Olfactory receptor neuron responses coding for rapid odour sampling

A mechanosensory feedback that uncouples external and self-generated sensory responses in the olfactory cortex

Sampling behaviors have sensory consequences that can hinder perceptual stability. In olfaction, sniffing affects early odor encoding, mimicking a sudden change in odor concentration. We examined how the inhalation speed affects the representation of odor concentration in the main olfactory cortex. Neurons combine the odor input with a global top-down signal preceding the sniff and a mechanosensory feedback generated by the air passage through the nose during inhalation. Still, the population representation of concentration is remarkably sniff invariant. This is because the mechanosensory and olfactory responses are uncorrelated within and across neurons. Thus, faster odor inhalation and an increase in concentration change the cortical activity pattern in distinct ways. This encoding strategy affords tolerance to potential concentration fluctuations caused by varying inhalation speeds. Since mechanosensory reafferences are widespread across sensory systems, the coding scheme described here may be a canonical strategy to mitigate the sensory ambiguities caused by movements.

Odorant-responsive biological receptors and electronic noses for volatile organic compounds with aldehyde for human health and diseases: A perspective review

Volatile organic compounds (VOCs) are gaseous chemicals found in ambient air and exhaled breath. In particular, highly reactive aldehydes are frequently found in polluted air and have been linked to various diseases. Thus, extensive studies have been carried out to elucidate disease-specific aldehydes released from the body to develop potential biomarkers for diagnostic purposes. Mammals possess innate sensory systems, such as receptors and ion channels, to detect these VOCs and maintain physiological homeostasis. Recently, electronic biosensors such as the electronic nose have been developed for disease diagnosis. This review aims to present an overview of natural sensory receptors that can detect reactive aldehydes, as well as electronic noses that have the potential to diagnose certain diseases. In this regard, this review focuses on eight aldehydes that are well-defined as biomarkers in human health and disease. It offers insights into the biological aspects and technological advances in detecting aldehyde-containing VOCs. Therefore, this review will aid in understanding the role of aldehyde-containing VOCs in human health and disease and the technological advances for improved diagnosis.

Coupling of Mouse Olfactory Bulb Projection Neurons to Fluctuating Odor Pulses

Odors are transported by turbulent air currents, creating complex temporal fluctuations in odor concentration that provide a potentially informative stimulus dimension. We have shown that mice are able to discriminate odor stimuli based on their temporal structure, indicating that information contained in the temporal structure of odor plumes can be extracted by the mouse olfactory system. Here, using in vivo extracellular and intracellular electrophysiological recordings, we show that mitral cells (MCs) and tufted cells (TCs) of the male C57BL/6 mouse olfactory bulb can encode the dominant temporal frequencies present in odor stimuli up to at least 20 Hz. A substantial population of cell-odor pairs showed significant coupling of their subthreshold membrane potential with the odor stimulus at both 2 Hz (29/70) and the suprasniff frequency 20 Hz (24/70). Furthermore, mitral/tufted cells (M/TCs) show differential coupling of their membrane potential to odor concentration fluctuations with tufted cells coupling more strongly for the 20 Hz stimulation. Frequency coupling was always observed to be invariant to odor identity, and M/TCs that coupled well to a mixture also coupled to at least one of the components of the mixture. Interestingly, pharmacological blocking of the inhibitory circuitry strongly modulated frequency coupling of cell-odor pairs at both 2 Hz (10/15) and 20 Hz (9/15). These results provide insight into how both cellular and circuit properties contribute to the encoding of temporal odor features in the mouse olfactory bulb.SIGNIFICANCE STATEMENT Odors in the natural environment have a strong temporal structure that can be extracted and used by mice in their behavior. Here, using in vivo extracellular and intracellular electrophysiological techniques, we show that the projection neurons in the olfactory bulb can encode and couple to the dominant frequency present in an odor stimulus. Furthermore, frequency coupling was observed to be differential between mitral and tufted cells and was odor invariant but strongly modulated by local inhibitory circuits. In summary, this study provides insight into how both cellular and circuit properties modulate encoding of odor temporal features in the mouse olfactory bulb.

Differences in spike generation instead of synaptic inputs determine the feature selectivity of two retinal cell types

Retinal ganglion cells (RGCs) are the spiking projection neurons of the eye that encode different features of the visual environment. The circuits providing synaptic input to different RGC types to drive feature selectivity have been studied extensively, but there has been less research aimed at understanding the intrinsic properties and how they impact feature selectivity. We introduce an RGC type in the mouse, the Bursty Suppressed-by-Contrast (bSbC) RGC, and compared it to the OFF sustained alpha (OFFsA). Differences in their contrast response functions arose from differences not in synaptic inputs but in their intrinsic properties. Spike generation was the key intrinsic property behind this functional difference; the bSbC RGC undergoes depolarization block while the OFFsA RGC maintains a high spike rate. Our results demonstrate that differences in intrinsic properties allow these two RGC types to detect and relay distinct features of an identical visual stimulus to the brain.

Voltage imaging in the olfactory bulb using transgenic mouse lines expressing the genetically encoded voltage indicator ArcLight

Genetically encoded voltage indicators (GEVIs) allow optical recordings of membrane potential changes in defined cell populations. Transgenic reporter animals that facilitate precise and repeatable targeting with high expression levels would further the use of GEVIs in the in vivo mammalian brain. However, the literature on developing and applying transgenic mouse lines as vehicles for GEVI expression is limited. Here we report the first in vivo experiments using a transgenic reporter mouse for the GEVI ArcLight, which utilizes a Cre/tTA dependent expression system (TIGRE 1.0). We developed two mouse lines with ArcLight expression restricted to either olfactory receptor neurons, or a subpopulation of interneurons located in the granule and glomerular layers in the olfactory bulb. The ArcLight expression in these lines was sufficient for in vivo imaging of odorant responses in single trials using epifluorescence and 2-photon imaging. The voltage responses were odor-specific and concentration-dependent, which supported earlier studies about perceptual transformations carried out by the bulb that used calcium sensors of neural activity. This study demonstrates that the ArcLight transgenic line is a flexible genetic tool that can be used to record the neuronal electrical activity of different cell types with a signal-to-noise ratio that is comparable to previous reports using viral transduction.

Stimulus Driven Functional Transformations in the Early Olfactory System

Olfactory stimuli are encountered across a wide range of odor concentrations in natural environments. Defining the neural computations that support concentration invariant odor perception, odor discrimination, and odor-background segmentation across a wide range of stimulus intensities remains an open question in the field. In principle, adaptation could allow the olfactory system to adjust sensory representations to the current stimulus conditions, a well-known process in other sensory systems. However, surprisingly little is known about how adaptation changes olfactory representations and affects perception. Here we review the current understanding of how adaptation impacts processing in the first two stages of the vertebrate olfactory system, olfactory receptor neurons (ORNs), and mitral/tufted cells.

Sniff-synchronized, gradient-guided olfactory search by freely moving mice

For many organisms, searching for relevant targets such as food or mates entails active, strategic sampling of the environment. Finding odorous targets may be the most ancient search problem that motile organisms evolved to solve. While chemosensory navigation has been well characterized in microorganisms and invertebrates, spatial olfaction in vertebrates is poorly understood. We have established an olfactory search assay in which freely moving mice navigate noisy concentration gradients of airborne odor. Mice solve this task using concentration gradient cues and do not require stereo olfaction for performance. During task performance, respiration and nose movement are synchronized with tens of milliseconds precision. This synchrony is present during trials and largely absent during inter-trial intervals, suggesting that sniff-synchronized nose movement is a strategic behavioral state rather than simply a constant accompaniment to fast breathing. To reveal the spatiotemporal structure of these active sensing movements, we used machine learning methods to parse motion trajectories into elementary movement motifs. Motifs fall into two clusters, which correspond to investigation and approach states. Investigation motifs lock precisely to sniffing, such that the individual motifs preferentially occur at specific phases of the sniff cycle. The allocentric structure of investigation and approach indicates an advantage to sampling both sides of the sharpest part of the odor gradient, consistent with a serial-sniff strategy for gradient sensing. This work clarifies sensorimotor strategies for mouse olfactory search and guides ongoing work into the underlying neural mechanisms.

Odor sampling strategies in mice with genetically altered olfactory responses

Peripheral sensory cells and the central neuronal circuits that monitor environmental changes to drive behaviors should be adapted to match the behaviorally relevant kinetics of incoming stimuli, be it the detection of sound frequencies, the speed of moving objects or local temperature changes. Detection of odorants begins with the activation of olfactory receptor neurons in the nasal cavity following inhalation of air and airborne odorants carried therein. Thus, olfactory receptor neurons are stimulated in a rhythmic and repeated fashion that is determined by the breathing or sniffing frequency that can be controlled and altered by the animal. This raises the question of how the response kinetics of olfactory receptor neurons are matched to the imposed stimulation frequency and if, vice versa, the kinetics of olfactory receptor neuron responses determine the sniffing frequency. We addressed this question by using a mouse model that lacks the K⁺-dependent Na⁺/Ca²⁺ exchanger 4 (NCKX4), which results in markedly slowed response termination of olfactory receptor neuron responses and hence changes the temporal response kinetics of these neurons. We monitored sniffing behaviors of freely moving wildtype and NCKX4 knockout mice while they performed olfactory Go/NoGo discrimination tasks. Knockout mice performed with similar or, surprisingly, better accuracy compared to wildtype mice, but chose, depending on the task, different odorant sampling durations depending on the behavioral demands of the odorant identification task. Similarly, depending on the demands of the behavioral task, knockout mice displayed a lower basal breathing frequency prior to odorant sampling, a possible mechanism to increase the dynamic range for changes in sniffing frequency during odorant sampling. Overall, changes in sniffing behavior between wildtype and NCKX4 knockout mice were subtle, suggesting that, at least for the particular odorant-driven task we used, slowed response termination of the odorant-induced receptor neuron response either has a limited detrimental effect on odorant-driven behavior or mice are able to compensate via an as yet unknown mechanism.

Fast odour dynamics are encoded in the olfactory system and guide behaviour

Odours are transported in turbulent plumes, which result in rapid concentration fluctuations^1,2 that contain rich information about the olfactory scenery, such as the composition and location of an odour source^2-4. However, it is unclear whether the mammalian olfactory system can use the underlying temporal structure to extract information about the environment. Here we show that ten-millisecond odour pulse patterns produce distinct responses in olfactory receptor neurons. In operant conditioning experiments, mice discriminated temporal correlations of rapidly fluctuating odours at frequencies of up to 40 Hz. In imaging and electrophysiological recordings, such correlation information could be readily extracted from the activity of mitral and tufted cells-the output neurons of the olfactory bulb. Furthermore, temporal correlation of odour concentrations⁵ reliably predicted whether odorants emerged from the same or different sources in naturalistic environments with complex airflow. Experiments in which mice were trained on such tasks and probed using synthetic correlated stimuli at different frequencies suggest that mice can use the temporal structure of odours to extract information about space. Thus, the mammalian olfactory system has access to unexpectedly fast temporal features in odour stimuli. This endows animals with the capacity to overcome key behavioural challenges such as odour source separation⁵, figure-ground segregation⁶ and odour localization⁷ by extracting information about space from temporal odour dynamics.

The functional relevance of olfactory marker protein in the vertebrate olfactory system: a never-ending story

Olfactory marker protein (OMP) was first described as a protein expressed in olfactory receptor neurons (ORNs) in the nasal cavity. In particular, OMP, a small cytoplasmic protein, marks mature ORNs and is also expressed in the neurons of other nasal chemosensory systems: the vomeronasal organ, the septal organ of Masera, and the Grueneberg ganglion. While its expression pattern was more easily established, OMP's function remained relatively vague. To date, most of the work to understand OMP's role has been done using mice lacking OMP. This mostly phenomenological work has shown that OMP is involved in sharpening the odorant response profile and in quickening odorant response kinetics of ORNs and that it contributes to targeting of ORN axons to the olfactory bulb to refine the glomerular response map. Increasing evidence shows that OMP acts at the early stages of olfactory transduction by modulating the kinetics of cAMP, the second messenger of olfactory transduction. However, how this occurs at a mechanistic level is not understood, and it might also not be the only mechanism underlying all the changes observed in mice lacking OMP. Recently, OMP has been detected outside the nose, including the brain and other organs. Although no obvious logic has become apparent regarding the underlying commonality between nasal and extranasal expression of OMP, a broader approach to diverse cellular systems might help unravel OMP's functions and mechanisms of action inside and outside the nose.

Adaptive temporal processing of odor stimuli

The olfactory system translates chemical signals into neuronal signals that inform behavioral decisions of the animal. Odors are cues for source identity, but if monitored long enough, they can also be used to localize the source. Odor representations should therefore be robust to changing conditions and flexible in order to drive an appropriate behavior. In this review, we aim at discussing the main computations that allow robust and flexible encoding of odor information in the olfactory neural pathway.

Odor Concentration Change Coding in the Olfactory Bulb

Dynamical changes in the environment strongly impact our perception. Likewise, sensory systems preferentially represent stimulus changes, enhancing temporal contrast. In olfaction, odor concentration changes across consecutive inhalations (ΔC_t) can guide odor source localization, yet the neural representation of ΔC_t has not been studied in vertebrates. We have found that, in the mouse olfactory bulb, a subset of mitral/tufted (M/T) cells represents ΔC_t, enhancing the contrast between different concentrations. These concentration change responses are direction selective: they respond either to increments or decrements of concentration, reminiscent of ON and OFF selectivity in the retina. This contrast enhancement scales with the magnitude, but not the duration of the concentration step. Further, ΔC_t can be read out from the total spike count per sniff, unlike odor identity and intensity, which are represented by fast temporal spike patterns. Our results demonstrate that a subset of M/T cells represents ΔC_t, providing a signal that may instruct navigational decisions in downstream olfactory circuits.

Stimulus dynamics-dependent information transfer of olfactory and vomeronasal sensory neurons in mice

The parallel processing of chemical signals by the main olfactory system and the vomeronasal system has been known to control animal behavior. The physiological significance of peripheral parallel pathways consisting of olfactory sensory neurons and vomeronasal sensory neurons is not well understood. Here, we show complementary characteristics of the information transfer of the olfactory sensory neurons and vomeronasal sensory neurons. A difference in excitability between the sensory neurons was revealed by patch-clamp experiments. The olfactory and vomeronasal sensory neurons showed phasic and tonic firing, respectively. Intrinsic channel kinetics determining firing patterns was demonstrated by a Hodgkin-Huxley-style computation. Our estimation of the information carried by action potentials during one cycle of sinusoidal stimulation with variable durations revealed distinct characteristics of information transfer between the sensory neurons. Phasic firing of the olfactory sensory neurons was suitable to carry information about rapid changes in a shorter cycle (<200 ms). In contrast, tonic firing of the vomeronasal sensory neurons was able to convey information about smaller stimuli changing slowly with longer cycles (>500 ms). Thus, the parallel pathways of the two types of sensory neurons can convey information about a wide range of dynamic stimuli. A combination of complementary characteristics of olfactory information transfer may enhance the synergy of the interaction between the main olfactory system and the vomeronasal system.

Sniff Invariant Odor Coding

Sampling regulates stimulus intensity and temporal dynamics at the sense organ. Despite variations in sampling behavior, animals must make veridical perceptual judgments about external stimuli. In olfaction, odor sampling varies with respiration, which influences neural responses at the olfactory periphery. Nevertheless, rats were able to perform fine odor intensity judgments despite variations in sniff kinetics. To identify the features of neural activity supporting stable intensity perception, in awake mice we measured responses of mitral/tufted (MT) cells to different odors and concentrations across a range of sniff frequencies. Amplitude and latency of the MT cells' responses vary with sniff duration. A fluid dynamics (FD) model based on odor concentration kinetics in the intranasal cavity can account for this variability. Eliminating sniff waveform dependence of MT cell responses using the FD model allows for significantly better decoding of concentration. This suggests potential schemes for sniff waveform invariant odor concentration coding.

Sniffing Fast: Paradoxical Effects on Odor Concentration Discrimination at the Levels of Olfactory Bulb Output and Behavior

In awake mice, sniffing behavior is subject to complex contextual modulation. It has been hypothesized that variance in inhalation dynamics alters odor concentration profiles in the naris despite a constant environmental concentration. Using whole-cell recordings in the olfactory bulb of awake mice, we directly demonstrate that rapid sniffing mimics the effect of odor concentration increase at the level of both mitral and tufted cell (MTC) firing rate responses and temporal responses. Paradoxically, we find that mice are capable of discriminating fine concentration differences within short timescales despite highly variable sniffing behavior. One way that the olfactory system could differentiate between a change in sniffing and a change in concentration would be to receive information about the inhalation parameters in parallel with information about the odor. We find that the sniff-driven activity of MTCs without odor input is informative of the kind of inhalation that just occurred, allowing rapid detection of a change in inhalation. Thus, a possible reason for sniff modulation of the early olfactory system may be to directly inform downstream centers of nasal flow dynamics, so that an inference can be made about environmental concentration independent of sniff variance.

Calcium-activated chloride channels clamp odor-evoked spike activity in olfactory receptor neurons

The calcium-activated chloride channel anoctamin-2 (Ano2) is thought to amplify transduction currents in olfactory receptor neurons (ORNs), a hypothesis supported by previous studies in dissociated neurons from Ano2^−/− mice. Paradoxically, despite a reduction in transduction currents in Ano2^−/− ORNs, their spike output for odor stimuli may be higher. We examined the role of Ano2 in ORNs in their native environment in freely breathing mice by imaging activity in ORN axons as they arrive in the olfactory bulb glomeruli. Odor-evoked responses in ORN axons of Ano2^−/− animals were consistently larger for a variety of odorants and concentrations. In an open arena, Ano2^−/− animals took longer to approach a localized odor source than Ano2^+/+ animals, revealing clear olfactory behavioral deficits. Our studies provide the first in vivo evidence toward an alternative or additional role for Ano2 in the olfactory transduction cascade, where it may serve as a feedback mechanism to clamp ORN spike output.

Tracking of unfamiliar odors is facilitated by signal amplification through anoctamin 2 chloride channels in mouse olfactory receptor neurons

Many animals follow odor trails to find food, nesting sites, or mates, and they require only faint olfactory cues to do so. The performance of a tracking dog, for instance, poses the question on how the animal is able to distinguish a target odor from the complex chemical background around the trail. Current concepts of odor perception suggest that animals memorize each odor as an olfactory object, a percept that enables fast recognition of the odor and the interpretation of its valence. An open question still is how this learning process operates efficiently at the low odor concentrations that typically prevail when animals inspect an odor trail. To understand olfactory processing under these conditions, we studied the role of an amplification mechanism that boosts signal transduction at low stimulus intensities, a process mediated by calcium‐gated anoctamin 2 chloride channels. Genetically altered Ano2^−/− mice, which lack these channels, display an impaired cue‐tracking behavior at low odor concentrations when challenged with an unfamiliar, but not with a familiar, odor. Moreover, recordings from the olfactory epithelium revealed that odor coding lacks sensitivity and temporal resolution in anoctamin 2‐deficient mice. Our results demonstrate that the detection of an unfamiliar, weak odor, as well as its memorization as an olfactory object, require signal amplification in olfactory receptor neurons. This process may contribute to the phenomenal tracking abilities of animals that follow odor trails.

Antagonism in olfactory receptor neurons and its implications for the perception of odor mixtures

Natural environments feature mixtures of odorants of diverse quantities, qualities and complexities. Olfactory receptor neurons (ORNs) are the first layer in the sensory pathway and transmit the olfactory signal to higher regions of the brain. Yet, the response of ORNs to mixtures is strongly non-additive, and exhibits antagonistic interactions among odorants. Here, we model the processing of mixtures by mammalian ORNs, focusing on the role of inhibitory mechanisms. We show how antagonism leads to an effective ‘normalization’ of the ensemble ORN response, that is, the distribution of responses of the ORN population induced by any mixture is largely independent of the number of components in the mixture. This property arises from a novel mechanism involving the distinct statistical properties of receptor binding and activation, without any recurrent neuronal circuitry. Normalization allows our encoding model to outperform non-interacting models in odor discrimination tasks, leads to experimentally testable predictions and explains several psychophysical experiments in humans.

When ordering in a coffee shop, you probably recognize and enjoy the aroma of freshly roasted coffee beans. But as well as coffee, you can also smell the croissants behind the counter and maybe even the perfume or cologne of the person next to you. Each of these scents consists of a collection of chemicals, or odorants. To distinguish between the aroma of coffee and that of croissants, your brain must group the odorants appropriately and then keep the groups separate from each other.

This is not a trivial task. Odorants bind to proteins called odorant receptors found on the surface of cells in the nose called olfactory receptor neurons. But each odorant does not have its own dedicated receptor. Instead, a single odorant will bind to multiple types of odorant receptors, and thus, each olfactory receptor neuron may respond to multiple odorants. So how does the brain encode mixtures of odorants in a way that allows us to distinguish one aroma from another?

Reddy, Zak et al. have developed a computational model to explain how this process works. The model assumes that an odorant triggers a response in an olfactory receptor neuron via two steps. First, the odorant binds to an odorant receptor. Second, the bound odorant activates the receptor. But the odorant that binds most strongly to a receptor will not necessarily be the odorant that is best at activating that receptor.

This allows a phenomenon called competitive antagonism to occur. This is when one odorant in a mixture binds more strongly to a receptor than the other odorants, but only weakly activates that receptor. In so doing, the strongly bound odorant prevents the other odorants from binding to and activating the receptor. This helps tame the dominating influence of background odors, which might otherwise saturate the responses of individual olfactory receptor neurons.

Reddy, Zak et al. show that processes such as competitive antagonism enable olfactory receptor neurons to encode all of the odors within a mixture. The model can explain various phenomena observed in experiments and it adds to our understanding of how the brain generates our sense of smell. The model may also be relevant to other biological systems that must filter weak signals from a dominant background. These include the immune system, which must distinguish a small set of foreign proteins from the much larger number of proteins that make up our bodies.

Stimulus dependent diversity and stereotypy in the output of an olfactory functional unit

Olfactory inputs are organized in an array of functional units (glomeruli), each relaying information from sensory neurons expressing a given odorant receptor to a small population of output neurons, mitral/tufted (MT) cells. MT cells respond heterogeneously to odorants, and how the responses encode stimulus features is unknown. We recorded in awake mice responses from “sister” MT cells that receive input from a functionally characterized, genetically identified glomerulus, corresponding to a specific receptor (M72). Despite receiving similar inputs, sister MT cells exhibit temporally diverse, concentration-dependent, excitatory and inhibitory responses to most M72 ligands. In contrast, the strongest known ligand for M72 elicits temporally stereotyped, early excitatory responses in sister MT cells, consistent across a range of concentrations. Our data suggest that information about ligand affinity is encoded in the collective stereotypy or diversity of activity among sister MT cells within a glomerular functional unit in a concentration-tolerant manner.

Mitral/tufted (MT) cells connect to a single glomerulus and receive inputs from sensory neurons expressing the same odorant receptor. Here the authors report that sister MT cells connected to the M72 glomerulus exhibit variable responses to most M72 ligands but respond in a reproducible and stereotyped way to a high-affinity M72 ligand.

A primacy code for odor identity

Humans can identify visual objects independently of view angle and lighting, words independently of volume and pitch, and smells independently of concentration. The computational principles underlying invariant object recognition remain mostly unknown. Here we propose that, in olfaction, a small and relatively stable set comprised of the earliest activated receptors forms a code for concentration-invariant odor identity. One prediction of this “primacy coding” scheme is that decisions based on odor identity can be made solely using early odor-evoked neural activity. Using an optogenetic masking paradigm, we define the sensory integration time necessary for odor identification and demonstrate that animals can use information occurring <100 ms after inhalation onset to identify odors. Using multi-electrode array recordings of odor responses in the olfactory bulb, we find that concentration-invariant units respond earliest and at latencies that are within this behaviorally-defined time window. We propose a computational model demonstrating how such a code can be read by neural circuits of the olfactory system.

Odor identity remains stable despite changes in concentration yet the neural mechanisms are relatively unknown. Here the authors test a primacy coding model using an optogenetic masking paradigm in mice to show that a set of earliest activated receptors are sufficient to make decisions about odor identity across concentrations.

A Population Representation of Absolute Light Intensity in the Mammalian Retina

Environmental illumination spans many log units of intensity and is tracked for essential functions that include regulation of the circadian clock, arousal state, and hormone levels. Little is known about the neural representation of light intensity and how it covers the necessary range. This question became accessible with the discovery of mammalian photoreceptors that are required for intensity-driven functions, the M1 ipRGCs. The spike outputs of M1s are thought to uniformly track intensity over a wide range. We provide a different understanding: individual cells operate over a narrow range, but the population covers irradiances from moonlight to full daylight. The range of most M1s is limited by depolarization block, which is generally considered pathological but is produced intrinsically by these cells. The dynamics of block allow the population to code stimulus intensity with flexibility and efficiency. Moreover, although spikes are distorted by block, they are regularized during axonal propagation.

Cilia- and Flagella-Associated Protein 69 Regulates Olfactory Transduction Kinetics in Mice

Animals detect odorous chemicals through specialized olfactory sensory neurons (OSNs) that transduce odorants into neural electrical signals. We identified a novel and evolutionarily conserved protein, cilia- and flagella-associated protein 69 (CFAP69), in mice that regulates olfactory transduction kinetics. In the olfactory epithelium, CFAP69 is enriched in OSN cilia, where olfactory transduction occurs. Bioinformatic analysis suggests that a large portion of CFAP69 can form Armadillo-type α-helical repeats, which may mediate protein-protein interactions. OSNs lacking CFAP69, remarkably, displayed faster kinetics in both the on and off phases of electrophysiological responses at both the neuronal ensemble level as observed by electroolfactogram and the single-cell level as observed by single-cell suction pipette recordings. In single-cell analysis, OSNs lacking CFAP69 showed faster response integration and were able to fire APs more faithfully to repeated odor stimuli. Furthermore, both male and female mutant mice that specifically lack CFAP69 in OSNs exhibited attenuated performance in a buried food pellet test when a background of the same odor to the food pellet was present even though they should have better temporal resolution of coding olfactory stimulation at the peripheral. Therefore, the role of CFAP69 in the olfactory system seems to be to allow the olfactory transduction machinery to work at a precisely regulated range of response kinetics for robust olfactory behavior.SIGNIFICANCE STATEMENT Sensory receptor cells are generally thought to evolve to respond to sensory cues as fast as they can. This idea is consistent with mutational analyses in various sensory systems, where mutations of sensory receptor cells often resulted in reduced response size and slowed response kinetics. Contrary to this idea, we have found that there is a kinetic "damper" present in the olfactory transduction cascade of the mouse that slows down the response kinetics and, by doing so, it reduces the peripheral temporal resolution in coding odor stimuli and allows for robust olfactory behavior. This study should trigger a rethinking of the significance of the intrinsic speed of sensory transduction and the pattern of the peripheral coding of sensory stimuli.

Cyclic-nucleotide-gated cation current and Ca2+-activated Cl current elicited by odorant in vertebrate olfactory receptor neurons

Olfactory transduction in vertebrate olfactory receptor neurons (ORNs) involves primarily a cAMP-signaling cascade that leads to the opening of cyclic-nucleotide-gated (CNG), nonselective cation channels. The consequent Ca²⁺ influx triggers adaptation but also signal amplification, the latter by opening a Ca²⁺-activated Cl channel (ANO2) to elicit, unusually, an inward Cl current. Hence the olfactory response has inward CNG and Cl components that are in rapid succession and not easily separable. We report here success in quantitatively separating these two currents with respect to amplitude and time course over a broad range of odorant strengths. Importantly, we found that the Cl current is the predominant component throughout the olfactory dose-response relation, down to the threshold of signaling to the brain. This observation is very surprising given a recent report by others that the olfactory-signal amplification effected by the Ca²⁺-activated Cl current does not influence the behavioral olfactory threshold in mice.

The Ca2+-activated Cl- channel TMEM16B regulates action potential firing and axonal targeting in olfactory sensory neurons

TMEM16B is expressed in olfactory sensory neurons, but previous attempts to establish a physiological role in olfaction have been unsuccessful. Pietra et al. find that genetic ablation of TMEM16B results in defects in the olfactory behavior of mice and the cellular physiology of olfactory sensory neurons.

The Ca²⁺-activated Cl⁻ channel TMEM16B is highly expressed in the cilia of olfactory sensory neurons (OSNs). Although a large portion of the odor-evoked transduction current is carried by Ca²⁺-activated Cl⁻ channels, their role in olfaction is still controversial. A previous report (Billig et al. 2011. Nat. Neurosci.http://dx.doi.org/10.1038/nn.2821) showed that disruption of the TMEM16b/Ano2 gene in mice abolished Ca²⁺-activated Cl⁻ currents in OSNs but did not produce any major change in olfactory behavior. Here we readdress the role of TMEM16B in olfaction and show that TMEM16B knockout (KO) mice have behavioral deficits in odor-guided food-finding ability. Moreover, as the role of TMEM16B in action potential (AP) firing has not yet been studied, we use electrophysiological recording methods to measure the firing activity of OSNs. Suction electrode recordings from isolated olfactory neurons and on-cell loose-patch recordings from dendritic knobs of neurons in the olfactory epithelium show that randomly selected neurons from TMEM16B KO mice respond to stimulation with increased firing activity than those from wild-type (WT) mice. Because OSNs express different odorant receptors (ORs), we restrict variability by using a mouse line that expresses a GFP-tagged I7 OR, which is known to be activated by heptanal. In response to heptanal, we measure dramatic changes in the firing pattern of I7-expressing neurons from TMEM16B KO mice compared with WT: responses are prolonged and display a higher number of APs. Moreover, lack of TMEM16B causes a markedly reduced basal spiking activity in I7-expressing neurons, together with an alteration of axonal targeting to the olfactory bulb, leading to the appearance of supernumerary I7 glomeruli. Thus, TMEM16B controls AP firing and ensures correct glomerular targeting of OSNs expressing I7. Altogether, these results show that TMEM16B does have a relevant role in normal olfaction.

Properties of an optogenetic model for olfactory stimulation

KEY POINTS

In olfactory research it is difficult to deliver stimuli with defined intensity and duration to olfactory sensory neurons. Expression of channelrhodopsin 2 (ChR2) in olfactory sensory neurons provides a means to activate these neurons with light flashes. Appropriate mouse models are available. The present study explores the suitability of an established olfactory marker protein (OMP)/ChR2-yellow fluorescent protein (YFP) mouse model for ex vivo experimentation. Expression of ChR2 in sensory neurons of the main olfactory epithelium, the septal organ and vomeronasal organ is characterized. Expression pattern of ChR2 in olfactory receptor neurons and the properties of light responses indicate that light stimulation does not impact on signal transduction in the chemosensory cilia. Light-induced electro-olfactograms are characterized with light flashes of different intensities, durations and frequencies. The impact of light-induced afferent stimulation on the olfactory bulb is examined with respect to response amplitude, polarity and low-pass filtering.

ABSTRACT

For the examination of sensory processing, it is helpful to deliver stimuli in precisely defined temporal and spatial patterns with accurate control of stimulus intensity. This is challenging in experiments with the mammalian olfactory system because airborne odorants have to be transported into the intricate sensory structures of the nose and must dissolve in mucus to be detected by sensory neurons. Defined and reproducible activity can be generated in olfactory sensory neurons that express the light-gated ion channel channelrhodopsin 2 (ChR2). The neurons can be stimulated by light flashes in a controlled fashion by this optogenetic approach. Here we examined the application of an olfactory marker protein (OMP)/ChR2-yellow fluorescent protein (YFP) model for ex vivo exploration of the olfactory epithelium and the olfactory bulb of the mouse. We studied the expression patterns of ChR2 in the main olfactory system, the vomeronasal system, and the septal organ, and we found that ChR2 is absent from the sensory cilia of olfactory sensory neurons. In the olfactory epithelium, we characterized light-induced electro-olfactograms with respect to peripheral encoding of stimulus intensity, stimulus duration and stimulus frequency. In acute slices of the olfactory bulb, we identified specific aspects of the ChR2-induced input signal, concerning its dynamic range, its low-pass filter property and its response to prolonged stimulation. Our study describes the performance of the OMP/ChR2-YFP model for ex vivo experimentation on the peripheral olfactory system and documents its versatility and its limitations for olfactory research.

The Odorant Receptor-Dependent Role of Olfactory Marker Protein in Olfactory Receptor Neurons

Olfactory receptor neurons (ORNs) in the nasal cavity detect and transduce odorants into action potentials to be conveyed to the olfactory bulb. Odorants are delivered to ORNs via the inhaled air at breathing frequencies that can vary from 2 to 10 Hz in the mouse. Thus olfactory transduction should occur at sufficient speed such that it can accommodate repetitive and frequent stimulation. Activation of odorant receptors (ORs) leads to adenylyl cyclase III activation, cAMP increase, and opening of cyclic nucleotide-gated channels. This makes the kinetic regulation of cAMP one of the important determinants for the response time course. We addressed the dynamic regulation of cAMP during the odorant response and examined how basal levels of cAMP are controlled. The latter is particularly relevant as basal cAMP depends on the basal activity of the expressed OR and thus varies across ORNs. We found that olfactory marker protein (OMP), a protein expressed in mature ORNs, controls both basal and odorant-induced cAMP levels in an OR-dependent manner. Lack of OMP increases basal cAMP, thus abolishing differences in basal cAMP levels between ORNs expressing different ORs. Moreover, OMP speeds up signal transduction for ORNs to better synchronize their output with high-frequency stimulation and to perceive brief stimuli. Last, OMP also steepens the dose-response relation to improve concentration coding although at the cost of losing responses to weak stimuli. We conclude that OMP plays a key regulatory role in ORN physiology by controlling multiple facets of the odorant response.

From molecule to mind: an integrative perspective on odor intensity

A fundamental problem in systems neuroscience is mapping the physical properties of a stimulus to perceptual characteristics. In vision, wavelength translates into color; in audition, frequency translates into pitch. Although odorant concentration is a key feature of olfactory stimuli, we do not know how concentration is translated into perceived intensity by the olfactory system. A variety of neural responses at several levels of processing have been reported to vary with odorant concentration, suggesting specific coding models. However, it remains unclear which, if any, of these phenomena underlie the perception of odor intensity. Here, we provide an overview of current models at different stages of olfactory processing, and identify promising avenues for future research.

Calmodulin-dependent activation and inactivation of anoctamin calcium-gated chloride channels

Calcium-dependent chloride channels serve critical functions in diverse biological systems. Driven by cellular calcium signals, the channels codetermine excitatory processes and promote solute transport. The anoctamin (ANO) family of membrane proteins encodes three calcium-activated chloride channels, named ANO 1 (also TMEM16A), ANO 2 (also TMEM16B), and ANO 6 (also TMEM16F). Here we examined how ANO 1 and ANO 2 interact with Ca²⁺/calmodulin using nonstationary current analysis during channel activation. We identified a putative calmodulin-binding domain in the N-terminal region of the channel proteins that is involved in channel activation. Binding studies with peptides indicated that this domain, a regulatory calmodulin-binding motif (RCBM), provides two distinct modes of interaction with Ca²⁺/calmodulin, one at submicromolar Ca²⁺ concentrations and one in the micromolar Ca²⁺ range. Functional, structural, and pharmacological data support the concept that calmodulin serves as a calcium sensor that is stably associated with the RCBM domain and regulates the activation of ANO 1 and ANO 2 channels. Moreover, the predominant splice variant of ANO 2 in the brain exhibits Ca²⁺/calmodulin-dependent inactivation, a loss of channel activity within 30 s. This property may curtail ANO 2 activity during persistent Ca²⁺ signals in neurons. Mutagenesis data indicated that the RCBM domain is also involved in ANO 2 inactivation, and that inactivation is suppressed in the retinal ANO 2 splice variant. These results advance the understanding of Ca²⁺ regulation in anoctamin Cl⁻ channels and its significance for the physiological function that anoctamin channels subserve in neurons and other cell types.

Comparing thoracic and intra-nasal pressure transients to monitor active odor sampling during odor-guided decision making in the mouse

BACKGROUND

Recording of physiological parameters in behaving mice has seen an immense increase over recent years driven by, for example, increased miniaturization of recording devices. One parameter particularly important for odorant-driven behaviors is the breathing frequency, since the latter dictates the rate of odorant delivery to the nasal cavity and the olfactory receptor neurons located therein.

NEW METHOD

Typically, breathing patterns are monitored by either measuring the breathing-induced temperature or pressure changes in the nasal cavity. Both require the implantation of a nasal cannula and tethering of the mouse to either a cable or tubing. To avoid these limitations we used an implanted pressure sensor which reads the thoracic pressure and transmits the data telemetrically, thus making it suitable for experiments which require a freely moving animal.

RESULTS

Mice performed a Go/NoGo odorant-driven behavioral task with the implanted pressure sensor, which proved to work reliably to allow recording of breathing signals over several weeks from a given animal.

COMPARISON TO EXISTING METHOD(S)

We simultaneously recorded the thoracic and nasal pressure changes and found that measuring the thoracic pressure change yielded similar results compared to measurements of nasal pressure changes.

CONCLUSION

Telemetrically recorded breathing signals are a feasible method to monitor odorant-guided behavioral changes in breathing rates. Its advantages are most significant when recording from a freely moving animal over several weeks. The advantages and disadvantages of different methods to record breathing patterns are discussed.

Temporal processing in the olfactory system: can we see a smell?

Sensory processing circuits in the visual and olfactory systems receive input from complex, rapidly changing environments. Although patterns of light and plumes of odor create different distributions of activity in the retina and olfactory bulb, both structures use what appears on the surface similar temporal coding strategies to convey information to higher areas in the brain. We compare temporal coding in the early stages of the olfactory and visual systems, highlighting recent progress in understanding the role of time in olfactory coding during active sensing by behaving animals. We also examine studies that address the divergent circuit mechanisms that generate temporal codes in the two systems, and find that they provide physiological information directly related to functional questions raised by neuroanatomical studies of Ramon y Cajal over a century ago. Consideration of differences in neural activity in sensory systems contributes to generating new approaches to understand signal processing.

Intermediate intrinsic diversity enhances neural population coding

Cell-to-cell variability in molecular, genetic, and physiological features is increasingly recognized as a critical feature of complex biological systems, including the brain. Although such variability has potential advantages in robustness and reliability, how and why biological circuits assemble heterogeneous cells into functional groups is poorly understood. Here, we develop analytic approaches toward answering how neuron-level variation in intrinsic biophysical properties of olfactory bulb mitral cells influences population coding of fluctuating stimuli. We capture the intrinsic diversity of recorded populations of neurons through a statistical approach based on generalized linear models. These models are flexible enough to predict the diverse responses of individual neurons yet provide a common reference frame for comparing one neuron to the next. We then use Bayesian stimulus decoding to ask how effectively different populations of mitral cells, varying in their diversity, encode a common stimulus. We show that a key advantage provided by physiological levels of intrinsic diversity is more efficient and more robust encoding of stimuli by the population as a whole. However, we find that the populations that best encode stimulus features are not simply the most heterogeneous, but those that balance diversity with the benefits of neural similarity.

Testing the sorption hypothesis in olfaction: a limited role for sniff strength in shaping primary odor representations during behavior

The acquisition of sensory information during behavior shapes the neural representation, central processing, and perception of external stimuli. In mammals, a sniff represents the basic unit of odor sampling, yet how sniffing shapes odor representations remains poorly understood. Perhaps the earliest hypothesis of the role of sniffing in olfaction arises from the fact that odorants with different physicochemical properties exhibit different patterns of deposition across the olfactory epithelium, and that these patterns are differentially affected by flow rate. However, whether sniff flow rates shape odor representations during natural sniffing remains untested, and whether animals make use of odorant sorption-airflow relationships as part of an active odor-sampling strategy remains unclear. We tested these ideas in the intact rat using a threefold approach. First, we asked whether sniff strength shapes odor representations in vivo by imaging from olfactory receptor neuron (ORN) terminals during controlled changes in inhalation flow in the anesthetized rat. Second, we asked whether sniff strength shapes odor representations by imaging from ORNs during natural sniffing in the awake rat. Third, we asked whether rats actively modulate sniff strength during an odor discrimination task. We found that, while artificial changes in flow rate can alter ORN responses, sniff strength has negligible effect on odor representations during natural sniffing, and behaving rats do not modulate flow rate to improve odor discrimination. These data suggest that modulating sniff strength does not shape odor representations sufficiently to be part of a strategy for active odor sensing in the behaving animal.

Temporal response dynamics of Drosophila olfactory sensory neurons depends on receptor type and response polarity

Insect olfactory sensory neurons (OSN) express a diverse array of receptors from different protein families, i.e. ionotropic receptors (IR), gustatory receptors (GR) and odorant receptors (OR). It is well known that insects are exposed to a plethora of odor molecules that vary widely in both space and time under turbulent natural conditions. In addition to divergent ligand specificities, these different receptors might also provide an increased range of temporal dynamics and sensitivities for the olfactory system. To test this, we challenged different Drosophila OSNs with both varying stimulus durations (10–2000 ms), and repeated stimulus pulses of key ligands at various frequencies (1–10 Hz). Our results show that OR-expressing OSNs responded faster and with higher sensitivity to short stimulations as compared to IR- and Gr21a-expressing OSNs. In addition, OR-expressing OSNs could respond to repeated stimulations of excitatory ligands up to 5 Hz, while IR-expressing OSNs required ~5x longer stimulations and/or higher concentrations to respond to similar stimulus durations and frequencies. Nevertheless, IR-expressing OSNs did not exhibit adaptation to longer stimulations, unlike OR- and Gr21a-OSNs. Both OR- and IR-expressing OSNs were also unable to resolve repeated pulses of inhibitory ligands as fast as excitatory ligands. These differences were independent of the peri-receptor environment in which the receptors were expressed and suggest that the receptor expressed by a given OSN affects both its sensitivity and its response to transient, intermittent chemical stimuli. OR-expressing OSNs are better at resolving low dose, intermittent stimuli, while IR-expressing OSNs respond more accurately to long-lasting odor pulses. This diversity increases the capacity of the insect olfactory system to respond to the diverse spatiotemporal signals in the natural environment.

Functional and evolutionary aspects of chemoreceptors

The perception and processing of chemical signals from the environment is essential for any living systems and is most probably the first sense developed in life. This perspective discusses the physical limits of chemoreception and gives an overview on the receptor types developed during evolution to detect chemical signals from the outside world of an organism. It discusses the interaction of chemoreceptors with downstream signaling elements, especially the interaction between electrical and chemical signaling. It is further considered how the primary chemosignal is appropriately amplified. Three examples of chemosensory systems illustrate different strategies of such amplification.

Individual and synergistic effects of sniffing frequency and flow rate on olfactory bulb activity

Is faster or stronger sniffing important for the olfactory system? Odorant molecules are captured by sniffing. The features of sniffing constrain both the temporality and intensity of the input to the olfactory structures. In this context, it is clear that variations in both the sniff frequency and flow rate have a major impact on the activation of olfactory structures. However, the question of how frequency and flow rate individually or synergistically impact bulbar output has not been answered. We have addressed this question using multiple experimental approaches. In double-tracheotomized, anesthetized rats, we recorded both the bulbar local field potential (LFP) and mitral/tufted cells' activities when the sampling flow rate and frequency were controlled independently. We found that a tradeoff between the sampling frequency and the flow rate could maintain olfactory bulb sampling-related rhythmicity and that only an increase in flow rate could induce a faster, odor-evoked response. LFP and sniffing were recorded in awake rats. We found that sampling-related rhythmicity was maintained during high-frequency sniffing. Furthermore, we observed that the covariation between the frequency and flow rate, which was necessary for the tradeoff seen in the anesthetized preparations, also occurred in awake animals. Our study shows that the sampling frequency and flow rate can act either independently or synergistically on bulbar output to shape the neuronal message. The system likely takes advantage of this flexibility to adapt sniffing strategies to animal behavior. Our study provides additional support for the idea that sniffing and olfaction function in an integrated manner.