Determinants for nasal trigeminal detection of volatile organic compounds

Quantifying Peripheral Modulation of Olfaction by Trigeminal Agonists

In the mammalian nose, two chemosensory systems, the trigeminal and the olfactory mediate the detection of volatile chemicals. Most odorants are able to activate the trigeminal system, and vice versa, most trigeminal agonists activate the olfactory system as well. Although these two systems constitute two separate sensory modalities, trigeminal activation modulates the neural representation of an odor. The mechanisms behind the modulation of olfactory response by trigeminal activation are still poorly understood. We addressed this question by looking at the olfactory epithelium (OE), where olfactory sensory neurons (OSNs) and trigeminal sensory fibers co-localize and where the olfactory signal is generated. Our study was conducted in a mouse model. Both sexes, males and females, were included. We characterize the trigeminal activation in response to five different odorants by measuring intracellular Ca2+ changes from primary cultures of trigeminal neurons (TGNs). We also measured responses from mice lacking TRPA1 and TRPV1 channels known to mediate some trigeminal responses. Next, we tested how trigeminal activation affects the olfactory response in the olfactory epithelium using electro-olfactogram (EOG) recordings from wild-type (WT) and TRPA1/V1-knock out (KO) mice. The trigeminal modulation of the olfactory response was determined by measuring responses to the odorant, 2-phenylethanol (PEA), an odorant with little trigeminal potency after stimulation with a trigeminal agonist. Trigeminal agonists induced a decrease in the EOG response to PEA, which depended on the level of TRPA1 and TRPV1 activation induced by the trigeminal agonist. This suggests that trigeminal activation can alter odorant responses even at the earliest stage of the olfactory sensory transduction.SIGNIFICANCE STATEMENT Most odorants reaching the olfactory epithelium (OE) can simultaneously activate olfactory and trigeminal systems. Although these two systems constitute two separate sensory modalities, trigeminal activation can alter odor perception. Here, we analyzed the trigeminal activity induced by different odorants proposing an objective quantification of their trigeminal potency independent from human perception. We show that trigeminal activation by odorants reduces the olfactory response in the olfactory epithelium and that such modulation correlates with the trigeminal potency of the trigeminal agonist. These results show that the trigeminal system impacts the olfactory response from its earliest stage.

Study protocol: randomised controlled trial of conditioned open-label placebo (COLP) for perioperative pain management in patients with head and neck cancer

INTRODUCTION

Patients with head and neck cancer have a substantial risk of chronic opioid dependence following surgery due to pain and psychosocial consequences from both the disease process and its treatments. Conditioned open-label placebos (COLPs) have been effective for reducing the dose of active medication required for a clinical response across a wide range of medical conditions. We hypothesise that the addition of COLPs to standard multimodal analgesia will be associated with reduced baseline opioid consumption by 5 days after surgery in comparison to standard multimodal analgesia alone in patients with head and neck cancer.

METHODS AND ANALYSIS

This randomised controlled trial will evaluate the use of COLP for adjunctive pain management in patients with head and neck cancer. Participants will be randomised with 1:1 allocation to either the treatment as usual or COLP group. All participants will receive standard multimodal analgesia, including opioids. The COLP group will additionally receive conditioning (ie, exposure to a clove oil scent) paired with active and placebo opioids for 5 days. Participants will complete surveys on pain, opioid consumption and depression symptoms through 6 months after surgery. Average change in baseline opioid consumption by postoperative day 5 and average pain levels and opioid consumption through 6 months will be compared between groups.

ETHICS AND DISSEMINATION

There remains a demand for more effective and safer strategies for postoperative pain management in patients with head and neck cancer as chronic opioid dependence has been associated with decreased survival in this patient population. Results from this study may lay the groundwork for further investigation of COLPs as a strategy for adjunctive pain management in patients with head and neck cancer. This clinical trial has been approved by the Johns Hopkins University Institutional Review Board (IRB00276225) and is registered on the National Institutes of Health Clinical Trials Database.

TRIAL REGISTRATION NUMBER

NCT04973748.

Quantifying peripheral modulation of olfaction by trigeminal agonists

In the mammalian nose, two chemosensory systems, the trigeminal and the olfactory mediate the detection of volatile chemicals. Most odorants in fact are able to activate the trigeminal system, and vice versa, most trigeminal agonists activate the olfactory system as well. Although these two systems constitute two separate sensory modalities, trigeminal activation modulates the neural representation of an odor. The mechanisms behind the modulation of olfactory response by trigeminal activation are still poorly understood. In this study, we addressed this question by looking at the olfactory epithelium, where olfactory sensory neurons and trigeminal sensory fibers co-localize and where the olfactory signal is generated. We characterize the trigeminal activation in response to five different odorants by measuring intracellular Ca2+ changes from primary cultures of trigeminal neurons (TGNs). We also measured responses from mice lacking TRPA1 and TRPV1 channels known to mediate some trigeminal responses. Next, we tested how trigeminal activation affects the olfactory response in the olfactory epithelium using electro-olfactogram (EOG) recordings from WT and TRPA1/V1-KO mice. The trigeminal modulation of the olfactory response was determined by measuring responses to the odorant, 2-phenylethanol (PEA), an odorant with little trigeminal potency after stimulation with a trigeminal agonist. Trigeminal agonists induced a decrease in the EOG response to PEA, which depended on the level of TRPA1 and TRPV1 activation induced by the trigeminal agonist. This suggests that trigeminal activation can alter odorant responses even at the earliest stage of the olfactory sensory transduction.

Chemosensory Perception: A Review on Electrophysiological Methods in “Cognitive Neuro-Olfactometry”

Various brain imaging techniques are available, but few are specifically designed to visualize chemical sensory and, in particular, olfactory processing. This review describes the results of quantitative and qualitative studies that have used electroencephalography (EEG) and magneto-encephalography (MEG) to evaluate responses to olfactory stimulation (OS). EEG and MEG are able to detect the components of chemosensory event-related potentials (CSERPs) and the cortical rhythms associated with different types of OS. Olfactory studies are filling the gaps in both the developmental field of the life cycle (from newborns to geriatric age) and the clinical and basic research fields, in a way that can be considered the modern “cognitive neuro-olfactometry”.

AutonoMouse: High throughput operant conditioning reveals progressive impairment with graded olfactory bulb lesions

Operant conditioning is a crucial tool in neuroscience research for probing brain function. While molecular, anatomical and even physiological techniques have seen radical increases in throughput, efficiency, and reproducibility in recent years, behavioural tools have somewhat lagged behind. Here we present a fully automated, high-throughput system for self-initiated conditioning of up to 25 group-housed, radio-frequency identification (RFID) tagged mice over periods of several months and >106 trials. We validate this "AutonoMouse" system in a series of olfactory behavioural tasks and show that acquired data is comparable to previous semi-manual approaches. Furthermore, we use AutonoMouse to systematically probe the impact of graded olfactory bulb lesions on olfactory behaviour, demonstrating that while odour discrimination in general is robust to even most extensive disruptions, small olfactory bulb lesions already impair odour detection. Discrimination learning of similar mixtures as well as learning speed are in turn reliably impacted by medium lesion sizes. The modular nature and open-source design of AutonoMouse should allow for similar robust and systematic assessments across neuroscience research areas.

Somatosensory Response to Trigeminal Stimulation: A Functional Near-Infrared Spectroscopy (fNIRS) Study

Functional near-infrared spectroscopy (fNIRS) is an optical imaging technique measuring relative hemodynamic changes in superficial cortical structures. It has successfully been applied to detect a hemodynamic response in the somatosensory cortex evoked by irritating mechanical, electrical, and heat stimulations of limbs or the face. The aim of the current study was to explore the feasibility of fNIRS to detect respective responses evoked by irritating chemical stimulations of the nasal divisions of the trigeminal nerve. In two experiments, healthy subjects were exposed to acetic acid and ethyl acetate presented using a respiration-synchronized olfactometer. Results demonstrated that fNIRS can detect a signal in both hemispheres after birhinal (experiment 1: n = 14) and monorhinal (experiment 2: n = 12) stimulations using acetic acid but not ethyl acetate. This is a first evidence that fNIRS might be a suitable imaging technique to assess chemosensory neuronal correlates in the somatosensory cortex thereby offering a new, portable method to evaluate the irritating properties of certain volatiles in an objective, nonverbal, easy, and comparably inexpensive manner.

An assessment of air quality reflecting the chemosensory irritation impact of mixtures of volatile organic compounds

We present a method to assess the air quality of an environment based on the chemosensory irritation impact of mixtures of volatile organic compounds (VOCs) present in such environment. We begin by approximating the sigmoid function that characterizes psychometric plots of probability of irritation detection (Q) versus VOC vapor concentration to a linear function. First, we apply an established equation that correlates and predicts human sensory irritation thresholds (SIT) (i.e., nasal and eye irritation) based on the transfer of the VOC from the gas phase to biophases, e.g., nasal mucus and tear film. Second, we expand the equation to include other biological data (e.g., odor detection thresholds) and to include further VOCs that act mainly by "specific" effects rather than by transfer (i.e., "physical") effects as defined in the article. Then we show that, for 72 VOCs in common, Q values based on our calculated SITs are consistent with the Threshold Limit Values (TLVs) listed for those same VOCs on the basis of sensory irritation by the American Conference of Governmental Industrial Hygienists (ACGIH). Third, we set two equations to calculate the probability (Qmix) that a given air sample containing a number of VOCs could elicit chemosensory irritation: one equation based on response addition (Qmix scale: 0.00 to 1.00) and the other based on dose addition (1000*Qmix scale: 0 to 2000). We further validate the applicability of our air quality assessment method by showing that both Qmix scales provide values consistent with the expected sensory irritation burden from VOC mixtures present in a wide variety of indoor and outdoor environments as reported on field studies in the literature. These scales take into account both the concentration of VOCs at a particular site and the propensity of the VOCs to evoke sensory irritation.

Compilation and analysis of types and concentrations of airborne chemicals measured in various indoor and outdoor human environments

The main purpose of this article is to summarize and illustrate the results of a literature search on the types, levels, relative concentrations, concentration spread of individual chemicals, and number of airborne compounds (mostly volatile organic compounds, VOCs) that have been found, measured, and reported both indoors and outdoors. Two broad categories of indoor environments are considered: (1) home/school, and (2) commercial spaces. Also, two categories of outdoor environments are considered: (1) non-industrial and (2) industrial (the latter represented by the vicinity of a pig farm and the vicinity of an oil refinery). The outcome is presented as a series of graphs and tables containing the following statistics: geometric mean, arithmetic mean, median, standard deviation, variance, standard error, interquartile distance, minimum value, maximum value, and number of data (data count) for the air concentration of each reported compound in a given environment. A Supplementary Table allows interested readers to match each single value included in this compilation with its corresponding original reference.

Transient receptor potential channels encode volatile chemicals sensed by rat trigeminal ganglion neurons

Primary sensory afferents of the dorsal root and trigeminal ganglia constantly transmit sensory information depicting the individual’s physical and chemical environment to higher brain regions. Beyond the typical trigeminal stimuli (e.g. irritants), environmental stimuli comprise a plethora of volatile chemicals with olfactory components (odorants). In spite of a complete loss of their sense of smell, anosmic patients may retain the ability to roughly discriminate between different volatile compounds. While the detailed mechanisms remain elusive, sensory structures belonging to the trigeminal system seem to be responsible for this phenomenon. In order to gain a better understanding of the mechanisms underlying the activation of the trigeminal system by volatile chemicals, we investigated odorant-induced membrane potential changes in cultured rat trigeminal neurons induced by the odorants vanillin, heliotropyl acetone, helional, and geraniol. We observed the dose-dependent depolarization of trigeminal neurons upon application of these substances occurring in a stimulus-specific manner and could show that distinct neuronal populations respond to different odorants. Using specific antagonists, we found evidence that TRPA1, TRPM8, and/or TRPV1 contribute to the activation. In order to further test this hypothesis, we used recombinantly expressed rat and human variants of these channels to investigate whether they are indeed activated by the odorants tested. We additionally found that the odorants dose-dependently inhibit two-pore potassium channels TASK1 and TASK3 heterologously expressed In Xenopus laevis oocytes. We suggest that the capability of various odorants to activate different TRP channels and to inhibit potassium channels causes neuronal depolarization and activation of distinct subpopulations of trigeminal sensory neurons, forming the basis for a specific representation of volatile chemicals in the trigeminal ganglia.

In vivo monitoring of chemically evoked activity patterns in the rat trigeminal ganglion

Albeit lacking a sense of smell, anosmic patients maintain a reduced ability to distinguish different volatile chemicals by relying exclusively on their trigeminal system (TS). To elucidate differences in the neuronal representation of these volatile substances in the TS, we performed voltage-sensitive dye imaging (VSDI) in the rat trigeminal ganglion (TG) in vivo. We demonstrated that stimulus-specific patterns of bioelectrical activity occur within the TG upon nasal administration of ten different volatile chemicals. With regard to spatial differences between the evoked trigeminal response patterns, these substances could be sorted into three groups. Signal intensity and onset latencies were also dependent on the administered stimulus and its concentration. We conclude that particular compounds detected by the TS are represented by (1) a specific spatial response pattern, (2) the signal intensity, and (3) onset latencies within the pattern. Jointly, these trigeminal representations may contribute to the surprisingly high discriminative skills of anosmic patients.

Predicting odor perceptual similarity from odor structure

To understand the brain mechanisms of olfaction we must understand the rules that govern the link between odorant structure and odorant perception. Natural odors are in fact mixtures made of many molecules, and there is currently no method to look at the molecular structure of such odorant-mixtures and predict their smell. In three separate experiments, we asked 139 subjects to rate the pairwise perceptual similarity of 64 odorant-mixtures ranging in size from 4 to 43 mono-molecular components. We then tested alternative models to link odorant-mixture structure to odorant-mixture perceptual similarity. Whereas a model that considered each mono-molecular component of a mixture separately provided a poor prediction of mixture similarity, a model that represented the mixture as a single structural vector provided consistent correlations between predicted and actual perceptual similarity (r≥0.49, p<0.001). An optimized version of this model yielded a correlation of r = 0.85 (p<0.001) between predicted and actual mixture similarity. In other words, we developed an algorithm that can look at the molecular structure of two novel odorant-mixtures, and predict their ensuing perceptual similarity. That this goal was attained using a model that considers the mixtures as a single vector is consistent with a synthetic rather than analytical brain processing mechanism in olfaction.

Infants’ hedonic responsiveness to food odours: a longitudinal study during and after weaning (8, 12 and 22 months)

Background

Olfaction is a highly salient sensory modality in early human life. Neonates show keen olfactory sensitivity and hedonic responsiveness. However, little is known about hedonic olfactory responsiveness between the neonatal period and 2 years of age. In an attempt to fill this gap, this longitudinal follow-up study aimed at investigating hedonic responses to food odours in infants during the first 2 years of life. The second objective was to evaluate whether gender has an influence on hedonic responses during this early period. Four control stimuli and eight odours (four rated by adults as a priori pleasant and four a priori unpleasant) were presented in bottles to 235 infants at 8, 12 and 22 months of age. The infant’s exploratory behaviour towards odorized and control bottles was measured in terms of mouthing defined as direct contact with perioral and/or perinasal areas. For each odorized bottle, duration proportions of mouthing were calculated relative to the control bottles.

Results

For the three ages, shorter duration of mouthing was found for unpleasantly scented bottles compared to pleasantly scented bottles. This contrast between pleasant and unpleasant odours was similar for girls and boys. Correlations of responses between ages were modest in number and level, and concerned mostly unpleasant odours.

Conclusion

During the first two years of life, infants discriminate the hedonic valence of odours. They avoid most of the food odours considered as unpleasant by adults, but their attraction towards food-odours judged pleasant by adults does not appear to be fully shaped at this early age. Taken as a whole, the present results highlight both the plasticity of hedonic responses to food odours, and relatively stable avoidance behaviours towards some unpleasant odours.

Dissociated representations of pleasant and unpleasant olfacto-trigeminal mixtures: an FMRI study

How the pleasantness of chemosensory stimuli such as odorants or intranasal trigeminal compounds is processed in the human brain has been the focus of considerable recent interest. Yet, so far, only the unimodal form of this hedonic processing has been explored, and not its bimodal form during crossmodal integration of olfactory and trigeminal stimuli. The main purpose of the present study was to investigate this question. To this end, functional magnetic resonance imaging (fMRI) was used in an experiment comparing brain activation related to a pleasant and a relatively unpleasant olfacto-trigeminal mixture, and to their individual components (CO(2) alone, Orange alone, Rose alone). Results revealed first common neural activity patterns in response to both mixtures in a number of regions: notably the superior temporal gyrus and the caudate nucleus. Common activations were also observed in the insula, although the pleasant mixture activated the right insula whereas the unpleasant mixture activated the left insula. However, specific activations were observed in anterior cingulate gyrus and the ventral tegmental area only during the perception of the pleasant mixture. These findings emphasized for the firs time the involvement of the latter structures in processing of pleasantness during crossmodal integration of chemosensory stimuli.

Oral cavity discrimination of vapor-phase long-chain 18-carbon fatty acids

Linoleic, oleic, and stearic fatty acids, presented vapor-phase retronasally, were discriminable from blanks and each other, but the same concentrations, oral-cavity-only (OCO), were not discriminable from blanks. It remained possible that higher concentrations might be discriminable OCO. To evaluate this, participants attempted to discriminate undiluted linoleic, oleic, or stearic acids, vapor-phase OCO, from blanks. For each fatty acid, participants received 5 stimulus delivery containers (SDCs) in 2 trials; 4 SDC held blanks, the fifth, a fatty acid. As a "positive control" in 2 trials, participants received vapor-phase OCO peppermint extract and blanks. For all trials, the task was to select the 1 different SDC. It was found that the 1 different SDC was selected in 24% of stearic, 32% of linoleic, 47% of oleic acid, and in 92% of peppermint trials; discriminations (the 1 different SDC selected in both trials) occurred in 0%, 16%, 26%, and 84% of pairs, respectively. Correct selections for oleic acid differed from chance, P = 0.0004, but not for linoleic acid, P = 0.125, or stearic acid, P = 0.345, Bonferroni corrected. Vapor-phase oleic acid can be an oral cavity trigeminal stimulus, linoleic acid might be (uncorrected P = 0.0384), but vapor-phase stearic acid cannot be.

An algorithm for 353 odor detection thresholds in humans

One hundred and ninety three odor detection thresholds, ODTs, obtained by Nagata using the Japanese triangular bag method can be correlated as log (1/ODT) by a linear equation with R(2) = 0.748 and a standard deviation, SD, of 0.830 log units; the latter may be compared with our estimate of 0.66 log units for the self-consistency of Nagata's data. Aldehydes, acids, unsaturated esters, and mercaptans were included in the equation through indicator variables that took into account the higher potency of these compounds. The ODTs obtained by Cometto-Muñiz and Cain, by Cometto-Muñiz and Abraham, and by Hellman and Small could be put on the same scale as those of Nagata to yield a linear equation for 353 ODTs with R(2) = 0.759 and SD = 0.819 log units. The compound descriptors are available for several thousand compounds, and can be calculated from structure, so that further ODT values on the Nagata scale can be predicted for a host of volatile or semivolatile compounds.

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.

The airway sensory hyperreactivity syndrome

After exclusion of diverse pulmonary illnesses, the remaining explanations for chronic cough include medication with angiotensin-converting enzyme (ACE) inhibitor, gastroesophageal reflux disease (GERD), and post-nasal drip. Different clinics report shifting frequencies for both the causes of chronic cough and the success of treatment. However, after all evaluations, differential diagnosis still leaves a group of patients with unexplained cough. This unexplained cough is also known as chronic idiopathic cough (CIC), though there are widely varying opinions as to its existence. Among patients previously diagnosed with CIC, a subgroup has been identified with both upper and lower airway symptoms, including cough induced by odours and chemicals, and with increased cough sensitivity to inhaled capsaicin, which is known to stimulate the airway sensory nerves. A suggested explanation for this condition is a hyperreactivity of the sensory nerves of the entire airways, and hence the condition is known as sensory hyperreactivity (SHR). SHR affects more than 6% of the adult population in Sweden. It is a longstanding condition, and is clearly associated with significant social and psychological impacts.

Odor detection by humans of lineal aliphatic aldehydes and helional as gauged by dose-response functions

We have measured concentration detection (i.e., psychometric) functions to determine the odor detectability of homologous aliphatic aldehydes (propanal, butanal, hexanal, octanal, and nonanal) and helional. Subjects (16 < or = n < or = 18) used a 3-alternative forced-choice procedure against carbon-filtered air (blanks), under an ascending concentration approach. Generation, delivery, and control of each vapor were achieved via an 8-station vapor delivery device. Gas chromatography served to quantify the concentrations presented. Group and individual functions were modeled by a sigmoid (logistic) equation. Odor detection thresholds (ODTs) were defined as the concentration producing a detectability (P) halfway (P = 0.5) between chance (P = 0.0) and perfect detection (P = 1.0). ODTs decreased with carbon chain length: 2.0, 0.46, 0.33, and 0.17 ppb, respectively, from propanal to octanal, but the threshold increased for nonanal (0.53 ppb), revealing maximum sensitivity for the 8-carbon member. The strong olfactory receptor (OR) ligands octanal and helional (0.14 ppb) showed the lowest thresholds. ODTs fell at the lower end of previously reported values. Interindividual variability (ODT ratios) amounted to a factor ranging from 10 to 50, lower than typically reported, and was highest for octanal and hexanal. The behavioral dose-response functions emerge at concentrations 2-5 orders of magnitude lower than those required for functions tracing the activation of specific human ORs by the same aldehydes in cell/molecular studies, after all functions were expressed as vapor concentrations.

The biological and toxicological activity of gases and vapors

A large amount of data on the biological and toxicological activity of gases and vapors has been collected from the literature. Processes include sensory irritation thresholds, the Alarie mouse test, inhalation anesthesia, etc. It is shown that a single equation using only five descriptors (properties of the gases and vapors) plus a set of indicator variables for the given processes can correlate 643 biological and non-lethal toxicological activities of 'non-reactive' compounds with a standard deviation of 0.36 log unit. The equation is scaled to sensory irritation thresholds obtained by the procedure of Cometto-Muñiz, and Cain provides a general equation for the prediction of sensory irritation thresholds in man. It is suggested that differences in biological/toxicological activity arise primarily from transport from the gas phase to a receptor phase or area, except for odor detection thresholds where interaction with a receptor(s) is important.

Effects on perceived air quality and symptoms of exposure to microbially produced metabolites and compounds emitted from damp building materials

This work investigated perceived air quality and health effects from exposure to low to high levels of volatile organic compounds (VOCs) emitted from damp building materials and a mixture of molds growing on the materials. A mixture of Wallemia sebi, Fusarium culmorum, Penicillium chrysogenum, Ulocladium botrytis, and Aspergillus versicolor was inoculated on pine wood and particle board. In Study 1, each of 27 participants took part in two exposure conditions, one with air from molds growing on building materials (low levels of emissions from the building materials and the mold mixture) and one with blank air, both conditions during 60 min. In Study 2, each of 24 participants was exposed (10 min) four times in a 2 x 2 design randomly to air from moldy building materials (high levels) and blank, with and without nose-clip. The participants rated air quality and symptoms before, during, and after each exposure. Self-reported tear-film break-up time and attention and processing speed (Study 1) was also measured. Exposure to high VOC levels increased the reports of perceived poor air quality, and in the condition without nose-clip enhanced skin symptoms were also noted. No such outcome was observed when exposing the participants to low VOC levels.

PRACTICAL IMPLICATIONS

Emissions from building materials caused by dampness and microbial growth may be involved in indoor air health problems. This study showed that exposure to high levels of VOC emitted from damp building materials and a mixture of mold may cause perceived poor air quality. It also indicated that stimulation of chemical warning systems (the nasal chemosensory part of the trigeminal system and the olfactory system) may enhance skin symptoms.

Olfactory detectability of homologous n-alkylbenzenes as reflected by concentration-detection functions in humans

As part of our systematic exploration of chemical determinants for the olfactory potency of vapors towards humans, we measured concentration-detection functions for the odor of the homologous n-alkylbenzenes toluene, ethylbenzene, butylbenzene, hexylbenzene, and octylbenzene. A vapor delivery device based on dynamic olfactometry and calibrated by gas chromatography, served to test groups of 16 to 17 participants. Subjects were young adults from both genders, normosmics, and nonsmokers. Odor functions were tightly modeled by a sigmoid (logistic) function, both at the group and the individual level. Odor detection thresholds (ODTs), defined as the concentration producing a detectability halfway between chance and perfect detection, decreased with alkyl chain length from toluene (79 ppb) to butylbenzene (2.5 ppb), and then increased form butyl to octylbenzene (89 ppb). The "U"-shaped trend of ODTs as a function of alkyl chain length indicated a loss of odor potency beyond a certain molecular size, a phenomenon recently described for chemosensory irritation (chemesthesis) and that will need consideration in structure-activity models of chemosensory potency. Interindividual ODTs' variability for any single odorant amounted to one order of magnitude, in agreement with recent studies of other homologous series but quite smaller than commonly depicted.

Mechanisms of increased airway sensitivity to occupational chemicals and odors

PURPOSE OF REVIEW

Airway symptoms induced by chemicals and odors are common problems that are also reported after contact with substances normally regarded as nontoxic. This article reviews current findings and opinions regarding mechanisms of increased airway sensitivity to occupational chemicals and odors.

RECENT FINDINGS

Individuals exposed to organic solvents during work had more nasal irritation and lower threshold to pyridine odor compared with a nonexposed control group. Six percentage of a general population in Sweden had pronounced airway chemical sensitivity and augmented capsaicin cough sensitivity, known to reflect the sensory nerve reactivity of the airways. The cough sensitivity was associated with changed levels of nerve growth factor in nasal lavage and such patients had longstanding symptoms influencing their working capacity. Positron emission tomography activation studies with several different odorants showed in patients with odor-associated symptoms an odorant-related increase in activation of the anterior cingulate cortex and cuneus-precuneus in comparison with a control group.

SUMMARY

In subgroups of individuals with airway symptoms induced by chemicals and odors, there seems to be a sustainable physiological mechanism behind the reactions. An increased vulnerability to stress cannot be neglected as a confounding factor in some sensitive individuals.

No oral-cavity-only discrimination of purely olfactory odorants

The purely olfactory odorants coumarin, octanoic acid, phenylethyl alcohol, and vanillin had been found to be consistently identified when presented retronasally but could not be identified when presented oral-cavity only (OCO). However, OCO discrimination of these odorants was not tested. Consequently, it remained possible that the oral cavity trigeminal system might provide sufficient information to differentiate these purely olfactory odorants. To evaluate this, 20 participants attempted to discriminate vapor-phase coumarin, octanoic acid, phenylethyl alcohol, and vanillin and, as a control, the trigeminal stimulus peppermint extract, from their glycerin solvent, all presented OCO. None of the purely olfactory odorants could be discriminated OCO, but, as expected, peppermint extract was consistently discriminated. This inability to discriminate clarifies and expands the previous report of lack of OCO identification of purely olfactory odorants. Taken together with prior data, these results suggest that the oral cavity trigeminal system is fully unresponsive to these odorants in vapor phase and that coumarin, octanoic acid, phenylethyl alcohol, and vanillin are indeed purely olfactory stimuli. The OCO discrimination of peppermint extract demonstrated that the absence of discrimination for the purely olfactory odorants was odorant dependent and confirmed that the oral cavity trigeminal system will provide differential response information to some vapor-phase stimuli.

Improved smell pleasantness after odor-taste associative learning in humans

Whereas some aspects of olfactory hedonism in humans are present from birth, others form during development and throughout adulthood. Although it is generally agreed that such hedonic representations emerge by associative learning, it is not yet clear which learning parameters are prominent. The present study investigated the influence of number of trials on odor preference acquisition in human adults. Forty-eight subjects randomly assigned to three groups were tested in three sessions. In the first session, subjects ranked eight food odors from most pleasant to most unpleasant. The second session consisted in an associative learning, the two most neutral odors were randomly associated with a drink: one odor with water (CS-) and the other odor with a pleasant sweet solution (CS+). In the third session subjects ranked the eight food odors again. In group A, CS+ was paired three times with the US, and in group B only once; in group C, CS+ was paired only once with the US but with a total duration identical to that in group A. Results showed that CS+ was ranked as significantly more pleasant after learning in group A (p<.05), but not in groups B and C (p>.05). In conclusion, the study showed that a neutral smell can acquire positive emotional features after being paired with a pleasant taste, and that this learning depends on the number of associations between smell and taste.

An oral-cavity component in retronasal smelling of natural extracts

Retronasal and oral-cavity-only identifications of six natural extract odorants, presented in air-phase, were compared in an initial experiment. Prior to identification testing, the 21 participants were given experience with air-phase presentations, and with the odorants and their correct identifications. Retronasal correct identifications for anise, cinnamon, coffee, orange, peppermint, and strawberry were 88%, 81%, 98%, 95%, 91%, and 83%; oral-cavity-only, 19%, 21%, 19%, 21%, 33%, and 24%. All participants correctly identified retronasal odorants above chance. Across participants only peppermint received correct oral-cavity-only identifications, but two participants gave correct oral-cavity-only identifications for all odorants. In a second experiment, different participants attempted to discriminate oral-cavity-only odorants from their solvents. Fifteen participants discriminated orange, peppermint, and strawberry odorants from their solvents, and five discriminated all odorants from their solvents. It had been hypothesized that peppermint would provide unique trigeminal stimulation; this was supported by correct oral-cavity-only identification of only peppermint. A second hypothesis posited oral-cavity-only discrimination of orange and peppermint from their solvents because of trigeminal stimuli, but strawberry extract discrimination was unexpected. Furthermore, oral-cavity-only discrimination of all odorants by one-quarter of the participants was not anticipated. Overall, these outcomes suggest that peppermint-like odorants can initiate sufficiently differential responses in the oral cavity to permit identification, indicate that not only odorants with known trigeminal stimulus components but also others may elicit oral-cavity-only air-phase responses, and imply that for a substantial minority of individuals, trigeminal input may enhance oral-cavity effectiveness of many odorants during retronasal smelling.

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.

Trigeminally-mediated health effects of air pollutants: sources of inter-individual variability

Trigeminal (ocular and nasal) irritation comprises the dominant symptom complex in so-called "problem buildings". Imputed etiologic agents in indoor air include extremes of temperature and humidity, the presence of volatile organic compounds, combustion products (including tobacco smoke), ozone (from office machines), and products of indoor air chemistry. In addition to producing primary irritation, mucosal irritants trigger a variety of secondary reflex symptoms, such as nasal congestion, rhinorrhea, and sinus pressure, and may predispose to infection in the form of sinusitis and otitis media. Marked variability in self-reported sensitivity to indoor air pollutants has been observed, with females, younger individuals, and people with allergies reporting more symptoms. We report on a series of experiments designed to uncover demographic patterns of "nasal irritant sensitivity", as well as potential mechanism(s) involved in observed chemesthetic variability.

Concentration-detection functions for eye irritation evoked by homologous n-alcohols and acetates approaching a cut-off point

We measured the concentration-detection (i.e., psychometric) functions for the eye irritation evoked by three homologous n-alcohols (1-nonanol, 1-decanol and 1-undecanol) and two homologous acetates (nonyl and decyl acetate). A vapor delivery device based on a dynamic dilution of stimuli in nitrogen served to present various concentrations of each compound, including the undiluted vapor, to the subjects (n >or= 26). Delivered concentrations were quantified by gas chromatography. Detection probability (P) was assessed via a three-alternative, forced-choice procedure and quantified on a scale ranging from P = 0.0 (chance detection) to P = 1.0 (perfect detection). Flow rate to the eye equaled 2.5 l/min and time of exposure was 6 s. The functions for 1-undecanol and decyl acetate plateaued at P approximately 0.5 and P approximately 0.25, respectively, such that further increases in concentration failed to increase detection notably. Thus, both series reached a break point, or cut-off, in the detection of ocular irritation. The present outcome provides additional evidence that the cut-off does not rest on the low vapor concentration of the homolog but, more likely, on the homolog exceeding a critical molecular dimension(s), which prevents it from interacting effectively with the appropriate receptors.

Cutoff in detection of eye irritation from vapors of homologous carboxylic acids and aliphatic aldehydes

Using neat vapors of selected homologous aldehydes (decanal, undecanal, dodecanal) and carboxylic acids (pentanoic, hexanoic, heptanoic, octanoic, nonanoic), we explored the point where a certain homolog (and all larger ones) becomes undetectable by eye irritation (i.e. by ocular chemesthesis). This phenomenon has been observed in other homologous series that also reach a break-point, or cutoff, in chemesthetic detection. Participants (11<or=n<or=32) were tested using a three-alternative, forced-choice procedure. Flow rate to the eye equaled 4 or 8 l/min and time of exposure was 6 s. The outcome showed that dodecanal and heptanoic acid were the shortest undetectable homologs. When the vapor concentration of the stimuli was increased by heating the liquid source to 37 degrees C, homologs located before the cutoff point (e.g. hexanoic acid) became readily detected by all subjects, whereas homologs located at the cutoff remained largely undetected. In addition, a comparison of calculated values of eye irritation thresholds for aldehydes and acids (from a successful model of ocular chemesthetic potency) with values of saturated vapor concentration at 23 and 37 degrees C indicated that the vapor concentration of dodecanal and heptanoic acid should have been enough to produce detection. The outcome suggests that the cutoff observed does not result from a low vapor concentration but from limitations in the structure or dimension(s) of the molecules that render them unsuitable to interact effectively with human chemesthetic receptors.

The neural representation of odor is modulated by the presence of a trigeminal stimulus during odor encoding

OBJECTIVES

Odor perception does not simply consist in hierarchical processing from transduction to a single "true" cerebral representation. Odor sensation may be modulated by available sensory information during encoding. The present study set out to examine whether the presence of a pure trigeminal stimulus during odor encoding may modulate odor perception at both behavioral and cortical levels.

METHODS

Participants were tested in a 2-session within-subject design: first, an odor encoding session included a delay conditioning procedure in which relatively selective olfactory stimulants (phenyl ethyl alcohol or vanillin, Conditioned Stimulus+, CS+) were presented either with a pulse of CO(2) (Unconditioned Stimulus, US), or alone (Conditioned Stimulus-, CS-); then, in the second session, both pure odorants (CS+ and CS-) were presented alone. During this second session, olfactory event-related potentials were simultaneously recorded and analyzed at different electrode sites including Cz and Pz (sites known to have maximal amplitudes for trigeminal and olfactory stimuli, respectively). After each trial, subjects were asked to rate odor intensity and hedonics.

RESULTS

The results showed that CS+ intensity ratings increased in 8 subjects and decreased in 6. Cortically, a group effect was observed for P2 amplitude, which increased in the "CS+ intensity increase" group vs. the "CS+ intensity decrease" group at Cz (p<0.05) but not at Pz (p>0.05).

CONCLUSIONS

This result suggests that the presence of a pure trigeminal stimulus (CO(2)) during odor encoding alters the neural representation of a pure odor.

SIGNIFICANCE

The neural representation of odors comprises not only the odor itself but also contextual information (trigeminal in the present case) presented during encoding.

Intranasal trigeminal function in subjects with and without an intact sense of smell

The intranasal trigeminal system is involved in the perception of odors. To investigate the cerebral processing of sensory information from the trigeminal nerve in detail we studied subjects with and without olfactory function using functional magnetic resonance imaging. A normosmic group (n=12) was compared with a group of anosmic subjects (n=11). For trigeminal stimulation gaseous CO(2) was used. Following right-sided stimulation with CO(2) controls exhibited a stronger right-sided cerebral activation than anosmic subjects. Stronger activation was found in controls compared to anosmic subjects for the right prefrontal cortex, the right somatosensory cortex (SI), and the left parietal insula. In contrast, relatively higher activation was found in anosmic subjects for the left supplementary motor area in the frontal lobe, the right superior and middle temporal lobe, the left parahippocampal gyrus in the limbic lobe, and the sub-lobar region of the left putamen and right insula which was mostly due to a decreased BOLD signal of controls in these areas. Additional conjunction analysis revealed that activated areas common to the two groups were the cerebellum and the right premotor frontal cortex. These data suggest that the processing of the trigeminally mediated information is different in the presence or absence of an intact sense of smell, pointing towards the intimate connection between the two chemosensory systems.

Does Haber's law apply to human sensory irritation?

Irritation of the eyes, nose, and throat by airborne chemicals--also referred to as "sensory irritation"--is an important endpoint in both occupational and environmental toxicology. Modeling of human sensory irritation relies on knowledge of the physical chemistry of the compound(s) involved, as well as the exposure parameters (concentration and duration). A reciprocal relationship between these two exposure variables is postulated under Haber's law, implying that protracted, low-level exposures may be toxicologically equivalent to brief, high-level exposures. Although time is recognized as having an influence on sensory irritation, the quantitative predictions of Haber's Law have been addressed for only a handful of compounds in human experimental studies. We have conducted a systematic literature review that includes a semiquantitative comparison of psychophysical data extracted from controlled human exposure studies versus. the predictions of Haber's law. Studies containing relevant data involved exposures to ammonia (2), chlorine (2), formaldehyde (1), inorganic dusts such as calcium oxide (1), and the volatile organic compound 1-octene (1). With the exception of dust exposure, varying exposure concentration has a proportionally greater effect on sensory irritation than does changing exposure duration. For selected time windows, a more generalized power law model (c(n) x t = k) rather than Haber's law per se (c x t = k) yields reasonably robust predictions. Complicating this picture, however, is the frequent observation of intensity-time "plateauing," with time effects disappearing, or even reversing, after a relatively short period, depending on the test compound. The implications of these complex temporal dynamics for risk assessment and standard setting have been incompletely explored to date.

Chemical Boundaries for Detection of Eye Irritation in Humans from Homologous Vapors

In a series of experiments, we looked at a "cutoff" effect for the detection of eye irritation from neat vapors of homologous n-alkylbenzenes and 2-ketones. Stimuli comprised pentyl, hexyl, and heptyl benzene, 2-dodecanone, and 2-tridecanone, presented to each eye at 4 and 8 l/min for 6 sec, using a three-alternative forced-choice procedure against blanks. Detection probability corrected for chance (i.e., detectability) decreased with carbon chain length such that heptyl benzene and 2-tridecanone were virtually undetectable, irrespective of flow rate to the eye. Heating both stimuli sources to 37 degrees C (body temperature) from 23 degrees C (room temperature) increased vapor concentration by 5.0 and 6.9 times, respectively, for heptyl benzene and 2-tridecanone. Still, both chemicals failed to show increased detection for 13 of the 21 participants. In addition, plots of experimentally measured and calculated eye irritation thresholds as a function of carbon chain length for each series indicated that, based on the trend, the concentration of the two cutoff homologs at 37 degrees C should have been high enough to allow detection. Taken together, the results suggest that these cutoffs rest on limitations related to the dimension of the molecules rather than on limitations related to their vapor concentration. For example, the stimulus molecule could exceed the size that allows it to fit into the receptor pocket of a receptive protein. Plots of calculated molecular dimensions across homologous alkylbenzenes, from ethyl to dodecylbenzene, and across 2-ketones, from 2-octanone to 2-octadecanone, provided additional support to the above conclusion.

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

Specific anosmia is a term that describes an inability to perceive a particular odorant in the context of an otherwise normal olfactory acuity. The most common example, for the odor of androstenone, has been ascribed a prevalence ranging from 2 to 45%. In two experiments we sought to determine whether this wide range could be explained by the difference in steroid concentrations used, and by the degree to which the trigeminal system contributes to perception of androstenone. Experiment 1 demonstrated that high concentrations of androstenone stimulated the trigeminal system, as indicated by electrophysiological recordings. Experiment 2 demonstrated that conscious detection of androstenone is possible based solely on the trigeminal system. Interestingly, detection seems to interact with olfactory acuity in that subjects with a low olfactory sensitivity to androstenone were better able to detect its trigeminal component. The agreement between conscious experience and behavioral discrimination was not well calibrated, in that subjects demonstrated a clear overconfidence in their abilities. Altogether, the current study suggests that androstenone is an odorant that produces a concentration-dependent degree of trigeminal stimulation. This trigeminal component explains the diversity of the reported prevalence of specific anosmia for androstenone and might have implications on future use of specific anosmia as a tool to understand odor processing.