Abstract: “Sniffin’ Sticks” is a test of nasal chemosensory performance that is based on penlike odor-dispensing devices. It is comprised of three tests of olfactory function: tests for odor threshold, discrimination and identification. Previous work has already established its test-retest reliability and validity in comparison to established measures of olfactory sensitivity. The results of this test are presented as a composite TDI score – i.e., the sum of results obtained for threshold, discrimination and identification measures. The present multicenter investigation aimed at providing normative values in relation to different age groups. To this end, 966 patients were investigated in 11 centers. An additional study tried to establish values for the identification of anosmic patients, with 70 anosmics investigated in five specialized centers where the presence of anosmia was confirmed by means of olfactory evoked potentials. For healthy subjects, the TDI score at the 10th percentile was 24.5 in subjects younger than 15 years, 30.3 for ages from 16 to 35 years, 28.8 for ages from 36 to 55 years and 27.5 for subjects older than 55 years. While these data can be used to estimate individual olfactory abilities in relation to a subject’s age, hyposmia was defined as the 10th percentile score of 16- to 35-year-old subjects. Our latter study revealed that none of 70 anosmics reached a TDI score higher than 15. This score of 15 is regarded as the cut-off value for functional anosmia. These results provide the basis for the routine clinical evaluation of patients with olfactory disorders using “Sniffin’ Sticks.”

Abstract: In adult rodents, neurons are continually generated in the subventricular zone of the forebrain, from where they migrate tangentially toward the olfactory bulb, the only known target for these neuronal precursors. Within the main olfactory bulb, they ascend radially into the granule and periglomerular cell layers, where they differentiate mainly into local interneurons. The functional consequences of this permanent generation and integration of new neurons into existing circuits are unknown. To address this question, we used neural cell adhesion molecule-deficient mice that have documented deficits in the migration of olfactory-bulb neuron precursors, leading to about 40% size reduction of this structure. Our anatomical study reveals that this reduction is restricted to the granule cell layer, a structure that contains exclusively γ-aminobutyric acid (GABA)ergic interneurons. Furthermore, mutant mice were subjected to experiments designed to examine the behavioral consequences of such anatomical alteration. We found that the specific reduction in the newly generated interneuron population resulted in an impairment of discrimination between odors. In contrast, both the detection thresholds for odors and short-term olfactory memory were unaltered, demonstrating that a critical number of bulbar granule cells is crucial only for odor discrimination but not for general olfactory functions.

Abstract: Candidate Drosophila olfactory receptors (ORs) provide molecular tools to investigate how the organization of the Drosophila olfactory system determines the coding of olfactory stimuli. Neurons in the third antennal segment and maxillary palp appear to express different ORs. Individual olfactory neurons send axonal projections to glomeruli in the antennal lobe. Using transgenic flies, we provide evidence that the neurons expressing a given OR gene, which have cell bodies distributed among neurons expressing other ORs, converge in their projections to topographically fixed glomeruli in the antennal lobe. This convergence allows for the formation of an odotopic map in the antennal lobe whose organization could provide a basis for olfactory discrimination in Drosophila.

Abstract: Neurons are generated from neural progenitor cells not only during development but also in the mature brain. To develop an in vivo system for analyzing neurogenesis, we generated transgenic mice expressing green fluorescent protein (GFP) under the control of regulatory regions of the nestin gene. GFP fluorescence was observed in areas and during periods connected with neurogenesis, including embryonic neuroepithelium, neonatal cerebellum, and hippocampal dentate gyrus and rostral migratory pathway from the subventricular zone to the olfactory bulb in the adult. GFP-positive cells in the adult brain included immature neuronal cells expressing polysialylated NCAM. BrdU labeling experiments revealed that newly generated interneurons which migrated rostrally from the subventricular zone expressed GFP until they reached the olfactory bulb. These results indicate that nestin promoter-GFP transgenic mice can be utilized to visualize the regions of neurogenesis throughout the life of the animals and to follow the migration and differentiation of newly generated neurons.

Abstract: The olfactory ensheathing cell (OEC) has attracted much interest recently because of its potential for transplantation-based therapy of CNS disease. Rat OECs are able to remyelinate demyelinated axons and support regeneration of damaged axons. Although OECs can be grown readily from the rat, a macrosmatic species, it has been uncertain whether it would be similarly straightforward to obtain these cells from the human, a microsmatic species with a relatively poorly developed olfactory system. In this study, we have identified a human OEC which shares many properties with its rat counterpart, including expression of the low-affinity nerve growth factor receptor (L-NGFr) and similar growth factor requirements. Purified populations of human OECs obtained by selection with L-NGFr antibodies have extremely high viability in tissue culture, and are capable of remyelinating persistently demyelinated CNS axons following transplantation into experimentally induced demyelinating lesions in the rat spinal cord. Thus, the human OEC represents an important new cell for the development of transplant therapy of CNS diseases.

Abstract: Objectives To evaluate histological changes in the olfactory mucosa of patients with chronic rhinosinusitis. These results are analyzed in light of current understanding of the pathophysiology of anosmia secondary to nasal and sinus disease. Study Design Prospective study of olfaction on patients undergoing sinus surgery for the management of chronic rhinosinusitis. Methods Thirty patients, aged 22 to 39 years, underwent olfactory biopsy at the time of surgery with evaluation by a pathologist. Inflammatory changes were graded as mild (normal), moderate, or severe. Clinical olfactory function was evaluated using the University of Pennsylvania Smell Identification Test (UPSIT). The results correlated with the degree of olfactory dysfunction. Results Of the 30 patients 19 had unequivocal olfactory mucosa in the biopsy specimen. Eleven had only respiratory or indeterminate mucosa. Nine patients demonstrated normal olfactory mucosa and normal olfactory function (UPSIT >35). Ten patients demonstrated pathological changes in the olfactory mucosa with an influx of lymphocytes, macrophages, and eosinophils. Of these 10 patients, 7 had olfactory deficits as determined by UPSIT. The remaining three patients had normal olfactory function despite moderate chronic inflammation. These studies indicate that the olfactory mucosa is capable of mounting an inflammatory response similar to that seen in the respiratory mucosa of patients with chronic sinusitis. These data suggest that the olfactory deficits in these patients may be the result of inflammatory changes within the olfactory mucosa in addition to any alteration in airflow to the olfactory cleft.

Abstract: Sensory adaptation allows organisms to reach behavioral equilibrium with the ambient environment and respond primarily to changes in stimulation. Given its functional significance, it is not surprising that adaptation in the olfactory system exhibits many of the same characteristics as adaptation in other sensory systems, including vision. Repeated or prolonged exposure to an odorant typically leads to stimulus-specific decreases in olfactory sensitivity to that odorant, but sensitivity recovers over time in the absence of further exposure. Psychophysical analysis shows that olfactory adaptation results in elevations in odor thresholds and in reduced responsiveness to suprathreshold stimulation. Further, the magnitude of the decrease and the time course of adaptation and recovery are dependent on the concentration of the odor and on the duration of exposure. It is generally agreed that olfactory adaptation can occur at multiple levels in the olfactory system and can involve both peripheral (receptor level) and more central (post-receptor) components. Evidence for peripheral and central involvement comes from studies showing that monorhinal stimulation results in adaptation in both the ipsilateral and contralateral nostril, although the degree of adaptation in the ipsilateral nostril is more profound and recovery is slower. Additional evidence for central involvement comes from studies that have found relatively small decreases in peripheral response following repeated stimulation despite substantial reductions in perceived intensity. Most psychophysical studies of adaptation, however, have not differentiated the peripheral and central processes. Although relatively few in number, studies of the parametric features of olfactory adaptation in both vertebrate (e.g. rat) and invertebrate (e.g. Drosophila, Caenorhabditis elegans) animal models appear to replicate the findings in psychophysical studies of adult humans. Despite the broad overall similarity of olfactory adaptation to adaptation in other sensory systems, olfactory adaptation exhibits some unique features. Adaptation in olfaction has been shown to be very long-lasting in some cases and may be modulated by the contribution of pre-neural events and physico-chemical properties of the odorant molecules that govern diffusion to receptor sites and post-receptor clearance.

Abstract: These experiments examined the sufficiency of pairing an odor with either intrabulbar activation of noradrenergic β-receptors or pharmacological stimulation of the locus coeruleus to support learned odor preferences in Postnatal Day 6-7 rat pups. The results showed that pups exposed to odor paired with β-receptor activation limited to the olfactory bulb (isoproterenol, 50 μM) displayed a conditioned approach response on subsequent exposure to that odor. Furthermore, putative stimulation of the locus coeruleus (2 μM idazoxan or 2 mM acetylcholine) paired with odor produced a subsequent preference for that odor. The effects of locus coeruleus stimulation could be blocked by a pretraining injection of the β-receptor antagonist propranolol (20 mg/kg). Together these results suggest that convergence of odor input with norepinephrine release from the locus coeruleus terminals within the olfactory bulb is sufficient to support olfactory learning.

Abstract: For those interested in the functional organization of the olfactory pathway, this year marks two milestones: a half century since Lord Adrian (1950) suggested that odors are encoded by their spatial pattern of activity in the olfactory bulb, and a quarter of a century since the 2-deoxyglucose (2-DG) method provided evidence for the mechanisms underlying those patterns (Sharp et al., 1975). The article by Johnson and Leon in this issue (Johnson and Leon, 2000) on 2-DG patterns adds to the increasing evidence supporting the hypothesis that odor maps are part of the neural basis for perceptual discrimination of both odor quality and odor intensity. Despite this increasing evidence, it is difficult to conceive of how information carried in odor molecules can be represented by two-dimensional spatial patterns in the brain. The significance of the odor patterns, therefore, has not been widely appreciated outside the field of olfaction and has been controversial to some degree even within the field. For the purposes of this review, we consider as “odor maps” those spatial activity patterns within the olfactory bulb that are elicited by odor stimulation. There is also a rich variety of spatial patterns identified by other means, such as by horseradish peroxidase (HRP) tracing (Jastreboff et al., 1984; Astic and Saucier, 1986), monoclonal antibodies (Schwob, 1992; Mori and Yoshihara, 1995), and olfactory receptor gene-labelled projections (Ressler et al., 1994; Vassar et al., 1994; Mombaerts et al., 1996). These patterns will map onto the activity maps when functional correlations are made. The first steps toward those correlations are being taken (Bozza et al., 1999), which will ultimately lead to odor maps that have a combined functional, structural, and molecular basis. The different approaches have given rise to a number of different terms for the maps in the olfactory bulb. In addition to “odor maps,” there are “odotope maps” (encoding the determinants of odor molecules) and equivalents such as “odor determinant maps” and “olfactophore maps”; “epitope maps” (in analogy with the immune system); “chemotopic maps” (encompassing the general idea of encoding chemicals); and “odor images” (in analogy with the patterns laid down in vision). For the purposes of this review, the terms matter less than the principle, that information carried in odor molecules can be mapped into neural space. We first summarize the principles underlying odor maps that have been obtained from 2-DG and the growing number of other activity marking methods. We emphasize the accumulation of experimental evidence to support both the general idea of odor maps, as well as specific properties of the maps. We then review some of the controversies surrounding the significance of the odor maps and attempt to indicate how they can be resolved. Our focus is on mammals, but results in other vertebrate and invertebrate species will be noted where relevant.

Abstract: Olfactory processing in the human brain was examined using positron emission tomography. Twelve normal volunteers were scanned while smelling pairs of odors: they were asked to judge which odor was more pleasant in one condition, and which was more intense in a second condition; they also were scanned while sniffing an odorless stimulus. As in prior studies, greater cerebral blood flow was found in the right orbitofrontal cortex during both pleasantness and intensity judgments as compared to baseline. Cerebellar activity was also seen, but contrary to expectations no activity was detected in the primary olfactory (piriform) cortex. Only the pleasantness judgment elicited additional activity within the hypothalamus, suggesting that this structure may be involved in affective processing that requires access to information about internal state.

Abstract: The prevailing paradigm for G protein-coupled receptors is that each receptor is narrowly tuned to its ligand and closely related agonists. An outstanding problem is whether this paradigm applies to olfactory receptor (ORs), which is the largest gene family in the genome, in which each of 1,000 different G protein-coupled receptors is believed to interact with a range of different odor molecules from the many thousands that comprise “odor space.” Insights into how these interactions occur are essential for understanding the sense of smell. Key questions are: (i) Is there a binding pocket? (ii) Which amino acid residues in the binding pocket contribute to peak affinities? (iii) How do affinities change with changes in agonist structure? To approach these questions, we have combined single-cell PCR results [Malnic, B., Hirono, J., Sato, T. & Buck, L. B. (1999) Cell 96, 713–723] and well-established molecular dynamics methods to model the structure of a specific OR (OR S25) and its interactions with 24 odor compounds. This receptor structure not only points to a likely odor-binding site but also independently predicts the two compounds that experimentally best activate OR S25. The results provide a mechanistic model for olfactory transduction at the molecular level and show how the basic G protein-coupled receptor template is adapted for encoding the enormous odor space. This combined approach can significantly enhance the identification of ligands for the many members of the OR family and also may shed light on other protein families that exhibit broad specificities, such as chemokine receptors and P450 oxidases.

Abstract: The accessory olfactory system mediates intraspecies pheromonal communication. Two subsets of spatially segregated vomeronasal sensory neurons, presumably handling functionally and structurally different sets of ligand molecules, can be distinguished. The two subsets of sensory neurons project their axons to segregated zones of the accessory olfactory bulb (AOB) and connect with zonally separated mitral/tufted (M/T) cells, suggesting that the accessory olfactory system is divided into two distinct pathways up to the level of the AOB. To examine whether the segregation is maintained at the accessory olfactory cortical (AOC) regions, we selectively tracer-labelled mitral/tufted cells located in the rostral, caudal or in both zones of the adult mouse AOB. The results demonstrate that the axonal projection patterns of rostral zone and caudal zone M/T cells were indistinguishable in the AOC regions. Mitral/tufted cell axons from either zone of the AOB covered the entire area of all four AOC regions: the bed nucleus of the accessory olfactory tract, the medial amygdaloid nucleus, the posteromedial cortical amygdaloid nucleus and the bed nucleus of the stria terminalis. Therefore, over the entire area of each AOC region, ensembles of cortical neurons receive input from both zonal subsets of M/T cells of the AOB. However, the present results do not rule out the possibility that individual cortical neurons sample information from M/T cells of a single zone. These results are consistent with the idea that the segregation of zonal pathways collapses in the AOC regions. Clusters of cortical neurons in each AOC region may combine information from both families of pheromone receptors and thus handle signals from structurally and functionally different categories of pheromone molecules.

Abstract: Recent work in the anterior piriform cortex (aPCX) has demonstrated that cortical odor receptive fields are highly dynamic, showing rapid changes of both firing rate and temporal patterning within relatively few inhalations of an odor, despite relatively maintained, patterned input from olfactory bulb mitral/tufted cells. The present experiment examined the precision (odor-specificity) of this receptive field plasticity and compared it with the primary cortical afferent, olfactory bulb mitral/tufted cells. Adult Long-Evans hooded rats, urethan anesthetized and freely breathing, were used for single-unit recording from mitral/tufted and aPCX layer II/III neurons. Partial mapping of receptive fields to alkane odors (pentane, heptane, and nonane) was performed before and immediately after habituation (50-s exposure) to one of the alkanes. The results demonstrated that odor habituation of aPCX responses was odor specific, with minimal cross-habituation between alkanes differing by as few as two carbons. Mitral/tufted cells, however, showed strong cross-habituation within the odor set with the most profound cross effects to carbon chains shorter than the habituating stimulus. The results suggest that although mitral/tufted cells and aPCX neurons have roughly similar odor receptive fields, aPCX neurons have significantly better odor discrimination within their receptive field. The results have important implications for understanding the underlying bases of receptive fields in olfactory system neurons and the mechanisms of odor discrimination and memory.

Abstract: This review examines interactions in the mammalian central nervous system (CNS) between carnosine and the endogenous transition metals zinc and copper. Although the relationship between these substances may be applicable to other brain regions, the focus is on the olfactory system where these substances may have special significance. Carnosine is not only highly concentrated in the olfactory system, but it is also contained in neurons (in contrast to glia cells in most of the brain) and has many features of a neurotransmitter. Whereas the function of carnosine in the CNS is not well understood, we review evidence that suggests that it may act as both a neuromodulator and a neuroprotective agent. Although zinc and/or copper are found in many neuronal pathways in the brain, the concentrations of zinc and copper in the olfactory bulb (the target of afferent input from sensory neurons in the nose) are among the highest in the CNS. Included in the multitude of physiological roles that zinc and copper play in the CNS is modulation of neuronal excitability. However, zinc and copper also have been implicated in a variety of neurologic conditions including Alzheimer's disease, Parkinson's disease, stroke, and seizures. Here we review the modulatory effects that carnosine can have on zinc and copper's abilities to influence neuronal excitability and to exert neurotoxic effects in the olfactory system. Other aspects of carnosine in the CNS are reviewed elsewhere in this issue.

Abstract: Purpose. The aim of the study was to investigate whetherdopamine is transferred along the olfactory pathway to the brain followingnasal administration to mice.

Abstract: Partitioning of synaptic neuropil into glomeruli is a common feature of primary olfactory centers in most animal species. The functional significance of glomeruli, however, is not yet well understood. The present study is part of our effort to test the hypothesis that each glomerulus is a functional unit dedicated to processing information about a particular odorant or attribute of odor molecules and that the glomerular array constitutes a map of "odor space." We investigated the physiological and morphological features of uniglomerular projection neurons (PNs) associated with an identified glomerulus in each antennal lobe of the female sphinx moth, Manduca sexta. This "lateral large female glomerulus" (latLFG) is sexually dimorphic and therefore may play a female-specific role, such as processing of information about one or more odorants important for orientation of a female to host plants for oviposition. Together with the medial LFG (medLFG), the latLFG resides outside the array of spheroidal ordinary glomeruli, near the entrance of the antennal (olfactory) nerve. Each LFG is innervated by four to five PNs. Using intracellular recording and staining, we examined the responses of latLFG-PNs to odorants that represent major classes of volatiles released by host plants of M. sexta. All latLFG-PNs were excited when the ipsilateral antenna was stimulated with low concentrations of the monoterpenoid linalool. Dose-response analysis showed that neither other monoterpenoids nor representatives of other classes of host plant volatiles were similarly stimulatory to latLFG-PNs. These findings are consistent with the idea that each glomerulus has a characteristic, limited molecular receptive range.

Abstract: Dysosmia and anosmia are reported to occur following human exposure to hydrogen sulfide (H2S) gas. The clinical association between H2S exposure and olfactory dysfunction in humans necessitates evaluation of the nasal cavity and olfactory system in experimental animals used to study H2S toxicity. The purpose of this study was to subchronically expose 10-week-old male CD rats to relatively low concentrations of H2S and to histologically evaluate the nasal cavity for exposure-related lesions. Rats (n = 12/group) were exposed via inhalation to 0, 10, 30, or 80 ppm H2S 6 h/d and 7 d/wk for 10 weeks. Following exposure to 30 and 80 ppm H2S, a significant increase in nasal lesions limited to the olfactory mucosa was observed. The lesions, which consisted of olfactory neuron loss and basal cell hyperplasia, were multifocal, bilaterally symmetrical, and had a characteristic rostrocaudal distribution pattern. Regions of the nasal cavity affected included the dorsal medial meatus and the dorsal and medial portions of the ethmoid recess. The no observed adverse effect level for olfactory lesions in this study was 10 ppm. For perspective, the American Conference of Governmental Industrial Hygienists threshold limit value (TLV) recommendation for H2S is currently 10 ppm (proposed revision: 5 ppm), so the concentrations employed in the present study were 3 and 8 times the TLV. These findings suggest that subchronic inhalation exposure to a relatively low level of H2S (30 ppm) can result in olfactory toxicity in rats. However, because of differences in the breathing style and nasal anatomy of rats and humans, additional research is required to determine the significance of these results for human health risk assessment.

Abstract: Zonal organization is one of the characteristic features observed in both main and accessory olfactory systems. In the main olfactory system, most of the odorant receptors are classified into four groups according to their zonal expression patterns in the olfactory epithelium. Each group of odorant receptors is expressed by sensory neurons distributed within one of four circumscribed zones. Olfactory sensory neurons in a given zone of the epithelium project their axons to the glomeruli in a corresponding zone of the main olfactory bulb. Glomeruli in the same zone tend to represent similar odorant receptors having similar tuning specificity to odorants. Vomeronasal receptors (or pheromone receptors) are classified into two groups in the accessory olfactory system. Each group of receptors is expressed by vomeronasal sensory neurons in either the apical or basal zone of the vomeronasal epithelium. Sensory neurons in the apical zone project their axons to the rostral zone of the accessory olfactory bulb and form synaptic connections with mitral tufted cells belonging to the rostral zone. Signals originated from basal zone sensory neurons are sent to mitral tufted cells in the caudal zone of the accessory olfactory bulb. We discuss functional implications of the zonal organization in both main and accessory olfactory systems.

Abstract: The glomerular sheet in the olfactory bulb (OB) provides an olfactory sensory map identifying which odorant receptors (ORs) in the nose are activated by inhaled odorants. How are the glomeruli spatially arranged in the OB? Using OCAM and neuropilin-1 (NP1) as molecular markers for target glomeruli of distinct subsets of olfactory axons, we demonstrate here that glomeruli are parceled into topographically distinct domains. Spatial arrangement of these domains suggests that each OB contains two mirror-image maps of the glomeruli. In situ hybridization shows that the glomeruli representing the same OR are symmetrically arranged; one in a domain in the lateral hemisphere and the other in a corresponding domain in the medial hemisphere of the OB. These results suggest that OB contains two symmetrical OR maps with similar domain organization.

Abstract: Goats have a well-developed vomeronasal (VN) system and exhibit pheromone-induced reproductive facilitation, but there are no reports on the projection pattern of VN neurons in this species. Rodent, guinea pig and opossum accessory olfactory bulbs (AOBs) have been shown to have a segregated pattern of projection of the VN neurons, which express the two alpha-subtypes of the G-protein, namely Gi2 and Go, to the rostral and caudal regions of the AOB, respectively. In this study we investigated the projection pattern of VN nerve terminals by immunocytochemical staining of the goat vomeronasal organ (VNO) and the AOB with antibodies to Gi2 and Go. Gi2-immunoreactivity was found on the luminal surface of the sensory epithelium of the VNO, and in the VN nerve and glomerular layer throughout the AOB. On the other hand, Go-immunoreactivity was not identified in either the VNO or the VN nerve layer of the AOB. These results indicate that the projection pattern of VN neurons from the VNO to the AOB in the goat is considerably different from that in rodents which show a distinct segregated pattern.