Humans as an animal model for systems-level organization of olfaction

Sensing odorants and pheromones with chemosensory receptors

Olfaction is a critical sensory modality that allows living things to acquire chemical information from the external world. The olfactory system processes two major classes of stimuli: (a) general odorants, small molecules derived from food or the environment that signal the presence of food, fire, or predators, and (b) pheromones, molecules released from individuals of the same species that convey social or sexual cues. Chemosensory receptors are broadly classified, by the ligands that activate them, into odorant or pheromone receptors. Peripheral sensory neurons expressing either odorant or pheromone receptors send signals to separate odor- and pheromone-processing centers in the brain to elicit distinct behavioral and neuroendocrinological outputs. General odorants activate receptors in a combinatorial fashion, whereas pheromones activate narrowly tuned receptors that activate sexually dimorphic neural circuits in the brain. We review recent progress on chemosensory receptor structure, function, and circuitry in vertebrates and invertebrates from the point of view of the molecular biology and physiology of these sensory systems.

Odorant concentration dependence in electroolfactograms recorded from the human olfactory epithelium

Electroolfactograms (EOGs) are the summated generator potentials of olfactory receptor neurons measured directly from the olfactory epithelium. To validate the sensory origin of the human EOG, we set out to ask whether EOGs measured in humans were odorant concentration dependent. Each of 22 subjects (12 women, mean age = 23.3 yr) was tested with two odorants, either valeric acid and linalool (n = 12) or isovaleric acid and l-carvone (n = 10), each delivered at four concentrations diluted with warm (37 degrees C) and humidified (80%) odorless air. In behavior, increased odorant concentration was associated with increased perceived intensity (all F > 5, all P < 0.001). In EOG, increased odorant concentration was associated with increased area under the EOG curve (all F > 8, all P < 0.001). These findings substantiate EOG as a tool for probing olfactory coding directly at the level of olfactory receptor neurons in humans.

Odor quality coding and categorization in human posterior piriform cortex

Efficient recognition of odorous objects universally shapes animal behavior and is crucial for survival. To distinguish kin from nonkin, mate from nonmate and food from nonfood, organisms must be able to create meaningful perceptual representations of odor qualities and categories. It is currently unknown where and in what form the brain encodes information about odor quality. By combining functional magnetic resonance imaging (fMRI) with multivariate (pattern-based) techniques, we found that spatially distributed ensemble activity in human posterior piriform cortex (PPC) coincides with perceptual ratings of odor quality, such that odorants with more (or less) similar fMRI patterns were perceived as more (or less) alike. We did not observe these effects in anterior piriform cortex, amygdala or orbitofrontal cortex, indicating that ensemble coding of odor categorical perception is regionally specific for PPC. These findings substantiate theoretical models emphasizing the importance of distributed piriform templates for the perceptual reconstruction of odor object quality.

The influence of learning on sleep slow oscillations and associated spindles and ripples in humans and rats

The mechanisms underlying off-line consolidation of memory during sleep are elusive. Learning of hippocampus-dependent tasks increases neocortical slow oscillation synchrony, and thalamocortical spindle and hippocampal ripple activity during subsequent non-rapid eye movement sleep. Slow oscillations representing an oscillation between global neocortical states of increased (up-state) and decreased (down-state) neuronal firing temporally group thalamic spindle and hippocampal ripple activity, which both occur preferentially during slow oscillation up-states. Here we examined whether slow oscillations also group learning-induced increases in spindle and ripple activity, thereby providing time-frames of facilitated hippocampus-to-neocortical information transfer underlying the conversion of temporary into long-term memories. Learning (word-pairs in humans, odor-reward associations in rats) increased slow oscillation up-states and, in humans, shaped the timing of down-states. Slow oscillations grouped spindle and rat ripple activity into up-states under basal conditions. Prior learning produced in humans an increase in spindle activity focused on slow oscillation up-states. In rats, learning induced a distinct increase in spindle and ripple activity that was not synchronized to up-states. Event-correlation histograms indicated an increase in spindle activity with the occurrence of ripples. This increase was prolonged after learning, suggesting a direct temporal tuning between ripples and spindles. The lack of a grouping effect of slow oscillations on learning-induced spindles and ripples in rats, together with the less pronounced effects of learning on slow oscillations, presumably reflects a weaker dependence of odor learning on thalamo-neocortical circuitry. Slow oscillations might provide an effective temporal frame for hippocampus-to-neocortical information transfer only when thalamo-neocortical systems are already critically involved during learning.

Olfactory hypersensitivity in migraineurs: a H(2)(15)O-PET study

Olfactory hypersensitivity (OHS) may occur during migraine attacks and seems to be very specific to this form of headache. OHS is also observed during migraine-free periods and is associated with the presence of odour-triggered attacks. Yet the pathophysiology of OHS remains unknown. The aim of our study was to evaluate olfactory processing in migraineurs with OHS and to investigate whether regional cerebral blood flow (rCBF) associated with olfactory stimulation is modified in these patients compared with controls. Eleven migraineurs with OHS and 12 controls participated in a H(2)(15)O-positron emission tomography study, including three scans in which odours were delivered and three scans where only odourless air was delivered. rCBF during olfactory condition was compared with that for the odourless baseline condition. Between-group analyses were performed using voxel-based and region-of-interest analyses. During both olfactory and non-olfactory conditions, we observed higher rCBF in the left piriform cortex and antero-superior temporal gyrus in migraineurs compared with controls. During odour stimulation, migraineurs also showed significantly higher activation than controls in the left temporal pole and significantly lower activation in the frontal (left inferior as well as left and right middle frontal gyri) and temporo-parietal (left and right angular, and right posterior superior temporal gyri) regions, posterior cingulate gyrus and right locus coeruleus. These results could reflect a particular role of both the piriform cortex and antero-superior temporal gyrus in OHS and odour-triggered migraine. Whether these rCBF changes are the cause or a consequence of odour-triggered migraines and interictal OHS remains unknown. Further comparisons between migraineurs with and without OHS are warranted to address this issue. The abnormal cerebral activation patterns during olfactory stimulation might reflect altered cerebrovascular response to olfactory stimulation due to the migraine disease, or an abnormal top-down regulation process related to OHS.

Expressing fear enhances sensory acquisition

It has been proposed that facial expression production originates in sensory regulation. Here we demonstrate that facial expressions of fear are configured to enhance sensory acquisition. A statistical model of expression appearance revealed that fear and disgust expressions have opposite shape and surface reflectance features. We hypothesized that this reflects a fundamental antagonism serving to augment versus diminish sensory exposure. In keeping with this hypothesis, when subjects posed expressions of fear, they had a subjectively larger visual field, faster eye movements during target localization and an increase in nasal volume and air velocity during inspiration. The opposite pattern was found for disgust. Fear may therefore work to enhance perception, whereas disgust dampens it. These convergent results provide support for the Darwinian hypothesis that facial expressions are not arbitrary configurations for social communication, but rather, expressions may have originated in altering the sensory interface with the physical world.

Effects of bilateral deep brain stimulation of the subthalamic nucleus on olfactory function in Parkinson's disease patients

OBJECTIVE

The goal of the present study was to evaluate the effects of bilateral deep brain stimulation (DBS) of the subthalamic nucleus (STN) on olfaction in patients with Parkinson's disease (PD).

METHODS

15 patients suffering from sporadic PD-related dysosmia were implanted with bilateral electrodes aimed at the STN. One week before the surgery, odor detection threshold (DT) and identification threshold (IT) were evaluated in all patients using the 'five odor olfactory detection arrays' in both medication-off and medication-on conditions. 15 healthy age-matched controls also received the same olfactory evaluation. Patient evaluations were repeated at 6 and 12 months postoperatively in a medication-off/stimulator-on or medication-off/stimulator-off condition. Odor DT and IT scores were compared pre- and postoperatively, as well as between the medication-off/stimulator-on or -off conditions.

RESULTS

The motor symptoms of all 15 PD patients, including rigidity, tremor, bradykinesia, postural instability, and gait were significantly improved after stimulator implantation. The UPDRS motor (UPDRS III) scores decreased significantly in the medication-off/stimulator-on condition (p < 0.01). The odor DT and IT scores of PD patients were higher than those of healthy controls (p < 0.01). In the medication-off/stimulator-off condition, there was no significant difference in the odor DT and IT scores in PD patients pre- vs. postoperatively (p > 0.05). Notably, there were no significant alterations to DT scores in the stimulator-on and -off conditions at the 6- and 12-month follow-up (p > 0.05), whereas IT scores were significantly improved in the stimulation-on relative to the stimulation-off condition at the 6- and 12-month follow-up.

CONCLUSIONS

STN DBS can significantly improve olfactory cognitive function in PD patients. The possible mechanisms include an improvement in striatal metabolism and neuronal activity in the orbitofrontal cortex mediated by STN DBS, as well as increased glucose metabolism in the striatum, midbrain, cingulate gyrus, and motor and higher-order somatosensory association cortices.

Hedonic-specific activity in piriform cortex during odor imagery mimics that during odor perception

Although it is known that visual imagery is accompanied by activity in visual cortical areas, including primary visual cortex, whether olfactory imagery exists remains controversial. Here we asked whether cue-dependent olfactory imagery was similarly accompanied by activity in olfactory cortex, and in particular whether hedonic-specific patterns of activity evident in olfactory perception would also be present during olfactory imagery. We used functional magnetic resonance imaging to measure activity in subjects who alternated between smelling and imagining pleasant and unpleasant odors. Activity induced by imagining odors mimicked that induced by perceiving real odorants, not only in the particular brain regions activated, but also in its hedonic-specific pattern. For both real and imagined odors, unpleasant stimuli induced greater activity than pleasant stimuli in the left frontal portion of piriform cortex and left insula. These findings combine with findings from other modalities to suggest activation of primary sensory cortical structures during mental imagery of sensory events.

Chemotopic odorant coding in a mammalian olfactory system

Systematic mapping studies involving 365 odorant chemicals have shown that glomerular responses in the rat olfactory bulb are organized spatially in patterns that are related to the chemistry of the odorant stimuli. This organization involves the spatial clustering of principal responses to numerous odorants that share key aspects of chemistry such as functional groups, hydrocarbon structural elements, and/or overall molecular properties related to water solubility. In several of the clusters, responses shift progressively in position according to odorant carbon chain length. These response domains appear to be constructed from orderly projections of sensory neurons in the olfactory epithelium and may also involve chromatography across the nasal mucosa. The spatial clustering of glomerular responses may serve to "tune" the principal responses of bulbar projection neurons by way of inhibitory interneuronal networks, allowing the projection neurons to respond to a narrower range of stimuli than their associated sensory neurons. When glomerular activity patterns are viewed relative to the overall level of glomerular activation, the patterns accurately predict the perception of odor quality, thereby supporting the notion that spatial patterns of activity are the key factors underlying that aspect of the olfactory code. A critical analysis suggests that alternative coding mechanisms for odor quality, such as those based on temporal patterns of responses, enjoy little experimental support.

The effect of a salient odor context on memory retrieval in young infants

Three-month-old infants were trained to move a mobile in the presence of a coconut or cherry odor (context). One or 5 days later, the infants were tested for retrieval in the presence of either the same odor, the alternate odor, or no odor. Infants tested with the training odor displayed retention at both intervals; retention was not seen at either interval in the alternate odor or no odor conditions. These data suggest that the odor combines with the mobile to form a compound-stimulus representation of the learned task whose presence after both short (1 day) and long (5 days) intervals is a necessary retrieval cue.

Associative encoding in posterior piriform cortex during odor discrimination and reversal learning

Recent proposals have conceptualized piriform cortex as an association cortex, capable of integrating incoming olfactory information with descending input from higher order associative regions such as orbitofrontal cortex and basolateral amygdala (ABL). If true, encoding in piriform cortex should reflect associative features prominent in these areas during associative learning involving olfactory cues. We recently reported that neurons in anterior piriform cortex (APC) in rats exhibited significant plasticity in their responses to odor cues during associative learning. Here, we have repeated this study, recording from neurons in posterior piriform cortex (PPC), a region of piriform cortex that receives much stronger input from ABL. If associative encoding in piriform cortex is driven by inputs from ABL, then we should see more plasticity in PPC neurons than we observed in APC. Consistent with this hypothesis, we found that PPC neurons were highly associative and appeared to be somewhat more likely than neurons recorded in APC to alter their responses to the odor cues after reversal of the odor-outcome associations in the task. Further, odor-selective PPC populations exhibited markedly different firing patterns based on the valence of the odor cue. These results suggest associative encoding in piriform cortex is represented in a topographical fashion, reflecting the stronger and more specific input from olfactory bulb concerning the sensory features of odors in anterior regions and stronger input from ABL concerning the meaning of odors in posterior regions.

Odor cues during slow-wave sleep prompt declarative memory consolidation

Sleep facilitates memory consolidation. A widely held model assumes that this is because newly encoded memories undergo covert reactivation during sleep. We cued new memories in humans during sleep by presenting an odor that had been presented as context during prior learning, and so showed that reactivation indeed causes memory consolidation during sleep. Re-exposure to the odor during slow-wave sleep (SWS) improved the retention of hippocampus-dependent declarative memories but not of hippocampus-independent procedural memories. Odor re-exposure was ineffective during rapid eye movement sleep or wakefulness or when the odor had been omitted during prior learning. Concurring with these findings, functional magnetic resonance imaging revealed significant hippocampal activation in response to odor re-exposure during SWS.

Odour identification in frontotemporal lobar degeneration

Little information is available concerning olfactory processing in frontotemporal lobar degeneration (FTLD). We undertook a case-control study of olfactory processing in three male patients fulfilling clinical criteria for FTLD. Odour identification (semantic analysis) and odour discrimination (perceptual analysis) were investigated using tests adapted from the University of Pennsylvania Smell Identification Test. General neuropsychometry and structural volumetric brain magnetic resonance imaging (MRI) were also performed. The three patients with FTLD exhibited a disorder of olfactory processing with the characteristics of a predominantly semantic (odour identification) deficit. This olfactory deficit was more prominent in patients with greater involvement of the temporal lobes on MRI. Central deficits of odour identification may be more common in FTLD than previously recognised, and these deficits may assist in clinical characterisation.

Methods for building an olfactometer with known concentration outcomes

We provide detailed instructions and part selections for construction of a five-channel air dilution olfactometer capable of generating neat odorants and binary mixtures at a range of known concentrations. At the heart of the olfactometer is an odorant canister that is (1) cheap and readily available, (2) safe and durable, (3) has minimal odor adherence, (4) is easily incorporated into any olfactometer, and critically (5) produces a highly consistent stimulus. By flowing a given carrier gas at a given flowrate through a given odorant in this canister, the same end-vapor is achieved. Flow/concentration outcomes are provided for several odorants routinely used in olfactometry. This tool will enable researchers to generate known concentrations without expensive analytical machinery.

Validation and optimization of statistical approaches for modeling odorant-induced fMRI signal changes in olfactory-related brain areas

Recent neuroimaging studies have converged to show that odorant-induced responses to prolonged stimulation in primary olfactory cortex (POC) are characterized by a rapidly habituating time course. Different statistical approaches have effectively modeled this time course. One approach explicitly modeled rapid habituation using an exponentially decaying reference waveform that decreased to baseline levels within 30 to 40 s. A second approach modeled an early transient response by simply shortening the odorant 'ON' period to be less than the actual stimulation period (i.e., 9 of 40 s). The goal of the current study was to validate, compare, and optimize these methodological approaches by applying them to an olfactory fMRI block-design dataset from 10 healthy young subjects presented with odorants for 12 s (ON), alternating with 30 s of clear air (OFF). Both approaches significantly improved sensitivity to odorant-induced signal changes in POC relative to a square-wave model based on the actual stimulation period. Our findings further demonstrate that the 'optimal' model fit to the data was achieved by shortening the odorant 'ON' period to approximately 6 s. These results suggest that sensitivity to odorant-induced POC activity in block-design experiments can be optimized by modeling an early phasic response followed by a precipitous rather than specific exponential decrease to baseline levels. Notably, whole brain voxel-wise analyses further established that modeling rapid habituation in this way is not only sensitive, but also highly specific to odorant-induced activation in a well-established network of olfactory-related brain areas.

Molecular biology of peptide pheromone production and reception in mice

Intraspecies communication via pheromones plays an important role in social and sexual behaviors, which are critical for survival and reproduction in many animal species. In mice, pheromonal signals are processed by the parallel action of two olfactory systems: the main olfactory system and the vomeronasal pathway. Pheromones are recognized by chemosensory receptors expressed in the main olfactory epithelium and by V1R- and V2R-type receptors expressed in the vomeronasal organ (VNO). Mice take advantage of the chemical properties of both types of pheromones (i.e., volatile/nonvolatile) to precisely control the spatial and temporal transmission of their individual signals. The recent discovery of the exocrine gland-secreting peptide (ESP) family, which appears to encode a VNO-specific ligand repertoire, should open a new avenue to understanding peptide pheromone-mediated communication via the vomeronasal pathway in mice. In this chapter, I will review the current knowledge on genetic and molecular aspects of peptide pheromones and their receptors, by focusing primarily on the mouse VNO system. It is also an intriguing aspect to discuss peptide pheromones in the context of the evolutionary importance of species-specific chemical communication.

The Molecular Basis for Ligand Specificity in a Mouse Olfactory Receptor

Sequence differences between members of the mouse olfac-tory receptor MOR42 subfamily (MOR42-3 and MOR42-1) are likely to be the basis for variation in ligand binding preference among these receptors. We investigated the specificity of MOR42-3 for a variety of dicarboxylic acids. We used site-directed mutagenesis, guided by homology modeling and ligand docking studies, to locate functionally important residues. Receptors were expressed in Xenopus oocytes and assayed using high throughput electrophysiology. The importance of the Val-113 residue, located deep within the receptor, was analyzed in the context of interhelical interactions. We also screened additional residues predicted to be involved in ligand binding site, based on comparison of ortholog/paralog pairs from the mouse and human olfactory receptor genomes (Man, O., Gilad, Y., and Lancet, D. (2004) Protein Sci. 13, 240-254). A network of 8 residues in transmembrane domains III, V, and VI was identified. These residues form part of the ligand binding pocket of MOR42-3. C12 dicarboxylic acid did not activate the receptor in our functional assay, yet our docking simulations predicted its binding site in MOR42-3. Binding without activation implied that C12 dicarboxylic acid might act as an antagonist. In our functional assay, C12 dicarboxylic acid did indeed act as an antagonist of MOR42-3, in agreement with molecular docking studies. Our results demonstrate a powerful approach based on the synergy between computational predictions and physiological assays.

Olfactory dysfunction as a predictor of neurodegenerative disease

Olfactory dysfunction is present in patients diagnosed with Alzheimer's disease or idiopathic Parkinson's disease and can differentiate each of these disorders from related disorders with similar clinical presentations. The pathologic hallmarks of each disease are present in brain regions involved in processing olfactory input. Both the olfactory functional deficits and the corroborating pathologic lesions are present in asymptomatic subjects with increased risk of developing these diseases. Preclinical detection of neurodegenerative diseases is necessary to control their devastating effects on individuals and societies. We address whether olfactory dysfunction can be used to assess risk for developing Alzheimer's disease or Parkinson's disease in asymptomatic individuals. We argue that further characterization and a deeper understanding of olfactory deficits in these neurodegenerative diseases at the molecular, cellular, and systems levels will augment our acumen for preclinical detection and elucidate pathogenic mechanisms to guide the development of new therapeutic modalities.

Olfactory neuronal dynamics in behaving animals

More than 50 years have passed since the first recording of neuronal responses to an odor stimulus from the primary olfactory brain area, the main olfactory bulb. During this time very little progress has been achieved in understanding neuronal dynamics in the olfactory bulb in awake behaving animals, which is very different from that in anesthetized preparations. In this paper we formulate a new framework containing the main reasons for studying olfactory neuronal dynamics in awake animals and review advances in the field within this new framework.