Rapid encoding and perception of novel odors in the rat

Do rodents smell with sound?

Chemosensation via olfaction is a critical process underlying social interactions in many different species. Past studies of olfaction in mammals often have focused on its mechanisms in isolation from other systems, limiting the generalizability of findings from olfactory research to perceptual processes in other modalities. Studies of chemical communication, in particular, have progressed independently of research on vocal behavior and acoustic communication. Those bioacousticians who have considered how sound production and reception might interact with olfaction often portray odors as cues to the kinds of vocalizations that might be functionally useful. In the olfaction literature, vocalizations are rarely mentioned. Here, we propose that ultrasonic vocalizations may affect what rodents smell by altering the deposition of inhaled particles and that rodents coordinate active sniffing with sound production specifically to enhance reception of pheromones. In this scenario, rodent vocalizations may contribute to a unique mode of active olfactory sensing, in addition to whatever roles they serve as social signals. Consideration of this hypothesis highlights the perceptual advantages that parallel coordination of multiple sensorimotor processes may provide to individuals exploring novel situations and environments, especially those involving dynamic social interactions.

Recent odor experience selectively modulates olfactory sensitivity across the glomerular output in the mouse olfactory bulb

Although animals can reliably locate and recognize odorants embedded in complex environments, the neural circuits for accomplishing these tasks remain incompletely understood. Adaptation is likely to be important as it could allow neurons in a brain area to adjust to the broader sensory environment. Adaptive processes must be flexible enough to allow the brain to make dynamic adjustments, while maintaining sufficient stability so that organisms do not forget important olfactory associations. Processing within the mouse olfactory bulb is likely involved in generating adaptation, although there are conflicting models of how it transforms the glomerular output of the mouse olfactory bulb. Here we performed 2-photon Ca 2+ imaging from mitral/tufted glomeruli in awake mice to determine the time course of recovery from adaptation, and whether it acts broadly or selectively across the glomerular population. Individual glomerular responses, as well as the overall population odor representation was similar across imaging sessions. However, odor-concentration pairings presented with interstimulus intervals upwards of 30-s evoked heterogeneous adaptation that was concentration-dependent. We demonstrate that this form of adaptation is unrelated to variations in respiration, and olfactory receptor neuron glomerular measurements indicate that it is unlikely to be inherited from the periphery. Our results indicate that the olfactory bulb output can reliably transmit stable odor representations, but recent odor experiences can selectively shape neural responsiveness for upwards of 30 seconds. We propose that neural circuits that allow for non-uniform adaptation across mitral/tufted glomerular could be important for making dynamic adjustments in complex odor environments.

Sniffing can be initiated by dopamine's actions on ventral striatum neurons

Sniffing is a motivated behavior displayed by nearly all terrestrial vertebrates. While sniffing is associated with acquiring and processing odors, sniffing is also intertwined with affective and motivated states. The neuromodulatory systems which influence the display of sniffing are unclear. Here, we report that dopamine release into the ventral striatum is coupled with bouts of sniffing and that stimulation of dopaminergic terminals in these regions drives increases in respiratory rate to initiate sniffing whereas inhibition of these terminals reduces respiratory rate. Both the firing of individual neurons and the activity of post-synaptic D1 and D2 receptor-expressing neurons in the ventral striatum are also coupled with sniffing and local antagonism of D1 and D2 receptors squelches sniffing. Together, these results support a model whereby sniffing can be initiated by dopamine's actions upon ventral striatum neurons. The nature of sniffing being integral to both olfaction and motivated behaviors implicates this circuit in a wide array of functions.

Preclinical investigation of the effect of stress on the binding of [18F]F13640, a 5-HT1A radiopharmaceutical

BACKGROUND

[18F]F13640 is a new PET radiopharmaceutical for brain molecular imaging of serotonin 5-HT1A receptors. Since we intend to use this radiopharmaceutical in psychiatric studies, it is crucial to establish possible sensitivity modification of 5-HT1A receptors availability during an acute stress exposure. In this study, we first assessed the cerebrometabolic effects of a new animal model of stress with [18F]FDG and then proceeded to test for effects of this model on the cerebral binding of [18F]F13640, a 5-HT1A receptors PET radiopharmaceutical.

METHODS

Four groups of male Sprague-Dawley were used to identify the optimal model: "stressed group" (n = 10), "post-traumatic stress disorder (PTSD) group" (n = 9) and "restraint group" (n = 8), compared with a control group (n = 8). All rats performed neuroimaging [18F]FDG μPET-CT to decipher which model was the most appropriate to test effects of stress on radiotracer binding. Subsequently, a group of rats (n = 10) underwent two PET imaging acquisitions (baseline and PTSD condition) using the PET radiopharmaceutical [18F]F13640 to assess influence of stress on its binding. Voxel-based analysis was performed to assess [18F]FDG or [18F]F13640 changes.

RESULTS

In [18F]FDG experiments, the PTSD group showed a pattern of cerebrometabolic activation in various brain regions previously implicated in stress (amygdala, perirhinal cortex, olfactory bulb and caudate). [18F]F13640 PET scans showed increased radiotracer binding in the PTSD condition in caudate nucleus and brainstem.

CONCLUSIONS

The present study demonstrated stress-induced cerebrometabolic activation or inhibition of various brain regions involved in stress model. Applying this model to our radiotracer, [18F]F13640 showed few influence of stress on its binding. This will enable to rule out any confounding effect of stress during imaging studies.

Adult-neurogenesis allows for representational stability and flexibility in early olfactory system

In the early olfactory system, adult-neurogenesis, a process of neuronal replacement results in the continuous reorganization of synaptic connections and network architecture throughout the animal's life. This poses a critical challenge: How does the olfactory system maintain stable representations of odors and therefore allow for stable sensory perceptions amidst this ongoing circuit instability? Utilizing a detailed spiking network model of early olfactory circuits, we uncovered dual roles for adult-neurogenesis: one that both supports representational stability to faithfully encode odor information and also one that facilitates plasticity to allow for learning and adaptation. In the main olfactory bulb, adult-neurogenesis affects neural codes in individual mitral and tufted cells but preserves odor representations at the neuronal population level. By contrast, in the olfactory piriform cortex, both individual cell responses and overall population dynamics undergo progressive changes due to adult-neurogenesis. This leads to representational drift, a gradual alteration in sensory perception. Both processes are dynamic and depend on experience such that repeated exposure to specific odors reduces the drift due to adult-neurogenesis; thus, when the odor environment is stable over the course of adult-neurogenesis, it is neurogenesis that actually allows the representations to remain stable in piriform cortex; when those olfactory environments change, adult-neurogenesis allows the cortical representations to track environmental change. Whereas perceptual stability and plasticity due to learning are often thought of as two distinct, often contradictory processing in neuronal coding, we find that adult-neurogenesis serves as a shared mechanism for both. In this regard, the quixotic presence of adult-neurogenesis in the mammalian olfactory bulb that has been the focus of considerable debate in chemosensory neuroscience may be the mechanistic underpinning behind an array of complex computations.

Wireless monitoring of respiration with EEG reveals relationships between respiration, behavior, and brain activity in freely moving mice

Active sampling in the olfactory domain is a fundamental aspect of mouse behavior, and there is increasing evidence that respiration-entrained neural activity outside of the olfactory system sets an important global brain rhythm. It is therefore crucial to accurately measure breathing during natural behaviors. We develop a new approach to do this in freely moving animals, by implanting a telemetry-based pressure sensor into the right jugular vein, which allows for wireless monitoring of thoracic pressure. After verifying this technique against standard head-fixed respiration measurements, we combined it with EEG and EMG recording and used evolving partial coherence analysis to investigate the relationship between respiration and brain activity across a range of experiments in which the mice could move freely. During voluntary exploration of odors and objects, we found that the association between respiration and cortical activity in the delta and theta frequency range decreased, whereas the association between respiration and cortical activity in the alpha range increased. During sleep, however, the presentation of an odor was able to cause a transient increase in sniffing without changing dominant sleep rhythms (delta and theta) in the cortex. Our data align with the emerging idea that the respiration rhythm could act as a synchronizing scaffold for specific brain rhythms during wakefulness and exploration, but suggest that respiratory changes are less able to impact brain activity during sleep. Combining wireless respiration monitoring with different types of brain recording across a variety of behaviors will further increase our understanding of the important links between active sampling, passive respiration, and neural activity.NEW & NOTEWORTHY Animals can alter their respiration rate to actively sample their environment, and increasing evidence suggests that neurons across the brain align their firing to this changing rhythm. We developed a new approach to measure sniffing in freely moving mice while simultaneously recording brain activity, and uncovered how specific cortical rhythms changed their coherence with respiration rhythm during natural behaviors and across arousal states.

Simultaneous recording of ultrasonic vocalizations and sniffing from socially interacting individual rats using a miniature microphone

Vocalizations are pivotal in mammalian communication, especially in humans. Rodents accordingly rely on ultrasonic vocalizations (USVs) that reflect their internal state as a primary channel during social interactions. However, attributing vocalizations to specific individuals remains challenging, impeding internal state assessment. Rats emit 50-kHz USVs to indicate positive states and intensify sniffing during alertness and social interactions. Here, we present a method involving a miniature microphone attached to the rat nasal cavity that allows to capture both male and female individual rat vocalizations and sniffing patterns during social interactions. We found that while the emission of 50-kHz USVs increases during close interactions, these signals lack specific behavioral associations. Moreover, a previously unreported low-frequency vocalization type marking rat social interactions was uncovered. Finally, different dynamics of sniffing and vocalization activities point to distinct underlying internal states. Thus, our method facilitates the exploration of internal states concurrent with social behaviors.

Odor encoding by signals in the olfactory bulb

To understand the operation of the olfactory system, it is essential to know how information is encoded in the olfactory bulb. We applied Shannon information theoretic methods to address this, with signals from up to 57 glomeruli simultaneously optically imaged from presynaptic inputs in glomeruli in the mouse dorsal (dOB) and lateral (lOB) olfactory bulb, in response to six exemplar pure chemical odors. We discovered that, first, the tuning of these signals from glomeruli to a set of odors is remarkably broad, with a mean sparseness of 0.83 and a mean signal correlation of 0.64. Second, both of these factors contribute to the low information that is available from the responses of even populations of many tens of glomeruli, which was only 1.35 bits across 33 glomeruli on average, compared with the 2.58 bits required to perfectly encode these six odors. Third, although there is considerable interest in the possibility of temporal encoding of stimulus including odor identity, the amount of information in the temporal aspects of the presynaptic glomerular responses was low (mean 0.11 bits) and, importantly, was redundant with respect to the information available from the rates. Fourth, the information from simultaneously recorded glomeruli asymptotes very gradually and nonlinearly, showing that glomeruli do not have independent responses. Fifth, the information from a population became available quite rapidly, within 100 ms of sniff onset, and the peak of the glomerular response was at 200 ms. Sixth, the information from the lOB was not additive with that of the dOB.NEW & NOTEWORTHY We report broad tuning and low odor information available across the lateral and dorsal bulb populations of glomeruli. Even though response latencies can be significantly predictive of stimulus identity, such contained very little information and none that was not redundant with information based on rate coding alone. Last, in line with the emerging notion of the important role of earliest stages of responses ("primacy"), we report a very rapid rise in information after each inhalation.

Understanding Categorical Learning in Neural Circuits Through the Primary Olfactory Cortex

Knowing which elements in the environment are associated with various opportunities and dangers is advantageous. A major role of mammalian sensory systems is to provide information about the identity of such elements which can then be used for adaptive action planning by the animal. Identity-tuned sensory representations are categorical, invariant to nuances in the sensory stream and depend on associative learning. Although categorical representations are well documented across several sensory modalities, these tend to situate synaptically far from the sensory organs which reduces experimenter control over input-output transformations. The formation of such representations is a fundamental neural computation that remains poorly understood. Odor representations in the primary olfactory cortex have several characteristics that qualify them as categorical and identity-tuned, situated only two synapses away from the sensory epithelium. The formation of categorical representations is likely critically dependent on—and dynamically controlled by—recurrent circuitry within the primary olfactory cortex itself. Experiments suggest that the concerted activity of several neuromodulatory systems plays a decisive role in shaping categorical learning through complex interactions with recurrent activity and plasticity in primary olfactory cortex circuits. In this perspective we discuss missing pieces of the categorical learning puzzle, and why several features of olfaction make it an attractive model system for this challenge.

Whole-brain studies of spontaneous behavior in head-fixed rats enabled by zero echo time MB-SWIFT fMRI

Understanding the link between the brain activity and behavior is a key challenge in modern neuroscience. Behavioral neuroscience, however, lacks tools to record whole-brain activity in complex behavioral settings. Here we demonstrate that a novel Multi-Band SWeep Imaging with Fourier Transformation (MB-SWIFT) functional magnetic resonance imaging (fMRI) approach enables whole-brain studies in spontaneously behaving head-fixed rats. First, we show anatomically relevant functional parcellation. Second, we show sensory, motor, exploration, and stress-related brain activity in relevant networks during corresponding spontaneous behavior. Third, we show odor-induced activation of olfactory system with high correlation between the fMRI and behavioral responses. We conclude that the applied methodology enables novel behavioral study designs in rodents focusing on tasks, cognition, emotions, physical exercise, and social interaction. Importantly, novel zero echo time and large bandwidth approaches, such as MB-SWIFT, can be applied for human behavioral studies, allowing more freedom as body movement is dramatically less restricting factor.

Top-down feedback enables flexible coding strategies in the olfactory cortex

In chemical sensation, multiple models have been proposed to explain how odors are represented in the olfactory cortex. One hypothesis is that the combinatorial identity of active neurons within sniff-related time windows is critical, whereas another model proposes that it is the temporal structure of neural activity that is essential for encoding odor information. We find that top-down feedback to the main olfactory bulb dictates the information transmitted to the piriform cortex and switches between these coding strategies. Using a detailed network model, we demonstrate that feedback control of inhibition influences the excitation-inhibition balance in mitral cells, restructuring the dynamics of piriform cortical cells. This results in performance improvement in odor discrimination tasks. These findings present a framework for early olfactory computation, where top-down feedback to the bulb flexibly shapes the temporal structure of neural activity in the piriform cortex, allowing the early olfactory system to dynamically switch between two distinct coding models.

Minimal cross-trial generalization in learning the representation of an odor-guided choice task

There is no single way to represent a task. Indeed, despite experiencing the same task events and contingencies, different subjects may form distinct task representations. As experimenters, we often assume that subjects represent the task as we envision it. However, such a representation cannot be taken for granted, especially in animal experiments where we cannot deliver explicit instruction regarding the structure of the task. Here, we tested how rats represent an odor-guided choice task in which two odor cues indicated which of two responses would lead to reward, whereas a third odor indicated free choice among the two responses. A parsimonious task representation would allow animals to learn from the forced trials what is the better option to choose in the free-choice trials. However, animals may not necessarily generalize across odors in this way. We fit reinforcement-learning models that use different task representations to trial-by-trial choice behavior of individual rats performing this task, and quantified the degree to which each animal used the more parsimonious representation, generalizing across trial types. Model comparison revealed that most rats did not acquire this representation despite extensive experience. Our results demonstrate the importance of formally testing possible task representations that can afford the observed behavior, rather than assuming that animals' task representations abide by the generative task structure that governs the experimental design.

Protocol of controlled odorant stimulation for reducing apnoeic episodes in premature newborns: a randomised open-label Latin-square study with independent evaluation of the main endpoint (PREMODEUR)

INTRODUCTION

Apnoea affects 85% of premature infants under 34 weeks of age and would be an important risk factor for subsequent neuropsychological disorders. Currently, premature children with life-threatening apnoeas receive stimulants such as methylxanthines (mainly, caffeine) or doxapram (an analeptic unlicensed in children under 15). However, these products have undesirable effects (hyperarousal, irritability, sleep disorders, tachycardia) and are not always effective because apnoea does persist in some premature newborns. Previous studies have indicated that odorant stimulation, a non-invasive intervention, may stimulate the respiratory rhythm. The objective of the present protocol is to reduce the occurrence of apnoeic episodes in premature newborns by controlled odorant stimulation added to current pharmacological treatments.

METHODS AND ANALYSIS

The project is a randomised open-label Latin-square trial with independent evaluation of the main endpoint. It will include 60 preterm neonates from two university hospital neonatal intensive care units over 2 years (2021-2023). Each newborn will receive no (S0), sham (S1) or real olfactory stimulation (S2) in random order. During S2, three distinct odorants (mint, grapefruit and vanilla) will be delivered successively, in puffs, over 24 hours. Mint and grapefruit odours stimulate the main and the trigeminal olfactory pathways, whereas vanilla odour stimulates only the main olfactory pathway. A statistical analysis will compare the incidence of apnoeic episodes during S1 versus S2 using a mixed effects Poisson model.

ETHICS AND DISSEMINATION

Ethical approval was obtained from the Comité de Protection des Personnes Île-de-France XI (# 2017-AO13-50-53). The results will be disseminated through various scientific meetings, specialised peer-reviewed journals and, whenever possible, posted on appropriate public websites.

TRIAL REGISTRATION NUMBER

NCT02851979; Pre-results.

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.

Finding the Brain in the Nose

Olfaction is fundamentally distinct from other sensory modalities. Natural odor stimuli are complex mixtures of volatile chemicals that interact in the nose with a receptor array that, in rodents, is built from more than 1,000 unique receptors. These interactions dictate a peripheral olfactory code, which in the brain is transformed and reformatted as it is broadcast across a set of highly interconnected olfactory regions. Here we discuss the problems of characterizing peripheral population codes for olfactory stimuli, of inferring the specific functions of different higher olfactory areas given their extensive recurrence, and of ultimately understanding how odor representations are linked to perception and action. We argue that, despite the differences between olfaction and other sensory modalities, addressing these specific questions will reveal general principles underlying brain function.

Neuroscience: Illuminating Principles of Odor Coding

Neuroscientists still are not sure what makes any two odors smell alike. A new study uses light to manipulate the sensory cells in our nose that respond to odors and reveals that both the timing and identity of activated cells influence odor perception.

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.

A Neural Mechanism of Cue-Outcome Expectancy Generated by the Interaction Between Orbitofrontal Cortex and Amygdala

Taste perception is important for animals to take adequate nutrients and avoid toxins for their survival. Appetitive and aversive behaviors are produced by value evaluation of taste and taste expectation caused by other sensations. The value evaluation, coupled with a cue presentation, produces outcome expectation and guides flexible behaviors when the environment is changed. Experimental studies demonstrated distinct functional roles of basolateral amygdala (ABL) and orbitofrontal cortex (OFC) in value evaluation and adaptive behavior. ABL is involved in generating a cue-outcome association, whereas OFC makes a contribution of generating a cue-triggered expectation to guide adaptive behavior. However, it remains unclear how ABL and OFC form their functional roles, with the learning of adaptive behavior. To address this issue, we focus on an odor discrimination task of rats and develop a computational model that consists of OFC and ABL, interacting with reward and decision systems. We present the neural mechanisms underlying the rapid formation of cue-outcome association in ABL and late behavioral adaptation mediated by OFC. Moreover, we offer 2 functions of cue-selective neurons in OFC: one is that the activation of cue-selective neurons transmits value information to decision area to guide behavior and another is that persistent activity of cue-selective neurons evokes a weak activity of taste-sensitive OFC neurons, leading to cue-outcome expectation. Our model further accounts for ABL and OFC responses caused by lesions of these areas. The results provide a computational framework of how ABL and OFC are functionally linked through their interactions with the reward and decision systems.

Odor coding in piriform cortex: mechanistic insights into distributed coding

Olfaction facilitates a large variety of animal behaviors such as feeding, mating, and communication. Recent work has begun to reveal the logic of odor transformations that occur throughout the olfactory system to form the odor percept. In this review, we describe the coding principles and mechanisms by which the piriform cortex and other olfactory areas encode three key odor features: odor identity, intensity, and valence. We argue that the piriform cortex produces a multiplexed odor code that allows non-interfering representations of distinct features of the odor stimulus to facilitate odor recognition and learning, which ultimately drives behavior.

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

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

Precise Holographic Manipulation of Olfactory Circuits Reveals Coding Features Determining Perceptual Detection

Sensory systems transform the external world into time-varying spike trains. What features of spiking activity are used to guide behavior? In the mouse olfactory bulb, inhalation of different odors leads to changes in the set of neurons activated, as well as when neurons are activated relative to each other (synchrony) and the onset of inhalation (latency). To explore the relevance of each mode of information transmission, we probed the sensitivity of mice to perturbations across each stimulus dimension (i.e., rate, synchrony, and latency) using holographic two-photon optogenetic stimulation of olfactory bulb neurons with cellular and single-action-potential resolution. We found that mice can detect single action potentials evoked synchronously across <20 olfactory bulb neurons. Further, we discovered that detection depends strongly on the synchrony of activation across neurons, but not the latency relative to inhalation.

Respiration-Locking of Olfactory Receptor and Projection Neurons in the Mouse Olfactory Bulb and Its Modulation by Brain State

For sensory systems of the brain, the dynamics of an animal’s own sampling behavior has a direct consequence on ensuing computations. This is particularly the case for mammalian olfaction, where a rhythmic flow of air over the nasal epithelium entrains activity in olfactory system neurons in a phenomenon known as sniff-locking. Parameters of sniffing can, however, change drastically with brain states. Coupled to the fact that different observation methods have different kinetics, consensus on the sniff-locking properties of neurons is lacking. To address this, we investigated the sniff-related activity of olfactory sensory neurons (OSNs), as well as the principal neurons of the olfactory bulb (OB), using 2-photon calcium imaging and intracellular whole-cell patch-clamp recordings in vivo, both in anesthetized and awake mice. Our results indicate that OSNs and OB output neurons lock robustly to the sniff rhythm, but with a slight temporal shift between behavioral states. We also observed a slight delay between methods. Further, the divergent sniff-locking by tufted cells (TCs) and mitral cells (MCs) in the absence of odor can be used to determine the cell type reliably using a simple linear classifier. Using this classification on datasets where morphological identification is unavailable, we find that MCs use a wider range of temporal shifts to encode odors than previously thought, while TCs have a constrained timing of activation due to an early-onset hyperpolarization. We conclude that the sniff rhythm serves as a fundamental rhythm but its impact on odor encoding depends on cell type, and this difference is accentuated in awake mice.

Top-Down Control of Inhibitory Granule Cells in the Main Olfactory Bulb Reshapes Neural Dynamics Giving Rise to a Diversity of Computations

Growing evidence shows that top-down projections from excitatory neurons in piriform cortex selectively synapse onto local inhibitory granule cells in the main olfactory bulb, effectively gating their own inputs by controlling inhibition. An open question in olfaction is the role this feedback plays in shaping the dynamics of local circuits, and the resultant computational benefits it provides. Using rate models of neuronal firing in a network consisting of excitatory mitral and tufted cells, inhibitory granule cells and top-down piriform cortical neurons, we found that changes in the weight of feedback to inhibitory neurons generated diverse network dynamics and complex transitions between these dynamics. Changes in the weight of top-down feedback supported a number of computations, including both pattern separation and oscillatory synchrony. Additionally, the network could generate gamma oscillations though a mechanism we termed Top-down control of Inhibitory Neuron Gamma (TING). Collectively, these functions arose from a codimension-2 bifurcation in the dynamical system. Our results highlight a key role for this top-down feedback, gating inhibition to facilitate often diametrically different computations.

Differential Impacts of Repeated Sampling on Odor Representations by Genetically-Defined Mitral and Tufted Cell Subpopulations in the Mouse Olfactory Bulb

Sniffing, the active control of breathing beyond passive respiration, is used by mammals to modulate olfactory sampling. Sniffing allows animals to make odor-guided decisions within ∼200 ms, but animals routinely engage in bouts of high-frequency sniffing spanning several seconds; the impact of such repeated odorant sampling on odor representations remains unclear. We investigated this question in the mouse olfactory bulb (OB), where mitral and tufted cells (MTCs) form parallel output streams of odor information processing. To test the impact of repeated odorant sampling on MTC responses, we used two-photon imaging in anesthetized male and female mice to record activation of MTCs while precisely varying inhalation frequency. A combination of genetic targeting and viral expression of GCaMP6 reporters allowed us to access mitral cell (MC) and superficial tufted cell (sTC) subpopulations separately. We found that repeated odorant sampling differentially affected responses in MCs and sTCs, with MCs showing more diversity than sTCs over the same time period. Impacts of repeated sampling among MCs included both increases and decreases in excitation, as well as changes in response polarity. Response patterns across simultaneously-imaged MCs reformatted over time, with representations of different odorants becoming more distinct. Individual MCs responded differentially to changes in inhalation frequency, whereas sTC responses were more uniform over time and across frequency. Our results support the idea that MCs and TCs comprise functionally distinct pathways for odor information processing, and suggest that the reformatting of MC odor representations by high-frequency sniffing may serve to enhance the discrimination of similar odors.SIGNIFICANCE STATEMENT Repeated sampling of odorants during high-frequency respiration (sniffing) is a hallmark of active odorant sampling by mammals; however, the adaptive function of this behavior remains unclear. We found distinct effects of repeated sampling on odor representations carried by the two main output channels from the mouse olfactory bulb (OB), mitral and tufted cells (MTCs). Mitral cells (MCs) showed more diverse changes in response patterns over time as compared with tufted cells (TCs), leading to odorant representations that were more distinct after repeated sampling. These results support the idea that MTCs contribute different aspects to encoding odor information, and they indicate that MCs (but not TCs) may play a primary role in the modulation of olfactory processing by sampling behavior.

Spatiotemporal dynamics of odor responses in the lateral and dorsal olfactory bulb

The mammalian olfactory bulb (OB) plays an essential role in odor processing during the perception of smell. Optical imaging of the OB has proven to be a key tool in elucidating the spatial odor mapping and temporal dynamics that underlie higher-order odor processing. Much is known about the activation of olfactory sensory neuron (OSN) glomerular responses in the dorsal olfactory bulb (dOB) during odor presentation. However, the dorsal bulb provides access to only approximately 25% of all glomeruli, and little is known about how the lateral bulb functions during this critical process. Here, we report, for the first time, simultaneous measurements of OSN glomerular activity from both the dOB and the lateral olfactory bulb (lOB), thus describing odor-specific spatial mapping and the temporal dynamics of olfactory input to both the dorsal and lateral bulb. Odor responses in the lateral bulb tended to be most prominent in the dorso-lateral (D-L) region. Lateral glomeruli became active in a dorso-ventral (D-V) sequence upon odor inhalation, unlike the anterio-posterior (A-P) activity wave typical of the dorsal glomeruli. Across the entire D-L bulb, the spatial organization of these dynamics can be explained neither by the purely mechanosensitive dynamics (to breathing clean air) nor by the response amplitudes across glomeruli. Instead, these dynamics can be explained by a combination of zonal receptor distributions, associated OB projections, and air flow paths across the epithelium upon inhalation. Remarkably, we also found that a subset of OSN glomeruli in the lOB was highly sensitive to extranasal air pressure changes, a response type that has not been reported in dorsal glomeruli.

The mammalian olfactory bulb plays an essential role in odor processing during the perception of smell, but most studies have focused on the dorsal olfactory bulb, which contains only a quarter of all glomeruli. In this study, imaging of the lateral olfactory bulb reveals new properties in smell processing.

Primacy coding facilitates effective odor discrimination when receptor sensitivities are tuned

The olfactory system faces the difficult task of identifying an enormous variety of odors independent of their intensity. Primacy coding, where the odor identity is encoded by the receptor types that respond earliest, might provide a compact and informative representation that can be interpreted efficiently by the brain. In this paper, we analyze the information transmitted by a simple model of primacy coding using numerical simulations and statistical descriptions. We show that the encoded information depends strongly on the number of receptor types included in the primacy representation, but only weakly on the size of the receptor repertoire. The representation is independent of the odor intensity and the transmitted information is useful to perform typical olfactory tasks with close to experimentally measured performance. Interestingly, we find situations in which a smaller receptor repertoire is advantageous for discriminating odors. The model also suggests that overly sensitive receptor types could dominate the entire response and make the whole array useless, which allows us to predict how receptor arrays need to adapt to stay useful during environmental changes. Taken together, we show that primacy coding is more useful than simple binary and normalized coding, essentially because the sparsity of the odor representations is independent of the odor statistics, in contrast to the alternatives. Primacy coding thus provides an efficient odor representation that is independent of the odor intensity and might thus help to identify odors in the olfactory cortex.

Humans can identify odors independent of their intensity. Experimental data suggest that this is accomplished by representing the odor identity by the earliest responding receptor types. Using theoretical modeling, we here show that such a primacy code outperforms alternative encodings and allows discriminating odors with close to experimentally measured performance. This performance depends strongly on the number of receptors considered in the primacy code, but the receptor repertoire size is less important. The model also suggests a strong evolutionary pressure on the receptor sensitivities, which could explain observed receptor copy number adaptations. By predicting psycho-physical experiments, the model will thus contribute to our understanding of the olfactory system.

Temporal Dynamics of Inhalation-Linked Activity across Defined Subpopulations of Mouse Olfactory Bulb Neurons Imaged In Vivo

In mammalian olfaction, inhalation drives the temporal patterning of neural activity that underlies early olfactory processing. It remains poorly understood how the neural circuits that process incoming olfactory information are engaged in the context of inhalation-linked dynamics. Here, we used artificial inhalation and two-photon calcium imaging to compare the dynamics of activity evoked by odorant inhalation across major cell types of the mouse olfactory bulb (OB). We expressed GCaMP6f or jRGECO1a in mitral and tufted cell (MTC) subpopulations, olfactory sensory neurons (OSNs), and two major juxtaglomerular interneuron classes and imaged responses to a single inhalation of odorant. Activity in all cell types was strongly linked to inhalation, and all cell types showed some variance in the latency, rise times, and durations of their inhalation-linked response. Juxtaglomerular interneuron dynamics closely matched that of sensory inputs, while MTCs showed the highest diversity in responses, with a range of latencies and durations that could not be accounted for by heterogeneity in sensory input dynamics. Diversity was apparent even among “sister” tufted cells innervating the same glomerulus. Surprisingly, inhalation-linked responses of MTCs were highly overlapping and could not be distinguished on the basis of their inhalation-linked dynamics, with the exception of a subpopulation of superficial tufted cells expressing cholecystokinin (CCK). Our results are consistent with a model in which diversity in inhalation-linked patterning of OB output arises first at the level of sensory input and is enhanced by feedforward inhibition from juxtaglomerular interneurons which differentially impact different subpopulations of OB output neurons.

Somatostatin Serves a Modulatory Role in the Mouse Olfactory Bulb: Neuroanatomical and Behavioral Evidence

Somatostatin (SOM) and somatostatin receptors (SSTR1-4) are present in all olfactory structures, including the olfactory bulb (OB), where SOM modulates physiological gamma rhythms and olfactory discrimination responses. In this work, histological, viral tracing and transgenic approaches were used to characterize SOM cellular targets in the murine OB. We demonstrate that SOM targets all levels of mitral dendritic processes in the OB with somatostatin receptor 2 (SSTR2) detected in the dendrites of previously uncharacterized mitral-like cells. We show that inhibitory interneurons of the glomerular layer (GL) express SSTR4 while SSTR3 is confined to the granule cell layer (GCL). Furthermore, SOM cells in the OB receive synaptic inputs from olfactory cortical afferents. Behavioral studies demonstrate that genetic deletion of SSTR4, SSTR2 or SOM differentially affects olfactory performance. SOM or SSTR4 deletion have no major effect on olfactory behavioral performances while SSTR2 deletion impacts olfactory detection and discrimination behaviors. Altogether, these results describe novel anatomical and behavioral contributions of SOM, SSTR2 and SSTR4 receptors in olfactory processing.

Methods in Rodent Chemosensory Cognition

Olfactory information processing and learning are highly developed computational abilities of rodents. These attributes can be exploited to ask questions at several levels of complexity, from aspects of odorant binding by olfactory receptors to higher order learning about the predictive consequences of odorant stimulus presentation. Quantitative understanding of rodent odorant sampling patterns, both baseline nasal breathing and odorant-stimulated sniffing, is critical to elucidating mechanisms of olfactory information processing, from primary olfactory receptors to cortical centers that synthesize olfactory percepts from preprocessed multimodal inputs. This chapter outlines an innovative new method for measuring breathing and sniffing rates in unrestrained mice while the mice are performing odor-guided tasks in a computer controlled olfactometer.The method described here involves implantation of a wireless pressure sensor in the mouse that reports on thoracic pressure transients caused by breathing and sniffing. Recordings of pressure sensor outputs are made simultaneously with optically-sensed nose pokes by the mouse into an odor delivery port or a water delivery port. Odorant delivery timing and water reward delivery are also recorded simultaneously. This method allows for breathing and sniffing dependent thoracic pressure transients to be recorded with high temporal precision before, during, and after the mouse approaches an odor delivery port, samples the delivered odor, and obtains a water reward contingent on the identity of the odor that was presented and sampled.

Centrifugal Inputs to the Main Olfactory Bulb Revealed Through Whole Brain Circuit-Mapping

Neuronal activity in sensory regions can be modulated by attention, behavioral state, motor output, learning, and memory. This is often done through direct feedback or centrifugal projections originating from higher processing areas. Though, functionally important, the identity and organization of these feedback connections remain poorly characterized. Using a retrograde monosynaptic g-deleted rabies virus and whole-brain reconstructions, we identified the organization of feedback projecting neurons to the main olfactory bulb of the mouse. In addition to previously described projections from regions such as the Anterior Olfactory Nucleus (AON) and the piriform cortex, we characterized direct projections from pyramidal cells in the ventral CA1 region of hippocampus and the entorhinal cortex to the granule cell layer (GCL) of the main olfactory bulb (MOB). These data suggest that areas involved in stress, anxiety, learning and memory are all tethered to olfactory coding, two synapses away from where chemical compounds are first detected. Consequently, we hypothesize that understanding olfactory perception, even at the earliest stages, may require studying memory and behavior in addition to studying the physiochemical features of odors.

Awake and behaving mouse fMRI during Go/No-Go task

Functional magnetic imaging (fMRI) has been widely used to examine the functional neural networks in both the evoked and resting states. However, most fMRI studies in rodents are performed under anesthesia, which greatly limits the scope of their application, and behavioral relevance. Efforts have been made to image rodents in the awake condition, either in the resting state or in response to sensory or optogenetic stimulation. However, fMRI in awake behaving rodents has not yet been achieved. In the current study, a novel fMRI paradigm for awake and behaving mice was developed, allowing functional imaging of the mouse brain in an olfaction-based go/no-go task. High resolution functional imaging with limited motion and image distortion were achieved at 9.4T with a cryogenic coil in awake and behaving mice. Distributed whole-brain spatiotemporal patterns were revealed, with drastically different activity profiles for go versus no-go trials. Therefore, we have demonstrated the feasibility of functional imaging of an olfactory behavior in awake mice. This fMRI paradigm in awake behaving mice could lead to novel insights into neural mechanisms underlying behaviors at a whole-brain level.

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.

Algorithms for Olfactory Search across Species

Localizing the sources of stimuli is essential. Most organisms cannot eat, mate, or escape without knowing where the relevant stimuli originate. For many, if not most, animals, olfaction plays an essential role in search. While microorganismal chemotaxis is relatively well understood, in larger animals the algorithms and mechanisms of olfactory search remain mysterious. In this symposium, we will present recent advances in our understanding of olfactory search in flies and rodents. Despite their different sizes and behaviors, both species must solve similar problems, including meeting the challenges of turbulent airflow, sampling the environment to optimize olfactory information, and incorporating odor information into broader navigational systems.

Imaging Cortical Dynamics in GCaMP Transgenic Rats with a Head-Mounted Widefield Macroscope

Widefield imaging of calcium dynamics is an emerging method for mapping regional neural activity but is currently limited to restrained animals. Here we describe cScope, a head-mounted widefield macroscope developed to image large-scale cortical dynamics in rats during natural behavior. cScope provides a 7.8 × 4 mm field of view and dual illumination paths for both fluorescence and hemodynamic correction and can be fabricated at low cost using readily attainable components. We also report the development of Thy-1 transgenic rat strains with widespread neuronal expression of the calcium indicator GCaMP6f. We combined these two technologies to image large-scale calcium dynamics in the dorsal neocortex during a visual evidence accumulation task. Quantitative analysis of task-related dynamics revealed multiple regions having neural signals that encode behavioral choice and sensory evidence. Our results provide a new transgenic resource for calcium imaging in rats and extend the domain of head-mounted microscopes to larger-scale cortical dynamics. VIDEO ABSTRACT.

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.

Behavioral readout of spatio-temporal codes in olfaction

Neural recordings performed at an increasing scale and resolution have revealed complex, spatio-temporally precise patterns of activity in the olfactory system. Multiple models may explain the functional consequences of the spatio-temporal olfactory code, but the link to behavior remains unclear. Recent evidence in the field suggests a behavioral sensitivity to both fine spatial and temporal features in the code. How these features and combinations of features give rise to olfactory behavior is the subject of active research in the field. Modern genetic and optogenetic methods show great promise in testing the link between olfactory codes and behavior.

Learning-Dependent and -Independent Enhancement of Mitral/Tufted Cell Glomerular Odor Responses Following Olfactory Fear Conditioning in Awake Mice

Associative fear learning produces fear toward the conditioned stimulus (CS) and often generalization, the expansion of fear from the CS to similar, unlearned stimuli. However, how fear learning affects early sensory processing of learned and unlearned stimuli in relation to behavioral fear responses to these stimuli remains unclear. We subjected male and female mice expressing the fluorescent calcium indicator GCaMP3 in olfactory bulb mitral and tufted cells to a classical olfactory fear conditioning paradigm. We then used awake, in vivo calcium imaging to quantify learning-induced changes in glomerular odor responses, which constitute the first site of olfactory processing in the brain. The results demonstrate that odor-shock pairing nonspecifically enhances glomerular odor representations in a learning-dependent manner and increases representational similarity between the CS and nonconditioned odors, potentially priming the system toward generalization of learned fear. Additionally, CS-specific glomerular enhancements remain even when associative learning is blocked, suggesting two separate mechanisms lead to enhanced glomerular responses following odor-shock pairings.SIGNIFICANCE STATEMENT In the olfactory bulb (OB), odors are uniquely coded in a spatial map that represents odor identity, making the OB a unique model system for investigating how learned fear alters sensory processing. Classical fear conditioning causes fear of the conditioned stimulus (CS) and of neutral stimuli, known as generalization. Combining fear conditioning with fluorescent calcium imaging of OB glomeruli, we found enhanced glomerular responses of the CS as well as neutral stimuli in awake mice, which mirrors fear generalization. We report that CS and neutral stimuli enhancements are, respectively, learning-independent and learning-dependent. Together, these results reveal distinct mechanisms leading to enhanced OB processing of fear-inducing stimuli and provide important implications for altered sensory processing in fear generalization.

A transformation from temporal to ensemble coding in a model of piriform cortex

Different coding strategies are used to represent odor information at various stages of the mammalian olfactory system. A temporal latency code represents odor identity in olfactory bulb (OB), but this temporal information is discarded in piriform cortex (PCx) where odor identity is instead encoded through ensemble membership. We developed a spiking PCx network model to understand how this transformation is implemented. In the model, the impact of OB inputs activated earliest after inhalation is amplified within PCx by diffuse recurrent collateral excitation, which then recruits strong, sustained feedback inhibition that suppresses the impact of later-responding glomeruli. We model increasing odor concentrations by decreasing glomerulus onset latencies while preserving their activation sequences. This produces a multiplexed cortical odor code in which activated ensembles are robust to concentration changes while concentration information is encoded through population synchrony. Our model demonstrates how PCx circuitry can implement multiplexed ensemble-identity/temporal-concentration odor coding.

High-Throughput Automatic Training System for Odor-Based Learned Behaviors in Head-Fixed Mice

Understanding neuronal mechanisms of learned behaviors requires efficient behavioral assays. We designed a high-throughput automatic training system (HATS) for olfactory behaviors in head-fixed mice. The hardware and software were constructed to enable automatic training with minimal human intervention. The integrated system was composed of customized 3D-printing supporting components, an odor-delivery unit with fast response, Arduino based hardware-controlling and data-acquisition unit. Furthermore, the customized software was designed to enable automatic training in all training phases, including lick-teaching, shaping and learning. Using HATS, we trained mice to perform delayed non-match to sample (DNMS), delayed paired association (DPA), Go/No-go (GNG), and GNG reversal tasks. These tasks probed cognitive functions including sensory discrimination, working memory, decision making and cognitive flexibility. Mice reached stable levels of performance within several days in the tasks. HATS enabled an experimenter to train eight mice simultaneously, therefore greatly enhanced the experimental efficiency. Combined with causal perturbation and activity recording techniques, HATS can greatly facilitate our understanding of the neural-circuitry mechanisms underlying learned behaviors.

Task dependence of odor discrimination: choosing between speed and accuracy

Odor discrimination is a complex task that may be improved by increasing sampling time to facilitate evidence accumulation. However, experiments testing this phenomenon in olfaction have produced conflicting results. To resolve this disparity, Frederick et al. (Frederick DE, Brown A, Tacopina S, Mehta N, Vujovic M, Brim E, Amina T, Fixsen B, Kay LM. J Neurosci 37: 4416-4426, 2017) conducted experiments that suggest that sampling time and performance are task dependent. Their findings have implications for understanding olfactory processing and experimental design, specifically the effect of subtle differences in experimental design on study results.

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.

Theta Oscillations Rapidly Convey Odor-Specific Content in Human Piriform Cortex

Olfactory oscillations are pervasive throughout vertebrate and invertebrate nervous systems. Such observations have long implied that rhythmic activity patterns play a fundamental role in odor coding. Using intracranial EEG recordings from rare patients with medically resistant epilepsy, we find that theta oscillations are a distinct electrophysiological signature of olfactory processing in the human brain. Across seven patients, odor stimulation enhanced theta power in human piriform cortex, with robust effects at the level of single trials. Importantly, classification analysis revealed that piriform oscillatory activity conveys olfactory-specific information that can be decoded within 110-518 ms of a sniff, and maximally within the theta frequency band. This temporal window was also associated with increased theta-specific phase coupling between piriform cortex and hippocampus. Together these findings suggest that human piriform cortex has access to olfactory content in the time-frequency domain and can utilize these signals to rapidly differentiate odor stimuli.

Gamma and Beta Oscillations Define a Sequence of Neurocognitive Modes Present in Odor Processing

Olfactory system beta (15-35 Hz) and gamma (40-110 Hz) oscillations of the local field potential in mammals have both been linked to odor learning and discrimination. Gamma oscillations represent the activity of a local network within the olfactory bulb, and beta oscillations represent engagement of a systemwide network. Here, we test whether beta and gamma oscillations represent different cognitive modes using the different demands of go/no-go and two-alternative choice tasks that previously were suggested to favor beta or gamma oscillations, respectively. We reconcile previous studies and show that both beta and gamma oscillations occur in both tasks, with gamma dominating the early odor sampling period (2-4 sniffs) and beta dominating later. The relative power and coherence of both oscillations depend separately on multiple factors within both tasks without categorical differences across tasks. While the early/gamma-associated period occurs in all trials, rats can perform above chance without the later/beta-associated period. Longer sampling, which includes beta oscillations, is associated with better performance. Gamma followed by beta oscillations therefore represents a sequence of cognitive and neural states during odor discrimination, which can be separately modified depending on the demands of a task and odor discrimination. Additionally, fast (85 Hz) and slow (70 Hz) olfactory bulb gamma oscillation sub-bands have been hypothesized to represent tufted and mitral cell networks, respectively (Manabe and Mori, 2013). We find that fast gamma favors the early and slow gamma the later (beta-dominated) odor-sampling period and that the relative contributions of these oscillations are consistent across tasks.

SIGNIFICANCE STATEMENT

Olfactory system gamma (40-110 Hz) and beta (15-35 Hz) oscillations of the local field potential indicate different neural firing statistics and functional circuits. We show that gamma and beta oscillations occur in stereotyped sequence during odor sampling in associative tasks, with local gamma dominating the first 250 ms of odor sniffing, followed by systemwide beta as behavioral responses are prepared. Oscillations and coupling strength between brain regions are modulated by task, odor, and learning, showing that task features can dramatically adjust the dynamics of a cortical sensory system, which changes state every ∼250 ms. Understanding cortical circuits, even at the biophysical level, depends on careful use of multiple behavioral contexts and stimuli.

Cell-Type-Specific Modulation of Sensory Responses in Olfactory Bulb Circuits by Serotonergic Projections from the Raphe Nuclei

Serotonergic neurons in the brainstem raphe nuclei densely innervate the olfactory bulb (OB), where they can modulate the initial representation and processing of olfactory information. Serotonergic modulation of sensory responses among defined OB cell types is poorly characterized in vivo Here, we used cell-type-specific expression of optical reporters to visualize how raphe stimulation alters sensory responses in two classes of GABAergic neurons of the mouse OB glomerular layer, periglomerular (PG) and short axon (SA) cells, as well as mitral/tufted (MT) cells carrying OB output to piriform cortex. In PG and SA cells, brief (1-4 s) raphe stimulation elicited a large increase in the magnitude of responses linked to inhalation of ambient air, as well as modest increases in the magnitude of odorant-evoked responses. Near-identical effects were observed when the optical reporter of glutamatergic transmission iGluSnFR was expressed in PG and SA cells, suggesting enhanced excitatory input to these neurons. In contrast, in MT cells imaged from the dorsal OB, raphe stimulation elicited a strong increase in resting GCaMP fluorescence with only a slight enhancement of inhalation-linked responses to odorant. Finally, optogenetically stimulating raphe serotonergic afferents in the OB had heterogeneous effects on presumptive MT cells recorded extracellularly, with an overall modest increase in resting and odorant-evoked responses during serotonergic afferent stimulation. These results suggest that serotonergic afferents from raphe dynamically modulate olfactory processing through distinct effects on multiple OB targets, and may alter the degree to which OB output is shaped by inhibition during behavior.

SIGNIFICANCE STATEMENT

Modulation of the circuits that process sensory information can profoundly impact how information about the external world is represented and perceived. This study investigates how the serotonergic system modulates the initial processing of olfactory information by the olfactory bulb, an obligatory relay between sensory neurons and cortex. We find that serotonergic projections from the raphe nuclei to the olfactory bulb dramatically enhance the responses of two classes of inhibitory interneurons to sensory input, that this effect is mediated by increased glutamatergic drive onto these neurons, and that serotonergic afferent activation alters the responses of olfactory bulb output neurons in vivo These results elucidate pathways by which neuromodulatory systems can dynamically regulate brain circuits during behavior.

Spontaneous Rapid Odor Source Localization Behavior Requires Interhemispheric Communication

Navigation, finding food sources, and avoiding danger critically depend on the identification and spatial localization of airborne chemicals. When monitoring the olfactory environment, rodents spontaneously engage in active olfactory sampling behavior, also referred to as exploratory sniffing [1]. Exploratory sniffing is characterized by stereotypical high-frequency respiration, which is also reliably evoked by novel odorant stimuli [2, 3]. To study novelty-induced exploratory sniffing, we developed a novel, non-contact method for measuring respiration by infrared (IR) thermography in a behavioral paradigm in which novel and familiar stimuli are presented to head-restrained mice. We validated the method by simultaneously performing nasal pressure measurements, a commonly used invasive approach [2, 4], and confirmed highly reliable detection of inhalation onsets. We further discovered that mice actively orient their nostrils toward novel, previously unexperienced, smells. In line with the remarkable speed of olfactory processing reported previously [3, 5, 6], we find that mice initiate their response already within the first sniff after odor onset. Moreover, transecting the anterior commissure (AC) disrupted orienting, indicating that the orienting response requires interhemispheric transfer of information. This suggests that mice compare odorant information obtained from the two bilaterally symmetric nostrils to locate the source of the novel odorant. We further demonstrate that asymmetric activation of the anterior olfactory nucleus (AON) is both necessary and sufficient for eliciting orienting responses. These findings support the view that the AON plays an important role in the internostril difference comparison underlying rapid odor source localization.

Differences in peripheral sensory input to the olfactory bulb between male and female mice

Female mammals generally have a superior sense of smell than males, but the biological basis of this difference is unknown. Here, we demonstrate sexually dimorphic neural coding of odorants by olfactory sensory neurons (OSNs), primary sensory neurons that physically contact odor molecules in the nose and provide the initial sensory input to the brain’s olfactory bulb. We performed in vivo optical neurophysiology to visualize odorant-evoked OSN synaptic output into olfactory bub glomeruli in unmanipulated (gonad-intact) adult mice from both sexes, and found that in females odorant presentation evoked more rapid OSN signaling over a broader range of OSNs than in males. These spatiotemporal differences enhanced the contrast between the neural representations of chemically related odorants in females compared to males during stimulus presentation. Removing circulating sex hormones makes these signals slower and less discriminable in females, while in males they become faster and more discriminable, suggesting opposite roles for gonadal hormones in influencing male and female olfactory function. These results demonstrate that the famous sex difference in olfactory abilities likely originates in the primary sensory neurons, and suggest that hormonal modulation of the peripheral olfactory system could underlie differences in how males and females experience the olfactory world.