Reciprocal intraglomerular excitation and intra- and interglomerular lateral inhibition between mouse olfactory bulb mitral cells

Fast-spiking interneuron detonation drives high-fidelity inhibition in the olfactory bulb

Inhibitory circuits in the mammalian olfactory bulb (OB) dynamically reformat olfactory information as it propagates from peripheral receptors to downstream cortex. To gain mechanistic insight into how specific OB interneuron types support this sensory processing, we examine unitary synaptic interactions between excitatory mitral and tufted cells (MTCs), the OB projection cells, and a conserved population of anaxonic external plexiform layer interneurons (EPL-INs) using pair and quartet whole-cell recordings in acute mouse brain slices. Physiological, morphological, neurochemical, and synaptic analyses divide EPL-INs into distinct subtypes and reveal that parvalbumin-expressing fast-spiking EPL-INs (FSIs) perisomatically innervate MTCs with release-competent dendrites and synaptically detonate to mediate fast, short-latency recurrent and lateral inhibition. Sparse MTC synchronization supralinearly increases this high-fidelity inhibition, while sensory afferent activation combined with single-cell silencing reveals that individual FSIs account for a substantial fraction of total network-driven MTC lateral inhibition. OB output is thus powerfully shaped by detonation-driven high-fidelity perisomatic inhibition.

Activity-dependent local protection and lateral inhibition control synaptic competition in developing mitral cells in mice

In developing brains, activity-dependent remodeling facilitates the formation of precise neuronal connectivity. Synaptic competition is known to facilitate synapse elimination; however, it has remained unknown how different synapses compete with one another within a post-synaptic cell. Here, we investigate how a mitral cell in the mouse olfactory bulb prunes all but one primary dendrite during the developmental remodeling process. We find that spontaneous activity generated within the olfactory bulb is essential. We show that strong glutamatergic inputs to one dendrite trigger branch-specific changes in RhoA activity to facilitate the pruning of the remaining dendrites: NMDAR-dependent local signals suppress RhoA to protect it from pruning; however, the subsequent neuronal depolarization induces neuron-wide activation of RhoA to prune non-protected dendrites. NMDAR-RhoA signals are also essential for the synaptic competition in the mouse barrel cortex. Our results demonstrate a general principle whereby activity-dependent lateral inhibition across synapses establishes a discrete receptive field of a neuron.

Stability and flexibility of odor representations in the mouse olfactory bulb

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

An ErbB4-Positive Neuronal Network in the Olfactory Bulb for Olfaction

Olfactory information is relayed and processed in the olfactory bulb (OB). Mitral cells, the principal output excitatory neurons of the OB, are controlled by multiple types of interneurons. However, mechanisms that regulate the activity of OB interneurons are not well understood. We provide evidence that the transmembrane tyrosine kinase ErbB4 is selectively expressed in subsets of OB inhibitory neurons in both male and female mice. ErbB4-positive (ErbB4+) neurons are mainly located in the glomerular layer (GL) and granule cell layer (GCL) and do not express previously defined markers. Optogenetic activation of GL-ErbB4+ neurons promotes theta oscillation, whereas activation of those in the GCL generates γ oscillations. Stimulation of OB slices with NRG1, a ligand that activates ErbB4, increases GABA transmission onto mitral cells, suggesting a role of OB NRG1-ErbB4 signaling in olfaction. In accord, ErbB4 mutant mice or acute inhibition of ErbB4 by a chemical genetic approach diminishes GABA transmission, reduces bulbar local field potential power, increases the threshold of olfactory sensitivity, and impairs odor discrimination. Together, these results identified a bulbar inhibitory network of ErbB4+ neurons for olfaction. Considering that both Nrg1 and Erbb4 are susceptibility genes for neuropsychiatric disorders, our study provides insight into pathologic mechanisms of olfactory malfunctions in these disorders.SIGNIFICANCE STATEMENT This study demonstrates that ErbB4+ neurons are a new subset of olfactory bulb inhibitory neurons in the glomerular layer and granule cell layer that innervate mitral cells and ErbB4- cells. They regulate olfaction by controlling local synchrony and distinct oscillations. ErbB4 inhibition diminishes GABA transmission, reduces bulbar local field potential power, increases the threshold of olfactory sensitivity, and impairs odor discrimination. Our results provide insight into pathophysiological mechanism of olfaction deficits in brain disorders associated with Nrg1 or Erbb4 mutations.

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

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

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

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

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

Cell and circuit origins of fast network oscillations in the mammalian main olfactory bulb

Neural synchrony generates fast network oscillations throughout the brain, including the main olfactory bulb (MOB), the first processing station of the olfactory system. Identifying the mechanisms synchronizing neurons in the MOB will be key to understanding how network oscillations support the coding of a high-dimensional sensory space. Here, using paired recordings and optogenetic activation of glomerular sensory inputs in MOB slices, we uncovered profound differences in principal mitral cell (MC) vs. tufted cell (TC) spike-time synchrony: TCs robustly synchronized across fast- and slow-gamma frequencies, while MC synchrony was weaker and concentrated in slow-gamma frequencies. Synchrony among both cell types was enhanced by shared glomerular input but was independent of intraglomerular lateral excitation. Cell-type differences in synchrony could also not be traced to any difference in the synchronization of synaptic inhibition. Instead, greater TC than MC synchrony paralleled the more periodic firing among resonant TCs than MCs and emerged in patterns consistent with densely synchronous network oscillations. Collectively, our results thus reveal a mechanism for parallel processing of sensory information in the MOB via differential TC vs. MC synchrony, and further contrast mechanisms driving fast network oscillations in the MOB from those driving the sparse synchronization of irregularly firing principal cells throughout cortex.

The Neurotransmitter Receptor Architecture of the Mouse Olfactory System

The uptake, transmission and processing of sensory olfactory information is modulated by inhibitory and excitatory receptors in the olfactory system. Previous studies have focused on the function of individual receptors in distinct brain areas, but the receptor architecture of the whole system remains unclear. Here, we analyzed the receptor profiles of the whole olfactory system of adult male mice. We examined the distribution patterns of glutamatergic (AMPA, kainate, mGlu_2/3, and NMDA), GABAergic (GABA_A, GABA_A(BZ), and GABA_B), dopaminergic (D_1/5) and noradrenergic (α₁ and α₂) neurotransmitter receptors by quantitative in vitro receptor autoradiography combined with an analysis of the cyto- and myelo-architecture. We observed that each subarea of the olfactory system is characterized by individual densities of distinct neurotransmitter receptor types, leading to a region- and layer-specific receptor profile. Thereby, the investigated receptors in the respective areas and strata showed a heterogeneous expression. Generally, we detected high densities of mGlu_2/3Rs, GABA_A(BZ)Rs and GABA_BRs. Noradrenergic receptors revealed a highly heterogenic distribution, while the dopaminergic receptor D_1/5 displayed low concentrations, except in the olfactory tubercle and the dorsal endopiriform nucleus. The similarities and dissimilarities of the area-specific multireceptor profiles were analyzed by a hierarchical cluster analysis. A three-cluster solution was found that divided the areas into the (1) olfactory relay stations (main and accessory olfactory bulb), (2) the olfactory cortex (anterior olfactory cortex, dorsal peduncular cortex, taenia tecta, piriform cortex, endopiriform nucleus, entorhinal cortex, orbitofrontal cortex) and the (3) olfactory tubercle, constituting its own cluster. The multimodal receptor-architectonic analysis of each component of the olfactory system provides new insights into its neurochemical organization and future possibilities for pharmaceutic targeting.

Changes in pairwise correlations during running reshape global network state in the main olfactory bulb

Neural codes for sensory inputs have been hypothesized to reside in a broader space defined by ongoing patterns of spontaneous activity. To understand the structure of this spontaneous activity in the olfactory system, we performed high-density recordings of neural populations in the main olfactory bulb of awake mice. We observed changes in pairwise correlations of spontaneous activity between mitral and tufted (M/T) cells when animals were running, which resulted in an increase in the entropy of the population. Surprisingly, pairwise maximum entropy models that described the population activity using only assumptions about the firing rates and correlations of neurons were better at predicting the global structure of activity when animals were stationary as compared to when they were running, implying that higher order (3rd, 4th order) interactions governed population activity during locomotion. Taken together, we found that locomotion alters the functional interactions that shape spontaneous population activity at the earliest stages of olfactory processing, one synapse away from the sensory receptors in the nasal epithelium. These data suggest that the coding space available for sensory representations responds adaptively to the animal's behavioral state.NEW & NOTEWORTHY The organization and structure of spontaneous population activity in the olfactory system places constraints of how odor information is represented. Using high-density electrophysiological recordings of mitral and tufted cells, we found that running increases the dimensionality of spontaneous activity, implicating higher order interactions among neurons during locomotion. Behavior, thus, flexibly alters neuronal activity at the earliest stages of sensory processing.

Dendritic integration in olfactory bulb granule cells upon simultaneous multispine activation: Low thresholds for nonlocal spiking activity

The inhibitory axonless olfactory bulb granule cells form reciprocal dendrodendritic synapses with mitral and tufted cells via large spines, mediating recurrent and lateral inhibition. As a case in point for dendritic transmitter release, rat granule cell dendrites are highly excitable, featuring local Na+ spine spikes and global Ca2+- and Na+-spikes. To investigate the transition from local to global signaling, we performed holographic, simultaneous 2-photon uncaging of glutamate at up to 12 granule cell spines, along with whole-cell recording and dendritic 2-photon Ca2+ imaging in acute juvenile rat brain slices. Coactivation of less than 10 reciprocal spines was sufficient to generate diverse regenerative signals that included regional dendritic Ca2+-spikes and dendritic Na+-spikes (D-spikes). Global Na+-spikes could be triggered in one third of granule cells. Individual spines and dendritic segments sensed the respective signal transitions as increments in Ca2+ entry. Dendritic integration as monitored by the somatic membrane potential was mostly linear until a threshold number of spines was activated, at which often D-spikes along with supralinear summation set in. As to the mechanisms supporting active integration, NMDA receptors (NMDARs) strongly contributed to all aspects of supralinearity, followed by dendritic voltage-gated Na+- and Ca2+-channels, whereas local Na+ spine spikes, as well as morphological variables, barely mattered. Because of the low numbers of coactive spines required to trigger dendritic Ca2+ signals and thus possibly lateral release of GABA onto mitral and tufted cells, we predict that thresholds for granule cell-mediated bulbar lateral inhibition are low. Moreover, D-spikes could provide a plausible substrate for granule cell-mediated gamma oscillations.

Parallel astrocyte calcium signaling modulates olfactory bulb responses

Astrocytes are the most abundant glial cell in the central nervous system. They modulate synaptic function through a variety of mechanisms, and yet remain relatively understudied with respect to overall neuronal circuit function. Exploiting the tractability of the mouse olfactory system, we manipulated astrocyte activity and examined how astrocytes modulate olfactory bulb responses. Toward this, we genetically targeted both astrocytes and neurons for in vivo widefield imaging of Ca2+ responses to odor stimuli. We found that astrocytes exhibited odor response maps that overlap with excitatory neuronal activity. By manipulating Ca2+ activity in astrocytes using chemical genetics we found that odor-evoked neuronal activity was reciprocally affected, suggesting that astrocyte activation inhibits neuronal odor responses. Subsequently, behavioral experiments revealed that astrocyte manipulations affect both odor detection threshold and discrimination, suggesting that astrocytes play an active role in olfactory sensory processing circuits. Together, these studies show that astrocyte calcium signaling contributes to olfactory behavior through modulation of sensory circuits.

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

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

fMRI study of olfactory processing in mice under three anesthesia protocols: Insight into the effect of ketamine on olfactory processing

Functional magnetic resonance imaging (fMRI) is a valuable tool for studying neural activations in the central nervous system of animals due to its wide spatial coverage and non-invasive nature. However, the advantages of fMRI have not been fully realized in functional studies in mice, especially in the olfactory system, possibly due to the lack of suitable anesthesia protocols with spontaneous breathing. Since mice are widely used in biomedical research, it is desirable to evaluate different anesthesia protocols for olfactory fMRI studies in mice. Dexmedetomidine (DEX) as a sedative/anesthetic has been introduced to fMRI studies in mice, but it has a limited anesthesia duration. To extend the anesthesia duration, DEX has been combined with a low dose of isoflurane (ISO) or ketamine (KET) in previous functional studies in mice. In this report, olfactory fMRI studies were performed under three anesthesia protocols (DEX alone, DEX/ISO, and DEX/KET) in three different groups of mice. Isoamyl-acetate was used as an odorant, and the odorant-induced neural activations were measured by blood oxygenation-level dependent (BOLD) fMRI. BOLD fMRI responses were observed in the olfactory bulb (OB), anterior olfactory nuclei (AON), and piriform cortex (Pir). Interestingly, BOLD fMRI activations were also observed in the prefrontal cortical region (PFC), which are most likely caused by the draining vein effect. The response in the OB showed no adaptation to either repeated odor stimulations or continuous odor exposure, but the response in the Pir showed adaptation during the continuous odor exposure. The data also shows that ISO suppresses the olfactory response in the OB and AON, while KET enhances the olfactory response in the Pir. Thus, DEX/KET should be an attractive anesthesia for olfactory fMRI in mice.

Diversity of Axonal and Dendritic Contributions to Neuronal Output

Our general understanding of neuronal function is that dendrites receive information that is transmitted to the axon, where action potentials (APs) are initiated and propagated to eventually trigger neurotransmitter release at synaptic terminals. Even though this canonical division of labor is true for a number of neuronal types in the mammalian brain (including neocortical and hippocampal pyramidal neurons or cerebellar Purkinje neurons), many neuronal types do not comply with this classical polarity scheme. In fact, dendrites can be the site of AP initiation and propagation, and even neurotransmitter release. In several interneuron types, all functions are carried out by dendrites as these neurons are devoid of a canonical axon. In this article, we present a few examples of "misbehaving" neurons (with a non-canonical polarity scheme) to highlight the diversity of solutions that are used by mammalian neurons to transmit information. Moreover, we discuss how the contribution of dendrites and axons to neuronal excitability may impose constraints on the morphology of these compartments in specific functional contexts.

Zinc Modulates Olfactory Bulb Kainate Receptors

Kainate receptors (KARs) are glutamate receptors with ionotropic and metabotropic activity composed of the GluK1-GluK5 subunits. We previously reported that KARs modulate excitatory and inhibitory transmission in the olfactory bulb (OB). Zinc, which is highly concentrated in the OB, also appears to modulate OB synaptic transmission via actions at other ionotropic glutamate receptors (i.e., AMPA, NMDA). However, few reports of effects of zinc on recombinant and/or native KARs exist and none have involved the OB. In the present study, we investigated the effects of exogenously applied zinc on OB KARs expressed by mitral/tufted (M/T) cells. We found that 100 µM zinc inhibits currents evoked by various combinations of KAR agonists (kainate or SYM 2081) and the AMPA receptor antagonist SYM 2206. The greatest degree of zinc-mediated inhibition was observed with coapplication of zinc with the GluK1- and GluK2-preferring agonist SYM 2081 plus SYM 2206. This finding is consistent with prior reports of zinc's inhibitory effects on some recombinant (homomeric GluK1 and GluK2 and heteromeric GluK2/GluK4 and GluK2/GluK5) KARs, although potentiation of other (GluK3, GluK2/3) KARs has also been described. It is also of potential importance given our previously reported molecular data suggesting that OB neurons express relatively high levels of GluK1 and GluK2. Our present findings suggest that a physiologically relevant concentration of zinc modulates KARs expressed by M/T cells. As M/T cells are targets of zinc-containing olfactory sensory neurons, synaptically released zinc may influence odor information-encoding synaptic circuits in the OB via actions at KARs.

Decreased amplitude and reliability of odor-evoked responses in two mouse models of autism

Sensory processing deficits are increasingly recognized as core symptoms of autism spectrum disorders (ASDs). However the molecular and circuit mechanisms that lead to sensory deficits are unknown. We show that two molecularly disparate mouse models of autism display similar deficits in sensory-evoked responses in the mouse olfactory system. We find that both Cntnap2- and Shank3-deficient mice of both sexes exhibit reduced response amplitude and trial-to-trial reliability during repeated odor presentation. Mechanistically, we show that both mouse models have weaker and fewer synapses between olfactory sensory nerve (OSN) terminals and olfactory bulb tufted cells and weaker synapses between OSN terminals and inhibitory periglomerular cells. Consequently, deficits in sensory processing provide an excellent candidate phenotype for analysis in ASDs.NEW & NOTEWORTHY The genetics of autism spectrum disorder (ASD) are complex. How the many risk genes generate the similar sets of symptoms that define the disorder is unknown. In particular, little is understood about the functional consequences of these genetic alterations. Sensory processing deficits are important aspects of the ASD diagnosis and may be due to unreliable neural circuits. We show that two mouse models of autism, Cntnap2- and Shank3-deficient mice, display reduced odor-evoked response amplitudes and reliability. These data suggest that altered sensory-evoked responses may constitute a circuit phenotype in ASDs.

Balancing Extrasynaptic Excitation and Synaptic Inhibition within Olfactory Bulb Glomeruli

Glutamatergic transmission in the brain typically occurs at well-defined synaptic connections, but increasing evidence indicates that neural excitation can also occur through activation of “extrasynaptic” glutamate receptors. Here, we investigated the underlying mechanisms and functional properties of extrasynaptic signals that are part of a feedforward path of information flow in the olfactory bulb. This pathway involves glutamatergic interneurons, external tufted cells (eTCs), that are excited by olfactory sensory neurons (OSNs) and in turn excite output mitral cells (MCs) extrasynaptically. Using pair-cell and triple-cell recordings in rat bulb slices (of either sex), combined with ultrastructural approaches, we first present evidence that eTC-to-MC signaling results from “spillover” of glutamate released at eTC synapses onto GABAergic periglomerular (PG) cells in glomeruli. Thus, feedforward excitation is an indirect result of and must cooccur with activation of inhibitory circuitry. Next, to examine the dynamics of the competing signals, we assayed the relationship between the number of spikes in eTCs and excitation of MCs or PG cells in pair-cell recordings. This showed that extrasynaptic excitation in MCs is very weak due to single spikes but rises sharply and supralinearly with increasing spikes, differing from sublinear behavior for synaptic excitation of PG cells. Similar dynamics leading to a preference for extrasynaptic excitation were also observed during recordings of extrasynaptic and inhibitory currents in response to OSN input of increasing magnitude. The observed alterations in the balance between extrasynaptic excitation and inhibition in glomeruli with stimulus strength could underlie an intraglomerular mechanism for olfactory contrast enhancement.

The olfactory bulb is a source of high-frequency oscillations (130-180 Hz) associated with a subanesthetic dose of ketamine in rodents

High-frequency neuronal population oscillations (HFO, 130-180 Hz) are robustly potentiated by subanesthetic doses of ketamine. This frequency band has been recorded in functionally and neuroanatomically diverse cortical and subcortical regions, notably ventral striatal areas. However, the locus of generation remains largely unknown. There is compelling evidence that olfactory regions can drive oscillations in distant areas. Here we tested the hypothesis that the olfactory bulb (OB) is a locus for the generation of HFO following a subanesthetic dose of ketamine. The effect of ketamine on the electrophysiological activity of the OB and ventral striatum of male Wistar rats was examined using field potential and unit recordings, local inhibition, naris blockade, current source density and causality estimates. Ketamine-HFO was of larger magnitude and was phase-advanced in the OB relative to ventral striatum. Granger causality analysis was consistent with the OB as the source of HFO. Unilateral local inhibition of the OB and naris blockade both attenuated HFO recorded locally and in the ventral striatum. Within the OB, current source density analysis revealed HFO current dipoles close to the mitral layer and unit firing of mitral/tufted cells was phase locked to HFO. Our results reveal the OB as a source of ketamine-HFO which can contribute to HFO in the ventral striatum, known to project diffusely to many other brain regions. These findings provide a new conceptual understanding on how changes in olfactory system function may have implications for neurological disorders involving NMDA receptor dysfunction such as schizophrenia and depression.

Kainate Receptors Play a Role in Modulating Synaptic Transmission in the Olfactory Bulb

Glutamate is the neurotransmitter used at most excitatory synapses in the mammalian brain, including those in the olfactory bulb (OB). There, ionotropic glutamate receptors including N-methyl-d-aspartate receptors (NMDARs) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) play a role in processes such as reciprocal inhibition and glomerular synchronization. Kainate receptors (KARs) represent another type of ionotropic glutamate receptor, which are composed of five (GluK1-GluK5) subunits. Whereas KARs appear to be heterogeneously expressed in the OB, evidence as to whether these KARs are functional, found at synapses, or modify synaptic transmission is limited. In the present study, coapplication of KAR agonists (kainate, SYM 2081) and AMPAR antagonists (GYKI 52466, SYM 2206) demonstrated that functional KARs are expressed by OB neurons, with a subset of receptors located at synapses. Application of kainate and the GluK1-selective agonist ATPA had modulatory effects on excitatory postsynaptic currents (EPSCs) evoked by stimulation of the olfactory nerve layer. Application of kainate and ATPA also had modulatory effects on reciprocal inhibitory postsynaptic currents (IPSCs) evoked using a protocol that evokes dendrodendritic inhibition. The latter finding suggests that KARs, with relatively slow kinetics, may play a role in circuits in which the relatively brief duration of AMPAR-mediated currents limits the role of AMPARs in synaptic transmission (e.g., reciprocal inhibition at dendrodendritic synapses). Collectively, our findings suggest that KARs, including those containing the GluK1 subunit, modulate excitatory and inhibitory transmission in the OB. These data further suggest that KARs participate in the regulation of synaptic circuits that encode odor information.

Blocking of Dendrodendritic Inhibition Unleashes Widely Spread Lateral Propagation of Odor-evoked Activity in the Mouse Olfactory Bulb

The olfactory circuitry in mice involves a well-characterized, vertical receptor type-specific organization, but the localized inhibitory effect from granule cells on action potentials that propagate laterally in secondary dendrites of mitral cell remains open to debate. To understand the functional dynamics of the lateral (horizontal) circuits, we analyzed odor-induced signaling using transgenic mice expressing a genetically encoded Ca²⁺ indicator specifically in mitral/tufted and some juxtaglomerular cells. Optical imaging of the dorsal olfactory bulb (dOB) revealed specific patterns of glomerular activation in response to odor presentation or direct electric stimulation of the olfactory nerve (ON). Application of a mixture of ionotropic and metabotropic glutamate receptor antagonists onto the exposed dOB completely abolished the responses to direct stimulation of the ON as well as discrete odor-evoked glomerular responses patterns, while a spatially more widespread response component increased and expanded into previously nonresponsive regions. To test whether the widespread odor response component represented signal propagation along mitral cell secondary dendrites, an NMDA receptor antagonist alone was applied to the dOB and was found to also increase and expand odor-evoked response patterns. Finally, with dOB excitatory synaptic transmission completely blocked, application of 1 mM muscimol (a GABA_A receptor agonist) to a circumscribed volume in the deep external plexiform layer (EPL) induced an odor non-responsive area. These results indicate that odor stimulation can activate olfactory reciprocal synapses and control lateral interactions among olfactory glomerular modules along a wide range of mitral cell secondary dendrites by modulating the inhibitory effect from granule cells.

Quantitative Association of Anatomical and Functional Classes of Olfactory Bulb Neurons

Juxtaglomerular cells (JGCs) of the olfactory bulb (OB) glomerular layer (GL) play a fundamental role in olfactory information processing. Their variability in morphology, physiology, and connectivity suggests distinct functions. The quantitative understanding of population-wise morphological and physiological properties and a comprehensive classification based on quantitative parameters, however, is still lacking, impeding the analysis of microcircuits. Here, we provide multivariate clustering of 95 in vitro sampled cells from the GL of the mouse (male or female C57BL/6) OB and perform detailed morphological and physiological characterization for the seven computed JGC types. Using a classifier based on a subselection of parameters, we identified the neuron types in paired recordings to characterize their functional connectivity. We found that 4 of the 7 clusters comply with prevailing concepts of GL cell types, whereas the other 3 represent own distinct entities. We have labeled these entities horizontal superficial tufted cell (hSTC), vertical superficial tufted cell, and microglomerular cell (MGC): The hSTC is a tufted cell with a lateral dendrite that much like mitral cells and tufted cells receives excitatory inputs from the external tufted cell but likewise serves as an excitatory element for glomerular interneurons. The vertical superficial tufted cell, on the other hand, represents a tufted cell type with vertically projecting basal dendrites. We further define the MGC, characterized by a small dendritic tree and plateau action potentials. In addition to olfactory nerve-driven and external tufted cell driven interneurons, these MGCs represent a third functionally distinct type, the hSTC-driven interneurons. The presented correlative analysis helps to bridge the gap between branching patterns and cellular functional properties, permitting the integration of results from in vivo recordings, advanced morphological tools, and connectomics.

SIGNIFICANCE STATEMENT The variance of neuron properties is a feature across mammalian cerebral circuits, contributing to signal processing and adding computational robustness to the networks. It is particularly noticeable in the glomerular layer of the olfactory bulb, the first site of olfactory information processing. We provide the first unbiased population-wise multivariate analysis to correlate morphological and physiological parameters of juxtaglomerular cells. We identify seven cell types, including four previously described neuron types, and identify further three distinct classes. The presented correlative analysis of morphological and physiological parameters gives an opportunity to predict morphological classes from physiological measurements or the functional properties of neurons from morphology and opens the way to integrate results from in vivo recordings, advanced morphological tools, and connectomics.

Cholecystokinin selectively activates short axon cells to enhance inhibition of olfactory bulb output neurons

KEY POINTS

Cholecystokinin (CCK) via CCK-B receptors significantly enhances the GABA_A receptor-mediated synaptic inhibition of principal olfactory bulb (OB) output neurons. This CCK action requires action potentials in presynaptic neurons. The enhanced inhibition of OB output neurons is a result of CCK-elevated inhibitory input from the glomerular circuit. CCK modulation of the glomerular circuit also leads to potentiated presynaptic inhibition of olfactory nerve terminals and postsynaptic inhibition of glomerular neurons. Selective excitation of short axon cells underlies the CCK-potentiated glomerular inhibition.

ABSTRACT

Neuropeptides such as cholecystokinin (CCK) are important for many brain functions, including sensory processing. CCK is predominantly present in a subpopulation of excitatory neurons and activation of CCK receptors is implicated in olfactory signal processing in the olfactory bulb (OB). However, the cellular and circuit mechanisms underlying the actions of CCK in the OB remain elusive. In the present study, we characterized the effects of CCK on synaptic inhibition of the principal OB output neurons mitral/tufted cells (MTCs) followed by mechanistic analyses at both circuit and cellular levels. First, we found that CCK via CCK-B receptors enhances the GABA_A receptor-mediated spontaneous IPSCs in MTCs. Second, CCK does not affect the action potential independent miniature IPSCs in MTCs. Third, CCK potentiates glomerular inhibition resulting in increased GABA_B receptor-mediated presynaptic inhibition of olfactory nerve terminals and enhanced spontaneous IPSCs in MTCs and glomerular neurons. Fourth, CCK enhances miniature IPSCs in the excitatory external tufted cells, although neither in the inhibitory short axon cells (SACs) nor in periglomerular cells (PGCs). Finally, CCK excites all tested SACs and a very small minority of GABAergic neurons in the granule cell layer or in periglomerular cells, but not in deep SACs. These results demonstrate that CCK selectively activates SACs to engage the SAC-formed interglomerular circuit and thus elevates inhibition broadly in the OB glomerular layer. This modulation may prevent the system from saturating in response to a high concentration of odourants or facilitate the detection of weak stimuli by increasing signal-to-noise ratio.

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

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

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

Intraglomerular gap junctions enhance interglomerular synchrony in a sparsely connected olfactory bulb network

KEY POINTS

Despite sparse connectivity, population-level interactions between mitral cells (MCs) and granule cells (GCs) can generate synchronized oscillations in the rodent olfactory bulb. Intraglomerular gap junctions between MCs at the same glomerulus can greatly enhance synchronized activity of MCs at different glomeruli. The facilitating effect of intraglomerular gap junctions on interglomerular synchrony is through triggering of mutually synchronizing interactions between MCs and GCs. Divergent connections between MCs and GCs make minimal direct contribution to synchronous activity.

ABSTRACT

A dominant feature of the olfactory bulb response to odour is fast synchronized oscillations at beta (15-40 Hz) or gamma (40-90 Hz) frequencies, thought to be involved in integration of olfactory signals. Mechanistically, the bulb presents an interesting case study for understanding how beta/gamma oscillations arise. Fast oscillatory synchrony in the activity of output mitral cells (MCs) appears to result from interactions with GABAergic granule cells (GCs), yet the incidence of MC-GC connections is very low, around 4%. Here, we combined computational and experimental approaches to examine how oscillatory synchrony can nevertheless arise, focusing mainly on activity between 'non-sister' MCs affiliated with different glomeruli (interglomerular synchrony). In a sparsely connected model of MCs and GCs, we found first that interglomerular synchrony was generally quite low, but could be increased by a factor of 4 by physiological levels of gap junctional coupling between sister MCs at the same glomerulus. This effect was due to enhanced mutually synchronizing interactions between MC and GC populations. The potent role of gap junctions was confirmed in patch-clamp recordings in bulb slices from wild-type and connexin 36-knockout (KO) mice. KO reduced both beta and gamma local field potential oscillations as well as synchrony of inhibitory signals in pairs of non-sister MCs. These effects were independent of potential KO actions on network excitation. Divergent synaptic connections did not contribute directly to the vast majority of synchronized signals. Thus, in a sparsely connected network, gap junctions between a small subset of cells can, through population effects, greatly amplify oscillatory synchrony amongst unconnected cells.

Sensory experience shapes the integration of adult-born neurons into the olfactory bulb

Olfaction is an ancient sensory modality which is heavily involved in viscerally-important tasks like finding food and identifying mates. Olfactory processing involves interpreting stimuli from a non-continuous odor space, and translating them into an organized pattern of neuronal activity in the olfactory bulb. Additionally, olfactory processing is rapidly modulated by behavioral states and vice versa. This implies strong bidirectional neuromodulation between the olfactory bulb and other brain regions that include the cortex, hippocampus, and basal forebrain. Intriguingly, the olfactory bulb is one of the only brain regions where adult-born neurons are integrated into existing networks throughout life. The ongoing integration of adult-born neurons is known to be important for olfactory processing, odor discrimination, and odor learning. Furthermore, the survival and integration of the adult-born neurons is regulated by neuromodulatory signaling, sensory experience, and olfactory learning. Studies making use of new genetic markers to label and manipulate immature adult-born neurons reveal an increase in their population response to odors as they mature. Importantly, this reflects a period of developmental plasticity where adult-born neurons are especially sensitive to sensory experience and olfactory learning. In this review, we discuss the contribution of adult neurogenesis to olfactory bulb plasticity and information processing, with a focus on the developmental plasticity of adult born neurons, and how it is influenced by sensory experience and olfactory learning. Ultimately, recent studies raise important questions about behavioral-state-dependent effects on adult-born neurons, and the consequences of neuromodulation on the developmental plasticity of newborn neurons in the olfactory bulb.

Inhibitory circuits of the mammalian main olfactory bulb

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

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.

Early Odorant Exposure Increases the Number of Mitral and Tufted Cells Associated with a Single Glomerulus

The highly specific organization of the olfactory bulb (OB) is well known, but the impact of early odorant experience on its circuit structure is unclear. Olfactory sensory neurons (OSNs) project axons from the olfactory epithelium to the OB, where they form spherical neuropil structures called glomeruli. These glomeruli and the postsynaptic targets of OSNs, including mitral and tufted cells (M/TCs) and juxtaglomerular cells, form glomerular modules, which represent the basic odor-coding units of the OB. Here, we labeled M/TCs within a single glomerular module of the mouse OB and show that odorant exposure that starts prenatally and continues through postnatal day 25 has a major impact on the structure of the glomerular module. We confirm that exposure increases the volume of the activated glomeruli and show that exposure increases M/TC number by >40% in a glomerulus-specific fashion. Given the role of M/TCs in OB output and in lateral inhibition, increasing the number of M/TCs connected to a single glomerulus may also increase the influence of that glomerulus on the OB network and on OB output. Our results show that early odorant exposure has a profound effect on OB connectivity and thus may affect odorant processing significantly.

SIGNIFICANCE STATEMENT

Experience shapes neural circuits in a variety of ways, most commonly by changing the strength of activated connections. Relatively little is known about how experience changes circuitry in the olfactory system. Here, we show that for a genetically identified glomerulus in the mouse olfactory bulb, early odorant exposure increases the number of associated mitral and tufted cells by 40% and 100%, respectively. Understanding the structural changes induced by early odorant experience can provide insight into how bulbar organization gives rise to efficient processing. We find that odorant experience increases the number of projection neurons associated with a single glomerulus significantly, a dramatic and long-lasting structural change that may have important functional implications.

Different forms of structural plasticity in the adult olfactory bulb

The adult olfactory bulb (OB) continuously receives new interneurons that integrate into the functional neuronal network and that play an important role in odor information processing and olfactory behavior. Adult neuronal progenitors are derived from neural stem cells in the subventricular zone (SVZ) bordering the lateral ventricle. They migrate long distances along the rostral migratory stream (RMS) toward the OB where they differentiate into interneurons, mature, and establish synapses with tufted or mitral cells (MC), the principal neurons in the OB. The plasticity provided by both adult-born and pre-existing early-born neurons depends on the formation and pruning of new synaptic contacts that adapt the functioning of the bulbar network to changing environmental conditions. However, the formation of new synapses occurs over a long time scale (hours-days), whereas some changes in environmental conditions can occur more rapidly, requiring a much faster adjustment of neuronal networks. A new form of structural remodeling of adult-born, but not early-born, neurons was recently brought to light. This plasticity, which is based on the activity-dependent relocation of mature spines of GCs toward the dendrites of active principal cells, may allow a more rapid adjustment of the neuronal network in response to quick and persistent changes in sensory inputs. In this mini-review we discuss the different forms of structural plasticity displayed by adult-born and early-born neurons and the possibility that these different forms of structural remodeling may fulfill distinct roles in odor information processing.

Wiring variations that enable and constrain neural computation in a sensory microcircuit

Neural network function can be shaped by varying the strength of synaptic connections. One way to achieve this is to vary connection structure. To investigate how structural variation among synaptic connections might affect neural computation, we examined primary afferent connections in the Drosophila olfactory system. We used large-scale serial section electron microscopy to reconstruct all the olfactory receptor neuron (ORN) axons that target a left-right pair of glomeruli, as well as all the projection neurons (PNs) postsynaptic to these ORNs. We found three variations in ORN→PN connectivity. First, we found a systematic co-variation in synapse number and PN dendrite size, suggesting total synaptic conductance is tuned to postsynaptic excitability. Second, we discovered that PNs receive more synapses from ipsilateral than contralateral ORNs, providing a structural basis for odor lateralization behavior. Finally, we found evidence of imprecision in ORN→PN connections that can diminish network performance.

DOI:http://dx.doi.org/10.7554/eLife.24838.001

Differential serotonergic modulation across the main and accessory olfactory bulbs

KEY POINTS

There are serotonergic projections to both the main (MOB) and the accessory olfactory bulb (AOB). Current-clamp experiments demonstrate that serotonergic afferents are largely excitatory for mitral cells (MCs) in the MOB where 5-HT_2A receptors mediate a direct excitatory action. Serotonergic afferents are predominately inhibitory for MCs in the AOB. There are two types of inhibition: indirect inhibition mediated through the 5-HT₂ receptors on GABAergic interneurons and direct inhibition via the 5-HT₁ receptors on MCs. Differential 5-HT neuromodulation of MCs across the MOB and AOB could contribute to select behaviours such as olfactory learning or aggression.

ABSTRACT

Mitral cells (MCs) contained in the main (MOB) and accessory (AOB) olfactory bulb have distinct intrinsic membrane properties but the extent of neuromodulation across the two systems has not been widely explored. Herein, we investigated a widely distributed CNS modulator, serotonin (5-HT), for its ability to modulate the biophysical properties of MCs across the MOB and AOB, using an in vitro, brain slice approach in postnatal 15-30 day mice. In the MOB, 5-HT elicited three types of responses in 93% of 180 cells tested. Cells were either directly excited (70%), inhibited (10%) or showed a mixed response (13%)- first inhibition followed by excitation. In the AOB, 82% of 148 cells were inhibited with 18% of cells showing no response. Albeit located in parallel partitions of the olfactory system, 5-HT largely elicited MC excitation in the MOB while it evoked two different kinetic rates of MC inhibition in the AOB. Using a combination of pharmacological agents, we found that the MC excitatory responses in the MOB were mediated by 5-HT_2A receptors through a direct activation. In comparison, 5-HT-evoked inhibitory responses in the AOB arose due to a polysynaptic, slow-onset inhibition attributed to 5-HT₂ receptor activation exciting GABAergic interneurons. The second type of inhibition had a rapid onset as a result of direct inhibition mediated by the 5-HT₁ class of receptors. The distinct serotonergic modulation of MCs between the MOB and AOB could provide a molecular basis for differential chemosensory behaviours driven by the brainstem raphe nuclei into these parallel systems.

Synchronous Infra-Slow Bursting in the Mouse Accessory Olfactory Bulb Emerge from Interplay between Intrinsic Neuronal Dynamics and Network Connectivity

Rhythmic neuronal activity of multiple frequency bands has been described in many brain areas and attributed to numerous brain functions. Among these, little is known about the mechanism and role of infra-slow oscillations, which have been demonstrated recently in the mouse accessory olfactory bulb (AOB). Along with prolonged responses to stimuli and distinct network connectivity, they inexplicably affect the AOB processing of social relevant stimuli. Here, we show that assemblies of AOB mitral cells are synchronized by lateral interactions through chemical and electrical synapses. Using a network model, we demonstrate that the synchronous oscillations in these assemblies emerge from interplay between intrinsic membrane properties and network connectivity. As a consequence, the AOB network topology, in which each mitral cell receives input from multiple glomeruli, enables integration of chemosensory stimuli over extended time scales by interglomerular synchrony of infra-slow bursting. These results provide a possible functional significance for the distinct AOB physiology and topology. Beyond the AOB, this study presents a general model for synchronous infra-slow bursting in neuronal networks.SIGNIFICANCE STATEMENT Infra-slow rhythmic neuronal activity with a very long (>10 s) duration has been described in many brain areas, but little is known about the role of this activity and the mechanisms that produce it. Here, we combine experimental and computational methods to show that synchronous infra-slow bursting activity in mitral cells of the mouse accessory olfactory bulb (AOB) emerges from interplay between intracellular dynamics and network connectivity. In this novel mechanism, slow intracellular Na⁺ dynamics endow AOB mitral cells with a weak tendency to burst, which is further enhanced and stabilized by chemical and electrical synapses between them. Combined with the unique topology of the AOB network, infra-slow bursting enables integration and binding of multiple chemosensory stimuli over a prolonged time scale.

Interglomerular Connectivity within the Canonical and GC-D/Necklace Olfactory Subsystems

The mammalian main olfactory system contains several subsystems that differ not only in the receptors they express and the glomerular targets they innervate within the main olfactory bulb (MOB), but also in the strategies they use to process odor information. The canonical main olfactory system employs a combinatorial coding strategy that represents odorant identity as a pattern of glomerular activity. By contrast, the "GC-D/necklace" olfactory subsystem—formed by olfactory sensory neurons expressing the receptor guanylyl cyclase GC-D and their target necklace glomeruli (NGs) encircling the caudal MOB—is critical for the detection of a small number of semiochemicals that promote the acquisition of food preferences. The formation of these socially-transmitted food preferences requires the animal to integrate information about two types of olfactory stimuli: these specialized social chemosignals and the food odors themselves. However, the neural mechanisms with which the GC-D/necklace subsystem processes this information are unclear. We used stimulus-induced increases in intrinsic fluorescence signals to map functional circuitry associated with NGs and canonical glomeruli (CGs) in the MOB. As expected, CG-associated activity spread laterally through both the glomerular and external plexiform layers associated with activated glomeruli. Activation of CGs or NGs resulted in activity spread between the two types of glomeruli; there was no evidence of preferential connectivity between individual necklace glomeruli. These results support previous anatomical findings that suggest the canonical and GC-D/necklace subsystems are functionally connected and may integrate general odor and semiochemical information in the MOB.

Calretinin-Periglomerular Interneurons in Mice Olfactory Bulb: Cells of Few Words

Within the olfactory bulb (OB), periglomerular (PG) cells consist of various types of interneurons, generally classified by their chemical properties such as neurotransmitter and calcium binding proteins. Calretinin (CR) characterizes morphologically and functionally the more numerous and one of the less known subpopulation of PG cells in the OB. Using of transgenic mice expressing eGFP under the CR promoter, we have tried to obtain the first functional characterization of these cells. Electrophysiological recordings were made in these cells using the patch-clamp technique in thin slices. Using ion substitution methods and specific blockers, we dissected the main voltage-dependent conductances present, obtaining a complete kinetic description for each of them. The more peculiar property of these cells from the electrophysiological point of view is the presence only of a single K-current, A-type – there is no trace of delayed rectifier or of Ca-dependent K-current. Other currents identified, isolated and fully characterized are a fast sodium current, a small L-type calcium current, and an inward rectifier, h-type cationic current. As a consequence of the peculiar complement of voltage-dependent conductances present in these cells, and in particular the absence of delayed-rectifier potassium currents, under the functional point of view these cells present two interesting properties. First, in response to prolonged depolarisations, after the inactivation of the A-current these cells behave as a purely ohmic elements, showing no outward rectification. Second, the CR cells studied can respond only with a single action potential to excitatory inputs; since they send inhibitory synapses to projection neurones, they seem to be designed to inhibit responses of the main neurones to isolated, random excitatory signals, rapidly losing their vetoing effect in response to more structured, repetitive excitatory signals. We propose that a possible role for these rather untalkative interneurons in the intense exchange of messages within the OB might be that of improving the signal-to-noise ratio in the first stages of the olfactory information processing.

Three-dimensional synaptic analyses of mitral cell and external tufted cell dendrites in rat olfactory bulb glomeruli

Recent studies have suggested that the two excitatory cell classes of the mammalian olfactory bulb, the mitral cells (MCs) and tufted cells (TCs), differ markedly in physiological responses. For example, TCs are more sensitive and broadly tuned to odors than MCs and also are much more sensitive to stimulation of olfactory sensory neurons (OSNs) in bulb slices. To examine the morphological bases for these differences, we performed quantitative ultrastructural analyses of glomeruli in rat olfactory bulb under conditions in which specific cells were labeled with biocytin and 3,3'-diaminobenzidine. Comparisons were made between MCs and external TCs (eTCs), which are a TC subtype in the glomerular layer with large, direct OSN signals and capable of mediating feedforward excitation of MCs. Three-dimensional analysis of labeled apical dendrites under an electron microscope revealed that MCs and eTCs in fact have similar densities of several chemical synapse types, including OSN inputs. OSN synapses also were distributed similarly, favoring a distal localization on both cells. Analysis of unlabeled putative MC dendrites further revealed gap junctions distributed uniformly along the apical dendrite and, on average, proximally with respect to OSN synapses. Our results suggest that the greater sensitivity of eTCs vs. MCs is due not to OSN synapse number or absolute location but rather to a conductance in the MC dendrite that is well positioned to attenuate excitatory signals passing to the cell soma. Functionally, such a mechanism could allow rapid and dynamic control of OSN-driven action potential firing in MCs through changes in gap junction properties. J. Comp. Neurol. 525:592-609, 2017. © 2016 Wiley Periodicals, Inc.

Parallel processing of afferent olfactory sensory information

KEY POINTS

The functional synaptic connectivity between olfactory receptor neurons and principal cells within the olfactory bulb is not well understood. One view suggests that mitral cells, the primary output neuron of the olfactory bulb, are solely activated by feedforward excitation. Using focal, single glomerular stimulation, we demonstrate that mitral cells receive direct, monosynaptic input from olfactory receptor neurons. Compared to external tufted cells, mitral cells have a prolonged afferent-evoked EPSC, which serves to amplify the synaptic input. The properties of presynaptic glutamate release from olfactory receptor neurons are similar between mitral and external tufted cells. Our data suggest that afferent input enters the olfactory bulb in a parallel fashion.

ABSTRACT

Primary olfactory receptor neurons terminate in anatomically and functionally discrete cortical modules known as olfactory bulb glomeruli. The synaptic connectivity and postsynaptic responses of mitral and external tufted cells within the glomerulus may involve both direct and indirect components. For example, it has been suggested that sensory input to mitral cells is indirect through feedforward excitation from external tufted cells. We also observed feedforward excitation of mitral cells with weak stimulation of the olfactory nerve layer; however, focal stimulation of an axon bundle entering an individual glomerulus revealed that mitral cells receive monosynaptic afferent inputs. Although external tufted cells had a 4.1-fold larger peak EPSC amplitude, integration of the evoked currents showed that the synaptic charge was 5-fold larger in mitral cells, reflecting the prolonged response in mitral cells. Presynaptic afferents onto mitral and external tufted cells had similar quantal amplitude and release probability, suggesting that the larger peak EPSC in external tufted cells was the result of more synaptic contacts. The results of the present study indicate that the monosynaptic afferent input to mitral cells depends on the strength of odorant stimulation. The enhanced spiking that we observed in response to brief afferent input provides a mechanism for amplifying sensory information and contrasts with the transient response in external tufted cells. These parallel input paths may have discrete functions in processing olfactory sensory input.

Glomerular and Mitral-Granule Cell Microcircuits Coordinate Temporal and Spatial Information Processing in the Olfactory Bulb

The olfactory bulb processes inputs from olfactory receptor neurons (ORNs) through two levels: the glomerular layer at the site of input, and the granule cell level at the site of output to the olfactory cortex. The sequence of action of these two levels has not yet been examined. We analyze this issue using a novel computational framework that is scaled up, in three-dimensions (3D), with realistic representations of the interactions between layers, activated by simulated natural odors, and constrained by experimental and theoretical analyses. We suggest that the postulated functions of glomerular circuits have as their primary role transforming a complex and disorganized input into a contrast-enhanced and normalized representation, but cannot provide for synchronization of the distributed glomerular outputs. By contrast, at the granule cell layer, the dendrodendritic interactions mediate temporal decorrelation, which we show is dependent on the preceding contrast enhancement by the glomerular layer. The results provide the first insights into the successive operations in the olfactory bulb, and demonstrate the significance of the modular organization around glomeruli. This layered organization is especially important for natural odor inputs, because they activate many overlapping glomeruli.

Disruption of Kcc2-dependent inhibition of olfactory bulb output neurons suggests its importance in odour discrimination

Synaptic inhibition in the olfactory bulb (OB), the first relay station of olfactory information, is believed to be important for odour discrimination. We interfered with GABAergic inhibition of mitral and tufted cells (M/T cells), the principal neurons of the OB, by disrupting their potassium-chloride cotransporter 2 (Kcc2). Roughly, 70% of mice died around 3 weeks, but surviving mice appeared normal. In these mice, the resulting increase in the intracellular Cl(-) concentration nearly abolished GABA-induced hyperpolarization of mitral cells (MCs) and unexpectedly increased the number of perisomatic synapses on MCs. In vivo analysis of odorant-induced OB electrical activity revealed increased M/T cell firing rate, altered phasing of action potentials in the breath cycle and disrupted separation of odour-induced M/T cell activity patterns. Mice also demonstrated a severely impaired ability to discriminate chemically similar odorants or odorant mixtures. Our work suggests that precisely tuned GABAergic inhibition onto M/T cells is crucial for M/T cell spike pattern separation needed to distinguish closely similar odours.

Granule cell excitability regulates gamma and beta oscillations in a model of the olfactory bulb dendrodendritic microcircuit

Odors evoke gamma (40-100 Hz) and beta (20-30 Hz) oscillations in the local field potential (LFP) of the mammalian olfactory bulb (OB). Gamma (and possibly beta) oscillations arise from interactions in the dendrodendritic microcircuit between excitatory mitral cells (MCs) and inhibitory granule cells (GCs). When cortical descending inputs to the OB are blocked, beta oscillations are extinguished whereas gamma oscillations become larger. Much of this centrifugal input targets inhibitory interneurons in the GC layer and regulates the excitability of GCs, which suggests a causal link between the emergence of beta oscillations and GC excitability. We investigate the effect that GC excitability has on network oscillations in a computational model of the MC-GC dendrodendritic network with Ca(2+)-dependent graded inhibition. Results from our model suggest that when GC excitability is low, the graded inhibitory current mediated by NMDA channels and voltage-dependent Ca(2+) channels (VDCCs) is also low, allowing MC populations to fire in the gamma frequency range. When GC excitability is increased, the activation of NMDA receptors and other VDCCs is also increased, allowing the slow decay time constants of these channels to sustain beta-frequency oscillations. Our model argues that Ca(2+) flow through VDCCs alone could sustain beta oscillations and that the switch between gamma and beta oscillations can be triggered by an increase in the excitability state of a subpopulation of GCs.

Spatio-Temporal Characteristics of Inhibition Mapped by Optical Stimulation in Mouse Olfactory Bulb

Mitral and tufted cells (MTCs) of the mammalian olfactory bulb are connected via dendrodendritic synapses with inhibitory interneurons in the external plexiform layer. The range, spatial layout, and temporal properties of inhibitory interactions between MTCs mediated by inhibitory interneurons remain unclear. Therefore, we tested for inhibitory interactions using an optogenetic approach. We optically stimulated MTCs expressing channelrhodopsin-2 in transgenic mice, while recording from individual MTCs in juxtacellular or whole-cell configuration in vivo. We used a spatial noise stimulus for mapping interactions between MTCs belonging to different glomeruli in the dorsal bulb. Analyzing firing responses of MTCs to the stimulus, we did not find robust lateral inhibitory effects that were spatially specific. However, analysis of sub-threshold changes in the membrane potential revealed evidence for inhibitory interactions between MTCs that belong to different glomerular units. These lateral inhibitory effects were short-lived and spatially specific. MTC response maps showed hyperpolarizing effects radially extending over more than five glomerular diameters. The inhibitory maps exhibited non-symmetrical yet distance-dependent characteristics.

Astroglial Connexin 43 Hemichannels Modulate Olfactory Bulb Slow Oscillations

An emergent concept in neurosciences consists in considering brain functions as the product of dynamic interactions between neurons and glial cells, particularly astrocytes. Although the role played by astrocytes in synaptic transmission and plasticity is now largely documented, their contribution to neuronal network activity is only beginning to be appreciated. In mouse olfactory bulb slices, we observed that the membrane potential of mitral cells oscillates between UP and DOWN states at a low frequency (<1 Hz). Such slow oscillations are correlated with glomerular local field potentials, indicating spontaneous local network activity. Using a combination of genetic and pharmacological tools, we showed that the activity of astroglial connexin 43 hemichannels, opened in an activity-dependent manner, increases UP state amplitude and impacts mitral cell firing rate. This effect requires functional adenosine A1 receptors, in line with the observation that ATP is released via connexin 43 hemichannels. These results highlight a new mechanism of neuroglial interaction in the olfactory bulb, where astrocyte connexin hemichannels are both targets and modulators of neuronal circuit function.

SIGNIFICANCE STATEMENT

An emergent concept in neuroscience consists in considering brain function as the product of dynamic interactions between neurons and glial cells, particularly astrocytes. A typical feature of astrocytes is their high expression level of connexins, the molecular constituents of gap junction channels and hemichannels. Although hemichannels represent a powerful medium for intercellular communication between astrocytes and neurons, their function in physiological conditions remains largely unexplored. Our results show that in the olfactory bulb, connexin 43 hemichannel function is promoted by neuronal activity and, in turn, modulates neuronal network slow oscillations. This novel mechanism of neuroglial interaction could influence olfactory information processing by directly impacting the output of the olfactory bulb.

Synaptic clusters function as odor operators in the olfactory bulb

How the olfactory bulb organizes and processes odor inputs through fundamental operations of its microcircuits is largely unknown. To gain new insight we focus on odor-activated synaptic clusters related to individual glomeruli, which we call glomerular units. Using a 3D model of mitral and granule cell interactions supported by experimental findings, combined with a matrix-based representation of glomerular operations, we identify the mechanisms for forming one or more glomerular units in response to a given odor, how and to what extent the glomerular units interfere or interact with each other during learning, their computational role within the olfactory bulb microcircuit, and how their actions can be formalized into a theoretical framework in which the olfactory bulb can be considered to contain "odor operators" unique to each individual. The results provide new and specific theoretical and experimentally testable predictions.

Cortical Feedback Decorrelates Olfactory Bulb Output in Awake Mice

The olfactory bulb receives rich glutamatergic projections from the piriform cortex. However, the dynamics and importance of these feedback signals remain unknown. Here, we use multiphoton calcium imaging to monitor cortical feedback in the olfactory bulb of awake mice and further probe its impact on the bulb output. Responses of feedback boutons were sparse, odor specific, and often outlasted stimuli by several seconds. Odor presentation either enhanced or suppressed the activity of boutons. However, any given bouton responded with stereotypic polarity across multiple odors, preferring either enhancement or suppression. Feedback representations were locally diverse and differed in dynamics across bulb layers. Inactivation of piriform cortex increased odor responsiveness and pairwise similarity of mitral cells but had little impact on tufted cells. We propose that cortical feedback differentially impacts these two output channels of the bulb by specifically decorrelating mitral cell responses to enable odor separation.

Intraglomerular lateral inhibition promotes spike timing variability in principal neurons of the olfactory bulb

The activity of mitral and tufted cells, the principal neurons of the olfactory bulb, is modulated by several classes of interneurons. Among them, diverse periglomerular (PG) cell types interact with the apical dendrites of mitral and tufted cells inside glomeruli at the first stage of olfactory processing. We used paired recording in olfactory bulb slices and two-photon targeted patch-clamp recording in vivo to characterize the properties and connections of a genetically identified population of PG cells expressing enhanced yellow fluorescent protein (EYFP) under the control of the Kv3.1 potassium channel promoter. Kv3.1-EYFP(+) PG cells are axonless and monoglomerular neurons that constitute ∼30% of all PG cells and include calbindin-expressing neurons. They respond to an olfactory nerve stimulation with a short barrage of excitatory inputs mediated by mitral, tufted, and external tufted cells, and, in turn, they indiscriminately release GABA onto principal neurons. They are activated by even the weakest olfactory nerve input or by the discharge of a single principal neuron in slices and at each respiration cycle in anesthetized mice. They participate in a fast-onset intraglomerular lateral inhibition between principal neurons from the same glomerulus, a circuit that reduces the firing rate and promotes spike timing variability in mitral cells. Recordings in other PG cell subtypes suggest that this pathway predominates in generating glomerular inhibition. Intraglomerular lateral inhibition may play a key role in olfactory processing by reducing the similarity of principal cells discharge in response to the same incoming input.

Circuit motifs for contrast-adaptive differentiation in early sensory systems: the role of presynaptic inhibition and short-term plasticity

In natural signals, such as the luminance value across of a visual scene, abrupt changes in intensity value are often more relevant to an organism than intensity values at other positions and times. Thus to reduce redundancy, sensory systems are specialized to detect the times and amplitudes of informative abrupt changes in the input stream rather than coding the intensity values at all times. In theory, a system that responds transiently to fast changes is called a differentiator. In principle, several different neural circuit mechanisms exist that are capable of responding transiently to abrupt input changes. However, it is unclear which circuit would be best suited for early sensory systems, where the dynamic range of the natural input signals can be very wide. We here compare the properties of different simple neural circuit motifs for implementing signal differentiation. We found that a circuit motif based on presynaptic inhibition (PI) is unique in a sense that the vesicle resources in the presynaptic site can be stably maintained over a wide range of stimulus intensities, making PI a biophysically plausible mechanism to implement a differentiator with a very wide dynamical range. Moreover, by additionally considering short-term plasticity (STP), differentiation becomes contrast adaptive in the PI-circuit but not in other potential neural circuit motifs. Numerical simulations show that the behavior of the adaptive PI-circuit is consistent with experimental observations suggesting that adaptive presynaptic inhibition might be a good candidate neural mechanism to achieve differentiation in early sensory systems.

Tasks for inhibitory interneurons in intact brain circuits

Synaptic inhibition, brought about by a rich variety of interneuron types, counters excitation, modulates the gain, timing, tuning, bursting properties of principal cell firing, and exerts selective filtering of synaptic excitation. At the network level, it allows for coordinating transient interactions among the principal cells to form cooperative assemblies for efficient transmission of information and routing of excitatory activity across networks, typically in the form of brain oscillations. Recent techniques based on targeted expression of neuronal activity modulators, such as optogenetics, allow physiological identification and perturbation of specific interneuron subtypes in the intact brain. Combined with large-scale recordings or imaging techniques, these approaches facilitate our understanding of the multiple roles of inhibitory interneurons in shaping circuit functions.

Spiking neural circuits with dendritic stimulus processors : encoding, decoding, and identification in reproducing kernel Hilbert spaces

We present a multi-input multi-output neural circuit architecture for nonlinear processing and encoding of stimuli in the spike domain. In this architecture a bank of dendritic stimulus processors implements nonlinear transformations of multiple temporal or spatio-temporal signals such as spike trains or auditory and visual stimuli in the analog domain. Dendritic stimulus processors may act on both individual stimuli and on groups of stimuli, thereby executing complex computations that arise as a result of interactions between concurrently received signals. The results of the analog-domain computations are then encoded into a multi-dimensional spike train by a population of spiking neurons modeled as nonlinear dynamical systems. We investigate general conditions under which such circuits faithfully represent stimuli and demonstrate algorithms for (i) stimulus recovery, or decoding, and (ii) identification of dendritic stimulus processors from the observed spikes. Taken together, our results demonstrate a fundamental duality between the identification of the dendritic stimulus processor of a single neuron and the decoding of stimuli encoded by a population of neurons with a bank of dendritic stimulus processors. This duality result enabled us to derive lower bounds on the number of experiments to be performed and the total number of spikes that need to be recorded for identifying a neural circuit.