juveniles to chemical signals before the taste system. Embryos that have hatched from coating but that do not yet feed exhibit nonspecialized motor responses to olfactory stimuli already. Immediately after the transition to exogenous nutrition, olfactory sensitivity to signals which elicit defensive and feeding behavioral responses begins to form and the ability to differentiate between similar odors develops. The reception of a limited number of taste stimuli occurs in the larvae during the transition to exogenous nutrition. With age, the spectrum of effective taste substances expands and the time spent on the definition of palatability by juvenile fishes reduces. Functional development of individual components of the taste system arises heterochronously, i.e., the outer (extraoral) form of taste reception arises earlier and more rapidly, and the buccal (intraoral) form of taste reception arises slower. No information is available about the functional development of the common chemical sense in the ontogeny of fish. It is assumed that the function of the chemosensory system arises in fish in early larval instar.     

Functional development of chemosensory systems in the fish ontogeny

Regularities of the functional development of chemosensory systems in the ontogeny of fish has been studied, i.e., the olfactory system, the taste system, and the common chemical sense. The olfactory system begins to function and provides response of juveniles to chemical signals before the taste system. Embryos that have hatched from eggs but that do not yet take food exhibit nonspecialized motor responses to olfactory stimuli already. Immediately after the transition to exogenous feeding, olfactory sensitivity to signals which elicit defensive and feeding behavioral responses begins to form and the ability to differentiate between similar odors develops. The reception of a limited number of taste stimuli occurs in the larvae during the transition to exogenous feeding. With age, the spectrum of effective taste substances expands and the time spent on the definition of palatability by juvenile fishes reduces. Functional development of individual components of the taste system arises heterochronously, i.e., the external (extraoral) form of taste reception arises earlier and more rapidly, and the oral (intraoral) form of taste reception arises slower. No information is available about the functional development of the common chemical sense in the ontogeny of fish. It is assumed that the function of the chemosensory system arises in fish in early larval stages.     

Selective removal of a target stimulus localized by taste in humans

Recent studies have shown that people can localize a punctate gustatory stimulus on the lingual epithelium in the absence of discriminative tactile cues. The present studies examined the human ability to localize taste sensations on the tongue and to use this information to remove selectively a target stimulus (a flavored, 1 cm(3) gelatin cube) from the mouth when presented with non-target distractors that vary in number and taste. Findings indicate that humans are capable of localizing and removing either an aversive or an appetitive gustatory target from a field of tactile distractors via taste sensations alone, although this ability diminishes as the number of distractors increases (implicating serial searches, rather than parallel). In addition, humans can localize and selectively remove a target taste in the presence of distractors of another distinct taste quality. Under these conditions performance is either unaffected or reduced, which indicates that contrast with the distinct taste of the distractors does not enhance performance. Humans also are capable of removing a nearly tasteless cube from a field of flavored distractors, but this is clearly a more difficult task, suggesting that 'tactile capture' of taste occurs for the tasteless target cube and interferes with the localization of taste. Finally, perceived suprathreshold stimulus intensity did not seem to be related to the ability to localize and remove a target stimulus via taste sensations and failed to account for variations in performance across individuals.     

Liposome-mediated transfection of mature taste cells

The introduction and expression of exogenous DNA in neurons is valuable for analyzing a range of cellular and molecular processes in the periphery, e.g., the roles of transduction-related proteins, the impact of growth factors on development and differentiation, and the function of promoters specific to cell type. However, sensory receptor cells, particularly chemosensory cells, have been difficult to transfect. We have successfully introduced plasmids expressing green and Discosoma Red fluorescent proteins (GFP and DsRed) into rat taste buds in primary culture. Transfection efficiency increased when delaminated taste epithelium was redigested with fresh protease, suggesting that a protective barrier of extracellular matrix surrounding taste cells may normally be present. Because taste buds are heterogeneous aggregates of cells, we used alpha-gustducin, neuronal cell adhesion molecule (NCAM), and neuronal ubiquitin carboxyl terminal hydrolase (PGP9.5), markers for defined subsets of mature taste cells, to demonstrate that liposome-mediated transfection targets multiple taste cell types. After testing eight commercially available lipids, we identified one, Transfast, that is most effective on taste cells. We also demonstrate the effectiveness of two common "promiscuous" promoters and one promoter that taste cells use endogenously. These studies should permit ex vivo strategies for studying development and cellular function in taste cells.     

Olfaction and Taste: Invasive Odours and Disappearing Objects

The metaphorisation of sight and hearing, the objective senses, dominate the founding ideas, or philosophemes, of Western philosophy. The senses of taste and smell are of little relevance in the formation of conceptual knowledge or in classificatory systems; they are, by virtue of their dissolving objects, incapable of giving objective knowledge in Western metaphysics. Derrida and Ulmer developed a metaphorology that exploits the chemical basis of the subjective senses of taste and smell. The anthropology of the senses takes this questioning of metaphysics into issues of how olfaction and taste function in sociality. In the routine practices of everyday life, is olfaction able to create the sense of community that it does in rituals? Or, has the repression of smell in humanity's evolution towards ‘civilisation’ muted the connective ability of multiple odour particles? In a culture and metaphysics that presumes the separability of the self from the other and the self from the object, is there a place for senses that make a nonsense of separation and objectivity through their state of meaningful dissolution? Through philosophy's metaphorisation, has taste been stripped of its sensuousness and made a sense for aesthetics and not flavours and textures? In a metaphorics premised in judgement and discernment, can taste be a sense that founds sociality? In blurring the boundaries between self and other that are necessary to form and maintain the distinction, the dissolvability of smell and taste makes another metaphorics and other socialities possible. Of all the senses, that of smell—which is attracted without objectifying—bears clearest witness to the urge to lose oneself in and become the ‘other’. As perception and the perceived—both are united—smell is more expressive than the other senses (Horkheimer and Adorno 1979:184).     

Taste and smell GPCRs in the lung: Evidence for a previously unrecognized widespread chemosensory system

Taste and smell receptor expression has been traditionally limited to the tongue and nose. We have identified bitter taste receptors (TAS2Rs) and olfactory receptors (ORs) on human airway smooth muscle (HASM) cells. TAS2Rs signal to PLCβ evoking an increase in [Ca^2 +]_i causing membrane hyperpolarization and marked HASM relaxation ascertained by single cell, ex vivo, and in vivo methods. The presence of TAS2Rs in the lung was unexpected, as was the bronchodilatory function which has been shown to be due to signaling within specific microdomains of the cell. Unlike β₂-adrenergic receptor-mediated bronchodilation, TAS2R function is not impaired in asthma and shows little tachyphylaxis. HASM ORs do not bronchodilate, but rather modulate cytoskeletal remodeling and hyperplasia, two cardinal features of asthma. We have shown that short chain fatty acids, byproducts of fermentation of polysaccharides by the gut microbiome, activate HASM ORs. This establishes a non-immune gut-lung mechanism that ties observations on gut microbial communities to asthma phenotypes. Subsequent studies by multiple investigators have revealed expression and specialized functions of TAS2Rs and ORs in multiple cell-types and organs throughout the body. Collectively, the data point towards a previously unrecognized chemosensory system which recognizes endogenous and exogenous agonists. These receptors and their ligands play roles in normal homeostatic functions, predisposition or adaptation to disease, and represent drug targets for novel therapeutics.     

Chemesthesis and the chemical senses as components of a "chemofensor complex"

An important function of the chemical senses is to warn against dangerous biological and chemical agents in the environment. The discovery in recent years of "taste" receptor cells outside the oral cavity that appear to have protective functions has raised new questions about the nature and scope of the chemical senses in general and of chemesthesis in particular. The present paper briefly reviews these findings within the context of what is currently known about the body's chemically sensitive protective mechanisms, including nonsensory processes that help to expel or neutralize threatening agents once they have been encountered. It is proposed that this array of defense mechanisms constitutes a "chemofensor complex" in which chemesthesis is the most ubiquitous, functionally diverse, and interactive chemosensory component.     

Expression of taste receptors in Solitary Chemosensory Cells of rodent airways

Background

Chemical irritation of airway mucosa elicits a variety of reflex responses such as coughing, apnea, and laryngeal closure. Inhaled irritants can activate either chemosensitive free nerve endings, laryngeal taste buds or solitary chemosensory cells (SCCs). The SCC population lies in the nasal respiratory epithelium, vomeronasal organ, and larynx, as well as deeper in the airway. The objective of this study is to map the distribution of SCCs within the airways and to determine the elements of the chemosensory transduction cascade expressed in these SCCs.

Methods

We utilized a combination of immunohistochemistry and molecular techniques (rtPCR and in situ hybridization) on rats and transgenic mice where the Tas1R3 or TRPM5 promoter drives expression of green fluorescent protein (GFP).

Results

Epithelial SCCs specialized for chemoreception are distributed throughout much of the respiratory tree of rodents. These cells express elements of the taste transduction cascade, including Tas1R and Tas2R receptor molecules, α-gustducin, PLCβ2 and TrpM5. The Tas2R bitter taste receptors are present throughout the entire respiratory tract. In contrast, the Tas1R sweet/umami taste receptors are expressed by numerous SCCs in the nasal cavity, but decrease in prevalence in the trachea, and are absent in the lower airways.

Conclusions

Elements of the taste transduction cascade including taste receptors are expressed by SCCs distributed throughout the airways. In the nasal cavity, SCCs, expressing Tas1R and Tas2R taste receptors, mediate detection of irritants and foreign substances which trigger trigeminally-mediated protective airway reflexes. Lower in the respiratory tract, similar chemosensory cells are not related to the trigeminal nerve but may still trigger local epithelial responses to irritants. In total, SCCs should be considered chemoreceptor cells that help in preventing damage to the respiratory tract caused by inhaled irritants and pathogens.