Dendritic processing within olfactory bulb circuits

Quantitative morphometry of electrophysiologically identified CA3b interneurons reveals robust local geometry and distinct cell classes

The morphological and electrophysiological diversity of inhibitory cells in hippocampal area CA3 may underlie specific computational roles and is not yet fully elucidated. In particular, interneurons with somata in strata radiatum (R) and lacunosum-moleculare (L-M) receive converging stimulation from the dentate gyrus and entorhinal cortex as well as within CA3. Although these cells express different forms of synaptic plasticity, their axonal trees and connectivity are still largely unknown. We investigated the branching and spatial patterns, plus the membrane and synaptic properties, of rat CA3b R and L-M interneurons digitally reconstructed after intracellular labeling. We found considerable variability within but no difference between the two layers, and no correlation between morphological and biophysical properties. Nevertheless, two cell types were identified based on the number of dendritic bifurcations, with significantly different anatomical and electrophysiological features. Axons generally branched an order of magnitude more than dendrites. However, interneurons on both sides of the R/L-M boundary revealed surprisingly modular axodendritic arborizations with consistently uniform local branch geometry. Both axons and dendrites followed a lamellar organization, and axons displayed a spatial preference toward the fissure. Moreover, only a small fraction of the axonal arbor extended to the outer portion of the invaded volume, and tended to return toward the proximal region. In contrast, dendritic trees demonstrated more limited but isotropic volume occupancy. These results suggest a role of predominantly local feedforward and lateral inhibitory control for both R and L-M interneurons. Such a role may be essential to balance the extensive recurrent excitation of area CA3 underlying hippocampal autoassociative memory function.

Neocortical disynaptic inhibition requires somatodendritic integration in interneurons

In his theory of functional polarity, Ramon y Cajal first identified the soma and dendrites as the principal recipient compartments of a neuron and the axon as its main output structure. Despite notable exceptions in other parts of the nervous system (Schoppa and Urban, 2003; Wässle, 2004; Howard et al., 2005), this route of signal propagation has been shown to underlie the functional properties of most neocortical circuits studied so far. Recent evidence, however, suggests that neocortical excitatory cells may trigger the release of the inhibitory neurotransmitter GABA by directly depolarizing the axon terminals of inhibitory interneurons, thus bypassing their somatodendritic compartments (Ren et al., 2007). By using a combination of optical and electrophysiological approaches, we find that synaptically released glutamate fails to trigger GABA release through a direct action on GABAergic terminals under physiological conditions. Rather, our evidence suggests that glutamate triggers GABA release only after somatodendritic depolarization and action potential generation at GABAergic interneurons. These data indicate that neocortical inhibition is recruited by classical somatodendritic integration rather than direct activation of interneuron axon terminals.

From Pheromones to Behavior

In recent years, considerable progress has been achieved in the comprehension of the profound effects of pheromones on reproductive physiology and behavior. Pheromones have been classified as molecules released by individuals and responsible for the elicitation of specific behavioral expressions in members of the same species. These signaling molecules, often chemically unrelated, are contained in body fluids like urine, sweat, specialized exocrine glands, and mucous secretions of genitals. The standard view of pheromone sensing was based on the assumption that most mammals have two separated olfactory systems with different functional roles: the main olfactory system for recognizing conventional odorant molecules and the vomeronasal system specifically dedicated to the detection of pheromones. However, recent studies have reexamined this traditional interpretation showing that both the main olfactory and the vomeronasal systems are actively involved in pheromonal communication. The current knowledge on the behavioral, physiological, and molecular aspects of pheromone detection in mammals is discussed in this review.

Inhibition shapes sex selectivity in the mouse accessory olfactory bulb

Laterally connected inhibitory circuitry is found throughout the nervous system, including many early sensory processing systems. The extent to which it plays a role in shaping neuronal stimulus selectivity in systems like olfaction, however, which lack a simple two-dimensional representation of their stimulus space, has remained controversial. We examined this issue using an experimental preparation that allowed electrophysiological recording from the accessory olfactory bulb of an anesthetized mouse during the controlled delivery of pheromonal stimuli, in this case derived from the urine of male and female mice. We found that individual neurons were often highly selective for the sex of the urine donor. Examination of both explicitly inhibitory responses, as well as responses to mixtures of male and female urine, revealed that laterally connected inhibition was both prevalent and of large magnitude, particularly for male-selective neurons. Pharmacological manipulation of this inhibition resulted in a shift in many neurons' stimulus selectivities. Finally, we found that a behavioral response (pregnancy block) evoked by the presence of unfamiliar male urine could be suppressed by the addition of female urine to the stimulus, demonstrating that this system displays a behavioral opponency consistent with neural inhibition. Together, these results indicate that laterally connected inhibitory circuitry in the accessory olfactory bulb plays an important role in shaping neural selectivity for natural stimuli.

Excitatory actions of noradrenaline and metabotropic glutamate receptor activation in granule cells of the accessory olfactory bulb

Modulation of dendrodendritic synapses by the noradrenergic system in the accessory olfactory bulb (AOB) plays a key role in the formation of memory in olfactory-mediated behaviors. We have recently shown that noradrenaline (NA) inhibits mitral cells by increasing gamma-aminobutyric acid inhibitory input onto mitral cells in the AOB, suggesting an excitatory action of NA on granule cells (GCs). Here, we show that NA (10 microM) elicits a long-lasting depolarization of GCs. This effect is mediated by activation of alpha(1)-adrenergic receptors as the depolarization is mimicked by phenylephrine (PE, 30 microM) and completely blocked by the alpha(1)-adrenergic receptor antagonist prazosin (300 nM). In addition to this depolarization, application of NA induced the appearance of a slow afterdepolarization (sADP) following a stimulus-elicited train of action potentials. Similarly, the group I metabotropic glutamate receptor (mGluR1) agonist DHPG (10-30 microM) also produced a depolarization of GCs and the appearance of a stimulus-induced sADP. The ionic and voltage dependence and sensitivity to blockers of the sADP suggest that it is mediated by the nonselective cationic conductance I(CAN). Thus the excitatory action resulting from the activation of these receptors could be mediated by a common transduction target. Surprisingly, the excitatory effect of PE on GCs was completely blocked by the mGluR1 antagonist LY367385 (100 microM). Conversely, the effect of DHPG was not antagonized by the alpha(1)-adrenergic receptor antagonist prazosin (300 nM). These results suggest that most of the noradrenergic effect on GCs in the AOB is mediated by potentiation of a basal activity of mGluR1s.

Parallel processing strategies of the primate visual system

Incoming sensory information is sent to the brain along modality-specific channels corresponding to the five senses. Each of these channels further parses the incoming signals into parallel streams to provide a compact, efficient input to the brain. Ultimately, these parallel input signals must be elaborated on and integrated in the cortex to provide a unified and coherent percept. Recent studies in the primate visual cortex have greatly contributed to our understanding of how this goal is accomplished. Multiple strategies including retinal tiling, hierarchical and parallel processing and modularity, defined spatially and by cell type-specific connectivity, are used by the visual system to recover the intricate detail of our visual surroundings.

Olfactory behavior and physiology are disrupted in prion protein knockout mice

The prion protein PrP^C is infamous for its role in disease, yet its normal physiological function remains unknown. Here we report a novel behavioral phenotype of PrP^−/− mice in an odor-guided task. This phenotype is manifest in three PrP knockout lines on different genetic backgrounds, strong evidence it is specific to the lack of PrP^C rather than other genetic factors. PrP^−/− mice also display altered behavior in a second olfactory task, suggesting the phenotype is olfactory specific. Furthermore, PrP^C deficiency affects oscillatory activity in the deep layers of the main olfactory bulb, as well as dendrodendritic synaptic transmission between olfactory bulb granule and mitral cells. Importantly, both the behavioral and electrophysiological alterations found in PrP^−/− mice are rescued by transgenic neuronal-specific expression of PrP^C. These data suggest a critical role for PrP^C in the normal processing of sensory information by the olfactory system.

Temporal structure of receptor neuron input to the olfactory bulb imaged in behaving rats

The dynamics of sensory input to the nervous system play a critical role in shaping higher-level processing. In the olfactory system, the dynamics of input from olfactory receptor neurons (ORNs) are poorly characterized and depend on multiple factors, including respiration-driven airflow through the nasal cavity, odorant sorption kinetics, receptor-ligand interactions between odorant and receptor, and the electrophysiological properties of ORNs. Here, we provide a detailed characterization of the temporal organization of ORN input to the mammalian olfactory bulb (OB) during natural respiration, using calcium imaging to monitor ORN input to the OB in awake, head-fixed rats expressing odor-guided behaviors. We report several key findings. First, across a population of homotypic ORNs, each inhalation of odorant evokes a burst of action potentials having a rise time of about 80 ms and a duration of about 100 ms. This rise time indicates a relatively slow, progressive increase in ORN activation as odorant flows through the nasal cavity. Second, the dynamics of ORN input differ among glomeruli and for different odorants and concentrations, but remain reliable across successive inhalations. Third, inhalation alone (in the absence of odorant) evokes ORN input to a significant fraction of OB glomeruli. Finally, high-frequency sniffing of odorant strongly reduces the temporal coupling between ORN inputs and the respiratory cycle. These results suggest that the dynamics of sensory input to the olfactory system may play a role in coding odor information and that, in the awake animal, strategies for processing odor information may change as a function of sampling behavior.

Profound context-dependent plasticity of mitral cell responses in olfactory bulb

On the basis of its primary circuit it has been postulated that the olfactory bulb (OB) is analogous to the retina in mammals. In retina, repeated exposure to the same visual stimulus results in a neural representation that remains relatively stable over time, even as the meaning of that stimulus to the animal changes. Stability of stimulus representation at early stages of processing allows for unbiased interpretation of incoming stimuli by higher order cortical centers. The alternative is that early stimulus representation is shaped by previously derived meaning, which could allow more efficient sampling of odor space providing a simplified yet biased interpretation of incoming stimuli. This study helps place the olfactory system on this continuum of subjective versus objective early sensory representation. Here we show that odor responses of the output cells of the OB, mitral cells, change transiently during a go-no-go odor discrimination task. The response changes occur in a manner that increases the ability of the circuit to convey information necessary to discriminate among closely related odors. Remarkably, a switch between which of the two odors is rewarded causes mitral cells to switch the polarity of their divergent responses. Taken together these results redefine the function of the OB as a transiently modifiable (active) filter, shaping early odor representations in behaviorally meaningful ways.

Neurobiology of a Simple Memory

Habituation is one of the simplest forms of memory, yet its neurobiological mechanisms remain largely unknown in mammalian systems. This review summarizes recent multidisciplinary analyses of the neurobiology of mammalian odor habituation including in vitro and in vivo synaptic physiology, sensory physiology, behavioral pharmacology, and computational modeling approaches. The findings show that a metabotropic glutamate receptor–mediated depression of afferent synapses to the olfactory cortex is necessary and perhaps sufficient to account for cortical sensory adaptation and short-term behavioral habituation. Furthermore, long-term habituation is an N-methyl-d-aspartate (NMDA) receptor–dependent process within the olfactory bulb. Thus there is both a pharmacological and anatomical distinction between short-term and long-term memory for habituation. The differential locus of change underlying short- and long-term memory leads to predictable differences in their behavioral characteristics, such as specificity.

Ultrastructural analysis of chemical synapses and gap junctions between Drosophila brain neurons in culture

Dissociated cultures from many species have been important tools for exploring factors that regulate structure and function of central neuronal synapses. We have previously shown that cells harvested from brains of late stage Drosophila pupae can regenerate their processes in vitro. Electrophysiological recordings demonstrate the formation of functional synaptic connections as early as 3 days in vitro (DIV), but no information about synapse structure is available. Here, we report that antibodies against pre-synaptic proteins Synapsin and Bruchpilot result in punctate staining of regenerating neurites. Puncta density increases as neuritic plexuses develop over the first 4 DIV. Electron microscopy reveals that closely apposed neurites can form chemical synapses with both pre- and postsynaptic specializations characteristic of many inter-neuronal synapses in the adult brain. Chemical synapses in culture are restricted to neuritic processes and some neurite pairs form reciprocal synapses. GABAergic synapses have a significantly higher percentage of clear core versus granular vesicles than non-GABA synapses. Gap junction profiles, some adjacent to chemical synapses, suggest that neurons in culture can form purely electrical as well as mixed synapses, as they do in the brain. However, unlike adult brain, gap junctions in culture form between neuronal somata as well as neurites, suggesting soma ensheathing glia, largely absent in culture, regulate gap junction location in vivo. Thus pupal brain cultures, which support formation of interneuronal synapses with structural features similar to synapses in adult brain, are a useful model system for identifying intrinsic and extrinsic regulators of central synapse structure as well as function.

Presynaptic glutamate receptors: physiological functions and mechanisms of action

Glutamate acts on postsynaptic glutamate receptors to mediate excitatory communication between neurons. The discovery that additional presynaptic glutamate receptors can modulate neurotransmitter release has added complexity to the way we view glutamatergic synaptic transmission. Here we review evidence of a physiological role for presynaptic glutamate receptors in neurotransmitter release. We compare the physiological roles of ionotropic and metabotropic glutamate receptors in short- and long-term regulation of synaptic transmission. Furthermore, we discuss the physiological conditions that are necessary for their activation, the source of the glutamate that activates them, their mechanisms of action and their involvement in higher brain function.

Inhibitory interactions among olfactory glomeruli do not necessarily reflect spatial proximity

Inhibitory interactions shape the activity of output neurons in primary olfactory centers and promote contrast enhancement of odor representations. Patterns of interglomerular connectivity, however, are largely unknown. To test whether the proximity of glomeruli to one another is correlated with interglomerular inhibitory interactions, we used intracellular recording and staining methods to record the responses of projection (output) neurons (PNs) associated with glomeruli of known olfactory tuning in the primary olfactory center of the moth Manduca sexta. We focused on Toroid I, a glomerulus in the male-specific macroglomerular complex (MGC) specialized to one of the two key components of the conspecific females' sex pheromone, and the adjacent, sexually isomorphic glomerulus 35, which is highly sensitive to Z-3-hexenyl acetate (Z3-6:OAc). We used the two odorants to activate these reference glomeruli and tested the effects of olfactory activation in other glomeruli. We found that Toroid-I PNs were not inhibited by input to G35, whereas G35 PNs were inhibited by input to Toroid-I PNs. We also recorded the responses of PNs arborizing in other sexually isomorphic glomeruli to stimulation with the sex pheromone and Z3-6:OAc. We found that inhibitory responses were not related to proximity to the MGC and G35: both distant and adjacent PNs were inhibited by stimulation with the sex pheromone, some others were affected by only one odorant, and yet others by neither. Similar results were obtained in female PNs recorded in proximity to female-specific glomeruli. Our findings indicate that inhibitory interactions among glomeruli are widespread and independent of their spatial proximity.

Activity-dependent gating of lateral inhibition in the mouse olfactory bulb

Lateral inhibition is a circuit motif found throughout the nervous system that often generates contrast enhancement and center-surround receptive fields. We investigated the functional properties of the circuits mediating lateral inhibition between olfactory bulb principal neurons (mitral cells) in vitro. We found that the lateral inhibition received by mitral cells is gated by postsynaptic firing, such that a minimum threshold of postsynaptic activity is required before effective lateral inhibition is recruited. This dynamic regulation allows the strength of lateral inhibition to be enhanced between cells with correlated activity. Simulations show that this regulation of lateral inhibition causes decorrelation of mitral cell activity that is evoked by similar stimuli, even when stimuli have no clear spatial structure. These results show that this previously unknown mechanism for specifying lateral inhibitory connections allows functional inhibitory connectivity to be dynamically remapped to relevant populations of neurons.

Multiple modes of synaptic excitation of olfactory bulb granule cells

Inhibition generated by granule cells, the most common GABAergic cell type in the olfactory bulb, plays a critical role in shaping the output of the olfactory bulb. However, relatively little is known about the synaptic mechanisms responsible for activating these interneurons in addition to the specialized dendrodendritic synapses located on distal dendrites. Using two-photon guided minimal stimulation in acute rat brain slices, we found that distal and proximal excitatory synapses onto granule cells are functionally distinct. Proximal synapses arise from piriform cortical neurons and facilitate with paired-pulse stimulation, whereas distal dendrodendritic synapses generate EPSCs with slower kinetics that depress with paired stimulation. Proximal cortical feedback inputs can relieve the tonic Mg block of NMDA receptors (NMDARs) at distal synapses and gate dendrodendritic inhibition onto mitral cells. Most excitatory synapses we examined onto granule cells activated both NMDARs and AMPA receptors, whereas a subpopulation appeared to be NMDAR silent. The convergence of two types of excitatory inputs onto GABAergic granule cells provides a novel mechanism for regulating the degree of interglomerular processing of sensory input in the olfactory bulb through piriform cortex/olfactory bulb synaptic interactions.

Genetic analysis of brain circuits underlying pheromone signaling

Molecular approaches and genetic manipulations have provided novel insights into the processing of pheromone-mediated information by the olfactory and vomeronasal systems of mammals. We will review and discuss the specific contribution of each of the two chemosensory systems that ensure specific behavioral responses to conspecific animals.

Spatiotemporal representations in the olfactory system

A complete understanding of the mechanisms underlying any kind of sensory, motor or cognitive task requires analysis from the systems to the cellular level. In olfaction, new behavioural evidence in rodents has provided temporal limits on neural processing times that correspond to less than 150ms--the timescale of a single sniff. Recent in vivo data from the olfactory bulb indicate that, within each sniff, odour representation is not only spatially organized, but also temporally structured by odour-specific patterns of onset latencies. Thus, we propose that the spatial representation of odour is not a static one, but rather evolves across a sniff, whereby for difficult discriminations of similar odours, it is necessary for the olfactory system to "wait" for later-activated components. Based on such evidence, we have devised a working model to assess further the relevance of such spatiotemporal processes in odour representation.

Opposing inward and outward conductances regulate rebound discharges in olfactory mitral cells

The olfactory bulb, a second-order sensory brain region, relays afferent input from olfactory receptor neurons to piriform cortex and other higher brain centers. Although large inhibitory postsynaptic potentials (IPSPs) are evident in in vivo intracellular recordings from mitral cells, the functional significance of these synaptic responses has not been defined. In many brain regions, IPSPs can function to either inhibit spiking by transiently suppressing activity or can evoke spiking directly by triggering rebound discharges. We used whole cell patch-clamp recordings from mitral cells in olfactory bulb slices to investigate the mechanisms by which IPSPs regulate mitral cell spike discharges. Mitral cells have unusual intrinsic membrane properties that support rebound spike generation in response to small-amplitude (3-5 mV) but not large-amplitude hyperpolarizing current injections or IPSPs. Rebound spiking occurring in mitral cells was dependent on recovery of subthreshold Na currents, and could be blocked by tetrodotoxin (TTX, 1 microM) or the subthreshold Na channel blocker riluzole (10 microM). Surprisingly, larger-amplitude hyperpolarizing stimuli impeded spike generation by recruiting a transient outward I(A)-like current that was sensitive to high concentrations of 4-aminopyridine and Ba. The interplay of voltage-gated subthreshold Na channels and transient outward current produces a narrow range of IPSP amplitudes that generates rebound spikes. We also found that subthreshold Na channels boost subthreshold excitatory stimuli to produce membrane voltages where granule-cell-mediated IPSPs can produce rebound spikes. These results demonstrate how the intrinsic membrane properties of mitral cells enable inhibitory inputs to bidirectionally control spike output from the olfactory bulb.

Information processing in the olfactory systems of insects and vertebrates

Insects and vertebrates separately evolved remarkably similar mechanisms to process olfactory information. Odors are sampled by huge numbers of receptor neurons, which converge type-wise upon a much smaller number of principal neurons within glomeruli. There, odor information is transformed by inhibitory interneuron-mediated, cross-glomerular circuit interactions that impose slow temporal structures and fast oscillations onto the firing patterns of principal neurons. The transformations appear to improve signal-to-noise characteristics, define odor categories, achieve precise odor identification, extract invariant features, and begin the process of sparsening the neural representations of odors for efficient discrimination, memorization, and recognition.

Neurogenic correlates of an olfactory discrimination task in the adult olfactory bulb

In the main olfactory bulb, stimuli are coded within the spatio-temporal pattern of mitral cells' activity. Granule cells are interneurons that shape the mitral cells' activity, and are continuously generated in the adult main olfactory bulb. However, the role of granule cell renewal remains elusive. We show here that an associative olfactory discrimination task reduces the survival of newborn neurons. However, when the olfactory task involves perceptually related odorants, the learning process is slower and does not induce such a reduction in the number of new neurons. Mapping newborn cells within the granule cell layer of the main olfactory bulb reveals a clustered distribution that evolves with learning as a function of odorant similarity and partly overlaps with the immediate-early gene Zif268 expression pattern. These data provide insight into the functional mechanisms underlying olfactory discrimination learning, and promote the importance of neurogenesis as a cellular basis for the restructuring of odor images in the main olfactory bulb.

Glomerulus-specific, long-latency activity in the olfactory bulb granule cell network

Reliable, stimulus-specific temporal patterns of action potentials have been proposed to encode information in many brain areas, perhaps most notably in the olfactory system. Analysis of such temporal coding has focused almost exclusively on excitatory neurons. Thus, the role of networks of inhibitory interneurons in establishing and maintaining this reliability is unclear. Here we use imaging of population activity in vitro to investigate the mechanisms of temporal pattern generation in mouse olfactory bulb inhibitory interneurons. We show that activity of these interneurons evolves slowly in time but that individual neurons fire at reliable times, with a timescale similar to the slow changes in the patterns of odor-evoked activity and to odor discrimination. Most strikingly, the latency of a single granule cell is highly reliable from trial to trial during repeated stimulation of the same glomerulus, whereas this same cell will have a markedly different latency when a different glomerulus is activated. These data suggest that the timing of granule cell-mediated inhibition in the olfactory bulb is tightly regulated by the source of input and that inhibition may contribute to the generation of reliable temporal patterns of mitral cell activity.

Olfactory discrimination learning increases the survival of adult-born neurons in the olfactory bulb

In the olfactory bulb (OB), new neurons are added throughout life, forming an integral part of the functioning circuit. Yet only some of them survive more than a month. To determine whether this turnover depends on olfactory learning, we examined the survival of adult newborn cells labeled with the cell division marker BrdU, administered before learning in an olfactory discrimination task. We report that discrimination learning increases the number of newborn neurons in the adult OB by prolonging their survival. Simple exposure to the pair of olfactory cues did not alter neurogenesis, indicating that the mere activation of sensory inputs during the learning task was insufficient to alter neurogenesis. The increase in cell survival after learning was not uniformly distributed throughout angular sectors of coronal sections of the OB. Monitoring odor activation maps using patterns of Zif268 immediate early gene expression revealed that survival was greater in regions more activated by the non-reinforced odorant. We conclude that sensory activation in a learning context not only controls the total number of newborn neurons in the adult OB, but also refines their precise location. Shaping the distribution of newborn neurons by influencing their survival could optimize the olfactory information processing required for odor discrimination.

Fast neurotransmitter release triggered by Ca influx through AMPA-type glutamate receptors

Feedback inhibition at reciprocal synapses between A17 amacrine cells and rod bipolar cells (RBCs) shapes light-evoked responses in the retina. Glutamate-mediated excitation of A17 cells elicits GABA (gamma-aminobutyric acid)-mediated inhibitory feedback onto RBCs, but the mechanisms that underlie GABA release from the dendrites of A17 cells are unknown. If, as observed at all other synapses studied, voltage-gated calcium channels (VGCCs) couple membrane depolarization to neurotransmitter release, feedforward excitatory postsynaptic potentials could spread through A17 dendrites to elicit 'surround' feedback inhibitory transmission at neighbouring synapses. Here we show, however, that GABA release from A17 cells in the rat retina does not depend on VGCCs or membrane depolarization. Instead, calcium-permeable AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors (AMPARs), activated by glutamate released from RBCs, provide the calcium influx necessary to trigger GABA release from A17 cells. The AMPAR-mediated calcium signal is amplified by calcium-induced calcium release (CICR) from intracellular calcium stores. These results describe a fast synapse that operates independently of VGCCs and membrane depolarization and reveal a previously unknown form of feedback inhibition within a neural circuit.

Spontaneous field potentials in the glomeruli of the olfactory bulb: the leading role of juxtaglomerular cells

Field potentials recorded in the olfactory bulb glomerular layer (GL) are thought to result mainly from activation of mitral and tufted cells. The contribution of juxtaglomerular cells (JG) is unknown. We tested the hypothesis that JG are the main driving force to novel spontaneous glomerular layer field potentials (sGLFPs), which were recorded in rat olfactory bulb slices maintained in an interface chamber. We found that sGLFPs have comparable magnitudes, durations and frequencies both in standard horizontal slices, where all layers with all cell types were present, and in isolated GL slices, where only JG cells were preserved. Hence, the impact of mitral and deep/medium tufted cells to sGLFPs turned out to be minor. Therefore, we propose that the main generators of sGLFPs are JG neurons. We further explored the mechanism of generation of sGLFPs using a neuronal ensemble model comprising all types of cells associated with a single glomerulus. Random orientation and homogenous distribution of dendrites in the glomerular neuropil along with surrounding shell of cell bodies of JG neurons resulted in substantial spatial restriction of the generated field potential. The model predicts that less than 20% of sGLFP can spread from one glomerulus to an adjacent one. The contribution of JG cells to the total field in the center of the glomerulus is estimated as approximately 50% ( approximately 34% periglomerular and approximately 16% external tufted cells), whereas deep/medium tufted cells provide approximately 39% and mitral cells only approximately 10%. Occasionally, some sGLFPs recorded in adjacent or remote glomeruli were cross-correlated, suggesting involvement of interglomerular communication in information coding. These results demonstrate a leading role of JG cells in activation of the main olfactory bulb (MOB) functional modules. Finally, we hypothesize that the GL is not a set of independent modules, but it represents a subsystem in the MOB network, which can perform initial processing of odors.

Heterosynaptic modulation of the dorsal root potential in the turtle spinal cord in vitro

In the somatosensory system, the flow of sensory information is regulated at early stages by presynaptic inhibition. Recent findings have shown that the mechanisms generating the primary afferent depolarization (PAD) associated with presynaptic inhibition are complex, with some components mediated by a non-spiking mechanism. How sensory inputs carried by neighbouring afferent fibres interact to regulate the generation of PAD, and thus presynaptic inhibition, is poorly known. Here, we investigated the interaction between neighbouring primary afferents for the generation of PAD in an in vitro preparation of the turtle spinal cord. To monitor PAD we recorded the dorsal root potential (DRP), while the simultaneous cord dorsum potential (CDP) was recorded to assess the population postsynaptic response. We found that the DRP and the CDP evoked by a primary afferent test stimulus was greatly reduced by a conditioning activation of neighbouring primary afferents. This depression had early and late components, mediated in part by GABAA and GABAB receptors, since they were reduced by bicuculline and SCH 50911 respectively. However, with the selective stimulation of C and Adelta fibres in the presence of TTX, the early and late depression of the DRP was replaced by facilitation of the GABAergic and glutamatergic components of the TTX-resistant DRP. Our findings suggest a subtle lateral excitatory interaction between primary afferents for the generation of PAD mediated by a non-spiking mechanism that may contribute to shaping of information transmitted by C and Adelta fibres in a spatially confined scale in analogy with the retina and olfactory bulb.

Potentiation of cortical inhibition by visual deprivation

The fine-tuning of circuits in sensory cortex requires sensory experience during an early critical period. Visual deprivation during the critical period has catastrophic effects on visual function, including loss of visual responsiveness to the deprived eye, reduced visual acuity, and loss of tuning to many stimulus characteristics. These changes occur faster than the remodelling of thalamocortical axons, but the intracortical plasticity mechanisms that underlie them are incompletely understood. Long-term depression of excitatory intracortical synapses has been proposed as a general candidate mechanism for the loss of cortical responsiveness after visual deprivation. Alternatively (or in addition), the decreased ability of the deprived eye to activate cortical neurons could be due to enhanced intracortical inhibition. Here we show that visual deprivation leaves excitatory connections in layer 4 (the primary input layer to cortex) unaffected, but markedly potentiates inhibitory feedback between fast-spiking basket cells (FS cells) and star pyramidal neurons (star pyramids). Further, a previously undescribed form of long-term potentiation of inhibition (LTPi) could be induced at synapses from FS cells to star pyramids, and was occluded by previous visual deprivation. These data suggest that potentiation of inhibition is a major cellular mechanism underlying the deprivation-induced degradation of visual function, and that this form of LTPi is important in fine-tuning cortical circuitry in response to visual experience.

Adjusting neurophysiological computations in the adult olfactory bulb

The olfactory bulb receives signals from olfactory sensory neurons and conveys them to higher centers. The mapping of the sensory inputs generates a reproducible spatial pattern in the glomerular layer of the olfactory bulb for each odorant. Then, this restricted activation is transformed into highly distributed patterns by lateral interactions between relay neurons and local interneurons. Thus, odor information processing requires the spatial patterning of both sensory inputs and synaptic interactions. In other words, odor representation is highly dynamic and temporally orchestrated. Here, we describe how the local inhibitory network shapes the global oscillations and the precise synchronization of relay neurons. We discuss how local inhibitory interneurons transpose the spatial dimension into temporal patterning. Remarkably, this transposition is not fixed but highly flexible to continuously optimize olfactory information processing.

Dynamic connectivity in the mitral cell-granule cell microcircuit

The interactions between excitatory mitral cells and inhibitory granule cells are critical for the regulation of olfactory bulb activity. Here we review anatomical and physiological data on the mitral cell-granule cell circuit and provide a quantitative estimate of how this connectivity varies as a function of distance between mitral cells. We also discuss the ways in which the functional connectivity can be altered rapidly during olfactory bulb activity.

Correlation-induced synchronization of oscillations in olfactory bulb neurons

Oscillations are a common feature of odor-evoked and spontaneous activity in the olfactory system in vivo and in vitro and are thought to play an important role in information processing and memory in a variety of brain areas. Theoretical and experimental studies have described several mechanisms by which oscillations can be generated and synchronized. Here, we investigate the hypothesis that correlated noisy inputs are able to generate synchronous oscillations in olfactory bulb mitral cells in vitro. We consider several alternative mechanisms and conclude that olfactory bulb synchronous oscillations are likely to arise because of the response of uncoupled oscillating neurons to aperiodic but correlated inputs. This mechanism has been described theoretically, but we provide the first experimental evidence that such a mechanism may underlie synchronization in real neurons. In physiological experiments, we show that this mechanism can generate gamma-band oscillations in populations of olfactory bulb mitral cells. This mechanism synchronizes oscillatory firing by using shared fast fluctuations in stochastic inputs across neurons, without requiring any synaptic or electrical coupling. We discuss the properties and limitations of synchronization by this mechanism and suggest that it may underlie fast oscillations in many brain areas.

Adrenergic enhancement of inhibitory transmission in the accessory olfactory bulb

Noradrenergic modulation of dendrodendritic synapses between the mitral and granule cells in the accessory olfactory bulb (AOB) is postulated to play a key role in the formation of memory in olfactory-mediated behaviors. Current models propose that noradrenaline (NA) increases excitation of mitral/tufted cells (M/TCs) by decreasing the release of GABA from granule cells. However, surprisingly little is known about the actions of NA at the cellular level in the AOB. Here, in recordings from AOB slices, we show that NA decreases the firing frequency of M/TCs in response to stimulation. This effect is attributable to an increase in the GABA inhibitory input to M/TCs. Application of NA (10 microM) produced an approximately 20-fold increase in the frequency of GABA-induced miniature IPSCs (mIPSCs) without changing their amplitude. A pharmacological analysis indicated that the increase in mIPSCs frequency results from activation of alpha1 adrenergic receptors. In addition to increasing the mIPSC frequency, NA also potentiated GABA inhibitory currents induced by direct stimulation of granule cells. Together, our results suggest that NA increases the release of GABA from granule cells by acting on presynaptic receptors. Thus, the role of the noradrenergic activity in the AOB may be opposite than suggested previously: we find that the overall effect of NA in the AOB is inhibition of M/TCs.

Changing Roles for Temporal Representation of Odorant During the Oscillatory Response of the Olfactory Bulb

It has been hypothesized that the brain uses combinatorial as well as temporal coding strategies to represent stimulus properties. The mechanisms and properties of the temporal coding remain undetermined, although it has been postulated that oscillations can mediate formation of this type of code. Here we use a generic model of the vertebrate olfactory bulb to explore the possible role of oscillatory behavior in temporal coding. We show that three mechanisms—synaptic inhibition, slow self-inhibition and input properties—mediate formation of a temporal sequence of simultaneous activations of glomerular modules associated with specific odorants within the oscillatory response. The sequence formed depends on the relative properties of odorant features and thus may mediate discrimination of odorants activating overlapping sets of glomeruli. We suggest that period-doubling transitions may be driven through excitatory feedback from a portion of the olfactory network acting as a coincidence modulator. Furthermore, we hypothesize that the period-doubling transition transforms the temporal code from a roster of odorant components to a signal of odorant identity and facilitates discrimination of individual odorants within mixtures.

Circuit Properties Generating Gamma Oscillations in a Network Model of the Olfactory Bulb

The study of the neural basis of olfaction is important both for understanding the sense of smell and for understanding the mechanisms of neural computation. In the olfactory bulb (OB), the spatial patterning of both sensory inputs and synaptic interactions is crucial for processing odor information, although this patterning alone is not sufficient. Recent studies have suggested that representations of odor may already be distributed and dynamic in the first olfactory relay. The growing evidence demonstrating a functional role for the temporal structure of bulbar neuronal activity supports this assumption. However, the detailed mechanisms underlying this temporal structure have never been thoroughly studied. Our study focused on gamma (40–100 Hz) network oscillations in the mammalian OB, which is a form of temporal patterning in bulbar activity elicited by olfactory stimuli. We used computational modeling combined with electrophysiological recordings to investigate the basic synaptic organization necessary and sufficient to generate sustained gamma rhythms. We found that features of gamma oscillations obtained in vitro were identical to those of a model based on lateral inhibition as the coupling modality (i.e., low irregular firing rate and high oscillation stability). In contrast, they differed substantially from those of a model based on lateral excitatory coupling (i.e., high regular firing rate and instable oscillations). Therefore we could precisely tune the oscillation frequency by changing the kinetics of inhibitory events supporting the lateral inhibition. Moreover, gradually decreasing GABAergic synaptic transmission decreased the degree of relay neuron synchronization in response to sensory inputs, both theoretically and experimentally. Thus we have shown that lateral inhibition provides a mechanism by which the dynamic processing of odor information might be finely tuned within the OB circuit.

Detecting activity in olfactory bulb glomeruli with astrocyte recording

In the olfactory bulb, axons of olfactory sensory neurons (OSNs) expressing the same olfactory receptor converge on specific glomeruli. These afferents form axodendritic synapses with mitral/tufted and periglomerular cell dendrites, whereas the dendrites of mitral/tufted cells and periglomerular interneurons form dendrodendritic synapses. The two types of intraglomerular synapses appear to be spatially isolated in subcompartments delineated by astrocyte processes. Because each astrocyte sends processes into a single glomerulus, we used astrocyte recording as an intraglomerular detector of neuronal activity. In glomerular astrocytes, a single shock in the olfactory nerve layer evoked a prolonged inward current, the major part of which was attributable to a barium-sensitive potassium current. The K+ current closely reflected the time course of depolarization of mitral/tufted cells, indicating that K+ accumulation mainly reflects the activity of mitral/tufted cells. The astrocyte K+ current was dependent on AMPA and NMDA receptors in mitral/tufted cells as well as on a previously undescribed metabotropic glutamate receptor 1 component. Block of the K+ current with barium unmasked a synaptic glutamate transporter current. Perhaps surprisingly, the transporter current had components caused by glutamate released at both olfactory nerve terminals and mitral/tufted cell dendrites. The time course of the transporter currents suggested that rapid synchronous glutamate release at OSN terminals triggers asynchronous glutamate release from mitral/tufted cells. Glomerular astrocyte recording provides a sensitive means to examine functional compartmentalization within and between olfactory bulb glomeruli.

Chemosensory selectivity of output neurons innervating an identified, sexually isomorphic olfactory glomerulus

The antennal lobe (AL) of insects, like the olfactory bulb of vertebrates, is characterized by discrete modules of synaptic neuropil called glomeruli. In some insects (e.g., moths and cockroaches), a few glomeruli are sexually dimorphic and function in labeled lines for processing of sensory information about sex pheromones. Controversy still exists, however, about whether projection (output) neurons (PNs) of glomeruli in the main AL are also narrowly tuned. We examined this critical issue in the AL of the moth Manduca sexta. We used intracellular recording and staining techniques to investigate the chemosensory tuning of PNs innervating an identifiable, sexually isomorphic glomerulus, G35, in the main AL. We found that the morphological features and chemosensory tuning of G35-PNs were nearly identical in females and males. G35-PNs responded to low concentrations of the plant-derived volatile compound cis-3-hexenyl acetate (c3HA), but the sensitivity threshold of female PNs was lower than that of male PNs. The propionate and butyrate homologs of c3HA could evoke excitatory responses but only at moderate-to-high concentrations. Other plant volatiles did not evoke responses from G35-PNs. Moreover, PNs innervating glomeruli near G35 (in females) showed little or no response to c3HA. Female G35-PNs were hyperpolarized by (+/-)linalool, a compound that excites PNs in an adjacent glomerulus, thus providing evidence for lateral-inhibitory interactions between glomeruli. Our results show that PNs arborizing in an identified glomerulus in the main olfactory pathway are morphologically and physiologically equivalent in both sexes and have characteristic, limited molecular receptive ranges that are highly conserved across individuals.

Olfactory bulb external tufted cells are synchronized by multiple intraglomerular mechanisms

In rat olfactory bulb slices, external tufted (ET) cells spontaneously generate spike bursts. Only ET cells affiliated with the same glomerulus exhibit significant synchronous activity, suggesting that synchrony results mainly from intraglomerular interactions. The intraglomerular mechanisms underlying their synchrony are unknown. Using dual extracellular and patch-clamp recordings from ET cell pairs of the same glomerulus, we found that the bursting of ET cells is synchronized by several mechanisms. First, ET cell pairs of the same glomerulus receive spontaneous synchronous fast excitatory synaptic input that can also be evoked by olfactory nerve stimulation. Second, they exhibit correlated spontaneous slow excitatory synaptic currents that can also be evoked by stimulation of the external plexiform layer. These slow currents may reflect the repetitive release of glutamate via spillover from the dendritic tufts of other ET or mitral/tufted cells affiliated with the same glomerulus. Third, ET cells exhibit correlated bursts of inhibitory synaptic activity immediately after the synchronous fast excitatory input. These bursts of IPSCs were eliminated by CNQX and may therefore reflect correlated feedback inhibition from periglomerular cells that are driven by ET cell spike bursts. Fourth, in the presence of fast synaptic blockers, ET cell pairs exhibit synchronous slow membrane current oscillations associated with rhythmic spikelets, which were sensitive to the gap junction blocker carbenoxolone. These findings suggest that coordinated synaptic transmission and gap junction coupling synchronize the spontaneous bursting of ET cells of the same glomerulus.

Lateral excitation within the olfactory bulb

Lateral inhibition is a common feature of cortical networks, serving such functions as contrast enhancement. In the olfactory bulb, inhibition is imbedded in the local connectivity at dendrodendritic synapses between mitral cells and interneurons. However, there is also evidence for excitatory interactions between mitral cells despite the lack of direct synaptic connections. This lateral excitation, although a less well recognized feature of the circuit, provides a potentially powerful mechanism to enhance coordinated activity. We examined lateral excitation in paired recordings between mitral cells projecting to the same glomerulus. Trains of action potentials in one mitral cell evoked autoexcitation in the stimulated cell and a prolonged depolarization in the second cell. This lateral excitation was absent in connexin36(-/-) mice, which lack mitral-mitral cell gap junctions. However, spillover of dendritically released glutamate contributed to lateral excitation during concerted mitral cell excitation or by single-cell activity if glutamate uptake was blocked. Our results suggest that electrical coupling and spillover create a lateral excitatory network within the glomerulus, thus markedly amplifying the sensitivity of each glomerulus to incoming sensory input.

In vivo imaging of juxtaglomerular neuron turnover in the mouse olfactory bulb

As a consequence of adult neurogenesis, the olfactory bulb (OB) receives a continuous influx of newborn neurons well into adulthood. However, their rates of generation and turnover, the factors controlling their survival, and how newborn neurons intercalate into adult circuits are largely unknown. To visualize the dynamics of adult neurogenesis, we produced a line of transgenic mice expressing GFP in approximately 70% of juxtaglomerular neurons (JGNs), a population that undergoes adult neurogenesis. Using in vivo two-photon microscopy, time-lapse analysis of identified JGN cell bodies revealed a neuronal turnover rate of approximately 3% of this population per month. Although new neurons appeared and older ones disappeared, the overall number of JGNs remained constant. This approach provides a dynamic view of the actual appearance and disappearance of newborn neurons in the vertebrate central nervous system, and provides an experimental substrate for functional analysis of adult neurogenesis.

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.

Long-term fate and distribution of newborn cells in the adult mouse olfactory bulb: Influences of olfactory deprivation

The adult subventricular zone produces neuroblasts that migrate to the main olfactory bulb, where they differentiate into interneurons in the glomerular and granular layers. Using bromodeoxyuridine labeling, the survival of newborn cells was assessed in these two layers of the MOB in control mice and in mice unilaterally deprived from sensory input by naris occlusion. In control main olfactory bulbs, bromodeoxyuridine-positive cell density decreased about 70% between 15 and 180 days post-bromodeoxyuridine administration but earlier in the glomerular layer than in the granular layer. At all time points examined, newborn cell density was higher in the deep granular layer than in the superficial granular layer. Occlusion started at the age of 2 months and lasted for 15, 30, 45, 60 or 180 days. The newborn cell survival was similarly reduced in both layers by occlusion, during a critical period 15 and 45 days post-occlusion. Interestingly, olfactory deprivation decreased bromodeoxyuridine-positive cell density in the deep granular layer only, indicating a greater dependence of cell fate on sensory input in this sub-layer. Neuronal differentiation was assessed in the granular layer and glomerular layer by multiple double-labeling 45 days post-bromodeoxyuridine-injections, the time point at which the proportion of bromodeoxyuridine-positive cells expressing a neuronal marker reached approximately 85% in the granular layer and approximately 50% in the glomerular layer. Naris occlusion did not significantly affect these proportions. Taken together, our results reveal that the survival of newborn cells has a different time course in the glomerular layer and in the granular layer, but is similarly decreased in each layer by olfactory deprivation. In addition, our data suggest a functional heterogeneity of neurogenesis within the granular layer.

An in vitro study of long-term potentiation in the carp (Cyprinus carpio L.) olfactory bulb

Long-term potentiation (LTP) of synaptic transmission is considered a cellular mechanism for neural plasticity and memory formation. Previously, we showed that in the carp olfactory bulb, LTP occurs at the dendrodendritic mitral-to-granule cell synapse following tetanic electrical stimulation applied to the olfactory tract, and suggested that it is involved in the process of olfactory memory formation. As a first step towards understanding mechanisms underlying plasticity at this synapse, we examined the effects of various drugs (glutamate and GABA receptor agonists and antagonists, noradrenaline, and drugs affecting cAMP signaling) on dendrodendritic mitral-to-granule cell synaptic transmission in an in vitro preparation. Two forms of LTP are involved: a postsynaptic form (tetanus-evoked LTP) and a presynaptic form. The postsynaptic form is evoked at the granule cell dendrite following tetanic olfactory tract stimulation and is suppressed by the NMDA receptor antagonist, D-AP5, enhanced by noradrenaline, and occluded by the metabotropic glutamate receptor agonist, trans-ACPD. The presynaptic form occurs at the mitral cell dendrite following blockade of the GABA(A) receptor by picrotoxin and bicuculline, or via activation of cAMP signaling by forskolin and 8-Br-cAMP.

Mechanisms of odor discrimination: neurophysiological and behavioral approaches

Understanding how complex neuronal circuits in the brain perform advanced computations is a central question in neuroscience that can only be addressed using a combination of approaches, including neurophysiology and behavioral analyses. In the olfactory bulb, neurophysiological studies have revealed that neuronal interactions reorganize odor-evoked activity patterns so that their discriminability is enhanced. Recent behavioral studies have examined the role of this computation in odor discrimination tasks and generated working models of behavioral odor discrimination strategies. The results appear consistent with a role of pattern reorganization in odor discrimination behavior but further studies are necessary to resolve this issue. These studies advance the understanding of neuronal circuit function in the olfactory bulb and illustrate benefits and caveats of comparing behavioral and neurophysiological results.

Odor response properties of neighboring mitral/tufted cells in the rat olfactory bulb

Olfactory perception initiates in the nasal epithelium wherefrom olfactory receptor neurons--expressing the same receptor protein--project and converge in two different glomeruli within each olfactory bulb. Recent evidence suggests that glomeruli are isolated functional units, arranged in a chemotopic manner in the olfactory bulb. Exposure to odorants leads to the activation of specific populations of glomeruli. In rodents, about 25-50 mitral/tufted cells project their primary dendrites to a single glomerulus receiving similar sensory input. Yet, little is known about the properties of neighboring mitral/tufted cells connected to one or a few neighboring glomeruli. We used tetrodes to simultaneously record multiple single-unit activity in the mitral cell layer of anesthetized, freely breathing rats while exposed to mixtures of chemically related compounds. First, we characterized the odorant-induced modifications in firing rate of neighboring mitral/tufted cells and found that they do not share odorant response profiles. Individual units showed a long silent (11.01 ms) period with no oscillatory activity. Cross-correlation analysis between neighboring mitral/tufted cells revealed negligible synchronous activity among them. Finally, we show that respiratory-related temporal patterns are dissimilar among neighboring mitral/tufted cells and also that odorant stimulation results in an individual modification that is not necessarily shared by neighboring mitral/tufted cells. These results show that neighboring mitral/tufted cells frequently exhibit dissimilar response properties, which are not consistent with a precise chemotopic map at the mitral/tufted cell layer in the olfactory bulb.

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.

Oscillations in the Olfactory Bulb Carry Information About Odorant History

While odorant-evoked oscillations in the vertebrate olfactory bulb have been studied extensively, information about their possible cognitive role has been missing. Using voltage-sensitive dye imaging, we show that repeated odorant presentations with interstimulus intervals of 2–12 s had dramatic and diverse effects on the three oscillations that occur in the turtle olfactory bulb. Two of the oscillations are strikingly depressed in response to the second stimulation even of a new odorant was presented. The third oscillation is enhanced if the odorant is the same but suppressed if the odorant is new. The effects suggest that the oscillations carry information about odorant novelty and consistency.

Contrasting short-term plasticity at two sides of the mitral-granule reciprocal synapse in the mammalian olfactory bulb

The mitral-granule reciprocal synapse shapes the response of the olfactory bulb to odour stimuli by mediating lateral and reciprocal inhibition. We investigated the short-term plasticity of both the mitral-to-granule excitatory synapse and the granule-to-mitral inhibitory synapse in rat olfactory bulb slices, using whole-cell patch clamp recordings. The granule-to-mitral inhibitory synapse invariably exhibited paired-pulse depression at interstimulus intervals of less than a second, while the mitral-to-granule excitatory synapse showed heterogeneous responses, which on average yielded a moderate facilitation. Trains of stimuli led to a much greater depression at the granule-to-mitral synapse than at the mitral-to-granule synapse. Since mitral cells commonly respond to odours by burst firing with each inhalation cycle, we used bursts of stimuli to study recovery from depression. We found that recovery from depression induced by fast trains of stimuli was more rapid at the mitral-to-granule synapse than at the granule-to-mitral synapse. In addition, depression was enhanced by higher calcium concentrations, suggesting at least partial contribution of presynaptic mechanisms to short-term depression. The observed short-term plasticity could enable mitral cells to overcome autoinhibition and increase action potential propagation along lateral dendrites by burst firing.

Olfactory coding in Drosophila larvae investigated by cross-adaptation

In order to reveal aspects of olfactory coding, the effects of sensory adaptation on the olfactory responses of first-instar Drosophila melanogaster larvae were tested. Larvae were pre-stimulated with a homologous series of acetic esters (C3-C9), and their responses to each of these odours were then measured. The overall patterns suggested that methyl acetate has no specific pathway but was detected by all the sensory pathways studied here, that butyl and pentyl acetate tended to have similar effects to each other and that hexyl acetate was processed separately from the other odours. In a number of cases, cross-adaptation transformed a control attractive response into a repulsive response; in no case was an increase in attractiveness observed. This was investigated by studying changes in dose-response curves following pre-stimulation. These findings are discussed in light of the possible intra- and intercellular mechanisms of adaptation and the advantage of altered sensitivity for the larva.

Mapping and reconstruction of domoic acid-induced neurodegeneration in the mouse brain

Domoic acid, a potent neurotoxin and glutamate analog produced by certain species of the marine diatom Pseudonitzschia, is responsible for several human and wildlife intoxication events. The toxin characteristically damages the hippocampus in exposed humans, rodents, and marine mammals. Histochemical studies have identified this, and other regions of neurodegeneration, though none have sought to map all brain regions affected by domoic acid. In this study, mice exposed (i.p.) to 4 mg/kg domoic acid for 72 h exhibited behavioral and pathological signs of neurotoxicity. Brains were fixed by intracardial perfusion and processed for histochemical analysis. Serial coronal sections (50 microm) were stained using the degeneration-sensitive cupric silver staining method of DeOlmos. Degenerated axons, terminals, and cell bodies, which stained black, were identified and the areas of degeneration were mapped onto Paxinos mouse atlas brain plates using Adobe Illustrator CS. The plates were then combined to reconstruct a 3-dimensional image of domoic acid-induced neurodegeneration using Amira 3.1 software. Affected regions included the olfactory bulb, septal area, and limbic system. These findings are consistent with behavioral and pathological studies demonstrating the effects of domoic acid on cognitive function and neurodegeneration in rodents.

Aging results in reduced epidermal growth factor receptor signaling, diminished olfactory neurogenesis, and deficits in fine olfactory discrimination

Previous studies demonstrating olfactory interneuron involvement in olfactory discrimination and decreased proliferation in the forebrain subventricular zone with age led us to ask whether olfactory neurogenesis and, consequently, olfactory discrimination were impaired in aged mice. Pulse labeling showed that aged mice (24 months of age) had fewer new interneurons in the olfactory bulb than did young adult (2 months of age) mice. However, the aged mice had more olfactory interneurons in total than their younger counterparts. Aged mice exhibited no differences from young adult mice in their ability to discriminate between two discrete odors but were significantly poorer at performing discriminations between similar odors (fine olfactory discrimination). Leukemia inhibitory factor receptor heterozygote mice, which have less neurogenesis and fewer olfactory interneurons than their wild-type counterparts, performed more poorly at fine olfactory discrimination than the wild types, suggesting that olfactory neurogenesis, rather than the total number of interneurons, was responsible for fine olfactory discrimination. Immunohistochemistry and Western blot analyses revealed a selective reduction in expression levels of epidermal growth factor (EGF) receptor (EGFR) signaling elements in the aged forebrain subventricular zone. Waved-1 mutant mice, which express reduced quantities of transforming growth factor-alpha, the predominant EGFR ligand in adulthood, phenocopy aged mice in olfactory neurogenesis and performance on fine olfactory discrimination tasks. These results suggest that the impairment in fine olfactory discrimination with age may result from a reduction in EGF-dependent olfactory neurogenesis.