Cholinergic modulation of sensory representations in the olfactory bulb

The Response Dynamics and Function of Cholinergic and GABAergic Neurons in the Basal Forebrain During Olfactory Learning

Modulation of neural circuits is essential for flexible sensory perception and decision-making in a changing environment. Cholinergic and GABAergic projections to the olfactory system from the horizontal limb of the diagonal band of Broca (HDB) in the basal forebrain are crucial for odor detection and olfactory learning. Although studies have demonstrated that HDB neurons respond during olfactory learning, how cholinergic and GABAergic neurons differ in their response dynamics and roles in olfactory learning remains unclear. In this study, we examined the response profiles of these two subpopulations of neurons during passive odor exposure and associative olfactory learning. We show that the excitatory responses in both cholinergic and GABAergic neurons tended to habituate during repeated passive odor exposure. However, while these habituated responses were also observed in GABAergic neurons during a go-go task, there was no such habituation in cholinergic neurons. Moreover, the responses to S+ and S− trials diverged in cholinergic neurons once mice learned a go/no-go task. Furthermore, the chemogenetic inactivation of cholinergic neurons in the HDB impaired odor discrimination. Together, these findings suggest that cholinergic neurons in the HDB reflect attention to positive reinforcement and may regulate odor discrimination via top–down inputs to the olfactory system.

Rapid Effects of Vagus Nerve Stimulation on Sensory Processing Through Activation of Neuromodulatory Systems

After sensory information is encoded into neural signals at the periphery, it is processed through multiple brain regions before perception occurs (i.e., sensory processing). Recent work has begun to tease apart how neuromodulatory systems influence sensory processing. Vagus nerve stimulation (VNS) is well-known as an effective and safe method of activating neuromodulatory systems. There is a growing body of studies confirming VNS has immediate effects on sensory processing across multiple sensory modalities. These immediate effects of VNS on sensory processing are distinct from the more well-documented method of inducing lasting neuroplastic changes to the sensory pathways through repeatedly delivering a brief VNS burst paired with a sensory stimulus. Immediate effects occur upon VNS onset, often disappear upon VNS offset, and the modulation is present for all sensory stimuli. Conversely, the neuroplastic effect of pairing sub-second bursts of VNS with a sensory stimulus alters sensory processing only after multiple pairing sessions, this alteration remains after cessation of pairing sessions, and the alteration selectively affects the response properties of neurons encoding the specific paired sensory stimulus. Here, we call attention to the immediate effects VNS has on sensory processing. This review discusses existing studies on this topic, provides an overview of the underlying neuromodulatory systems that likely play a role, and briefly explores the potential translational applications of using VNS to rapidly regulate sensory processing.

Connectivity and dynamics in the olfactory bulb

Dendrodendritic interactions between excitatory mitral cells and inhibitory granule cells in the olfactory bulb create a dense interaction network, reorganizing sensory representations of odors and, consequently, perception. Large-scale computational models are needed for revealing how the collective behavior of this network emerges from its global architecture. We propose an approach where we summarize anatomical information through dendritic geometry and density distributions which we use to calculate the connection probability between mitral and granule cells, while capturing activity patterns of each cell type in the neural dynamical systems theory of Izhikevich. In this way, we generate an efficient, anatomically and physiologically realistic large-scale model of the olfactory bulb network. Our model reproduces known connectivity between sister vs. non-sister mitral cells; measured patterns of lateral inhibition; and theta, beta, and gamma oscillations. The model in turn predicts testable relationships between network structure and several functional properties, including lateral inhibition, odor pattern decorrelation, and LFP oscillation frequency. We use the model to explore the influence of cortex on the olfactory bulb, demonstrating possible mechanisms by which cortical feedback to mitral cells or granule cells can influence bulbar activity, as well as how neurogenesis can improve bulbar decorrelation without requiring cell death. Our methodology provides a tractable tool for other researchers.

Cortical feedback and gating in odor discrimination and generalization

A central question in neuroscience is how context changes perception. In the olfactory system, for example, experiments show that task demands can drive divergence and convergence of cortical odor responses, likely underpinning olfactory discrimination and generalization. Here, we propose a simple statistical mechanism for this effect based on unstructured feedback from the central brain to the olfactory bulb, which represents the context associated with an odor, and sufficiently selective cortical gating of sensory inputs. Strikingly, the model predicts that both convergence and divergence of cortical odor patterns should increase when odors are initially more similar, an effect reported in recent experiments. The theory in turn predicts reversals of these trends following experimental manipulations and in neurological conditions that increase cortical excitability.

Role of Adult-Born Versus Preexisting Neurons Born at P0 in Olfactory Perception in a Complex Olfactory Environment in Mice

Olfactory perceptual learning is defined as an improvement in the discrimination of perceptually close odorants after passive exposure to these odorants. In mice, simple olfactory perceptual learning involving the discrimination of two odorants depends on an increased number of adult-born neurons in the olfactory bulb, which refines the bulbar output. However, the olfactory environment is complex, raising the question of the adjustment of the bulbar network to multiple discrimination challenges. Perceptual learning of 1 to 6 pairs of similar odorants led to discrimination of all learned odor pairs. Increasing complexity did not increase adult-born neuron survival but enhanced the number of adult-born neurons responding to learned odorants and their spine density. Moreover, only complex learning induced morphological changes in neurons of the granule cell layer born during the first day of life (P0). Selective optogenetic inactivation of either population confirmed functional involvement of adult-born neurons regardless of the enrichment complexity, while preexisting neurons were required for complex discrimination only.

Long-Range GABAergic Inhibition Modulates Spatiotemporal Dynamics of the Output Neurons in the Olfactory Bulb

Local interneurons of the olfactory bulb (OB) are densely innervated by long-range GABAergic neurons from the basal forebrain (BF), suggesting that this top-down inhibition regulates early processing in the olfactory system. However, how GABAergic inputs modulate the OB output neurons, the mitral/tufted cells, is unknown. Here, in male and female mice acute brain slices, we show that optogenetic activation of BF GABAergic inputs produced distinct local circuit effects that can influence the activity of mitral/tufted cells in the spatiotemporal domains. Activation of the GABAergic axons produced a fast disinhibition of mitral/tufted cells consistent with a rapid and synchronous release of GABA onto local interneurons in the glomerular and inframitral circuits of the OB, which also reduced the spike precision of mitral/tufted cells in response to simulated stimuli. In addition, BF GABAergic inhibition modulated local oscillations in a layer-specific manner. The intensity of locally evoked θ oscillations was decreased on activation of top-down inhibition in the glomerular circuit, while evoked γ oscillations were reduced by inhibition of granule cells. Furthermore, BF GABAergic input reduced dendrodendritic inhibition in mitral/tufted cells. Together, these results suggest that long-range GABAergic neurons from the BF are well suited to influence temporal and spatial aspects of processing by OB circuits.SIGNIFICANCE STATEMENT Disruption of GABAergic inhibition from the basal forebrain (BF) to the olfactory bulb (OB) impairs the discrimination of similar odors, yet how this centrifugal inhibition influences neuronal circuits in the OB remains unclear. Here, we show that the BF GABAergic neurons exclusively target local inhibitory neurons in the OB, having a functional disinhibitory effect on the output neurons, the mitral cells. Phasic inhibition by BF GABAergic neurons reduces spike precision of mitral cells and lowers the intensity of oscillatory activity in the OB, while directly modulating the extent of dendrodendritic inhibition. These circuit-level effects of this centrifugal inhibition can influence the temporal and spatial dynamics of odor coding in the OB.

Olfactory bulb surroundings can help to distinguish Parkinson's disease from non-parkinsonian olfactory dysfunction

BACKGROUND

The olfactory bulb is one of the first regions of insult in Parkinson's disease (PD), consistent with the early onset of olfactory dysfunction. Investigations of the olfactory bulb may, therefore, help early pre-motor diagnosis. We aimed to investigate olfactory bulb and its surrounding regions in PD-related olfactory dysfunction when specifically compared to other forms of non-parkinsonian olfactory dysfunction (NPOD) and healthy controls.

METHODS

We carried out MRI-based olfactory bulb volume measurements from T2-weighted imaging in scans from 15 patients diagnosed with PD, 15 patients with either post-viral or sinonasal NPOD and 15 control participants. Further, we applied a deep learning model (convolutional neural network; CNN) to scans of the olfactory bulb and its surrounding area to classify PD-related scans from NPOD-related scans.

RESULTS

Compared to controls, both PD and NPOD patients had smaller olfactory bulbs, when measured manually (both p < .001) whereas no difference was found between PD and NPOD patients. In contrast, when a CNN was used to differentiate between PD patients and NPOD patients, an accuracy of 88.3% was achieved. The cortical area above the olfactory bulb which stretches around and into the olfactory sulcus appears to be a region of interest in the differentiation between PD and NPOD patients.

CONCLUSION

Measures from and around the olfactory bulb in combination with the use of a deep learning model may help differentiate PD patients from patients with NPOD, which may be used to develop early diagnostic tools based on olfactory dysfunction.

Input dependent modulation of olfactory bulb activity by HDB GABAergic projections

Basal forebrain modulation of central circuits is associated with active sensation, attention, and learning. While cholinergic modulations have been studied extensively the effect of non-cholinergic basal forebrain subpopulations on sensory processing remains largely unclear. Here, we directly compare optogenetic manipulation effects of two major basal forebrain subpopulations on principal neuron activity in an early sensory processing area, i.e. mitral/tufted cells (MTCs) in the olfactory bulb. In contrast to cholinergic projections, which consistently increased MTC firing, activation of GABAergic fibers from basal forebrain to the olfactory bulb leads to differential modulation effects: while spontaneous MTC activity is mainly inhibited, odor-evoked firing is predominantly enhanced. Moreover, sniff-triggered averages revealed an enhancement of maximal sniff evoked firing amplitude and an inhibition of firing rates outside the maximal sniff phase. These findings demonstrate that GABAergic neuromodulation affects MTC firing in a bimodal, sensory-input dependent way, suggesting that GABAergic basal forebrain modulation could be an important factor in attention mediated filtering of sensory information to the brain.

Acetylcholine Regulates Olfactory Perceptual Learning through Effects on Adult Neurogenesis

Learning to perceptually discriminate between chemical signals in the environment (olfactory perceptual learning [OPL]) is critical for survival. Multiple mechanisms have been implicated in OPL, including modulation of neurogenesis and manipulation of cholinergic activity. However, whether these represent distinct processes regulating OPL or if cholinergic effects on OPL depend upon neurogenesis has remained an unresolved question. Using a combination of pharmacological and optogenetic approaches, cholinergic activity was shown to be both necessary and sufficient to drive OPL, and this process was dependent on the presence of newly born cells in the olfactory bulb (OB). This study is the first to directly demonstrate that cholinergic effects on OPL require adult OB neurogenesis.

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Acetylcholine modulates olfactory perceptual learning

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Cholinergic modulation alters olfactory bulb neurogenesis

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Cholinergic effects on olfactory perceptual learning require adult neurogenesis

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Cholinergic excitation does not alter the phenotype of newborn olfactory bulb cells

Expression Profiling of Cytokine, Cholinergic Markers, and Amyloid-β Deposition in the APPSWE/PS1dE9 Mouse Model of Alzheimer's Disease Pathology

Background:

Alzheimer’s disease (AD), a neurodegenerative disease, is associated with dysfunction of the olfactory and the entorhinal cortex of the brain that control memory and cognitive functions and other daily activities. Pro-inflammatory cytokines, amyloid-β (Aβ), and the cholinergic system play vital roles in the pathophysiology of AD. However, the role of changes in cholinergic system components, Aβ accumulation, and cytokines in both the olfactory and entorhinal cortex is not known clearly.

Objective:

The present study is aimed to evaluate the changes of cholinergic system components, Aβ accumulation, and cytokines in both the olfactory bulb (OB) and entorhinal cortex (EC) of young and aged APP_SWE/PS1dE9 transgenic (Tg) mice.

Methods:

We have explored the changes of cholinergic system components, Aβ accumulation, and expression profiling of cytokines in the OB and EC of aged APPswe transgenic mice and age-matched wild type mice using quantitative Real-Time PCR assays and immunohistochemistry techniques.

Results:

In aged Tg mice, a significant increase of expression of interleukin (IL)-1β, tumor necrosis factor (TNF)-α, and chemokine MCP1 (p < 0.001, p < 0.001, and p = 0.001, respectively) and a significant reduction of nAChRα4 (p = 0.048) and AChE (p = 0.023) was observed when compared with age-matched wild type mice. Higher levels of AChE and BuChE are expressed in OB and EC of the APP_SWE/PS1dE9 of Tg mice. Aβ accumulation was observed in OB and EC of the APP_SWE/PS1dE9 of Tg mice.

Conclusion:

The study demonstrates the expression profiling of pro-inflammatory cytokines and cholinergic markers as well as Aβ accumulation in OB and EC of the APP_SWE/PS1dE9 Tg mice. Moreover, the study also demonstrated that the APP_SWE/PS1dE9 Tg mice can be useful as a mouse model to understand the role of pro-inflammatory cytokines and cholinergic markers in pathophysiology of AD.

Intrinsic and Extrinsic Neuromodulation of Olfactory Processing

Neuromodulation is a ubiquitous feature of neural systems, allowing flexible, context specific control over network dynamics. Neuromodulation was first described in invertebrate motor systems and early work established a basic dichotomy for neuromodulation as having either an intrinsic origin (i.e., neurons that participate in network coding) or an extrinsic origin (i.e., neurons from independent networks). In this conceptual dichotomy, intrinsic sources of neuromodulation provide a “memory” by adjusting network dynamics based upon previous and ongoing activation of the network itself, while extrinsic neuromodulators provide the context of ongoing activity of other neural networks. Although this dichotomy has been thoroughly considered in motor systems, it has received far less attention in sensory systems. In this review, we discuss intrinsic and extrinsic modulation in the context of olfactory processing in invertebrate and vertebrate model systems. We begin by discussing presynaptic modulation of olfactory sensory neurons by local interneurons (LNs) as a mechanism for gain control based on ongoing network activation. We then discuss the cell-class specific effects of serotonergic centrifugal neurons on olfactory processing. Finally, we briefly discuss the integration of intrinsic and extrinsic neuromodulation (metamodulation) as an effective mechanism for exerting global control over olfactory network dynamics. The heterogeneous nature of neuromodulation is a recurring theme throughout this review as the effects of both intrinsic and extrinsic modulation are generally non-uniform.

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.

Preservation of Essential Odor-Guided Behaviors and Odor-Based Reversal Learning after Targeting Adult Brain Serotonin Synthesis

The neurotransmitter serotonin (5-HT) is considered a powerful modulator of sensory system organization and function in a wide range of animals. The olfactory system is innervated by midbrain 5-HT neurons into both its primary and secondary odor-processing stages. Facilitated by this circuitry, 5-HT and its receptors modulate olfactory system function, including odor information input to the olfactory bulb. It is unknown, however, whether the olfactory system requires 5-HT for even its most basic behavioral functions. To address this question, we established a conditional genetic approach to specifically target adult brain tryptophan hydroxylase 2 (Tph2), encoding the rate-limiting enzyme in brain 5-HT synthesis, and nearly eliminate 5-HT from the mouse forebrain. Using this novel model, we investigated the behavior of 5-HT-depleted mice during performance in an olfactory go/no-go task. Surprisingly, the near elimination of 5-HT from the forebrain, including the olfactory bulbs, had no detectable effect on the ability of mice to perform the odor-based task. Tph2-targeted mice not only were able to learn the task, but also had levels of odor acuity similar to those of control mice when performing coarse odor discrimination. Both groups of mice spent similar amounts of time sampling odors during decision-making. Furthermore, odor reversal learning was identical between 5-HT-depleted and control mice. These results suggest that 5-HT neurotransmission is not necessary for the most essential aspects of olfaction, including odor learning, discrimination, and certain forms of cognitive flexibility.

Structural basis for cholinergic regulation of neural circuits in the mouse olfactory bulb

Odor information is regulated by olfactory inputs, bulbar interneurons, and centrifugal inputs in the olfactory bulb (OB). Cholinergic neurons projecting from the nucleus of the horizontal limb of the diagonal band of Broca and the magnocellular preoptic nucleus are one of the primary centrifugal inputs to the OB. In this study, we focused on cholinergic regulation of the OB and analyzed neural morphology with a particular emphasis on the projection pathways of cholinergic neurons. Single-cell imaging of a specific neuron within dense fibers is critical to evaluate the structure and function of the neural circuits. We labeled cholinergic neurons by infection with virus vector and then reconstructed them three-dimensionally. We also examined the ultramicrostructure of synapses by electron microscopy tomography. To further clarify the function of cholinergic neurons, we performed confocal laser scanning microscopy to investigate whether other neurotransmitters are present within cholinergic axons in the OB. Our results showed the first visualization of complete cholinergic neurons, including axons projecting to the OB, and also revealed frequent axonal branching within the OB where it innervated multiple glomeruli in different areas. Furthermore, electron tomography demonstrated that cholinergic axons formed asymmetrical synapses with a morphological variety of thicknesses of the postsynaptic density. Although we have not yet detected the presence of other neurotransmitters, the range of synaptic morphology suggests multiple modes of transmission. The present study elucidates the ways that cholinergic neurons could contribute to the elaborate mechanisms involved in olfactory processing in the OB. J. Comp. Neurol. 525:574-591, 2017. © 2016 Wiley Periodicals, Inc.

Plasticity in the Olfactory System: Lessons for the Neurobiology of Memory

We are rapidly advancing toward an understanding of the molecular events underlying odor transduction, mechanisms of spatiotemporal central odor processing, and neural correlates of olfactory perception and cognition. A thread running through each of these broad components that define olfaction appears to be their dynamic nature. How odors are processed, at both the behavioral and neural level, is heavily dependent on past experience, current environmental context, and internal state. The neural plasticity that allows this dynamic processing is expressed nearly ubiquitously in the olfactory pathway, from olfactory receptor neurons to the higher-order cortex, and includes mechanisms ranging from changes in membrane excitability to changes in synaptic efficacy to neurogenesis and apoptosis. This review will describe recent findings regarding plasticity in the mammalian olfactory system that are believed to have general relevance for understanding the neurobiology of memory.

Basal forebrain dynamics during nonassociative and associative olfactory learning

Cholinergic and GABAergic projections from the horizontal diagonal band (HDB) and medial preoptic area (MCPO) of the basal forebrain to the olfactory system are associated with odor discrimination and odor learning, as well as modulation of neural responses in olfactory structures. Whereas pharmacological and lesion studies give insights into the functional role of these modulatory inputs on a slow timescale, the response dynamics of neurons in the HDB/MCPO during olfactory behaviors have not been investigated. In this study we examined how these neurons respond during two olfactory behaviors: spontaneous investigation of odorants and odor-reward association learning. We observe rich heterogeneity in the response dynamics of individual HDB/MCPO neurons, with a substantial fraction of neurons exhibiting task-related modulation. HDB/MCPO neurons show both rapid and transient responses during bouts of odor investigation and slow, long-lasting modulation of overall response rate based on behavioral demands. Specifically, baseline rates were higher during the acquisition phase of an odor-reward association than during spontaneous investigation or the recall phase of an odor reward association. Our results suggest that modulatory projections from the HDB/MCPO are poised to influence olfactory processing on multiple timescales, from hundreds of milliseconds to minutes, and are therefore capable of rapidly setting olfactory network dynamics during odor processing and learning.

Differential Muscarinic Modulation in the Olfactory Bulb

Neuromodulation of olfactory circuits by acetylcholine (ACh) plays an important role in odor discrimination and learning. Early processing of chemosensory signals occurs in two functionally and anatomically distinct regions, the main and accessory olfactory bulbs (MOB and AOB), which receive extensive cholinergic input from the basal forebrain. Here, we explore the regulation of AOB and MOB circuits by ACh, and how cholinergic modulation influences olfactory-mediated behaviors in mice. Surprisingly, despite the presence of a conserved circuit, activation of muscarinic ACh receptors revealed marked differences in cholinergic modulation of output neurons: excitation in the AOB and inhibition in the MOB. Granule cells (GCs), the most abundant intrinsic neuron in the OB, also exhibited a complex muscarinic response. While GCs in the AOB were excited, MOB GCs exhibited a dual muscarinic action in the form of a hyperpolarization and an increase in excitability uncovered by cell depolarization. Furthermore, ACh influenced the input-output relationship of mitral cells in the AOB and MOB differently showing a net effect on gain in mitral cells of the MOB, but not in the AOB. Interestingly, despite the striking differences in neuromodulatory actions on output neurons, chemogenetic inhibition of cholinergic neurons produced similar perturbations in olfactory behaviors mediated by these two regions. Decreasing ACh in the OB disrupted the natural discrimination of molecularly related odors and the natural investigation of odors associated with social behaviors. Thus, the distinct neuromodulation by ACh in these circuits could underlie different solutions to the processing of general odors and semiochemicals, and the diverse olfactory behaviors they trigger.

SIGNIFICANCE STATEMENT

State-dependent cholinergic modulation of brain circuits is critical for several high-level cognitive functions, including attention and memory. Here, we provide new evidence that cholinergic modulation differentially regulates two parallel circuits that process chemosensory information, the accessory and main olfactory bulb (AOB and MOB, respectively). These circuits consist of remarkably similar synaptic arrangement and neuronal types, yet cholinergic regulation produced strikingly opposing effects in output and intrinsic neurons. Despite these differences, the chemogenetic reduction of cholinergic activity in freely behaving animals disrupted odor discrimination of simple odors, and the investigation of social odors associated with behaviors signaled by the Vomeronasal system.

Functional differentiation of cholinergic and noradrenergic modulation in a biophysical model of olfactory bulb granule cells

Olfactory bulb granule cells are modulated by both acetylcholine (ACh) and norepinephrine (NE), but the effects of these neuromodulators have not been clearly distinguished. We used detailed biophysical simulations of granule cells, both alone and embedded in a microcircuit with mitral cells, to measure and distinguish the effects of ACh and NE on cellular and microcircuit function. Cholinergic and noradrenergic modulatory effects on granule cells were based on data obtained from slice experiments; specifically, ACh reduced the conductance densities of the potassium M current and the calcium-dependent potassium current, whereas NE nonmonotonically regulated the conductance density of an ohmic potassium current. We report that the effects of ACh and NE on granule cell physiology are distinct and functionally complementary to one another. ACh strongly regulates granule cell firing rates and afterpotentials, whereas NE bidirectionally regulates subthreshold membrane potentials. When combined, NE can regulate the ACh-induced expression of afterdepolarizing potentials and persistent firing. In a microcircuit simulation developed to investigate the effects of granule cell neuromodulation on mitral cell firing properties, ACh increased spike synchronization among mitral cells, whereas NE modulated the signal-to-noise ratio. Coapplication of ACh and NE both functionally improved the signal-to-noise ratio and enhanced spike synchronization among mitral cells. In summary, our computational results support distinct and complementary roles for ACh and NE in modulating olfactory bulb circuitry and suggest that NE may play a role in the regulation of cholinergic function.

Computational modeling suggests distinct, location-specific function of norepinephrine in olfactory bulb and piriform cortex

Noradrenergic modulation from the locus coerulus is often associated with the regulation of sensory signal-to-noise ratio. In the olfactory system, noradrenergic modulation affects both bulbar and cortical processing, and has been shown to modulate the detection of low concentration stimuli. We here implemented a computational model of the olfactory bulb and piriform cortex, based on known experimental results, to explore how noradrenergic modulation in the olfactory bulb and piriform cortex interact to regulate odor processing. We show that as predicted by behavioral experiments in our lab, norepinephrine can play a critical role in modulating the detection and associative learning of very low odor concentrations. Our simulations show that bulbar norepinephrine serves to pre-process odor representations to facilitate cortical learning, but not recall. We observe the typical non-uniform dose—response functions described for norepinephrine modulation and show that these are imposed mainly by bulbar, but not cortical processing.

Distinct roles of bulbar muscarinic and nicotinic receptors in olfactory discrimination learning

The olfactory bulb (OB) and piriform cortex receive dense cholinergic projections from the basal forebrain. Cholinergic modulation within the piriform cortex has long been proposed to serve important functions in olfactory learning and memory. We here investigate how olfactory discrimination learning is regulated by cholinergic modulation of the OB inputs to the piriform cortex. We examined rats' performance on a two-alternative choice odor discrimination task following local, bilateral blockade of cholinergic nicotinic and/or muscarinic receptors in the OB. Results demonstrate that acquisition, but not recall, of novel discrimination problems is impaired following blockade of OB cholinergic receptors, although the relative contribution of muscarinic and nicotinic receptors depends on task difficulty. Blocking muscarinic receptors impairs learning for nearly all odor sets, whereas blocking nicotinic receptors only affects performance for perceptually similar odors. This pattern of behavioral effects is consistent with predictions from a model of cholinergic modulation in the OB and piriform cortex (de Almeida et al., 2013). Model simulations suggest that muscarinic and nicotinic receptors may serve complementary roles in regulating coherence and sparseness of the OB network output, which in turn differentially regulate the strength and overlap in cortical odor representations. Overall, our results suggest that muscarinic receptor blockade results in a bona fide learning impairment that may arise because cortical neurons are activated less often. Behavioral impairment following nicotinic receptor blockade may not be due to the inability of the cortex to learn, but rather arises because the cortex is unable to resolve highly overlapping input patterns.

The orexinergic system influences conditioned odor aversion learning in the rat: a theory on the processes and hypothesis on the circuit involved

A large variety of behaviors that are essential for animal survival depend on the perception and processing of surrounding smells present in the natural environment. In particular, food-search behavior, which is conditioned by hunger, is directly driven by the perception of odors associated with food, and feeding status modulates olfactory sensitivity. The orexinergic hypothalamic peptide orexin A (OXA), one of the central and peripheral hormones that triggers food intake, has been shown to increase olfactory sensitivity in various experimental conditions including the conditioned odor aversion learning paradigm (COA). COA is an associative task that corresponds to the association between an olfactory conditioned stimulus (CS) and a delayed gastric malaise. Previous studies have shown that this association is formed only if the delay separating the CS presentation from the malaise is short, suggesting that the memory trace of the odor is relatively unstable. To test the selectivity of the OXA system in olfactory sensitivity, a recent study compared the effects of fasting and of central infusion of OXA during the acquisition of COA. Results showed that the increased olfactory sensitivity induced by fasting and by OXA infusion was accompanied by enhanced COA learning performances. In reference to the duration of action of OXA, the present work details the results obtained during the successive COA extinction tests and suggests a hypothesis concerning the role of the OXA component of fasting on the memory processes underlying CS-malaise association during COA. Moreover, referring to previous data in the literature we suggest a functional circuit model where fasting modulates olfactory memory processes through direct and/or indirect activation of particular OXA brain targets including the olfactory bulb, the locus coeruleus (LC) and the amygdala.

Context-driven activation of odor representations in the absence of olfactory stimuli in the olfactory bulb and piriform cortex

Sensory neural activity is highly context dependent and shaped by experience and expectation. In the olfactory bulb (OB), the first cerebral relay of olfactory processing, responses to odorants are shaped by previous experiences including contextual information thanks to strong feedback connections. In the present experiment, mice were conditioned to associate an odorant with a visual context and were then exposed to the visual context alone. We found that the visual context alone elicited exploration of the odor port similar to that elicited by the stimulus when it was initially presented. In the OB, the visual context alone elicited a neural activation pattern, assessed by mapping the expression of the immediate early gene zif268 (egr-1) that was highly similar to that evoked by the conditioned odorant, but not other odorants. This OB activation was processed by olfactory network as it was transmitted to the piriform cortex. Interestingly, a novel context abolished neural and behavioral responses. In addition, the neural representation in response to the context was dependent on top-down inputs, suggesting that context-dependent representation is initiated in cortex. Modeling of the experimental data suggests that odor representations are stored in cortical networks, reactivated by the context and activate bulbar representations. Activation of the OB and the associated behavioral response in the absence of physical stimulus showed that mice are capable of internal representations of sensory stimuli. The similarity of activation patterns induced by imaged and the corresponding physical stimulus, triggered only by the relevant context provides evidence for an odor-specific internal representation.

Duration and specificity of olfactory nonassociative memory

Olfactory habituation is a simple form of nonassociative memory in which responsiveness to stable but behaviorally nonsignificant stimuli is decreased. Olfactory habituation has recently become a paradigm widely used to probe the neural substrate underlying olfactory perception and memory. This simple behavioral paradigm has been used successfully used to probe many aspects of olfactory processing, and it has recently become clear that the neural processes underlying olfactory habituation can depend on the task parameters used. We here further investigate memory specificity and duration using 2 variations in task parameters: the number of habituation trials and the time delay between habituation and cross-habituation testing. We find that memory specificity increases with the number of habituation trials but decreases with time after the last habituation trial.

Noradrenergic but not cholinergic modulation of olfactory bulb during processing of near threshold concentration stimuli

Neuromodulatory systems such as noradrenaline (NE), acetylcholine (ACh), and serotonin (5HT) serve important functions in sensory perception. We use the olfactory bulb (OB) as a model system to study the roles of individual neuromodulators in sensory perception. Using a spontaneous, nonreward motivated detection task, as well as a reward-motivated task, we show that rats can easily respond to odorants at very low concentrations when motivated to do so in a food-rewarded task, despite not showing spontaneous responses to these low concentration odorants. Using the same tasks paired with local bulbar infusions of noradrenergic and cholinergic drugs, we then show that rats engage their noradrenergic, but not their cholinergic system, to better respond to near threshold odorants. These results suggest that while cholinergic modulation of OB function is mostly important for odor decorrelation and discrimination, noradrenergic modulation is important for signal-to-noise modulation.

Cholinergic modulation during acquisition of olfactory fear conditioning alters learning and stimulus generalization in mice

We investigated the role of cholinergic neurotransmission in olfactory fear learning. Mice receiving pairings of odor and foot shock displayed fear to the trained odor the following day. Pretraining injections of the nicotinic antagonist mecamylamine had no effect on subsequent freezing, while the muscarinic antagonist scopolamine significantly reduced freezing. To test whether cholinergic manipulation affected fear generalization, mice were presented with odors similar to the trained odor. Generalization was increased following pretraining scopolamine, while the muscarinic agonist oxotremorine decreased generalization. These results suggest that muscarinic neurotransmission during the acquisition of olfactory association modulates both the strength and specificity of learning.

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.

Optogenetic activation of basal forebrain cholinergic neurons modulates neuronal excitability and sensory responses in the main olfactory bulb

The main olfactory bulb (MOB) in mammals receives massive centrifugal input from cholinergic neurons in the horizontal limb of the diagonal band of Broca (HDB) in the basal forebrain, the activity of which is thought to be correlated with animal behaving states, such as attention. Cholinergic signals in the bulb facilitate olfactory discrimination and learning, but it has remained controversial how the activity of HDB cholinergic neurons modulates neuronal excitability and olfactory responses in the MOB. In this study, we used an optogenetic approach to selectively activate HDB cholinergic neurons and recorded the effect of this activation on the spontaneous firing activity and odor-evoked responses of mouse MOB neurons. Cells were juxtacellularly labeled and their specific types were morphologically determined. We find that light stimulation of HDB cholinergic neurons inhibits the spontaneous firing activity of all major cell types, including mitral/tufted (M/T) cells, periglomerular (PG) cells, and GABAergic granule cells. Inhibitions are significantly produced by stimulation at 10 Hz and further enhanced at higher frequencies. In addition, cholinergic activation sharpens the olfactory tuning curves of a majority of M/T cells but broadly potentiates odor-evoked responses of PG cells and granule cells. These results demonstrate strong effects of the basal forebrain cholinergic system on modulating neuronal excitability in the MOB and support the hypothesis that cholinergic activity increases olfactory discrimination capability.

Blocking muscarinic receptors in the olfactory bulb impairs performance on an olfactory short-term memory task

Cholinergic inputs to cortical processing networks have long been associated with attentional and top-down processing. Experimental and theoretical studies suggest that cholinergic inputs to the main olfactory bulb (OB) can modulate both neural and behavioral odor discrimination. Previous experiments from our laboratory and others demonstrate that blockade of nicotinic receptors directly impairs olfactory discrimination, whereas blockade of muscarinic receptors only measurably impairs olfactory perception when task demands are made more challenging, such as when very low-concentration odors are used or rats are required to maintain sensory memory over long durations. To further investigate the role of muscarinic signaling in the OB, we developed an olfactory delayed match-to-sample task using a digging-based behavioral paradigm. We find that rats are able to maintain robust short-term odor memory for 10–100 s. To investigate the role of muscarinic signaling in task performance, we bilaterally infused scopolamine into the OB. We find that high dosages of scopolamine (38 mM) impair performance on the task across all delays tested, including the baseline condition with no delay, whereas lower dosages (7.6 mM and 22.8 mM) had no measureable effects. These results indicate that general execution of the match-to-sample task, even with no delay, is at least partially dependent on muscarinic signaling in the OB.

Noradrenergic and cholinergic modulation of olfactory bulb sensory processing

Neuromodulation in sensory perception serves important functions such as regulation of signal to noise ratio, attention, and modulation of learning and memory. Neuromodulators in specific sensory areas often have highly similar cellular, but distinct behavioral effects. To address this issue, we here review the function and role of two neuromodulators, acetylcholine (Ach) and noradrenaline (NE) for olfactory sensory processing in the adult main olfactory bulb. We first describe specific bulbar sensory computations, review cellular effects of each modulator and then address their specific roles in bulbar sensory processing. We finally put these data in a behavioral and computational perspective.

Nicotinic receptor-mediated filtering of mitral cell responses to olfactory nerve inputs involves the α3β4 subtype

Acetylcholine (ACh) plays a major role in the processing of sensory inputs. Cholinergic input to the mammalian olfactory bulb modulates odor discrimination and perceptual learning by mechanisms that have yet to be elucidated. We have used the mouse olfactory bulb to examine the role of nicotinic ACh receptors (nAChRs) in regulating the responses of mitral cells (MCs), the output neurons of the olfactory bulb, to olfactory nerve input. We show that ACh activates α3β4* nAChRs (* denotes the possible presence of other subunits) on MCs, leading to their excitation. Despite depolarizing MCs directly, the net effect of nAChR activation is to suppress olfactory nerve-evoked responses in these cells via activity-dependent feedback GABAergic mechanisms. Our results indicate that nAChRs gate incoming olfactory nerve input wherein weak input stimuli are filtered out, whereas strong stimuli are transmitted via the MCs. Based on our observations, we provide a mechanistic model for the sharpening of MC receptive fields by nAChRs, which could aid in odor discrimination and perceptual learning.

Functional and laminar dissociations between muscarinic and nicotinic cholinergic neuromodulation in the tree shrew primary visual cortex

Acetylcholine is an important neuromodulator involved in cognitive function. The impact of cholinergic neuromodulation on computations within the cortical microcircuit is not well understood. Here we investigate the effects of layer-specific cholinergic drug application in the tree shrew primary visual cortex during visual stimulation with drifting grating stimuli of varying contrast and orientation. We describe differences between muscarinic and nicotinic cholinergic effects in terms of both the layer of cortex and the attribute of visual representation. Nicotinic receptor activation enhanced the contrast response in the granular input layer of the cortex, while tending to reduce neural selectivity for orientation across all cortical layers. Muscarinic activation modestly enhanced the contrast response across cortical layers, and tended to improve orientation tuning. This resulted in highest orientation selectivity in the supra- and infragranular layers, where orientation selectivity was already greatest in the absence of pharmacological stimulation. Our results indicate that laminar position plays a crucial part in functional consequences of cholinergic stimulation, consistent with the differential distribution of cholinergic receptors. Nicotinic receptors function to enhance sensory representations arriving in the cortex, whereas muscarinic receptors act to boost the cortical computation of orientation tuning. Our findings suggest close homology between cholinergic mechanisms in tree shrew and primate visual cortices.

Functional Differences between Global Pre- and Postsynaptic Inhibition in the Drosophila Olfactory Circuit

The Drosophila antennal lobe is subdivided into multiple glomeruli, each of which represents a unique olfactory information processing channel. In each glomerulus, feedforward input from olfactory receptor neurons (ORNs) is transformed into activity of projection neurons (PNs), which represent the output. Recent investigations have indicated that lateral presynaptic inhibitory input from other glomeruli controls the gain of this transformation. Here, we address why this gain control acts “pre”-synaptically rather than “post”-synaptically. Postsynaptic inhibition could work similarly to presynaptic inhibition with regard to regulating the firing rates of PNs depending on the stimulus intensity. We investigate the differences between pre- and postsynaptic gain control in terms of odor discriminability by simulating a network model of the Drosophila antennal lobe with experimental data. We first demonstrate that only presynaptic inhibition can reproduce the type of gain control observed in experiments. We next show that presynaptic inhibition decorrelates PN responses whereas postsynaptic inhibition does not. Due to this effect, presynaptic gain control enhances the accuracy of odor discrimination by a linear decoder while its postsynaptic counterpart only diminishes it. Our results provide the reason gain control operates “pre”-synaptically but not “post”-synaptically in the Drosophila antennal lobe.

Sequential mechanisms underlying concentration invariance in biological olfaction

Concentration invariance-the capacity to recognize a given odorant (analyte) across a range of concentrations-is an unusually difficult problem in the olfactory modality. Nevertheless, humans and other animals are able to recognize known odors across substantial concentration ranges, and this concentration invariance is a highly desirable property for artificial systems as well. Several properties of olfactory systems have been proposed to contribute to concentration invariance, but none of these alone can plausibly achieve full concentration invariance. We here propose that the mammalian olfactory system uses at least six computational mechanisms in series to reduce the concentration-dependent variance in odor representations to a level at which different concentrations of odors evoke reasonably similar representations, while preserving variance arising from differences in odor quality. We suggest that the residual variance then is treated like any other source of stimulus variance, and categorized appropriately into "odors" via perceptual learning. We further show that naïve mice respond to different concentrations of an odorant just as if they were differences in quality, suggesting that, prior to odor categorization, the learning-independent compensatory mechanisms are limited in their capacity to achieve concentration invariance.

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.

Neural correlates of olfactory learning: Critical role of centrifugal neuromodulation

The mammalian olfactory system is well established for its remarkable capability of undergoing experience-dependent plasticity. Although this process involves changes at multiple stages throughout the central olfactory pathway, even the early stages of processing, such as the olfactory bulb and piriform cortex, can display a high degree of plasticity. As in other sensory systems, this plasticity can be controlled by centrifugal inputs from brain regions known to be involved in attention and learning processes. Specifically, both the bulb and cortex receive heavy inputs from cholinergic, noradrenergic, and serotonergic modulatory systems. These neuromodulators are shown to have profound effects on both odor processing and odor memory by acting on both inhibitory local interneurons and output neurons in both regions.

Cholinergic modulation of neuronal excitability in the accessory olfactory bulb

The accessory olfactory bulb (AOB), the first relay of chemosensory information in the Vomeronasal system, receives extensive cholinergic innervation from the basal forebrain. Cholinergic modulation of neuronal activity in the olfactory bulb has been hypothesized to play an important role in olfactory processing; however, little is known about the cellular actions of acetylcholine (ACh) within the AOB. Here using in vitro slice preparation, we show that muscarinic acetylcholine receptor (mAChR) activation increases neuronal excitability of granule and mitral/tufted cells (GCs and MCs) in the AOB. Activation of mAChRs increased excitability of GCs by three distinct mechanisms: induction of a long-lasting depolarization, activation of a slow afterdepolarization (sADP), and an increase in excitatory glutamatergic input due to MC depolarization. The depolarization and sADP were elicited by the selective agonist 4-[[[(3-chlorophenyl)amino]carbonyl]oxy]-N,N,N-trimethyl-2-butyn-1-aminium chloride (100 μM) and blocked by low concentrations of pirenzepine (300 nM), indicating that they result from activation of M1-like mAChRs. In contrast, cholinergic stimulation increased the excitability of MCs via recruitment of nicotinic AChRs (nAChRs) and M1-like mAChRs. Submaximal activation of these receptors, however, decreased the excitability of MCs. Surprisingly, we found that unlike GCs in the main olfactory bulb, GCs in the AOB are excited by mAChR activation in young postnatal neurons, suggesting marked differences in cholinergic regulation of development between these two regions of the olfactory bulb.

Compensatory responses to age-related decline in odor quality acuity: cholinergic neuromodulation and olfactory enrichment

The perceptual differentiation of odors can be measured behaviorally using generalization gradients. The steepness of these gradients defines a form of olfactory acuity for odor quality that depends on neural circuitry within the olfactory bulb and is regulated by cholinergic activity therein as well as by associative learning. Using this system as a reduced model for age-related cognitive decline, we show that aged mice, while maintaining almost the same baseline behavioral performance as younger mice, are insensitive to the effects of acutely elevated acetylcholine, which sharpens generalization gradients in young adult mice. Moreover, older mice exhibit evidence of chronically elevated acetylcholine levels in the olfactory bulb, suggesting that their insensitivity to further elevated levels of acetylcholine may arise because the maximum capacity of the system to respond to acetylcholine has already been reached. We propose a model in which an underlying, age-related, progressive deficit is mitigated by a compensatory cholinergic feedback loop that acts to retard the behavioral effects of what would otherwise be a substantial age-related decline in olfactory plasticity. We also treated mice with 10-day regimens of olfactory environmental enrichment and/or repeated systemic injections of the acetylcholinesterase inhibitor physostigmine. Each treatment alone sharpened odor quality acuity, but administering both treatments together had no greater effect than either alone. Age was not a significant main effect in this study, suggesting that some capacity for acetylcholine-dependent plasticity is still present in aged mice despite their sharply reduced ability to respond to acute increases in acetylcholine levels. These results suggest a dynamical framework for understanding age-related decline in neural circuit processing in which the direct effects of aging can be mitigated, at least temporarily, by systemic compensatory responses. In particular, a decline in cholinergic efficacy can precede any breakdown in cholinergic production, which may help explain the limited effectiveness of cholinergic replacement therapies in combating cognitive decline.

Ablation of mouse adult neurogenesis alters olfactory bulb structure and olfactory fear conditioning

Adult neurogenesis replenishes olfactory bulb (OB) interneurons throughout the life of most mammals, yet during this constant flux it remains unclear how the OB maintains a constant structure and function. In the mouse OB, we investigated the dynamics of turnover and its impact on olfactory function by ablating adult neurogenesis with an x-ray lesion to the sub-ventricular zone (SVZ). Regardless of the magnitude of the lesion to the SVZ, we found no change in the survival of young adult born granule cells (GCs) born after the lesion, and a gradual decrease in the population of GCs born before the lesion. After a lesion producing a 96% reduction of incoming adult born GCs to the OB, we found a diminished behavioral fear response to conditioned odor cues but not to audio cues. Interestingly, despite this behavioral deficit and gradual anatomical changes, we found no electrophysiological changes in the GC population assayed in vivo through dendro-dendritic synaptic plasticity and odor-evoked local field potential oscillations. These data indicate that turnover in the granule cell layer is generally decoupled from the rate of adult neurogenesis, and that OB adult neurogenesis plays a role in a wide behavioral system extending beyond the OB.

Dopaminergic modulation of olfactory bulb processing affects odor discrimination learning in rats

Olfactory behavioral studies have shown that, when modulated through systemic injections, D1 and D2 receptors have opposing effects on odor discrimination learning. In the present study, cannulated male Sprague-Dawley rats were used to investigate how the modulation of these 2 types of dopaminergic receptors through direct infusion of D1/D2 agonists and antagonists into the olfactory bulb affect olfactory perception. Dopaminergic modulation was locally altered by manipulations of D1 (agonist SKF 82958: 14.6, 43.8, & 143.6 mM; antagonist SCH-23390: 13.4, 40.1, & 60.1 mM) and D2 (agonists quinpirole: 78.2, 117.3, & 156.4 mM; antagonist sulpiride: 0.3, 0.9, & 2.9 mM) receptors during a simultaneous odor discrimination task. The authors found that modulation of D2, but not D1, receptors significantly affected rats' odor discrimination performance. A significant positive correlation between blockade of D2 receptors and discrimination performance, as well as a significant negative correlation between D2 receptor activation and discrimination performance, was observed.

Odor-specific habituation arises from interaction of afferent synaptic adaptation and intrinsic synaptic potentiation in olfactory cortex

Segmentation of target odorants from background odorants is a fundamental computational requirement for the olfactory system and is thought to be behaviorally mediated by olfactory habituation memory. Data from our laboratory have shown that odor-specific adaptation in piriform neurons, mediated at least partially by synaptic adaptation between the olfactory bulb outputs and piriform cortex pyramidal cells, is highly odor specific, while that observed at the synaptic level is specific only to certain odor features. Behavioral data show that odor habituation memory at short time constants corresponding to synaptic adaptation is also highly odor specific and is blocked by the same pharmacological agents as synaptic adaptation. Using previously developed computational models of the olfactory system we show here how synaptic adaptation and potentiation interact to create the observed specificity of response adaptation. The model analyzes the mechanisms underlying the odor specificity of habituation, the dependence on functioning cholinergic modulation, and makes predictions about connectivity to and within the piriform neural network. Predictions made by the model for the role of cholinergic modulation are supported by behavioral results.

Modeling nicotinic neuromodulation from global functional and network levels to nAChR based mechanisms

Neuromodulator action has received increasing attention in theoretical neuroscience. Yet models involving both neuronal populations dynamics at the circuit level and detailed receptor properties are only now being developed. Here we review recent computational approaches to neuromodulation, focusing specifically on acetylcholine (ACh) and nicotine. We discuss illustrative examples of models ranging from functional top-down to neurodynamical bottom-up. In the top-down approach, a computational theory views ACh as encoding the uncertainty expected in an environment. A different line of models accounts for neural population dynamics treating ACh as toggling neuronal networks between read-in of information and recall of memory. Building on the neurodynamics idea we discuss two models of nicotine's action with increasing degree of biological realism. Both consider explicitly receptor-level mechanisms but with different scales of detail. The first is a large-scale model of nicotine-dependent modulation of dopaminergic signaling that is capable of simulating nicotine self-administration. The second is a novel approach where circuit-level neurodynamics of the ventral tegmental area (VTA) are combined with explicit models of the dynamics of specific nicotinic ACh receptor subtypes. We show how the model is constructed based on local anatomy, electrophysiology and receptor properties and provide an illustration of its potential. In particular, we show how the model can shed light on the specific mechanisms by which nicotine controls dopaminergic neurotransmission in the VTA. This model serves us to conclude that detailed accounts for neuromodulator action at the basis of behavioral and cognitive models are crucial to understand how neuromodulators mediate their functional properties.

Multiple learning parameters differentially regulate olfactory generalization

Sensory representations depend strongly on the descending regulation of perceptual processing. Generalization among similar stimuli is a fundamental cognitive process that defines the extent of the variance in physical stimulus properties that becomes categorized together and associated with a common contingency, thereby establishing units of meaning. The olfactory system provides an experimentally tractable model system in which to study the interactions of these physical and psychological factors within the framework of their underlying neurophysiological mechanisms. The authors here show that olfactory associative learning systematically regulates gradients of odor generalization. Specifically, increasing odor-reward pairings, odor concentration, or reward quality--each a determinant of associative learning--significantly transformed olfactory generalization gradients, each narrowing the range of variance in odor quality perceived as likely to share the learned contingency of a conditioned odor stimulus. However, differences in the qualitative features of these three transformations suggest that these different determinants of learning are not necessarily theoretically interchangeable. These results demonstrate that odor representations are substantially shaped by experience and descending influences.

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.

State dependence of network output: modeling and experiments

Emerging experimental evidence suggests that both networks and their component neurons respond to similar inputs differently, depending on the state of network activity. The network state is determined by the intrinsic dynamical structure of the network and may change as a function of neuromodulation, the balance or stochasticity of synaptic inputs to the network, and the history of network activity. Much of the knowledge on state-dependent effects comes from comparisons of awake and sleep states of the mammalian brain. Yet, the mechanisms underlying these states are difficult to unravel. Several vertebrate and invertebrate studies have elucidated cellular and synaptic mechanisms of state dependence resulting from neuromodulation, sensory input, and experience. Recent studies have combined modeling and experiments to examine the computational principles that emerge when network state is taken into account; these studies are highlighted in this article. We discuss these principles in a variety of systems (mammalian, crustacean, and mollusk) to demonstrate the unifying theme of state dependence of network output.

Muscarinic receptor activation modulates granule cell excitability and potentiates inhibition onto mitral cells in the rat olfactory bulb

The olfactory bulb is a second-order brain region that connects sensory neurons with cortical areas. However, the olfactory bulb does not appear to play a simple relay role and is subject instead to extensive local and extrinsic synaptic influences. Prime among the external, or centrifugal, inputs is the dense cholinergic innervation from the basal forebrain, which terminates in both the granule cell and plexiform layers. Cholinergic inputs to the bulb have been implicated in olfactory working memory tasks in rodents and may be related to olfactory deficits reported in people with neurodegenerative disorders that involve basal forebrain neurons. In this study, we use whole-cell recordings from acute rat slices to demonstrate that one function of this input is to potentiate the excitability of GABAergic granule cells and thereby modulate inhibitory drive onto mitral cells. This increase in granule cell excitability is mediated by a concomitant decrease in the normal afterhyperpolarization response and augmentation of an afterdepolarization, both triggered by pirenzepine-sensitive M1 receptors. The afterdepolarization was dependent on elevations in intracellular calcium and appeared to be mediated by a calcium-activated nonselective cation current (I(CAN)). Near firing threshold, depolarizing inputs could evoke quasipersistent firing characterized by irregular discharges that lasted, on average, for 2 min. In addition to regulating the excitability of the primary interneuronal subtype in the bulb, M1 receptors regulate the degree of adaptation that occurs during repetitive sniffing-like inputs and may therefore play a critical role in regulating short-term plasticity in the olfactory system.

Olfactory bulb gamma oscillations are enhanced with task demands

Fast oscillations in neural assemblies have been proposed as a mechanism to facilitate stimulus representation in a variety of sensory systems across animal species. In the olfactory system, intervention studies suggest that oscillations in the gamma frequency range play a role in fine odor discrimination. However, there is still no direct evidence that such oscillations are intrinsically altered in intact systems to aid in stimulus disambiguation. Here we show that gamma oscillatory power in the rat olfactory bulb during a two-alternative choice task is modulated in the intact system according to task demands with dramatic increases in gamma power during discrimination of molecularly similar odorants in contrast to dissimilar odorants. This elevation in power evolves over the course of criterion performance, is specific to the gamma frequency band (65-85 Hz), and is independent of changes in the theta or beta frequency band range. Furthermore, these high amplitude gamma oscillations are restricted to the olfactory bulb, such that concurrent piriform cortex recordings show no evidence of enhanced gamma power during these high-amplitude events. Our results display no modulation in the power of beta oscillations (15-28 Hz) shown previously to increase with odor learning in a Go/No-go task, and we suggest that the oscillatory profile of the olfactory system may be influenced by both odor discrimination demands and task type. The results reported here indicate that enhancement of local gamma power may reflect a switch in the dynamics of the system to a strategy that optimizes stimulus resolution when input signals are ambiguous.