Action potential propagation in dendrites of rat mitral cells in vivo

In vivo spontaneous activity and coital-evoked inhibition of mouse accessory olfactory bulb output neurons

Little is known about estrous effects on brain microcircuits. We examined the accessory olfactory bulb (AOB) in vivo, in anesthetized naturally cycling females, as model microcircuit receiving coital somatosensory information. Whole-cell recordings demonstrate that output neurons are relatively hyperpolarized in estrus and unexpectedly fire high frequency bursts of action potentials. To mimic coitus, a calibrated artificial vagino-cervical stimulation (aVCS) protocol was devised. aVCS evoked stimulus-locked local field responses in the interneuron layer independent of estrous stage. The response is sensitive to α1-adrenergic receptor blockade, as expected since aVCS increases norepinephrine release in AOB. Intriguingly, only in estrus does aVCS inhibit AOB spike output. Estrus-specific output reduction coincides with prolonged aVCS activation of inhibitory interneurons. Accordingly, in estrus the AOB microcircuit sets the stage for coital stimulation to inhibit the output neurons, possibly via high frequency bursting-dependent enhancement of reciprocal synapse efficacy between inter- and output neurons.

Anatomical and Functional Connectivity at the Dendrodendritic Reciprocal Mitral Cell–Granule Cell Synapse: Impact on Recurrent and Lateral Inhibition

In the vertebrate olfactory bulb, reciprocal dendrodendritic interactions between its principal neurons, the mitral and tufted cells, and inhibitory interneurons in the external plexiform layer mediate both recurrent and lateral inhibition, with the most numerous of these interneurons being granule cells. Here, we used recently established anatomical parameters and functional data on unitary synaptic transmission to simulate the strength of recurrent inhibition of mitral cells specifically from the reciprocal spines of rat olfactory bulb granule cells in a quantitative manner. Our functional data allowed us to derive a unitary synaptic conductance on the order of 0.2 nS. The simulations predicted that somatic voltage deflections by even proximal individual granule cell inputs are below the detection threshold and that attenuation with distance is roughly linear, with a passive length constant of 650 μm. However, since recurrent inhibition in the wake of a mitral cell action potential will originate from hundreds of reciprocal spines, the summated recurrent IPSP will be much larger, even though there will be substantial mutual shunting across the many inputs. Next, we updated and refined a preexisting model of connectivity within the entire rat olfactory bulb, first between pairs of mitral and granule cells, to estimate the likelihood and impact of recurrent inhibition depending on the distance between cells. Moreover, to characterize the substrate of lateral inhibition, we estimated the connectivity via granule cells between any two mitral cells or all the mitral cells that belong to a functional glomerular ensemble (i.e., which receive their input from the same glomerulus), again as a function of the distance between mitral cells and/or entire glomerular mitral cell ensembles. Our results predict the extent of the three regimes of anatomical connectivity between glomerular ensembles: high connectivity within a glomerular ensemble and across the first four rings of adjacent glomeruli, substantial connectivity to up to eleven glomeruli away, and negligible connectivity beyond. Finally, in a first attempt to estimate the functional strength of granule-cell mediated lateral inhibition, we combined this anatomical estimate with our above simulation results on attenuation with distance, resulting in slightly narrowed regimes of a functional impact compared to the anatomical connectivity.

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.

Active Dendrites and Local Field Potentials: Biophysical Mechanisms and Computational Explorations

Neurons and glial cells are endowed with membranes that express a rich repertoire of ion channels, transporters, and receptors. The constant flux of ions across the neuronal and glial membranes results in voltage fluctuations that can be recorded from the extracellular matrix. The high frequency components of this voltage signal contain information about the spiking activity, reflecting the output from the neurons surrounding the recording location. The low frequency components of the signal, referred to as the local field potential (LFP), have been traditionally thought to provide information about the synaptic inputs that impinge on the large dendritic trees of various neurons. In this review, we discuss recent computational and experimental studies pointing to a critical role of several active dendritic mechanisms that can influence the genesis and the location-dependent spectro-temporal dynamics of LFPs, spanning different brain regions. We strongly emphasize the need to account for the several fast and slow dendritic events and associated active mechanisms - including gradients in their expression profiles, inter- and intra-cellular spatio-temporal interactions spanning neurons and glia, heterogeneities and degeneracy across scales, neuromodulatory influences, and activitydependent plasticity - towards gaining important insights about the origins of LFP under different behavioral states in health and disease. We provide simple but essential guidelines on how to model LFPs taking into account these dendritic mechanisms, with detailed methodology on how to account for various heterogeneities and electrophysiological properties of neurons and synapses while studying LFPs.

In vivo beta and gamma subthreshold oscillations in rat mitral cells: origin and gating by respiratory dynamics

In mammals, olfactory bulb (OB) dynamics are paced by slow and fast oscillatory rhythms at multiple levels: local field potential, spike discharge, and/or membrane potential oscillations. Interactions between these levels have been well studied for the slow rhythm linked to animal respiration. However, less is known regarding rhythms in the fast beta (10-35 Hz) and gamma (35-100 Hz) frequency ranges, particularly at the membrane potential level. Using a combination of intracellular and extracellular recordings in the OB of freely breathing rats, we show that beta and gamma subthreshold oscillations (STOs) coexist intracellularly and are related to extracellular local field potential (LFP) oscillations in the same frequency range. However, they are differentially affected by changes in cell excitability and by odor stimulation. This leads us to suggest that beta and gamma STOs may rely on distinct mechanisms: gamma STOs would mainly depend on mitral cell intrinsic resonance, while beta STOs could be mainly driven by synaptic activity. In a second study, we find that STO occurrence and timing are constrained by the influence of the slow respiratory rhythm on mitral and tufted cells. First, respiratory-driven excitation seems to favor gamma STOs, while respiratory-driven inhibition favors beta STOs. Second, the respiratory rhythm is needed at the subthreshold level to lock gamma and beta STOs in similar phases as their LFP counterparts and to favor the correlation between STO frequency and spike discharge. Overall, this study helps us to understand how the interaction between slow and fast rhythms at all levels of OB dynamics shapes its functional output. NEW & NOTEWORTHY In the mammalian olfactory bulb of a freely breathing anesthetized rat, we show that both beta and gamma membrane potential fast oscillation ranges exist in the same mitral and tufted (M/T) cell. Importantly, our results suggest they have different origins and that their interaction with the slow subthreshold oscillation (respiratory rhythm) is a key mechanism to organize their dynamics, favoring their functional implication in olfactory bulb information processing.

Two-Photon Na+ Imaging Reports Somatically Evoked Action Potentials in Rat Olfactory Bulb Mitral and Granule Cell Neurites

Dendrodendritic synaptic interactions are a hallmark of neuronal processing in the vertebrate olfactory bulb. Many classes of olfactory bulb neurons including the principal mitral cells (MCs) and the axonless granule cells (GCs) dispose of highly efficient propagation of action potentials (AP) within their dendrites, from where they can release transmitter onto each other. So far, backpropagation in GC dendrites has been investigated indirectly via Ca²⁺ imaging. Here, we used two-photon Na⁺ imaging to directly report opening of voltage-gated sodium channels due to AP propagation in both cell types. To this end, neurons in acute slices from juvenile rat bulbs were filled with 1 mM SBFI via whole-cell patch-clamp. Calibration of SBFI signals revealed that a change in fluorescence ΔF/F by 10% corresponded to a Δ[Na⁺]_i of ∼22 mM. We then imaged proximal axon segments of MCs during somatically evoked APs (sAP). While single sAPs were detectable in ∼50% of axons, trains of 20 sAPs at 50 Hz always resulted in substantial ΔF/F of ∼15% (∼33 mM Δ[Na⁺]_i). ΔF/F was significantly larger for 80 Hz vs. 50 Hz trains, and decayed with half-durations τ_1/2 ∼0.6 s for both frequencies. In MC lateral dendrites, AP trains yielded small ΔF/F of ∼3% (∼7 mM Δ[Na⁺]_i). In GC apical dendrites and adjacent spines, single sAPs were not detectable. Trains resulted in an average dendritic ΔF/F of 7% (16 mM Δ[Na⁺]_i) with τ_1/2 ∼1 s, similar for 50 and 80 Hz. Na⁺ transients were indistinguishable between large GC spines and their adjacent dendrites. Cell-wise analysis revealed two classes of GCs with the first showing a decrease in ΔF/F along the dendrite with distance from the soma and the second an increase. These classes clustered with morphological parameters. Simulations of Δ[Na⁺]_i replicated these behaviors via negative and positive gradients in Na⁺ current density, assuming faithful AP backpropagation. Such specializations of dendritic excitability might confer specific temporal processing capabilities to bulbar principal cell-GC subnetworks. In conclusion, we show that Na⁺ imaging provides a valuable tool for characterizing AP invasion of MC axons and GC dendrites and spines.

Control of Mitral/Tufted Cell Output by Selective Inhibition among Olfactory Bulb Glomeruli

Inhibition is fundamental to information processing by neural circuits. In the olfactory bulb (OB), glomeruli are the functional units for odor information coding, but inhibition among glomeruli is poorly characterized. We used two-photon calcium imaging in anesthetized and awake mice to visualize both odorant-evoked excitation and suppression in OB output neurons (mitral and tufted, MT cells). MT cell response polarity mapped uniformly to discrete OB glomeruli, allowing us to analyze how inhibition shapes OB output relative to the glomerular map. Odorants elicited unique patterns of suppression in only a subset of glomeruli in which such suppression could be detected, and excited and suppressed glomeruli were spatially intermingled. Binary mixture experiments revealed that interglomerular inhibition could suppress excitatory mitral cell responses to odorants. These results reveal that inhibitory OB circuits nonlinearly transform odor representations and support a model of selective and nonrandom inhibition among glomerular ensembles.

Biophysical constraints on lateral inhibition in the olfactory bulb

The mitral cells (MCs) of the mammalian olfactory bulb (OB) constitute one of two populations of principal neurons (along with middle/deep tufted cells) that integrate afferent olfactory information with top-down inputs and intrinsic learning and deliver output to downstream olfactory areas. MC activity is regulated in part by inhibition from granule cells, which form reciprocal synapses with MCs along the extents of their lateral dendrites. However, with MC lateral dendrites reaching over 1.5 mm in length in rats, the roles of distal inhibitory synapses pose a quandary. Here, we systematically vary the properties of a MC model to assess the capacity of inhibitory synaptic inputs on lateral dendrites to influence afferent information flow through MCs. Simulations using passivized models with varying dendritic morphologies and synaptic properties demonstrated that, even with unrealistically favorable parameters, passive propagation fails to convey effective inhibitory signals to the soma from distal sources. Additional simulations using an active model exhibiting action potentials, subthreshold oscillations, and a dendritic morphology closely matched to experimental values further confirmed that distal synaptic inputs along the lateral dendrite could not exert physiologically relevant effects on MC spike timing at the soma. Larger synaptic conductances representative of multiple simultaneous inputs were not sufficient to compensate for the decline in signal with distance. Reciprocal synapses on distal MC lateral dendrites may instead serve to maintain a common fast oscillatory clock across the OB by delaying spike propagation within the lateral dendrites themselves.

An arterially perfused nose-olfactory bulb preparation of the rat

A main feature of the mammalian olfactory bulb network is the presence of various rhythmic activities, in particular, gamma, beta, and theta oscillations, with the latter coupled to the respiratory rhythm. Interactions between those oscillations as well as the spatial distribution of network activation are likely to determine olfactory coding. Here, we describe a novel semi-intact perfused nose-olfactory bulb-brain stem preparation in rats with both a preserved olfactory epithelium and brain stem, which could be particularly suitable for the study of oscillatory activity and spatial odor mapping within the olfactory bulb, in particular, in hitherto inaccessible locations. In the perfused olfactory bulb, we observed robust spontaneous oscillations, mostly in the theta range. Odor application resulted in an increase in oscillatory power in higher frequency ranges, stimulus-locked local field potentials, and excitation or inhibition of individual bulbar neurons, similar to odor responses reported from in vivo recordings. Thus our method constitutes the first viable in situ preparation of a mammalian system that uses airborne odor stimuli and preserves these characteristic features of odor processing. This preparation will allow the use of highly invasive experimental procedures and the application of techniques such as patch-clamp recording, high-resolution imaging, and optogenetics within the entire olfactory bulb.

Distributed organization of a brain microcircuit analyzed by three-dimensional modeling: the olfactory bulb

The functional consequences of the laminar organization observed in cortical systems cannot be easily studied using standard experimental techniques, abstract theoretical representations, or dimensionally reduced models built from scratch. To solve this problem we have developed a full implementation of an olfactory bulb microcircuit using realistic three-dimensional (3D) inputs, cell morphologies, and network connectivity. The results provide new insights into the relations between the functional properties of individual cells and the networks in which they are embedded. To our knowledge, this is the first model of the mitral-granule cell network to include a realistic representation of the experimentally-recorded complex spatial patterns elicited in the glomerular layer (GL) by natural odor stimulation. Although the olfactory bulb, due to its organization, has unique advantages with respect to other brain systems, the method is completely general, and can be integrated with more general approaches to other systems. The model makes experimentally testable predictions on distributed processing and on the differential backpropagation of somatic action potentials in each lateral dendrite following odor learning, providing a powerful 3D framework for investigating the functions of brain microcircuits.

Dissecting local circuits: parvalbumin interneurons underlie broad feedback control of olfactory bulb output

In the mouse olfactory bulb, information from sensory neurons is extensively processed by local interneurons before being transmitted to the olfactory cortex by mitral and tufted (M/T) cells. The precise function of these local networks remains elusive because of the vast heterogeneity of interneurons, their diverse physiological properties, and their complex synaptic connectivity. Here we identified the parvalbumin interneurons (PVNs) as a prominent component of the M/T presynaptic landscape by using an improved rabies-based transsynaptic tracing method for local circuits. In vivo two-photon-targeted patch recording revealed that PVNs have exceptionally broad olfactory receptive fields and exhibit largely excitatory and persistent odor responses. Transsynaptic tracing indicated that PVNs receive direct input from widely distributed M/T cells. Both the anatomical and functional extent of this M/T→PVN→M/T circuit contrasts with the narrowly confined M/T→granule cell→M/T circuit, suggesting that olfactory information is processed by multiple local circuits operating at distinct spatial scales.

Sparse distributed representation of odors in a large-scale olfactory bulb circuit

In the olfactory bulb, lateral inhibition mediated by granule cells has been suggested to modulate the timing of mitral cell firing, thereby shaping the representation of input odorants. Current experimental techniques, however, do not enable a clear study of how the mitral-granule cell network sculpts odor inputs to represent odor information spatially and temporally. To address this critical step in the neural basis of odor recognition, we built a biophysical network model of mitral and granule cells, corresponding to 1/100th of the real system in the rat, and used direct experimental imaging data of glomeruli activated by various odors. The model allows the systematic investigation and generation of testable hypotheses of the functional mechanisms underlying odor representation in the olfactory bulb circuit. Specifically, we demonstrate that lateral inhibition emerges within the olfactory bulb network through recurrent dendrodendritic synapses when constrained by a range of balanced excitatory and inhibitory conductances. We find that the spatio-temporal dynamics of lateral inhibition plays a critical role in building the glomerular-related cell clusters observed in experiments, through the modulation of synaptic weights during odor training. Lateral inhibition also mediates the development of sparse and synchronized spiking patterns of mitral cells related to odor inputs within the network, with the frequency of these synchronized spiking patterns also modulated by the sniff cycle.

In the paper we address the role of lateral inhibition in a neuronal network. It is an essential and widespread mechanism of neural processing that has been demonstrated in many brain systems. A key finding that would reveal how and to what extent it can modulate input signals and give rise to some form of perception would involve network-wide recording of individual cells during in vivo behavioral experiments. While this problem has been intensely investigated, it is beyond current methods to record from a reasonable set of cells experimentally to decipher the emergent properties and behavior of the network, leaving the underlying computational and functional roles of lateral inhibition still poorly understood. We addressed this problem using a large-scale model of the olfactory bulb. The model demonstrates how lateral inhibition modulates the evolving dynamics of the olfactory bulb network, generating mitral and granule cell responses that account for critical experimental findings. It also suggests how odor identity can be represented by a combination of temporal and spatial patterns of mitral cell activity, with both feedforward excitation and lateral inhibition via dendrodendritic synapses as the underlying mechanisms facilitating network self-organization and the emergence of synchronized oscillations.

Regulation of spike timing-dependent plasticity of olfactory inputs in mitral cells in the rat olfactory bulb

The recent history of activity input onto granule cells (GCs) in the main olfactory bulb can affect the strength of lateral inhibition, which functions to generate contrast enhancement. However, at the plasticity level, it is unknown whether and how the prior modification of lateral inhibition modulates the subsequent induction of long-lasting changes of the excitatory olfactory nerve (ON) inputs to mitral cells (MCs). Here we found that the repetitive stimulation of two distinct excitatory inputs to the GCs induced a persistent modification of lateral inhibition in MCs in opposing directions. This bidirectional modification of inhibitory inputs differentially regulated the subsequent synaptic plasticity of the excitatory ON inputs to the MCs, which was induced by the repetitive pairing of excitatory postsynaptic potentials (EPSPs) with postsynaptic bursts. The regulation of spike timing-dependent plasticity (STDP) was achieved by the regulation of the inter-spike-interval (ISI) of the postsynaptic bursts. This novel form of inhibition-dependent regulation of plasticity may contribute to the encoding or processing of olfactory information in the olfactory bulb.

Distinct intrinsic membrane properties determine differential information processing between main and accessory olfactory bulb mitral cells

Most mammals rely on semiochemicals, such as pheromones, to mediate their social interactions. Recent studies found that semiochemicals are perceived by at least two distinct chemosensory systems: the main and accessory olfactory systems, which share many molecular, cellular, and anatomical features. Nevertheless, the division of labor between these systems remained unclear. Previously we suggested that the two olfactory systems differ in the way they process sensory information. In this study we found that mitral cells of the main and accessory olfactory bulbs, the first brain stations of both systems, display markedly different passive and active intrinsic properties which permit distinct types of information processing. Moreover, we found that accessory olfactory bulb mitral cells are divided into three neuronal sub-populations with distinct firing properties. These neuronal sub-populations can be integrated in a simulated neuronal network that neglects episodic stimuli while amplifying reaction to long-lasting signals.

Timescale-dependent shaping of correlation by olfactory bulb lateral inhibition

Neurons respond to sensory stimuli by altering the rate and temporal pattern of action potentials. These spike trains both encode and propagate information that guides behavior. Local inhibitory networks can affect the information encoded and propagated by neurons by altering correlations between different spike trains. Correlations introduce redundancy that can reduce encoding but also facilitate propagation of activity to downstream targets. Given this trade-off, how can networks maximize both encoding and propagation efficacy? Here, we examine this problem by measuring the effects of olfactory bulb inhibition on the pairwise statistics of mitral cell spiking. We evoked spiking activity in the olfactory bulb in vitro and measured how lateral inhibition shapes correlations across timescales. We show that inhibitory circuits simultaneously increase fast correlation (i.e., synchrony increases) and decrease slow correlation (i.e., firing rates become less similar). Further, we use computational models to show the benefits of fast correlation/slow decorrelation in the context of odor coding. Olfactory bulb inhibition enhances population-level discrimination of similar inputs, while improving propagation of mitral cell activity to cortex. Our findings represent a targeted strategy by which a network can optimize the correlation structure of its output in a dynamic, activity-dependent manner. This trade-off is not specific to the olfactory system, but rather our work highlights mechanisms by which neurons can simultaneously accomplish multiple, and sometimes competing, aspects of sensory processing.

Correlated firing in tufted cells of mouse olfactory bulb

Temporally correlated spike discharges are proposed to be important for the coding of olfactory stimuli. In the olfactory bulb, correlated spiking is known in two classes of output neurons, the mitral cells and external tufted cells. We studied a third major class of bulb output neurons, the middle tufted cells, analyzing their bursting and spike timing correlations, and their relation to mitral cells. Using patch-clamp and fluorescent tracing, we recorded spontaneous spiking from tufted-tufted or mitral-tufted cell pairs with visualized dendritic projections in mouse olfactory bulb slices. We found peaks in spike cross-correlograms indicating correlated activity on both fast (peak width 1-50 ms) and slow (peak width>50 ms) time scales, only in pairs with convergent glomerular projections. Coupling appeared tighter in tufted-tufted pairs, which showed correlated firing patterns and smaller mean width and lag of narrow peaks. Some narrow peaks resolved into 2-3 sub-peaks (width 1-12 ms), indicating multiple modes of fast correlation. Slow correlations were related to bursting activity, while fast correlations were independent of slow correlations, occurring in both bursting and non-bursting cells. The AMPA receptor antagonist NBQX (20 microM) failed to abolish broad or narrow peaks in either tufted-tufted or mitral-tufted pairs, and changes of peak height and width in NBQX were not significantly different from spontaneous drift. Thus, AMPA-receptors are not required for fast and slow spike correlations. Electrical coupling was observed in all convergent tufted-tufted and mitral-tufted pairs tested, suggesting a potential role for gap junctions in concerted firing. Glomerulus-specific correlation of spiking offers a useful mechanism for binding the output signals of diverse neurons processing and transmitting different sensory information encoded by common olfactory receptors.

An intrinsic vasopressin system in the olfactory bulb is involved in social recognition

Many peptides, when released as chemical messengers within the brain, have powerful influences on complex behaviours. Most strikingly, vasopressin and oxytocin, once thought of as circulating hormones whose actions were confined to peripheral organs, are now known to be released in the brain where they play fundamentally important roles in social behaviours1. In humans, disruptions of these peptide systems have been linked to several neurobehavioural disorders, including Prader-Willi syndrome, affective disorders, and obsessive-compulsive disorder, and polymorphisms of the vasopressin V1a receptor have been linked to autism2,3. Here we report that the rat olfactory bulb contains a large population of interneurones which express vasopressin, that blocking the actions of vasopressin in the olfactory bulb impairs the social recognition abilities of rats, and that vasopressin agonists and antagonists can modulate the processing of information by olfactory bulb neurones. The findings indicate that social information is processed in part by a vasopressin system intrinsic to the olfactory system.

Early transformations in odor representation

Sensory representations are repeatedly transformed by neural computations that determine which of their attributes can be effectively processed at each stage. Whereas some early computations are common across multiple sensory systems, they can utilize dissimilar underlying mechanisms depending on the properties of each modality. Recent work in the olfactory bulb has substantially clarified the neural algorithms underlying early odor processing. The high-dimensionality of odor space strictly limits the utility of topographical representations, forcing similarity-dependent computations such as decorrelation to employ unusual neural algorithms. The distinct architectures and properties of the two prominent computational layers in the olfactory bulb suggest that the bulb is directly comparable not only to the retina but also to primary visual cortex.

Functional roles of distributed synaptic clusters in the mitral-granule cell network of the olfactory bulb

Odors are encoded in spatio-temporal patterns within the olfactory bulb, but the mechanisms of odor recognition and discrimination are poorly understood. It is reasonable to postulate that the olfactory code is sculpted by lateral and feedforward inhibition mediated by granule cells onto the mitral cells. Recent viral tracing and physiological studies revealed patterns of distributed granule cell synaptic clusters that provided additional clues to the possible mechanisms at the network level. The emerging properties and functional roles of these patterns, however, are unknown. Here, using a realistic model of 5 mitral and 100 granule cells we show how their synaptic network can dynamically self-organize and interact through an activity-dependent dendrodendritic mechanism. The results suggest that the patterns of distributed mitral-granule cell connectivity may represent the most recent history of odor inputs, and may contribute to the basic processes underlying mixture perception and odor qualities. The model predicts how and why the dynamical interactions between the active mitral cells through the granule cell synaptic clusters can account for a variety of puzzling behavioral results on odor mixtures and on the emergence of synthetic or analytic perception.

Respiration-gated formation of gamma and beta neural assemblies in the mammalian olfactory bulb

A growing body of data suggests that information coding can be achieved not only by varying neuronal firing rate, but also by varying spike timing relative to network oscillations. In the olfactory bulb (OB) of a freely breathing anaesthetized mammal, odorant stimulation induces prominent oscillatory local field potential (LFP) activity in the beta (10-35 Hz) and gamma (40-80 Hz) ranges, which alternate during a respiratory cycle. At the same time, mitral/tufted (M/T) cells display respiration-modulated spiking patterns. Using simultaneous recordings of M/T unitary activities and LFP activity, we conducted an analysis of the temporal relationships between M/T cell spiking activity and both OB beta and gamma oscillations. We observed that M/T cells display a respiratory pattern that pre-tunes instantaneous frequencies to a gamma or beta regime. Consequently, M/T cell spikes become phase-locked to either gamma or beta LFP oscillations according to their frequency range and respiratory pattern. Our results suggest that slow respiratory dynamics pre-tune M/T cells to a preferential fast rhythm (beta or gamma) such that a spike-LFP coupling might occur when units and oscillation frequencies are in a compatible range. This double-coupling process might define two complementary beta- and gamma-neuronal assemblies along the course of a respiratory cycle.

Odor-induced persistent discharge of mitral cells in the mouse olfactory bulb

Short-term retention of sensory information in the form of persistent activity of central neurons plays a key role in transforming a brief sensory stimulation into longer-lasting brain responses. The olfactory system uses this transformation for various functional purposes, but the underlying neuronal mechanisms remain elusive. Here, we recorded odor-evoked, single-unit spike responses of mitral and tufted (M/T) cells in the mouse olfactory bulb (OB) under urethane anesthesia and examined the neuronal mechanisms of the persistent discharge (PD) of M/T cells that outlasts the odor stimulus for tens of seconds. The properties of the persistent afterdischarge that occurred after odor stimulation were distinct from those of odor-induced immediate spike responses in terms of the magnitude, odorant specificity, and odorant concentration-response relationship. This suggests that neuronal mechanisms other than prolonged input from olfactory sensory neurons are involved in generating these afterdischarges. Metabotropic glutamate receptor 1 (mGluR1) is expressed in the dendrites of M/T cells and is thought to participate in intraglomerular interactions among M/T cells. In OBs lacking mGluR1, or treated locally with an mGluR1-selective antagonist, the duration of the odor-induced spike responses was significantly lower than that in control OBs, indicating that mGluR1 within the bulbar neuronal circuits participates in the PD generation. These results suggest that neuronal circuits in the OB can actively prolong the odor-induced spike activity of bulbar output neurons and thus transform a brief odor input into longer-lasting activity in the central olfactory system.

Two-photon imaging of capillary blood flow in olfactory bulb glomeruli

Two-photon laser scanning microscopy (TPLSM) is an efficient tool to study cerebral blood flow (CBF) and cellular activity in depth in the brain. We describe here the advantages and weaknesses of the olfactory bulb as a model to study neurovascular coupling using TPLSM. By combining intra- and extracellular recordings, TPLSM of CBF in individual capillaries, local application of drugs, we show that odor triggers odorant-specific and concentration-dependent increases in CBF. We also demonstrate that activation of neurons is required to trigger blood flow responses.

New Angles on Neuronal Dendrites In Vivo

Imaging technologies are well suited to study neuronal dendrites, which are key elements for synaptic integration in the CNS. Dendrites are, however, frequently oriented perpendicular to tissue surfaces, impeding in vivo imaging approaches. Here we introduce novel laser-scanning modes for two-photon microscopy that enable in vivo imaging of spatiotemporal activity patterns in dendrites. First, we developed a method to image planes arbitrarily oriented in 3D, which proved particularly beneficial for calcium imaging of parallel fibers and Purkinje cell dendrites in rat cerebellar cortex. Second, we applied free linescans—either through multiple dendrites or along a single vertically oriented dendrite—to reveal fast dendritic calcium dynamics in neocortical pyramidal neurons. Finally, we invented a ribbon-type 3D scanning method for imaging user-defined convoluted planes enabling simultaneous measurements of calcium signals along multiple apical dendrites. These novel scanning modes will facilitate optical probing of dendritic function in vivo.

Lateral dendritic shunt inhibition can regularize mitral cell spike patterning

Mitral cells, the principal output neurons of the olfactory bulb, receive direct synaptic activation from primary sensory neurons. Shunting inhibitory inputs delivered by granule cell interneurons onto mitral cell lateral dendrites, while poorly positioned to prevent spike initiation, are believed to influence spike timing and underlie coordinated field potential oscillations. We investigated this phenomenon in a reduced compartmental mitral cell model suitable for incorporation into network simulations. Lateral dendritic shunt conductances delayed spiking to a degree dependent on both their electrotonic distance and phase of onset. Moreover, when the afferent activation of mitral cells was loosely coordinated in time, recurrent inhibition significantly narrowed the distribution of mitral cell spike times, illustrating a tendency towards coordinated synchronous activity. However, if mitral cell activity was initially disorganized, recurrent inhibition actually increased the variance in spike timing. This result suggests an essential role for early mechanisms of temporal coordination in olfaction, such as sniffing and the initial synchronization of mitral cell intrinsic oscillations by periglomerular cell-mediated inhibition.

Dendritic action potentials connect distributed dendrodendritic microcircuits

Lateral inhibition of cells surrounding an excited area is a key property of sensory systems, sharpening the preferential tuning of individual cells in the presence of closely related input signals. In the olfactory pathway, a dendrodendritic synaptic microcircuit between mitral and granule cells in the olfactory bulb has been proposed to mediate this type of interaction through granule cell inhibition of surrounding mitral cells. However, it is becoming evident that odor inputs result in broad activation of the olfactory bulb with interactions that go beyond neighboring cells. Using a realistic modeling approach we show how backpropagating action potentials in the long lateral dendrites of mitral cells, together with granule cell actions on mitral cells within narrow columns forming glomerular units, can provide a mechanism to activate strong local inhibition between arbitrarily distant mitral cells. The simulations predict a new role for the dendrodendritic synapses in the multicolumnar organization of the granule cells. This new paradigm gives insight into the functional significance of the patterns of connectivity revealed by recent viral tracing studies. Together they suggest a functional wiring of the olfactory bulb that could greatly expand the computational roles of the mitral-granule cell network.

In vivo simultaneous tracing and Ca(2+) imaging of local neuronal circuits

A central question about the brain is how information is processed by large populations of neurons embedded in intricate local networks. Answering this question requires not only monitoring functional dynamics of many neurons simultaneously, but also interpreting such activity patterns in the context of neuronal circuitry. Here, we introduce a versatile approach for loading Ca(2+) indicators in vivo by local electroporation. With this method, Ca(2+) imaging can be performed both at neuron population level and with exquisite subcellular resolution down to dendritic spines and axon boutons. This enabled mitral cell odor-evoked ensemble activity to be analyzed simultaneously with revealing their specific connectivity to different glomeruli. Colabeling of Purkinje cell dendrites and intersecting parallel fibers allowed Ca(2+) imaging of both presynaptic boutons and postsynaptic dendrites. This approach thus provides an unprecedented capability for in vivo visualizing active cell ensembles and tracing their underlying local neuronal circuits.

The olfactory granule cell: from classical enigma to central role in olfactory processing

The granule cell of the olfactory bulb was first described by Golgi in 1875 and Cajal and his contemporaries in the 1890s as an enigmatic cell without an axon, whose status as a nerve cell was questionable. Insight into its functions began in the 1960s with evidence that it acted as an interneuron to mediate powerful inhibition of mitral cells. The circuit was found to involve dendrodendritic synapses for activation by mitral cell lateral dendrites of the granule cell dendritic spines and inhibition of the same and neighboring mitral cell lateral dendrites. Subsequent studies established the roles of glutamatergic receptors and GABAergic receptors in this circuit. The lateral inhibition is believed to be involved in contrast enhancement between mitral cells responding to different odor molecules. Current studies are analysing how the lateral inhibition can be mediated over arbitrary distances between columns of granule cells through action potential propagation in the mitral cell secondary dendrites. Among other important properties, granule cells undergo neurogenesis from precursor cells throughout adult life. This originally enigmatic cell thus appears to play a critical role in olfactory processing.

Dendritic excitability and calcium signalling in the mitral cell distal glomerular tuft

The processing of odour information starts at the level of the olfactory glomerulus, where the mitral cell distal dendritic tuft not only receives olfactory nerve sensory input but also generates dendrodendritic output to form complicated glomerular synaptic circuits. Analysing the membrane properties and calcium signalling mechanisms in these tiny dendritic branches is crucial for understanding how the glomerular tuft transmits and processes olfactory signals. With the use of two-photon Ca2+ imaging in rat olfactory bulb slices, we found that these distal dendritic branches displayed a significantly larger Ca2+ signal than the soma and primary dendrite trunk. A back-propagating action potential was able to trigger a Ca2+ increase throughout the entire glomerular tuft, indicative of the presence of voltage-gated Ca2+ conductances in all branches at different levels of ramification. In response to a train of action potentials evoked at 60 Hz from the soma, the tuft Ca2+ signal increased linearly with the number of action potentials, suggesting that these glomerular branches were able to support repetitive penetration of Na+ action potentials. When a strong olfactory nerve excitatory input was paired with an inhibition from mitral cell basal dendrites, a small spike-like fast prepotential was revealed at both the soma and distal primary dendrite trunk. Corresponding to this fast prepotential was a Ca2+ increase confined locally within the glomerular tuft. In summary, the mitral cell distal dendritic tuft possesses both Na+ and Ca2+ voltage-dependent conductances which can mediate glomerular Ca2+ responsiveness critical for dendrodendritic output and synaptic plasticity.

Non-topographical contrast enhancement in the olfactory bulb

Background

Contrast enhancement within primary stimulus representations is a common feature of sensory systems that regulates the discrimination of similar stimuli. Whereas most sensory stimulus features can be mapped onto one or two dimensions of quality or location (e.g., frequency or retinotopy), the analogous similarities among odor stimuli are distributed high-dimensionally, necessarily yielding a chemotopically fragmented map upon the surface of the olfactory bulb. While olfactory contrast enhancement has been attributed to decremental lateral inhibitory processes among olfactory bulb projection neurons modeled after those in the retina, the two-dimensional topology of this mechanism is intrinsically incapable of mediating effective contrast enhancement on such fragmented maps. Consequently, current theories are unable to explain the existence of olfactory contrast enhancement.

Results

We describe a novel neural circuit mechanism, non-topographical contrast enhancement (NTCE), which enables contrast enhancement among high-dimensional odor representations exhibiting unpredictable patterns of similarity. The NTCE algorithm relies solely on local intraglomerular computations and broad feedback inhibition, and is consistent with known properties of the olfactory bulb input layer. Unlike mechanisms based upon lateral projections, NTCE does not require a built-in foreknowledge of the similarities in molecular receptive ranges expressed by different olfactory bulb glomeruli, and is independent of the physical location of glomeruli within the olfactory bulb.

Conclusion

Non-topographical contrast enhancement demonstrates how intrinsically high-dimensional sensory data can be represented and processed within a physically two-dimensional neural cortex while retaining the capacity to represent stimulus similarity. In a biophysically constrained computational model of the olfactory bulb, NTCE successfully mediates contrast enhancement among odorant representations in the natural, high-dimensional similarity space defined by the olfactory receptor complement and underlies the concentration-independence of odor quality representations.

Olfactory nerve stimulation-induced calcium signaling in the mitral cell distal dendritic tuft

Olfactory receptor neuron axons form the olfactory nerve (ON) and project to the glomerular layer of the olfactory bulb, where they form excitatory synapses with terminal arborizations of the mitral cell (MC) tufted primary dendrite. Clusters of MC dendritic tufts define olfactory glomeruli, where they involve in complex synaptic interactions. The computational function of these cellular interactions is not clear. We used patch-clamp electrophysiology combined with whole field or two-photon Ca2+ imaging to study ON stimulation-induced Ca2+ signaling at the level of individual terminal branches of the MC primary dendrite in mice. ON-evoked subthreshold excitatory postsnaptic potentials induced Ca2+ transients in the MC tuft dendrites that were spatially inhomogeneous, exhibiting discrete "hot spots." In contrast, Ca2+ transients induced by backpropagating action potentials occurred throughout the dendritic tuft, being larger in the thin terminal dendrites than in the base of the tuft. Single ON stimulation-induced Ca2+ transients were depressed by the NMDA receptor antagonist D-aminophosphonovaleric acid (D-APV), increased with increasing stimulation intensity, and typically showed a prolonged rising phase. The synaptically induced Ca2+ signals reflect, at least in part, dendrodendritic interactions that support intraglomerular coupling of MCs and generation of an output that is common to all MCs associated with one glomerulus.

Computation in the olfactory system

Computational models are increasingly essential to systems neuroscience. Models serve as proofs of concept, tests of sufficiency, and as quantitative embodiments of working hypotheses and are important tools for understanding and interpreting complex data sets. In the olfactory system, models have played a particularly prominent role in framing contemporary theories and presenting novel hypotheses, a role that will only grow as the complexity and intricacy of experimental data continue to increase. This review will attempt to provide a comprehensive, functional overview of computational ideas in olfaction and outline a computational framework for olfactory processing based on the insights provided by these diverse models and their supporting data.

Synaptic integration in dendritic trees

Most neurons have elaborate dendritic trees that receive tens of thousands of synaptic inputs. Because postsynaptic responses to individual synaptic events are usually small and transient, the integration of many synaptic responses is needed to depolarize most neurons to action potential threshold. Over the past decade, advances in electrical and optical recording techniques have led to new insights into how synaptic responses propagate and interact within dendritic trees. In addition to their passive electrical and morphological properties, dendrites express active conductances that shape individual synaptic responses and influence synaptic integration locally within dendrites. Dendritic voltage-gated Na(+) and Ca(2+) channels support action potential backpropagation into the dendritic tree and local initiation of dendritic spikes, whereas K(+) conductances act to dampen dendritic excitability. While all dendrites investigated to date express active conductances, different neuronal types show specific patterns of dendritic channel expression leading to cell-specific differences in the way synaptic responses are integrated within dendritic trees. This review explores the way active and passive dendritic properties shape synaptic responses in the dendrites of central neurons, and emphasizes their role in synaptic integration.

Applications of multi-variate analysis of variance (MANOVA) to multi-electrode array electrophysiology data

We have developed an adaptation of multi-variate analysis of variance (MANOVA) to analyze statistically both local and global patterns of multi-electrode array (MEA) electrophysiology data where the activities of many (typically >100) neurons have been recorded simultaneously. Whereas simple application of standard MANOVA techniques prohibits extraction of useful information in this kind of data, our new approach, MEANOVA (=MEA+MANOVA), allows a more useful and powerful approach to analyze such complex neurophysiological data. The MEANOVA test enables the detection of the "hot-spots" in the MEA data and has been validated using recordings from the rat olfactory bulb. To further validate the power of this approach, we have also applied the MEANOVA test to data obtained from a simple computational network model. This MEANOVA software and other useful statistical methods for MEA data can be downloaded from http://www.sussex.ac.uk/Users/pmh20

The role of distal dendritic gap junctions in synchronization of mitral cell axonal output

One of the first and most important stages of odor processing occurs in the glomerular units of the olfactory bulb and most likely involves mitral cell synchronization. Using a detailed model constrained by a number of experimental findings, we show how the intercellular coupling mediated by intraglomerular gap junctions (GJs) in the tuft dendrites could play a major role in sychronization of mitral cell action potential output in spite of their distal dendritic location. The model suggests that the high input resistance and active properties of the fine tuft dendrites are instrumental in generating local spike synchronization and an efficient forward and backpropagation of action potentials between the tuft and the soma. The model also gives insight into the physiological significance of long primary dendrites in mitral cells, and provides evidence against the use of reduced single compartmental models to investigate network properties of cortical pyramidal neurons.

Nonlinear microscopy: new techniques and applications

Nonlinear microscopy, a general term that embraces any microscopy technique based on nonlinear optics, is further establishing itself as an important tool in neurobiology. Recent advances in labels, labeling techniques, and the use of native or genetically encoded contrast agents have bolstered the capacity of nonlinear microscopes to image the structure and function of not just single cells but of entire networks of cells. Along with novel strategies to image over exceptionally long durations and with increased depth penetration in living brains, these advances are opening new opportunities in neurobiology that were previously unavailable.

Information processing in the mammalian olfactory system

Recently, modern neuroscience has made considerable progress in understanding how the brain perceives, discriminates, and recognizes odorant molecules. This growing knowledge took over when the sense of smell was no longer considered only as a matter for poetry or the perfume industry. Over the last decades, chemical senses captured the attention of scientists who started to investigate the different stages of olfactory pathways. Distinct fields such as genetic, biochemistry, cellular biology, neurophysiology, and behavior have contributed to provide a picture of how odor information is processed in the olfactory system as it moves from the periphery to higher areas of the brain. So far, the combination of these approaches has been most effective at the cellular level, but there are already signs, and even greater hope, that the same is gradually happening at the systems level. This review summarizes the current ideas concerning the cellular mechanisms and organizational strategies used by the olfactory system to process olfactory information. We present findings that exemplified the high degree of olfactory plasticity, with special emphasis on the first central relay of the olfactory system. Recent observations supporting the necessity of such plasticity for adult brain functions are also discussed. Due to space constraints, this review focuses mainly on the olfactory systems of vertebrates, and primarily those of mammals.

Backpropagating action potentials in neurones: measurement, mechanisms and potential functions

Here we review some properties and functions of backpropagating action potentials in the dendrites of mammalian CNS neurones. We focus on three main aspects: firstly the current techniques available for measuring backpropagating action potentials, secondly the morphological parameters and voltage gated ion channels that determine action potential backpropagation and thirdly the potential functions of backpropagating action potentials in real neuronal networks.

Pre- and postsynaptic GABAA receptors at reciprocal dendrodendritic synapses in the olfactory bulb

Presynaptic ionotropic receptors are important regulators of synaptic function; however, little is known about their organization in the presynaptic membrane. We show here a different spatial organization of presynaptic and postsynaptic GABA(A) receptors at reciprocal dendrodendritic synapses between mitral and granule cells in the rat olfactory bulb. Using postembedding electron microscopy, we have found that mitral cell dendrites express GABA(A) receptors at postsynaptic specializations of symmetric (GABAergic) synapses, as well as at presynaptic sites of asymmetric (glutamatergic) synapses. Analysis of the subsynaptic distribution of gold particles revealed that in symmetric synapses GABA(A) receptors are distributed along the entire postsynaptic membrane, whereas in asymmetric synapses they are concentrated at the edge of the presynaptic specialization. To assess the specificity of immunogold labelling, we analysed the olfactory bulbs of mutant mice lacking the alpha1 subunit of GABA(A) receptors. We found that in wild-type mice alpha1 subunit immunoreactivity was similar to that observed in rats, whereas in knockout mice the immunolabelling was abolished. These results indicate that in mitral cell dendrites GABA(A) receptors are distributed in a perisynaptic domain that surrounds the presynaptic specialization. Such presynaptic receptors may be activated by spillover of GABA from adjacent inhibitory synapses and modulate glutamate release, thereby providing a novel mechanism regulating dendrodendritic inhibition in the olfactory bulb.

Analytical solutions of the Frankenhaeuser-Huxley equations I: minimal model for backpropagation of action potentials in sparsely excitable dendrites

Hodgkin and Huxley's ionic theory of the nerve impulse embodies principles, applicable also to the impulses in vertebrate nerve fibers, as demonstrated by Bernhard Frankenhaeuser and Andrew Huxley 40 years ago. Frankenhaeuser and Huxley reformulated the classical Hodgkin-Huxley equations, in terms of electrodiffusion theory, and computed action potentials specifically for saltatory conduction in myelinated axons. In this paper, we obtain analytical solutions to the most difficult nonlinear partial differential equations in classical neurophysiology. We solve analytically the Frankenhaeuser-Huxley equations pertaining to a model of sparsely excitable, nonlinear dendrites with clusters of transiently activating, TTX-sensitive Na(+) channels, discretely distributed as point sources of inward current along a continuous (non-segmented) leaky cable structure. Each cluster or hot-spot, corresponding to a mesoscopic level description of Na(+) ion channels, includes known cumulative inactivation kinetics observed at the microscopic level. In such a third-order system, the 'recovery' variable is an electrogenic sodium-pump imbedded in the passive membrane, and the system is stabilized by the presence of a large leak conductance mediated by a composite number of ligand-gated channels permeable to monovalent cations Na(+) and K(+). In order to reproduce antidromic propagation and attenuation of action potentials, a nonlinear integral equation must be solved (in the presence of suprathreshold input, and a constant-field equation of electrodiffusion at each hot-spot with membrane gates controlling the flow of current). A perturbative expansion of the non-dimensional membrane potential (Phi) is used to obtain time-dependent analytical solutions, involving a voltage-dependent Na(+) activation (micro) and a state-dependent inactivation (eta) gating variables. It is shown that action potentials attenuate in amplitude in accordance with experimental findings, and that the spatial density distribution of transient Na(+) channels along a long dendrite contributes significantly to their discharge patterns. A major significance of the analytical modeling, in contrast to the computational modeling of backpropagating action potentials, is the provision of a continuous description of the voltage as a function of position, allowing for greater feasibility in developing large-scale biophysical neural networks, without the need for ad hoc computational modeling.

Repeated cocaine administration suppresses HVA-Ca2+ potentials and enhances activity of K+ channels in rat nucleus accumbens neurons

The nucleus accumbens (NAc) is an important forebrain area involved in sensitization, withdrawal effects, and self-administration of cocaine. However, little is known about cocaine-induced alterations in the neuronal excitability and whole cell neuroplasticity in this region that may affect behaviors. Our recent investigations have demonstrated that repeated cocaine administration decreases voltage-sensitive sodium and calcium currents (VSSCs and VSCCs, respectively) in freshly dissociated NAc neurons of rats. In this study, current-clamp recordings were performed in slice preparations to determine the effects of chronic cocaine on evoked Ca(2+) potentials and voltage-sensitive K(+) currents in NAc neurons. Repeated cocaine administration with 3-4 days of withdrawal caused significant alterations in Ca(2+) potentials, including suppression of Ca(2+)-mediated spikes, increase in the intracellular injected current intensity required for generation of Ca(2+) potentials (rheobase), reduced duration of Ca(2+) plateau potentials, and abolishment of secondary Ca(2+) potentials associated with the primary Ca(2+) plateau potential. Application of nickel (Ni(2+)), which blocks low-voltage activated T-type Ca(2+) channels, had no impact on evoked Ca(2+) plateau potentials in NAc neurons, indicating that these Ca(2+) potentials are high-voltage activated (HVA). In addition, repeated cocaine pretreatment also hyperpolarized the resting membrane potential, increased the amplitude of afterhyperpolarization in Ca(2+) spikes, and enhanced the outward rectification observed during membrane depolarization. These findings indicate that repeated cocaine administration not only suppressed HVA-Ca(2+) potentials but also significantly enhanced the activity of various K(+) channels in NAc neurons. They also demonstrate an integrative role of whole cell neuroplasticity during cocaine withdrawal, by which the subthreshold membrane excitability of NAc neurons is significantly decreased.

The control of dendrite development

Dendrite development is an important and unsolved problem in neuroscience. The nervous system is composed of a vast number of neurons with strikingly different morphology. Neurons are highly polarized cells with distinct subcellular compartments, including one or multiple dendritic processes arising from the cell body, and a single, extended axon. Communications between neurons involve synapses formed between axons of the presynaptic neurons and dendrites of the postsynaptic neurons. Extensive studies over the past decade have identified many molecules underlying axonal outgrowth and pathfinding. In contrast, the control of dendrite development is still much less well understood. However, recent progress has begun to shed light on the molecular mechanisms that orchestrate dendrite growth, arborization, and guidance.