Investigation of the role of interneurons and their modulation by centrifugal fibers in a neural model of the olfactory bulb

Mesoscopic neuron population modeling of normal/epileptic brain dynamics

Simulations of EEG data provide the understanding of how the limbic system exhibits normal and abnormal states of the electrical activity of the brain. While brain activity exhibits a type of homeostasis of excitatory and inhibitory mesoscopic neuron behavior, abnormal neural firings found in the seizure state exhibits brain instability due to runaway oscillatory entrained neural behavior. We utilize a model of mesoscopic brain activity, the KIV model, where each network represents the areas of the limbic system, i.e., hippocampus, sensory cortex, and the amygdala. Our model initially demonstrates oscillatory entrained neural behavior as the epileptogenesis, and then by increasing the external weights that join the three networks that represent the areas of the limbic system, seizure activity entrains the entire system. By introducing an external signal into the model, simulating external electrical titration therapy, the modeled seizure behavior can be 'rebalanced' back to its normal state.

A coupled-oscillator model of olfactory bulb gamma oscillations

The olfactory bulb transforms not only the information content of the primary sensory representation, but also its underlying coding metric. High-variance, slow-timescale primary odor representations are transformed by bulbar circuitry into secondary representations based on principal neuron spike patterns that are tightly regulated in time. This emergent fast timescale for signaling is reflected in gamma-band local field potentials, presumably serving to efficiently integrate olfactory sensory information into the temporally regulated information networks of the central nervous system. To understand this transformation and its integration with interareal coordination mechanisms requires that we understand its fundamental dynamical principles. Using a biophysically explicit, multiscale model of olfactory bulb circuitry, we here demonstrate that an inhibition-coupled intrinsic oscillator framework, pyramidal resonance interneuron network gamma (PRING), best captures the diversity of physiological properties exhibited by the olfactory bulb. Most importantly, these properties include global zero-phase synchronization in the gamma band, the phase-restriction of informative spikes in principal neurons with respect to this common clock, and the robustness of this synchronous oscillatory regime to multiple challenging conditions observed in the biological system. These conditions include substantial heterogeneities in afferent activation levels and excitatory synaptic weights, high levels of uncorrelated background activity among principal neurons, and spike frequencies in both principal neurons and interneurons that are irregular in time and much lower than the gamma frequency. This coupled cellular oscillator architecture permits stable and replicable ensemble responses to diverse sensory stimuli under various external conditions as well as to changes in network parameters arising from learning-dependent synaptic plasticity.

Internal Cholinergic Regulation of Learning and Recall in a Model of Olfactory Processing

In the olfactory system, cholinergic modulation has been associated with contrast modulation and changes in receptive fields in the olfactory bulb, as well the learning of odor associations in olfactory cortex. Computational modeling and behavioral studies suggest that cholinergic modulation could improve sensory processing and learning while preventing pro-active interference when task demands are high. However, how sensory inputs and/or learning regulate incoming modulation has not yet been elucidated. We here use a computational model of the olfactory bulb, piriform cortex (PC) and horizontal limb of the diagonal band of Broca (HDB) to explore how olfactory learning could regulate cholinergic inputs to the system in a closed feedback loop. In our model, the novelty of an odor is reflected in firing rates and sparseness of cortical neurons in response to that odor and these firing rates can directly regulate learning in the system by modifying cholinergic inputs to the system. In the model, cholinergic neurons reduce their firing in response to familiar odors—reducing plasticity in the PC, but increase their firing in response to novel odor—increasing PC plasticity. Recordings from HDB neurons in awake behaving rats reflect predictions from the model by showing that a subset of neurons decrease their firing as an odor becomes familiar.

A model of cholinergic modulation in olfactory bulb and piriform cortex

In this work we investigate in a computational model how cholinergic inputs to the olfactory bulb (OB) and piriform cortex (PC) modulate odor representations. We use experimental data derived from different physiological studies of ACh modulation of the bulbar and cortical circuitry and the interaction between these two areas. The results presented here indicate that cholinergic modulation in the OB significantly increases contrast and synchronization in mitral cell output. Each of these effects is derived from distinct neuronal interactions, with different groups of interneurons playing different roles. Both bulbar modulation effects contribute to more stable learned representations in PC, with pyramidal networks trained with cholinergic-modulated inputs from the bulb exhibiting more robust learning than those trained with unmodulated bulbar inputs. This increased robustness is evidenced as better recovery of memories from corrupted patterns and lower-concentration inputs as well as increased memory capacity.

On-Center/Inhibitory-Surround Decorrelation via Intraglomerular Inhibition in the Olfactory Bulb Glomerular Layer

Classical lateral inhibition, which relies on spatially ordered neural representations of physical stimuli, cannot decorrelate sensory representations in which stimulus properties are represented non-topographically. Recent theoretical and experimental studies indicate that such a non-topographical representation of olfactory stimuli predominates in olfactory bulb, thereby refuting the classical view that olfactory decorrelation is mediated by lateral inhibition comparable to that in the retina. Questions persist, however, regarding how well non-topographical decorrelation models can replicate the inhibitory “surround” that has been observed experimentally (with respect to odor feature-similarity) in olfactory bulb principal neurons, analogous to the spatial inhibitory surround generated by lateral inhibition in retina. Using two contrasting scenarios of stimulus representation – one “retinotopically” organized and one in which receptive fields are unpredictably distributed as they are in olfactory bulb – we here show that intracolumnar inhibitory interactions between local interneurons and principal neurons successfully decorrelate similar sensory representations irrespective of the scenario of representation. In contrast, lateral inhibitory interactions between these same neurons in neighboring columns are only able to effectively decorrelate topographically organized representations. While anatomical substrates superficially consistent with both types of inhibition exist in olfactory bulb, of the two only local intraglomerular inhibition suffices to mediate olfactory decorrelation.

Nonlinear effects of noradrenergic modulation of olfactory bulb function in adult rodents

The mammalian main olfactory bulb receives a significant noradrenergic input from the locus coeruleus. Norepinephrine (NE) is involved in acquisition of conditioned odor preferences in neonatal animals, in some species-specific odor-dependent behaviors, and in adult odor perception. We provide a detailed review of the functional role of NE in adult rodent main olfactory bulb function. We include cellular, synaptic, network, and behavioral data and use computational simulations to tie these different types of data together.

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.

Bulbar acetylcholine enhances neural and perceptual odor discrimination

Experimental and modeling data suggest that the circuitry of the main olfactory bulb (OB) plays a critical role in olfactory discrimination. Processing of such information arises from the interaction between OB output neurons local interneurons, as well as interactions between the OB network and centrifugal inputs. Cholinergic input to the OB in particular has been hypothesized to regulate mitral cell odorants receptive fields (ORFs) and behavioral discrimination of similar odorants. We recorded from individual mitral cells in the OB in anesthetized rats to determine the degree of overlap in ORFs of individual mitral cells after exposure to odorant stimuli. Increasing the efficacy of the cholinergic neurotransmission in the OB by addition of the anticholinesterase drug neostigmine (20 mM) sharpened the ORF responses of mitral cells. Furthermore, coaddition of either the nicotinic antagonist methyllycaconitine citrate hydrate (MLA) (20 mM) or muscarinic antagonist scopolamine (40 mM) together with neostigmine (20 mM) attenuated the neostigmine-dependent sharpening of ORFs. These electrophysiological findings are predictive of accompanying behavioral experiments in which cholinergic modulation was manipulated by direct infusion of neostigmine, MLA, and scopolamine into the OB during olfactory behavioral tasks. Increasing the efficacy of cholinergic action in the OB increased perceptual discrimination of odorants in these experiments, whereas blockade of nicotinic or muscarinic receptors decreased perceptual discrimination. These experiments show that behavioral discrimination is modulated in a manner predicted by the changes in mitral cell ORFs by cholinergic drugs. These results together present a first direct comparison between neural and perceptual effects of a bulbar neuromodulator.

Noradrenergic regulation of GABAergic inhibition of main olfactory bulb mitral cells varies as a function of concentration and receptor subtype

The main olfactory bulb (MOB) receives a rich noradrenergic innervation from the pontine nucleus locus coeruleus (LC). Previous studies indicate that norepinephrine (NE) modulates the strength of GABAergic inhibition in MOB. However, the nature of this modulation and the NE receptors involved remain controversial. The goal of this study was to investigate the role of NE receptor subtypes in modulating the GABAergic inhibition of mitral cells using patch-clamp electrophysiology in rat MOB slices. NE concentration dependently and bi-directionally modulated GABA(A) receptor-mediated spontaneous and miniature inhibitory postsynaptic currents (sIPSCs/mIPSCs) recorded in mitral cells. Low doses of NE suppressed sIPSCs and mIPSCs because of activation of alpha2 receptors. Intermediate concentrations of NE increased sIPSCs and mIPSCs primarily because of activation of alpha1 receptors. In contrast, activation of beta receptors increased sIPSCs but not mIPSCs. These results indicate that NE release regulates the strength of GABAergic inhibition of mitral cells depending on the NE receptor subtype activated. Functionally, the differing affinity of noradrenergic receptor subtypes seems to allow for dynamic modulation of GABAergic inhibition in MOB as function of the extracellular NE concentration, which in turn, is regulated by behavioral state.

Olfactory system gamma oscillations: the physiological dissection of a cognitive neural system

Oscillatory phenomena have been a focus of dynamical systems research since the time of the classical studies on the pendulum by Galileo. Fast cortical oscillations also have a long and storied history in neurophysiology, and olfactory oscillations have led the way with a depth of explanation not present in the literature of most other cortical systems. From the earliest studies of odor-evoked oscillations by Adrian, many reports have focused on mechanisms and functional associations of these oscillations, in particular for the so-called gamma oscillations. As a result, much information is now available regarding the biophysical mechanisms that underlie the oscillations in the mammalian olfactory system. Recent studies have expanded on these and addressed functionality directly in mammals and in the analogous insect system. Sub-bands within the rodent gamma oscillatory band associated with specific behavioral and cognitive states have also been identified. All this makes oscillatory neuronal networks a unique interdisciplinary platform from which to study neurocognitive and dynamical phenomena in intact, freely behaving animals. We present here a summary of what has been learned about the functional role and mechanisms of gamma oscillations in the olfactory system as a guide for similar studies in other cortical systems.

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.

An argument for an olfactory thalamus

The mammalian olfactory system is unique in that sensory receptors synapse directly into the olfactory bulb of the forebrain without the thalamic relay that is common to all other sensory pathways. We argue that the olfactory bulb has an equivalent role to the thalamus, because the two regions have very similar structures and functions. Both the thalamus and the olfactory bulb are the final stage in sensory processing before reaching target cortical regions, at which there is a massive increase in neuron and synapse numbers. Thus, both structures act as a bottleneck that is a target for various modulatory inputs, and this arrangement enables efficient control of information flow before cortical processing occurs.

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.

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.

Differential effects of unilateral olfactory deprivation on noradrenergic and cholinergic systems in the main olfactory bulb of the rat

The lack of environmental olfactory stimulation produced by sensory deprivation causes significant changes in the deprived olfactory bulb. Olfactory transmission in the main olfactory bulb (MOB) is strongly modulated by centrifugal systems. The present report examines the effects of unilateral deprivation on the noradrenergic and cholinergic centrifugal systems innervating the MOB. The morphology, distribution, and density of positive axons were studied in the MOBs of control and deprived rats, using dopamine-beta-hydroxylase (DBH)-immunohistochemistry and acetylcholinesterase (AChE) histochemistry in serial sections. Catecholamine content was compared among the different groups of MOBs (control, contralateral, and ipsilateral to the deprivation) using high-performance liquid chromatography analysis. Sensory deprivation revealed that the noradrenergic system developed adaptive plastic changes after olfactory deprivation, including important modifications in its fiber density and distribution, while no differences in cholinergic innervation were observed under the same conditions. The noradrenergic system underwent an important alteration in the glomerular layer, in which some glomeruli showed a dense noradrenergic innervation that was not detected in control animals. The DBH-positive glomeruli with the highest noradrenergic fiber density were compared with AChE-stained sections and it was observed that the strongly noradrenergic-innervated glomeruli were always atypical glomeruli (characterized by their strong degree of cholinergic innervation). In addition to the morphological findings, our biochemical data revealed that olfactory deprivation caused a decrease in the content of dopamine and its metabolite 3,4-dihydroxyphenylacetic acid in the ipsilateral MOB in comparison to the contralateral and control MOBs, together with an increase in noradrenaline levels in both the ipsilateral and contralateral MOBs. Our results show that regulation of the noradrenergic centrifugal system in the MOB depends on environmental olfactory stimulation and that it is highly reactive to sensory deprivation. By contrast, the cholinergic system is fairly stable and does not exhibit clear changes after the loss of sensory inputs.

Learning-induced modulation of oscillatory activities in the mammalian olfactory system: the role of the centrifugal fibres

In the mammalian olfactory system, oscillations related to odour representation have been described in field potential activities. Previous results showed that in olfactory bulb (OB) of awake rats engaged in an olfactory learning, odour presentation produced a decrease of oscillations in gamma frequency range (60-90 Hz) associated with a power increase in beta frequency range (15-40 Hz). This response pattern was strongly amplified in trained animals. The aim of this work was twofold: whether learning also induces similar changes in OB target structures and whether such OB response depends on its centrifugal inputs. Local field potentials (LFPs) were recorded through chronically implanted electrodes in the OB, piriform and enthorhinal cortices of freely moving rats performing an olfactory discrimination. Oscillatory activities characteristics (amplitude, frequency and time-course) were extracted in beta and gamma range by a wavelet analysis. First, we found that odour induced beta oscillatory activity was present not only in the OB, but also in the other olfactory structures. In each recording site, characteristics of the beta oscillatory responses were dependent of odour, structure and learning level. Unilateral section of the olfactory peduncle was made before training, and LFPs were symmetrically recorded in the two bulbs all along the acquisition of the learning task. Data showed that deprivation of centrifugal feedback led to an increase of spontaneous gamma activity. Moreover, under this condition olfactory learning was no longer associated with the typical large beta band. As a whole, learning modulation of the beta oscillatory response in olfactory structures may reflect activity of a distributed functional network involved in odour representation.

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.

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.

Computational modeling suggests that response properties rather than spatial position determine connectivity between olfactory glomeruli

Olfactory responses require the representation of high-dimensional olfactory stimuli within the constraints of two-dimensional neural networks. We used a computational model of the honeybee antennal lobe to test how inhibitory interactions in the antennal lobe should be organized to best reproduce the experimentally measured input-output function in this structure. Our simulations show that a functionally organized inhibitory network, as opposed to an anatomically or all-to-all organized inhibitory network, best reproduces the input-output function of the antennal lobe observed with calcium imaging. In this network, inhibition between each pair of glomeruli was proportional to the similarity of their odor-response profiles. We conclude that contrast enhancement between odorants in the honeybee antennal lobe is best achieved when interglomerular inhibition is organized based on glomerular odor response profiles rather than on anatomical neighborhood relations.

Configurational and elemental odor mixture perception can arise from local inhibition

Contrast enhancement via lateral inhibitory circuits is a common mechanism in sensory systems. We here employ a computational model to show that, in addition to shaping experimentally observed molecular receptive fields in the olfactory bulb, functionally lateral inhibitory circuits can also mediate the elemental and configurational properties of odor mixture perception. To the extent that odor perception can be predicted by slow-timescale neural activation patterns in the olfactory bulb, and to the extent that interglomerular inhibitory projections map onto a space of odorant similarity, the model shows that these inhibitory processes in the olfactory bulb suffice to generate the behaviorally observed inverse relationship between two odorants' perceptual similarities and the perceptual similarities between either of these same odorants and their binary mixture.

Retrograde tracing of zinc-enriched (ZEN) neuronal somata projecting to the olfactory bulb

Zinc ions seem to be important to several neurological functions and have been connected to the pathophysiology of epilepsy, neuronal cell death after seizure or stroke, and Alzheimer's disease. Both epilepsy and Alzheimer's disease are clinical conditions believed to involve the olfactory bulb. The mammalian olfactory bulb is densely innervated by zinc-enriched (ZEN) neurons, and the distribution of the ZEN terminals in the mouse olfactory bulb has previously been described. The aim of this study was to describe the origins of ZEN terminals projecting into the main olfactory bulb of the rat. Selective labeling of ZEN terminals was accomplished by intracerebral infusion of sodium selenide, whereby zinc selenium clusters are created in the ZEN terminals. Some of these clusters move by retrograde axonal transport to the somata where they can be silver-enhanced by autometallography (AMG). After infusion of sodium selenide into the main olfactory bulb, retrogradely labeled ZEN somata were found (1) ipsilaterally in all anterior olfactory nuclei, taenia tecta, piriform cortex and lateral entorhinal cortex, and (2) contralaterally in anterior olfactory nuclei except the external division. The ipsilateral anterior olfactory nucleus had the densest population of ZEN somata, and it was found that these somata originated mainly from pyramidal neurons in layers II and III of each area. The olfactory-related centrifugal afferents to the main olfactory bulb are discussed.

Cholinergic modulation of sensory representations in the olfactory bulb

We present a computational model of the mammalian olfactory bulb (OB) designed to investigate how cholinergic inputs modulate olfactory sensory representations. The model integrates experimental data derived from diverse physiological studies of cholinergic modulation of OB circuitry into a simulation of bulbar responses to realistic odorants. Experimentally-observed responses to a homologous series of odorants (unbranched aliphatic aldehydes) were simulated; realistic cholinergic inputs to the OB model served to increase the discriminability of the bulbar responses generated to very similar odorants. This simulation predicted, correctly, that missing cholinergic inputs to the OB would result in greater generalization between similar aliphatic aldehydes. Based on the assumption that the overlap between the neural representations of two sensory stimuli is predictive of their perceptual similarity, we tested this prediction in a behavioral experiments with rats. We show that, indeed, rats with selective lesions of cholinergic neurons that project to the OB and cortex discriminate less well between aliphatic aldehydes with similar carbon chain lengths than do rats that received sham lesions.

How synchronization properties among second-order sensory neurons can mediate stimulus salience

Spatial patterns of glomerular activity in the vertebrate olfactory bulb and arthropod antennal lobe reflect an important component of first-order olfactory representation and contribute to odorant identification. Higher concentration odor stimuli evoke broader glomerular activation patterns, resulting in greater spatial overlap among different odor representations. However, behavioral studies demonstrate results contrary to what these data might suggest: Honeybees are more, not less, able to discriminate among odorants applied at higher concentrations. Using a computational model of the honeybee antennal lobe, the authors show that changes in synchronization patterns among antennal lobe projection neurons, as observed electrophysiologically, could parsimoniously underlie these observations. The results suggest that stimulus salience, as defined behaviorally, is directly correlated with the degree of synchronization among second-order olfactory neurons.

Disruption of GABA(A) receptors on GABAergic interneurons leads to increased oscillatory power in the olfactory bulb network

Synchronized neural activity is believed to be essential for many CNS functions, including neuronal development, sensory perception, and memory formation. In several brain areas GABA(A) receptor-mediated synaptic inhibition is thought to be important for the generation of synchronous network activity. We have used GABA(A) receptor beta3 subunit deficient mice (beta3-/-) to study the role of GABAergic inhibition in the generation of network oscillations in the olfactory bulb (OB) and to reveal the role of such oscillations in olfaction. The expression of functional GABA(A) receptors was drastically reduced (>93%) in beta3-/- granule cells, the local inhibitory interneurons of the OB. This was revealed by a large reduction of muscimol-evoked whole-cell current and the total current mediated by spontaneous, miniature inhibitory postsynaptic currents (mIPSCs). In beta3-/- mitral/tufted cells (principal cells), there was a two-fold increase in mIPSC amplitudes without any significant change in their kinetics or frequency. In parallel with the altered inhibition, there was a significant increase in the amplitude of theta (80% increase) and gamma (178% increase) frequency oscillations in beta3-/- OBs recorded in vivo from freely moving mice. In odor discrimination tests, we found beta3-/- mice to be initially the same as, but better with experience than beta3+/+ mice in distinguishing closely related monomolecular alcohols. However, beta3-/- mice were initially better and then worse with practice than control mice in distinguishing closely related mixtures of alcohols. Our results indicate that the disruption of GABA(A) receptor-mediated synaptic inhibition of GABAergic interneurons and the augmentation of IPSCs in principal cells result in increased network oscillations in the OB with complex effects on olfactory discrimination, which can be explained by an increase in the size or effective power of oscillating neural cell assemblies among the mitral cells of beta3-/- mice.

Selective loss of cholinergic neurons projecting to the olfactory system increases perceptual generalization between similar, but not dissimilar, odorants

The neuromodulator acetylcholine is thought to modulate information processing in the olfactory system. The authors used 192 IgG-saporin, a lesioning agent selective for basal forebrain cholinergic neurons, to determine whether selective lesions of cholinergic neurons projecting to the olfactory bulb and cortex affect odor perception in rats. Lesioned and sham-operated rats were tested in an olfactory generalization paradigm with sets of chemically related odorants (n-aliphatic aldehydes, acids, and alcohols). Lesioned rats generalized more between chemically similar odorants but did not differ from controls in their response to chemically unrelated odorants or in acquisition of the conditioned odor. Results show that cholinergic inputs to the olfactory system influence perceptual qualities of odorants and confirm predictions made by computational models of this system.

Model for olfactory discrimination and learning in Limax procerebrum incorporating oscillatory dynamics and wave propagation

We extend our model of the procerebral (PC) lobe of Limax, which is comprised of a layer of coupled oscillators and a layer of memory neurons, each layer 4 rows by 20 columns, corresponding to the cell body layer (burster cells) and neuropil layer (nonburster cells) of the PC lobe. A gradient of connections in the layer of model burster cells induces periodic wave propagation, as measured in the PC lobe. We study odor representations in the biological PC lobe using the technique of Kimura and coworkers. Lucifer yellow injection into intact Limax after appetitive or aversive odor learning results in a band or patch of labeled cells in the PC lobe with the band long axis normal to the axis of wave propagation. Learning two odors yields two parallel bands of labeled PC cells. We introduce olfactory input to our model PC lobe such that each odor maximally activates a unique row of four cells which produces a short-term memory trace of odor stimulation. A winner-take-all synaptic competition enabled by collapse of the phase gradient during odor presentation produces a single short-term memory band for each odor. The short-term memory is converted to long-term memory if odor stimulation is followed by activation of an input pathway for the unconditioned stimulus (US) which presumably results in release of one or more neuromodulatory amines or peptides in the PC lobe.

Zinc-enriched (ZEN) terminals in mouse olfactory bulb

The present study was designed to localize zinc-enriched (ZEN) terminals in mouse olfactory bulb by means of ZnT3 immunocytochemistry (ICC) and zinc autometallography (AMG). The immunocytochemical staining of ZnT3 was closely correlated with the AMG pattern. ZEN terminals were defined as terminals showing both ZnT3 immunoreactivities and AMG granules. At the light microscopic level, dense staining patterns for ZnT3 immunoreactivity were seen in the granule cell layer and the olfactory glomerular layer. At the ultrastructural level, ZEN terminals were restricted to presynaptic terminals with single or multiple postsynaptic thickenings. The postsynaptic profiles contacting ZEN terminals appeared to be dendrites or somata of granule cells in the granule cell layer and periglomerular cells and mitral/tufted (M/T) cells in the olfactory glomerular layer. This suggests that two main sources of ZEN terminals are present in mouse olfactory bulb: (1) centrifugal fibres making asymmetrical synapses with granule cells and periglomerular cells, and (2) olfactory receptor terminals contacting dendritic profiles of M/T cells or periglomerular cells. The close correlation between ZEN terminals and the glutamatergic system is discussed.

Neural activity in the horizontal limb of the diagonal band of broca can be modulated by electrical stimulation of the olfactory bulb and cortex in rats

Previously published theoretical models of olfactory processing suggest that cholinergic modulatory inputs to the olfactory system should be regulated by neural activity in the olfactory bulb. We tested these predictions using in vivo electrophysiology in rats. We show that the activity of approximately 20% of neurons recorded in the horizontal limb of the diagonal band of Broca (HDB), which is the source of cholinergic projections to the olfactory system, can be modulated by electrical stimulation of either the lateral olfactory tract or the cell body layer of piriform cortex. These data suggest a possible physiological pathway for the proposed regulation of neural activity in the HDB by activity in the olfactory bulb or cortex.

Generalization between binary odor mixtures and their components in the rat

We have adopted a conditioning paradigm to investigate generalization between odor mixtures and components. Rats were conditioned to find a reward buried in odor-scented cups. The conditioned odor was either a mixture (O1 + O2) or a pure component (O1). Once they learned the task to criterion, they were tested in random sequence for response to that O1, O1 + O2 and to an unrelated odor (O3). Generalization was consistently the strongest from O1 to O1 + O2 or from O1 + O2 to O1. Furthermore. the degree of generalization depended on the odorants used as O1, O2, and O3. This latter finding in a particular indicates that this assay can be used to assess properties of mixtures, which could arise at either peripheral or more central locations.

Electrical stimulation of the horizontal limb of the diagonal band of broca modulates population EPSPs in piriform cortex

Electrical stimulation of the horizontal limb of the diagonal band of Broca (HDB) was coupled with recording of evoked potentials in the piriform cortex. Stimulation of the HDB caused an enhancement of the late, disynaptic component of the evoked potential elicited by stimulation of the lateral olfactory tract but caused a suppression of the synaptic potential elicited by stimulation of the posterior piriform cortex. The muscarinic antagonist scopolamine blocked both effects of HDB stimulation. The enhancement of disynaptic potentials could be due to cholinergic depolarization of pyramidal cells, whereas the suppression of potentials evoked by posterior piriform stimulation could be due to presynaptic inhibition of intrinsic fiber synaptic transmission by acetylcholine.

Characterizing complex chemosensors: information-theoretic analysis of olfactory systems

The mechanisms that underlie a wine lover's ability to identify a favorite vintage and a dog's ability to track the scent of a lost child are still deep mysteries. Our understanding of these olfactory phenomena is confounded by the difficulty encountered when attempting to identify the parameters that define odor stimuli, by the broad tuning and variability of neurons in the olfactory pathway,and by the distributed nature of olfactory encoding. These issues pertain to both biological systems and to newly developed 'artificial noses' that seek to mimic these natural processes. Information theory, which quantifies explicitly the extent to which the state of one system (for example, the universe of all odors) relates to the state of another (for example, the responses of an odor-sensing device),can serve as a basis for analysing both natural and engineered odor sensors. This analytical approach can be used to explore the problems of defining stimulus dimensions, assessing strategies of neuronal processing, and examining the properties of biological systems that emerge from interactions among their complex components. It can also serve to optimize the design of artificial olfactory devices for a variety of applications, which include process control, medical diagnostics and the detection of explosives.

Glutamate and synaptic plasticity at mammalian primary olfactory synapses

Glutamate is the transmitter at synapses from the olfactory nerve (ON) to mitral (Mi)/tufted cells, but very little is known about the functional properties of this synapse. This report summarizes in vitro physiological and computational modeling studies investigating glutamatergic neurotransmission at ON-->Mi cell synapses. Single ON shocks in rat main olfactory bulb (MOB) slices elicit distinct early and late spiking components triggered, respectively, by (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)/kainic acid (KA) and N-methyl-D-aspartate (NMDA) receptor activation. Modeling simulations showed that the placement of both AMPA/KA and NMDA receptors on Mi apical dendrites replicates the experimentally observed early and late Mi spiking responses to ON shocks. Brief, tetanic ON stimulation in vitro induced robust, selective long-term potentiation (LTP) of NMDA receptor-dependent spiking. Modeling experiments disclosed several potential mechanisms underlying the selective LTP of NMDA receptor-dependent spiking. These findings demonstrate that ON-->Mi cell transmission exhibits a novel form of plasticity whereby high frequency synaptic activity induces selective LTP of NMDA receptor-dependent spiking.

Computational models of neuromodulation

Computational modeling of neural substrates provides an excellent theoretical framework for the understanding of the computational roles of neuromodulation. In this review, we illustrate, with a large number of modeling studies, the specific computations performed by neuromodulation in the context of various neural models of invertebrate and vertebrate preparations. We base our characterization of neuromodulations on their computational and functional roles rather than on anatomical or chemical criteria. We review the main framework in which neuromodulation has been studied theoretically (central pattern generation and oscillations, sensory processing, memory and information integration). Finally, we present a detailed mathematical overview of how neuromodulation has been implemented at the single cell and network levels in modeling studies. Overall, neuromodulation is found to increase and control computational complexity.

A re-estimation of the number of glomeruli and mitral cells in the olfactory bulb of rabbit

Although morphological characteristics of mammalian olfactory bulb (OB) are well documented in rodents (rat, mice), only one study has been performed in rabbit, which is also commonly used in olfactory research. The study carried out by Allison and Warwick in 1949 led to surprising results still quoted in recent literature. The present study re-examined this question in young rabbit OB, while it was also conducted with rat as control. In five animals of each species, areas and coordinates of glomeruli and mitral profiles were measured in 10 coronal sections uniformly distributed along the rostrocaudal axis of the OB, and a distribution-free stereological method was used to compute values along this axis. For glomeruli, the estimated number was 4200 in rat and 6300 in rabbit. While this estimation matched with those found in literature in rat, it strikingly differed from the Allison and Warwick's value of 1900 only. For mitral cell number, we found 59,600 while the preceding study found 45,000 only. In contrast to the number of glomeruli, the mitral cell number in rat and rabbit were very close. Indeed, results showed 56, 200 in rat. The results suggest that the numbers of olfactory glomeruli and mitral cells has been previously underestimated in rabbit, and that the number of glomeruli changes as a function of species. In addition, both the results of the present study and reports in the literature suggest the number of mitral cells to be rather similar in mouse, rat and rabbit. As a consequence, the glomerular/mitral cell ratio is likely to varied in a wide range across species.

Modulation of inhibition in a model of olfactory bulb reduces overlap in the neural representation of olfactory stimuli

In a neural model of olfactory bulb processing, we demonstrate the putative role of the modulation of two types of inhibition, inspired by electrophysiological data on the effect of acetylcholine and noradrenaline on olfactory bulb synaptic transmission. Feedback regulation of modulation based on bulbar activity serves to 'normalize' the activity of output neurons in response to different levels of input activities. This mechanism also decreases the overlap between pairs of output patterns (Mitral cell activities), enhancing the discrimination between overlapping olfactory input patterns. The effect of the modulation at the two levels of interneurons is complementary: while an increase in periglomerular inhibition decreases the number of responding output neurons, a decrease in granule cell inhibition increases the firing frequencies of these neurons.