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At present, no satisfactory theory exists to explain how a given molecule results in the perception of a particular smell. One theory is that olfactory sensory neurons detect intramolecular vibrations of the odorous molecule. We used psychophysical methods in humans to test this vibration theory of olfaction and found no evidence to support it.
Induction of olfactory sensitivity in humans was first illustrated when men and women who were initially unable to smell the volatile steroid androstenone (5alpha-androst-16-en-3-one) developed that ability after repeated, brief exposures. Because this finding has not been replicated with other compounds in humans, it has been assumed that olfactory induction is a narrowly constrained phenomenon, occurring only in individuals with specific anosmias, perhaps only to androstenone (compare ref. 2). Here we show that induction of enhanced olfactory sensitivity seems to be a more general phenomenon, with marked changes in olfactory acuity occurring during repeated test exposures to several odorants among people with average baseline sensitivity to these compounds. This increased sensitivity (averaging five orders of magnitude) was observed only among females of reproductive age. These observations provide convincing evidence that female olfactory acuity to a variety of odorants can vastly improve with repeated test exposures. They also suggest a sensory basis for the anecdotal observation of greater olfactory sensitivities among females and raise the possibility that the olfactory-induction process may be associated with female reproductive behaviors such as pair bonding and kin recognition.
Research successes over the past decade have provided a broad outline of the neuroscience of olfaction and taste. Our understanding of these systems now spans the molecular to the psychological. It will soon reach critical mass and begin to generate a variety of practical applications with commercial potential. Given the ubiquity of smell and taste and their importance to health, nutrition and quality of life, these applications could have a major impact on consumer product markets and create entirely new ones. Sensory biotechnology could be the first post-genomic application to break through to the consumer market. We describe odor modulation technologies with implications for food intake, health care and other arenas. Our deeper understanding of olfaction and taste in animal behavior and reproduction provides opportunities in pest control and animal husbandry, where environmentally neutral interventions are much in demand.
OBJECTIVE: How much do the sensory properties of food influence the way people select their food and how much they eat? The objective of this paper is to review results from studies investigating the link between the sensory perception of food and human appetite regulation. CONTENT OF THE REVIEW: The influence of palatability on appetite and food intake in humans has been investigated in several studies. All reviewed studies have shown increased intake as palatability increased, whereas assessments of the effect of palatability using measures of subjective appetite sensations have shown diverging results, for example, subjects either feel more hungry and less full after a palatable meal compared to a less palatable meal, or they feel the opposite, or there is no difference. Whether palatability has an effect on appetite in the period following consumption of a test meal is unclear. Several studies have investigated which sensory properties of food are involved in sensory-specific satiety. Taste, smell, texture and appearance-specific satieties have been identified, whereas studies on the role of macronutrients and the energy content of the food in sensory-specific satiety have given equivocal results. Different studies have shown that macronutrients and energy content play a role in sensory-specific satiety or that macronutrients and energy content are not a factor in sensory-specific satiety. Sensory-specific satiety may have an important influence on the amount of food eaten. Studies have shown that increasing the food variety can increase food and energy intake and in the short to medium term alter energy balance. Further knowledge about the importance of flavour in appetite regulation is needed, for example, which flavour combinations improve satiety most, the possible connection between flavour intensity and satiety, the effect of persistence of chemesthetic sensation on palatability and satiety, and to what extent genetic variation in taste sensitivity and perception influences dietary habits and weight control.
The sense of smell is mediated by the initiation of action potential in olfactory sensory neurons during odor stimulation. However, little is known about odorant-olfactory receptor (OR) recognition mechanisms. In the present work, we identified the structural motifs of odorant molecules required to activate mouse OR912-93 by detection of the odorant response using calcium measurement in cells transfected with OR and G(alpha)q and G(alpha)15 proteins. The use of sets of odorants led to the identification of ketones with an aliphatic carbon chain length >or= four carbon atoms and a carbonyl group preferentially located in position C2 or C3. The threshold of detection of these odorants is as low as 10(-6)-10(-8)m. No other odorant ligand, out of 70 representatives of the odorant world, was active. The human ortholog of OR912-93 is not functional, suggesting that apart from a stop-mutation located at the 5'-end that was corrected in the construct, it incurred other deleterious mutations during evolution.
Odours are important in emotional processing; yet relatively little is known about the representations of the affective qualities of odours in the human brain. We found that three pleasant and three unpleasant odours activated dissociable parts of the human brain. Pleasant but not unpleasant odours were found to activate a medial region of the rostral orbitofrontal cortex. Further, there was a correlation between the subjective pleasantness ratings of the six odours given during the investigation with activation of a medial region of the rostral orbitofrontal cortex. In contrast, a correlation between the subjective unpleasantness ratings of the six odours was found in regions of the left and more lateral orbitofrontal cortex. Moreover, a double dissociation was found with the intensity ratings of the odours, which were not correlated with the BOLD signal in the orbitofrontal cortex, but were correlated with the signal in medial olfactory cortical areas including the pyriform and anterior entorhinal cortex. Activation was also found in the anterior cingulate cortex, with a middle part of the anterior cingulate activated by both pleasant and unpleasant odours, and a more anterior part of the anterior cingulate cortex showing a correlation with the subjective pleasantness ratings of the odours. Thus the results suggest that there is a hedonic map of the sense of smell in brain regions such as the orbitofrontal cortex, and these results have implications for understanding the psychiatric and related problems that follow damage to these brain areas.
Studies to investigate the relative value of sight, hearing and smell in mutual recognition between ewes and lambs are described. The method used was to alter clues that could aid in recognition, rather than interfering with the animal's sensory perception. When lambs were coloured with powdered dyes to change appearance, this produced marked avoidance by the dams of the treated lambs. When lambs were partially coloured, the ewes' greatest reaction was shown to lambs whose heads were coloured. Other experiments compared the role of vision and hearing by observing recognition when the ewes or lambs were hidden behind screening, or muted. The results indicate that while olfaction is important for recognition when the ewe and lamb are close together, visual clues are of major importance in maternal discrimination and auditory clues are important for the lambs as they get older.
The role played by the lamb in the regulation of maternal behaviour of the ewe around parturition was investigated in three experiments. In the first, the spontaneous manifestation of that behaviour was observed in parturient ewes when their lambs were swapped with either newborn or 12 to 24-hr old alien lambs. The ewes were undisturbed, or only slightly so, when their lambs were exchanged for alien newborn. In contrast, very obvious and significant differences were observed when swapping involved 12 to 24-hr old lambs. In the second experiment, ewes totally separated from their lambs at birth and for 12 hrs accepted a newborn alien lamb more readily than their own 12-hr old lamb at the time of reunion. In the third experiment, ewes were maintained in partial contact with their lambs for 12 hrs and maternal behaviour was tested at that time. The ewes remained maternal up to the time of testing only if they could smell their lambs. It was concluded that some characteristics of the newborn lamb are necessary for the development of adequate maternal behaviour.
BACKGROUND: It is controversially discussed whether strictly olfactory stimuli are able to evoke changes of breathing patterns. A number work groups argue that the more or less noticeable irritative component of odors are responsible for a respiratory response. Other work groups regard changes of breathing pattern as an indicator for an intact smell ability during "pure" olfactory stimulation. METHODS: 12 normosmic young volunteers were repeatedly challenged with hydrogensulfide (H (2)S) for two seconds on inspiration in concentrations just above the threshold using a flow olfactometer. Clean air stimuli were presented randomly as control blanks. The breathing patterns were registered with a differential pressure transducer occluding one nostril via a tube. The signals were digitized and recorded on a PC. Nine parameters were calculated for each breath using a custom made LabView (R) software. The breath during stimulation was compared to the average of five breathing cycles prior to stimulation. An increase or reduction of each parameter was equally regarded by means of their absolute value. The sum was termed respiration index (RI). RESULTS: The mean RI (all trials of each test person) stimulated with H (2)S varied from 201 to 570; stimulated with clean air from 85 to 188. The difference was significant (p < 0,001). A significant group-difference for each single trial was seen as well. There was no significant increase or decrease of RI during 15 successive stimulations with H (2)S. CONCLUSIONS: Detectable short H (2)S-stimuli are able to evoke alterations of breathing patterns which can be identified by computer. Respiration-olfactometry is suitable to objectify odor perception.
The characteristics of olfactory memory during development were investigated and the hypothesis that the pleasantness of smells may be affected by previous associations with pleasant or unpleasant objects or events was tested. This type of emotional memory was compared in the immediate and long-term recognition of olfactory stimuli. Children from three different age groups (mean ages: 6 years 6 months; 8 years 9 months; and 10 years 5 months) were subdivided into two groups. One group was presented with six different odours, each with a slide depicting a pleasant picture. The other group was presented with the odours accompanied by unpleasant pictures. Immediately after stimulus presentation the subjects underwent a recognition test. One month later the subjects underwent a second recognition test, at the end of which they were required to give an evaluation of the pleasantness of each odour on a nine-point scale. At no age level did the pictures matched to the odours affect the recognition score. Olfactory memory varied with age, chiefly because memory decay increased with age, perhaps because of greater proactive interference. With increasing age more rapid decay was set against better immediate recognition. The hypothesis that the hedonic characteristics of odours are partially learned and are affected by events experienced in other modalities was supported.
We explored the ability of humans to identify individual cats by smell. Twenty-five cat owners were required to indicate which of two odours (one belonging to their own cat and one belonging to an unfamiliar cat) smelt the strongest, most pleasant, and which belonged to their own cat. Only thirteen (52%) of the participants were able to recognise the odour of their own cat.
In this experiment, the effects of prior experience on odour perception and discrimination were explored. Participants repeatedly sniffed a mixture composed of two odours, AX, as well as smelling two further odours alone, B and Y. After this training phase, participants were asked to rate the similarity of the odours A and X, B and Y, and a non-exposed pair C and Z. A and X were judged as significantly more alike than the other pairs. Exactly the same pattern emerged on a second test, in which participants were asked to select the odd odour out of sets of three. It was consistently harder for participants to pick the odd odour when the stimuli were drawn from the AX pair (eg A versus AX versus AX). Not only do these findings demonstrate that prior experience can affect odour perception, a finding not predicted by theories of odour perception based solely upon the physiochemical properties of odours, they also suggest that experience can act to selectively decrease discriminability.
To facilitate communication about fragrances, one can use the colours people tend to associate with their smells. We investigated to what extent odour-colour correspondences for fine fragrances can be accounted for by underlying emotional associations. Odour-colour matches and degree-of-fit judgments revealed that odours were matched to colours non-randomly. Matching colours differed mainly on blackness (brightness), and less on chromaticness (saturation) and hue. Furthermore, we found a consistent negative relationship between odour-colour degree-of-fit ratings and the difference between the odour scores and the colour scores on one of the emotion dimensions (pleasure). This suggests that emotional associations may partly underlie odour-colour correspondences.
Anopheles gambiae is a highly anthropophilic mosquito responsible for the majority of malaria transmission in Africa. The biting and host preference behavior of this disease vector is largely influenced by its sense of smell, which is presumably facilitated by G protein-coupled receptor signaling [Takken, W. & Knols, B. (1999) Annu. Rev. Entomol. 44, 131-157]. Because of the importance of host preference to the mosquitoes' ability to transmit disease, we have initiated studies intended to elucidate the molecular mechanisms underlying olfaction in An. gambiae. In the course of these studies, we have identified a number of genes potentially involved in signal transduction, including a family of candidate odorant receptors. One of these receptors, encoded by GPRor7 (hereafter referred to as AgOr7), is remarkably similar to an odorant receptor that is expressed broadly in olfactory tissues and has been identified in Drosophila melanogaster and other insects [Krieger, J., Klink, O., Mohl, C., Raming, K. & Breer, H. (2003) J. Comp. Physiol. A 189, 519-526; Vosshall, L. B., Amrein, H., Morozov, P. S., Rzhetsky, A. & Axel, R. (1999) Cell 96, 725-736]. We have observed AgOr7 expression in olfactory and gustatory tissues in adult An. gambiae and during several stages of the mosquitoes' development. Within the female adult peripheral chemosensory system, antiserum against the AgOR7 polypeptide labels most sensilla of the antenna and maxillary palp as well as a subset of proboscis sensilla. Furthermore, AgOR7 antiserum labeling is observed within the larval antenna and maxillary palpus. These results are consistent with a role for AgOr7 in both olfaction and gustation in An. gambiae and raise the possibility that AgOr7 orthologs may also be of general importance to both modalities of chemosensation in other insects.
The testosterone derivative 4,16-androstadien-3-one (AND) and the estrogen-like steroid estra-1,3,5(10),16-tetraen-3-ol (EST) are candidate compounds for human pheromones. AND is detected primarily in male sweat, whereas EST has been found in female urine. In a previous positron emission tomography study, we found that smelling AND and EST activated regions covering sexually dimorphic nuclei of the anterior hypothalamus, and that this activation was differentiated with respect to sex and compound. In the present study, the pattern of activation induced by AND and EST was compared among homosexual men, heterosexual men, and heterosexual women. In contrast to heterosexual men, and in congruence with heterosexual women, homosexual men displayed hypothalamic activation in response to AND. Maximal activation was observed in the medial preoptic area/anterior hypothalamus, which, according to animal studies, is highly involved in sexual behavior. As opposed to putative pheromones, common odors were processed similarly in all three groups of subjects and engaged only the olfactory brain (amygdala, piriform, orbitofrontal, and insular cortex). These findings show that our brain reacts differently to the two putative pheromones compared with common odors, and suggest a link between sexual orientation and hypothalamic neuronal processes.
The role of cilia in recognition of olfactory stimuli has been controversial. Cilia from the intact olfactory rosettes of the rainbow trout Salmo gairdneri were isolated, characterized biochemically, and examined by electron microscopy. The markers studied are those associated with cilia in other organisms. Dynein arms contain Mg2+-AtPase; this enzyme was enriched in the isolated cilia preparation. Guanine nucleotides are associated with the outer microtubule doublets of cilia but adenine nucleotides are not; a substantial enrichment in guanine, relative to adenine, was found in the cilia preparation. Tubulin, the structural protein component of microtubules, occurs in large amounts in cilia. Disc gel electrophoresis indicated tubulin in the cilia preparation. Electron microscopy confirmed the presence of cilia in the isolated preparation. Rainbow trout have an acute sense of smell and many amino acids are odorants to this species. Functional activity of the cilia preparation relevant to odorant recognition was assessed by using binding of radioactively labeled odorant amino acids. L-Alanine, L-serine, L-threonine, L-lysine, and D-alanine bound to the cilia preparation. This study provides direct biochemical evidence that olfactory cilia bind odorant molecules and supports the hypothesis that odorant recognition sites are integral parts of the cilia.
The aim of this investigation was to study the impact of stress on ligature-induced periodontal disease in rats by means of a variable moderate chronic stress model. Thirty male Wistar rats were randomly assigned to six groups. Control groups received only ligatures around the second maxillary molars, while experimental groups were exposed to stress in additional. Stress was imposed by means of flashing light, isolation, rat blood smelling, new environment exposure, immobilization in cold temperature and immobilization at room temperature. Stress was applied randomly, thereby diminishing adaptation of the animals to the model. The animals were killed after 29, 43, and 57 days. The distance between the cementum-enamel junction and the alveolar bone (CEJ-AB) was measured. Alveolar bone loss was statistically different between stressed and control animals, whereas differences were not observed between experimental periods. The mean CEJ-AB distance in animals exposed to stress was 154.50 microm smaller than the corresponding distance in the controls. It might be concluded that variable moderate chronic stress decreased alveolar bone loss in a ligature-induced periodontal disease model.
OBJECTIVE: To compare the effect on olfaction of topical glucocorticoid treatment versus placebo given for an extended period in addition to oral short-term glucocorticoids in patients with anosmia/hyposmia. MATERIAL AND METHODS: This was a randomized, double-blind, placebo-controlled study. The criterion for inclusion in the blinded phase was an improvement of at least 2 steps in the butanol odor threshold test following open treatment for 10 days with oral and nasal corticosteroids. Forty patients were included: 20 were randomized to treatment with fluticasone propionate, 10 to placebo and 10 others as controls. The topical treatment was continued for 6 months. RESULTS: The 3 groups showed similar improvements in their sense of smell after the initial 10-day treatment with combined oral and nasal corticosteroids. Patients who continued the local treatment maintained the same level of improvement throughout the study, whether or not they had been given nasal corticosteroids or placebo. We found no significant differences between the treatment groups. CONCLUSIONS: In patients with anosmia/hyposmia partly caused by local inflammation, no further improvement in the olfactory threshold is achieved by continuing with a topical intranasal glucocorticoid after an initial combined topical and systemic glucocorticoid treatment.