On the trigeminal percept of androstenone and its implications on the rate of specific anosmia

Olfactory perception of 5α-androst-16-en-3-one: Data obtained in the residents of central Russia

This dataset describes olfactory perception of androstenone (5α-androst-16-en-3-one) in residents of central Russia (n=807, 9-84 years old). Based on previous studies, 3.13 × 10-3% androstenone was utilized for sensory testing. A modified two-alternative choice task with "no-difference" options was applied as an odor discrimination procedure. The participants rated the perceived odor intensity and pleasantness on 3-point categorical scales. The subsequent task was free odor identification. We used a survey to determine characteristics of our study population such as age, sex, ethnicity, olfactory abilities, smoking habits, respiratory tract diseases, perfume use, blood group. To examine sex differences in characteristics of participants, as well as in their perception of androstenone, we have applied one-way ANOVA for age variable and Chi-square tests for categorical variables. The dataset will be useful for assessing the impact of the aforementioned factors on the perception of volatile steroids. A wide range of researchers studying verbal descriptors of specific odors and cross-cultural differences in olfactory perception may reuse this dataset. Primarily, this dataset may benefit meat industry, as androstenone, which is perceived as an unpleasant odor by about a quarter of the population, significantly contributes to pork flavor.

Interpretive review: Semiochemicals in domestic pigs and dogs

This interpretive review includes discussion of the available scientific literature with interpretations by the authors. The broad field of semiochemicals can be confusing to scientists and consumers. This review attempts to summarize the known scientific studies for pig and dog semiochemicals while at the same time attempting to refine our use of terminology. The specific objectives of this interpretive review are to summarize and interpret much of the key scientific literature (but not the lay literature) on semiochemicals in pigs and dogs to include (1) definitions of semiochemicals and related molecules including pheromones, (2) to briefly summarize olfactory organs, and (3) and to examine the scientific literature for semiochemical mechanisms and applications in dogs and pigs (two domesticated species with known olfactory acuity). Dogs and pigs have olfactory features that are similar in that they both lack certain olfactory organs (Grueneberg ganglion and Septal Organ) and they have a small vomeronasal organ (VNO) without some major receptors that are found in other species. The primary olfactory organs for both pigs and dogs are the main olfactory epithelium and perhaps the trigeminal nerve. Several examples of pheromones activating the brain via the MOE or Trigeminal nerve rather than the VNO challenge the concept that the VNO is the site of pheromone sensing. We believe it is not appropriate to label something a pheromone when evidence is not available to show that it is a pheromone. We offer definitions for the terms semiochemicals, pheromones, interomones and others and then determine if the evidence is sufficient to call certain semiochemicals a pheromone. Here we review mixed, largely negative, scientific reports of the efficacy of some products labeled as "pheromones" that are more appropriately called semiochemicals. Interomones can have a more powerful effect on dog behavior and physiology than semiochemicals marketed as pheromones. Because marketing of semiochemicals is far ahead of the science, bringing some logic and uniformity to the field will benefit animals and hopefully cause less consumer confusion. Semiochemicals have the potential to offer powerful solutions to behavioral problems using more naturally occurring molecules.

Influence of gender and culture on the perception of acidic compounds of human body odor

There is increasing evidence that human body odor is involved in interpersonal communication. However, among the wide variety of substances excreted by the human body that might act as chemosignals, much attention has been granted to androstenes to the detriment of other categories. Here, we focused on the acidic fraction of human body odor. We investigated men and women's perceptual descriptions and detection thresholds of the sexually dimorphic (male) compound 3-hydroxy-3-methylhexanoic acid (HMHA) in two contrasted cultures, France and Madagascar. Perceptual responses to HMHA in both countries were very similar. HMHA proved to be more typical of body odor than another chemically-related major compound of human body odor 3MHA (3-methylhex-2-enoic acid also known as 3M2H). A significant portion of the samples studied (between 8 and 19%) was likely to be anosmic to HMHA (and to 3MHA: 25%). Although differences would be expected between men and women's perceptual responses to HMHA, based on the assumption that this compound would have a function in human partner choice, no sex differences were found for any of the perceptual variables. However, in Malagasy women, perceived intensity of HMHA was higher in participants who were in the fertile phase of the menstrual cycle. Whether HMHA is relevant in the perception of a potential partner thus requires further explorations, with more implicit approaches for example and/or by investigating the repercussions of HMHA specific anosmia on interpersonal relationships.

The intranasal trigeminal system

Many odors activate the intranasal chemosensory trigeminal system where they produce cooling and other somatic sensations such as tingling, burning, or stinging. Specific trigeminal receptors are involved in the mediation of these sensations. Importantly, the trigeminal system also mediates sensitivity to airflow. The intranasal trigeminal and the olfactory system are closely connected. With regard to central nervous processing, it is most interesting that trigeminal stimuli can activate the piriform cortex, which is typically viewed as the primary olfactory cortex. This suggests that interactions between the two systems may form at a relatively early stage of processing. For example, there is evidence showing that acquired olfactory loss leads to reduced trigeminal sensitivity, probably on account of the lack of interaction in the central nervous system. Decreased trigeminal sensitivity may also be responsible for changes in airflow perception, leading to the impression of congested nasal airways.

Individual variability of human olfactory sensitivity to volatile steroids: Environmental and genetic factors

The sensitivity to androstenone and possible factors, determining the sensitivity were investigated for the large sample of inhabitants of central Russia (n = 860). Specific anosmia was detected in 48.8% of subjects. Women were more sensitive to androstenone than men. The proportion of men, but not women perceiving the smell of androstenone as a strong one in the concentration used decreased with age. Smoking, blood group, or ethnicity had no significant effect on the expression of specific anosmia and the perception of androstenone odor intensity.

Responsiveness of human neonates to the odor of 5α-androst-16-en-3-one: a behavioral paradox?

The odorous steroid 5α-androst-16-en-3-one (AND) occurs in numerous biological fluids in mammals, including man, where it is believed to play a chemocommunicative role. As AND was recently detected in milk and amniotic fluid, sensitivity and hedonic responses to this substance were assessed in human neonates. To this aim, respiration and facial expressions were recorded in 3-day-old newborns in response to aqueous solutions of AND, ranging from 500ng/mL to 0.5 fg/mL. Although analyses of respiratory rate did not lead to clear-cut results, the newborns changed their facial expressions at concentrations not detected by adults in a triangle test. Newborns displayed negative facial actions of longer duration to AND relative to an odorless control. Thus, AND may be considered to be offensive to newborns, which is a counterintuitive outcome as they are exposed to this compound in the womb (and it should therefore be familiar), in milk, and on the mother's skin surface (and it should therefore be conditioned as positive). Multiple reasons for this perceptual-behavioral paradox are discussed.

Human olfactory lateralization requires trigeminal activation

Rats are able to lateralize odors. This ability involves specialized neurons in the orbitofrontal cortex which are able to process the left, right and bilateral presentation of stimuli. However, it is not clear whether this function is preserved in humans. Humans are in general not able to differentiate whether a selective olfactory stimulant has been applied to the left or right nostril; however exceptions have been reported. Following a screening of 152 individuals with an olfactory lateralization test, we identified 19 who could lateralize odors above chance level. 15 of these "lateralizers" underwent olfactory fMRI scanning in a block design and were compared to 15 controls matched for age and sex distribution. As a result, both groups showed comparable activation of olfactory eloquent brain areas. However, subjects with lateralization ability had a significantly enhanced activation of cerebral trigeminal processing areas (somatosensory cortex, intraparietal sulcus). In contrast to controls, lateralizers furthermore exhibited no suppression in the area of the trigeminal principal sensory nucleus. An exploratory study with an olfactory change detection paradigm furthermore showed that lateralizers oriented faster towards changes in the olfactory environment. Taken together, our study suggests that the trigeminal system is activated to a higher degree by the odorous stimuli in the group of "lateralizers". We conclude that humans are not able to lateralize odors based on the olfactory input alone, but vary in the degree to which the trigeminal system is recruited.

Brain responses to odor mixtures with sub-threshold components

Although most odorants we encounter in daily life are mixtures of several chemical substances, we still lack significant information on how we perceive and how the brain processes mixtures of odorants. We aimed to investigate the processing of odor mixtures using behavioral measures and functional magnetic resonance imaging (fMRI). The odor mixture contained a target odor (ambroxan) in a concentration at which it could be perceived by half of the subjects (sensitive group); the other half could not perceive the odor (insensitive group). In line with previous findings on multi-component odor mixtures, both groups of subjects were not able to distinguish a complex odor mixture containing or not containing the target odor. However, sensitive subjects had stronger activations than insensitive subjects in chemosensory processing areas such as the insula when exposed to the mixture containing the target odor. Furthermore, the sensitive group exhibited larger brain activations when presented with the odor mixture containing the target odor compared to the odor mixture without the target odor; this difference was smaller, though present for the insensitive group. In conclusion, we show that a target odor presented within a mixture of odors can influence brain activations although on a psychophysical level subjects are not able to distinguish the mixture with and without the target. On the practical side these results suggest that the addition of a certain compound to a mixture of odors may not be detected on a cognitive level; however, this additional odor may significantly change the cerebral processing of this mixture. In this context, FMRI offers unique possibilities to look at the subliminal effects of odors.

Stimulus selection for intranasal sensory isolation: eugenol is an irritant

Both the olfactory and the trigeminal systems are able to respond to intranasal presentations of chemical vapor. Accordingly, when the nose detects a volatile chemical, it is often unclear whether we smell it, feel it, or both. The distinction may often be unimportant in our everyday perception of fragrances or aromas, but it can matter in experiments that purport to isolate olfactory processes or study the interaction between olfaction and chemesthesis. Researchers turn to a small pool of compounds that are believed to be "pure olfactory" stimuli with little or no trigeminal impact. The current report reexamines one such commonly used compound, namely eugenol, a flavor and fragrance ingredient that has anesthetic properties under some conditions. Using a standard method involving many trials during an experimental session (Experiment 1), subjects were unable to reliably lateralize eugenol, consistent with claims that this compound is detected primarily through olfaction. However, with more limited exposure (Experiments 2 and 3), subjects were able to lateralize eugenol. We speculate that anesthetic properties of eugenol could blunt its trigeminal impact in some paradigms. Regardless, the current experiments suggest that eugenol can in fact stimulate the trigeminal nerve but in a complex concentration-dependent manner. Implications and strategies for selection of model odorants are discussed.

Electrical responses to chemosensory stimulation recorded from the vomeronasal duct and the respiratory epithelium in humans

The physiological significance of the human vomeronasal duct (VND) is still unclear. The aim of the present study was to investigate the question whether mucosal responses obtained from the VND are different from those obtained from the respiratory epithelium. There were 15 healthy subjects (8 male, 7 female; age range 19-45 years; 14 normosmic subjects, 1 anosmic subject). All subjects participated in two sessions whereby the first session was used to acquaint them with the experimental conditions. For chemical stimulation, an olfactometer was used which delivered chemical stimulants without altering mechanical or thermal conditions at the stimulated nasal mucosa. For stimulation we used substances previously reported to produce vomeronasal activation ("estra"=estra-1,3,5(10),16-tetraen-3ol and "andro"=androsta-4,16-dien-3-on); in addition, gaseous CO(2) was used as a non-odorous, relatively specific stimulant of the trigeminal nerve. Placement of electrodes either in the VND or on the respiratory epithelium was performed under endoscopical guidance. Subjects rated the overall intensity of the stimuli, the strength of trigeminally mediated sensations, and the hedonic tone of the stimulants. Responses could not be recorded from all subjects. For the remaining 7 subjects, intensity was strongest for CO(2) stimuli (p<0.001), whereas no significant difference was observed between "andro" and "estra" (p=0.33). All three stimulants produced responses at the respiratory epithelium with largest responses obtained after stimulation with CO(2). Similar findings were made for recordings inside the VND. Due to the small sample size sexual dimorphisms could not be addressed. In summary, these results seem to indicate that the presently used stimulants produce similar responses at the respiratory epithelium and in the VND which argues against a specific responsiveness of the VND epithelium to chemosensory stimuli although it has to be kept in mind that the effective sample size was small.

Perception of specific trigeminal chemosensory agonists

The intranasal trigeminal system is a third chemical sense in addition to olfaction and gustation. As opposed to smell and taste, we still lack knowledge on the relationship between receptor binding and perception for the trigeminal system. We therefore investigated the sensitivity of the intranasal trigeminal system towards agonists of the trigeminal receptors TRPM8 and TRPA1 by assessing subjects' ability to identify which nostril has been stimulated in a monorhinal stimulation design. We summed the number of correct identifications resulting in a lateralization score. Stimuli were menthol (activating TRPM8 receptors), eucalyptol (TRPM8), mustard oil (TRPA1) and two mixtures thereof (menthol/eucalyptol and menthol/mustard oil). In addition, we examined the relationship between intensity and lateralization scores and investigated whether intensity evaluation and lateralization scores of the mixtures show additive effects. All stimuli were correctly lateralized significantly above chance. Across subjects the lateralization scores for single compounds activating the same receptor showed a stronger correlation than stimuli activating different receptors. Although single compounds were isointense, the mixture of menthol and eucalyptol (activating only TRPM8) was perceived as weaker and was lateralized less accurately than the mixture of menthol and mustard oil (activating both TRPM8 and TRPA1) suggesting suppression effects in the former mixture. In conclusion, sensitivity of different subpopulations of trigeminal sensory neurons seems to be related, but only to a certain degree. The large coherence in sensitivity between various intranasal trigeminal stimuli suggests that measuring sensitivity to one single trigeminal chemical stimulus may be sufficient to generally assess the trigeminal system's chemosensitivity. Further, for stimuli activating the same receptor a mixture suppression effect appears to occur similar to that observed in the other chemosensory systems.

Localisation of unilateral nasal stimuli across sensory systems

Odor stimuli presented to one nostril can only be localised if they additionally activate the trigeminal nerve's chemosensitive fibers. In this study we aimed to investigate characteristics in the localisation of unilateral trigeminal, olfactory and somatosensory nasal stimuli. We compared the ability of healthy young subjects to localise monorhinally presented (a) pure olfactory stimuli (phenyl ethyl alcohol), (b) mixed olfactory trigeminal stimuli (eucalyptol), and (c) somatosensory stimuli (air puffs). As expected, subjects could localise the air puffs and eucalyptol, but could not phenyl ethyl alcohol. Interestingly, we observed a significant correlation between localisation performance for eucalyptol and phenyl ethyl alcohol but not between the ability to localise somatosensory and trigeminal or olfactory stimuli. These observations show that on a behavioural level, the trigeminal chemosensory system is more intimately connected to the olfactory system than to the somatosensory system despite the fact that anatomically its information is conveyed via same nerve as the latter. Furthermore, they show that the trigeminal chemosensory system should therefore be considered a self-confined contributor to chemosensory perception.

Current issues in the study of androstenes in human chemosignaling

We review research on the 16-androstenes and their special claim, born originally of the finding that androstenes function as boar pheromones, to be human chemosignals. Microbial fauna in human axillae act upon the 16-androstenes to produce odorous volatiles. Both individual variation and sex differences in perception of these odors suggest that they may play a role in mediating social behavior, and there is now much evidence that they modulate changes in interpersonal perception, and individual mood, behavior, and physiology. Many of these changes are sensitive to the context in which the compounds are experienced. However, many key outstanding questions remain. These include identification of the key active compounds, better quantification of naturally occurring concentrations and understanding how experimentally administered concentrations elicit realistic effects, and elucidation of individual differences (e.g., sex differences) in production rates. Until such issues are addressed, the question of whether the androstenes play a special role in human interactions will remain unresolved.

Genetics of taste and smell: poisons and pleasures

Eating is dangerous. While food contains nutrients and calories that animals need to produce heat and energy, it may also contain harmful parasites, bacteria, or chemicals. To guide food selection, the senses of taste and smell have evolved to alert us to the bitter taste of poisons and the sour taste and off-putting smell of spoiled foods. These sensory systems help people and animals to eat defensively, and they provide the brake that helps them avoid ingesting foods that are harmful. But choices about which foods to eat are motivated by more than avoiding the bad; they are also motivated by seeking the good, such as fat and sugar. However, just as not everyone is equally capable of sensing toxins in food, not everyone is equally enthusiastic about consuming high-fat, high-sugar foods. Genetic studies in humans and experimental animals strongly suggest that the liking of sugar and fat is influenced by genotype; likewise, the abilities to detect bitterness and the malodors of rotting food are highly variable among individuals. Understanding the exact genes and genetic differences that affect food intake may provide important clues in obesity treatment by allowing caregivers to tailor dietary recommendations to the chemosensory landscape of each person.

Self-ratings of olfactory function reflect odor annoyance rather than olfactory acuity

OBJECTIVE/HYPOTHESIS

Self-ratings of olfactory function often correlates poorly with results of objective smell tests. We explored these ratings relative to self-rating of odor annoyance, to odor identification ability, and to mean perceived intensity of odors, and estimated relative genetic and environmental contributions to these traits.

PARTICIPANTS AND METHODS

A total of 1,311 individual twins from the general population (62% females and 38% males, aged 10-83 years, mean age 29 years) including 191 monozygous and 343 dizygous complete twin pairs from Australia, Denmark, Finland, and the United Kingdom rated their sense of smell and annoyance caused by ambient smells (e.g., smells of foods) using seven categories, and performed odor identification and evaluation task for six scratch-and-sniff odor stimuli.

RESULTS

The self-rating of olfactory function correlated with the self-rating of odor annoyance (r = 0.30) but neither correlated with the odor identification score. Quantitative genetic modeling revealed no unambiguously significant genetic contribution to variation in any of the studied traits.

CONCLUSION

The results suggest that environmental rather than genetic factors modify the self-rating of olfactory function and support earlier findings of discrepancy between subjective and objective measures of olfactory function. In addition, the results imply that the self-rating of olfactory function arises from experienced odor annoyance rather than from actual olfactory acuity.

Interactions between the chemical senses: Trigeminal function in patients with olfactory loss

The intranasal trigeminal and the olfactory system are intimately connected. There is evidence showing that acquired olfactory loss leads to reduced trigeminal sensitivity due to the lack of a central-nervous interaction. Both, the orbitofrontal cortex and the rostral insula appear to be of significance in the amplification of trigeminal input which is missing in patients with olfactory loss. On peripheral levels, however, adaptive mechanisms seem to produce an increase in the trigeminal responsiveness of patients with hyposmia or anosmia.