Robust odor coding via inhalation-coupled transient activity in the mammalian olfactory bulb

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.

The Vomeronasal System Can Learn Novel Stimulus Response Pairings

Behavioral responses can be classified as innate or learned and are often mediated by distinct neuronal pathways. In many animals, chemical cues are crucial for directing behaviors, and multiple chemosensory subsystems serve this purpose. The major subsystems in vertebrates are the main olfactory system (MOS) and the vomeronasal system (VNS). While the MOS has well-documented associative capabilities, the VNS is known for its role in mediating innate responses to sensory cues with clear ethological significance. However, it remains unknown whether the VNS can map arbitrary sensory activation to novel behavioral outputs. To address this question, we used several optogenetic strategies for selective vomeronasal activation and tested whether mice could associate stimulation patterns with particular reward locations. Our experiments indicate that mice can, indeed, exploit VNS activity to direct novel behavioral responses, implying that the VNS holds a substantial capacity for redirecting and adapting behavioral responses to given stimulation patterns.

Rapid Learning of Odor-Value Association in the Olfactory Striatum

Rodents can successfully learn multiple novel stimulus-response associations after only a few repetitions when the contingencies predict reward. The circuits modified during such reinforcement learning to support decision-making are not known, but the olfactory tubercle (OT) and posterior piriform cortex (pPC) are candidates for decoding reward category from olfactory sensory input and relaying this information to cognitive and motor areas. Through single-cell recordings in behaving male and female C57BL/6 mice, we show here that an explicit representation for reward category emerges in the OT within minutes of learning a novel odor-reward association, whereas the pPC lacks an explicit representation even after weeks of overtraining. The explicit reward category representation in OT is visible in the first sniff (50-100 ms) of an odor on each trial, and precedes the motor action. Together, these results suggest that the coding of stimulus information required for reward prediction does not occur within olfactory cortex, but rather in circuits involving the olfactory striatum.SIGNIFICANCE STATEMENT Rodents are olfactory specialists and can use odors to learn contingencies quickly and well. We have found that mice can readily learn to place multiple odors into rewarded and unrewarded categories. Once they have learned the rule, they can do such categorization in a matter of minutes (<10 trials). We found that neural activity in olfactory cortex largely reflects sensory coding, with very little explicit information about categories. By contrast, neural activity in a brain region in the ventral striatum is rapidly modified in a matter of minutes to reflect reward category. Our experiments set up a paradigm for studying rapid sensorimotor reinforcement in a circuit that is right at the interface of sensory input and reward areas.

Synchronous Infra-Slow Oscillations Organize Ensembles of Accessory Olfactory Bulb Projection Neurons into Distinct Microcircuits

The accessory olfactory system controls social and sexual behavior. In the mouse accessory olfactory bulb, the first central stage of information processing along the accessory olfactory pathway, projection neurons (mitral cells) display infra-slow oscillatory discharge with remarkable periodicity. The physiological mechanisms that underlie this default output state, however, remain controversial. Moreover, whether such rhythmic infra-slow activity patterns exist in awake behaving mice and whether such activity reflects the functional organization of the accessory olfactory bulb circuitry remain unclear. Here, we hypothesize that mitral cell ensembles form synchronized microcircuits that subdivide the accessory olfactory bulb into segregated functional clusters. We use a miniature microscope to image the Ca²⁺ dynamics within the apical dendritic compartments of large mitral cell ensembles in vivo. We show that infra-slow periodic patterns of concerted neural activity, indeed, reflect the idle state of accessory olfactory bulb output in awake male and female mice. Ca²⁺ activity profiles are distinct and glomerulus-specific. Confocal time-lapse imaging in acute slices reveals that groups of mitral cells assemble into microcircuits that exhibit correlated Ca²⁺ signals. Moreover, electrophysiological profiling of synaptic connectivity indicates functional coupling between mitral cells. Our results suggest that both intrinsically rhythmogenic neurons and neurons entrained by fast synaptic drive are key elements in organizing the accessory olfactory bulb into functional microcircuits, each characterized by a distinct default pattern of infra-slow rhythmicity.

SIGNIFICANCE STATEMENT Information processing in the accessory olfactory bulb (AOB) plays a central role in conspecific chemosensory communication. Surprisingly, many basic physiological principles that underlie neuronal signaling in the AOB remain elusive. Here, we show that AOB projection neurons (mitral cells) form parallel synchronized ensembles both in vitro and in vivo. Infra-slow synchronous oscillatory activity within AOB microcircuits thus adds a new dimension to chemosensory coding along the accessory olfactory pathway.

Excitability of Neural Activity is Enhanced, but Neural Discrimination of Odors is Slightly Decreased, in the Olfactory Bulb of Fasted Mice

Olfaction and satiety status influence each other: cues from the olfactory system modulate eating behavior, and satiety affects olfactory abilities. However, the neural mechanisms governing the interactions between olfaction and satiety are unknown. Here, we investigate how an animal’s nutritional state modulates neural activity and odor representation in the mitral/tufted cells of the olfactory bulb, a key olfactory center that plays important roles in odor processing and representation. At the single-cell level, we found that the spontaneous firing rate of mitral/tufted cells and the number of cells showing an excitatory response both increased when mice were in a fasted state. However, the neural discrimination of odors slightly decreased. Although ongoing baseline and odor-evoked beta oscillations in the local field potential in the olfactory bulb were unchanged with fasting, the amplitude of odor-evoked gamma oscillations significantly decreased in a fasted state. These neural changes in the olfactory bulb were independent of the sniffing pattern, since both sniffing frequency and mean inhalation duration did not change with fasting. These results provide new information toward understanding the neural circuit mechanisms by which olfaction is modulated by nutritional status.

Bilateral and unilateral odor processing and odor perception

Imagine smelling a novel perfume with only one nostril and then smelling it again with the other nostril. Clearly, you can tell that it is the same perfume both times. This simple experiment demonstrates that odor information is shared across both hemispheres to enable perceptual unity. In many sensory systems, perceptual unity is believed to be mediated by inter-hemispheric connections between iso-functional cortical regions. However, in the olfactory system, the underlying neural mechanisms that enable this coordination are unclear because the two olfactory cortices are not topographically organized and do not seem to have homotypic inter-hemispheric mapping. This review presents recent advances in determining which aspects of odor information are processed unilaterally or bilaterally, and how odor information is shared across the two hemispheres. We argue that understanding the mechanisms of inter-hemispheric coordination can provide valuable insights that are hard to achieve when focusing on one hemisphere alone.

Direct Comparison of Odor Responses of Homologous Glomeruli in the Medial and Lateral Maps of the Mouse Olfactory Bulb

Olfactory sensory neurons (OSNs) expressing same-type odorant receptors typically project to a pair of glomeruli in the medial and lateral sides of the olfactory bulbs (OBs) in rodents. This multiple glomerular representation of homologous inputs is considered to have more important functional roles for odor information processing than the redundant backup system. However, a consensus idea is lacking and this hinders interpretation of the phenomenon. In addition, the shared and unique odorant response properties of the homologous glomeruli remain unclear because the majority of medial glomeruli are hidden in the septal OB, and thus it is difficult to directly compare them. OSNs, which express trace amine-associated odorant receptors (TAARs), were recently identified that project to a pair of glomeruli uniquely located in the dorsal OB. In this study, we measured the odorant-induced calcium responses of homologous pairs of TAAR glomeruli simultaneously in anesthetized mice and directly compared their response patterns. We found that they exhibited similar temporal response patterns and could not find differences in onset latency, rise time, decay time, or response amplitude. However, the medial glomeruli had significantly larger respiration-locked calcium fluctuations than the lateral glomeruli. This trend was observed with/without odorant stimulation in postsynaptic neurons of GABAergic, dopaminergic, and mitral/tufted cells, but not in presynaptic olfactory sensory axon terminals. This indicates that, at least in these TAAR glomeruli, the medial rather than the lateral OB map enhances the respiration-locked rhythm and transfers this information to higher brain centers.

The contribution of temporal coding to odor coding and odor perception in humans

Whether neurons encode information through their spike rates, their activity times or both is an ongoing debate in systems neuroscience. Here, we tested whether humans can discriminate between a pair of temporal odor mixtures (TOMs) composed of the same two components delivered in rapid succession in either one temporal order or its reverse. These TOMs presumably activate the same olfactory neurons but at different times and thus differ mainly in the time of neuron activation. We found that most participants could hardly discriminate between TOMs, although they easily discriminated between a TOM and one of its components. By contrast, participants succeeded in discriminating between the TOMs when they were notified of their successive nature in advance. We thus suggest that the time of glomerulus activation can be exploited to extract odor-related information, although it does not change the odor perception substantially, as should be expected from an odor code per se.

Learning improves decoding of odor identity with phase-referenced oscillations in the olfactory bulb

Local field potential oscillations reflect temporally coordinated neuronal ensembles-coupling distant brain regions, gating processing windows, and providing a reference for spike timing-based codes. In phase amplitude coupling (PAC), the amplitude of the envelope of a faster oscillation is larger within a phase window of a slower carrier wave. Here, we characterized PAC, and the related theta phase-referenced high gamma and beta power (PRP), in the olfactory bulb of mice learning to discriminate odorants. PAC changes throughout learning, and odorant-elicited changes in PRP increase for rewarded and decrease for unrewarded odorants. Contextual odorant identity (is the odorant rewarded?) can be decoded from peak PRP in animals proficient in odorant discrimination, but not in naïve mice. As the animal learns to discriminate the odorants the dimensionality of PRP decreases. Therefore, modulation of phase-referenced chunking of information in the course of learning plays a role in early sensory processing in olfaction.

Complex neural representation of odour information in the olfactory bulb

The most important task of the olfactory system is to generate a precise representation of odour information under different brain and behavioural states. As the first processing stage in the olfactory system and a crucial hub, the olfactory bulb plays a key role in the neural representation of odours, encoding odour identity, intensity and timing. Although the neural circuits and coding strategies used by the olfactory bulb for odour representation were initially identified in anaesthetized animals, a large number of recent studies focused on neural representation of odorants in the olfactory bulb in awake behaving animals. In this review, we discuss these recent findings, covering (a) the neural circuits for odour representation both within the olfactory bulb and the functional connections between the olfactory bulb and the higher order processing centres; (b) how related factors such as sniffing affect and shape the representation; (c) how the representation changes under different states; and (d) recent progress on the processing of temporal aspects of odour presentation in awake, behaving rodents. We highlight discussion of the current views and emerging proposals on the neural representation of odorants in the olfactory bulb.

Discovering Precise Temporal Patterns in Large-Scale Neural Recordings through Robust and Interpretable Time Warping

Though the temporal precision of neural computation has been studied intensively, a data-driven determination of this precision remains a fundamental challenge. Reproducible spike patterns may be obscured on single trials by uncontrolled temporal variability in behavior and cognition and may not be time locked to measurable signatures in behavior or local field potentials (LFP). To overcome these challenges, we describe a general-purpose time warping framework that reveals precise spike-time patterns in an unsupervised manner, even when these patterns are decoupled from behavior or are temporally stretched across single trials. We demonstrate this method across diverse systems: cued reaching in nonhuman primates, motor sequence production in rats, and olfaction in mice. This approach flexibly uncovers diverse dynamical firing patterns, including pulsatile responses to behavioral events, LFP-aligned oscillatory spiking, and even unanticipated patterns, such as 7 Hz oscillations in rat motor cortex that are not time locked to measured behaviors or LFP.

Task-Demand-Dependent Neural Representation of Odor Information in the Olfactory Bulb and Posterior Piriform Cortex

In awake rodents, the neural representation of olfactory information in the olfactory bulb is largely dependent on brain state and behavioral context. Learning-modified neural plasticity has been observed in mitral/tufted cells, the main output neurons of the olfactory bulb. Here, we propose that the odor information encoded by mitral/tufted cell responses in awake mice is highly dependent on the behavioral task demands. We used fiber photometry to record calcium signals from the mitral/tufted cell population in awake, head-fixed male mice under different task demands. We found that the mitral/tufted cell population showed similar responses to two distinct odors when the odors were presented in the context of a go/go task, in which the mice received a water reward regardless of the identity of the odor presented. However, when the same odors were presented in a go/no-go task, in which one odor was rewarded and the other was not, then the mitral cell population responded very differently to the two odors, characterized by a robust reduction in the response to the nonrewarded odor. Thus, the representation of odors in the mitral/tufted cell population depends on whether the task requires discrimination of the odors. Strikingly, downstream of the olfactory bulb, pyramidal neurons in the posterior piriform cortex also displayed a task-demand-dependent neural representation of odors, but the anterior piriform cortex did not, indicating that these two important higher olfactory centers use different strategies for neural representation.SIGNIFICANCE STATEMENT The most important task of the olfactory system is to generate a precise representation of odor information under different brain states. Whether the representation of odors by neurons in olfactory centers such as the olfactory bulb and the piriform cortex depends on task demands remains elusive. We find that odor representation in the mitral/tufted cells of the olfactory bulb depends on whether the task requires odor discrimination. A similar neural representation is found in the posterior piriform cortex but not the anterior piriform cortex, indicating that these higher olfactory centers use different representational strategies. The task-demand-dependent representational strategy is likely important for facilitating information processing in higher brain centers responsible for decision making and encoding of salience.

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.

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.

Signal Detection and Coding in the Accessory Olfactory System

In many mammalian species, the accessory olfactory system plays a central role in guiding behavioral and physiological responses to social and reproductive interactions. Because of its relatively compact structure and its direct access to amygdalar and hypothalamic nuclei, the accessory olfactory pathway provides an ideal system to study sensory control of complex mammalian behavior. During the last several years, many studies employing molecular, behavioral, and physiological approaches have significantly expanded and enhanced our understanding of this system. The purpose of the current review is to integrate older and newer studies to present an updated and comprehensive picture of vomeronasal signaling and coding with an emphasis on early accessory olfactory system processing stages. These include vomeronasal sensory neurons in the vomeronasal organ, and the circuitry of the accessory olfactory bulb. Because the overwhelming majority of studies on accessory olfactory system function employ rodents, this review is largely focused on this phylogenetic order, and on mice in particular. Taken together, the emerging view from both older literature and more recent studies is that the molecular, cellular, and circuit properties of chemosensory signaling along the accessory olfactory pathway are in many ways unique. Yet, it has also become evident that, like the main olfactory system, the accessory olfactory system also has the capacity for adaptive learning, experience, and state-dependent plasticity. In addition to describing what is currently known about accessory olfactory system function and physiology, we highlight what we believe are important gaps in our knowledge, which thus define exciting directions for future investigation.

Efficient calculation of heterogeneous non-equilibrium statistics in coupled firing-rate models

Understanding nervous system function requires careful study of transient (non-equilibrium) neural response to rapidly changing, noisy input from the outside world. Such neural response results from dynamic interactions among multiple, heterogeneous brain regions. Realistic modeling of these large networks requires enormous computational resources, especially when high-dimensional parameter spaces are considered. By assuming quasi-steady-state activity, one can neglect the complex temporal dynamics; however, in many cases the quasi-steady-state assumption fails. Here, we develop a new reduction method for a general heterogeneous firing-rate model receiving background correlated noisy inputs that accurately handles highly non-equilibrium statistics and interactions of heterogeneous cells. Our method involves solving an efficient set of nonlinear ODEs, rather than time-consuming Monte Carlo simulations or high-dimensional PDEs, and it captures the entire set of first and second order statistics while allowing significant heterogeneity in all model parameters.

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.

Narrowly Confined and Glomerulus-Specific Onset Latencies of Odor-Evoked Calcium Transients in the Juxtaglomerular Cells of the Mouse Main Olfactory Bulb

Odor information is transmitted from olfactory sensory neurons to principal neurons at the glomeruli of the olfactory bulb. The intraglomerular neuronal circuit also includes hundreds of interneurons referred to as juxtaglomerular (JG) cells. Stimulus selectivity is well correlated among many JG cells that are associated with the same glomerulus, consistent with their highly homogeneous sensory inputs. However, much less is known about the temporal aspects of their activity, including the temporal coordination of their odor-evoked responses. As many JG cells within a glomerular module respond to the same stimulus, the extent to which their activity is temporally aligned will affect the temporal profile of their population inhibitory inputs. Using random-access high-speed two-photon microscopy, we recorded the odor-evoked calcium transients of mouse JG cells and compared the onset latency and rise time among neurons putatively associated with the same and different glomeruli. Whereas the overall onset latencies of odor-evoked transients were distributed across a ∼150 ms time window, those from cells putatively associated with the same glomerulus were confined to a much narrower window of several tens of milliseconds. This result suggests that onset latency primarily depends on the associated glomerulus. We also observed glomerular specificity in the rise time. The glomerulus-specific temporal pattern of odor-evoked activity implies that the temporal patterns of inputs from the intraglomerular circuit are unique to individual glomerulus–odor pairs, which may contribute to efficient shaping of the temporal pattern of activity in the principal neurons.

Odor Concentration Change Coding in the Olfactory Bulb

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

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.

Effect of adenosine on short-term synaptic plasticity in mouse piriform cortex in vitro: adenosine acts as a high-pass filter

We examined the effect of adenosine and of adenosine A1 receptor blockage on short-term synaptic plasticity in slices of adult mouse anterior piriform cortex maintained in vitro in an in vivo-like ACSF. Extracellular recording of postsynaptic responses was performed in layer 1a while repeated electrical stimulation (5-pulse-trains, frequency between 3.125 and 100 Hz) was applied to the lateral olfactory tract. Our stimulation protocol was aimed at covering the frequency range of oscillatory activities observed in the olfactory bulb in vivo. In control condition, postsynaptic response amplitude showed a large enhancement for stimulation frequencies in the beta and gamma frequency range. A phenomenological model of short-term synaptic plasticity fitted to the data suggests that this frequency-dependent enhancement can be explained by the interplay between a short-term facilitation mechanism and two short-term depression mechanisms, with fast and slow recovery time constants. In the presence of adenosine, response amplitude evoked by low-frequency stimulation decreased in a dose-dependent manner (IC50  = 70 μmol/L). Yet short-term plasticity became more dominated by facilitation and less influenced by depression. Both changes compensated for the initial decrease in response amplitude in a way that depended on stimulation frequency: compensation was strongest at high frequency, up to restoring response amplitudes to values similar to those measured in control condition. The model suggested that the main effects of adenosine were to decrease neurotransmitter release probability and to attenuate short-term depression mechanisms. Overall, these results suggest that adenosine does not merely inhibit neuronal activity but acts in a more subtle, frequency-dependent manner.

Coordinated electrical activity in the olfactory bulb gates the oscillatory entrainment of entorhinal networks in neonatal mice

Although the developmental principles of sensory and cognitive processing have been extensively investigated, their synergy has been largely neglected. During early life, most sensory systems are still largely immature. As a notable exception, the olfactory system is functional at birth, controlling mother–offspring interactions and neonatal survival. Here, we elucidate the structural and functional principles underlying the communication between olfactory bulb (OB) and lateral entorhinal cortex (LEC)—the gatekeeper of limbic circuitry—during neonatal development. Combining optogenetics, pharmacology, and electrophysiology in vivo with axonal tracing, we show that mitral cell–dependent discontinuous theta bursts in OB drive network oscillations and time the firing in LEC of anesthetized mice via axonal projections confined to upper cortical layers. Acute pharmacological silencing of OB activity diminishes entorhinal oscillations, whereas odor exposure boosts OB–entorhinal coupling at fast frequencies. Chronic impairment of olfactory sensory neurons disrupts OB–entorhinal activity. Thus, OB activity shapes the maturation of entorhinal circuits.

Cognitive performance is maximized only through permanent interactions with the environment, yet the contribution of sensory stimuli to cognitive processing has been largely neglected. This is especially true when considering the maturation of limbic circuits accounting for memory and executive abilities. Rodents are blind and deaf, do not whisker, and have limited motor abilities during the first days of life, and therefore, the contribution of sensory inputs to limbic ontogeny has been deemed negligible. As a notable exception, olfactory inputs are processed already early in life and might shape the limbic development. To test this hypothesis, we investigate the principles of communication between the olfactory bulb (OB), the first processing station of olfactory inputs, and lateral entorhinal cortex (LEC)—the gatekeeper of limbic circuits centered on hippocampus and prefrontal cortex—of mice during the first and second postnatal weeks. We show that spontaneously generated patterns of electrical activity in the OB activate the entorhinal circuits via mono- and polysynaptic axonal projections. The activity within the circuitry connecting the OB to the LEC is boosted by odors and disrupted by chronic lesion of the olfactory periphery. Thus, spontaneous and stimulus-induced activity in the OB controls the maturation of neuronal networks in the LEC.

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.

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.

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.

Non-monotonic accumulation of spike time variance during membrane potential oscillations

A spike-phase neural code has been proposed as a mechanism to encode stimuli based on the precise timing of spikes relative to the phase of membrane potential oscillations. This form of coding has been reported in both in vivo and in vitro experiments across several regions of the brain, yet there are concerns that such precise timing may be compromised by an effect referred to as variance accumulation, wherein spike timing variance increases over the phase of an oscillation. Here, we provide a straightforward explanation of this effect based on the theoretical spike time variance. The proposed theory is consistent with recordings of mitral neurons. It shows that spike time variance can increase in a nonlinear fashion with spike number, in a way that is dependent upon the frequency and amplitude of the oscillation. Further, non-monotonic accumulation of variance can arise from different combinations of oscillation parameters. Nonlinear accumulation sometimes leads to lower variance than that of a mean rate-matched homogeneous Poisson process, particularly for spikes that occur in later phases of oscillation. However, such an advantage is limited to a narrow range of oscillation amplitudes and frequencies. These results suggest fundamental constraints on spike-phase coding, and reveal how certain spikes in a sequence may exhibit increased firing time precision relative to their neighbors.

Recurrent cortical circuits implement concentration-invariant odor coding

Animals rely on olfaction to find food, attract mates, and avoid predators. To support these behaviors, they must be able to identify odors across different odorant concentrations. The neural circuit operations that implement this concentration invariance remain unclear. We found that despite concentration-dependence in the olfactory bulb (OB), representations of odor identity were preserved downstream, in the piriform cortex (PCx). The OB cells responding earliest after inhalation drove robust responses in sparse subsets of PCx neurons. Recurrent collateral connections broadcast their activation across the PCx, recruiting global feedback inhibition that rapidly truncated and suppressed cortical activity for the remainder of the sniff, discounting the impact of slower, concentration-dependent OB inputs. Eliminating recurrent collateral output amplified PCx odor responses rendered the cortex steeply concentration-dependent and abolished concentration-invariant identity decoding.

Respiratory modulation of cognitive performance during the retrieval process

Recent research suggests that cognitive performance might be altered by the respiratory-synchronized activity generated in the brain. Previous human studies, however, have yielded inconsistent results when assessing task performance during distinct respiratory phases (inspiratory phase vs. expiratory phase). We therefore tested whether cognitive performance was regulated based on the timing of breathing components (e.g., expiratory-to-inspiratory (EI) phase transition) during the retrieval process. To determine the role of respiration in performance, the present study employed healthy subjects (n = 18) in a delayed matching-to-sample visual recognition task where a test cue was given in the respiratory phase-locked (Phased) or regularly paced (Non-phased) presentation paradigm. During the Phased session but not during the Non-phased session, the response time (RT) of the task increased by 466 ms (p = 0.003), and accuracy decreased by 21.4% (p = 0.004) when the retrieval process encompassed the EI transition. Breathing-dependent changes were particularly prominent when the EI transition occurred during the middle step of the retrieval process. Meanwhile, changes in the RT and accuracy were not observed when the retrieval process encompassed the inspiratory-to-expiratory phase transition. This is the first time that a certain phase transition in the respiratory cycle has been shown to modulate performance on a time scale of several seconds in a cognitive task. We propose that attenuation of these breathing-dependent cognitive fluctuations might be crucial for the maintenance and stability of successful performance in daily life and sports.

Dynamic contrast enhancement and flexible odor codes

Sensory stimuli evoke spiking activities patterned across neurons and time that are hypothesized to encode information about their identity. Since the same stimulus can be encountered in a multitude of ways, how stable or flexible are these stimulus-evoked responses? Here we examine this issue in the locust olfactory system. In the antennal lobe, we find that both spatial and temporal features of odor-evoked responses vary in a stimulus-history dependent manner. The response variations are not random, but allow the antennal lobe circuit to enhance the uniqueness of the current stimulus. Nevertheless, information about the odorant identity is conf ounded due to this contrast enhancement computation. Notably, predictions from a linear logical classifier (OR-of-ANDs) that can decode information distributed in flexible subsets of neurons match results from behavioral experiments. In sum, our results suggest that a trade-off between stability and flexibility in sensory coding can be achieved using a simple computational logic.

Sensory stimuli are encountered in multiple ways necessitating a flexible and adaptive neural population code for identification. Here, the authors show that the dynamics of odor coding in the locust antennal lobe varies with stimulus context so as to enhance the target stimulus representation.

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.

Respiratory fluctuations in pupil diameter are not maintained during cognitive tasks

Pupil diameter fluctuation throughout the respiratory cycle is autonomically controlled in the resting state, as pupils dilate during inspiration and constrict during expiration. Furthermore, pupil size is differentially modulated by cognitive states between task engagement and disengagement. To determine whether respiratory-dependent fluctuations in pupil size are maintained during a cognitive task, we employed healthy human subjects performing a delayed matching-to-sample task with a short delay and measured their pupil sizes and R wave-to-R wave intervals (RRIs). We detected respiratory fluctuations in pupil size and the RRI during the delay period immediately before the discrimination stage of the task. During the discrimination stage, the cognitive state with the higher task engagement yielded more pupil dilation. However, respiratory fluctuations in pupil size were abolished, whereas those in the RRI were still discernible during the discrimination stage. Our results suggest that an alternative control mechanism involving the cognitive state associated with task engagement overrides the respiratory-related autonomic control of pupil diameter.

Local cortical activity of distant brain areas can phase-lock to the olfactory bulb's respiratory rhythm in the freely behaving rat

An important unresolved question about neural processing is the mechanism by which distant brain areas coordinate their activities and relate their local processing to global neural events. A potential candidate for the local-global integration are slow rhythms such as respiration. In this study, we asked if there are modulations of local cortical processing that are phase-locked to (peripheral) sensory-motor exploratory rhythms. We studied rats on an elevated platform where they would spontaneously display exploratory and rest behaviors. Concurrent with behavior, we monitored whisking through electromyography and the respiratory rhythm from the olfactory bulb (OB) local field potential (LFP). We also recorded LFPs from dorsal hippocampus, primary motor cortex, primary somatosensory cortex, and primary visual cortex. We defined exploration as simultaneous whisking and sniffing above 5 Hz and found that this activity peaked at ~8 Hz. We considered rest as the absence of whisking and sniffing, and in this case, respiration occurred at ~3 Hz. We found a consistent shift across all areas toward these rhythm peaks accompanying behavioral changes. We also found, across areas, that LFP gamma (70-100 Hz) amplitude could phase-lock to the animal's OB respiratory rhythm, a finding indicative of respiration-locked changes in local processing. In a subset of animals, we also recorded the hippocampal theta activity and found that occurred at frequencies overlapped with respiration but was not spectrally coherent with it, suggesting a different oscillator. Our results are consistent with the notion of respiration as a binder or integrator of activity between brain regions.

Temporal Response Properties of Accessory Olfactory Bulb Neurons: Limitations and Opportunities for Decoding

The vomeronasal system (VNS) is a major vertebrate chemosensory system that functions in parallel to the main olfactory system (MOS). Despite many similarities, the two systems dramatically differ in the temporal domain. While MOS responses are governed by breathing and follow a subsecond temporal scale, VNS responses are uncoupled from breathing and evolve over seconds. This suggests that the contribution of response dynamics to stimulus information will differ between these systems. While temporal dynamics in the MOS are widely investigated, similar analyses in the accessory olfactory bulb (AOB) are lacking. Here, we have addressed this issue using controlled stimulus delivery to the vomeronasal organ of male and female mice. We first analyzed the temporal properties of AOB projection neurons and demonstrated that neurons display prolonged, variable, and neuron-specific characteristics. We then analyzed various decoding schemes using AOB population responses. We showed that compared with the simplest scheme (i.e., integration of spike counts over the entire response period), the division of this period into smaller temporal bins actually yields poorer decoding accuracy. However, optimal classification accuracy can be achieved well before the end of the response period by integrating spike counts within temporally defined windows. Since VNS stimulus uptake is variable, we analyzed decoding using limited information about stimulus uptake time, and showed that with enough neurons, such time-invariant decoding is feasible. Finally, we conducted simulations that demonstrated that, unlike the main olfactory bulb, the temporal features of AOB neurons disfavor decoding with high temporal accuracy, and, rather, support decoding without precise knowledge of stimulus uptake time.SIGNIFICANCE STATEMENT A key goal in sensory system research is to identify which metrics of neuronal activity are relevant for decoding stimulus features. Here, we describe the first systematic analysis of temporal coding in the vomeronasal system (VNS), a chemosensory system devoted to socially relevant cues. Compared with the main olfactory system, timescales of VNS function are inherently slower and variable. Using various analyses of real and simulated data, we show that the consideration of response times relative to stimulus uptake can aid the decoding of stimulus information from neuronal activity. However, response properties of accessory olfactory bulb neurons favor decoding schemes that do not rely on the precise timing of stimulus uptake. Such schemes are consistent with the variable nature of VNS stimulus uptake.

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.

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

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

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

A 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.

Mechanosensory-Based Phase Coding of Odor Identity in the Olfactory Bulb

Mitral and tufted (M/T) cells in the olfactory bulb produce rich temporal patterns of activity in response to different odors. However, it remains unknown how these temporal patterns are generated and how they are utilized in olfaction. Here we show that temporal patterning effectively discriminates between the two sensory modalities detected by olfactory sensory neurons (OSNs): odor and airflow-driven mechanical signals. Sniff-induced mechanosensation generates glomerulus-specific oscillatory activity in M/T cells, whose phase was invariant across airflow speed. In contrast, odor stimulation caused phase shifts (phase coding). We also found that odor-evoked phase shifts are concentration invariant and stable across multiple sniff cycles, contrary to the labile nature of rate coding. The loss of oscillatory mechanosensation impaired the precision and stability of phase coding, demonstrating its role in olfaction. We propose that phase, not rate, coding is a robust encoding strategy of odor identity and is ensured by airflow-induced mechanosensation in OSNs.

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.

A theoretical framework for analyzing coupled neuronal networks: Application to the olfactory system

Determining how synaptic coupling within and between regions is modulated during sensory processing is an important topic in neuroscience. Electrophysiological recordings provide detailed information about neural spiking but have traditionally been confined to a particular region or layer of cortex. Here we develop new theoretical methods to study interactions between and within two brain regions, based on experimental measurements of spiking activity simultaneously recorded from the two regions. By systematically comparing experimentally-obtained spiking statistics to (efficiently computed) model spike rate statistics, we identify regions in model parameter space that are consistent with the experimental data. We apply our new technique to dual micro-electrode array in vivo recordings from two distinct regions: olfactory bulb (OB) and anterior piriform cortex (PC). Our analysis predicts that: i) inhibition within the afferent region (OB) has to be weaker than the inhibition within PC, ii) excitation from PC to OB is generally stronger than excitation from OB to PC, iii) excitation from PC to OB and inhibition within PC have to both be relatively strong compared to presynaptic inputs from OB. These predictions are validated in a spiking neural network model of the OB–PC pathway that satisfies the many constraints from our experimental data. We find when the derived relationships are violated, the spiking statistics no longer satisfy the constraints from the data. In principle this modeling framework can be adapted to other systems and be used to investigate relationships between other neural attributes besides network connection strengths. Thus, this work can serve as a guide to further investigations into the relationships of various neural attributes within and across different regions during sensory processing.

Sensory processing is known to span multiple regions of the nervous system. However, electrophysiological recordings during sensory processing have traditionally been limited to a single region or brain layer. With recent advances in experimental techniques, recorded spiking activity from multiple regions simultaneously is feasible. However, other important quantities— such as inter-region connection strengths—cannot yet be measured. Here, we develop new theoretical tools to leverage data obtained by recording from two different brain regions simultaneously. We address the following questions: what are the crucial neural network attributes that enable sensory processing across different regions, and how are these attributes related to one another? With a novel theoretical framework to efficiently calculate spiking statistics, we can characterize a high dimensional parameter space that satisfies data constraints. We apply our results to the olfactory system to make specific predictions about effective network connectivity. Our framework relies on incorporating relatively easy-to-measure quantities to predict hard-to-measure interactions across multiple brain regions. Because this work is adaptable to other systems, we anticipate it will be a valuable tool for analysis of other larger scale brain recordings.

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.

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.

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.

Neural Representation of Odor-Guided Behavior in the Rat Olfactory Thalamus

The mediodorsal thalamus (MDT) is a higher-order corticocortical thalamic nucleus involved in cognition and memory. However, anatomically, the MDT is also the primary site of olfactory representation in the thalamus, receiving strong inputs from olfactory cortex and having reciprocal connections with orbitofrontal cortex (OFC). Nonetheless, its role in olfaction remains unclear. Here, we recorded single units in the MDT, as well as local field potentials in the MDT, piriform cortex (PCX), and OFC in rats performing a two-alternative odor discrimination task. We show that subsets of MDT units display odorant selectivity during sampling, as well as encoding of spatio-motor aspects of the task. Furthermore, the olfactory trans-thalamic network rapidly switches functional connectivity between MDT and cortical areas depending on current task demands, with, for example, MDT-PCX coupling enhanced during odor sampling and MDT-OFC coupling enhanced during the decision/goal approach compared with baseline and presampling. These results demonstrate MDT representation of diverse sensorimotor components of an olfactory task.

SIGNIFICANCE STATEMENT

The mediodorsal thalamus (MDT) is the major olfactory thalamic nucleus and links the olfactory archicortex with the prefrontal neocortex. The MDT is well known to be involved in higher-order cognitive and memory functions, but its role in olfaction is poorly understood. Here, using single-unit and local field potential analyses, we explored MDT function during an odor-guided decision task in rats. We describe MDT odor and multisensory coding and demonstrate behavior-dependent functional connectivity within the MDT/sensory cortex/prefrontal cortex network. Our results suggest a rich representation of olfactory and other information within MDT required to perform this odor-guided task. Our work opens a new model system for understanding MDT function and exploring the important role of MDT in cortical-cortical communication.

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.

Complementary codes for odor identity and intensity in olfactory cortex

The ability to represent both stimulus identity and intensity is fundamental for perception. Using large-scale population recordings in awake mice, we find distinct coding strategies facilitate non-interfering representations of odor identity and intensity in piriform cortex. Simply knowing which neurons were activated is sufficient to accurately represent odor identity, with no additional information about identity provided by spike time or spike count. Decoding analyses indicate that cortical odor representations are not sparse. Odorant concentration had no systematic effect on spike counts, indicating that rate cannot encode intensity. Instead, odor intensity can be encoded by temporal features of the population response. We found a subpopulation of rapid, largely concentration-invariant responses was followed by another population of responses whose latencies systematically decreased at higher concentrations. Cortical inhibition transforms olfactory bulb output to sharpen these dynamics. Our data therefore reveal complementary coding strategies that can selectively represent distinct features of a stimulus.

DOI:http://dx.doi.org/10.7554/eLife.22630.001

Context- and Output Layer-Dependent Long-Term Ensemble Plasticity in a Sensory Circuit

Sensory information is translated into ensemble representations by various populations of projection neurons in brain circuits. The dynamics of ensemble representations formed by distinct channels of output neurons in diverse behavioral contexts remains largely unknown. We studied the two output neuron layers in the olfactory bulb (OB), mitral and tufted cells, using chronic two-photon calcium imaging in awake mice. Both output populations displayed similar odor response profiles. During passive sensory experience, both populations showed reorganization of ensemble odor representations yet stable pattern separation across days. Intriguingly, during active odor discrimination learning, mitral but not tufted cells exhibited improved pattern separation, although both populations showed reorganization of ensemble representations. An olfactory circuitry model suggests that cortical feedback on OB interneurons can trigger both forms of plasticity. In conclusion, we show that different OB output layers display unique context-dependent long-term ensemble plasticity, allowing parallel transfer of non-redundant sensory information to downstream centers.

Video Abstract

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Mitral and tufted cells in the olfactory bulb show similar odor-evoked responses

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Passive odor experience reorganizes ensemble odor representations in both cell types

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Associative odor learning specifically improves pattern separation in mitral cells

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Cortical feedback can trigger both forms of plasticity in a network model

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.

Differences in Glomerular-Layer-Mediated Feedforward Inhibition onto Mitral and Tufted Cells Lead to Distinct Modes of Intensity Coding

Understanding how each of the many interneuron subtypes affects brain network activity is critical. In the mouse olfactory system, mitral cells (MCs) and tufted cells (TCs) comprise parallel pathways of olfactory bulb output that are thought to play distinct functional roles in odor coding. Here, in acute mouse olfactory bulb slices, we test how the two major classes of olfactory bulb interneurons differentially contribute to differences in MC versus TC response properties. We show that, whereas TCs respond to olfactory sensory neuron (OSN) stimulation with short latencies regardless of stimulation intensity, MC latencies correlate negatively with stimulation intensity. These differences between MCs and TCs are caused in part by weaker excitatory and stronger inhibitory currents onto MCs than onto TCs. These differences in inhibition between MCs and TCs are most pronounced during the first 150 ms after stimulation and are mediated by glomerular layer circuits. Therefore, blocking inhibition originating in the glomerular layer, but not granule-cell-mediated inhibition, reduces MC spike latency at weak stimulation intensities and distinct temporal patterns of odor-evoked responses in MCs and TCs emerge in part due to differences in glomerular-layer-mediated inhibition.SIGNIFICANCE STATEMENT Olfactory bulb mitral and tufted cells display different odor-evoked responses and are thought to form parallel channels of olfactory bulb output. Therefore, determining the circuit-level causes that drive these differences is vital. Here, we find that longer-latency responses in mitral cells, compared with tufted cells, are due to weaker excitation and stronger glomerular-layer-mediated inhibition.

Respiration Gates Sensory Input Responses in the Mitral Cell Layer of the Olfactory Bulb

Respiration plays an essential role in odor processing. Even in the absence of odors, oscillating excitatory and inhibitory activity in the olfactory bulb synchronizes with respiration, commonly resulting in a burst of action potentials in mammalian mitral/tufted cells (MTCs) during the transition from inhalation to exhalation. This excitation is followed by inhibition that quiets MTC activity in both the glomerular and granule cell layers. Odor processing is hypothesized to be modulated by and may even rely on respiration-mediated activity, yet exactly how respiration influences sensory processing by MTCs is still not well understood. By using optogenetics to stimulate discrete sensory inputs in vivo, it was possible to temporally vary the stimulus to occur at unique phases of each respiration. Single unit recordings obtained from the mitral cell layer were used to map spatiotemporal patterns of glomerular evoked responses that were unique to stimulations occurring during periods of inhalation or exhalation. Sensory evoked activity in MTCs was gated to periods outside phasic respiratory mediated firing, causing net shifts in MTC activity across the cycle. In contrast, odor evoked inhibitory responses appear to be permitted throughout the respiratory cycle. Computational models were used to further explore mechanisms of inhibition that can be activated by respiratory activity and influence MTC responses. In silico results indicate that both periglomerular and granule cell inhibition can be activated by respiration to internally gate sensory responses in the olfactory bulb. Both the respiration rate and strength of lateral connectivity influenced inhibitory mechanisms that gate sensory evoked responses.