Imaging and coding in the olfactory system

Olfaction: Allosteric modulation

New research indicates that the odor-evoked responses of human olfactory receptors can be enhanced via the non-competitive binding of an allosteric modulator. This modulatory mechanism adds an additional layer of complexity to the peripheral encoding of odors.

The behavioral sensitivity of mice to acetate esters

Measures of behavioral sensitivity provide an important guide for choosing the stimulus concentrations used in functional experiments. This information is particularly valuable in the olfactory system as the neural representation of an odorant changes with concentration. This study focuses on acetate esters because they are commonly used to survey neural activity in a variety of olfactory regions, probe the behavioral limits of odor discrimination, and assess odor structure–activity relationships in mice. Despite their frequent use, the relative sensitivity of these odorants in mice is not available. Thus, we assayed the ability of C57BL/6J mice to detect seven different acetates (propyl acetate, butyl acetate, pentyl acetate, hexyl acetate, octyl acetate, isobutyl acetate, and isoamyl acetate) using a head-fixed Go/No-Go operant conditioning assay combined with highly reproducible stimulus delivery. To aid in the accessibility and applicability of our data, we have estimated the vapor-phase concentrations of these odorants in five different solvents using a photoionization detector-based approach. The resulting liquid-/vapor-phase equilibrium equations successfully corrected for behavioral sensitivity differences observed in animals tested with the same odorant in different solvents. We found that mice are most sensitive to isobutyl acetate and least sensitive to propyl acetate. These updated measures of sensitivity will hopefully guide experimenters in choosing appropriate stimulus concentrations for experiments using these odorants.

Olfactory Detection Thresholds for Primary Aliphatic Alcohols in Mice

Probing the neural mechanisms that underlie each sensory system requires the presentation of perceptually appropriate stimulus concentrations. This is particularly relevant in the olfactory system as additional odorant receptors typically respond with increasing stimulus concentrations. Thus, perceptual measures of olfactory sensitivity provide an important guide for functional experiments. This study focuses on aliphatic alcohols because they are commonly used to survey neural activity in a variety of olfactory regions, probe the behavioral limits of odor discrimination, and assess odor-structure activity relationships in mice. However, despite their frequent use, a systematic study of the relative sensitivity of these odorants in mice is not available. Thus, we assayed the ability of C57BL/6J mice to detect a homologous series of primary aliphatic alcohols (1-propanol to 1-heptanol) using a head-fixed Go/No-Go operant conditioning assay combined with highly reproducible stimulus delivery. To aid in the accessibility of our data, we report the animal’s threshold to each odorant according to the 1) ideal gas condition, 2) nonideal gas condition (factoring in the activity of the odorant in the solvent), and 3) the liquid dilution of the odorant in the olfactometer. Of the odorants tested, mice were most sensitive to 1-hexanol and least sensitive to 1-butanol. These updated measures of murine sensitivity will hopefully guide experimenters in choosing appropriate stimulus concentrations for experiments using these odorants.

Similarity and Strength of Glomerular Odor Representations Define a Neural Metric of Sniff-Invariant Discrimination Time

The olfactory environment is first represented by glomerular activity patterns in the olfactory bulb. It remains unclear how these representations intersect with sampling behavior to account for the time required to discriminate odors. Using different chemical classes, we investigate glomerular representations and sniffing behavior during olfactory decision-making. Mice rapidly discriminate odorants and learn to increase sniffing frequency at a fixed latency after trial initiation, independent of odor identity. Relative to the increase in sniffing frequency, monomolecular odorants are discriminated within 10-40 ms, while binary mixtures require an additional 60-70 ms. Intrinsic imaging of glomerular activity in anesthetized and awake mice reveals that Euclidean distance between activity patterns and the time needed for discriminations are anti-correlated. Therefore, the similarity of glomerular patterns and their activation strengths, rather than sampling behavior, define the extent of neuronal processing required for odor discrimination, establishing a neural metric to predict olfactory discrimination time.

An efficient approach for preprocessing data from a large-scale chemical sensor array

In this paper, an artificial olfactory system (Electronic Nose) that mimics the biological olfactory system is introduced. The device consists of a Large-Scale Chemical Sensor Array (16, 384 sensors, made of 24 different kinds of conducting polymer materials) that supplies data to software modules, which perform advanced data processing. In particular, the paper concentrates on the software components consisting, at first, of a crucial step that normalizes the heterogeneous sensor data and reduces their inherent noise. Cleaned data are then supplied as input to a data reduction procedure that extracts the most informative and discriminant directions in order to get an efficient representation in a lower dimensional space where it is possible to more easily find a robust mapping between the observed outputs and the characteristics of the odors in input to the device. Experimental qualitative proofs of the validity of the procedure are given by analyzing data acquired for two different pure analytes and their binary mixtures. Moreover, a classification task is performed in order to explore the possibility of automatically recognizing pure compounds and to predict binary mixture concentrations.

Sense of achievement

Computational techniques developed to predict if odorants will interact with receptors in the olfactory system have achieved a success rate of 70%.

Direction selectivity in the visual system of the zebrafish larva

Neural circuits in the vertebrate retina extract the direction of object motion from visual scenes and convey this information to sensory brain areas, including the optic tectum. It is unclear how computational layers beyond the retina process directional inputs. Recent developmental and functional studies in the zebrafish larva, using minimally invasive optical imaging techniques, indicate that direction selectivity might be a genetically hardwired property of the zebrafish brain. Axons from specific direction-selective (DS) retinal ganglion cells appear to converge on distinct laminae in the superficial tectal neuropil where they serve as inputs to DS postsynaptic neurons of matching specificity. In addition, inhibitory recurrent circuits in the tectum might strengthen the DS response of tectal output neurons. Here we review these recent findings and discuss some controversies with a particular focus on the zebrafish tectum’s role in extracting directional features from moving visual scenes.

Non-redundant coding of aversive odours in the main olfactory pathway

Many species are critically dependent on olfaction for survival. In the main olfactory system of mammals, odours are detected by sensory neurons which express a large repertoire of canonical odorant receptors (ORs) and a much smaller repertoire of Trace Amine-Associated Receptors (TAARs)^1–4. Odours are encoded in a combinatorial fashion across glomeruli in the main olfactory bulb, with each glomerulus corresponding to a different receptor^5–7. The degree to which individual receptor genes contribute to odour perception is unclear. Here we show that genetic deletion of the olfactory TAAR gene family, or even a single TAAR gene, eliminates aversion that mice display to low concentrations of volatile amines and to the odour of predator urine. Our findings identify a role for the TAARs in olfaction, namely in the high-sensitivity detection of innately aversive odours. In addition, our data reveal that aversive amines are represented in a non-redundant fashion, and that individual main olfactory receptor genes can contribute significantly to odour perception.

Synthetic nanoelectronic probes for biological cells and tissues

Research at the interface between nanoscience and biology could yield breakthroughs in fundamental science and lead to revolutionary technologies. In this review, we focus on the interfaces between nanoelectronics and biology. First, we discuss nanoscale field effect transistors (nanoFETs) as probes to study cellular systems; specifically, we describe the development of nanoFETs that are comparable in size to biological nanostructures involved in communication through synthesized nanowires. Second, we review current progress in multiplexed extracellular sensing using planar nanoFET arrays. Third, we describe the designs and implementation of three distinct nanoFETs used to perform the first intracellular electrical recording from single cells. Fourth, we present recent progress in merging electronic and biological systems at the three-dimensional tissue level by use of macro-porous nanoelectronic scaffolds. Finally, we discuss future developments in this research area, unique challenges and opportunities, and the tremendous impact these nanoFET-based technologies might have on biological and medical sciences.

A computational study of odorant transport and deposition in the canine nasal cavity: implications for olfaction

Olfaction begins when an animal draws odorant-laden air into its nasal cavity by sniffing, thus transporting odorant molecules from the external environment to olfactory receptor neurons (ORNs) in the sensory region of the nose. In the dog and other macrosmatic mammals, ORNs are relegated to a recess in the rear of the nasal cavity that is comprised of a labyrinth of scroll-like airways. Evidence from recent studies suggests that nasal airflow patterns enhance olfactory sensitivity by efficiently delivering odorant molecules to the olfactory recess. Here, we simulate odorant transport and deposition during steady inspiration in an anatomically correct reconstructed model of the canine nasal cavity. Our simulations show that highly soluble odorants are deposited in the front of the olfactory recess along the dorsal meatus and nasal septum, whereas moderately soluble and insoluble odorants are more uniformly deposited throughout the entire olfactory recess. These results demonstrate that odorant deposition patterns correspond with the anatomical organization of ORNs in the olfactory recess. Specifically, ORNs that are sensitive to a particular class of odorants are located in regions where that class of odorants is deposited. The correlation of odorant deposition patterns with the anatomical organization of ORNs may partially explain macrosmia in the dog and other keen-scented species.

Alteration of sensory-evoked metabolic and oscillatory activities in the olfactory bulb of GLAST-deficient mice

Astrocytes are key cellular elements in both the tripartite synapse and the neurovascular unit. To fulfill this dual role in synaptic activity and metabolism, they express a panel of receptors and transporters that sense glutamate. Among them, the GLT-1 and GLAST transporters are known to regulate extracellular glutamate concentrations at excitatory synapses and consequently modulate glutamate receptor signaling. These major uptake systems are also involved in energy supply to neurons. However, the functional role of GLAST in concurrent regulation of metabolic and neuronal activity is currently unknown. We took advantage of the attractive structural and functional features of the main olfactory bulb to explore the impact of GLAST on sensory information processing while probing both glutamate uptake and neuronal activity in glomeruli and deeper cellular layers, respectively. Using odor-evoked 2-deoxyglucose imaging and local field potential recordings in GLAST knockout mice, we show in vivo that deletion of GLAST alters both glucose uptake and neuronal oscillations in olfactory bulb networks.

Memory coding in plastic neuronal subpopulations within the amygdala

Specific neuronal subpopulations within specific brain areas are responsible for learning and memory. A fear memory engages a subset of lateral amygdala neurons, but whether multiple contextual fear memories engage the same or different subsets of lateral amygdala neurons remains unclear. Here, we demonstrate the representation of multiple contextual fear memories in the amygdala with cellular and temporal resolution using a large-scale imaging method. Mice were conditioned with a footshock in 2 separate chambers. They were then re-exposed to either the same conditioning chamber twice or 2 different conditioning chambers. The activities of individual neurons related to the re-exposures were determined by the subcellular distribution of Arc/Arg3.1 RNA. Reactivation of different memories activated partially (about 50%) overlapping neurons, whereas reactivation of the same memory activated more overlapping (about 65%) neurons. These findings indicate that lateral amygdala neurons related to different fear memories are partly common, and that a small but significant neuronal population (2.7% of total lateral amygdala neurons) encodes differences in individual fear memories. Moreover, memory retrieval increased the size of the neuronal subpopulation activated during subsequent retrieval. Taken together, our findings indicate that small plastic subsets of neurons encode fear memories from individual contexts.

Olfactory discrimination of complex mixtures of amino acids by the black bullhead Ameiurus melas

On the basis of previous findings of behavioural discrimination of amino acids and on the knowledge of electrophysiology of the catfish (genera Ictalurus and Ameiurus) olfactory organs, behavioural experiments that investigated olfactory discrimination of amino acid mixtures were carried out on the black bullhead Ameiurus melas. Repeated presentations of food-rewarded mixtures released increased swimming activity measured by counting the number of turns >90° within 90 s of stimulus addition. Non-rewarded amino acids and their mixtures released little swimming activity, indicating that A. melas discriminated between the conditioned and the non-conditioned stimuli. Two questions of mixture discrimination were addressed: (1) Are A. melas able to detect components within simple and complex amino acid mixtures? (2) What are the smallest differences between two complex mixtures that A. melas can detect? Three and 13 component mixtures tested were composed primarily of equipotent amino acids [determined by equal electroolfactogram (EOG) amplitude] that contained L-Cys at ×100 the equipotent concentration. Ameiurus melas initially perceived the ternary amino acid mixture as its more stimulatory component alone [i.e. cysteine (Cys)], whereas the conditioned 13 component mixture containing the more stimulatory L-Cys was perceived immediately as different from L-Cys alone. The results indicate that components of ternary mixtures are detectable by A. melas but not those of more complex mixtures. To test for the smallest detectable differences in composition between similar multimixtures, all mixture components were equipotent. Initially, A. melas were unable to discriminate the mixtures of six amino acids from the conditioned mixtures of seven amino acids, whereas they discriminated immediately the mixtures of four and five amino acids from the conditioned mixture. Experience with dissimilar mixtures enabled the A. melas to start discriminating the seven-component conditioned mixture from its six-component counterparts. After fewer than five training trials, A. melas discriminated the mixtures of nine and 10 amino acids from a conditioned mixture of 12 equipotent amino acids; however, irrespective of the number of training trials, A. melas were unable to discriminate the 12 component mixture from its 11 component counterparts.

Odor representations in the mammalian olfactory bulb

A first key step in studying a sensory modality is to define how the brain represents the features of the sensory stimulus. This has proven to be a challenge in olfaction, where even the stimulus features have been a matter of considerable debate. In this review, we focus on olfactory representations in the first stage of the olfactory pathway, the olfactory bulb (OB). We examine the diverging viewpoints on spatially organized versus distributed representations. We then consider how odor sampling through respiration is a key part of the odorant code. Finally, we ask how the bulb handles the challenging task of representing mixtures. We suggest that current evidence points toward a representation that is spatially organized at the inputs but later distributed, with the spatial organization not being used for much computation. Nevertheless, this is a simple representation that effectively represents multiple individual odorants, as well as odor mixtures.

Synaptic inhibition in the olfactory bulb accelerates odor discrimination in mice

Local inhibitory circuits are thought to shape neuronal information processing in the central nervous system, but it remains unclear how specific properties of inhibitory neuronal interactions translate into behavioral performance. In the olfactory bulb, inhibition of mitral/tufted cells via granule cells may contribute to odor discrimination behavior by refining neuronal representations of odors. Here we show that selective deletion of the AMPA receptor subunit GluA2 in granule cells boosted synaptic Ca(2+) influx, increasing inhibition of mitral cells. On a behavioral level, discrimination of similar odor mixtures was accelerated while leaving learning and memory unaffected. In contrast, selective removal of NMDA receptors in granule cells slowed discrimination of similar odors. Our results demonstrate that inhibition of mitral cells controlled by granule cell glutamate receptors results in fast and accurate discrimination of similar odors. Thus, spatiotemporally defined molecular perturbations of olfactory bulb granule cells directly link stimulus similarity, neuronal processing time, and discrimination behavior to synaptic inhibition.

Histological properties of the nasal cavity and olfactory bulb of the Japanese jungle crow Corvus macrorhynchos

The nasal cavity and olfactory bulb (OB) of the Japanese jungle crow (Corvus macrorhynchos) were studied using computed tomography (CT) and histochemical staining. The nasal septum divided the nasal cavity in half. The anterior and maxillary conchae were present on both sides of the nasal cavity, but the posterior concha was indistinct. A small OB was present on the ventral surface of the periphery of the cerebrum. The OB-brain ratio--the ratio of the size of the OB to that of the cerebral hemisphere--was 6.13. The olfactory nerve bundles projected independently to the OB, which appeared fused on gross examination. Histochemical analysis confirmed the fusion of all OB layers. Using a neural tracer, we found that the olfactory nerve bundles independently projected to the olfactory nerve layer (ONL) and glomerular layer (GL) of the left and right halves of the fused OB. Only 4 of 21 lectins bound to the ONL and GL. Thus, compared with mammals and other birds, the jungle crow may have a poorly developed olfactory system and an inferior sense of olfaction. However, it has been contended recently that the olfactory abilities of birds cannot be judged from anatomical findings alone. Our results indicate that the olfactory system of the jungle crow is an interesting research model to evaluate the development and functions of vertebrate olfactory systems.

Manganese enhanced MRI reveals functional circuitry in response to odorant stimuli

To investigate the circuitry involved in detecting odorants in the rodent brain, we developed a method using manganese-enhanced MRI (MEMRI) to map the flow of neural information from the olfactory sensory neurons (OSNs) to the central layers of the olfactory bulb. Studies have shown that Mn(2+) enters active neurons and is transported anterogradely to axon terminals where it can cross synapses to functionally trace neural networks. Thus, by delivering MnCl(2) directly into the nasal cavity of mice and then exposing them to defined odorants, Mn(2+) is preferentially taken up by activated OSNs. Using the time course of the MRI signal, we generated maps of Mn(2+) accumulation in the olfactory bulb for both glomerular and mitral cell layers. Results demonstrated that overlapping yet distinct enhancement patterns were produced by exposure to either octanal, acetophenone, or carvone. Notably, areas of Mn(2+) accumulation in the mitral cell layer were similar to those in the glomerular layer consistent with neural information that passes from specific OSNs to specific mitral cells. Finally, by correlating specific Mn(2+) signal peaks to genetically labeled glomeruli that are known to be activated by the odorant octanal, we show that MEMRI maps can be resolved at the level of individual glomeruli.

Olfactometry with mice

A computer-controlled, multiple-channel odor generator, operant test chamber, and discrimination training procedure for mice are described. The odor generator allows controlled presentation of any one or combinations of eight odors to an odor-sampling port that contains a liquid reinforcement delivery tube that also serves to detect responses. A modified discrete trial operant conditioning procedure provides measures of both response accuracy and response rate during anticipation of stimulus delivery and in the presence of two different odor stimuli. Results from numerous experiments demonstrate that, with these methods, mice reliably and rapidly acquire odor detection and a variety of odor-discrimination tasks, and that response rate in the presence of the odor reflects the incentive motivation of stimuli that are or are not associated with reward.

The role of chemical communication in mate choice

Chemical signals are omnipresent in sexual communication in the vast majority of living organisms. The traditional paradigm was that their main purpose in sexual behaviour was to coordinate mate and species recognition and thus pheromones were conserved in structure and function. In recent years, this view has been challenged by theoretical analyses on the evolution of pheromones and empirical reports of mate choice based on chemical signals. The ability to measure precisely the quantity and quality of chemicals emitted by single individuals has also revealed considerable individual variation in chemical composition and release rates, and there is mounting evidence that prospecting mates respond to this variation. Here, we review the evidence for pheromones as indicators of mate quality and examine the extent of their use in individual mate assessment. We begin by briefly defining the levels of mate choice--species recognition, mate recognition and mate assessment. We then explore the degree to which pheromones satisfy the key criteria necessary for their evolution and maintenance as cues in mate assessment; that is, they should exhibit variation across individuals within a sex and species; they should honestly reflect an individual's quality and thus be costly to produce and/or maintain; they should display relatively high levels of heritability. There is now substantial empirical evidence that pheromones can satisfy all these criteria and, while measurements of the actual metabolic cost of pheromone production remain to some degree lacking, trade-offs between pheromone production and various fitness-related characters such as growth rate, immunocompetence and longevity have been reported for a range of species. In the penultimate section, we outline the growing number of studies where the consequences of chemical-based mate assessment have been investigated, specifically focussing on the reported direct and genetic benefits accrued by the receiver. Finally, we highlight potential areas for future research and in particular emphasise the need for interdisciplinary research that combines exploration of chemical, physiological and behavioural processes to further our understanding of the role of chemical cues in mate assessment.

Neurogenic correlates of an olfactory discrimination task in the adult olfactory bulb

In the main olfactory bulb, stimuli are coded within the spatio-temporal pattern of mitral cells' activity. Granule cells are interneurons that shape the mitral cells' activity, and are continuously generated in the adult main olfactory bulb. However, the role of granule cell renewal remains elusive. We show here that an associative olfactory discrimination task reduces the survival of newborn neurons. However, when the olfactory task involves perceptually related odorants, the learning process is slower and does not induce such a reduction in the number of new neurons. Mapping newborn cells within the granule cell layer of the main olfactory bulb reveals a clustered distribution that evolves with learning as a function of odorant similarity and partly overlaps with the immediate-early gene Zif268 expression pattern. These data provide insight into the functional mechanisms underlying olfactory discrimination learning, and promote the importance of neurogenesis as a cellular basis for the restructuring of odor images in the main olfactory bulb.

Learning-induced oscillatory activities correlated to odour recognition: a network activity

In trained behaving rats, the expression of a prominent beta oscillatory activity in the olfactory system was previously identified as a correlate of odour recognition. The aim of the present study was to assess the putative role of a functional coupling between the olfactory bulb (OB) and higher structures in this activity. We performed a unilateral inactivation of the medial part of the olfactory peduncle by lidocaine infusion. Inactivation deprived the OB from most of its centrifugal afferences, including feedback connections from the piriform cortex (PC) while sparing the ascending fibres from the OB to higher cortical structures. This treatment reduced the amplitude of odour-induced oscillatory beta responses both in OB and PC. In parallel, gamma activity classically observed in these two structures during spontaneous activity displayed a strong enhancement. Results suggest that odour-induced oscillatory response could be the emergent feature of an olfactory functional network set up through learning.

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

The past 15 years have seen significant advances in the study of olfaction, with particular emphasis on elucidating the molecular building blocks of the sensory process. However, much of the systems-level organization of olfaction remains unexplored. Here, we provide an overview at this level, highlighting results obtained from studying humans, whom we think provide an underutilized, yet critical, animal model for olfaction.

Spatio-temporal Ca2+ dynamics of moth olfactory projection neurones

We studied the Ca2+ dynamics of odour-evoked glomerular patterns in the antennal lobe of the moth Spodoptera littoralis using optical imaging. Here we selectively stained a large population of antennal lobe output neurones, projection neurones, by retrograde filling with FURA-dextran from the inner antennocerebral tract in the protocerebrum. Different plant-associated odorants evoked distributed patterns of activated glomeruli that were odour dependent and repeatable. These patterns were, however, dynamic during the period of odour exposure. Temporal responses differed across glomeruli and were stimulus dependent. Next we examined how the correlations between patterns evoked by different odorants changed with time. Initially, responses to structurally similar compounds were highly correlated, whereas responses to structurally different compounds differed. Within the period of odour exposure (1 s) we found a significant reduction in similarity of responses evoked by different odours, irrespective of initial similarity, whereas trial-to-trial correlations remained high. Our results suggested an ability for coarse classification at the initial encounter with an odour source. With time, however, the discrimination ability increases and structurally similar odours can be distinguished.

3D atlas describing the ontogenic evolution of the primary olfactory projections in the olfactory bulb of Xenopus laevis

The adult Xenopus presents the unique capability to smell odors both in water and air thanks to two different olfactory pathways. Nevertheless, the tadpole can initially perceive only water-borne odorants, as the olfactory receptor neurons (ORN) that will detect air-borne odorants develop later. Such a phenomenon requires major reorganization processes. Here we focused on the precise description of the neuroanatomical modifications occurring in the olfactory bulb (OB) of the tadpole throughout metamorphosis. Using both carbocyanine dyes and lectin staining, we investigated the evolution of ORN projection patterns into the OB from Stages 47 to 66, thus covering the period of time when all the modifications take place. Although our results confirm previous works (Reiss and Burd [1997] Semin Cell Dev Biol 8:171-179), we showed for the first time that the main olfactory bulb (MOB) is subdivided into seven zones at Stage 47 plus the accessory olfactory bulb (AOB). These seven zones receive fibers dedicated to aquatic olfaction ("aquatic fibers") and are conserved until Stage 66. At Stage 48 the first fibers dedicated to the aerial olfaction constitute a new dorsomedial zone that grows steadily, pushing the seven original zones ventrolaterally. Only the part of the OB receiving aquatic fibers is fragmented, reminiscent of the organization described in fish. This raises the question of whether such an organization in zones constitutes a plesiomorphy or is linked to aquatic olfaction. We generated a 3D atlas at several stages which are representative of the reorganization process. This will be a useful tool for future studies of development and function.

Stereotyped odor-evoked activity in the mushroom body of Drosophila revealed by green fluorescent protein-based Ca2+ imaging

To study the representation of olfactory information in higher brain centers, we expressed a green fluorescent protein-based Ca2+ sensor, G-CaMP, in the Drosophila mushroom body (MB). Using two-photon microscopy, we imaged odor-evoked G-CaMP fluorescence transients in MB neurons [Kenyon cells (KCs)] with single-cell resolution. Odors produced large fluorescence transients in a subset of KC somata and in restricted regions of the calyx, the neuropil of the MB. In different KCs, odor-evoked fluorescence transients showed diverse changes with odor concentration: in some KCs, fluorescence transients were evoked by an odor at concentrations spanning several orders of magnitude, whereas in others only at a narrow concentration range. Different odors produced fluorescence transients in different subsets of KCs. The spatial distributions of KCs showing fluorescence transients evoked by a given odor were similar across individuals. For some odors, individual KCs with fluorescence transients evoked by a particular odor could be found in similar locations in different flies with spatial precisions on the order of the size of KC somata. These results indicate that odor-evoked activity can have remarkable spatial specificity in the MB.

Information processing in the mammalian olfactory system

Recently, modern neuroscience has made considerable progress in understanding how the brain perceives, discriminates, and recognizes odorant molecules. This growing knowledge took over when the sense of smell was no longer considered only as a matter for poetry or the perfume industry. Over the last decades, chemical senses captured the attention of scientists who started to investigate the different stages of olfactory pathways. Distinct fields such as genetic, biochemistry, cellular biology, neurophysiology, and behavior have contributed to provide a picture of how odor information is processed in the olfactory system as it moves from the periphery to higher areas of the brain. So far, the combination of these approaches has been most effective at the cellular level, but there are already signs, and even greater hope, that the same is gradually happening at the systems level. This review summarizes the current ideas concerning the cellular mechanisms and organizational strategies used by the olfactory system to process olfactory information. We present findings that exemplified the high degree of olfactory plasticity, with special emphasis on the first central relay of the olfactory system. Recent observations supporting the necessity of such plasticity for adult brain functions are also discussed. Due to space constraints, this review focuses mainly on the olfactory systems of vertebrates, and primarily those of mammals.

Maintaining accuracy at the expense of speed: stimulus similarity defines odor discrimination time in mice

Odor discrimination times and their dependence on stimulus similarity were evaluated to test temporal and spatial models of odor representation in mice. In a go/no-go operant conditioning paradigm, discrimination accuracy and time were determined for simple monomolecular odors and binary mixtures of odors. Mice discriminated simple odors with an accuracy exceeding 95%. Binary mixtures evoking highly overlapping spatiotemporal patterns of activity in the olfactory bulb were discriminated equally well. However, while discriminating simple odors in less than 200 ms, mice required 70-100 ms more time to discriminate highly similar binary mixtures. We conclude that odor discrimination in mice is fast and stimulus dependent. Thus, the underlying neuronal mechanisms act on a fast timescale, requiring only a brief epoch of odor-specific spatiotemporal representations to achieve rapid discrimination of dissimilar odors. The fine discrimination of highly similar stimuli, however, requires temporal integration of activity, suggesting a tradeoff between accuracy and speed.

A biophysical model of vertebrate olfactory epithelium and bulb exhibiting gap junction dependent odor-evoked spatiotemporal patterns of activity

This work describes a biophysical model of the initial stages of vertebrate olfactory system containing structures representing the olfactory epithelium and bulb. Its main novelty is the introduction of gap junctions connecting neurons both in the epithelium and bulb, and of biologically detailed dendrodendritic synapses between granule and mitral cells in the bulb. The model was used to simulate the effect of an odor presentation on the neural activity pattern in the epithelium and bulb. During the time for which an odor is presented with a constant concentration, there are spatiotemporal patterns in the epithelium and bulb generated by the couplings due to the gap junctions and/or dendrodendritic synapses. A study varying the strength of the gap junction coupling shows that the spatiotemporal patterns, both in the epithelium and bulb, are dependent of the coupling strength. It is also shown that the olfactory bulb's spatiotemporal pattern depends on the existence of the dendrodendritic connections between mitral and granule cells. If these spatiotemporal patterns really exist in the early processing stages of the olfactory system they may be used for odor coding and the gap junctions and dendrodendritic synapses might have a role on it.

Interplay between local GABAergic interneurons and relay neurons generates gamma oscillations in the rat olfactory bulb

Olfactory stimuli have been known for a long time to elicit oscillations in olfactory brain areas. In the olfactory bulb (OB), odors trigger synchronous oscillatory activity that is believed to arise from the coherent and rhythmic discharges of large numbers of neurons. These oscillations are known to take part in encoding of sensory information before their transfer to higher subcortical and cortical areas. To characterize the cellular mechanisms underlying gamma (30-80 Hz) local field potential (LFP) oscillations, we simultaneously recorded multiunit discharges, intracellular responses, and LFP in rat OB slices. We showed that a single and brief electrical stimulation of olfactory nerve elicited LFP oscillations in the mitral cell body layer lasting >1 sec. Both action potentials and subthreshold oscillations of mitral/tufted cells, the bulbar output neurons, were precisely synchronized with LFP oscillations. This synchronization arises from the interaction between output neurons and granule cells, the main population of local circuit inhibitory interneurons, through dendrodendritic synapses. Interestingly enough, the synchronization exerted by reciprocal synaptic interactions did not require action potentials initiated in granule cell somata. Finally, local application of a GABA(A) receptor antagonist at the mitral cell and external plexiform layers confirmed the exclusive role of the granule cell reciprocal synapses in generating the evoked oscillations. We concluded that interneurons located in the granule cell layer generate synaptic activity capable of synchronizing activity of output neurons by interacting with both their subthreshold and spiking activity.

Olfactory Learning

The olfactory nervous systems of insects and mammals exhibit many similarities, suggesting that the mechanisms for olfactory learning may be shared. Neural correlates of olfactory memory are distributed among many neurons within the olfactory nervous system. Perceptual olfactory learning may be mediated by alterations in the odorant receptive fields of second and/or third order olfactory neurons, and by increases in the coherency of activity among ensembles of second order neurons. Operant olfactory conditioning is associated with an increase in the coherent population activity of these neurons. Olfactory classical conditioning increases the odor responsiveness and synaptic activity of second and perhaps third order neurons. Operant and classical conditioning both produce an increased responsiveness to conditioned odors in neurons of the basolateral amygdala. Molecular genetic studies of olfactory learning in Drosophila have revealed numerous molecules that function within the third order olfactory neurons for normal olfactory learning.

Local neurons play key roles in the mammalian olfactory bulb

Over the past few decades, research exploring how the brain perceives, discriminates, and recognizes odorant molecules has received a growing interest. Today, olfaction is no longer considered a matter of poetry. Chemical senses entered the biological era when an increasing number of scientists started to elucidate the early stages of the olfactory pathway. A combination of genetic, biochemical, cellular, electrophysiological and behavioral methods has provided a picture of how odor information is processed in the olfactory system as it moves from the periphery to higher areas of the brain. Our group is exploring the physiology of the main olfactory bulb, the first processing relay in the mammalian brain. From different electrophysiological approaches, we are attempting to understand the cellular rules that contribute to the synaptic transmission and plasticity at this central relay. How olfactory sensory inputs, originating from the olfactory epithelium located in the nasal cavity, are encoded in the main olfactory bulb remains a crucial question for understanding odor processing. More importantly, the persistence of a high level of neurogenesis continuously supplying the adult olfactory bulb with newborn local neurons provides an attractive model to investigate how basic olfactory functions are maintained when a large proportion of local neurons are continuously renewed. For this purpose, we summarize the current ideas concerning the molecular mechanisms and organizational strategies used by the olfactory system to encode and process information in the main olfactory bulb. We discuss the degree of sensitivity of the bulbar neuronal network activity to the persistence of this high level of neurogenesis that is modulated by sensory experience. Finally, it is worth mentioning that analyzing the molecular mechanisms and organizational strategies used by the olfactory system to transduce, encode, and process odorant information in the olfactory bulb should aid in understanding the general neural mechanisms involved in both sensory perception and memory. Due to space constraints, this review focuses exclusively on the olfactory systems of vertebrates and primarily those of mammals.

Olfactory receptor gene expression in tiger salamander olfactory epithelium

Physiological studies of odor-elicited responses from the olfactory epithelium and bulb in the tiger salamander, Ambystoma tigrinum, have elucidated a number of features of olfactory coding that appear to be conserved across several vertebrate species. This animal model has provided an accessible in vivo system for observing individual and ensemble olfactory responses to odorant stimulation using biochemical, neurophysiological, and behavioral assays. In this paper we have complemented these studies by characterizing 35 candidate odorant receptor genes. These receptor sequences are similar to those of the large families of olfactory receptors found in mammals and fish. In situ hybridization, using RNA probes to 20 of these sequences, demonstrates differential distributions of labeled cells across the extent and within the depth of the olfactory epithelium. The distributions of cells labeled with probes to different receptors show spatially restricted patterns that are generally localized to different degrees in medial-lateral and anterior-posterior directions. The patterns of receptor expression in the ventral olfactory epithelium (OE) are mirrored in the dorsal OE. We present a hypothesis as to how the sensory neuron populations expressing different receptor types responding to a particular odorant may relate to the distribution patterns of epithelial and bulbar responses previously characterized using single-unit and voltage-sensitive dye recording methods.

Altered Representation of the Spatial Code for Odors after Olfactory Classical Conditioning

In the olfactory bulb of vertebrates or the homologous antennal lobe of insects, odor quality is represented by stereotyped patterns of neuronal activity that are reproducible within and between individuals. Using optical imaging to monitor synaptic activity in the Drosophila antennal lobe, we show here that classical conditioning rapidly alters the neural code representing the learned odor by recruiting new synapses into that code. Pairing of an odor-conditioned stimulus with an electric shock-unconditioned stimulus causes new projection neuron synapses to respond to the odor along with those normally activated prior to conditioning. Different odors recruit different groups of projection neurons into the spatial code. The change in odor representation after conditioning appears to be intrinsic to projection neurons. The rapid recruitment by conditioning of new synapses into the representation of sensory information may be a general mechanism underlying many forms of short-term memory.

Learning modulation of odor-induced oscillatory responses in the rat olfactory bulb: a correlate of odor recognition?

In the first relay of information processing, the olfactory bulb (OB), odors are known to generate specific spatial patterns of activity. Recently, in freely behaving rats, we demonstrated that learning modulated oscillatory activity in local field potential (LFP), in response to odors, in both beta (15-40 Hz) and gamma (60-90 Hz) bands. The present study further characterized this odor-induced oscillatory activity with emphasis on its spatiotemporal distribution over the olfactory bulb and on its relationship with improvement of behavioral performances along training. For that purpose, LFPs were simultaneously recorded from four locations in the OB in freely moving rats performing an olfactory discrimination task. Electrodes were chronically implanted near relay neurons in the mitral cell body layer. Time-frequency methods were used to extract signal characteristics (amplitude, frequency, and time course) in the two frequency bands. Before training, odor presentation produced, on each site, a power decrease in gamma oscillations and a weak but significant increase in power of beta oscillations (approximately 25 Hz). When the training was achieved, these two phenomena were amplified. Interestingly, the beta oscillatory response showed several significant differences between the anterodorsal and posteroventral regions of the OB. In addition, clear-cut beta responses occurred in the signal as soon as animals began to master the task. As a whole, our results point to the possible functional importance of beta oscillatory activity in the mammalian OB, particularly in the context of olfactory learning.

Functional glutamatergic and glycinergic inputs to several superior olivary nuclei of the rat revealed by optical imaging

Superior olivary complex (SOC) neurons receive excitatory and inhibitory inputs from both ears. We determined the nature of such inputs to the main SOC nuclei with an optical imaging system. To do so, brainstem slices of postnatal (P) rats (P3-13) were treated with the fast voltage-sensitive dye RH795, and ipsilateral and contralateral SOC inputs were activated electrically. Optical signals, equivalent to membrane potential changes, were detected by a 464-photodiode array. The signals consisted mostly of two components which were identified as pre- and postsynaptic potentials in experiments with Ca2+-free solutions. They correlated with morphological structures, i.e. the presynaptic components were prominent in neuropil regions whereas the postsynaptic components dominated in somata regions. Postsynaptic components were distinguished pharmacologically with the glycine receptor blocker strychnine and the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor blocker 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Concerning the lateral superior olive, we confirmed the known glutamatergic inputs from the ipsilateral side and the glycinergic inputs from the ipsilateral and contralateral sides. Furthermore, we identified a CNQX-sensitive input from the contralateral side. In the medial superior olive, we corroborated the glutamatergic and glycinergic inputs from the ipsilateral and contralateral sides. Both ipsi- and contralaterally, the glutamatergic input was more pronounced than the glycinergic input. In the superior paraolivary nucleus, we also identified ipsilateral and contralateral inputs. Besides the known glycinergic input from the contralateral side, we found a novel glycinergic input from the ipsilateral side and identified CNQX-sensitive inputs from the contralateral and ipsilateral sides. The latter was very weak and appeared only in 30% of the experiments. The data show the feasibility of identifying functional inputs to the SOC with voltage-sensitive dye recordings.

Olfactory processing in a changing brain

The perception of odorant molecules provides the essential information that allows animals to explore their surrounding. We describe here how the external world of scents may sculpt the activity of the first central relay of the olfactory system, i.e., the olfactory bulb. This structure is one of the few brain areas to continuously replace one of its neuronal populations: the local GABAergic interneurons. How the newly generated neurons integrate into a pre-existing neural network and how basic olfactory functions are maintained when a large percentage of neurons are subjected to continuous renewal, are important questions that have recently received new insights. Furthermore, we shall see how the adult neurogenesis is specifically subjected to experience-dependent modulation. In particular, we shall describe the sensitivity of the bulbar neurogenesis to the activity level of sensory inputs from the olfactory epithelium and, in turn, how this neurogenesis may adjust the neural network functioning to optimize odor information processing. Finally, we shall discuss the behavioral consequences of the bulbar neurogenesis and how it may be appropriate for the sense of smell. By maintaining a constitutive turnover of bulbar interneurons subjected to modulation by environmental cues, we propose that adult ongoing neurogenesis in the olfactory bulb is associated with improved olfactory memory. These recent findings not only provide new fuel for the molecular and cellular bases of sensory perception but should also shed light onto cellular bases of learning and memory.

Functional brain imaging of olfactory processing in monkeys

As a step toward bridging the gap between human and animal studies of olfactory brain systems, we report results from an fMRI study of olfaction in squirrel monkeys. High-resolution fMRI images at 3 T with 1.25 x 1.25 x 1.2 mm(3) voxels were obtained covering the whole brain using an 8-cm-diameter birdcage coil and a gradient-echo spiral pulse sequence. Data were acquired from six sedated adult males using a standard block design. All fMRI data were spatially normalized to a common template and analyzed at the individual and group levels with statistical parametric and nonparametric methods. Robust odorant-induced activations were detected in several brain regions previously implicated in conscious human olfactory processing, including the orbitofrontal cortex, cerebellum, and piriform cortex. Consistent with human data, no stimulus intensity effects were observed in any of these regions. Average signal changes in these regions exceeded 0.6%, more than three times the expected signal change based on human fMRI studies of olfaction adjusting for differences in voxel size. These results demonstrate the feasibility of studying olfaction in sedated monkeys with imaging techniques commonly used at 3 T in humans and help promote direct comparisons between humans and nonhuman primates. Our findings, for example, provide novel support for the hypothesis that the cerebellum is involved in sensory acquisition. More broadly, this study suggests that olfactory processing in sedated monkeys and nonsedated humans shares similar neural substrates both within and beyond the primary olfactory system.

Trafficking prerogatives of olfactory receptors

Olfactory receptors lead lives of exclusivity and privilege, the monarchs of fiefdoms organized solely to carry out their instructions. Each olfactory sensory neuron expresses one allele of one of approximately 1000 olfactory receptor genes. It is thought that olfactory receptor diversity is critical for the ability of animals to detect many thousands of odorants, but supporting functional evidence has been difficult to obtain because olfactory receptors expressed in heterologous cells are typically retained in the endoplasmic reticulum. The membrane trafficking entitlements enjoyed by olfactory receptors appear to be available only in mature olfactory sensory neurons. Evidence is accumulating that cell-type-specific accessory proteins regulate first the exit of olfactory receptors from the endoplasmic reticulum, and then the trafficking of olfactory receptors from post-Golgi compartments to the plasma membrane of the olfactory cilia where the receptors gain access to odorants. Critical olfactory receptor accessory proteins are known only in the nematode Caenorhabditis elegans, where the absence of a novel protein called ODR-4 or a clathrin adaptor, UNC-101, interferes with proper trafficking. Similar functional specificity also occurs in a parallel chemosensory system, the mammalian vomeronasal organ. Trafficking of the V2R type of vomeronasal receptors is mediated by a vomeronasal-specific family of major histocompatibility complex proteins. Removal of olfactory receptors from the plasma membrane may be regulated in a more conventional fashion because odor stimulation has been linked to receptor phosphorylation, to the function of G-protein coupled receptor kinase 3, and to an increase in vesicles retrieved from the plasma membrane.

Blocking sensory inputs to identified antennal glomeruli selectively modifies odorant perception in Drosophila

Neural coding of sensory input is a major unsolved issue in neuroscience. Current experimental methods rely on neural activity recording or visualization following sensory stimulation. Most of them, however, do not include behavioral correlates on the actual perception by the animal. We present a novel approach to address olfaction and coding in adult Drosophila. Sensory input was selectively blocked in two subsets of sensory neurons that project to different, albeit overlapping, groups of central targets, by means of tetanus toxin expressed under the control of the yeast transcription factor Gal4. Glomeruli DL1, DL2, VM1, and VM4 were tested following stimulation with benzaldehyde, ethyl acetate, propionic acid, butanol, or acetone at various concentrations. The behavioral response was found to be modified in an odorant-specific and a concentration-dependent manner. Sensory input to DL2 and, to a minor extent, VM1 and/or VM4, appear to be required for benzaldehyde perception, while acetone is processed through DL1. None of these glomeruli, however, seem necessary for butanol perception. In addition, sexual differences were observed for some stimuli. These results demonstrate the behavioral relevance of odor representation as maps of glomerular activity generated in the antennal lobes following specific sensory input. The strategy used here should be useful to characterize olfactory coding, as new and selective Gal4 lines become available.

Enhancing vapor sensor discrimination by mimicking a canine nasal cavity flow environment

This paper discusses the use of flow dynamics in a model nasal cavity as a novel way to expand the information available from a fiber optic vapor sensor. Nasal cavities are known to play an important role in vertebrate olfaction; however, they have not previously been incorporated into gas-phase sensor technology. By exposing identical sensors to different flow environments, we found that measurements of the combined sensor responses provided increased discrimination of a series of alcohols as compared to measurements obtained from a single sensor at the anterior of the cavity. This methodology is potentially useful for improving the discriminatory power of sensors employed in "artificial noses" and could be used to model the effects of flow dynamics in vertebrate olfaction.

Olfactory activation patterns in the antennal lobe of the sphinx moth, Manduca sexta

The sphinx moth Manduca sexta is a well-studied insect with regard to central olfactory functions. Until now, the innervation patterns of olfactory receptor neurons into the array of olfactory glomeruli in the antennal lobe have, however, been unclear. Using optical imaging to visualize calcium dynamics within the antennal lobe we demonstrate specific patterns elicited by sex pheromone components and plant-derived odours. These patterns mainly reflect receptor neuron activity. Within the male-specific macroglomerular complex the two major pheromone components evoke stereotyped activity in either of two macroglomerular complex glomeruli. Based on previous knowledge of output neuron specificity, our results suggest a matching of information between input and output in the macroglomerular complex. Plant odours evoked activity in the sexually isomorphic glomeruli. Two major results were obtained: (1). terpenes and aromatic compounds activate different clusters of glomeruli with only minor overlapping, and (2). the position of certain key glomeruli is fixed in both males and females, which suggests that host-plant related odorants are processed in a similar way in both sexes.

Olfactory learning modifies the expression of odour-induced oscillatory responses in the gamma (60-90 Hz) and beta (15-40 Hz) bands in the rat olfactory bulb

This study addressed the question of the possible functional relevance of two different oscillatory activities, beta and gamma (15-40 and 60-90 Hz, respectively) for perception and memory processes in olfactory areas of mammals. Local field potentials were recorded near relay olfactory bulb neurons while rats performed an olfactory discrimination task. Signals reflected the mass activity from this region and characteristics of oscillatory activities were used as an index of local synchrony. Beta and gamma oscillatory activities were quantified by time-frequency methods before during and after odour sampling. In rats early in their training, olfactory sampling was associated with a significant decrease in power in the gamma band in parallel with a weak but significant increase in the beta band (centred on 27 Hz). Several days later, in well-trained rats, the gamma oscillatory depression was significantly enhanced both in duration and amplitude. It appeared within the 500 ms time period preceding odour onset and was further reduced during the odour period. Concurrently the beta oscillatory response (now centred on 24 Hz) during odour sampling was amplified by a twofold factor. The beta band response was modulated according to the chemical nature of the stimuli and rat's behavioural response. This study showed for the first time that odour sampling in behaving animals is associated with a clear shift in the olfactory bulb neuronal activity from a gamma to a beta oscillatory regime. Moreover, the data stress the importance of studying the odour-induced beta activity and its relation to perception and memory.

Neuronal representation of odourants in the olfactory bulb of Xenopus laevis tadpoles

When an odourant enters the nose, olfactory receptor neurons (ORNs) convey information about it to the olfactory bulb (OB), where this information is processed and where the first central representations of the odourant are generated. In this paper we show how odourants are represented by ensembles of OB neurons, in particular mitral cells (MCs) which are the output neurons of the OB. We were able to demonstrate for the first time that the intracellular calcium concentrations ([Ca2+]i) in the somata of these neurons undergo specific changes and that different stimuli are represented by different neuronal [Ca2+]i patterns. The similarity of patterns was assessed by cross-correlation analysis. We further show that noradrenaline (NA), which is reported to be involved in olfactory memory formation and to modulate synaptic transmission at dendrodendritic synapses in the OB, profoundly changes the representation of odourants at the level of MCs.

Transmission of olfactory information between three populations of neurons in the antennal lobe of the fly

Three classes of neurons form synapses in the antennal lobe of Drosophila, the insect counterpart of the vertebrate olfactory bulb: olfactory receptor neurons, projection neurons, and inhibitory local interneurons. We have targeted a genetically encoded optical reporter of synaptic transmission to each of these classes of neurons and visualized population responses to natural odors. The activation of an odor-specific ensemble of olfactory receptor neurons leads to the activation of a symmetric ensemble of projection neurons across the glomerular synaptic relay. Virtually all excited glomeruli receive inhibitory input from local interneurons. The extent, odor specificity, and partly interglomerular origin of this input suggest that inhibitory circuits assemble combinatorially during odor presentations. These circuits may serve as dynamic templates that extract higher order features from afferent activity patterns.

Maturation and death of adult-born olfactory bulb granule neurons: role of olfaction

Young neurons born in the subventricular zone (SVZ) of adult mice migrate to the olfactory bulb (OB) where they differentiate into granule cells (GCs) and periglomerular interneurons. Using retroviral labeling of precursors in the SVZ, we describe five stages and the timing for the maturation of newly formed GCs: (1) tangentially migrating neuroblasts (days 2-7); (2) radially migrating young neurons (days 5-7); (3) GCs with a simple unbranched dendrite that does not extend beyond the mitral cell layer (days 9-13); (4) GCs with a nonspiny branched dendrite in the external plexiform layer (days 11-22); and (5) mature GCs (days 15-30). Using [3H]thymidine, we show that the maximum number of labeled GCs is observed around day 15 after injection. Interestingly, between days 15 and 45 after birth, soon after the cells developed spines, the number of [3H]thymidine-labeled GCs declined by 50%. Using anosmic mice, we found that sensory input was critical for the survival of GCs from day 15 to 45 after labeling. However, the number and morphology of 15-d-old cells in the granule cell layer was similar in anosmic and wild-type mice. We infer that the lack of activity did not have an effect on the generation, migration, and early differentiation of granule cells. Soon after young GCs matured, and presumably became synaptically connected, their survival depended on the level of activity that they received. This selection mechanism might allow the construction of specific OB circuits based on olfactory experience and suggests possible functions of OB cell replacement.

On the scents of smell in the salamander

Our sense of smell is based on a remarkable chemical-detection system that possesses high sensitivity, broad discriminability and plastic, yet stable, function. Understanding how olfactory stimuli translate into perception is a problem of daunting complexity. How do odour-coding events in single cells correlate with emergent properties from the ensemble, and with behaviour? For comprehensive descriptions of neural function, analysis must extend from examination of how elemental principles relate to the function of the whole. The tiger salamander has long been used as an experimental model in studies of olfaction, enabling general questions about olfactory function to be approached.