Precise circuitry links bilaterally symmetric olfactory maps

Laminar organization of the anterior olfactory nucleus-the interplay between neurogenesis timing and neuroblast migration

Introduction

The anterior olfactory nucleus (AON) is a laminar structure embedded within the olfactory peduncle which serves as the conduit for connectivity between the olfactory bulb (OB) and the central processing centers of the brain. The largest portion of the AON is a ring of neurons and fibers that surround the core of the peduncle, the pars principalis (AONpP). The AONpP is further subdivided into an outer plexiform layer, or layer 1 (L1), that contains axons and dendrites, and an inner cell zone, or layer 2 (L2), formed by densely packed pyramidal cells. Relative to other regions of the olfactory system, the development of the AON remains poorly understood.

Methods

We performed injections of thymidine analogs in pregnant mice from E10 to E18 to determine the timeline of AON neurogenesis and used immunohistochemistry to study neuronal phenotypes both at adult and embryonic stages. To better understand migration and differentiation of the AON neurons, we labeled AON precursors using in utero electroporations with the piggyBac transposon into the rostral lateral ganglionic eminence, the embryonic source of AON neurons.

Results

Our analyses established that the earliest neurons targeted to the AON laminae arose at E10 with neurogenesis peaking at E13. In L1, we found a caudal-to-rostral neurogenic gradient not detected in L2. Quantification across the cardinal axes showed no gradients in L2 and a medial-to-lateral gradient for L1. Using immunohistochemistry, we found that AON neurons express the most common cortical markers Tbr1, Ctip2, NeuroD1, Sox5 and Cux1+2 at adult stages without laminar distinction. Tbr1 and NeuroD1 first appeared embryonically at E12, while Ctip2 and Sox5 were present at E13, following a dorsal-ventral pattern. Cux1+2 was not detected embryonically. Embryonically, our data on neuroblasts migration revealed that AON neuroblasts use a scaffold of radial glia to migrate to their final destinations in both L1 and L2 through a caudal-to-rostral migratory gradient.

Conclusion

For the first time, our data show a comprehensive timeline for the AON neurogenesis across the anatomical axes, and a detailed analysis on neuroblast migration in the mouse embryo. These data are crucial to understanding the embryonic formation and relationship of relay stations along the olfactory pathway.

Bilateral Alignment of Receptive Fields in the Olfactory Cortex

Each olfactory cortical hemisphere receives ipsilateral odor information directly from the olfactory bulb and contralateral information indirectly from the other cortical hemisphere. Since neural projections to the olfactory cortex (OC) are disordered and nontopographic, spatial information cannot be used to align projections from the two sides like in the visual cortex. Therefore, how bilateral information is integrated in individual cortical neurons is unknown. We have found, in mice, that the odor responses of individual neurons to selective stimulation of each of the two nostrils are significantly correlated, such that odor identity decoding optimized with information arriving from one nostril transfers very well to the other side. Nevertheless, these aligned responses are asymmetric enough to allow decoding of stimulus laterality. Computational analysis shows that such matched odor tuning is incompatible with purely random connections but is explained readily by Hebbian plasticity structuring bilateral connectivity. Our data reveal that despite the distributed and fragmented sensory representation in the OC, odor information across the two hemispheres is highly coordinated.

One respiratory cycle as a minimum time unit for making behavioral decisions in the mammalian olfactory system

Voluntary behaviors such as sniffing, moving, and eating require decision-making accompanied by intentional respiration. Based on the study of respiration-coherent activity of rodent olfactory networks, we infer that during the inhalation phase of respiration, olfactory cortical areas process environmental odor information and transmit it to the higher multisensory cognitive areas via feedforward pathways to comprehensively evaluate the surrounding situation. We also infer that during the exhalation phase, the higher multisensory areas generate cognitive-signals and transmit them not only to the behavioral output system but also back to the olfactory cortical areas. We presume that the cortical mechanism couples the intentional respiration with the voluntary behaviors. Thus, in one respiratory cycle, the mammalian brain may transmit and process sensory information to cognize and evaluate the multisensory image of the external world, leading to one behavioral decision and one emotional expression. In this perspective article, we propose that one respiratory cycle provides a minimum time unit for decision making during wakefulness.

Olfactory-trigeminal integration in the primary olfactory cortex

Humans naturally integrate signals from the olfactory and intranasal trigeminal systems. A tight interplay has been demonstrated between these two systems, and yet the neural circuitry mediating olfactory-trigeminal (OT) integration remains poorly understood. Using functional magnetic resonance imaging (fMRI), combined with psychophysics, this study investigated the neural mechanisms underlying OT integration. Fifteen participants with normal olfactory function performed a localization task with air-puff stimuli, phenylethyl alcohol (PEA; rose odor), or a combination thereof while being scanned. The ability to localize PEA to either nostril was at chance. Yet, its presence significantly improved the localization accuracy of weak, but not strong, air-puffs, when both stimuli were delivered concurrently to the same nostril, but not when different nostrils received the two stimuli. This enhancement in localization accuracy, exemplifying the principles of spatial coincidence and inverse effectiveness in multisensory integration, was associated with multisensory integrative activity in the primary olfactory (POC), orbitofrontal (OFC), superior temporal (STC), inferior parietal (IPC) and cingulate cortices, and in the cerebellum. Multisensory enhancement in most of these regions correlated with behavioral multisensory enhancement, as did increases in connectivity between some of these regions. We interpret these findings as indicating that the POC is part of a distributed brain network mediating integration between the olfactory and trigeminal systems. PRACTITIONER POINTS: Psychophysical and neuroimaging study of olfactory-trigeminal (OT) integration. Behavior, cortical activity, and network connectivity show OT integration. OT integration obeys principles of inverse effectiveness and spatial coincidence. Behavioral and neural measures of OT integration are correlated.

Circuit formation and sensory perception in the mouse olfactory system

In the mouse olfactory system, odor information is converted to a topographic map of activated glomeruli in the olfactory bulb (OB). Although the arrangement of glomeruli is genetically determined, the glomerular structure is plastic and can be modified by environmental stimuli. If the pups are exposed to a particular odorant, responding glomeruli become larger recruiting the dendrites of connecting projection neurons and interneurons. This imprinting not only increases the sensitivity to the exposed odor, but also imposes the positive quality on imprinted memory. External odor information represented as an odor map in the OB is transmitted to the olfactory cortex (OC) and amygdala for decision making to elicit emotional and behavioral outputs using two distinct neural pathways, innate and learned. Innate olfactory circuits start to work right after birth, whereas learned circuits become functional later on. In this paper, the recent progress will be summarized in the study of olfactory circuit formation and odor perception in mice. We will also propose new hypotheses on the timing and gating of olfactory circuit activity in relation to the respiration cycle.

The anterior olfactory nucleus revisited - an emerging role for neuropathological conditions?

Olfaction is an important sensory modality for many species and greatly influences animal and human behavior. Still, much about olfactory perception remains unknown. The anterior olfactory nucleus is one of the brain's central early olfactory processing areas. Located directly posterior to the olfactory bulb in the olfactory peduncle with extensive in- and output connections and unique cellular composition, it connects olfactory processing centers of the left and right hemispheres. Almost 20 years have passed since the last comprehensive review on the anterior olfactory nucleus has been published and significant advances regarding its anatomy, function, and pathophysiology have been made in the meantime. Here we briefly summarize previous knowledge on the anterior olfactory nucleus, give detailed insights into the progress that has been made in recent years, and map out its emerging importance in translational research of neurological diseases.

Organizational Principles of the Centrifugal Projections to the Olfactory Bulb

Centrifugal projections in the olfactory system are critical to both olfactory processing and behavior. The olfactory bulb (OB), the first relay station in odor processing, receives a substantial number of centrifugal inputs from the central brain regions. However, the anatomical organization of these centrifugal connections has not been fully elucidated, especially for the excitatory projection neurons of the OB, the mitral/tufted cells (M/TCs). Using rabies virus-mediated retrograde monosynaptic tracing in Thy1-Cre mice, we identified that the three most prominent inputs of the M/TCs came from the anterior olfactory nucleus (AON), the piriform cortex (PC), and the basal forebrain (BF), similar to the granule cells (GCs), the most abundant population of inhibitory interneurons in the OB. However, M/TCs received proportionally less input from the primary olfactory cortical areas, including the AON and PC, but more input from the BF and contralateral brain regions than GCs. Unlike organizationally distinct inputs from the primary olfactory cortical areas to these two types of OB neurons, inputs from the BF were organized similarly. Furthermore, individual BF cholinergic neurons innervated multiple layers of the OB, forming synapses on both M/TCs and GCs. Taken together, our results indicate that the centrifugal projections to different types of OB neurons may provide complementary and coordinated strategies in olfactory processing and behavior.

Lessons from behavioral lateralization in olfaction

Sensory information, sampled by sensory organs positioned on each side of the body may play a crucial role in organizing brain lateralization. This question is of particular interest with regard to the growing evidence of alteration in lateralization in several psychiatric conditions. In this context, the olfactory system, an ancient, mostly ipsilateral and well-conserved system across phylogeny may prove an interesting model system to understand the behavioral significance of brain lateralization. Here, we focused on behavioral data in vertebrates and non-vertebrates, suggesting that the two hemispheres of the brain differentially processed olfactory cues to achieve diverse sensory operations, such as detection, discrimination, identification of behavioral valuable cues or learning. These include reports across different species on best performances with one nostril or the other or odorant active sampling by one nostril or the other, depending on odorants or contexts. In some species, hints from peripheral anatomical or functional asymmetry were proposed to explain these asymmetries in behavior. Instigations of brain activation or more rarely of brain connectivity evoked by odorants revealed a complex picture with regards to asymmetric patterns which is discussed with respect to behavioral data. Along the steps of the discussed literature, we propose avenues for future research.

Olfactory Optogenetics: Light Illuminates the Chemical Sensing Mechanisms of Biological Olfactory Systems

The mammalian olfactory system has an amazing ability to distinguish thousands of odorant molecules at the trace level. Scientists have made great achievements on revealing the olfactory sensing mechanisms in decades; even though many issues need addressing. Optogenetics provides a novel technical approach to solve this dilemma by utilizing light to illuminate specific part of the olfactory system; which can be used in all corners of the olfactory system for revealing the olfactory mechanism. This article reviews the most recent advances in olfactory optogenetics devoted to elucidate the mechanisms of chemical sensing. It thus attempts to introduce olfactory optogenetics according to the structure of the olfactory system. It mainly includes the following aspects: the sensory input from the olfactory epithelium to the olfactory bulb; the influences of the olfactory bulb (OB) neuron activity patterns on olfactory perception; the regulation between the olfactory cortex and the olfactory bulb; and the neuromodulation participating in odor coding by dominating the olfactory bulb. Finally; current challenges and future development trends of olfactory optogenetics are proposed and discussed.

Functional Relevance of Dual Olfactory Bulbs in Olfactory Coding

Bilateral convergence of external stimuli is a common feature of vertebrate sensory systems. This convergence of inputs from the bilateral receptive fields allows higher order sensory perception, such as depth perception in the vertebrate visual system and stimulus localization in the auditory system. The functional role of such bilateral convergence in the olfactory system is unknown. To test whether each olfactory bulb (OB) contributes a separate piece of olfactory information, and whether information from the bilateral OB is integrated, we synchronized the activation of OBs with blue light in mice expressing ChIEF in the olfactory sensory neurons (OSNs) and behaviorally assessed the relevance of dual OBs in olfactory perception. Our findings suggest that each OB contributes separate components of olfactory information, and the mice integrate the bilaterally synchronized olfactory information for olfactory identity.

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.

Olfaction in Lamprey Pallium Revisited-Dual Projections of Mitral and Tufted Cells

The presence of two separate afferent channels from the olfactory glomeruli to different targets in the brain is unravelled in the lamprey. The mitral-like cells send axonal projections directly to the piriform cortex in the ventral part of pallium, whereas the smaller tufted-like cells project separately and exclusively to a relay nucleus called the dorsomedial telencephalic nucleus (dmtn). This nucleus, located at the interface between the olfactory bulb and pallium, in turn projects to a circumscribed area in the anteromedial, ventral part of pallium. The tufted-like cells are activated with short latency from the olfactory nerve and terminate with mossy fibers on the dmtn cells, wherein they elicit large unitary excitatory postsynaptic potentials (EPSPs). In all synapses along this tufted-like cell pathway, there is no concurrent inhibition, in contrast to the mitral-like cell pathway. This is similar to recent findings in rodents establishing two separate exclusive projection patterns, suggesting an evolutionarily conserved organization.

The effects of olfactory bulb removal on single-pellet skilled reaching task in rats

We focused on how the rat uses olfactory cues in a single-pellet reaching task, which is composed of three successive learned responses, Orient, Transport, and Withdrawal. Orient comprised: front wall detection, slot localisation, and nose poke until reach start. High-speed video-recording enabled us to describe the temporal features of this sequence in controls vs. 3-5 and 12-14 days after bilateral bulbectomy in trials with (P trial) vs. without (no-P trial) pellet. In controls, the full sequence was complete in P trials, while it was interrupted after Orient in no P-trials. After bulbectomy, the full sequence was seen in both P and no-P trials at days 3-5 and 12-14 and there was an increase in Orient duration due to the increased time in slot/shelf localisation. Unlike in controls, in anosmic rats, the first nose contact with the front wall took place below the slot/shelf level, and the number of nose touches together with the number of whisker cycles was significantly higher at 3-5 but not at 12-14 days. The relationship between nose touches and whisker cycles was linear in all experimental conditions. Bulbectomy resulted in no changes in the Transport duration or the time the paw spent out of the slot. These findings suggest that olfaction allows the animal to orient itself in pellet localisation, and offers insight into the contribution of olfaction during different stages of natural behaviour in skilled reaching task.

Characterization of odor-evoked neural activity in the olfactory peduncle

The tenia tecta is extensively interconnected with the main olfactory bulb and olfactory cortical areas and is well positioned to contribute to olfactory processing. However, little is known about odor representation within its dorsal (DTT) and ventral (VTT) components. To address this need, spontaneous and odor-evoked activity of DTT and VTT neurons was recorded from urethane anesthetized mice and compared to activity recorded from adjacent areas within adjacent caudomedial aspects of the anterior olfactory nucleus (AON). Neurons recorded from DTT, VTT, and AON exhibited odor-selective alterations in firing rate in response to a diverse set of monomolecular odorants. While DTT and AON neurons exhibited similar tuning breadth, selectivity, and response topography, the proportion of odor-selective neurons was substantially higher in the DTT. These findings provide evidence that the tenia tecta may contribute to the encoding of specific stimulus attributes. Further work is needed to fully characterize functional organization of the tenia tecta and its contribution to sensory representation and utilization.

Olfactory Circuitry and Behavioral Decisions

In mammals, odor information detected by olfactory sensory neurons is converted to a topographic map of activated glomeruli in the olfactory bulb. Mitral cells and tufted cells transmit signals sequentially to the olfactory cortex for behavioral outputs. To elicit innate behavioral responses, odor signals are directly transmitted by distinct subsets of mitral cells from particular functional domains in the olfactory bulb to specific amygdala nuclei. As for the learned decisions, input signals are conveyed by tufted cells as well as by mitral cells to the olfactory cortex. Behavioral scene cells link the odor information to the valence cells in the amygdala to elicit memory-based behavioral responses. Olfactory decision and perception take place in relation to the respiratory cycle. How is the sensory quality imposed on the olfactory inputs for behavioral outputs? How are the two types of odor signals, innate and learned, processed during respiration? Here, we review recent progress on the study of neural circuits involved in decision making in the mouse olfactory system.

Nasal airflow engages central olfactory processing and shapes olfactory percepts

Binding of airborne odour molecules to olfactory receptors at the top of the nasal cavity gives rise to our rich olfactory experience. Whether airflow plays a role in human olfactory perception beyond the transportation of odorants is scantly known. Combining psychophysical measures with strict controls of nasal flow parameters, we demonstrate in four experiments that the perceived intensity of a unilaterally presented odour decreases systematically with the amount of contralateral nasal airflow, in manners that are independent of odour flow rate, nasal pressure, perceived sniff vigour or attentional allocation. Moreover, the effect is due to the sensed rather than the factual amount of nasal flow, as applying a local anaesthetic to the contralateral nostril produces the same effect as physically blocking it. Our findings indicate that nasal flow spontaneously engages central olfactory processing and serves as an integral part of the olfactory percept in humans.

Subpopulations of Projection Neurons in the Olfactory Bulb

Generation of neuronal diversity is a biological strategy widely used in the brain to process complex information. The olfactory bulb is the first relay station of olfactory information in the vertebrate central nervous system. In the olfactory bulb, axons of the olfactory sensory neurons form synapses with dendrites of projection neurons that transmit the olfactory information to the olfactory cortex. Historically, the olfactory bulb projection neurons have been classified into two populations, mitral cells and tufted cells. The somata of these cells are distinctly segregated within the layers of the olfactory bulb; the mitral cells are located in the mitral cell layer while the tufted cells are found in the external plexiform layer. Although mitral and tufted cells share many morphological, biophysical, and molecular characteristics, they differ in soma size, projection patterns of their dendrites and axons, and odor responses. In addition, tufted cells are further subclassified based on the relative depth of their somata location in the external plexiform layer. Evidence suggests that different types of tufted cells have distinct cellular properties and play different roles in olfactory information processing. Therefore, mitral and different types of tufted cells are considered as starting points for parallel pathways of olfactory information processing in the brain. Moreover, recent studies suggest that mitral cells also consist of heterogeneous subpopulations with different cellular properties despite the fact that the mitral cell layer is a single-cell layer. In this review, we first compare the morphology of projection neurons in the olfactory bulb of different vertebrate species. Next, we explore the similarities and differences among subpopulations of projection neurons in the rodent olfactory bulb. We also discuss the timing of neurogenesis as a factor for the generation of projection neuron heterogeneity in the olfactory bulb. Knowledge about the subpopulations of olfactory bulb projection neurons will contribute to a better understanding of the complex olfactory information processing in higher brain regions.

Dynamic Impairment of Olfactory Behavior and Signaling Mediated by an Olfactory Corticofugal System

Processing of olfactory information is modulated by centrifugal projections from cortical areas, yet their behavioral relevance and underlying neural mechanisms remain unclear in most cases. The anterior olfactory nucleus (AON) is part of the olfactory cortex, and its extensive connections to multiple upstream and downstream brain centers place it in a prime position to modulate early sensory information in the olfactory system. Here, we show that optogenetic activation of AON neurons in awake male and female mice was not perceived as an odorant equivalent cue. However, AON activation during odorant presentation reliably suppressed behavioral odor responses. This AON-mediated effect was fast and constant across odors and concentrations. Likewise, activation of glutamatergic AON projections to the olfactory bulb (OB) transiently inhibited the excitability of mitral/tufted cells (MTCs) that relay olfactory input to the cortex. Single-unit MTC recordings revealed that optogenetic activation of glutamatergic AON terminals in the OB transiently decreased sensory-evoked MTC spiking, regardless of the strength or polarity of the sensory response. The reduction in MTC firing during optogenetic stimulation was confirmed in recordings in awake mice. These findings suggest that glutamatergic AON projections to the OB impede early olfactory signaling by inhibiting OB output neurons, thereby dynamically gating sensory throughput to the cortex.SIGNIFICANCE STATEMENT The anterior olfactory nucleus (AON) as an olfactory information processing area sends extensive projections to multiple brain centers, but the behavioral consequences of its activation have been scarcely investigated. Using behavioral tests in combination with optogenetic manipulation, we show that, in contrast to what has been suggested previously, the AON does not seem to form odor percepts but instead suppresses behavioral odor responses across odorants and concentrations. Furthermore, this study shows that AON activation inhibits olfactory bulb output neurons in both anesthetized as well as awake mice, pointing to a potential mechanism by which the olfactory cortex can actively and dynamically gate sensory throughput to higher brain centers.

A Synaptic Circuit Required for Acquisition but Not Recall of Social Transmission of Food Preference

During social transmission of food preference (STFP), the combination of an olfactory sensory input with a social cue induces long-term memory of a food odor. How a social cue produces long-term learning of an olfactory input, however, remains unknown. Here we show that the neurons of the anterior olfactory nucleus (AON), which form abundant synaptic projections onto granule cells in the olfactory bulb (OB), express the synaptogenic molecule C1ql3. Deletion of C1ql3 in the dorsolateral AON impaired synaptic AON→OB connections and abolished acquisition, but not recall, of STFP memory without significantly affecting basal olfaction. Moreover, deletion in granule cells of the OB of Bai3, a postsynaptic GPCR that binds C1ql3, similarly suppressed synaptic transmission at AON→OB projections and abolished acquisition, but not recall, of STFP memory. Thus, synaptic AON→OB connections are selectively required for STFP memory acquisition and are formed by an essential interaction of presynaptic C1ql3 with postsynaptic Bai3.

Humans navigate with stereo olfaction

Human navigation relies on inputs to our paired eyes and ears. Although we also have two nasal passages, there has been little empirical indication that internostril differences yield directionality in human olfaction without involving the trigeminal system. By using optic flow that captures the pattern of apparent motion of surface elements in a visual scene, we demonstrate through formal psychophysical testing that a moderate binaral concentration disparity of a nontrigeminal odorant consistently biases recipients' perceived direction of self-motion toward the higher-concentration side, despite that they cannot verbalize which nostril smells a stronger odor. We further show that the effect depends on the internostril ratio of odor concentrations and not the numeric difference in concentration between the two nostrils. Taken together, our findings provide behavioral evidence that humans smell in stereo and subconsciously utilize stereo olfactory cues in spatial navigation.

Interhemispheric connections between olfactory bulbs improve odor detection

Interhemispheric connections enable interaction and integration of sensory information in bilaterian nervous systems and are thought to optimize sensory computations. However, the cellular and spatial organization of interhemispheric networks and the computational properties they mediate in vertebrates are still poorly understood. Thus, it remains unclear to what extent the connectivity between left and right brain hemispheres participates in sensory processing. Here, we show that the zebrafish olfactory bulbs (OBs) receive direct interhemispheric projections from their contralateral counterparts in addition to top-down inputs from the contralateral zebrafish homolog of olfactory cortex. The direct interhemispheric projections between the OBs reach peripheral layers of the contralateral OB and retain a precise topographic organization, which directly connects similarly tuned olfactory glomeruli across hemispheres. In contrast, interhemispheric top-down inputs consist of diffuse projections that broadly innervate the inhibitory granule cell layer. Jointly, these interhemispheric connections elicit a balance of topographically organized excitation and nontopographic inhibition on the contralateral OB and modulate odor responses. We show that the interhemispheric connections in the olfactory system enable the modulation of odor response and contribute to a small but significant improvement in the detection of a reproductive pheromone when presented together with complex olfactory cues by potentiating the response of the pheromone selective neurons. Taken together, our data show a previously unknown function for an interhemispheric connection between chemosensory maps of the olfactory system.

Interhemispheric connections enable interaction and integration of sensory information in bilaterian nervous systems and are thought to optimize sensory computations. This study shows that interhemispheric olfactory connections in the zebrafish brain improve the detection of a reproductive pheromone within a noisy odor background.

Interhemispheric asymmetry of c-Fos expression in glomeruli and the olfactory tubercle following repeated odor stimulation

Odor adaptation allows the olfactory system to regulate sensitivity to different stimulus intensities, which is essential for preventing saturation of the cell‐transducing machinery and maintaining high sensitivity to persistent and repetitive odor stimuli. Although many studies have investigated the structure and mechanisms of the mammalian olfactory system that responds to chemical sensation, few studies have considered differences in neuronal activation that depend on the manner in which the olfactory system is exposed to odorants, or examined activity patterns of olfactory‐related regions in the brain under different odor exposure conditions. To address these questions, we designed three different odor exposure conditions that mimicked diverse odor environments and analyzed c‐Fos‐expressing cells (c‐Fos+ cells) in the odor columns of the olfactory bulb (OB). We then measured differences in the proportions of c‐Fos‐expressing cell types depending on the odor exposure condition. Surprisingly, under the specific odor condition in which the olfactory system was repeatedly exposed to the odorant for 1 min at 5‐min intervals, one of the lateral odor columns and the ipsilateral hemisphere of the olfactory tubercle had more c‐Fos+ cells than the other three odor columns and the contralateral hemisphere of the olfactory tubercle. However, this interhemispheric asymmetry of c‐Fos expression was not observed in the anterior piriform cortex. To confirm whether the anterior olfactory nucleus pars externa (AONpE), which connects the left and right OB, contributes to this asymmetry, AONpE‐lesioned mice were analyzed under the specific odor exposure condition. Asymmetric c‐Fos expression was not observed in the OB or the olfactory tubercle. These data indicate that the c‐Fos expression patterns of the olfactory‐related regions in the brain are influenced by the odor exposure condition and that asymmetric c‐Fos expression in these regions was observed under a specific odor exposure condition due to synaptic linkage via the AONpE.

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.

Cell-Type-Specific Whole-Brain Direct Inputs to the Anterior and Posterior Piriform Cortex

The piriform cortex (PC) is a key brain area involved in both processing and coding of olfactory information. It is implicated in various brain disorders, such as epilepsy, Alzheimer’s disease, and autism. The PC consists of the anterior (APC) and posterior (PPC) parts, which are different anatomically and functionally. However, the direct input networks to specific neuronal populations within the APC and PPC remain poorly understood. Here, we mapped the whole-brain direct inputs to the two major neuronal populations, the excitatory glutamatergic principal neurons and inhibitory γ-aminobutyric acid (GABA)-ergic interneurons within the APC and PPC using the rabies virus (RV)-mediated retrograde trans-synaptic tracing system. We found that for both types of neurons, APC and PPC share some similarities in input networks, with dominant inputs originating from the olfactory region (OLF), followed by the cortical subplate (CTXsp), isocortex, cerebral nuclei (CNU), hippocampal formation (HPF) and interbrain (IB), whereas the midbrain (MB) and hindbrain (HB) were rarely labeled. However, APC and PPC also show distinct features in their input distribution patterns. For both types of neurons, the input proportion from the OLF to the APC was higher than that to the PPC; while the PPC received higher proportions of inputs from the HPF and CNU than the APC did. Overall, our results revealed the direct input networks of both excitatory and inhibitory neuronal populations of different PC subareas, providing a structural basis to analyze the diverse PC functions.

A Prism Method for Optical Glomerular Mapping of the Medial Olfactory Bulb in Mice

The processing of odor input in the brain begins in the olfactory bulb (OB), where odor information is represented by combinations of active glomeruli. Each glomerulus is associated with a specific odorant receptor type, of which there are ~1,000 in mice; thus different odors activate different subsets of glomeruli. Most receptor types have duplicate lateral and medial glomeruli in each of the left and right OBs. The two sets of glomeruli form separate but mirror-symmetric glomerular maps. It is not known whether the odor representations in these paired maps are exact copies of each other or potentially encode additional information. Previous studies of glomerular odor representations were mostly limited to the lateral map because the medial map is inaccessible with high-resolution activity mapping techniques, such as optical imaging. To address this, we developed a method for optical imaging of the medial bulb by replacing the contralateral bulb with a right-angle prism that has a mirror coating on the hypotenuse. With this method, we performed calcium imaging of corresponding subsets of glomeruli in the lateral map at the dorsal surface and the medial map at the medial wall. Thus, we demonstrate an experimental model system for comparing odor representations in these redundant sensory maps, enabling a better understanding of the role of paired maps and the neuronal coding of odor stimuli.

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.

A Novel Birthdate-Labeling Method Reveals Segregated Parallel Projections of Mitral and External Tufted Cells in the Main Olfactory System

A fundamental strategy in sensory coding is parallel processing, whereby unique, distinct features of sensation are computed and projected to the central target in the form of submodal maps. It remains unclear, however, whether such parallel processing strategy is employed in the main olfactory system, which codes the complex hierarchical odor and behavioral scenes. A potential scheme is that distinct subsets of projection neurons in the olfactory bulb (OB) form parallel projections to the targets. Taking advantage of the observation that the distinct projection neurons develop at different times, we developed a Cre-loxP-based method that allows for birthdate-specific labeling of cell bodies and their axon projections in mice. This birthdate tag analysis revealed that the mitral cells (MCs) born in an early developmental stage and the external tufted cells (TCs) born a few days later form segregated parallel projections. Specifically, the latter subset converges the axons onto only two small specific targets, one of which, located at the anterolateral edge of the olfactory tubercle (OT), excludes widespread MC projections. This target is made up of neurons that express dopamine D1 but not D2 receptor and corresponds to the most anterolateral isolation of the CAP compartments (aiCAP) that were defined previously. This finding of segregated projections suggests that olfactory sensing does indeed involve parallel processing of functionally distinct submodalities. Importantly, the birthdate tag method used here may pave the way for deciphering the functional meaning of these individual projection pathways in the future.

Cranial Nerve Stimulation for Olfaction (Cranial Nerve 1)

Like sensory maps in other systems, the sense of smell has an organizational structure based on converging projections of olfactory receptor neurons containing unique odorant receptors onto the olfactory bulb in synaptic aggregations termed glomeruli. This organizational structure provides the potential for electrical stimulation and restoration of smell. Prior animal and human studies support the feasibility of an olfactory stimulation device, encouraging ongoing work in development of olfactory implants.

Cortical Organization of Centrifugal Afferents to the Olfactory Bulb: Mono- and Trans-synaptic Tracing with Recombinant Neurotropic Viral Tracers

Sensory processing is strongly modulated by different brain and behavioral states, and this is based on the top-down modulation. In the olfactory system, local neural circuits in the olfactory bulb (OB) are innervated by centrifugal afferents in order to regulate the processing of olfactory information in the OB under different behavioral states. The purpose of the present study was to explore the organization of neural networks in olfactory-related cortices and modulatory nuclei that give rise to direct and indirect innervations to the glomerular layer (GL) of the OB at the whole-brain scale. Injection of different recombinant attenuated neurotropic viruses into the GL showed that it received direct inputs from each layer in the OB, centrifugal inputs from the ipsilateralanterior olfactory nucleus (AON), anterior piriform cortex (Pir), and horizontal limb of diagonal band of Broca (HDB), and various indirect inputs from bilateral cortical neurons in the AON, Pir, amygdala, entorhinal cortex, hippocampus, HDB, dorsal raphe, median raphe and locus coeruleus. These results provide a circuitry basis that will help further understand the mechanism by which olfactory information-processing in the OB is regulated.

Nostril-specific and structure-based olfactory learning of chiral discrimination in human adults

Practice makes perfect. In human olfaction, such plasticity is generally assumed to occur at the level of cortical synthetic processing that shares information from both nostrils. Here we present findings that challenge this view. In two experiments, we trained human adults unirhinally for the discrimination between odor enantiomers over a course of about 10 to 11 days. Results showed that training-induced perceptual gain was restricted to the trained nostril yet partially generalized to untrained odor enantiomers in a structure- rather than quality- based manner. In other words, learning enhanced the differentiation of chirality (molecular configuration) as opposed to overall odor quality (odor object) per se. These findings argue that, unlike earlier beliefs, one nostril does not readily know what the other learns. Moreover, the initial analytical processing of the structural features of uninarial olfactory input remains plastic in human adults.

Although we may only become consciously aware of our sense of smell when we encounter something pungent, it can greatly influence our quality of life. Smells are processed by our olfactory system, a collection of receptors and nerve cells in the nose and brain.

Odor molecules activate the olfactory system when they bind to receptors in the nostrils. These molecules can have a wide range of chemical and physical properties. However, some odor molecules are mirror images of each other. These variants are known as enantiomers. Some people can naturally smell the difference between enantiomers; others can be taught how to tell them apart. Studying this training process could help us to understand how the olfactory system adapts to new circumstances.

Feng and Zhou trained volunteers to distinguish between odor enantiomers – but only using one nostril. After training, the volunteers were better able to tell the difference between different enantiomers – even for certain scents they had not been trained to discriminate – when sniffing through the trained nostril. However, they got no better at distinguishing between enantiomers when they sniffed them using the other nostril.

Overall, the results reported by Feng and Zhou confirm that human adults can learn how to process the structural features of odorants. However, in contrast to earlier beliefs, they also suggest that one nostril does not readily know what the other learns. This new understanding of how the olfactory system adapts could ultimately help us to develop therapies to restore a lost sense of smell.

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

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

Functional brain stem circuits for control of nose motion

Rodents shift their nose from side to side when they actively explore and lateralize odors in the space. This motor action is driven by a pair of muscles, the deflector nasi. We studied the premotor control of this motion. We used replication-competent rabies virus to transsynaptically label inputs to the deflector nasi muscle and find putative premotor labeling throughout the parvocellular, intermediate, and gigantocellular reticular formations, as well as the trigeminal nuclei, pontine reticular formation, midbrain reticular formation, red nucleus, and superior colliculus. Two areas with extensive labeling were analyzed for their impact on nose movement. One area is in the reticular formation caudal to the facial motor nucleus and is denoted the nose retrofacial area. The second is in the caudal part of the intermediate reticular region near the oscillator for whisking (the nose IRt). Functionally, we find that optogenetic activation of glutamatergic cells in both areas drives deflection of the nose. Ablation of cells in the nose retrofacial area, but not the nose IRt, impairs movement of the nose in response to the presentation of odorants but otherwise leaves movement unaffected. These data suggest that the nose retrofacial area is a conduit for a sensory-driven orofacial motor action. Furthermore, we find labeling of neurons that are immediately upstream of premotor neurons in the preBötzinger complex that presumably synchronizes a small, rhythmic component of nose motion to breathing. NEW & NOTEWORTHY We identify two previously undescribed premotor areas in the medulla that control deflection of the nose. This includes a pathway for directed motion of the nose in response to an odorant.

Induction of smell through transethmoid electrical stimulation of the olfactory bulb

BACKGROUND

Anosmia has an estimated prevalence of 5% of the general population. Outside of inflammatory causes, therapeutic options are limited despite research advances. Bypassing peripheral neuronal damage through central stimulation is a potential therapeutic option that has shown success in other sensory systems, most notably with hearing. We performed a pilot study to determine the feasibility of inducing smell through artificial electrical stimulation of the olfactory bulbs in humans.

METHODS

Subjects with a history of sinus surgery, including total ethmoidectomy, with intact ability to smell were enrolled. The ability to smell was confirmed with a 40-item smell identification test. Awake subjects underwent nasal endoscopy and either a monopolar or bipolar electrode was positioned at 3 areas along the lateral lamella of the cribriform plate within the ethmoid sinus cavity. A graded stimulation current of 1-20 mA at 3.17 Hz was administered while cortical evoked potential (CEP) recordings were collected. Subjective responses of perceived smell along with reports of discomfort were recorded. Subjects with artificially induced smell underwent repeat stimulation after medically induced anosmia.

RESULTS

Five subjects (age, 43-72 years) were enrolled. Three subjects reported smell perception smell with electrical stimulation. This was reproducible after inducing anosmia, but CEP recordings could not provide objective support. All subjects tolerated the study with minimal discomfort.

CONCLUSION

This is the first report of induced smell through transethmoid electrical stimulation of the olfactory bulb. These results provide a proof of concept for efforts in development of an olfactory implant system.

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.

Nrp2 is sufficient to instruct circuit formation of mitral-cells to mediate odour-induced attractive social responses

Odour information induces various innate responses that are critical to the survival of the individual and for the species. An axon guidance molecule, Neuropilin 2 (Nrp2), is known to mediate targeting of olfactory sensory neurons (primary neurons), to the posteroventral main olfactory bulb (PV MOB) in mice. Here we report that Nrp2-positive (Nrp2⁺) mitral cells (MCs, second-order neurons) play crucial roles in transmitting attractive social signals from the PV MOB to the anterior part of medial amygdala (MeA). Semaphorin 3F, a repulsive ligand to Nrp2, regulates both migration of Nrp2⁺ MCs to the PV MOB and their axonal projection to the anterior MeA. In the MC-specific Nrp2 knockout mice, circuit formation of Nrp2⁺ MCs and odour-induced attractive social responses are impaired. In utero, electroporation demonstrates that activation of the Nrp2 gene in MCs is sufficient to instruct their circuit formation from the PV MOB to the anterior MeA.

Diverse Representations of Olfactory Information in Centrifugal Feedback Projections

Although feedback or centrifugal projections from higher processing centers of the brain to peripheral regions have long been known to play essential functional roles, the anatomical organization of these connections remains largely unknown. Using a virus-based retrograde labeling strategy and 3D whole-brain reconstruction methods, we mapped the spatial organization of centrifugal projections from two olfactory cortical areas, the anterior olfactory nucleus (AON) and the piriform cortex, to the granule cell layer of the main olfactory bulb in the mouse. Both regions are major recipients of information from the bulb and are the largest sources of feedback to the bulb, collectively constituting circuits essential for olfactory coding and olfactory behavior. We found that, although ipsilateral inputs from the AON were uniformly distributed, feedback from the contralateral AON had a strong ventral bias. In addition, we observed that centrifugally projecting neurons were spatially clustered in the piriform cortex, in contrast to the distributed feedforward axonal inputs that these cells receive from the principal neurons of the bulb. Therefore, information carried from the bulb to higher processing structures by anatomically stereotypic projections is likely relayed back to the bulb by organizationally distinct feedback projections that may reflect different coding strategies and therefore different functional roles.

SIGNIFICANCE STATEMENT

Principles of anatomical organization, sometimes instantiated as "maps" in the mammalian brain, have provided key insights into the structure and function of circuits in sensory systems. Generally, these characterizations focus on projections from early sensory processing areas to higher processing structures despite considerable evidence that feedback or centrifugal projections often constitute major conduits of information flow. Our results identify structure in the organization of centrifugal feedback projections to the olfactory bulb that is fundamentally different from the organization of feedforward circuits. Our study suggests that understanding computations performed in the olfactory bulb, and more generally in the olfactory system, requires understanding interactions between feedforward and feedback "maps" both structurally and functionally.

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.

Determination of the connectivity of newborn neurons in mammalian olfactory circuits

The mammalian olfactory bulb is a forebrain structure just one synapse downstream from the olfactory sensory neurons and performs the complex computations of sensory inputs. The formation of this sensory circuit is shaped through activity-dependent and cell-intrinsic mechanisms. Recent studies have revealed that cell-type specific connectivity and the organization of synapses in dendritic compartments are determined through cell-intrinsic programs already preset in progenitor cells. These progenitor programs give rise to subpopulations within a neuron type that have distinct synaptic organizations. The intrinsically determined formation of distinct synaptic organizations requires factors from contacting cells that match the cell-intrinsic programs. While certain genes control wiring within the newly generated neurons, other regulatory genes provide intercellular signals and are only expressed in neurons that will form contacts with the newly generated cells. Here, the olfactory system has provided a useful model circuit to reveal the factors regulating assembly of the highly structured connectivity in mammals.

Early Commissural Diencephalic Neurons Control Habenular Axon Extension and Targeting

Most neuronal populations form on both the left and right sides of the brain. Their efferent axons appear to grow synchronously along similar pathways on each side, although the neurons or their environment often differ between the two hemispheres [1-4]. How this coordination is controlled has received little attention. Frequently, neurons establish interhemispheric connections, which can function to integrate information between brain hemispheres (e.g., [5]). Such commissures form very early, suggesting their potential developmental role in coordinating ipsilateral axon navigation during embryonic development [4]. To address the temporal-spatial control of bilateral axon growth, we applied long-term time-lapse imaging to visualize the formation of the conserved left-right asymmetric habenular neural circuit in the developing zebrafish embryo [6]. Although habenular neurons are born at different times across brain hemispheres [7], we found that elongation of habenular axons occurs synchronously. The initiation of axon extension is not controlled within the habenular network itself but through an early developing proximal diencephalic network. The commissural neurons of this network influence habenular axons both ipsilaterally and contralaterally. Their unilateral absence impairs commissure formation and coordinated habenular axon elongation and causes their subsequent arrest on both sides of the brain. Thus, habenular neural circuit formation depends on a second intersecting commissural network, which facilitates the exchange of information between hemispheres required for ipsilaterally projecting habenular axons. This mechanism of network formation may well apply to other circuits, and has only remained undiscovered due to technical limitations.

The Pig Olfactory Brain: A Primer

Despite the fact that pigs are reputed to have excellent olfactory abilities, few studies have examined regions of the pig brain involved in the sense of smell. The present study provides an overview of the olfactory bulb, anterior olfactory nucleus, and piriform cortex of adult pigs using several approaches. Nissl, myelin, and Golgi stains were used to produce a general overview of the organization of the regions and confocal microscopy was employed to examine 1) projection neurons, 2) GABAergic local circuit neurons that express somatostatin, parvalbumin, vasoactive intestinal polypeptide, or calretinin, 3) neuromodulatory fibers (cholinergic and serotonergic), and 4) glia (astrocytes and microglia). The findings revealed that pig olfactory structures are quite large, highly organized and follow the general patterns observed in mammals.

A direct anterior cingulate pathway to the primate primary olfactory cortex may control attention to olfaction

Behavioral and functional studies in humans suggest that attention plays a key role in activating the primary olfactory cortex through an unknown circuit mechanism. We report that a novel pathway from the anterior cingulate cortex, an area which has a key role in attention, projects directly to the primary olfactory cortex in rhesus monkeys, innervating mostly the anterior olfactory nucleus. Axons from the anterior cingulate cortex formed synapses mostly with spines of putative excitatory pyramidal neurons and with a small proportion of a neurochemical class of inhibitory neurons that are thought to have disinhibitory effect on excitatory neurons. This novel pathway from the anterior cingulate is poised to exert a powerful excitatory effect on the anterior olfactory nucleus, which is a critical hub for odorant processing via extensive bilateral connections with primary olfactory cortices and the olfactory bulb. Acting on the anterior olfactory nucleus, the anterior cingulate may activate the entire primary olfactory cortex to mediate the process of rapid attention to olfactory stimuli.

Unilateral olfactory deficit in a hemiparkinson's disease mouse model

Parkinson's disease is a neurodegenerative disease which often presents hyposmia (80-90% of the cases). We characterized the olfactory behavior in the model of 6-hydroxydopamine of Parkinson's disease. Mice were trained to discriminate between two odorants in a radial maze. One of the odorants was associated with water as a reward. 6-hydroxydopamine was injected directly into the dorsal striatum; after complete striatal denervation, olfactory performance was evaluated in a radial maze. In the first evaluation, experimental mice performed as control mice. After the first evaluation, the narine of the contralateral side to the striatal injection was closed and mice were evaluated again. The experimental group completely lost the capacity to discriminate between the odorant associated with the reward (heptaldehyde) and the unconditioned odorant (2-heptanone). We propose that the olfactory deficit was caused by dopaminergic denervation to the olfactory tubercle and nucleus accumbens.

Segregated labeling of olfactory bulb projection neurons based on their birthdates

Mitral and tufted cells are the projection neurons of the olfactory bulb (OB). We previously reported that somata location and innervation patterns were different between early- and late-born mitral cells (Imamura et al., 2011). Here, we introduced a plasmid that drives the expression of a GFP gene into the mouse OB using in utero electroporation, and demonstrated that we can deliver the plasmid vectors into distinct subsets of OB projection neurons by changing the timing of electroporation after fertilisation. The electroporation performed at embryonic day (E)10 preferentially labeled mitral cells in the accessory OB and main OB mitral cells in dorsomedial mitral cell layer (MCL). In contrast, the E12 electroporation introduced the plasmid vectors preferentially into main OB mitral cells in the ventrolateral MCL and tufted cells. Combining these data with BrdU injections, we confirmed that E10 and E12 electroporation preferentially labeled early- and late-born projection neurons, respectively. This work introduces a novel method for segregated labeling of mouse olfactory bulb projection neurons based on their birthdates. With this technique we found that early- and late-born projection neurons extend their secondary dendrites in the deep and superficial external plexiform layer (EPL), respectively. Although a similar segregation has been suggested for mitral vs. tufted cell dendrites, we found mitral cells projecting secondary dendrites into the superficial EPL in E12-electroporated main OB. Our observations indicate that timing of neurogenesis regulates not only somata location and innervation patterns but also the laminar organisation of projection neuron dendrites in the EPL.

Neuronal organization of olfactory bulb circuits

Olfactory sensory neurons extend their axons solely to the olfactory bulb, which is dedicated to odor information processing. The olfactory bulb is divided into multiple layers, with different types of neurons found in each of the layers. Therefore, neurons in the olfactory bulb have conventionally been categorized based on the layers in which their cell bodies are found; namely, juxtaglomerular cells in the glomerular layer, tufted cells in the external plexiform layer, mitral cells in the mitral cell layer, and granule cells in the granule cell layer. More recently, numerous studies have revealed the heterogeneous nature of each of these cell types, allowing them to be further divided into subclasses based on differences in morphological, molecular, and electrophysiological properties. In addition, technical developments and advances have resulted in an increasing number of studies regarding cell types other than the conventionally categorized ones described above, including short-axon cells and adult-generated interneurons. Thus, the expanding diversity of cells in the olfactory bulb is now being acknowledged. However, our current understanding of olfactory bulb neuronal circuits is mostly based on the conventional and simplest classification of cell types. Few studies have taken neuronal diversity into account for understanding the function of the neuronal circuits in this region of the brain. This oversight may contribute to the roadblocks in developing more precise and accurate models of olfactory neuronal networks. The purpose of this review is therefore to discuss the expanse of existing work on neuronal diversity in the olfactory bulb up to this point, so as to provide an overall picture of the olfactory bulb circuit.