Complex neural representation of odour information in the olfactory bulb

Neurogenesis dynamics in the olfactory bulb: deciphering circuitry organization, function, and adaptive plasticity

Adult neurogenesis persists after birth in the subventricular zone, with new neurons migrating to the granule cell layer and glomerular layers of the olfactory bulb, where they integrate into existing circuitry as inhibitory interneurons. The generation of these new neurons in the olfactory bulb supports both structural and functional plasticity, aiding in circuit remodeling triggered by memory and learning processes. However, the presence of these neurons, coupled with the cellular diversity within the olfactory bulb, presents an ongoing challenge in understanding its network organization and function. Moreover, the continuous integration of new neurons in the olfactory bulb plays a pivotal role in regulating olfactory information processing. This adaptive process responds to changes in epithelial composition and contributes to the formation of olfactory memories by modulating cellular connectivity within the olfactory bulb and interacting intricately with higher-order brain regions. The role of adult neurogenesis in olfactory bulb functions remains a topic of debate. Nevertheless, the functionality of the olfactory bulb is intricately linked to the organization of granule cells around mitral and tufted cells. This organizational pattern significantly impacts output, network behavior, and synaptic plasticity, which are crucial for olfactory perception and memory. Additionally, this organization is further shaped by axon terminals originating from cortical and subcortical regions. Despite the crucial role of olfactory bulb in brain functions and behaviors related to olfaction, these complex and highly interconnected processes have not been comprehensively studied as a whole. Therefore, this manuscript aims to discuss our current understanding and explore how neural plasticity and olfactory neurogenesis contribute to enhancing the adaptability of the olfactory system. These mechanisms are thought to support olfactory learning and memory, potentially through increased complexity and restructuring of neural network structures, as well as the addition of new granule granule cells that aid in olfactory adaptation. Additionally, the manuscript underscores the importance of employing precise methodologies to elucidate the specific roles of adult neurogenesis amidst conflicting data and varying experimental paradigms. Understanding these processes is essential for gaining insights into the complexities of olfactory function and behavior.

Essential Role of the Anterior Piriform Cortex in Mediating Social Novelty Output via a Top-Down Circuit

Social novelty is indispensable for a wide range of social behaviors. The medial prefrontal cortex (mPFC), along with other social information hubs, composes the foundational circuitry of social novelty. However, the precise circuit mechanisms that govern social novelty processing remain elusive. The piriform cortex, as the largest olfactory cortex, receives extensive innervation from top-down centers that dictate social behavior. Here, it is shown that the anterior piriform cortex (APC) exhibited an increase in gamma event incidence during social engagement in male mice. In vivo electrophysiology and fiber photometry reveal that APC pyramidal neurons respond more intensely to novel mice than familiar ones. Intriguingly, silencing APC neurons selectively impairs social novelty processing, yet leaves the basic olfactory discrimination capabilities intact. Moreover, the APC inherits social cues from the mPFC and sends feedback projections to the olfactory bulb (OB) to modulate social novelty. These findings unveil the APC's role as extending well beyond olfaction, encompassing a specialized function in social novelty recognition in male mice.

Intranasal iron administration induces iron deposition, immunoactivation, and cell-specific vulnerability in the olfactory bulb of C57BL/6 mice

Iron is the most abundant transition metal in the brain and is essential for brain development and neuronal function; however, its abnormal accumulation is also implicated in various neurological disorders. The olfactory bulb (OB), an early target in neurodegenerative diseases, acts as a gateway for environmental toxins and contains diverse neuronal populations with distinct roles. This study explored the cell-specific vulnerability to iron in the OB using a mouse model of intranasal administration of ferric ammonium citrate (FAC). Olfactory function was assessed through olfactory discrimination tests, while iron levels in OB tissues, cerebrospinal fluid (CSF), and serum were quantified using inductively coupled plasma mass spectrometry (ICP-MS), immunohistochemical staining, and iron assays. Transcriptomic changes and immune responses were assessed using RNA sequencing and immune cell infiltration analysis. Results showed that intranasal FAC administration impaired olfactory function, accompanied by iron deposition in the olfactory mucosa and OB, as well as damage to olfactory sensory neurons. Notably, these effects occurred without elevations in CSF or serum iron levels. OB iron accumulation activated multiple immune cells, including microglia and astrocytes, but did not trigger ferroptosis. Spatial transcriptomic sequencing of healthy adult mouse OBs revealed significant cellular heterogeneity, with an abundance of neuroglia and neurons. Among neurons, GABAergic neurons were the most prevalent, followed by glutamatergic and dopaminergic neurons, while cholinergic and serotonergic neurons were sparsely distributed. Under iron-stressed conditions, oligodendrocytes, dopaminergic neurons, and glutamatergic neurons exhibited significant damage, while GABAergic neurons remained unaffected. These findings highlight the selective vulnerability of neuronal and glial populations to iron-induced stress, offering novel insights into the loss of specific cell types in the OB during iron dysregulation.

Noradrenergic inputs from the locus coeruleus to anterior piriform cortex and the olfactory bulb modulate olfactory outputs

Norepinephrine (NE) released from locus coeruleus (LC) noradrenergic (NAergic) neurons plays a pivotal role in the regulation of olfactory behaviors. However, the precise circuits and receptor mechanisms underlying this function are not well understood. Here, in DBH-Cre mice model, we show that LC NAergic neurons project directly to both anterior piriform cortex (aPC) and the olfactory bulb (OB). By using pharmacological and optogenetic manipulations in vitro and in vivo, we found that NE reduces the excitability of aPC pyramidal neurons directly via α2 receptors and that it bidirectionally regulates the activity of OB mitral cells via modulation of inhibitory inputs. Activation of the NAergic projection reduced both spontaneous and odor-evoked activity in the aPC/OB in awake mice, enhanced the odor-decoding ability of the aPC, and decreased the odor-decoding ability of the OB. Furthermore, activation of LC-aPC/OB NAergic projections accelerated odor discrimination and specific inactivation of the LC-aPC/OB NAergic pathway impaired olfactory detection and discrimination. These findings identify the mechanism underlying NAergic modulation of the aPC/OB and elucidate its role in odor processing and olfactory behaviors.

Preliminary Findings on the Morphometric Characteristics of the Olfactory Bulb in the Cat

The aim of this preliminary study was to morphologically and dimensionally characterize the cat's olfactory bulb in the sagittal plane and to establish potential relationships with the cranial conformation, based on the study of in vivo MRI images. Midsagittal and transverse T2-weighted images of the head of 40 cats subjected to MRI were selected. For each animal, the skull index was calculated to classify the cranial conformation. Then, for the olfactory bulb, the angle was calculated, the orientation was determined, and the sagittal section area was measured. It was established that animals classified as brachycephalic have more compact olfactory bulbs, with smaller cross-sectional areas, ventral orientation and smaller angles established with the line that goes from the hard palate and the intercondylar notch of the foramen magnum. Animals classified as dolichocephalic have more globose and wider olfactory bulbs, dorsal orientation, and larger angles. Mesocephalic animals present an intermediate position. Males and younger adult animals have olfactory bulbs with larger cross-sectional areas than females and older animals. This work allows for the preliminarily characterization of the olfactory bulb in cats in the sagittal plane, and the correlations identified with other head structures open doors for the use of the bulb as an early indicator for the establishment of alterations of varied etiology.

Ion Channels in Odor Information Processing of Neural Circuits of the Vertebrate Olfactory Bulb

Olfactory disorders and their associated complications present a considerable challenge to an individual's quality of life and emotional wellbeing. The current range of treatments, including surgical procedures, pharmacological interventions, and behavioral training, frequently proves ineffective in restoring olfactory function. The olfactory bulb (OB) is essential for odor processing and plays a pivotal role in the development of these disorders. Despite the acknowledged significance of ion channels in sensory functions and related pathologies, their specific involvement in OB remains unexplored. This review presents an overview of the functions of various ion channel families in regulating neuronal excitability, synaptic transmission, and the complex processes of olfactory perception. The objective of this review was to elucidate the role of ion channels in olfactory function, providing new insights into the diagnosis and treatment of olfactory dysfunction.

Odor representation and coding by the mitral/tufted cells in the olfactory bulb

The olfactory bulb (OB) is the first relay station in the olfactory system and functions as a crucial hub. It can represent odor information precisely and accurately in an ever-changing environment. As the only output neurons in the OB, mitral/tufted cells encode information such as odor identity and concentration. Recently, the neural strategies and mechanisms underlying odor representation and encoding in the OB have been investigated extensively. Here we review the main progress on this topic. We first review the neurons and circuits involved in odor representation, including the different cell types in the OB and the neural circuits within and beyond the OB. We will then discuss how two different coding strategies-spatial coding and temporal coding-work in the rodent OB. Finally, we discuss potential future directions for this research topic. Overall, this review provides a comprehensive description of our current understanding of how odor information is represented and encoded by mitral/tufted cells in the OB.

Serotonergic input from the dorsal raphe nucleus shapes learning-associated odor responses in the olfactory bulb

AIM

Neural activity in the olfactory bulb (OB) can represent odor information during different brain and behavioral states. For example, the odor responses of mitral/tufted (M/T) cells in the OB change during learning of odor-discrimination tasks and, at the network level, beta power increases and the high gamma (HG) power decreases during odor presentation in such tasks. However, the neural mechanisms underlying these observations remain poorly understood. Here, we investigate whether serotonergic modulation from the dorsal raphe nucleus (DRN) to the OB is involved in shaping activity during the learning process in a go/no-go task in mice.

METHODS

Fiber photometry was used to record the population activity of DRN serotonergic neurons during a go/no-go task. In vivo electrophysiology was used to record neural activity (single units and local field potentials) in the OB during the go/no-go task. Real-time place preference (RTPP) and intracranial light administration in a specific subarea (iClass) tests were used to assess the ability of mice to encoding reward information.

RESULTS

Odor-evoked population activity in serotonergic neurons in the DRN was shaped during the learning process in a go/no-go task. In the OB, neural activity from oscillations to single cells showed complex, learning-associated changes and ability to encode information during an odor discrimination task. However, these properties were not observed after ablation of DRN serotonergic neurons.

CONCLUSION

The activity of neural networks and single cells in the OB, and their ability to encode information about odor value, are shaped by serotonergic projections from the DRN.

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

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

Common principles for odour coding across vertebrates and invertebrates

The olfactory system is an ideal and tractable system for exploring how the brain transforms sensory inputs into behaviour. The basic tasks of any olfactory system include odour detection, discrimination and categorization. The challenge for the olfactory system is to transform the high-dimensional space of olfactory stimuli into the much smaller space of perceived objects and valence that endows odours with meaning. Our current understanding of how neural circuits address this challenge has come primarily from observations of the mechanisms of the brain for processing other sensory modalities, such as vision and hearing, in which optimized deep hierarchical circuits are used to extract sensory features that vary along continuous physical dimensions. The olfactory system, by contrast, contends with an ill-defined, high-dimensional stimulus space and discrete stimuli using a circuit architecture that is shallow and parallelized. Here, we present recent observations in vertebrate and invertebrate systems that relate the statistical structure and state-dependent modulation of olfactory codes to mechanisms of perception and odour-guided behaviour.

Behavioral discrimination and olfactory bulb encoding of odor plume intermittency

In order to survive, animals often need to navigate a complex odor landscape where odors can exist in airborne plumes. Several odor plume properties change with distance from the odor source, providing potential navigational cues to searching animals. Here, we focus on odor intermittency, a temporal odor plume property that measures the fraction of time odor is above a threshold at a given point within the plume and decreases with increasing distance from the odor source. We sought to determine if mice can use changes in intermittency to locate an odor source. To do so, we trained mice on an intermittency discrimination task. We establish that mice can discriminate odor plume samples of low and high intermittency and that the neural responses in the olfactory bulb can account for task performance and support intermittency encoding. Modulation of sniffing, a behavioral parameter that is highly dynamic during odor-guided navigation, affects both behavioral outcome on the intermittency discrimination task and neural representation of intermittency. Together, this work demonstrates that intermittency is an odor plume property that can inform olfactory search and more broadly supports the notion that mammalian odor-based navigation can be guided by temporal odor plume properties.

Amelioration of olfactory dysfunction in a mouse model of Parkinson's disease via enhancing GABAergic signaling

BACKGROUND

Olfactory dysfunction is among the earliest non-motor symptoms of Parkinson's disease (PD). As the foremost pathological hallmark, α-synuclein initiates the pathology in the olfactory pathway at the early stage of PD, particularly in the olfactory epithelium (OE) and olfactory bulb (OB). However, the local neural microcircuit mechanisms underlying olfactory dysfunction between OE and OB in early PD remain unknown.

RESULTS

We observed that odor detection and discrimination were impaired in 6-month-old SNCA-A53T mice, while their motor ability remained unaffected. It was confirmed that α-synuclein increased and accumulated in OB but not in OE. Notably, the hyperactivity of mitral/tufted cells and the excitation/inhibition imbalance in OB were found in 6-month-old SNCA-A53T mice, which was attributed to the impaired GABAergic transmission and aberrant expression of GABA transporter 1 and vesicular GABA transporter in OB. We further showed that tiagabine, a potent and selective GABA reuptake inhibitor, could reverse the impaired olfactory function and GABAergic signaling in OB of SNCA-A53T mice.

CONCLUSIONS

Taken together, our findings demonstrate potential synaptic mechanisms of local neural microcircuit underlying olfactory dysfunction at the early stage of PD. These results highlight the critical role of aberrant GABAergic signaling of OB in early diagnosis and provide a potential therapeutic strategy for early-stage PD.

Synaptic Development in Diverse Olfactory Neuron Classes Uses Distinct Temporal and Activity-Related Programs

Developing neurons must meet core molecular, cellular, and temporal requirements to ensure the correct formation of synapses, resulting in functional circuits. However, because of the vast diversity in neuronal class and function, it is unclear whether or not all neurons use the same organizational mechanisms to form synaptic connections and achieve functional and morphologic maturation. Moreover, it remains unknown whether neurons united in a common goal and comprising the same sensory circuit develop on similar timescales and use identical molecular approaches to ensure the formation of the correct number of synapses. To begin to answer these questions, we took advantage of the Drosophila antennal lobe (AL), a model olfactory circuit with remarkable genetic access and synapse-level resolution. Using tissue-specific genetic labeling of active zones, we performed a quantitative analysis of synapse formation in multiple classes of neurons of both sexes throughout development and adulthood. We found that olfactory receptor neurons (ORNs), projection neurons (PNs), and local interneurons (LNs) each have unique time courses of synaptic development, addition, and refinement, demonstrating that each class follows a distinct developmental program. This raised the possibility that these classes may also have distinct molecular requirements for synapse formation. We genetically altered neuronal activity in each neuronal subtype and observed differing effects on synapse number based on the neuronal class examined. Silencing neuronal activity in ORNs, PNs, and LNs impaired synaptic development but only in ORNs did enhancing neuronal activity influence synapse formation. ORNs and LNs demonstrated similar impairment of synaptic development with enhanced activity of a master kinase, GSK-3β, suggesting that neuronal activity and GSK-3β kinase activity function in a common pathway. ORNs also, however, demonstrated impaired synaptic development with GSK-3β loss-of-function, suggesting additional activity-independent roles in development. Ultimately, our results suggest that the requirements for synaptic development are not uniform across all neuronal classes with considerable diversity existing in both their developmental time frames and molecular requirements. These findings provide novel insights into the mechanisms of synaptic development and lay the foundation for future work determining their underlying etiologies.SIGNIFICANCE STATEMENT Distinct olfactory neuron classes in Drosophila develop a mature synaptic complement over unique timelines and using distinct activity-dependent and molecular programs, despite having the same generalized goal of olfactory sensation.

Olfactory Bulb Integrity in Frontotemporal Dementia and Alzheimer’s Disease

Background: Olfactory dysfunction is highly prevalent in dementia syndromes, including Alzheimer’s disease (AD) and frontotemporal dementia (FTD). The structural integrity of the olfactory bulb (OB) is thought to play a critical role in odor detection and identification, but no MRI study has measured OB volume in FTD, or measured OB volume longitudinally in AD. Objective: To measure OB volume in FTD and AD patients longitudinally using MRI. Methods: This study measured OB volumes using MRI in patients diagnosed with behavioral-variant FTD (n = 55), semantic dementia (n = 34), progressive non-fluent aphasia (n = 30), AD (n = 50), and healthy age-matched controls (n = 55) at their first visit to a dementia research clinic (‘baseline’). Imaging data in patients 12-months later were analyzed where available (n = 84) for longitudinal assessment. Volumes of subcortical and cortical olfactory regions (‘olfactory network’) were obtained via surface-based morphometry. Results: Results revealed that in AD and FTD at baseline, OB volumes were similar to controls, whereas volumes of olfactory network regions were significantly reduced in all patient groups except in progressive non-fluent aphasia. Longitudinal data revealed that OB volume became significantly reduced (10–25% volume reduction) in all dementia groups with disease progression. Conclusion: Olfactory dysfunction is common in patients diagnosed with AD or FTD, but our results indicate that there is no detectable volume loss to the OBs upon first presentation to the clinic. Our findings indicate that the OBs become detectably atrophied later in the disease process. OB atrophy indicates the potential usefulness for OBs to be targeted in interventions to improve olfactory function.

Neuronal Adenosine A1 Receptor is Critical for Olfactory Function but Unable to Attenuate Olfactory Dysfunction in Neuroinflammation

Adenine nucleotides, such as adenosine triphosphate (ATP), adenosine diphosphate (ADP), as well as the nucleoside adenosine are important modulators of neuronal function by engaging P1 and P2 purinergic receptors. In mitral cells, signaling of the G protein-coupled P1 receptor adenosine 1 receptor (A1R) affects the olfactory sensory pathway by regulating high voltage-activated calcium channels and two-pore domain potassium (K2P) channels. The inflammation of the central nervous system (CNS) impairs the olfactory function and gives rise to large amounts of extracellular ATP and adenosine, which act as pro-inflammatory and anti-inflammatory mediators, respectively. However, it is unclear whether neuronal A1R in the olfactory bulb modulates the sensory function and how this is impacted by inflammation. Here, we show that signaling via neuronal A1R is important for the physiological olfactory function, while it cannot counteract inflammation-induced hyperexcitability and olfactory deficit. Using neuron-specific A1R-deficient mice in patch-clamp recordings, we found that adenosine modulates spontaneous dendro-dendritic signaling in mitral and granule cells via A1R. Furthermore, neuronal A1R deficiency resulted in olfactory dysfunction in two separate olfactory tests. In mice with experimental autoimmune encephalomyelitis (EAE), we detected immune cell infiltration and microglia activation in the olfactory bulb as well as hyperexcitability of mitral cells and olfactory dysfunction. However, neuron-specific A1R activity was unable to attenuate glutamate excitotoxicity in the primary olfactory bulb neurons in vitro or EAE-induced olfactory dysfunction and disease severity in vivo. Together, we demonstrate that A1R modulates the dendro-dendritic inhibition (DDI) at the site of mitral and granule cells and impacts the processing of the olfactory sensory information, while A1R activity was unable to counteract inflammation-induced hyperexcitability.

VIP interneurons regulate olfactory bulb output and contribute to odor detection and discrimination

In the olfactory bulb (OB), olfactory information represented by mitral/tufted cells (M/Ts) is extensively modulated by local inhibitory interneurons before being transmitted to the olfactory cortex. While the crucial roles of cortical vasoactive-intestinal-peptide-expressing (VIP) interneurons have been extensively studied, their precise function in the OB remains elusive. Here, we identify the synaptic connectivity of VIP interneurons onto mitral cells (MCs) and demonstrate their important role in olfactory behaviors. Optogenetic activation of VIP interneurons reduced both spontaneous and odor-evoked activity of M/Ts in awake mice. Whole-cell recordings revealed that VIP interneurons decrease MC firing through direct inhibitory synaptic connections with MCs. Furthermore, inactivation of VIP interneurons leads to increased MC firing and impaired olfactory detection and odor discrimination. Therefore, our results demonstrate that VIP interneurons control OB output and play critical roles in odor processing and olfactory behaviors.

Ablation of microRNAs in VIP+ interneurons impairs olfactory discrimination and decreases neural activity in the olfactory bulb

AIM

MicroRNAs (miRNAs) are abundantly expressed in vasoactive intestinal peptide expressing (VIP⁺ ) interneurons and are indispensable for their functional maintenance and survival. Here, we blocked miRNA biogenesis in postmitotic VIP⁺ interneurons in mice by selectively ablating Dicer, an enzyme essential for miRNA maturation, to study whether ablation of VIP⁺ miRNA affects olfactory function and neural activity in olfactory centres such as the olfactory bulb, which contains a large number of VIP⁺ interneurons.

METHODS

A go/no-go odour discrimination task and a food-seeking test were used to assess olfactory discrimination and olfactory detection. In vivo electrophysiological techniques were used to record single units and local field potentials.

RESULTS

Olfactory detection and olfactory discrimination behaviours were impaired in VIP⁺ -specific Dicer-knockout mice. In vivo electrophysiological recordings in awake, head-fixed mice showed that both spontaneous and odour-evoked firing rates were decreased in mitral/tufted cells in knockout mice. The power of ongoing and odour-evoked beta local field potentials response of the olfactory bulb and anterior piriform cortex were dramatically decreased. Furthermore, the coherence of theta oscillations between the olfactory bulb and anterior piriform cortex was decreased. Importantly, Dicer knockout restricted to olfactory bulb VIP⁺ interneurons recapitulated the behavioural and electrophysiological results of the global knockout.

CONCLUSIONS

VIP⁺ miRNAs are an important factor in sensory processing, affecting olfactory function and olfactory neural activity.

α-Synuclein aggregation in the olfactory bulb induces olfactory deficits by perturbing granule cells and granular-mitral synaptic transmission

Olfactory dysfunction is an early pre-motor symptom of Parkinson's disease (PD) but the neural mechanisms underlying this dysfunction remain largely unknown. Aggregation of α-synuclein is observed in the olfactory bulb (OB) during the early stages of PD, indicating a relationship between α-synuclein pathology and hyposmia. Here we investigate whether and how α-synuclein aggregates modulate neural activity in the OB at the single-cell and synaptic levels. We induced α-synuclein aggregation specifically in the OB via overexpression of double-mutant human α-synuclein by an adeno-associated viral (AAV) vector. We found that α-synuclein aggregation in the OB decreased the ability of mice to detect odors and to perceive attractive odors. The spontaneous activity and odor-evoked firing rates of single mitral/tufted cells (M/Ts) were increased by α-synuclein aggregates with the amplitude of odor-evoked high-gamma oscillations increased. Furthermore, the decreased activity in granule cells (GCs) and impaired inhibitory synaptic function were responsible for the observed hyperactivity of M/Ts induced by α-synuclein aggregates. These results provide direct evidences of the role of α-synuclein aggregates on PD-related olfactory dysfunction and reveal the neural circuit mechanisms by which olfaction is modulated by α-synuclein pathology.

Principles of odor coding in vertebrates and artificial chemosensory systems

The biological olfactory system is the sensory system responsible for the detection of the chemical composition of the environment. Several attempts to mimic biological olfactory systems have led to various artificial olfactory systems using different technical approaches. Here we provide a parallel description of biological olfactory systems and their technical counterparts. We start with a presentation of the input to the systems, the stimuli, and treat the interface between the external world and the environment where receptor neurons or artificial chemosensors reside. We then delineate the functions of receptor neurons and chemosensors as well as their overall I-O relationships. Up to this point, our account of the systems goes along similar lines. The next processing steps differ considerably: while in biology the processing step following the receptor neurons is the "integration" and "processing" of receptor neuron outputs in the olfactory bulb, this step has various realizations in electronic noses. For a long period of time, the signal processing stages beyond the olfactory bulb, i.e., the higher olfactory centers were little studied. Only recently there has been a marked growth of studies tackling the information processing in these centers. In electronic noses, a third stage of processing has virtually never been considered. In this review, we provide an up-to-date overview of the current knowledge of both fields and, for the first time, attempt to tie them together. We hope it will be a breeding ground for better information, communication, and data exchange between very related but so far little connected fields.

Dynamics of Glutamatergic Drive Underlie Diverse Responses of Olfactory Bulb Outputs In Vivo

Mitral/tufted (MT) cells of the olfactory bulb (OB) show diverse temporal responses to odorant stimulation that are thought to encode odor information. Much of this diversity is thought to arise from inhibitory OB circuits, but the dynamics of excitatory input to MT cells, which is driven in a feedforward manner by sensory afferents, may also be important. To examine the contribution of excitatory input dynamics to generating temporal diversity in MT cells, we imaged glutamate signaling onto MT cell dendrites in anesthetized and awake mice. We found surprising diversity in the temporal dynamics of these signals. Inhalation-linked glutamate transients were variable in onset latency and duration, and in awake mice the degree of coupling to inhalation varied substantially with odorant identity and concentration. Successive inhalations of odorant produced nonlinear changes in glutamate signaling that included facilitating, adapting and suppressive responses and which varied with odorant identity and concentration. Dual-color imaging of glutamate and calcium signals from MT cells in the same glomerulus revealed highly correlated presynaptic and postsynaptic signals across these different response types. Suppressive calcium responses in MT cells were nearly always accompanied by suppression in the glutamate signal, providing little evidence for MT cell suppression by lateral or feedforward inhibition. These results indicate a high degree of diversity in the dynamics of excitatory input to MT cells, and suggest that these dynamics may account for much of the diversity in MT cell responses that underlies OB odor representations.

Distinct Characteristics of Odor-evoked Calcium and Electrophysiological Signals in Mitral/Tufted Cells in the Mouse Olfactory Bulb

Fiber photometry is a recently-developed method that indirectly measures neural activity by monitoring Ca²⁺ signals in genetically-identified neuronal populations. Although fiber photometry is widely used in neuroscience research, the relationship between the recorded Ca²⁺ signals and direct electrophysiological measurements of neural activity remains elusive. Here, we simultaneously recorded odor-evoked Ca²⁺ and electrophysiological signals [single-unit spikes and local field potentials (LFPs)] from mitral/tufted cells in the olfactory bulb of awake, head-fixed mice. Odors evoked responses in all types of signal but the response characteristics (e.g., type of response and time course) differed. The Ca²⁺ signal was correlated most closely with power in the β-band of the LFP. The Ca²⁺ signal performed slightly better at odor classification than high-γ oscillations, worse than single-unit spikes, and similarly to β oscillations. These results provide new information to help researchers select an appropriate method for monitoring neural activity under specific conditions.

Oxytocin modulates neural processing of mitral/tufted cells in the olfactory bulb

AIM

Oxytocin plays an important role in social recognition in rodents, which is mediated predominantly by the olfactory system. Although oxytocin modulates neural activity in the olfactory bulb, the underlying mechanism is largely unknown. Here, we studied how direct infusion of oxytocin into the olfactory bulb affect social interactions in mice and modulate the neural activity of mitral/tufted cells in the olfactory bulb.

METHODS

A three-chamber social interaction test was used in the behavioural test. For in vivo studies, single unit recordings, local field potential recordings and fibre photometry recordings were used to record the neural activity of olfactory bulb. For in vitro studies, we performed patch clamp recordings in the slice of the olfactory bulb.

RESULTS

Behaviourally, direct oxytocin infusion in olfactory bulb increased performance in a social interaction task. Moreover, odour-evoked responses of mitral/tufted cells and neural discrimination of odours were both enhanced by oxytocin, whereas the spontaneous firing rate of mitral/tufted cells was reduced. At the neural network level, oxytocin decreased the amplitude of odour-evoked high gamma responses. At the cell population level, oxytocin decreased odour-evoked calcium responses (reflecting neural activity) specifically in granule cells. Moreover, in vitro slice recordings revealed that the inhibitory effect of oxytocin on mitral cell activity is mediated mainly by modulation of ATP-sensitive potassium channels and involves the oxytocin receptor-Gq-PLC-IP₃ signalling pathway.

CONCLUSION

Oxytocin modulates social interaction, likely by increasing the signal-to-noise ratio of odour responses in mitral cells which is partly through ATP-sensitive potassium channel.

Extrinsic neuromodulation in the rodent olfactory bulb

Evolutionarily, olfaction is one of the oldest senses and pivotal for an individual's health and survival. The olfactory bulb (OB), as the first olfactory relay station in the brain, is known to heavily process sensory information. To adapt to an animal's needs, OB activity can be influenced by many factors either from within (intrinsic neuromodulation) or outside (extrinsic neuromodulation) the OB which include neurotransmitters, neuromodulators, hormones, and neuropeptides. Extrinsic sources seem to be of special importance as the OB receives massive efferent input from numerous brain centers even outweighing the sensory input from the nose. Here, we review neuromodulatory processes in the rodent OB from such extrinsic sources. We will discuss extrinsic neuromodulation according to points of origin, receptors involved, affected circuits, and changes in behavior. In the end, we give a brief outlook on potential future directions in research on neuromodulation in the OB.

In vivo odorant input induces distinct synaptic plasticity of GABAergic synapses in developing zebrafish olfactory bulb

In the first station of central odor processing, the main olfactory bulb, signal processing is regulated by synaptic interactions between glutamatergic and GABAergic inputs of the mitral cells (MCs), the major projection neurons. Our previous study has found that repetitive postsynaptic spiking within a critical time window after presynaptic activation by natural odorant stimulation results in persistent enhancement of glutamatergic inputs of MCs in larval zebrafish. Here we observed a long-term depression of GABAergic synapses induced by the same protocol. This long-term depression was mediated by presynaptic NMDA receptors (NMDARs). Further dissecting GABAergic neurotransmission revealed that the STDP-induction protocol induced persistent modification in recurrent and lateral inhibition with opposite directions and distinct requirements on NMDARs. Thus, at the plasticity level, different types of GABAergic inhibition may utilize different mechanisms to cooperate or compete with excitatory inputs to optimize patterns of olfactory bulb output.

Improved Separation of Odor Responses in Granule Cells of the Olfactory Bulb During Odor Discrimination Learning

In the olfactory bulb, olfactory information is translated into ensemble representations by mitral/tufted cells, and these representations change dynamically in a context-dependent manner. In particular, odor representations in mitral/tufted cells display pattern separation during odor discrimination learning. Although granule cells provide major inhibitory input to mitral/tufted cells and play an important role in pattern separation and olfactory learning, the dynamics of odor responses in granule cells during odor discrimination learning remain largely unknown. Here, we studied odor responses in granule cells of the olfactory bulb using fiber photometry recordings in awake behaving mice. We found that odors evoked reliable, excitatory responses in the granule cell population. Intriguingly, during odor discrimination learning, odor responses in granule cells exhibited improved separation and contained information about odor value. In conclusion, we show that granule cells in the olfactory bulb display learning-related plasticity, suggesting that they may mediate pattern separation in mitral/tufted cells.

Adult Neurogenesis in the Olfactory System: Improving Performance for Difficult Discrimination Tasks?

What is the function of new neurons entering the olfactory bulb? Many insights regarding the molecular control of adult neurogenesis have been uncovered, but the purpose of new neurons entering the olfactory bulb has been difficult to ascertain. Here, studies investigating the role of adult neurogenesis in olfactory discrimination in mice are reviewed. Studies in which adult neurogenesis is affected are highlighted, with a focus on the role of environment enrichment and what happens during ageing. There is evidence for a role of adult neurogenesis in fine discrimination tasks, as underscored by studies that enhance adult neurogenesis. This is also observed in ageing studies, where older mice with reduced levels of adult neurogenesis perform poorly in olfactory discrimination. Differences in methodology that could account for alternative conclusions, and the importance of specificity in methods being used to investigate the effect of adult neurogenesis in olfactory performance are emphasized.

In vivo odorant input induces spike timing-dependent plasticity of glutamatergic synapses in developing zebrafish olfactory bulb

Early odorant experience and neural activity are essential for refining developing neural connections. Although neural activity-induced synaptic plasticity is one of the most important cellular mechanisms underlying the refinement of neural circuits, whether and how natural odorant experience induces long-term plasticity in the olfactory bulb remains unknown. In vivo perforated whole-cell recording from mitral cells (MCs) in larval zebrafish showed that odorant experience induced persistent modification of developing olfactory bulb circuits via spike timing-dependent plasticity (STDP). Repetitive odorant stimuli paired with postsynaptic spiking in a critical time window (pre-post, positive timing) resulted in persistent enhancement of glutamatergic inputs from olfactory sensory neurons, but long-term depression within the opposite time window (post-pre, negative timing). Furthermore, spike-timing-dependent potentiation (tLTP) in STDP induced by repetitive odorant stimulation had similar cellular processes to those of electrical stimulation-induced tLTP. Finally, odorant input induced STDP required the activation of postsynaptic N-methyl-d-aspartate receptors (NMDARs). Thus, the NMDAR is likely to be a postsynaptic coincidence detector responsible for the sensory experience-dependent refinement of developing connections.

Mechanisms of competitive selection: A canonical neural circuit framework

Competitive selection, the transformation of multiple competing sensory inputs and internal states into a unitary choice, is a fundamental component of animal behavior. Selection behaviors have been studied under several intersecting umbrellas including decision-making, action selection, perceptual categorization, and attentional selection. Neural correlates of these behaviors and computational models have been investigated extensively. However, specific, identifiable neural circuit mechanisms underlying the implementation of selection remain elusive. Here, we employ a first principles approach to map competitive selection explicitly onto neural circuit elements. We decompose selection into six computational primitives, identify demands that their execution places on neural circuit design, and propose a canonical neural circuit framework. The resulting framework has several links to neural literature, indicating its biological feasibility, and has several common elements with prominent computational models, suggesting its generality. We propose that this framework can help catalyze experimental discovery of the neural circuit underpinnings of competitive selection.

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.

Serotonergic afferents from the dorsal raphe decrease the excitability of pyramidal neurons in the anterior piriform cortex

The olfactory system receives extensive serotonergic inputs from the dorsal raphe, a nucleus involved in control of behavior, regulation of mood, and modulation of sensory processing. Although many studies have investigated how serotonin modulates the olfactory bulb, few have focused on the anterior piriform cortex (aPC), a region important for olfactory learning and encoding of odor identity and intensity. Specifically, the mechanism and functional significance of serotonergic modulation of the aPC remain largely unknown. Here we used pharmacologic, optogenetic, and fiber photometry techniques to examine the serotonergic modulation of neural activity in the aPC in vitro and in vivo. We found that serotonin (5-HT) reduces the excitability of pyramidal neurons directly via 5-HT_2C receptors, phospholipase C, and calcium-activated potassium (BK) channels. Furthermore, endogenous serotonin attenuates odor-evoked calcium responses in aPC pyramidal neurons. These findings identify the mechanism underlying serotonergic modulation of the aPC and shed light on its potential role.

Plasticity of Sniffing Pattern and Neural Activity in the Olfactory Bulb of Behaving Mice During Odor Sampling, Anticipation, and Reward

The olfactory bulb (OB) is the first relay station in the olfactory system. In the OB, mitral/tufted cells (M/Ts), which are the main output neurons, play important roles in the processing and representation of odor information. Recent studies focusing on the function of M/Ts at the single-cell level in awake behaving mice have demonstrated that odor-evoked firing of single M/Ts displays transient/long-term plasticity during learning. Here, we tested whether the neural activity of M/Ts and sniffing patterns are dependent on anticipation and reward in awake behaving mice. We used an odor discrimination task combined with in vivo electrophysiological recordings in awake, head-fixed mice, and found that, while learning induced plasticity of spikes and beta oscillations during odor sampling, we also found plasticity of spikes, beta oscillation, sniffing pattern, and coherence between sniffing and theta oscillations during the periods of anticipation and/or reward. These results indicate that the activity of M/Ts plays important roles not only in odor representation but also in salience-related events such as anticipation and reward.

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.

An update on anatomy and function of the teleost olfactory system

About half of all extant vertebrates are teleost fishes. Although our knowledge about anatomy and function of their olfactory systems still lags behind that of mammals, recent advances in cellular and molecular biology have provided us with a wealth of novel information about the sense of smell in this important animal group. Its paired olfactory organs contain up to five types of olfactory receptor neurons expressing OR, TAAR, VR1- and VR2-class odorant receptors associated with individual transduction machineries. The different types of receptor neurons are preferentially tuned towards particular classes of odorants, that are associated with specific behaviors, such as feeding, mating or migration. We discuss the connections of the receptor neurons in the olfactory bulb, the differences in bulbar circuitry compared to mammals, and the characteristics of second order projections to telencephalic olfactory areas, considering the everted ontogeny of the teleost telencephalon. The review concludes with a brief overview of current theories about odor coding and the prominent neural oscillations observed in the teleost olfactory system.