Sparse and selective odor coding by mitral/tufted neurons in the main olfactory bulb

Amplification of olfactory transduction currents implements sparse stimulus encoding

Sensory systems must perform the dual and opposing tasks of being sensitive to weak stimuli while also maintaining information content in dense and variable sensory landscapes. This occurs in the olfactory system, where OSNs are highly sensitive to low concentrations of odors and maintain discriminability in complex odor environments. How olfactory sensory neurons (OSNs) maintain both sensitivity and sparsity is poorly understood. Here, we investigated whether the calcium-activated chloride channel, TMEM16B, may support these dual roles in OSNs in both male and female mice. We used multiphoton microscopy to image the stimulus-response density of OSNs in the olfactory epithelium. In TMEM16B knockout mice, we found that sensory representations were denser, and the magnitude of OSN responses was increased. Behaviorally, these changes in sensory representations were associated with an increased aversion to the odorant trimethylamine, which switches perceptual valence as its concentration increases, and a decreased efficiency of olfactory-guided navigation. Our results indicate that the calcium-activated chloride channel TMEM16B sparsens sensory representations in the peripheral olfactory system and contributes to efficient integrative olfactory-guided behaviors.Significance Statement Sensory systems must build internal neural representations of stimuli found in the external environment. In the olfactory system, molecules that give rise to the perception of odors are detected by olfactory sensory neurons within the nose. Upon odorant binding to sensory neurons, a biochemical signaling cascade transduces neural signals that other areas of the brain can then read out. A key component of this cascade is the calcium-activated chloride channel TMEM16B. We found that despite its role in amplifying transduction currents in olfactory sensory neurons, TMEM16B paradoxically constrains their output, thereby limiting information transfer to the brain. Our findings also indicate that TMEM16B plays an important role in how animals detect and perceive odors.

Recalibrating Olfactory Neuroscience to the Range of Naturally Occurring Odor Concentrations

Sensory systems enable organisms to detect and respond to environmental signals relevant for their survival and reproduction. A crucial aspect of any sensory signal is its intensity; understanding how sensory signals guide behavior requires probing sensory system function across the range of stimulus intensities naturally experienced by an organism. In olfaction, defining the range of natural odorant concentrations is difficult. Odors are complex mixtures of airborne chemicals emitting from a source in an irregular pattern that varies across time and space, necessitating specialized methods to obtain an accurate measurement of concentration. Perhaps as a result, experimentalists often choose stimulus concentrations based on empirical considerations rather than with respect to ecological or behavioral context. Here, we attempt to determine naturally relevant concentration ranges for olfactory stimuli by reviewing and integrating data from diverse disciplines. We compare odorant concentrations used in experimental studies in rodents and insects with those reported in different settings including ambient natural environments, the headspace of natural sources, and within the sources themselves. We also compare these values to psychophysical measurements of odorant detection threshold in rodents, where thresholds have been extensively measured. Odorant concentrations in natural regimes rarely exceed a few parts per billion, while most experimental studies investigating olfactory coding and behavior exceed these concentrations by several orders of magnitude. We discuss the implications of this mismatch and the importance of testing odorants in their natural concentration range for understanding neural mechanisms underlying olfactory sensation and odor-guided behaviors.

Olfactory perception complexity induced by key odorants perceptual interactions of alcoholic beverages: Wine as a focus case example

The odorants in alcoholic beverages are frequently experienced as complex mixtures, and there is a complex array of influence factors and interactions involved during consumption that deeply increase its olfactory perception complexity, especially the complexity induced by perceptual interactions between different odorants. In this review, the effect of olfactory perceptual interactions and other factors related to the complexity of olfactory perception of alcoholic beverages are discussed. The classification, influencing factors, and mechanisms of olfactory perceptual interactions are outlined. Recent research progress as well as the methodologies applied in these studies on perceptual interactions between odorants observed in representative alcoholic beverages, especially wine, are briefly summarized. In the future, unified theory or systematic research methodology need to be established, since up to now, the rules of perceptual interaction between multiple odorants, which is critical to the alcoholic beverage industry to improve the flavor of their products, are still not revealed.

Development and analysis of an artificial olfactory bulb

This article presents the development of an artificial olfactory bulb (OB) using an electronic nose with thermally modulated metal-oxide sensors. Inspired by animal OBs, our approach employs thermal modulation to replicate the spatial encoding patterns of glomeruli clusters and subclusters. This new approach enhances the classification capabilities of traditional electronic noses and offers new insights for biomimetic olfaction. Molecular receptive range (MRR) analysis confirms that our artificial OB effectively mimics the glomerular distribution of animal OBs. Additionally, the incorporation of a short axon cell (SAC) network, inspired by the animal olfactory system, significantly improves lifetime sparseness and qualitative ability of the artificial OB through extensive lateral inhibition, providing a theoretical framework for enhanced olfactory performance.

Amplification of olfactory transduction currents implements sparse stimulus encoding

Sensory systems must perform the dual and opposing tasks of being sensitive to weak stimuli while also maintaining information content in dense and variable sensory landscapes. This occurs in the olfactory system, where OSNs are highly sensitive to low concentrations of odors and maintain discriminability in complex odor environments. How olfactory sensory neurons (OSNs) maintain both sensitivity and sparsity is not well understood. Here, we investigated whether the calcium-activated chloride channel, TMEM16B, may support these dual roles in OSNs. We used multiphoton microscopy to image the stimulus-response density of OSNs in the olfactory epithelium. In TMEM16B knockout mice, we found that sensory representations were denser, and the magnitude of OSN responses was increased. Behaviorally, these changes in sensory representations were associated with an increased aversion to the odorant trimethylamine, which switches perceptual valence as its concentration increases, and a decreased efficiency of olfactory-guided navigation. Together, our results indicate that the calcium-activated chloride channel TMEM16B sparsens sensory representations in the peripheral olfactory system and contributes to efficient integrative olfactory-guided behaviors.

Dense and Persistent Odor Representations in the Olfactory Bulb of Awake Mice

Recording and analysis of neural activity are often biased toward detecting sparse subsets of highly active neurons, masking important signals carried in low-magnitude and variable responses. To investigate the contribution of seemingly noisy activity to odor encoding, we used mesoscale calcium imaging from mice of both sexes to record odor responses from the dorsal surface of bilateral olfactory bulbs (OBs). The outer layer of the mouse OB is comprised of dendrites organized into discrete "glomeruli," which are defined by odor receptor-specific sensory neuron input. We extracted activity from a large population of glomeruli and used logistic regression to classify odors from individual trials with high accuracy. We then used add-in and dropout analyses to determine subsets of glomeruli necessary and sufficient for odor classification. Classifiers successfully predicted odor identity even after excluding sparse, highly active glomeruli, indicating that odor information is redundantly represented across a large population of glomeruli. Additionally, we found that random forest (RF) feature selection informed by Gini inequality (RF Gini impurity, RFGI) reliably ranked glomeruli by their contribution to overall odor classification. RFGI provided a measure of "feature importance" for each glomerulus that correlated with intuitive features like response magnitude. Finally, in agreement with previous work, we found that odor information persists in glomerular activity after the odor offset. Together, our findings support a model of OB odor coding where sparse activity is sufficient for odor identification, but information is widely, redundantly available across a large population of glomeruli, with each glomerulus representing information about more than one odor.

Firing Patterns of Mitral Cells and Their Transformation in the Main Olfactory Bulb

Mitral cells (MCs) in the main olfactory bulb relay odor information to higher-order olfactory centers by encoding the information in the form of action potentials. The firing patterns of these cells are influenced by both their intrinsic properties and their synaptic connections within the neural network. However, reports on MC firing patterns have been inconsistent, and the mechanisms underlying these patterns remain unclear. Using whole-cell patch-clamp recordings in mouse brain slices, we discovered that MCs exhibit two types of integrative behavior: regular/rhythmic firing and bursts of action potentials. These firing patterns could be transformed both spontaneously and chemically. MCs with regular firing maintained their pattern even in the presence of blockers of fast synaptic transmission, indicating this was an intrinsic property. However, regular firing could be transformed into bursting by applying GABAA receptor antagonists to block inhibitory synaptic transmission. Burst firing could be reverted to regular firing by blocking ionotropic glutamate receptors, rather than applying a GABAA receptor agonist, indicating that ionotropic glutamatergic transmission mediated this transformation. Further experiments on long-lasting currents (LLCs), which generated burst firing, also supported this mechanism. In addition, cytoplasmic Ca2+ in MCs was involved in the transformation of firing patterns mediated by glutamatergic transmission. Metabotropic glutamate receptors also played a role in LLCs in MCs. These pieces of evidence indicate that odor information can be encoded on a mitral cell (MC) platform, where it can be relayed to higher-order olfactory centers through intrinsic and dendrodendritic mechanisms in MCs.

Cholecystokinin-expressing superficial tufted cells modulate odour representation in the olfactory bulb and olfactory behaviours

In mammals, odour information within the olfactory bulb (OB) is processed by complex neural circuits before being ultimately represented in the action potential activity of mitral/tufted cells (M/Ts). Cholecystokinin-expressing (CCK+) superficial tufted cells (sTCs) are a subset of tufted cells that potentially contribute to olfactory processing in the OB by orchestrating M/T activity. However, the exact role of CCK+ sTCs in modulating odour processing and olfactory function in vivo is largely unknown. Here, we demonstrate that manipulating CCK+ sTCs can generate perception and induce place avoidance. Optogenetic activation/inactivation of CCK+ sTCs exerted strong but differing effects on spontaneous and odour-evoked M/T firing. Furthermore, inactivation of CCK+ sTCs disrupted M/T odour encoding and impaired olfactory detection and odour discrimination. These results establish the role of CCK+ sTCs in odour representation and olfactory behaviours. KEY POINTS: Mice could perceive the activity of CCK+ sTCs and show place avoidance to CCK+ sTC inactivation. Optical activation of CCK+ sTCs increased the percentage of cells with odour response but reduced the odour-evoked response in M/Ts in awake mice. Optical inactivation of CCK+ sTCs greatly decreased spontaneous firing and odour-evoked response in M/Ts. Inactivation of CCK+ sTCs impairs the odour decoding performance of M/Ts and disrupts odour detection and discrimination behaviours in mice. These results indicate that CCK+ sTCs participate in modulating the odour representation and maintaining normal olfactory-related behaviours.

Biosensors for Odor Detection: A Review

Animals can easily detect hundreds of thousands of odors in the environment with high sensitivity and selectivity. With the progress of biological olfactory research, scientists have extracted multiple biomaterials and integrated them with different transducers thus generating numerous biosensors. Those biosensors inherit the sensing ability of living organisms and present excellent detection performance. In this paper, we mainly introduce odor biosensors based on substances from animal olfactory systems. Several instances of organ/tissue-based, cell-based, and protein-based biosensors are described and compared. Furthermore, we list some other biological materials such as peptide, nanovesicle, enzyme, and aptamer that are also utilized in odor biosensors. In addition, we illustrate the further developments of odor biosensors.

Parallel Olfactory Systems Synergistically Activate the Posteroventral Part of the Medial Amygdala Upon Alarm Pheromone Detection in Rats

In rats, a mixture of hexanal and 4-methylpentanal is a main component of the alarm pheromone. When detected by the main olfactory system (MOS) and the vomeronasal system, respectively, they activate the anterior part of the bed nucleus of the stria terminalis (BNSTa). Therefore, the information from the two olfactory systems is expected to be integrated before being transmitted to the BNSTa. To specify the integration site, we examined Fos expression in 16 brain regions in response to water (n = 10), hexanal (n = 9), 4-methylpentanal (n = 9), the mixture (n = 9), or the alarm pheromone (n = 9) in male rats. The posteroventral part of the medial amygdala showed increased Fos expression to hexanal and 4-methylpentanal. The expression was further increased by the mixture. Therefore, this region is suggested as the integration site. In addition, the BNSTa, paraventricular nucleus of the hypothalamus, and anteroventral, anterodorsal, and posterodorsal parts of the medial amygdala were suggested to be located downstream of the integrated site because only the mixture increased Fos expression. We suggest that the posterolateral part of the cortical amygdala is upstream of the integration site in the MOS because all stimuli increased Fos expression. The posterior part of the bed nucleus of the stria terminalis and posteromedial part of the cortical amygdala were suggested as being located upstream in the vomeronasal system because 4-methylpentanal and the mixture increased Fos expression. These results provide information about the neural pathway underlying the alarm pheromone effects.

Olfactory response is a potential sign of consciousness: electroencephalogram findings

Objective

This study aimed to explore whether olfactory response can be a sign of consciousness and represent higher cognitive processing in patients with disorders of consciousness (DoC) using clinical and electroencephalogram data.

Methods

Twenty-eight patients with DoC [13 vegetative states (VS)/unresponsive wakefulness syndrome (UWS) and 15 minimally conscious states (MCS)] were divided into two groups: the presence of olfactory response (ORES) group and the absence of olfactory response (N-ORES) group according to behavioral signs from different odors, i.e., vanillin, decanoic acid, and blank stimuli. We recorded an olfactory task-related electroencephalogram (EEG) and analyzed the relative power and functional connectivity at the whole-brain level in patients with DoC and healthy controls (HCs). After three months, the outcomes of DoC patients were followed up using the coma recovery scale-revised (CRS-R).

Results

A significant relationship was found between olfactory responses and the level of consciousness (χ²(1) = 6.892, p = 0.020). For olfactory EEG, N-ORES patients showed higher theta functional connectivity than ORES patients after stimulation with vanillin (p = 0.029; p = 0.027). Patients with N-ORES showed lower alpha and beta relative powers than HCs at the group level (p = 0.019; p = 0.033). After three months, 62.5% (10/16) of the ORES patients recovered consciousness compared to 16.7% (2/12) in the N-ORES group. The presence of olfactory response was significantly associated with an improvement in consciousness (χ²(1) = 5.882, p = 0.023).

Conclusion

Olfactory responses should be considered signs of consciousness. The differences in olfactory processing between DoC patients with and without olfactory responses may be a way to explore the neural correlates of olfactory consciousness in these patients. The olfactory response may help in the assessment of consciousness and may contribute to therapeutic orientation.

Olfactory-auditory sensory integration in the lateral entorhinal cortex

Multisensory integration plays an important role in animal cognition. Although many studies have focused on visual-auditory integration, studies on olfactory-auditory integration are rare. Here, we investigated neural activity patterns and odor decoding in the lateral entorhinal cortex (LEC) under uni-sensory and multisensory stimuli in awake, head-fixed mice. Using specific retrograde tracing, we verified that the LEC receives direct inputs from the primary auditory cortex (AC) and the medial geniculate body (MGB). Strikingly, we found that mitral/tufted cells (M/Ts) in the olfactory bulb (OB) and neurons in the LEC respond to both olfactory and auditory stimuli. Sound decreased the neural responses evoked by odors in both the OB and LEC, for both excitatory and inhibitory responses. Interestingly, significant changes in odor decoding performance and modulation of odor-evoked local field potentials (LFPs) were observed only in the LEC. These data indicate that the LEC is a critical center for olfactory-auditory multisensory integration, with direct projections from both olfactory and auditory centers.

Stability and flexibility of odor representations in the mouse olfactory bulb

Dynamic changes in sensory representations have been basic tenants of studies in neural coding and plasticity. In olfaction, relatively little is known about the dynamic range of changes in odor representations under different brain states and over time. Here, we used time-lapse in vivo two-photon calcium imaging to describe changes in odor representation by mitral cells, the output neurons of the mouse olfactory bulb. Using anesthetics as a gross manipulation to switch between different brain states (wakefulness and under anesthesia), we found that odor representations by mitral cells undergo significant re-shaping across states but not over time within state. Odor representations were well balanced across the population in the awake state yet highly diverse under anesthesia. To evaluate differences in odor representation across states, we used linear classifiers to decode odor identity in one state based on training data from the other state. Decoding across states resulted in nearly chance-level accuracy. In contrast, repeating the same procedure for data recorded within the same state but in different time points, showed that time had a rather minor impact on odor representations. Relative to the differences across states, odor representations remained stable over months. Thus, single mitral cells can change dynamically across states but maintain robust representations across months. These findings have implications for sensory coding and plasticity in the mammalian brain.

Changes in spontaneous and odorant-induced single-unit activity of mitral/tufted neurons of the rat olfactory bulb during xylazine-tiletamine-zolazepam anesthesia

The nature and severity of mitral/tufted (M/T) cells reactions to odorants presented in anesthesia depend on various factors, and, above all, the nature and concentration of the odor, anesthesia, and the functional state of the olfactory bulb (OB). Compared to wakefulness, under anesthesia, the intensity of OB M/T cells responses to the odorants presented increases. However, the influence of anesthesia dynamics on the intensity of such responses has not been studied. To address this problem in rats, the activity of M/T cells and the local field potentials (LFP) in OB were recorded in the course of xylazine-tiletamine-zolazepam (XTZ) anesthesia. It has been shown that in the course of the anesthesia, the average frequency of background and odorant-induced single-unit activity of M/T cells increases, while the dominant frequency value of LFP in the gamma frequency range (90–170 Hz), on the contrary, decreases. The observed effects are assumed to be associated with changes in the functional state of the OB and systems for processing olfactory information in anesthesia.

Mapping odorant sensitivities reveals a sparse but structured representation of olfactory chemical space by sensory input to the mouse olfactory bulb

In olfactory systems, convergence of sensory neurons onto glomeruli generates a map of odorant receptor identity. How glomerular maps relate to sensory space remains unclear. We sought to better characterize this relationship in the mouse olfactory system by defining glomeruli in terms of the odorants to which they are most sensitive. Using high-throughput odorant delivery and ultrasensitive imaging of sensory inputs, we imaged responses to 185 odorants presented at concentrations determined to activate only one or a few glomeruli across the dorsal olfactory bulb. The resulting datasets defined the tuning properties of glomeruli - and, by inference, their cognate odorant receptors - in a low-concentration regime, and yielded consensus maps of glomerular sensitivity across a wide range of chemical space. Glomeruli were extremely narrowly tuned, with ~25% responding to only one odorant, and extremely sensitive, responding to their effective odorants at sub-picomolar to nanomolar concentrations. Such narrow tuning in this concentration regime allowed for reliable functional identification of many glomeruli based on a single diagnostic odorant. At the same time, the response spectra of glomeruli responding to multiple odorants was best predicted by straightforward odorant structural features, and glomeruli sensitive to distinct odorants with common structural features were spatially clustered. These results define an underlying structure to the primary representation of sensory space by the mouse olfactory system.

Biological constraints on configural odour mixture perception

Animals, including humans, detect odours and use this information to behave efficiently in the environment. Frequently, odours consist of complex mixtures of odorants rather than single odorants, and mixtures are often perceived as configural wholes, i.e. as odour objects (e.g. food, partners). The biological rules governing this 'configural perception' (as opposed to the elemental perception of mixtures through their components) remain weakly understood. Here, we first review examples of configural mixture processing in diverse species involving species-specific biological signals. Then, we present the original hypothesis that at least certain mixtures can be processed configurally across species. Indeed, experiments conducted in human adults, newborn rabbits and, more recently, in rodents and honeybees show that these species process some mixtures in a remarkably similar fashion. Strikingly, a mixture AB (A, ethyl isobutyrate; B, ethyl maltol) induces configural processing in humans, who perceive a mixture odour quality (pineapple) distinct from the component qualities (A, strawberry; B, caramel). The same mixture is weakly configurally processed in rabbit neonates, which perceive a particular odour for the mixture in addition to the component odours. Mice and honeybees also perceive the AB mixture configurally, as they respond differently to the mixture compared with its components. Based on these results and others, including neurophysiological approaches, we propose that certain mixtures are convergently perceived across various species of vertebrates/invertebrates, possibly as a result of a similar anatomical organization of their olfactory systems and the common necessity to simplify the environment's chemical complexity in order to display adaptive behaviours.

Paradoxical relationship between speed and accuracy in olfactory figure-background segregation

In natural settings, many stimuli impinge on our sensory organs simultaneously. Parsing these sensory stimuli into perceptual objects is a fundamental task faced by all sensory systems. Similar to other sensory modalities, increased odor backgrounds decrease the detectability of target odors by the olfactory system. The mechanisms by which background odors interfere with the detection and identification of target odors are unknown. Here we utilized the framework of the Drift Diffusion Model (DDM) to consider possible interference mechanisms in an odor detection task. We first considered pure effects of background odors on either signal or noise in the decision-making dynamics and showed that these produce different predictions about decision accuracy and speed. To test these predictions, we trained mice to detect target odors that are embedded in random background mixtures in a two-alternative choice task. In this task, the inter-trial interval was independent of behavioral reaction times to avoid motivating rapid responses. We found that increased backgrounds reduce mouse performance but paradoxically also decrease reaction times, suggesting that noise in the decision making process is increased by backgrounds. We further assessed the contributions of background effects on both noise and signal by fitting the DDM to the behavioral data. The models showed that background odors affect both the signal and the noise, but that the paradoxical relationship between trial difficulty and reaction time is caused by the added noise.

Sensory systems are constantly stimulated by signals from many objects in the environment. Segmentation of important signals from the cluttered background is therefore a task that is faced by all sensory systems. For many mammalians, the sense of smell is the primary sense that guides many daily behaviors. As such, the olfactory system must be able to detect and identify odors of interest against varying and dynamic backgrounds. Here we studied how background odors interfere with the detection of target odors. We trained mice on a task in which they are presented with odor mixtures and are required to report whether they include either of two target odors. We analyze the behavioral data using a common model of sensory-guided decision-making—the drift-diffusion-model. In this model, decisions are influenced by two elements: a drift which is the signal produced by the stimulus, and noise. We show that the addition of background odors has a dual effect—a reduction in the drift, as well as an increase in the noise. The increased noise also causes more rapid decisions, thereby producing a paradoxical relationship between trial difficulty and decision speed; mice make faster decisions on more difficult trials.

Light-regulated voltage-gated potassium channels for acute interrogation of channel function in neurons and behavior

Voltage-gated potassium (Kv) channels regulate the membrane potential and conductance of excitable cells to control the firing rate and waveform of action potentials. Even though Kv channels have been intensely studied for over 70 year, surprisingly little is known about how specific channels expressed in various neurons and their functional properties impact neuronal network activity and behavior in vivo. Although many in vivo genetic manipulations of ion channels have been tried, interpretation of these results is complicated by powerful homeostatic plasticity mechanisms that act to maintain function following perturbations in excitability. To better understand how Kv channels shape network function and behavior, we have developed a novel optogenetic technology to acutely regulate Kv channel expression with light by fusing the light-sensitive LOV domain of Vaucheria frigida Aureochrome 1 to the N-terminus of the Kv1 subunit protein to make an Opto-Kv1 channel. Recording of Opto-Kv1 channels expressed in Xenopus oocytes, mammalian cells, and neurons show that blue light strongly induces the current expression of Opto-Kv1 channels in all systems tested. We also find that an Opto-Kv1 construct containing a dominant-negative pore mutation (Opto-Kv1(V400D)) can be used to down-regulate Kv1 currents in a blue light-dependent manner. Finally, to determine whether Opto-Kv1 channels can elicit light-dependent behavioral effect in vivo, we targeted Opto-Kv1 (V400D) expression to Kv1.3-expressing mitral cells of the olfactory bulb in mice. Exposure of the bulb to blue light for 2–3 hours produced a significant increase in sensitivity to novel odors after initial habituation to a similar odor, comparable to behavioral changes seen in Kv1.3 knockout animals. In summary, we have developed novel photoactivatable Kv channels that provide new ways to interrogate neural circuits in vivo and to examine the roles of normal and disease-causing mutant Kv channels in brain function and behavior.

Encoding innately recognized odors via a generalized population code

Odors carrying intrinsic values often trigger instinctive aversive or attractive responses. It is not known how innate valence is encoded. An intuitive model suggests that the information is conveyed through specific channels in hardwired circuits along the olfactory pathway, insulated from influences of other odors, to trigger innate responses. Here, we show that in mice, mixing innately aversive or attractive odors with a neutral odor and, surprisingly, mixing two odors with the same valence, abolish the innate behavioral responses. Recordings from the olfactory bulb indicate that odors are not masked at the level of peripheral activation and glomeruli independently encode components in the mixture. In contrast, crosstalk among the mitral and tufted (M/T) cells changes their patterns of activity such that those elicited by the mixtures can no longer be linearly decoded as separate components. The changes in behavioral and M/T cell responses are associated with reduced activation of brain areas linked to odor preferences. Thus, crosstalk among odor channels at the earliest processing stage in the olfactory pathway leads to re-coding of odor identity to abolish valence associated with the odors. These results are inconsistent with insulated labeled lines and support a model of a common mechanism of odor recognition for both innate and learned valence associations.

Effect of Interglomerular Inhibitory Networks on Olfactory Bulb Odor Representations

Lateral inhibition is a fundamental feature of circuits that process sensory information. In the mammalian olfactory system, inhibitory interneurons called short axon cells (SACs) comprise the first network mediating lateral inhibition between glomeruli, the functional units of early olfactory coding and processing. The connectivity of this network and its impact on odor representations is not well understood. To explore this question, we constructed a computational model of the interglomerular inhibitory network using detailed characterizations of SAC morphologies taken from mouse olfactory bulb (OB). We then examined how this network transformed glomerular patterns of odorant-evoked sensory input (taken from previously-published datasets) as a function of the selectivity of interglomerular inhibition. We examined three connectivity schemes: selective (each glomerulus connects to few others with heterogeneous strength), nonselective (glomeruli connect to most others with heterogenous strength), or global (glomeruli connect to all others with equal strength). We found that both selective and nonselective interglomerular networks could mediate heterogeneous patterns of inhibition across glomeruli when driven by realistic sensory input patterns, but that global inhibitory networks were unable to produce input-output transformations that matched experimental data and were poor mediators of intensity-dependent gain control. We further found that networks whose interglomerular connectivities were tuned by sensory input profile decorrelated odor representations moreeffectively. These results suggest that, despite their multiglomerular innervation patterns, SACs are capable of mediating odorant-specific patterns of inhibition between glomeruli that could, theoretically, be tuned by experience or evolution to optimize discrimination of particular odorants.SIGNIFICANCE STATEMENT Lateral inhibition is a key feature of circuitry in many sensory systems including vision, audition, and olfaction. We investigate how lateral inhibitory networks mediated by short axon cells (SACs) in the mouse olfactory bulb (OB) might shape odor representations as a function of their interglomerular connectivity. Using a computational model of interglomerular connectivity derived from experimental data, we find that SAC networks, despite their broad innervation patterns, can mediate heterogeneous patterns of inhibition across glomeruli, and that the canonical model of global inhibition does not generate experimentally observed responses to stimuli. In addition, inhibitory connections tuned by input statistics yield enhanced decorrelation of similar input patterns. These results elucidate how the organization of inhibition between neural elements may affect computations.

Cortical processing of configurally perceived odor mixtures

Most odors are not composed of a single volatile chemical species, but rather are mixtures of many different volatile molecules, the perception of which is dependent on the identity and relative concentrations of the components. Changing either the identity or ratio of components can lead to shifts between configural and elemental perception of the mixture. For example, a 30/70 ratio of ethyl isobutyrate (odorant A, a strawberry scent) and ethyl maltol (odorant B, a caramel scent) is perceived as pineapple by humans - a configural percept distinct from the components. In contrast, a 68/32 ratio of the same odorants is perceived elementally, and is identified as the component odors. Here, we examined single-unit responses in the anterior and posterior piriform cortex (aPCX and pPCX) of mice to these A and B mixtures. We first demonstrate that mouse behavior is consistent with a configural/elemental perceptual shift as concentration ratio varies. We then compared responses to the configural mixture to those evoked by the elemental mixture, as well as to the individual components. Hierarchical cluster analyses suggest that in the mouse aPCX, the configural mixture was coded as distinct from both components, while the elemental mixture was coded as similar to the components. In contrast, mixture perception did not predict pPCX ensemble coding. Similar electrophysiological results were also observed in rats. The results suggest similar perceptual characteristics of the AB mixture across species, and a division in the roles of aPCX and pPCX in the coding of configural and elemental odor mixtures.

A stochastic model for interacting neurons in the olfactory bulb

We focus on interacting neurons organized in a block-layered network devoted to the information processing from the sensory system to the brain. Specifically, we consider the firing activity of olfactory sensory neurons, periglomerular, granule and mitral cells in the context of the neuronal activity of the olfactory bulb. We propose and investigate a stochastic model of a layered and modular network to describe the dynamic behavior of each prototypical neuron, taking into account both its role (excitatory/inhibitory) and its location within the network. We adopt specific Gauss-Markov processes suitable to provide reliable estimates of the firing activity of the different neurons, given their linkages. Furthermore, we study the impact of selective excitation/inhibition on the information transmission by means of simulations and numerical estimates obtained through a Volterra integral approach.

Target specific functions of EPL interneurons in olfactory circuits

Inhibitory interneurons are integral to sensory processing, yet revealing their cell type-specific roles in sensory circuits remains an ongoing focus. To Investigate the mouse olfactory system, we selectively remove GABAergic transmission from a subset of olfactory bulb interneurons, EPL interneurons (EPL-INs), and assay odor responses from their downstream synaptic partners — tufted cells and mitral cells. Using a combination of in vivo electrophysiological and imaging analyses, we find that inactivating this single node of inhibition leads to differential effects in magnitude, reliability, tuning width, and temporal dynamics between the two principal neurons. Furthermore, tufted and not mitral cell responses to odor mixtures become more linearly predictable without EPL-IN inhibition. Our data suggest that olfactory bulb interneurons, through exerting distinct inhibitory functions onto their different synaptic partners, play a significant role in the processing of odor information.

The precise cell-type specific role of inhibitory interneurons in regulating sensory responses in the olfactory bulb is not known. Here, the authors report that removing GABAergic inhibition from one layer differentially affects response dynamics of the two main output cell types and changes odor mixture processing.

A Novel Olfactometer for Efficient and Flexible Odorant Delivery

Understanding how sensory space maps to neural activity in the olfactory system requires efficiently and flexibly delivering numerous odorants within single experimental preparations. Such delivery is difficult with current olfactometer designs, which typically include limited numbers of stimulus channels and are subject to intertrial and interchannel contamination of odorants. Here, we present a novel olfactometer design that is easily constructed, modular, and capable of delivering an unlimited number of odorants in air with temporal precision and no detectable intertrial or interchannel contamination. The olfactometer further allows for the flexible generation of odorant mixtures and flexible timing of odorant sequences. Odorant delivery from the olfactometer is turbulent but reliable from trial to trial, supporting operant conditioning of mice in an odorant discrimination task and permitting odorants and concentrations to be mapped to neural activity with a level of precision equivalent to that obtained with a flow dilution olfactometer. This novel design thus provides several unique advantages for interrogating olfactory perception and for mapping sensory space to neural activity in the olfactory system.

Strong, weak and neuron type dependent lateral inhibition in the olfactory bulb

In many sensory systems, different sensory features are transmitted in parallel by several different types of output neurons. In the mouse olfactory bulb, there are only two output neuron types, the mitral and tufted cells (M/T), which receive similar odor inputs, but they are believed to transmit different odor characteristics. How these two neuron types deliver different odor information is unclear. Here, by combining electrophysiology and optogenetics, it is shown that distinct inhibitory networks modulate M/T cell responses differently. Overall strong lateral inhibition was scarce, with most neurons receiving lateral inhibition from a handful of unorganized surrounding glomeruli (~5% on average). However, there was a considerable variability between different neuron types in the strength and frequency of lateral inhibition. Strong lateral inhibition was mostly found in neurons locked to the first half of the respiration cycle. In contrast, weak inhibition arriving from many surrounding glomeruli was relatively more common in neurons locked to the late phase of the respiration cycle. Proximal neurons could receive different levels of inhibition. These results suggest that there is considerable diversity in the way M/T cells process odors so that even neurons that receive the same odor input transmit different odor information to the cortex.

Sensory Adaptation to Chemical Cues by Vomeronasal Sensory Neurons

Sensory adaptation is a source of experience-dependent feedback that impacts responses to environmental cues. In the mammalian main olfactory system (MOS), adaptation influences sensory coding at its earliest processing stages. Sensory adaptation in the accessory olfactory system (AOS) remains incompletely explored, leaving many aspects of the phenomenon unclear. We investigated sensory adaptation in vomeronasal sensory neurons (VSNs) using a combination of in situ Ca²⁺ imaging and electrophysiology. Parallel studies revealed prominent short-term sensory adaptation in VSNs upon repeated stimulation with mouse urine and monomolecular bile acid ligands at interstimulus intervals (ISIs) less than 30 s. In such conditions, Ca²⁺ signals and spike rates were often reduced by more than 50%, leading to dramatically reduced chemosensory sensitivity. Short-term adaptation was reversible over the course of minutes. Population Ca²⁺ imaging experiments revealed the presence of a slower form of VSN adaptation that accumulated over dozens of stimulus presentations delivered over tens of minutes. Most VSNs showed strong adaptation, but in a substantial VSN subpopulation adaptation was diminished or absent. Investigation of same- and opposite-sex urine responses in male and female VSNs revealed that adaptation to same-sex cues occurred at ISIs up to 180 s, conditions that did not induce adaptation to opposite-sex cues. This result suggests that VSN sensory adaptation can be modulated by sensory experience. These studies comprehensively establish the presence of VSN sensory adaptation and provide a foundation for future inquiries into the molecular and cellular mechanisms of this phenomenon and its impact on mammalian behavior.

Differential inhibition of pyramidal cells and inhibitory interneurons along the rostrocaudal axis of anterior piriform cortex

The spatial representation of stimuli in sensory neocortices provides a scaffold for elucidating circuit mechanisms underlying sensory processing. However, the anterior piriform cortex (APC) lacks topology for odor identity as well as afferent and intracortical excitation. Consequently, olfactory processing is considered homogenous along the APC rostral-caudal (RC) axis. We recorded excitatory and inhibitory neurons in APC while optogenetically activating GABAergic interneurons along the RC axis. In contrast to excitation, we find opposing, spatially asymmetric inhibition onto pyramidal cells (PCs) and interneurons. PCs are strongly inhibited by caudal stimulation sites, whereas interneurons are strongly inhibited by rostral sites. At least two mechanisms underlie spatial asymmetries. Enhanced caudal inhibition of PCs is due to increased synaptic strength, whereas rostrally biased inhibition of interneurons is mediated by increased somatostatin-interneuron density. Altogether, we show differences in rostral and caudal inhibitory circuits in APC that may underlie spatial variation in odor processing along the RC axis.

A Mirror-Symmetric Excitatory Link Coordinates Odor Maps across Olfactory Bulbs and Enables Odor Perceptual Unity

Sensory input reaching the brain from bilateral and offset channels is nonetheless perceived as unified. This unity could be explained by simultaneous projections to both hemispheres, or inter-hemispheric information transfer between sensory cortical maps. Odor input, however, is not topographically organized, nor does it project bilaterally, making olfactory perceptual unity enigmatic. Here we report a circuit that interconnects mirror-symmetric isofunctional mitral/tufted cells between the mouse olfactory bulbs. Connected neurons respond to similar odors from ipsi- and contra-nostrils, whereas unconnected neurons do not respond to odors from the contralateral nostril. This connectivity is likely mediated through a one-to-one mapping from mitral/tufted neurons to the ipsilateral anterior olfactory nucleus pars externa, which activates the mirror-symmetric isofunctional mitral/tufted neurons glutamatergically. This circuit enables sharing of odor information across hemispheres in the absence of a cortical topographical organization, suggesting that olfactory glomerular maps are the equivalent of cortical sensory maps found in other senses.

Functional Plasticity of Odor Representations during Motherhood

Motherhood is accompanied by new behaviors aimed at ensuring the wellbeing of the offspring. Olfaction plays a key role in guiding maternal behaviors during this transition. We studied functional changes in the main olfactory bulb (OB) of mothers in mice. Using in vivo two-photon calcium imaging, we studied the sensory representation of odors by mitral cells (MCs). We show that MC responses to monomolecular odors become sparser and weaker in mothers. In contrast, responses to biologically relevant odors are spared from sparsening or strengthen. MC responses to mixtures and to a range of concentrations suggest that these differences between odor responses cannot be accounted for by mixture suppressive effects or gain control mechanisms. In vitro whole-cell recordings show an increase in inhibitory synaptic drive onto MCs. The increase of inhibitory tone may contribute to the general decrease in responsiveness and concomitant enhanced representation of specific odors.

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MCs of mothers show sparser responses for pure odors

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MCs of mothers have stronger inhibitory drive onto MCs

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MCs of mothers show stronger responses to natural odors

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MC ensemble coding is improved for natural but not pure odors

A wearable system for olfactory electrophysiological recording and animal motion control

BACKGROUND

Bran-computer interface (BCI) is an important technique used in brain science. However, the large size of equipment and wires severely limit its practical applications.

NEW METHODS

This study presents a wearable system with bidirectional brain-computer interface based on Wi-Fi technology, which can be used for olfactory electrophysiological recording and animal motion control.

RESULTS

On the "brain-to-computer" side, the results show that the wireless system can record high-quality olfactory electrophysiological signals for over a month. By analyzing the recorded data, we find that the same mitral/tufted (M/T) cells can be activated by many odorants and different M/T cells can be activated by a single odorant. Further, we find neurons in dorsal lateral OB are highly sensitive to isoamyl acetate. On the "computer-to-brain" side, the results show that we can efficiently control rats' motions by applying electrical stimulations to electrodes implanted in specific brain regions.

COMPARISON WITH EXISTING METHODS

Most existing wireless BCI systems are designed for either recording or stimulating while our system is a bidirectional BCI featured with both functions. Taking advantage of our years of experience in olfactory decoding, we developed the first wireless system for olfactory electrophysiological recording and animal motion control. It provides high-quality recording and efficient motion control for a long time.

CONCLUSIONS

The system provides possibility of practical BCI applications, such as in vivo bioelectronic nose and "rat-robot".

Odor Perception on the Two Sides of the Brain: Consistency Despite Randomness

Neurons in piriform cortex receive input from a random collection of glomeruli, resulting in odor representations that lack the stereotypic organization of the olfactory bulb. We have performed in vivo optical imaging and mathematical modeling to demonstrate that correlations are retained in the transformation from bulb to piriform cortex, a feature essential for generalization across odors. Random connectivity also implies that the piriform representation of a given odor will differ among different individuals and across brain hemispheres in a single individual. We show that these different representations can nevertheless support consistent agreement about odor quality across a range of odors. Our model also demonstrates that, whereas odor discrimination and categorization require far fewer neurons than reside in piriform cortex, consistent generalization may require the full complement of piriform neurons.

POU6f1 Mediates Neuropeptide-Dependent Plasticity in the Adult Brain

The mouse olfactory bulb (OB) features continued, activity-dependent integration of adult-born neurons, providing a robust model with which to examine mechanisms of plasticity in the adult brain. We previously reported that local OB interneurons secrete the neuropeptide corticotropin-releasing hormone (CRH) in an activity-dependent manner onto adult-born granule neurons and that local CRH signaling promotes expression of synaptic machinery in the bulb. This effect is mediated via activation of the CRH receptor 1 (CRHR1), which is developmentally regulated during adult-born neuron maturation. CRHR1 is a G_S-protein-coupled receptor that activates CREB-dependent transcription in the presence of CRH. Therefore, we hypothesized that locally secreted CRH activates CRHR1 to initiate circuit plasticity programs. To identify such programs, we profiled gene expression changes associated with CRHR1 activity in adult-born neurons of the OB. Here, we show that CRHR1 activity influences expression of the brain-specific Homeobox-containing transcription factor POU Class 6 Homeobox 1 (POU6f1). To elucidate the contributions of POU6f1 toward activity-dependent circuit remodeling, we targeted CRHR1⁺ neurons in male and female mice for cell-type-specific manipulation of POU6f1 expression. Whereas loss of POU6f1 in CRHR1⁺ neurons resulted in reduced dendritic complexity and decreased synaptic connectivity, overexpression of POU6f1 in CRHR1⁺ neurons promoted dendritic outgrowth and branching and influenced synaptic function. Together, these findings suggest that the transcriptional program directed by POU6f1 downstream of local CRH signaling in adult-born neurons influences circuit dynamics in response to activity-dependent peptide signaling in the adult brain.SIGNIFICANCE STATEMENT Elucidating mechanisms of plasticity in the adult brain is helpful for devising strategies to understand and treat neurodegeneration. Circuit plasticity in the adult mouse olfactory bulb is exemplified by both continued cell integration and synaptogenesis. We previously reported that these processes are influenced by local neuropeptide signaling in an activity-dependent manner. Here, we show that local corticotropin-releasing hormone (CRH) signaling induces dynamic gene expression changes in CRH receptor expressing adult-born neurons, including altered expression of the transcription factor POU6f1 We further show that POU6f1 is necessary for proper dendrite specification and patterning, as well as synapse development and function in adult-born neurons. Together, these findings reveal a novel mechanism by which peptide signaling modulates adult brain circuit plasticity.

The olfactory bulb as the entry site for prion-like propagation in neurodegenerative diseases

Olfactory deficits are present in numerous neurodegenerative disorders and are accompanied by pathology in related brain regions. In several of these disorders, olfactory disturbances appear early and are considered as prodromal symptoms of the disease. In addition, pathological protein aggregates affect olfactory regions prior to other regions, suggesting that the olfactory system might be particularly vulnerable to neurodegenerative diseases. Exposed to the external environment, the olfactory epithelium and olfactory bulb allow pathogen and toxin penetration into the brain, a process that has been proposed to play a role in neurodegenerative diseases. Determining whether the olfactory bulb could be a starting point of pathology and of pathology spread is crucial to understanding how neurodegenerative diseases evolve. We argue that pathological changes following environmental insults contribute to the initiation of protein aggregation in the olfactory bulb, which then triggers the spread of the pathology within the brain by a templating mechanism in a prion-like manner. We review the evidence for the early involvement of olfactory structures in neurodegenerative diseases and the relationship between neuropathology and olfactory function. We discuss the vulnerability and putative underlying mechanisms by which pathology could be initiated in the olfactory bulb, from the entry of pathogens (promoted by increased permeability of the olfactory epithelium with aging or inflammation) to the sensitivity of the olfactory system to oxidative stress and inflammation. Finally, we review changes in protein expression and neural excitability triggered by pathogenic proteins that can promote pathogenesis in the olfactory bulb and beyond.

Biomimetic Sensors for the Senses: Towards Better Understanding of Taste and Odor Sensation

Taste and smell are very important chemical senses that provide indispensable information on food quality, potential mates and potential danger. In recent decades, much progress has been achieved regarding the underlying molecular and cellular mechanisms of taste and odor senses. Recently, biosensors have been developed for detecting odorants and tastants as well as for studying ligand-receptor interactions. This review summarizes the currently available biosensing approaches, which can be classified into two main categories: in vitro and in vivo approaches. The former is based on utilizing biological components such as taste and olfactory tissues, cells and receptors, as sensitive elements. The latter is dependent on signals recorded from animals' signaling pathways using implanted microelectrodes into living animals. Advantages and disadvantages of these two approaches, as well as differences in terms of sensing principles and applications are highlighted. The main current challenges, future trends and prospects of research in biomimetic taste and odor sensors are discussed.

History-Dependent Odor Processing in the Mouse Olfactory Bulb

In nature, animals normally perceive sensory information on top of backgrounds. Thus, the neural substrate to perceive under background conditions is inherent in all sensory systems. Where and how sensory systems process backgrounds is not fully understood. In olfaction, just a few studies have addressed the issue of odor coding on top of continuous odorous backgrounds. Here, we tested how background odors are encoded by mitral cells (MCs) in the olfactory bulb (OB) of male mice. Using in vivo two-photon calcium imaging, we studied how MCs responded to odors in isolation versus their responses to the same odors on top of continuous backgrounds. We show that MCs adapt to continuous odor presentation and that mixture responses are different when preceded by background. In a subset of odor combinations, this history-dependent processing was useful in helping to identify target odors over background. Other odorous backgrounds were highly dominant such that target odors were completely masked by their presence. Our data are consistent in both low and high odor concentrations and in anesthetized and awake mice. Thus, odor processing in the OB is strongly influenced by the recent history of activity, which could have a powerful impact on how odors are perceived.

SIGNIFICANCE STATEMENT We examined a basic feature of sensory processing in the olfactory bulb. Specifically, we measured how mitral cells adapt to continuous background odors and how target odors are encoded on top of such background. Our results show clear differences in odor coding based on the immediate history of the stimulus. Our results support the argument that odor coding in the olfactory bulb depends on the recent history of the sensory environment.

Direct Recording of Dendrodendritic Excitation in the Olfactory Bulb: Divergent Properties of Local and External Glutamatergic Inputs Govern Synaptic Integration in Granule Cells

The olfactory bulb contains excitatory principal cells (mitral and tufted cells) that project to cortical targets as well as inhibitory interneurons. How the local circuitry in this region facilitates odor-specific output is not known, but previous work suggests that GABAergic granule cells plays an important role, especially during fine odor discrimination. Principal cells interact with granule cells through reciprocal dendrodendritic connections that are poorly understood. While many studies examined the GABAergic output side of these reciprocal connections, little is known about how granule cells are excited. Only two previous studies reported monosynaptically coupled mitral/granule cell connections and neither attempted to determine the fundamental properties of these synapses. Using dual intracellular recordings and a custom-built loose-patch amplifier, we have recorded unitary granule cell EPSPs evoked in response to mitral cell action potentials in rat (both sexes) brain slices. We find that the unitary dendrodendritic input is relatively weak with highly variable release probability and short-term depression. In contrast with the weak dendrodendritic input, the facilitating cortical input to granule cells is more powerful and less variable. Our computational simulations suggest that dendrodendritic synaptic properties prevent individual principal cells from strongly depolarizing granule cells, which likely discharge in response to either concerted activity among a large proportion of inputs or coactivation of a smaller subset of local dendrodendritic inputs with coincidence excitation from olfactory cortex. This dual-pathway requirement likely enables the sparse mitral/granule cell interconnections to develop highly odor-specific responses that facilitate fine olfactory discrimination.SIGNIFICANCE STATEMENT The olfactory bulb plays a central role in converting broad, highly overlapping, sensory input patterns into odor-selective population responses. How this occurs is not known, but experimental and theoretical studies suggest that local inhibition often plays a central role. Very little is known about how the most common local interneuron subtype, the granule cell, is excited during odor processing beyond the unusual anatomical arraignment of the interconnections (reciprocal dendrodendritic synapses). Using paired recordings and two-photon imaging, we determined the properties of the primary input to granule cells for the first time and show that these connections bias interneurons to fire in response to spiking in large populations of principal cells rather than a small group of highly active cells.

Inhibitory circuits of the mammalian main olfactory bulb

Synaptic inhibition critically influences sensory processing throughout the mammalian brain, including the main olfactory bulb (MOB), the first station of sensory processing in the olfactory system. Decades of research across numerous laboratories have established a central role for granule cells (GCs), the most abundant GABAergic interneuron type in the MOB, in the precise regulation of principal mitral and tufted cell (M/TC) firing rates and synchrony through lateral and recurrent inhibitory mechanisms. In addition to GCs, however, the MOB contains a vast diversity of other GABAergic interneuron types, and recent findings suggest that, while fewer in number, these oft-ignored interneurons are just as important as GCs in shaping odor-evoked M/TC activity. Here I challenge the prevailing centrality of GCs. In this review, I first outline the specific properties of each GABAergic interneuron type in the rodent MOB, with particular emphasis placed on direct interneuron recordings and cell type-selective manipulations. On the basis of these properties, I then critically reevaluate the contribution of GCs vs. other interneuron types to the regulation of odor-evoked M/TC firing rates and synchrony via lateral, recurrent, and other inhibitory mechanisms. This analysis yields a novel model in which multiple interneuron types with distinct abundances, connectivity patterns, and physiologies complement one another to regulate M/TC activity and sensory processing.

Behavioral Status Influences the Dependence of Odorant-Induced Change in Firing on Prestimulus Firing Rate

The firing rate of the mitral/tufted cells in the olfactory bulb is known to undergo significant trial-to-trial variability and is affected by anesthesia. Here we ask whether odorant-elicited changes in firing rate depend on the rate before application of the stimulus in the awake and anesthetized mouse. We find that prestimulus firing rate varies widely on a trial-to-trial basis and that the stimulus-induced change in firing rate decreases with increasing prestimulus firing rate. Interestingly, this prestimulus firing rate dependence was different when the behavioral task did not involve detecting the valence of the stimulus. Finally, when the animal was learning to associate the odor with reward, the prestimulus firing rate was smaller for false alarms compared with correct rejections, suggesting that intrinsic activity reflects the anticipatory status of the animal. Thus, in this sensory modality, changes in behavioral status alter the intrinsic prestimulus activity, leading to a change in the responsiveness of the second-order neurons. We speculate that this trial-to-trial variability in odorant responses reflects sampling of the massive parallel input by subsets of mitral cells.SIGNIFICANCE STATEMENT The olfactory bulb must deal with processing massive parallel input from ∼1200 distinct olfactory receptors. In contrast, the visual system receives input from a small number of photoreceptors and achieves recognition of complex stimuli by allocating processing for distinct spatial locations to different brain areas. Here we find that the change in firing rate elicited by the odorant in second-order mitral cells depends on the intrinsic activity leading to a change of magnitude in the responsiveness of these neurons relative to this prestimulus activity. Further, we find that prestimulus firing rate is influenced by behavioral status. This suggests that there is top-down modulation allowing downstream brain processing areas to perform dynamic readout of olfactory information.

Heterogeneous effects of norepinephrine on spontaneous and stimulus-driven activity in the male accessory olfactory bulb

Norepinephrine (NE) release has been linked to experience-dependent plasticity in many model systems and brain regions. Among these is the rodent accessory olfactory system (AOS), which is crucial for detecting and processing socially relevant environmental cues. The accessory olfactory bulb (AOB), the first site of chemosensory information processing in the AOS, receives dense centrifugal innervation by noradrenergic fibers originating in the locus coeruleus. Although NE release has been linked to behavioral plasticity through its actions in the AOB, the impacts of noradrenergic modulation on AOB information processing have not been thoroughly studied. We made extracellular single-unit recordings of AOB principal neurons in ex vivo preparations of the early AOS taken from adult male mice. We analyzed the impacts of bath-applied NE (10 μM) on spontaneous and stimulus-driven activity. In the presence of NE, we observed overall suppression of stimulus-driven neuronal activity with limited impact on spontaneous activity. NE-associated response suppression in the AOB came in two forms: one that was strong and immediate (21%) and one other that involved gradual, stimulus-dependent monotonic response suppression (47%). NE-associated changes in spontaneous activity were more modest, with an overall increase in spontaneous spike frequency observed in 25% of neurons. Neurons with increased spontaneous activity demonstrated a net decrease in chemosensory discriminability. These results reveal that noradrenergic signaling in the AOB causes cell-specific changes in chemosensory tuning, even among similar projection neurons.NEW & NOTEWORTHY Norepinephrine (NE) is released throughout the brain in many behavioral contexts, but its impacts on information processing are not well understood. We studied the impact of NE on chemosensory tuning in the mouse accessory olfactory bulb (AOB). Electrophysiological recordings from AOB neurons in ex vivo preparations revealed that NE, on balance, inhibited mitral cell responses to chemosensory cues. However, NE's effects were heterogeneous, indicating that NE signaling reshapes AOB output in a cell- and stimulus-specific manner.

Normalized Neural Representations of Complex Odors

The olfactory system removes correlations in natural odors using a network of inhibitory neurons in the olfactory bulb. It has been proposed that this network integrates the response from all olfactory receptors and inhibits them equally. However, how such global inhibition influences the neural representations of odors is unclear. Here, we study a simple statistical model of the processing in the olfactory bulb, which leads to concentration-invariant, sparse representations of the odor composition. We show that the inhibition strength can be tuned to obtain sparse representations that are still useful to discriminate odors that vary in relative concentration, size, and composition. The model reveals two generic consequences of global inhibition: (i) odors with many molecular species are more difficult to discriminate and (ii) receptor arrays with heterogeneous sensitivities perform badly. Comparing these predictions to experiments will help us to understand the role of global inhibition in shaping normalized odor representations in the olfactory bulb.

Dense encoding of natural odorants by ensembles of sparsely activated neurons in the olfactory bulb

Sensory information undergoes substantial transformation along sensory pathways, usually encompassing sparsening of activity. In the olfactory bulb, though natural odorants evoke dense glomerular input maps, mitral and tufted (M/T) cells tuning is considered to be sparse because of highly odor-specific firing rate change. However, experiments used to draw this conclusion were either based on recordings performed in anesthetized preparations or used monomolecular odorants presented at arbitrary concentrations. In this study, we evaluated the lifetime and population sparseness evoked by natural odorants by capturing spike temporal patterning of neuronal assemblies instead of individual M/T tonic activity. Using functional imaging and tetrode recordings in awake mice, we show that natural odorants at their native concentrations are encoded by broad assemblies of M/T cells. While reducing odorant concentrations, we observed a reduced number of activated glomeruli representations and consequently a narrowing of M/T tuning curves. We conclude that natural odorants at their native concentrations recruit M/T cells with phasic rather than tonic activity. When encoding odorants in assemblies, M/T cells carry information about a vast number of odorants (lifetime sparseness). In addition, each natural odorant activates a broad M/T cell assembly (population sparseness).

Balancing the Robustness and Efficiency of Odor Representations during Learning

For reliable stimulus identification, sensory codes have to be robust by including redundancy to combat noise, but redundancy sacrifices coding efficiency. To address how experience affects the balance between the robustness and efficiency of sensory codes, we probed odor representations in the mouse olfactory bulb during learning over a week, using longitudinal two-photon calcium imaging. When mice learned to discriminate between two dissimilar odorants, responses of mitral cell ensembles to the two odorants gradually became less discrete, increasing the efficiency. In contrast, when mice learned to discriminate between two very similar odorants, the initially overlapping representations of the two odorants became progressively decorrelated, enhancing the robustness. Qualitatively similar changes were observed when the same odorants were experienced passively, a condition that would induce implicit perceptual learning. These results suggest that experience adjusts odor representations to balance the robustness and efficiency depending on the similarity of the experienced odorants.

Adult Neurogenesis in the Songbird: Region-Specific Contributions of New Neurons to Behavioral Plasticity and Stability

Our understanding of the role of new neurons in learning and encoding new information has been largely based on studies of new neurons in the mammalian dentate gyrus and olfactory bulb - brain regions that may be specialized for learning. Thus the role of new neurons in regions that serve other functions has yet to be fully explored. The song system provides a model for studying new neuron function in brain regions that contribute differently to song learning, song auditory discrimination, and song motor production. These regions subserve learning as well as long-term storage of previously learned information. This review examines the differences between learning-based and activity-based retention of new neurons and explores the potential contributions of new neurons to behavioral stability in the song motor production pathway.

Population imaging at subcellular resolution supports specific and local inhibition by granule cells in the olfactory bulb

Information processing in early sensory regions is modulated by a diverse range of inhibitory interneurons. We sought to elucidate the role of olfactory bulb interneurons called granule cells (GCs) in odor processing by imaging the activity of hundreds of these cells simultaneously in mice. Odor responses in GCs were temporally diverse and spatially disperse, with some degree of non-random, modular organization. The overall sparseness of activation of GCs was highly correlated with the extent of glomerular activation by odor stimuli. Increasing concentrations of single odorants led to proportionately larger population activity, but some individual GCs had non-monotonic relations to concentration due to local inhibitory interactions. Individual dendritic segments could sometimes respond independently to odors, revealing their capacity for compartmentalized signaling in vivo. Collectively, the response properties of GCs point to their role in specific and local processing, rather than global operations such as response normalization proposed for other interneurons.

Decoding of Context-Dependent Olfactory Behavior in Drosophila

Odor information is encoded in the activity of a population of glomeruli in the primary olfactory center. However, how this information is decoded in the brain remains elusive. Here, we address this question in Drosophila by combining neuronal imaging and tracking of innate behavioral responses. We find that the behavior is accurately predicted by a model summing normalized glomerular responses, in which each glomerulus contributes a specific, small amount to odor preference. This model is further supported by targeted manipulations of glomerular input, which biased the behavior. Additionally, we observe that relative odor preference changes and can even switch depending on the context, an effect correctly predicted by our normalization model. Our results indicate that olfactory information is decoded from the pooled activity of a glomerular repertoire and demonstrate the ability of the olfactory system to adapt to the statistics of its environment.

Massive normalization of olfactory bulb output in mice with a 'monoclonal nose'

Perturbations in neural circuits can provide mechanistic understanding of the neural correlates of behavior. In M71 transgenic mice with a “monoclonal nose”, glomerular input patterns in the olfactory bulb are massively perturbed and olfactory behaviors are altered. To gain insights into how olfactory circuits can process such degraded inputs we characterized odor-evoked responses of olfactory bulb mitral cells and interneurons. Surprisingly, calcium imaging experiments reveal that mitral cell responses in M71 transgenic mice are largely normal, highlighting a remarkable capacity of olfactory circuits to normalize sensory input. In vivo whole cell recordings suggest that feedforward inhibition from olfactory bulb periglomerular cells can mediate this signal normalization. Together, our results identify inhibitory circuits in the olfactory bulb as a mechanistic basis for many of the behavioral phenotypes of mice with a “monoclonal nose” and highlight how substantially degraded odor input can be transformed to yield meaningful olfactory bulb output.

DOI:http://dx.doi.org/10.7554/eLife.16335.001

The lining of the nose contains cells called olfactory sensory neurons that allow different smells to be detected. Odor molecules bind to receptor proteins that are embedded in the surface of the olfactory sensory neuron. Different receptors respond to different odors, and the nose contains hundreds of different receptors that work together to distinguish thousands of scents. When an odor molecule binds to a receptor, it triggers a pattern of electrical activity in the neuron. These patterns are the building blocks that allow smells to be recognized and if necessary, acted upon – by not eating food that smells rancid, for example.

In 2008, researchers genetically engineered mice so that nearly all of their olfactory sensory neurons produced the same type of olfactory receptor. Unexpectedly, these mice could still detect and discriminate between many different smells. Now, Roland, Jordan, Sosulski et al. – including several of the researchers involved in the 2008 study – have tracked the brain activity of these mice as they were exposed to various smells to find out how they can recognize such a wide range of odors with such a limited repertoire of receptors.

The results of the experiments revealed that neural circuits in the brains of these modified mice still produce largely normal patterns of activity in response to an odor. This ‘normalization’ of activity relies on a fine balance between ‘excitatory’ processes that increase the activity of neurons and ‘inhibitory’ processes that reduce this activity.

Overall, the findings of Roland, Jordan, Sosulski et al. provide a link between how a scent is detected and how this information is processed in the brain. In future experiments, it will be important to determine how this processing of odor information is influenced by learning and experience to generate the long-lasting odor memories that guide behavior.

DOI:http://dx.doi.org/10.7554/eLife.16335.002

Spatial Mapping in the Rat Olfactory Bulb by Odor and Direct Electrical Stimulation

OBJECTIVES

To directly measure the spatial mapping in the olfactory bulb by odor presentation and by direct electrical stimulation.

STUDY DESIGN

Experimental (animal).

SETTING

University research laboratory.

SUBJECTS AND METHODS

Odor (n = 8) and electrical stimulation (n = 4) of the olfactory bulb in rats were used to demonstrate the spatial mapping of neural responses in the olfactory bulb. Both multiunit responses to odor stimulation and evoked potential responses to localized electrical stimulation were measured in different regions of the olfactory bulb.

RESULTS

Responses that were recorded simultaneously from an array of 32 electrodes positioned at different locations within the olfactory bulb were mapped. Results show different spatial patterns of neural activity for different odors (odor maps). Direct stimulation of the olfactory bulb with electrical current pulses from electrodes positioned at different locations was also effective in generating spatial patterns of neural activity.

CONCLUSION

These data suggest that by programming an array of stimulating electrodes, it should be possible to selectively activate different regions of the olfactory bulb, generating unique patterns of neural activity as seen in normal smell.

Rapid and continuous activity-dependent plasticity of olfactory sensory input

Incorporation of new neurons enables plasticity and repair of circuits in the adult brain. Adult neurogenesis is a key feature of the mammalian olfactory system, with new olfactory sensory neurons (OSNs) wiring into highly organized olfactory bulb (OB) circuits throughout life. However, neither when new postnatally generated OSNs first form synapses nor whether OSNs retain the capacity for synaptogenesis once mature, is known. Therefore, how integration of adult-born OSNs may contribute to lifelong OB plasticity is unclear. Here, we use a combination of electron microscopy, optogenetic activation and in vivo time-lapse imaging to show that newly generated OSNs form highly dynamic synapses and are capable of eliciting robust stimulus-locked firing of neurons in the mouse OB. Furthermore, we demonstrate that mature OSN axons undergo continuous activity-dependent synaptic remodelling that persists into adulthood. OSN synaptogenesis, therefore, provides a sustained potential for OB plasticity and repair that is much faster than OSN replacement alone.