Neurobiology of a Simple Memory

Plasticity in inhibitory networks improves pattern separation in early olfactory processing

Distinguishing between nectar and non-nectar odors is challenging for animals due to shared compounds and varying ratios in complex mixtures. Changes in nectar production throughout the day and over the animal's lifetime add to the complexity. The honeybee olfactory system, containing fewer than 1000 principal neurons in the early olfactory relay, the antennal lobe (AL), must learn to associate diverse volatile blends with rewards. Previous studies identified plasticity in the AL circuits, but its role in odor learning remains poorly understood. Using a biophysical computational model, tuned by in vivo electrophysiological data, and live imaging of the honeybee's AL, we explored the neural mechanisms of plasticity in the AL. Our findings revealed that when trained with a set of rewarded and unrewarded odors, the AL inhibitory network suppresses responses to shared chemical compounds while enhancing responses to distinct compounds. This results in improved pattern separation and a more concise neural code. Our calcium imaging data support these predictions. Analysis of a graph convolutional neural network performing an odor categorization task revealed a similar mechanism for contrast enhancement. Our study provides insights into how inhibitory plasticity in the early olfactory network reshapes the coding for efficient learning of complex odors.

Predictive filtering of sensory response via orbitofrontal top-down input

Habituation is a crucial sensory filtering mechanism whose dysregulation can lead to a continuously intense world in disorders with sensory overload. While habituation is considered to require top-down predictive signaling to suppress irrelevant inputs, the exact brain loci storing the internal predictive model and the circuit mechanisms of sensory filtering remain unclear. We found that daily neural habituation in the primary auditory cortex (A1) was reversed by inactivation of the orbitofrontal cortex (OFC). Top-down projections from the ventrolateral OFC, but not other frontal areas, carried predictive signals that grew with daily sound experience and suppressed A1 via somatostatin-expressing inhibitory neurons. Thus, prediction signals from the OFC cancel out behaviorally irrelevant anticipated stimuli by generating their "negative images" in sensory cortices.

Plasticity in inhibitory networks improves pattern separation in early olfactory processing

Distinguishing between nectar and non-nectar odors is challenging for animals due to shared compounds and varying ratios in complex mixtures. Changes in nectar production throughout the day - and potentially many times within a forager's lifetime - add to the complexity. The honeybee olfactory system, containing fewer than 1,000 principal neurons in the early olfactory relay, the antennal lobe (AL), must learn to associate diverse volatile blends with rewards. Previous studies identified plasticity in the AL circuits, but its role in odor learning remains poorly understood. Using a biophysical computational network model, tuned by in vivo electrophysiological data, and live imaging of the honeybee's AL, we explored the neural mechanisms and functions of plasticity in the early olfactory system. Our findings revealed that when trained with a set of rewarded and unrewarded odors, the AL inhibitory network suppresses shared chemical compounds while enhancing responses to distinct compounds. This results in improved pattern separation and a more concise neural code. Our calcium imaging data support these predictions. Analysis of a graph convolutional neural network performing an odor categorization task revealed a similar mechanism for contrast enhancement. Our study provides insights into how inhibitory plasticity in the early olfactory network reshapes the coding for efficient learning of complex odors.

Oxytocin induces the formation of distinctive cortical representations and cognitions biased toward familiar mice

Social recognition is essential for the formation of social structures. Many times, recognition comes with lesser exploration of familiar animals. This lesser exploration has led to the assumption that recognition may be a habituation memory. The underlying memory mechanisms and the thereby acquired cortical representations of familiar mice have remained largely unknown, however. Here, we introduce an approach directly examining the recognition process from volatile body odors among male mice. We show that volatile body odors emitted by mice are sufficient to identify individuals and that more salience is assigned to familiar mice. Familiarity is encoded by reinforced population responses in two olfactory cortex hubs and communicated to other brain regions. The underlying oxytocin-induced plasticity promotes the separation of the cortical representations of familiar from other mice. In summary, neuronal encoding of familiar animals is distinct and utilizes the cortical representational space more broadly, promoting storage of complex social relationships.

Seeing Neurodegeneration in a New Light Using Genetically Encoded Fluorescent Biosensors and iPSCs

Neurodegenerative diseases present a progressive loss of neuronal structure and function, leading to cell death and irrecoverable brain atrophy. Most have disease-modifying therapies, in part because the mechanisms of neurodegeneration are yet to be defined, preventing the development of targeted therapies. To overcome this, there is a need for tools that enable a quantitative assessment of how cellular mechanisms and diverse environmental conditions contribute to disease. One such tool is genetically encodable fluorescent biosensors (GEFBs), engineered constructs encoding proteins with novel functions capable of sensing spatiotemporal changes in specific pathways, enzyme functions, or metabolite levels. GEFB technology therefore presents a plethora of unique sensing capabilities that, when coupled with induced pluripotent stem cells (iPSCs), present a powerful tool for exploring disease mechanisms and identifying novel therapeutics. In this review, we discuss different GEFBs relevant to neurodegenerative disease and how they can be used with iPSCs to illuminate unresolved questions about causes and risks for neurodegenerative disease.

Virtual accident curb risk habituation in workers by restoring sensory responses to real-world warning

In high-risk work environments, workers become habituated to hazards they frequently encounter, subsequently underestimating risk and engaging in unsafe behaviors. This phenomenon has been termed "risk habituation" and identified as a vital root cause of fatalities and injuries at workplaces. Providing an effective intervention that curbs workers' risk habituation is critical in preventing occupational injuries and fatalities. However, there exists no empirically supported intervention for curbing risk habituation. To this end, here we investigated how experiencing an accident in a virtual reality (VR) environment affects workers' risk habituation toward repeatedly exposed workplace hazards. We examined an underlying mechanism of risk habituation at the sensory level and evaluated the effect of the accident intervention through electroencephalography (EEG). The results of pre- and posttreatment analyses indicate experiencing the virtual accident effectively curbs risk habituation at both the behavioral and sensory level. The findings open new vistas for occupational safety training.

Improving Network Training on Resource-Constrained Devices via Habituation Normalization

As a technique for accelerating and stabilizing training, the batch normalization (BN) is widely used in deep learning. However, BN cannot effectively estimate the mean and the variance of samples when training/fine-tuning with small batches of data on resource-constrained devices. It will lead to a decrease in the accuracy of the deep learning model. In the fruit fly olfactory system, the algorithm based on the "negative image" habituation model can filter redundant information and improve numerical stability. Inspired by the circuit mechanism, we propose a novel normalization method, the habituation normalization (HN). HN first eliminates the "negative image" obtained by habituation and then calculates the statistics for normalizing. It solves the problem of accuracy degradation of BN when the batch size is small. The experiment results show that HN can speed up neural network training and improve the model accuracy on vanilla LeNet-5, VGG16, and ResNet-50 in the Fashion MNIST and CIFAR10 datasets. Compared with four standard normalization methods, HN keeps stable and high accuracy in different batch sizes, which shows that HN has strong robustness. Finally, the applying HN to the deep learning-based EEG signal application system indicates that HN is suitable for the network fine-tuning and neural network applications under limited computing power and memory.

Genetics, molecular control and clinical relevance of habituation learning

Habituation is the most fundamental form of learning. As a firewall that protects our brain from sensory overload, it is indispensable for cognitive processes. Studies in humans and animal models provide increasing evidence that habituation is affected in autism and related monogenic neurodevelopmental disorders (NDDs). An integrated application of habituation assessment in NDDs and their animal models has unexploited potential for neuroscience and medical care. With the aim to gain mechanistic insights, we systematically retrieved genes that have been demonstrated in the literature to underlie habituation. We identified 258 evolutionarily conserved genes across species, describe the biological processes they converge on, and highlight regulatory pathways and drugs that may alleviate habituation deficits. We also summarize current habituation paradigms and extract the most decisive arguments that support the crucial role of habituation for cognition in health and disease. We conclude that habituation is a conserved, quantitative, cognition- and disease-relevant process that can connect preclinical and clinical work, and hence is a powerful tool to advance research, diagnostics, and treatment of NDDs.

Pairing-Dependent Plasticity in a Dissected Fly Brain Is Input-Specific and Requires Synaptic CaMKII Enrichment and Nighttime Sleep

In Drosophila, in vivo functional imaging studies revealed that associative memory formation is coupled to a cascade of neural plasticity events in distinct compartments of the mushroom body (MB). In-depth investigation of the circuit dynamics, however, will require an ex vivo model that faithfully mirrors these events to allow direct manipulations of circuit elements that are inaccessible in the intact fly. The current ex vivo models have been able to reproduce the fundamental plasticity of aversive short-term memory, a potentiation of the MB intrinsic neuron (Kenyon cells [KCs]) responses after artificial learning ex vivo However, this potentiation showed different localization and encoding properties from those reported in vivo and failed to generate the previously reported suppression plasticity in the MB output neurons (MBONs). Here, we develop an ex vivo model using the female Drosophila brain that recapitulates behaviorally evoked plasticity in the KCs and MBONs. We demonstrate that this plasticity accurately localizes to the MB α'3 compartment and is encoded by a coincidence between KC activation and dopaminergic input. The formed plasticity is input-specific, requiring pairing of the conditioned stimulus and unconditioned stimulus pathways; hence, we name it pairing-dependent plasticity. Pairing-dependent plasticity formation requires an intact CaMKII gene and is blocked by previous-night sleep deprivation but is rescued by rebound sleep. In conclusion, we show that our ex vivo preparation recapitulates behavioral and imaging results from intact animals and can provide new insights into mechanisms of memory formation at the level of molecules, circuits, and brain state.SIGNIFICANCE STATEMENT The mammalian ex vivo LTP model enabled in-depth investigation of the hippocampal memory circuit. We develop a parallel model to study the Drosophila mushroom body (MB) memory circuit. Pairing activation of the conditioned stimulus and unconditioned stimulus pathways in dissected brains induces a potentiation pairing-dependent plasticity (PDP) in the axons of α'β' Kenyon cells and a suppression PDP in the dendrites of their postsynaptic MB output neurons, localized in the MB α'3 compartment. This PDP is input-specific and requires the 3' untranslated region of CaMKII Interestingly, ex vivo PDP carries information about the animal's experience before dissection; brains from sleep-deprived animals fail to form PDP, whereas those from animals who recovered 2 h of their lost sleep form PDP.

Attenuation and recovery of an avoidance response to a chemical antipredator cue in an invasive fish: implications for use as a repellent in conservation

The detection of predation risk without direct engagement with a predator is an important driver of prey movement strategies. Consequently, the application of alarm cues may prove an effective tool in guiding the movements of fishes targeted for control or conservation. However, failure to contemplate the sensory, physiological and cognitive outcomes of repeated or persistent exposure to the cue will likely lead to poor performance of management practices. Using a series of behavioural tests and physiological recordings from the olfactory organ, we examined the timing of onset and recovery of the alarm response in sea lamprey (Petromyzon marinus L.) when exposed continuously or sporadically to its alarm cue. In the laboratory, sea lamprey exhibited short-term, reversible attenuation of the alarm response over 2-4 h with continuous exposure. The alarm response spontaneously recovered after 30-60 min of removal from the cue. In long-duration free-swimming tests, where the animals were allowed to move into and out of the odour plume volitionally, repeated but sporadic encounter with the alarm cue over 5 h did not alter the alarm response. Electro-olfactogram recordings from the main olfactory epithelium indicated that olfactory sensory neurons quickly adapt to alarm cue and recovered within 15 min. Our findings strongly implicate habituation as the mechanism that induces reduction in the alarm response and provide insight into the design of effective management practices that seek to use fish alarm cues as repellents.

Reducing local synthesis of estrogen in the tubular striatum promotes attraction to same-sex odors in female mice

Brain-derived 17β-estradiol (E2) confers rapid effects on neural activity. The tubular striatum (TuS, also called the olfactory tubercle) is both capable of local E2 synthesis due to its abundant expression of aromatase and is a critical locus for odor-guided motivated behavior and odor hedonics. TuS neurons also contain mRNA for estrogen receptors α, β, and the G protein-coupled estrogen receptor. We demonstrate here that mRNA for estrogen receptors appears to be expressed upon TuS dopamine 1 receptor-expressing neurons, suggesting that E2 may play a neuromodulatory role in circuits which are important for motivated behavior. Therefore, we reasoned that E2 in the TuS may influence attraction to urinary odors which are highly attractive. Using whole-body plethysmography, we examined odor-evoked high-frequency sniffing as a measure of odor attaction. Bilateral infusion of the aromatase inhibitor letrozole into the TuS of gonadectomized female adult mice induced a resistance to habituation over successive trials in their investigatory sniffing for female mouse urinary odors, indicative of an enhanced attraction. All males displayed resistance to habituation for female urinary odors, indicative of enhanced attraction that is independent from E2 manipulation. Letrozole's effects were not due to group differences in basal respiration, nor changes in the ability to detect or discriminate between odors (both monomolecular odorants and urinary odors). Therefore, de novo E2 synthesis in the TuS impacts females' but not males' attraction to female urinary odors, suggesting a sex-specific influence of E2 in odor hedonics.

De-risking in Tier I CNS safety assessments is the primary function of study design and technical training of laboratory staff observers

Derisking is not a pharmaceutical industry strategy to reduce time, effort, or costs in drug development. Derisking strategies originated within the National Institutes of Health as a predicate to good science. There is a growing sentiment within drug development programs to diminish the importance of behavioral measures in toxicological studies and in the Tiered Safety assessment plans of the U.S. Regulatory Agencies and the International Commission on Harmonization. The validity and reliability of the Functional Observational Batter (FOB) is critically dependent on consistency and technical quality in each risk assessment plan. US Federal and International drug approval organizations have universally adopted the concept of principles of test construction rather than delineating specific behavioral assay endpoints for inclusion of the FOB in nonclinical safety protocols. The validity and reliability of behavioral observations in standardized neurotoxicity screening is critically dependent on the FOB developed by the Study Director with the Sponsor throughout all stages of testing.. The individual risk factors selected for observation to be included in the early Tier 1 safety program should be determined by the mechanism and mode of action of the test article. The results of Tier I testing are the basis for Tier II testing designs. Critical to the compliance with Good Laboratory Practices is the documentation of training of the operational staff scheduled to conduct all aspects of the established protocol.

Gene therapy rescues olfactory perception in a clinically relevant ciliopathy model of Bardet-Biedl syndrome

Bardet-Biedl syndrome (BBS) is a hereditary genetic disorder that results in numerous clinical manifestations including olfactory dysfunction. Of at least 21 BBS-related genes that can carry multiple mutations, a pathogenic mutation, BBS1M390R, is the single most common mutation of clinically diagnosed BBS outcomes. While the deletion of BBS-related genes in mice can cause variable penetrance in different organ systems, the impact of the Bbs1M390R mutation in the olfactory system remains unclear. Using a clinically relevant knock-in mouse model homozygous for Bbs1M390R, we investigated the impact of the mutation on the olfactory system and tested the potential of viral-mediated, wildtype gene replacement therapy to rescue smell loss. The cilia of olfactory sensory neurons (OSNs) in Bbs1^M390R/M390R mice were significantly shorter and fewer than those of wild-type mice. Also, both peripheral cellular odor detection and synaptic-dependent activity in the olfactory bulb were significantly decreased in the mutant mice. Furthermore, to gain insight into the degree to which perceptual features are impaired in the mutant mice, we used whole-body plethysmography to quantitatively measure odor-evoked sniffing. The Bbs1^M390R/M390R mice showed significantly higher odor detection thresholds (reduced odor sensitivity) compared to wild-type mice; however, their odor discrimination acuity was still well maintained. Importantly, adenoviral expression of Bbs1 in OSNs restored cilia length and re-established both peripheral odorant detection and odor perception. Together, our findings further expand our understanding for the development of gene therapeutic treatment for congenital ciliopathies in the olfactory system.

Temporary Anosmia in Mice Following Nasal Lavage With Dilute Detergent Solution

Olfactory sensory deprivation induces anosmia and reduces tyrosine hydroxylase and dopamine levels in the olfactory bulb. The behavioral consequences specific to the loss of olfactory bulb dopamine are difficult to determine because sensory deprivation protocols are either confounded by side effects or leave the animal anosmic. A new method to both induce sensory deprivation and to measure the behavioral and circuit consequences is needed. We developed a novel, recoverable anosmia protocol using nasal lavage with a dilute detergent solution. Detergent treatment did not damage the olfactory epithelium as measured by scanning electron microscopy, alcian blue histology, and acetylated tubulin immunohistochemistry. One treatment-induced anosmia that lasted 24 to 48 h. Three treatments over 5 days reduced olfactory bulb tyrosine hydroxylase and dopamine levels indicating that anosmia persists between treatments. Importantly, even with multiple treatments, olfactory ability recovered within 48 h. This is the first report of a sensory deprivation protocol that induces recoverable anosmia and can be paired with biochemical, histological, and behavioral investigations of olfaction.

Structure and flexibility in cortical representations of odour space

The cortex organizes sensory information to enable discrimination and generalization1-4. As systematic representations of chemical odour space have not yet been described in the olfactory cortex, it remains unclear how odour relationships are encoded to place chemically distinct but similar odours, such as lemon and orange, into perceptual categories, such as citrus5-7. Here, by combining chemoinformatics and multiphoton imaging in the mouse, we show that both the piriform cortex and its sensory inputs from the olfactory bulb represent chemical odour relationships through correlated patterns of activity. However, cortical odour codes differ from those in the bulb: cortex more strongly clusters together representations for related odours, selectively rewrites pairwise odour relationships, and better matches odour perception. The bulb-to-cortex transformation depends on the associative network originating within the piriform cortex, and can be reshaped by passive odour experience. Thus, cortex actively builds a structured representation of chemical odour space that highlights odour relationships; this representation is similar across individuals but remains plastic, suggesting a means through which the olfactory system can assign related odour cues to common and yet personalized percepts.

Deficits in olfactory sensitivity in a mouse model of Parkinson's disease revealed by plethysmography of odor-evoked sniffing

Hyposmia is evident in over 90% of Parkinson’s disease (PD) patients. A characteristic of PD is intraneuronal deposits composed in part of α-synuclein fibrils. Based on the analysis of post-mortem PD patients, Braak and colleagues suggested that early in the disease α-synuclein pathology is present in the dorsal motor nucleus of the vagus, as well as the olfactory bulb and anterior olfactory nucleus, and then later affects other interconnected brain regions. Here, we bilaterally injected α-synuclein preformed fibrils into the olfactory bulbs of wild type male and female mice. Six months after injection, the anterior olfactory nucleus and piriform cortex displayed a high α-synuclein pathology load. We evaluated olfactory perceptual function by monitoring odor-evoked sniffing behavior in a plethysmograph at one-, three- and six-months after injection. No overt impairments in the ability to engage in sniffing were evident in any group, suggesting preservation of the ability to coordinate respiration. At all-time points, females injected with fibrils exhibited reduced odor detection sensitivity, which was observed with the semi-automated plethysmography apparatus, but not a buried pellet test. In future studies, this sensitive methodology for assessing olfactory detection deficits could be used to define how α-synuclein pathology affects other aspects of olfactory perception and to clarify the neuropathological underpinnings of these deficits.

Habituation as a neural algorithm for online odor discrimination

Habituation is a form of simple memory that suppresses neural activity in response to repeated, neutral stimuli. This process is critical in helping organisms guide attention toward the most salient and novel features in the environment. Here, we follow known circuit mechanisms in the fruit fly olfactory system to derive a simple algorithm for habituation. We show, both empirically and analytically, that this algorithm is able to filter out redundant information, enhance discrimination between odors that share a similar background, and improve detection of novel components in odor mixtures. Overall, we propose an algorithmic perspective on the biological mechanism of habituation and use this perspective to understand how sensory physiology can affect odor perception. Our framework may also help toward understanding the effects of habituation in other more sophisticated neural systems.

Behavioral and Neurobiological Convergence of Odor, Mood and Emotion: A Review

The affective state is the combination of emotion and mood, with mood reflecting a running average of sequential emotional events together with an underlying internal affective state. There is now extensive evidence that odors can overtly or subliminally modulate mood and emotion. Relying primarily on neurobiological literature, here we review what is known about how odors can affect emotions/moods and how emotions/moods may affect odor perception. We take the approach that form can provide insight into function by reviewing major brain regions and neural circuits underlying emotion and mood, and then reviewing the olfactory pathway in the context of that emotion/mood network. We highlight the extensive neuroanatomical opportunities for odor-emotion/mood convergence, as well as functional data demonstrating reciprocal interactions between these processes. Finally, we explore how the odor- emotion/mood interplay is, or could be, used in medical and/or commercial applications.

A Computational Model of Oxytocin Modulation of Olfactory Recognition Memory

Social recognition in mammals depends on complex interactions between sensory and other brain areas as well as modulatory inputs by specific neuropeptides such as oxytocin (OXT). Social recognition memory specifically has been shown to depend among others on olfactory processing, and can be probed using methods similar to those used when probing non-social odor memory. We here use a computational model of two interconnected olfactory networks in the mouse, the olfactory bulb (OB) and anterior olfactory nucleus, to propose a mechanism for olfactory short-term recognition memory and its modulation in social situations. Based on previous experiments, we propose one early locus for memory to be the OB. During social encounters in mice, pyramidal cells in the anterior olfactory nucleus, themselves driven by olfactory input, are rendered more excitable by OXT release, resulting in stronger feedback to OB local interneurons. This additional input to the OB creates stronger dynamics and improves signal-to-noise ratio of odor responses in the OB proper. As a consequence, mouse social olfactory memories are more strongly encoded and their duration is modulated.

Disrupted Habituation in the Early Stage of Psychosis

BACKGROUND

Learning and memory are impaired in schizophrenia. Some theories have proposed that one form of memory, habituation, is particularly impaired. Preliminary evidence suggests that memory impairment is associated with failed hippocampal habituation in patients with chronic schizophrenia. We studied how abnormal habituation of the hippocampus is related to relational memory deficits in the early stage of psychosis.

METHODS

We measured hippocampal activity in 62 patients with early psychosis and 70 healthy individuals using functional magnetic resonance imaging. Habituation was defined as the slope of functional magnetic resonance imaging signal change to repeated presentations of faces and objects. Relational memory ability was measured as the slope of preferential viewing during a face-scene pair eye movement task outside the scanner.

RESULTS

Patients with early psychosis showed impaired relational memory (p < .001) and less hippocampal habituation to objects (p = .01) than healthy control subjects. In the healthy control group, better relational memory was associated with faster anterior hippocampal habituation (faces, r = -.28, p = .03). In contrast, patients with early psychosis showed no brain-behavior relationship (r = .12, p = .40).

CONCLUSIONS

We found evidence for disrupted hippocampal habituation in the early stage of psychosis along with an altered association between hippocampal habituation and relational memory ability. These results suggest that neural habituation may provide a novel target for early cognitive interventions in psychosis.

Black tea aroma inhibited increase of salivary chromogranin-A after arithmetic tasks

Background

Growing attention has been paid to the effects of food flavor components on alleviating negative brain functions caused by stressful lifestyles. In this study, we investigated the alleviating effect of two kinds of black tea aromas on physical and psychological stress induced by the Uchida-Kraepelin test, based on salivary chromogranin-A (CgA) levels as a stress marker and subjective evaluations (Profile of Mood States).

Results

Compared with the water exposure control, inhaling black tea aroma (Darjeeling and Assam in this study) induced lower salivary CgA concentration levels after 30 min of mental stress load tasks. This anti-stress effect of black tea aroma did not differ between the two tea types even though the concentration of the anti-stress components in the Darjeeling tea aroma was higher than that in the Assam aroma. However, Darjeeling tea aroma tended to decrease the tension and/or anxiety score immediately after the first exposure.

Conclusions

Inhaling black tea aroma may diminish stress levels caused by arithmetic mental stress tasks, and Darjeeling tea aroma tended to improve mood before mental stress load.

Postnatal Odor Exposure Increases the Strength of Interglomerular Lateral Inhibition onto Olfactory Bulb Tufted Cells

Lateral inhibition between pairs of olfactory bulb (OB) mitral cells (MCs) and tufted cells (TCs) is linked to a variety of computations including gain control, decorrelation, and gamma-frequency synchronization. Differential effects of lateral inhibition onto MCs and TCs via distinct lateral inhibitory circuits are one of several recently described circuit-level differences between MCs and TCs that allow each to encode separate olfactory features in parallel. Here, using acute OB slices from mice, we tested whether lateral inhibition is affected by prior odor exposure and if these effects differ between MCs and TCs. We found that early postnatal odor exposure to the M72 glomerulus ligand acetophenone increased the strength of interglomerular lateral inhibition onto TCs, but not MCs, when the M72 glomerulus was stimulated. These increases were specific to exposure to M72 ligands because exposure to hexanal did not increase the strength of M72-mediated lateral inhibition. Therefore, early life experiences may be an important factor shaping TC odor responses.

SIGNIFICANCE STATEMENT

Responses of olfactory (OB) bulb mitral cells (MCs) and tufted cells (TCs) are known to depend on prior odor exposure, yet the specific circuit mechanisms underlying these experience-dependent changes are unknown. Here, we show that odor exposure alters one particular circuit element, interglomerular lateral inhibition, which is known to be critical for a variety of OB computations. Early postnatal odor exposure to acetophenone, a ligand of M72 olfactory sensory neurons, increases the strength of M72-mediated lateral inhibition onto TCs, but not MCs, that project to nearby glomeruli. These findings add to a growing list of differences between MCs and TCs suggesting that that these two cell types play distinct roles in odor coding.

Habituation is altered in neuropsychiatric disorders-A comprehensive review with recommendations for experimental design and analysis

Abnormalities in the simplest form of learning, habituation, have been reported in a variety of neuropsychiatric disorders as etiologically diverse as Autism Spectrum Disorder, Fragile X syndrome, Schizophrenia, Parkinson's Disease, Huntington's Disease, Attention Deficit Hyperactivity Disorder, Tourette's Syndrome, and Migraine. Here we provide the first comprehensive review of what is known about alterations in this form of non-associative learning in each disorder. Across several disorders, abnormal habituation is predictive of symptom severity, highlighting the clinical significance of habituation and its importance to normal cognitive function. Abnormal habituation is discussed within the greater framework of learning theory and how it may relate to disease phenotype either as a cause, symptom, or therapy. Important considerations for the design and interpretation of habituation experiments are outlined with the hope that these will aid both clinicians and basic researchers investigating how this simple form of learning is altered in disease.

Apolipoprotein E4 causes early olfactory network abnormalities and short-term olfactory memory impairments

While apolipoprotein (Apo) E4 is linked to increased incidence of Alzheimer's disease (AD), there is growing evidence that it plays a role in functional brain irregularities that are independent of AD pathology. However, ApoE4-driven functional differences within olfactory processing regions have yet to be examined. Utilizing knock-in mice humanized to ApoE4 versus the more common ApoE3, we examined a simple olfactory perceptual memory that relies on the transfer of information from the olfactory bulb (OB) to the piriform cortex (PCX), the primary cortical region involved in higher order olfaction. In addition, we have recorded in vivo resting and odor-evoked local field potentials (LPF) from both brain regions and measured corresponding odor response magnitudes in anesthetized young (6-month-old) and middle-aged (12-month-old) ApoE mice. Young ApoE4 compared to ApoE3 mice exhibited a behavioral olfactory deficit coinciding with hyperactive odor-evoked response magnitudes within the OB that were not observed in older ApoE4 mice. Meanwhile, middle-aged ApoE4 compared to ApoE3 mice exhibited heightened response magnitudes in the PCX without a corresponding olfactory deficit, suggesting a shift with aging in ApoE4-driven effects from OB to PCX. Interestingly, the increased ApoE4-specific response in the PCX at middle-age was primarily due to a dampening of baseline spontaneous activity rather than an increase in evoked response power. Our findings indicate that early ApoE4-driven olfactory memory impairments and OB network abnormalities may be a precursor to later network dysfunction in the PCX, a region that not only is targeted early in AD, but may be selectively vulnerable to ApoE4 genotype.

Fus1 KO Mouse As a Model of Oxidative Stress-Mediated Sporadic Alzheimer's Disease: Circadian Disruption and Long-Term Spatial and Olfactory Memory Impairments

Insufficient advances in the development of effective therapeutic treatments of sporadic Alzheimer's Disease (sAD) to date are largely due to the lack of sAD-relevant animal models. While the vast majority of models do recapitulate AD's hallmarks of plaques and tangles by virtue of tau and/or beta amyloid overexpression, these models do not reflect the fact that in sAD (unlike familial AD) these genes are not risk factors per se and that other mechanisms like oxidative stress, metabolic dysregulation and inflammation play key roles in AD etiology. Here we characterize and propose the Fus1 KO mice that lack a mitochondrial protein Fus1/Tusc2 as a new sAD model. To establish sAD relevance, we assessed sAD related deficits in Fus1 KO and WT adult mice of 4-5 months old, the equivalent human age when the earliest cognitive and olfactory sAD symptoms arise. Fus1 KO mice showed oxidative stress (increased levels of ROS, decreased levels of PRDX1), disruption of metabolic homeostasis (decreased levels of ACC2, increased phosphorylation of AMPK), autophagy (decreased levels of LC3-II), PKC (decreased levels of RACK1) and calcium signaling (decreased levels of Calb2) in the olfactory bulb and/or hippocampus. Mice were behaviorally tested using objective and accurate video tracking (Noldus), in which Fus1 KO mice showed clear deficits in olfactory memory (decreased habituation/cross-habituation in the short and long term), olfactory guided navigation memory (inability to reduce their latency to find the hidden cookie), spatial memory (learning impairments on finding the platform in the Morris water maze) and showed more sleep time during the diurnal cycle. Fus1 KO mice did not show clear deficits in olfactory perception (cross-habituation), association memory (passive avoidance) or in species-typical behavior (nest building) and no increased anxiety (open field, light-dark box) or depression/anhedonia (sucrose preference) at this relatively young age. These neurobehavioral deficits of the Fus1 KO mice at this relatively young age are highly relevant to sAD, making them suitable for effective research on pharmacological targets in the context of early intervention of sAD.

Neuromodulation of olfactory transformations

The olfactory bulb and piriform cortex are the best studied structures of the mammalian olfactory system and are heavily innervated by extrinsic neuromodulatory inputs. The state-dependent release of acetylcholine, norepinephrine, serotonin, and other neuromodulators into these olfactory structures alters a constellation of physiological parameters in neurons and synapses that together modify the computations performed on sensory signals. These modifications affect the specificity, detectability, discriminability, and other properties of odor representations and thereby govern perceptual performance. Whereas different neuromodulators have distinct cellular effects, and tend to be associated with nominally different functions, it also is clear that these purported functions overlap substantially, and that ad hoc hypotheses regarding the roles of particular neuromodulators may have reached the limits of their usefulness.

Learning about natural variation of odor mixtures enhances categorization in early olfactory processing

Natural odors are typically mixtures of several chemical components. Mixtures vary in composition among odor objects that have the same meaning. Therefore a central 'categorization' problem for an animal as it makes decisions about odors in natural contexts is to correctly identify odor variants that have the same meaning and avoid variants that have a different meaning. We propose that identified mechanisms of associative and non-associative plasticity in early sensory processing in the insect antennal lobe and mammalian olfactory bulb are central to solving this problem. Accordingly, this plasticity should work to improve categorization of odors that have the opposite meanings in relation to important events. Using synthetic mixtures designed to mimic natural odor variation among flowers, we studied how honey bees learn about and generalize among floral odors associated with food. We behaviorally conditioned honey bees on a difficult odor discrimination problem using synthetic mixtures that mimic natural variation among snapdragon flowers. We then used calcium imaging to measure responses of projection neurons of the antennal lobe, which is the first synaptic relay of olfactory sensory information in the brain, to study how ensembles of projection neurons change as a result of behavioral conditioning. We show how these ensembles become 'tuned' through plasticity to improve categorization of odors that have the different meanings. We argue that this tuning allows more efficient use of the immense coding space of the antennal lobe and olfactory bulb to solve the categorization problem. Our data point to the need for a better understanding of the 'statistics' of the odor space.

BOD1 Is Required for Cognitive Function in Humans and Drosophila

Here we report a stop-mutation in the BOD1 (Biorientation Defective 1) gene, which co-segregates with intellectual disability in a large consanguineous family, where individuals that are homozygous for the mutation have no detectable BOD1 mRNA or protein. The BOD1 protein is required for proper chromosome segregation, regulating phosphorylation of PLK1 substrates by modulating Protein Phosphatase 2A (PP2A) activity during mitosis. We report that fibroblast cell lines derived from homozygous BOD1 mutation carriers show aberrant localisation of the cell cycle kinase PLK1 and its phosphatase PP2A at mitotic kinetochores. However, in contrast to the mitotic arrest observed in BOD1-siRNA treated HeLa cells, patient-derived cells progressed through mitosis with no apparent segregation defects but at an accelerated rate compared to controls. The relatively normal cell cycle progression observed in cultured cells is in line with the absence of gross structural brain abnormalities in the affected individuals. Moreover, we found that in normal adult brain tissues BOD1 expression is maintained at considerable levels, in contrast to PLK1 expression, and provide evidence for synaptic localization of Bod1 in murine neurons. These observations suggest that BOD1 plays a cell cycle-independent role in the nervous system. To address this possibility, we established two Drosophila models, where neuron-specific knockdown of BOD1 caused pronounced learning deficits and significant abnormalities in synapse morphology. Together our results reveal novel postmitotic functions of BOD1 as well as pathogenic mechanisms that strongly support a causative role of BOD1 deficiency in the aetiology of intellectual disability. Moreover, by demonstrating its requirement for cognitive function in humans and Drosophila we provide evidence for a conserved role of BOD1 in the development and maintenance of cognitive features.

Alterations of the volatile metabolome in mouse models of Alzheimer's disease

In the present study, we tested whether the volatile metabolome was altered by mutations of the Alzheimer's disease (AD)-implicated amyloid precursor protein gene (APP) and comprehensively examined urinary volatiles that may potentially serve as candidate biomarkers of AD. Establishing additional biomarkers in screening populations for AD will provide enhanced diagnostic specificity and will be critical in evaluating disease-modifying therapies. Having strong evidence of gross changes in the volatile metabolome of one line of APP mice, we utilized three unique mouse lines which over-express human mutations of the APP gene and their respective non-transgenic litter-mates (NTg). Head-space gas chromatography/mass spectrometry (GC/MS) of urinary volatiles uncovered several aberrant chromatographic peak responses. We later employed linear discrimination analysis and found that the GC/MS peak responses provide accurate (>84%) genotype classification of urinary samples. These initial data in animal models show that mutant APP gene expression entails a uniquely identifiable urinary odor, which if uncovered in clinical AD populations, may serve as an additional biomarker for the disease.

De novo loss-of-function mutations in WAC cause a recognizable intellectual disability syndrome and learning deficits in Drosophila

Recently WAC was reported as a candidate gene for intellectual disability (ID) based on the identification of a de novo mutation in an individual with severe ID. WAC regulates transcription-coupled histone H2B ubiquitination and has previously been implicated in the 10p12p11 contiguous gene deletion syndrome. In this study, we report on 10 individuals with de novo WAC mutations which we identified through routine (diagnostic) exome sequencing and targeted resequencing of WAC in 2326 individuals with unexplained ID. All but one mutation was expected to lead to a loss-of-function of WAC. Clinical evaluation of all individuals revealed phenotypic overlap for mild ID, hypotonia, behavioral problems and distinctive facial dysmorphisms, including a square-shaped face, deep set eyes, long palpebral fissures, and a broad mouth and chin. These clinical features were also previously reported in individuals with 10p12p11 microdeletion syndrome. To investigate the role of WAC in ID, we studied the importance of the Drosophila WAC orthologue (CG8949) in habituation, a non-associative learning paradigm. Neuronal knockdown of Drosophila CG8949 resulted in impaired learning, suggesting that WAC is required in neurons for normal cognitive performance. In conclusion, we defined a clinically recognizable ID syndrome, caused by de novo loss-of-function mutations in WAC. Independent functional evidence in Drosophila further supported the role of WAC in ID. On the basis of our data WAC can be added to the list of ID genes with a role in transcription regulation through histone modification.

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.

Nasal Administration of Cholera Toxin as a Mucosal Adjuvant Damages the Olfactory System in Mice

Cholera toxin (CT) induces severe diarrhea in humans but acts as an adjuvant to enhance immune responses to vaccines when administered orally. Nasally administered CT also acts as an adjuvant, but CT and CT derivatives, including the B subunit of CT (CTB), are taken up from the olfactory epithelium and transported to the olfactory bulbs and therefore may be toxic to the central nervous system. To assess the toxicity, we investigated whether nasally administered CT or CT derivatives impair the olfactory system. In mice, nasal administration of CT, but not CTB or a non-toxic CT derivative, reduced the expression of olfactory marker protein (OMP) in the olfactory epithelium and olfactory bulbs and impaired odor responses, as determined with behavioral tests and optical imaging. Thus, nasally administered CT, like orally administered CT, is toxic and damages the olfactory system in mice. However, CTB and a non-toxic CT derivative, do not damage the olfactory system. The optical imaging we used here will be useful for assessing the safety of nasal vaccines and adjuvants during their development for human use and CT can be used as a positive control in this test.

Habituation of glomerular responses in the olfactory bulb following prolonged odor stimulation reflects reduced peripheral input

Following prolonged odor stimulation, output from olfactory bulb (OB) mitral/tufted (M/T) cells is decreased in response to subsequent olfactory stimulation. Currently, it is unclear if this decrease is a function of adaptation of peripheral olfactory sensory neuron (OSN) responses or reflects depression of bulb circuits. We used wide-field calcium imaging in anesthetized transgenic GCaMP2 mice to compare excitatory glomerular layer odor responses before and after a 30-s odor stimulation. Significant habituation of subsequent glomerular odor responses to both the same and structurally similar odorants was detected with our protocol. To test whether depression of OSN terminals contributed to this habituation, olfactory nerve layer (ON) stimulation was used to drive glomerular layer responses in the absence of peripheral odor activation of the OSNs. Following odor habituation, in contrast to odor-evoked glomerular responses, ON stimulation-evoked glomerular responses were not habituated. The difference in response between odor and electrical stimulation following odor habituation provides evidence that odor response reductions measured in the glomerular layer of the OB are most likely the result of OSN adaptation processes taking place in the periphery.

The olfactory tubercle encodes odor valence in behaving mice

Sensory information acquires meaning to adaptively guide behaviors. Despite odors mediating a number of vital behaviors, the components of the olfactory system responsible for assigning meaning to odors remain unclear. The olfactory tubercle (OT), a ventral striatum structure that receives monosynaptic input from the olfactory bulb, is uniquely positioned to transform odor information into behaviorally relevant neural codes. No information is available, however, on the coding of odors among OT neurons in behaving animals. In recordings from mice engaged in an odor discrimination task, we report that the firing rate of OT neurons robustly and flexibly encodes the valence of conditioned odors over identity, with rewarded odors evoking greater firing rates. This coding of rewarded odors occurs before behavioral decisions and represents subsequent behavioral responses. We predict that the OT is an essential region whereby odor valence is encoded in the mammalian brain to guide goal-directed behaviors.

Coding of odor stimulus features among secondary olfactory structures

Sensory systems must represent stimuli in manners dependent upon a wealth of factors, including stimulus intensity and duration. One way the brain might handle these complex functions is to assign the tasks throughout distributed nodes, each contributing to information processing. We sought to explore this important aspect of sensory network function in the mammalian olfactory system, wherein the intensity and duration of odor exposure are critical contributors to odor perception. This is a quintessential model for exploring processing schemes given the distribution of odor information by olfactory bulb mitral and tufted cells into several anatomically distinct secondary processing stages, including the piriform cortex (PCX) and olfactory tubercle (OT), whose unique contributions to odor coding are unresolved. We explored the coding of PCX and OT neuron responses to odor intensity and duration. We found that both structures similarly partake in representing descending intensities of odors by reduced recruitment and modulation of neurons. Additionally, while neurons in the OT adapt to odor exposure, they display reduced capacity to adapt to either repeated presentations of odor or a single prolonged odor presentation compared with neurons in the PCX. These results provide insights into manners whereby secondary olfactory structures may, at least in some cases, uniquely represent stimulus features.

Odor Memory and Discrimination Covary as a Function of Delay between Encoding and Recall in Rats

Nonassociative odor learning paradigms are often used to assess memory, social recognition and neuromodulation of olfactory pathways. We here use a modified object recognition paradigm to investigate how an important task parameter, delay between encoding and recall trials, affects the properties of this memory. We show that both memory for a previously investigated odorant and discrimination of a novel odorant decay with delay time and that rats can remember an odorant for up to 45min after a single trial encoding event. The number of odorants that can be encoded, as well as the specificity of the encoded memory, decrease with increased delay and also depend on stimulus concentration. Memory for an odorant and discrimination of a novel odorant decay at approximately the same rate, whereas the specificity of the formed memory decays faster than the memory itself. These results have important implications for the interpretation of behavioral data obtained with this paradigm.

Rivalry of homeostatic and sensory-evoked emotions: Dehydration attenuates olfactory disgust and its neural correlates

Neural correlates have been described for emotions evoked by states of homeostatic imbalance (e.g. thirst, hunger, and breathlessness) and for emotions induced by external sensory stimulation (such as fear and disgust). However, the neurobiological mechanisms of their interaction, when they are experienced simultaneously, are still unknown. We investigated the interaction on the neurobiological and the perceptional level using subjective ratings, serum parameters, and functional magnetic resonance imaging (fMRI) in a situation of emotional rivalry, when both a homeostatic and a sensory-evoked emotion were experienced at the same time. Twenty highly dehydrated male subjects rated a disgusting odor as significantly less repulsive when they were thirsty. On the neurobiological level, we found that this reduction in subjective disgust during thirst was accompanied by a significantly reduced neural activity in the insular cortex, a brain area known to be considerably involved in processing of disgust. Furthermore, during the experience of disgust in the satiated condition, we observed a significant functional connectivity between brain areas responding to the disgusting odor, which was absent during the stimulation in the thirsty condition. These results suggest interference of conflicting emotions: an acute homeostatic imbalance can attenuate the experience of another emotion evoked by the sensory perception of a potentially harmful external agent. This finding offers novel insights with regard to the behavioral relevance of biologically different types of emotions, indicating that some types of emotions are more imperative for behavior than others. As a general principle, this modulatory effect during the conflict of homeostatic and sensory-evoked emotions may function to safeguard survival.

Time-course of trigeminal versus olfactory stimulation: evidence from chemosensory evoked potentials

Habituation of responses to chemosensory signals has been explored in many ways. Strong habituation and adaptation processes can be observed at the various levels of processing. For example, with repeated exposure, amplitudes of chemosensory event-related potentials (ERP) decrease over time. However, long-term habituation has not been investigated so far and investigations of differences in habituation between trigeminal and olfactory ERPs are very rare. The present study investigated habituation over a period of approximately 80 min for two olfactory and one trigeminal stimulus, respectively. Habituation was examined analyzing the N1 and P2 amplitudes and latencies of chemosensory ERPs and intensity ratings. It was shown that amplitudes of both components - and intensity ratings - decreased from the first to the last block. Concerning ERP latencies no effects of habituation were seen. Amplitudes of trigeminal ERPs diminished faster than amplitudes of olfactory ERPs, indicating that the habituation of trigeminal ERPs is stronger than habituation of olfactory ERPs. Amplitudes of trigeminal ERPs were generally higher than amplitudes of olfactory ERPs, as it has been shown in various studies before. The results reflect relatively selective central changes in response to chemosensory stimuli over time.

17β-estradiol enhances memory duration in the main olfactory bulb in CD-1 mice

Rodents rely heavily on odor detection, discrimination, and memory to locate food, find mates, care for pups, and avoid predators. Estrogens have been shown to increase memory retention in rodents performing spatial memory and object placement tasks. Here we evaluate the extent to which 17β-estradiol modulates memory formation and duration in the olfactory system. Adult CD-1 mice were gonadectomized and given either systemic 17β-estradiol replacement, local 17β-estradiol in the main olfactory bulb, or no replacement. Before performing the behavioral task the mice were given saline or PHTPP (an estrogen receptor β [ER-β] antagonist) via bilateral infusion into the main olfactory bulb. As the beta-type estrogen receptor (ER-β) is more abundant than the alpha-type estrogen receptor in the murine main olfactory bulb, the current study focuses on 17β-estradiol and its interactions with ERβ. Habituation, a simple, nonassociative learning task in which an animal is exposed to the same odor over successive presentations, was used to evaluate the animals' ability to detect odors and form an olfactory memory. To evaluate memory duration, we added a final trial of intertrial interval time (30 or 60 min) in which we presented the habituated odor. Neither surgical nor drug manipulation affected the ability of mice to detect or habituate to an odor. After habituation, gonadectomized 17β-estradiol-treated mice retained memory of an odor for 30 min, whereas non-estradiol-treated, 17β-estradiol+ERβ antagonist (PHTPP), and untreated male mice did not remember an odor 30 min after habituation. The results show that both systemic and local bulbar infusions of 17β-estradiol enhance odor memory duration in mice.

Network plasticity in adaptive filtering and behavioral habituation

The ability of organisms to seamlessly ignore familiar, inconsequential stimuli improves their selective attention and response to salient features of the environment. Here, I propose that this fundamental but unexplained phenomenon substantially derives from the ability of any pattern of neural excitation to create an enhanced inhibitory (or "negative") image of itself through target-specific scaling of inhibitory inputs onto active excitatory neurons. Familiar stimuli encounter strong negative images and are therefore less likely to be transmitted to higher brain centers. Integrating historical and recent observations, the negative-image model described here provides a mechanistic framework for understanding habituation, which is connected to ideas on dynamic predictive coding. In addition, it suggests insights for understanding autism spectrum disorders.

Properties and mechanisms of olfactory learning and memory

Memories are dynamic physical phenomena with psychometric forms as well as characteristic timescales. Most of our understanding of the cellular mechanisms underlying the neurophysiology of memory, however, derives from one-trial learning paradigms that, while powerful, do not fully embody the gradual, representational, and statistical aspects of cumulative learning. The early olfactory system—particularly olfactory bulb—comprises a reasonably well-understood and experimentally accessible neuronal network with intrinsic plasticity that underlies both one-trial (adult aversive, neonatal) and cumulative (adult appetitive) odor learning. These olfactory circuits employ many of the same molecular and structural mechanisms of memory as, for example, hippocampal circuits following inhibitory avoidance conditioning, but the temporal sequences of post-conditioning molecular events are likely to differ owing to the need to incorporate new information from ongoing learning events into the evolving memory trace. Moreover, the shapes of acquired odor representations, and their gradual transformation over the course of cumulative learning, also can be directly measured, adding an additional representational dimension to the traditional metrics of memory strength and persistence. In this review, we describe some established molecular and structural mechanisms of memory with a focus on the timecourses of post-conditioning molecular processes. We describe the properties of odor learning intrinsic to the olfactory bulb and review the utility of the olfactory system of adult rodents as a memory system in which to study the cellular mechanisms of cumulative learning.

Quantitative proteomics reveals olfactory input-dependent alterations in the mouse olfactory bulb proteome

Olfactory sensory information is processed and integrated by circuits within the olfactory bulb (OB) before being sent to the olfactory cortex. In the mammalian OB, neural activity driven by external stimuli can lead to experience-dependent changes in structures and functions. In this study, quantitative proteomics techniques were employed to study proteome-wide changes in the OB under four levels of neural activity (from low to high): devoid of peripheral input (using a transgenic model), wild-type control, and short-term and long-term odor exposures. Our results revealed that proteins related to various processes were altered in the OBs of odor-deprived and odor-stimulated mice compared to the wild-type controls. These changes induced by odor stimulation were quite different from those induced by a deficit in peripheral olfactory inputs. Detailed analysis demonstrated that metabolic process and synaptic transmission were the most commonly altered pathways and that the effects of peripheral deprivation were more profound. Our comparative proteomics analysis indicated that olfactory deprivation and odor exposure lead to different alterations in the OB proteome, which provides new clues about the mechanisms underlying the olfactory deprivation- or odor stimulation-induced plasticity of OB function and organization.

BIOLOGICAL SIGNIFICANCE

By combining quantitative proteomics, bioinformatics and WB/IHC analysis, this study reports the results of the first comparative study on proteome-wide changes in the olfactory bulb under different levels of olfactory input. Odor deprivation and stimulation induced proteomic changes clearly demonstrate significant metabolic shifts and alterations on synaptic transmission. This quantitative system biology study leads to a new level of understanding in the development of olfactory bulb plasticity induced by odor deprivation or stimulation, and provides many new clues for the olfactory bulb related functional studies.

Functional neuromodulation of chemosensation in vertebrates

Neuromodulation can be defined as a biophysical process that serves to modify-or modulate-the computation performed by a neuron or network as a function of task demands and behavioral state of the animal. These modulatory effects often involve substances extrinsic to the network under observation, such as acetylcholine (ACh), norepinephrine (NE), histamine, serotonin (5-HT), dopamine (DA), and a variety of neuropeptides. Olfactory and gustatory processes especially need to be adaptive and respond flexibly to changing environments, availability of resources and physiological needs. It is therefore crucial to understand the neuromodulatory processes that regulate the function of these systems.

Olfactory habituation in Drosophila-odor encoding and its plasticity in the antennal lobe

A ubiquitous feature of an animal's response to an odorant is that it declines when the odorant is frequently or continuously encountered. This decline in olfactory response, termed olfactory habituation, can have temporally or mechanistically different forms. The neural circuitry of the fruit fly Drosophila melanogaster's olfactory system is well defined in terms of component cells, which are readily accessible to functional studies and genetic manipulation. This makes it a particularly useful preparation for the investigation of olfactory habituation. In addition, the insect olfactory system shares many architectural and functional similarities with mammalian olfactory systems, suggesting that olfactory mechanisms in insects may be broadly relevant. In this chapter, we discuss the likely mechanisms of olfactory habituation in context of the participating cell types, their connectivity, and their roles in sensory processing. We overview the structure and function of key cell types, the mechanisms that stimulate them, and how they transduce and process odor signals. We then consider how each stage of olfactory processing could potentially contribute to behavioral habituation. After this, we overview a variety of recent mechanistic studies that point to an important role for potentiation of inhibitory synapses in the primary olfactory processing center, the antennal lobe, in driving the reduced response to familiar odorants. Following the discussion of mechanisms for short- and long-term olfactory habituation, we end by considering how these mechanisms may be regulated by neuromodulators, which likely play key roles in the induction, gating, or suppression of habituated behavior, and speculate on the relevance of these processes for other forms of learning and memory.

Stressors impair odor recognition memory via an olfactory bulb-dependent noradrenergic mechanism

Non-associative habituation and odor recognition tasks have been widely used to probe questions of social recognition, odor memory duration, and odor memory specificity. Among others, these paradigms have provided valuable insight into how neuromodulation, and specifically norepinephrine/noradrenaline (NE) influences odor memory. In general, NE levels are modulated by arousal, stress, and behavioral state, but there is sparse evidence of a direct relationship between NE and odor memory in adult rodents. The present study uses simple mild psychological stressors (bright light and sound) to modulate NE levels physiologically in order to probe stressors NE-dependent effect on odor recognition memory. In rats with bilateral bulbar cannulations, we show that these stressors modulate olfactory memory and that this effect is at least partially mediated by the olfactory bulb. Specifically, we show that the presence of stressors during the acquisition of odor memory suppresses memory for an odor when tested 30 min after familiarization to that odor. This suppression is blocked by infusing NE antagonists into the olfactory bulb prior to odor acquisition. Additionally, we find that infusion of bulbar NE is sufficient to suppress odor memory in a manner mimicking that of our stressors. These effects are unlikely to be solely mediated by locomotor/exploratory changes produced by stressors, although these stressors influence certain behaviors not directly related to odor investigation. This study provides important information about how behaviorally relevant changes in NE can influence top-down sensory processing and odor memory.

Effects of odor on emotion, with implications

The sense of smell is found widely in the animal kingdom. Human and animal studies show that odor perception is modulated by experience and/or physiological state (such as hunger), and that some odors can arouse emotion, and can lead to the recall of emotional memories. Further, odors can influence psychological and physiological states. Individual odorants are mapped via gene-specified receptors to corresponding glomeruli in the olfactory bulb, which directly projects to the piriform cortex and the amygdala without a thalamic relay. The odors to which a glomerulus responds reflect the chemical structure of the odorant. The piriform cortex and the amygdala both project to the orbitofrontal cortex (OFC) which with the amygdala is involved in emotion and associative learning, and to the entorhinal/hippocampal system which is involved in long-term memory including episodic memory. Evidence that some odors can modulate emotion and cognition is described, and the possible implications for the treatment of psychological problems, for example in reducing the effects of stress, are considered.

A computational framework for understanding decision making through integration of basic learning rules

Nonassociative and associative learning rules simultaneously modify neural circuits. However, it remains unclear how these forms of plasticity interact to produce conditioned responses. Here we integrate nonassociative and associative conditioning within a uniform model of olfactory learning in the honeybee. Honeybees show a fairly abrupt increase in response after a number of conditioning trials. The occurrence of this abrupt change takes many more trials after exposure to nonassociative trials than just using associative conditioning. We found that the interaction of unsupervised and supervised learning rules is critical for explaining latent inhibition phenomenon. Associative conditioning combined with the mutual inhibition between the output neurons produces an abrupt increase in performance despite smooth changes of the synaptic weights. The results show that an integrated set of learning rules implemented using fan-out connectivities together with neural inhibition can explain the broad range of experimental data on learning behaviors.

Roles of GSK3β in odor habituation and spontaneous neural activity of the mouse olfactory bulb

Glycogen synthase kinase 3β (GSK3β), a multifaceted kinase, is abundantly expressed in the brain, including the olfactory bulb (OB). In resting cells, GSK3β is constitutively active, and its over-activation is presumably involved in numerous brain diseases, such as Alzheimer’s disease. However, the functions of the constitutively active GSK3β in the adult brain under physiological conditions are not well understood. Here, we studied the possible functions of GSK3β activity in the OB. Odor stimulation, or blockade of peripheral olfactory inputs caused by either transgenic knock-out or ZnSO4 irrigation to the olfactory epithelium, all affected the expression level of GSK3β in the OB. When GSK3β activity was reduced by a selective inhibitor, the spontaneous oscillatory activity was significantly decreased in the granule cell layer of the OB. Furthermore, local inhibition of GSK3β activity in the OB significantly impaired the odor habituation ability. These results suggest that GSK3β plays important roles in both spontaneous neural activity and odor information processing in the OB, deepening our understanding of the potential functions of the constitutively active GSK3β in the brain under physiological conditions.