Extinction reverses olfactory fear-conditioned increases in neuron number and glomerular size

Perceptual constancy for an odor is acquired through changes in primary sensory neurons

The ability to consistently recognize an object despite variable sensory input is termed perceptual constancy. This ability is not innate; rather, it develops with experience early in life. We show that, when mice are naïve to an odor object, perceptual constancy is absent across increasing concentrations. The perceptual change coincides with a rapid reduction in activity from a single olfactory receptor channel that is most sensitive to the odor. This drop in activity is not a property of circuit interactions within the olfactory bulb; instead, it is due to a sensitivity mismatch of olfactory receptor neurons within the nose. We show that, after forming an association of this odor with food, the sensitivity of the receptor channel is matched to the odor object, preventing transmission failure and promoting perceptual stability. These data show that plasticity of the primary sensory organ enables learning of perceptual constancy.

The influence of olfactory experience on the birthrates of olfactory sensory neurons with specific odorant receptor identities

Olfactory sensory neurons (OSNs) are one of a few neuron types that are generated continuously throughout life in mammals. The persistence of olfactory sensory neurogenesis beyond early development has long been thought to function simply to replace neurons that are lost or damaged through exposure to environmental insults. The possibility that olfactory sensory neurogenesis may also serve an adaptive function has received relatively little consideration, largely due to the assumption that the generation of new OSNs is stochastic with respect to OSN subtype, as defined by the single odorant receptor gene that each neural precursor stochastically chooses for expression out of hundreds of possibilities. Accordingly, the relative birthrates of different OSN subtypes are predicted to be constant and impervious to olfactory experience. This assumption has been called into question, however, by evidence that the birthrates of specific OSN subtypes can be selectively altered by manipulating olfactory experience through olfactory deprivation, enrichment, and conditioning paradigms. Moreover, studies of recovery of the OSN population following injury provide further evidence that olfactory sensory neurogenesis may not be strictly stochastic with respect to subtype. Here we review this evidence and consider mechanistic and functional implications of the prospect that specific olfactory experiences can regulate olfactory sensory neurogenesis rates in a subtype-selective manner.

Circuit formation and sensory perception in the mouse olfactory system

In the mouse olfactory system, odor information is converted to a topographic map of activated glomeruli in the olfactory bulb (OB). Although the arrangement of glomeruli is genetically determined, the glomerular structure is plastic and can be modified by environmental stimuli. If the pups are exposed to a particular odorant, responding glomeruli become larger recruiting the dendrites of connecting projection neurons and interneurons. This imprinting not only increases the sensitivity to the exposed odor, but also imposes the positive quality on imprinted memory. External odor information represented as an odor map in the OB is transmitted to the olfactory cortex (OC) and amygdala for decision making to elicit emotional and behavioral outputs using two distinct neural pathways, innate and learned. Innate olfactory circuits start to work right after birth, whereas learned circuits become functional later on. In this paper, the recent progress will be summarized in the study of olfactory circuit formation and odor perception in mice. We will also propose new hypotheses on the timing and gating of olfactory circuit activity in relation to the respiration cycle.

In mice, discrete odors can selectively promote the neurogenesis of sensory neuron subtypes that they stimulate

In mammals, olfactory sensory neurons (OSNs) are born throughout life, presumably solely to replace neurons lost via turnover or injury. This assumption follows from the hypothesis that olfactory neurogenesis is strictly stochastic with respect to neuron subtype, as defined by the single odorant receptor allele that each neural precursor stochastically chooses out of hundreds of possibilities. This hypothesis is challenged by recent findings that the birthrates of a fraction of subtypes are selectively diminished by olfactory deprivation. These findings raise questions about how, and why, olfactory stimuli are required to promote the neurogenesis of some OSN subtypes, including whether the stimuli are generic (e.g., broadly activating odors or mechanical stimuli) or specific (e.g., discrete odorants). Based on RNA-seq and scRNA-seq analyses, we hypothesized that the neurogenic stimuli are specific odorants that selectively activate the same OSN subtypes whose birthrates are accelerated. In support of this, we have found, using subtype-specific OSN birthdating, that exposure to male and musk odors can accelerate the birthrates of responsive OSNs. Collectively, our findings reveal that certain odor experiences can selectively "amplify" specific OSN subtypes, and that persistent OSN neurogenesis may serve, in part, an adaptive function.

Activity-dependent formation of the topographic map and the critical period in the development of mammalian olfactory system

Neural activity influences every aspect of nervous system development. In olfactory systems, sensory neurons expressing the same odorant receptor project their axons to stereotypically positioned glomeruli, forming a spatial map of odorant receptors in the olfactory bulb. As individual odors activate unique combinations of glomeruli, this map forms the basis for encoding olfactory information. The establishment of this stereotypical olfactory map requires coordinated regulation of axon guidance molecules instructed by spontaneous activity. Recent studies show that sensory experiences also modify innervation patterns in the olfactory bulb, especially during a critical period of the olfactory system development. This review examines evidence in the field to suggest potential mechanisms by which various aspects of neural activity regulate axon targeting. We also discuss the precise functions served by neural plasticity during the critical period.

Legacies of salient environmental experiences-insights from chemosensation

Evidence for parental environments profoundly influencing the physiology, biology, and neurobiology of future generations has been accumulating in the literature. Recent efforts to understand this phenomenon and its underlying mechanisms have sought to use species like rodents and insects to model multi-generational legacies of parental experiences like stress and nutritional exposures. From these studies, we have come to appreciate that parental exposure to salient environmental experiences impacts the cadence of brain development, hormonal responses to stress, and the expression of genes that govern cellular responses to stress in offspring. Recent studies using chemosensory exposure have emerged as a powerful tool to shed new light on how future generations come to be influenced by environments to which parents are exposed. With a specific focus on studies that have leveraged such use of salient chemosensory experiences, this review synthesizes our current understanding of the concept, causes, and consequences of the inheritance of chemosensory legacies by future generations and how this field of inquiry informs the larger picture of how parental experiences can influence offspring biology.

The Role of the Stimulus in Olfactory Plasticity

Plasticity, the term we use to describe the ability of a nervous system to change with experience, is the evolutionary adaptation that freed animal behavior from the confines of genetic determinism. This capacity, which increases with brain complexity, is nowhere more evident than in vertebrates, especially mammals. Though the scientific study of brain plasticity dates back at least to the mid-19th century, the last several decades have seen unprecedented advances in the field afforded by new technologies. Olfaction is one system that has garnered particular attention in this realm because it is the only sensory modality with a lifelong supply of new neurons, from two niches no less! Here, we review some of the classical and contemporary literature dealing with the role of the stimulus or lack thereof in olfactory plasticity. We have restricted our comments to studies in mammals that have used dual tools of the field: stimulus deprivation and stimulus enrichment. The former manipulation has been implemented most frequently by unilateral naris occlusion and, thus, we have limited our comments to research using this technique. The work reviewed on deprivation provides substantial evidence of activity-dependent processes in both developing and adult mammals at multiple levels of the system from olfactory sensory neurons through to olfactory cortical areas. However, more recent evidence on the effects of deprivation also establishes several compensatory processes with mechanisms at every level of the system, whose function seems to be the restoration of information flow in the face of an impoverished signal. The results of sensory enrichment are more tentative, not least because of the actual manipulation: What odor or odors? At what concentrations? On what schedule? All of these have frequently not been sufficiently rationalized or characterized. Perhaps it is not surprising, then, that discrepant results are common in sensory enrichment studies. Despite this problem, evidence has accumulated that even passively encountered odors can "teach" olfactory cortical areas to better detect, discriminate, and more efficiently encode them for future encounters. We discuss these and other less-established roles for the stimulus in olfactory plasticity, culminating in our recommended "aspirations" for the field going forward.

Transgenerational Epigenetic Inheritance of Traumatic Experience in Mammals

In recent years, we have seen an increasing amount of evidence pointing to the existence of a non-genetic heredity of the effects of events such as separation from parents, threat to life, or other traumatising experiences such as famine. This heredity is often mediated by epigenetic regulations of gene expression and may be transferred even across several generations. In this review, we focus on studies which involve transgenerational epigenetic inheritance (TEI), with a short detour to intergenerational studies focused on the inheritance of trauma or stressful experiences. The reviewed studies show a plethora of universal changes which stress exposure initiates on multiple levels of organisation ranging from hormonal production and the hypothalamic-pituitary-adrenal (HPA) axis modulation all the way to cognition, behaviour, or propensity to certain psychiatric or metabolic disorders. This review will also provide an overview of relevant methodology and difficulties linked to implementation of epigenetic studies. A better understanding of these processes may help us elucidate the evolutionary pathways which are at work in the course of emergence of the diseases and disorders associated with exposure to trauma, either direct or in a previous generation.

Conceptualizing traumatic stress and the structure of posttraumatic psychopathology through the lenses of RDoC and HiTOP

Trauma-related psychopathology, most notably posttraumatic stress disorder (PTSD), poses unique challenges for psychiatric nosology due to the wide range of symptoms and diagnoses associated with trauma and challenges representing the impact of trauma exposure on psychopathology. In this paper, we review the literature on categorical (i.e., Diagnostic and Statistical Manual of Mental Disorders and International Classification of Diseases systems) versus dimensional conceptualizations of trauma-related symptoms with an emphasis on the Research Domain Criteria (RDoC) and the Hierarchical Taxonomy of Psychopathology (HiTOP) frameworks. We identify strengths of each approach and challenges in accommodating the full range of trauma-related psychopathology and the clinical implications thereof. We discuss several potential approaches for improving the representation of traumatic stress, including the use of PTSD subtypes, trauma-related specifiers for psychiatric diagnoses, and the development of a dimension that we call the traumatic stress spectrum, which spans both adaptive and adverse reactions to trauma. These approaches to representing traumatic stress can be evaluated empirically and further refined. We also discuss how the use of an integrated RDoC-HiTOP approach to reconceptualize traumatic stress might maximize the ability to model valid and reliable trauma-related phenotypes, which would aid in the investigation of clinically relevant biological correlates.

The Power of Odor Persuasion: The Incorporation of Olfactory Cues in Virtual Environments for Personalized Relaxation

Olfaction is the most ancient sense and is directly connected with emotional areas in the brain. It gives rise to perception linked to emotion both in everyday life and in memory-recall activities. Despite its emotional primacy in perception and its role in sampling the real physical world, olfaction is rarely used in clinical psychological settings because it relies on stimuli that are difficult to deliver. However, recent developments in virtual-reality tools are creating novel possibilities for the engagement of the sense of smell in this field. In this article, we present the relevant features of olfaction for relaxation purposes and then discuss possible future applications of involving olfaction in virtual-reality interventions for relaxation. We also discuss clinical applications, the potential of new tools, and current obstacles and limitations.

Severe histomorphological alterations in post-mortem olfactory glomeruli in Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent form of dementia. Key AD symptoms include memory and cognitive decline; however, comorbid symptoms such as depression and sensory‐perceptual dysfunction are often reported. Among these, a deterioration of olfactory sensation is observed in approximately 90% of AD patients. However, the precise pathophysiological basis underlying olfactory deficits because of AD remains elusive. The olfactory glomeruli in the olfactory bulb (OB) receive sensory information in the olfactory processing pathway. Maintaining the structural and functional integrity of the olfactory glomerulus is critical to olfactory signalling. Herein, we conducted an in‐depth histopathological assessment to reveal detailed structural alterations in the olfactory glomeruli in AD patients. Fresh frozen post‐mortem OB specimens obtained from six AD patients and seven healthy age‐matched individuals were examined. We used combined immunohistochemistry and stereology to assess the gross morphology and histological alterations, such as those in the expression of Aβ protein, microglia, and neurotransmitters in the OB. Electron microscopy was employed to study the ultrastructural features in the glomeruli. Significant accumulation of Aβ, morphologic damage, altered neurotransmitter levels, and microgliosis in the olfactory glomeruli of AD patients suggests that glomerular damage could affect olfactory function. Moreover, greater neurodegeneration was observed in the ventral olfactory glomeruli of AD patients. The synaptic ultrastructure revealed distorted postsynaptic densities and a decline in presynaptic vesicles in AD specimens. These findings show that the primary olfactory pathway is affected by the pathogenesis of AD, and may provide clues to identifying the mechanism involved in olfactory dysfunction in AD.

Olfactory Stimulation Regulates the Birth of Neurons That Express Specific Odorant Receptors

In mammals, olfactory sensory neurons (OSNs) are born throughout life, ostensibly solely to replace damaged OSNs. During differentiation, each OSN precursor “chooses,” out of hundreds of possibilities, a single odorant receptor (OR) gene, which defines the identity of the mature OSN. The relative neurogenesis rates of the hundreds of distinct OSN “subtypes” are thought to be constant, as they are determined by a stochastic process in which each OR is chosen with a fixed probability. Here, using histological, single-cell, and targeted affinity purification approaches, we show that closing one nostril in mice selectively reduces the number of newly generated OSNs of specific subtypes. Moreover, these reductions depend on an animal’s age and/or environment. Stimulation-dependent changes in the number of new OSNs are not attributable to altered rates of cell survival but rather production. Our findings indicate that the relative birth rates of distinct OSN subtypes depend on olfactory experience.

In mammals, the odorant receptor identities of newly generated olfactory sensory neurons are thought to be determined by each progenitor cell’s random choice of a single receptor. Here, van der Linden et al. show that, in mice, the birth rates of neurons expressing a subset of receptors depend on olfactory stimulation.

Modulation of olfactory signal detection in the olfactory epithelium: focus on the internal and external environment, and the emerging role of the immune system

Detection and discrimination of odorants by the olfactory system plays a pivotal role in animal survival. Olfactory-based behaviors must be adapted to an ever-changing environment. Part of these adaptations includes changes of odorant detection by olfactory sensory neurons localized in the olfactory epithelium. It is now well established that internal signals such as hormones, neurotransmitters, or paracrine signals directly affect the electric activity of olfactory neurons. Furthermore, recent data have shown that activity-dependent survival of olfactory neurons is important in the olfactory epithelium. Finally, as olfactory neurons are directly exposed to environmental toxicants and pathogens, the olfactory epithelium also interacts closely with the immune system leading to neuroimmune modulations. Here, we review how detection of odorants can be modulated in the vertebrate olfactory epithelium. We choose to focus on three cellular types of the olfactory epithelium (the olfactory sensory neuron, the sustentacular and microvillar cells) to present the diversity of modulation of the detection of odorant in the olfactory epithelium. We also present some of the growing literature on the importance of immune cells in the functioning of the olfactory epithelium, although their impact on odorant detection is only just beginning to be unravelled.

NIH Workshop Report: sensory nutrition and disease

In November 2019, the NIH held the "Sensory Nutrition and Disease" workshop to challenge multidisciplinary researchers working at the interface of sensory science, food science, psychology, neuroscience, nutrition, and health sciences to explore how chemosensation influences dietary choice and health. This report summarizes deliberations of the workshop, as well as follow-up discussion in the wake of the current pandemic. Three topics were addressed: A) the need to optimize human chemosensory testing and assessment, B) the plasticity of chemosensory systems, and C) the interplay of chemosensory signals, cognitive signals, dietary intake, and metabolism. Several ways to advance sensory nutrition research emerged from the workshop: 1) refining methods to measure chemosensation in large cohort studies and validating measures that reflect perception of complex chemosensations relevant to dietary choice; 2) characterizing interindividual differences in chemosensory function and how they affect ingestive behaviors, health, and disease risk; 3) defining circuit-level organization and function that link and interact with gustatory, olfactory, homeostatic, visceral, and cognitive systems; and 4) discovering new ligands for chemosensory receptors (e.g., those produced by the microbiome) and cataloging cell types expressing these receptors. Several of these priorities were made more urgent by the current pandemic because infection with sudden acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and the ensuing coronavirus disease of 2019 has direct short- and perhaps long-term effects on flavor perception. There is increasing evidence of functional interactions between the chemosensory and nutritional sciences. Better characterization of this interface is expected to yield insights to promote health, mitigate disease risk, and guide nutrition policy.

Semiochemical responsive olfactory sensory neurons are sexually dimorphic and plastic

Understanding how genes and experience work in concert to generate phenotypic variability will provide a better understanding of individuality. Here, we considered this in the main olfactory epithelium, a chemosensory structure with over a thousand distinct cell types in mice. We identified a subpopulation of olfactory sensory neurons, defined by receptor expression, whose abundances were sexually dimorphic. This subpopulation of olfactory sensory neurons was over-represented in sex-separated mice and robustly responsive to sex-specific semiochemicals. Sex-combined housing led to an attenuation of the dimorphic representations. Single-cell sequencing analysis revealed an axis of activity-dependent gene expression amongst a subset of the dimorphic OSN populations. Finally, the pro-apoptotic gene Baxwas necessary to generate the dimorphic representations. Altogether, our results suggest a role of experience and activity in influencing homeostatic mechanisms to generate a robust sexually dimorphic phenotype in the main olfactory epithelium.

Analysis of Genetically Regulated Gene Expression Identifies a Prefrontal PTSD Gene, SNRNP35, Specific to Military Cohorts

To reveal post-traumatic stress disorder (PTSD) genetic risk influences on tissue-specific gene expression, we use brain and non-brain transcriptomic imputation. We impute genetically regulated gene expression (GReX) in 29,539 PTSD cases and 166,145 controls from 70 ancestry-specific cohorts and identify 18 significant GReX-PTSD associations corresponding to specific tissue-gene pairs. The results suggest substantial genetic heterogeneity based on ancestry, cohort type (military versus civilian), and sex. Two study-wide significant PTSD associations are identified in European and military European cohorts; ZNF140 is predicted to be upregulated in whole blood, and SNRNP35 is predicted to be downregulated in dorsolateral prefrontal cortex, respectively. In peripheral leukocytes from 175 marines, the observed PTSD differential gene expression correlates with the predicted differences for these individuals, and deployment stress produces glucocorticoid-regulated expression changes that include downregulation of both ZNF140 and SNRNP35. SNRNP35 knockdown in cells validates its functional role in U12-intron splicing. Finally, exogenous glucocorticoids in mice downregulate prefrontal Snrnp35 expression.

Transdiagnostic Approach to Olfactory Reference Syndrome: Neurobiological Considerations

Olfactory reference syndrome (ORS) describes a constellation of emotional and behavioral symptoms that cause clinically significant distress or impairment arising from the false belief that one is emitting an offensive odor. Despite cases of ORS reported throughout the world over the last century, our knowledge and understanding of ORS remain relatively poor because of the limited literature-mostly case studies and series, but no clinical trials. ORS continues to pose significant diagnostic challenges within our current frameworks of categorizing mental disorders, including the Diagnostic and Statistical Manual of Mental Disorders and International Classification of Diseases. We review the ORS literature and discuss diagnostic parallels and challenges of placing ORS within specific categories. We also review the current research on the neurocircuitry of olfaction and of disorders with potential clinical relevance to patients presenting with ORS. While no primary neuroscientific research has specifically investigated ORS, an overlapping circuitry has been implicated in the neurobiology of obsessive-compulsive, trauma and stressor, and psychotic spectrum disorders, suggesting that the phenomenology of ORS can best be understood through a dimensional, rather than categorical, approach.

Plasticity in olfactory bulb circuits

Olfaction is crucial for animal survival and human well-being. The olfactory bulb is the obligatory input station for olfactory information. In contrast to the traditional view as a static relay station, recent evidence indicates that the olfactory bulb dynamically processes olfactory information in an experience-dependent and context-dependent manner. Here, we review recent studies on experience-dependent plasticity of the main circuit components within the olfactory bulb of rodents. We argue that the olfactory bulb plasticity allows optimal representations of behaviorally-relevant odors in the continuously changing olfactory environment.

First step of odorant detection in the olfactory epithelium and olfactory preferences differ according to the microbiota profile in mice

We have previously provided the first evidence that the microbiota modulates the physiology of the olfactory epithelium using germfree mice. The extent to which changes to the olfactory system depend on the microbiota is still unknown. In the present work, we explored if different microbiota would differentially impact olfaction. We therefore studied the olfactory function of three groups of mice of the same genetic background, whose parents had been conventionalized before mating with microbiota from three different mouse strains. Caecal short chain fatty acids profiles and 16S rRNA gene sequencing ascertained that gut microbiota differed between the three groups. We then used a behavioural test to measure the attractiveness of various odorants and observed that the three groups of mice differed in their attraction towards odorants. Their olfactory epithelium properties, including electrophysiological responses recorded by electro-olfactograms and expression of genes related to the olfactory transduction pathway, also showed several differences. Overall, our data demonstrate that differences in gut microbiota profiles are associated with differences in olfactory preferences and in olfactory epithelium functioning.

Proximate causes and consequences of intergenerational influences of salient sensory experience

Salient sensory environments experienced by a parental generation can exert intergenerational influences on offspring. While these data provide an exciting new perspective on biological inheritance, questions remain about causes and consequences of intergenerational influences of salient sensory experience. We previously showed that exposing male mice to a salient olfactory experience, like olfactory fear conditioning, resulted in offspring demonstrating a sensitivity to the odor used to condition the paternal generation and possessing enhanced neuroanatomical representation for that odor. In this study, we first injected RNA extracted from sperm of male mice that underwent olfactory fear conditioning into naïve single-cell zygotes and found that adults that developed from these embryos had increased sensitivity and enhanced neuroanatomical representation for the odor (Odor A) with which the paternal male had been conditioned. Next, we found that female, but not male offspring sired by males conditioned with Odor A show enhanced consolidation of a weak single-trial Odor A + shock fear conditioning protocol. Our data provide evidence that RNA found in the paternal germline after exposure to salient sensory experiences can contribute to intergenerational influences of such experiences, and that such intergenerational influences confer an element of adaptation to the offspring. In so doing, our study of intergenerational influences of parental sensory experience adds to existing literature on intergenerational influences of parental exposures to stress and dietary manipulations and suggests that some causes (sperm RNA) and consequences (behavioral flexibility) of intergenerational influences of parental experiences may be conserved across a variety of parental experiences.

Deconstructing the Gestalt: Mechanisms of Fear, Threat, and Trauma Memory Encoding

Threat processing is central to understanding debilitating fear- and trauma-related disorders such as posttraumatic stress disorder (PTSD). Progress has been made in understanding the neural circuits underlying the "engram" of threat or fear memory formation that complements a decades-old appreciation of the neurobiology of fear and threat involving hub structures such as the amygdala. In this review, we examine key recent findings, as well as integrate the importance of hormonal and physiological approaches, to provide a broader perspective of how bodily systems engaged in threat responses may interact with amygdala-based circuits in the encoding and updating of threat-related memory. Understanding how trauma-related memories are encoded and updated throughout the brain and the body will ultimately lead to novel biologically-driven approaches for treatment and prevention.

Pavlovian Olfactory Fear Conditioning: Its Neural Circuity and Importance for Understanding Clinical Fear-Based Disorders

Odors have proven to be the most resilient trigger for memories of high emotional saliency. Fear associated olfactory memories pose a detrimental threat of potentially transforming into severe mental illness such as fear and anxiety-related disorders. Many studies have deliberated on auditory, visual and general contextual fear memory (CFC) processes; however, fewer studies have investigated mechanisms of olfactory fear memory. Evidence strongly suggests that the neuroanatomical representation of olfactory fear memory differs from that of auditory and visual fear memory. The aim of this review article is to revisit the literature regarding the understanding of the neurobiological process of fear conditioning and to illustrate the circuitry of olfactory fear memory.

Memory formation in the absence of experience

Memory is coded by patterns of neural activity in distinct circuits. Therefore, it should be possible to reverse engineer a memory by artificially creating these patterns of activity in the absence of a sensory experience. In olfactory conditioning, an odor conditioned stimulus (CS) is paired with an unconditioned stimulus (US; for example, a footshock), and the resulting CS-US association guides future behavior. Here we replaced the odor CS with optogenetic stimulation of a specific olfactory glomerulus and the US with optogenetic stimulation of distinct inputs into the ventral tegmental area that mediate either aversion or reward. In doing so, we created a fully artificial memory in mice. Similarly to a natural memory, this artificial memory depended on CS-US contingency during training, and the conditioned response was specific to the CS and reflected the US valence. Moreover, both real and implanted memories engaged overlapping brain circuits and depended on basolateral amygdala activity for expression.

Reversing Behavioral, Neuroanatomical, and Germline Influences of Intergenerational Stress

BACKGROUND

Stressors affect populations exposed to them as well as offspring. Strategies preventing the intergenerational propagation of effects of stress would benefit public health. Olfactory cue-based fear conditioning provides a framework to address this issue.

METHODS

We 1) exposed adult male mice to an odor, acetophenone (Ace) or Lyral (parental generation [F0]-Exposed), 2) trained mice to associate these odors with mild foot shocks (F0-Trained), and 3) trained mice to associate these odors with mild foot shocks and then extinguished their fear toward these odors with odor-only presentations (F0-Extinguished). We then examined sensitivity of future generation (F1) offspring to these odors, expression of M71 odorant (Ace-responsive) and MOR23 odorant (Lyral-responsive) receptor-expressing cell populations in F1 offspring, and DNA methylation at genes encoding the Ace- (Olfr151, Olfr160) and Lyral- (Olfr16) responsive receptors in F0 sperm.

RESULTS

Extinguishing fear toward Ace or Lyral of F0 male mice (F0-Extinguished) that had been fear conditioned with Ace or Lyral, respectively, results in F1-Extinguished offspring that do not demonstrate behavioral sensitivity to Ace or Lyral, respectively, and do not have enhanced representation for M71 or MOR23 odorant receptors in the olfactory system, as is observed in F1-Trained-Ace or F1-Trained-Lyral cohorts, respectively. The promoters of genes encoding Olfr151 and Olfr160 receptors are less methylated in F0-Trained-Ace sperm compared with F0-Exposed-Ace sperm. The Olfr16 promoter is less methylated in F0-Trained-Lyral sperm compared with F0-Exposed-Lyral sperm, and F0-Extinguished-Lyral sperm have methylation levels comparable to F0-Exposed-Lyral sperm.

CONCLUSIONS

Our study demonstrates the potential of using extinction-based behavioral strategies to reverse influences of parental stress in offspring and in the parental germline.

Aversive learning-induced plasticity throughout the adult mammalian olfactory system: insights across development

Experiences, such as sensory learning, are known to induce plasticity in mammalian sensory systems. In recent years aversive olfactory learning-induced plasticity has been identified at all stages of the adult olfactory pathway; however, the underlying mechanisms have yet to be identified. Much of the work regarding mechanisms of olfactory associative learning comes from neonates, a time point before which the brain or olfactory system is fully developed. In addition, pups and adults often express different behavioral outcomes when subjected to the same olfactory aversive conditioning paradigm, making it difficult to directly attribute pup mechanisms of plasticity to adults. Despite the differences, there is evidence of similarities between pups and adults in terms of learning-induced changes in the olfactory system, suggesting at least some conserved mechanisms. Identifying these conserved mechanisms of plasticity would dramatically increase our understanding of how the brain is able to alter encoding and consolidation of salient olfactory information even at the earliest stages following aversive learning. The focus of this review is to systematically examine literature regarding olfactory associative learning across developmental stages and search for similarities in order to build testable hypotheses that will inform future studies of aversive learning-induced sensory plasticity in adults.

Paradoxical olfactory function in combat veterans: The role of PTSD and odor factors

Stress- and trauma-related disorders, including posttraumatic stress disorder (PTSD), are characterized by an increased sensitivity to threat cues. Given that threat detection is a critical function of olfaction and that combat trauma is commonly associated with burning odors, we sought a better understanding of general olfactory function as well as response to specific trauma-related (i.e. burning) odors in combat-related PTSD. Trauma-exposed combat veterans with (N = 22) and without (N = 25) PTSD were assessed for general and specific odor sensitivities using a variety of tools. Both groups had similar general odor detection thresholds. However, the combat veterans with PTSD, compared to combat veterans with comparable trauma exposure, but without PTSD, had increased ratings of odor intensity, negative valence, and odor-triggered PTSD symptoms, along with a blunted heart rate in response to burning rubber odor. These findings are discussed within the context of healthy versus pathological changes in olfactory processing that occur over time after psychological trauma.

Electro-olfactogram Responses Before and After Aversive Olfactory Conditioning in Humans

The aim of the present study was to investigate whether repetitive aversive odor conditioning induced changes at the level of the peripheral olfactory system in humans. A total of 51 volunteers participated. A pair of indistinguishable odor enantiomers [(+)-rose oxide and (-)-rose oxide] were used as stimuli. During the pre-conditioning, participants' ability to discriminate between the two odors was assessed using a three-alternative, forced-choice discrimination test. In addition, electro-olfactograms (EOG) from the olfactory epithelium were recorded. Participants underwent three conditioning sessions on consecutive days. The experimental group received an electrical stimulus to the forearm only following (+)-rose oxide presentation, whereas its enantiomer sibling was never paired with the aversive stimulus; the control group did not receive any electrical stimulation. During the post-conditioning session, their ability to discriminate the two enantiomers was assessed again using the discrimination test and EOG recordings were obtained similarly to the pre-conditioning session. Results showed significant differences in the peripheral electrophysiological responses between the conditioned and the unconditioned stimulus, demonstrating contextually induced changes at the level of the first neuron in the olfactory system.

Activity-Dependent Gene Expression in the Mammalian Olfactory Epithelium

Activity-dependent processes are important to olfactory sensory neurons (OSNs) in several ways, such as cell survival and the specificity of axonal convergence. The identification of activity-dependent mRNAs has contributed to our understanding of OSN axon convergence, but has revealed surprisingly little about other processes. Published studies of activity-dependent mRNAs in olfactory mucosae overlap poorly, but by combining these agreements with meta-analysis of existing data we identify 443 mRNAs that respond to methods that alter OSN activity. Three hundred and fifty of them are expressed in mature OSNs, consistent with the expectation that activity-dependent responses are cell autonomous and driven by odor stimulation. Many of these mRNAs encode proteins that function at presynaptic terminals or support electrical activity, consistent with hypotheses linking activity dependence to synaptic plasticity and energy conservation. The lack of agreement between studies is due largely to underpowered experiments. In addition, methods used to alter OSN activity are susceptible to indirect or off-target effects. These effects deserve greater attention, not only to rigorously identify OSN mRNAs that respond to altered OSN activity, but also because these effects are of significant interest in their own right. For example, the mRNAs of some sustentacular cell enzymes believed to function in odorant clearance (Cyp2a4 and Cyp2g1) are sensitive to unilateral naris occlusion used to reduce odorant stimulation of the ipsilateral olfactory epithelium. Also problematic are odorant receptor mRNAs, which show little agreement across studies and are susceptible to differences in frequency of expression that masquerade as activity-dependent changes in mRNA abundance.

Prenatal and Early Postnatal Odorant Exposure Heightens Odor-Evoked Mitral Cell Responses in the Mouse Olfactory Bulb

Early sensory experience shapes the anatomy and function of sensory circuits. In the mouse olfactory bulb (OB), prenatal and early postnatal odorant exposure through odorized food (food/odorant pairing) not only increases the volume of activated glomeruli but also increases the number of mitral and tufted cells (M/TCs) connected to activated glomeruli. Given the importance of M/TCs in OB output and in mediating lateral inhibitory networks, increasing the number of M/TCs connected to a single glomerulus may significantly change odorant representation by increasing the total output of that glomerulus and/or by increasing the strength of lateral inhibition mediated by cells connected to the affected glomerulus. Here, we seek to understand the functional impact of this long-term odorant exposure paradigm on the population activity of mitral cells (MCs). We use viral expression of GCaMP6s to examine odor-evoked responses of MCs following prenatal and early postnatal odorant exposure to two dissimilar odorants, methyl salicylate (MS) and hexanal, which are both strong activators of glomeruli on the dorsal OB surface. Previous work suggests that odor familiarity may decrease odor-evoked MC response in rodents. However, we find that early food-based odorant exposure significantly changes MC responses in an unexpected way, resulting in broad increases in the amplitude, number, and reliability of excitatory MC responses across the dorsal OB.

Prenatal and Early Postnatal Odorant Exposure Heightens Odor-Evoked Mitral Cell Responses in the Mouse Olfactory Bulb

Early sensory experience shapes the anatomy and function of sensory circuits. In the mouse olfactory bulb (OB), prenatal and early postnatal odorant exposure through odorized food (food/odorant pairing) not only increases the volume of activated glomeruli but also increases the number of mitral and tufted cells (M/TCs) connected to activated glomeruli. Given the importance of M/TCs in OB output and in mediating lateral inhibitory networks, increasing the number of M/TCs connected to a single glomerulus may significantly change odorant representation by increasing the total output of that glomerulus and/or by increasing the strength of lateral inhibition mediated by cells connected to the affected glomerulus. Here, we seek to understand the functional impact of this long-term odorant exposure paradigm on the population activity of mitral cells (MCs). We use viral expression of GCaMP6s to examine odor-evoked responses of MCs following prenatal and early postnatal odorant exposure to two dissimilar odorants, methyl salicylate (MS) and hexanal, which are both strong activators of glomeruli on the dorsal OB surface. Previous work suggests that odor familiarity may decrease odor-evoked MC response in rodents. However, we find that early food-based odorant exposure significantly changes MC responses in an unexpected way, resulting in broad increases in the amplitude, number, and reliability of excitatory MC responses across the dorsal OB.

Development of Odor Hedonics: Experience-Dependent Ontogeny of Circuits Supporting Maternal and Predator Odor Responses in Rats

A major component of perception is hedonic valence: perceiving stimuli as pleasant or unpleasant. Here, we used early olfactory experiences that shape odor preferences and aversions to explore developmental plasticity in circuits mediating odor hedonics. We used 2-deoxyglucose autoradiographic mapping of neural activity to identify circuits differentially activated by biologically relevant preferred and avoided odors across rat development. We then further probed this system by increasing or decreasing hedonic value. Using both region of interest and functional connectivity analyses, we identified regions within primary olfactory, amygdala/hippocampal, and prefrontal cortical networks that were activated differentially by maternal and male odors. Although some activated regions remained stable across development (postnatal days 7-23), there was a developmental emergence of others that resulted in an age-dependent elaboration of hedonic-response-specific circuitry despite stable behavioral responses (approach/avoidance) to the odors across age. Hedonic responses to these biologically important odors were modified through diet suppression of the maternal odor and co-rearing with a male. This allowed assessment of hedonic circuits in isolation of the specific odor quality and/or intensity. Early experience significantly modified odor-evoked circuitry in an age-dependent manner. For example, co-rearing with a male, which induced pup attraction to male odor, reduced activity in amygdala regions normally activated by the unfamiliar avoided male odor, making this region more consistent with maternal odor. Understanding the development of odor hedonics, particularly within the context of altered early life experience, provides insight into the development of sensory processes, food preferences, and the formation of social affiliations, among other behaviors.

SIGNIFICANCE STATEMENT

Odor hedonic valence controls approach-avoidance behaviors, but also modulates ongoing behaviors ranging from food preferences and social affiliation with the caregiver to avoidance of predator odors. Experiences can shape hedonic valence. This study explored brain circuitry involved in odor hedonic encoding throughout development using maternal and predator odors and assessed the effects of early life experience on odor hedonic encoding by increasing/decreasing the hedonic value of these odors. Understanding the role of changing brain circuitry during development and its impact on behavioral function is critical for understanding sensory processing across development. These data converge with exciting literature on the brain's hedonic network and highlight the significant role of early life experience in shaping the neural networks of highly biologically relevant stimuli.

Amygdalar Gating of Early Sensory Processing through Interactions with Locus Coeruleus

Fear- and stress-induced activity in the amygdala has been hypothesized to influence sensory brain regions through the influence of the amygdala on neuromodulatory centers. To directly examine this relationship, we used optical imaging to observe odor-evoked activity in populations of olfactory bulb inhibitory interneurons and of synaptic terminals of olfactory sensory neurons (the primary sensory neurons of the olfactory system, which provide the initial olfactory input to the brain) during pharmacological inactivation of amygdala and locus coeruleus (LC) in mice. Although the amygdala does not directly project to the olfactory bulb, joint pharmacological inactivation of the central, basolateral, and lateral nuclei of the amygdala nonetheless strongly suppressed odor-evoked activity in GABAergic inhibitory interneuron populations in the OB. This suppression was prevented by inactivation of LC or pretreatment of the olfactory bulb with a broad-spectrum noradrenergic receptor antagonist. Visualization of synaptic output from olfactory sensory neuron terminals into the olfactory bulb of the brain revealed that amygdalar inactivation preferentially strengthened the odor-evoked synaptic output of weakly activated populations of sensory afferents from the nose, thus demonstrating a change in sensory gating potentially mediated by local inhibition of olfactory sensory neuron terminals. We conclude that amygdalar activity influences olfactory processing as early as the primary sensory input to the brain by modulating norepinephrine release from the locus coeruleus into the olfactory bulb. These findings show that the amygdala and LC state actively determines which sensory signals are selected for processing in sensory brain regions. Similar local circuitry operates in the olfactory, visual, and auditory systems, suggesting a potentially shared mechanism across modalities.SIGNIFICANCE STATEMENT The affective state is increasingly understood to influence early neural processing of sensory stimuli, not just the behavioral response to those stimuli. The present study elucidates one circuit by which the amygdala, a critical structure for emotional learning, valence coding, and stress, can shape sensory input to the brain and early sensory processing through its connections to the locus coeruleus. One function of this interaction appears to be sensory gating, because inactivating the central, basolateral, and lateral nuclei of the amygdala selectively strengthened the weakest olfactory inputs to the brain. This linkage of amygdalar and LC output to primary sensory signaling may have implications for affective disorders that include sensory dysfunctions like hypervigilance, attentional bias, and impaired sensory gating.

Mechanism of Notch Pathway Activation and Its Role in the Regulation of Olfactory Plasticity in Drosophila melanogaster

The neural plasticity of sensory systems is being increasingly recognized as playing a role in learning and memory. We have previously shown that Notch, part of an evolutionarily conserved intercellular signaling pathway, is required in adult Drosophila melanogaster olfactory receptor neurons (ORNs) for the structural and functional plasticity of olfactory glomeruli that is induced by chronic odor exposure. In this paper we address how long-term exposure to odor activates Notch and how Notch in conjunction with chronic odor mediates olfactory plasticity. We show that upon chronic odor exposure a non-canonical Notch pathway mediates an increase in the volume of glomeruli by a mechanism that is autonomous to ORNs. In addition to activating a pathway that is autonomous to ORNs, chronic odor exposure also activates the Notch ligand Delta in second order projection neurons (PNs), but this does not appear to require acetylcholine receptor activation in PNs. Delta on PNs then feeds back to activate canonical Notch signaling in ORNs, which restricts the extent of the odor induced increase in glomerular volume. Surprisingly, even though the pathway that mediates the increase in glomerular volume is autonomous to ORNs, nonproductive transsynaptic Delta/Notch interactions that do not activate the canonical pathway can block the increase in volume. In conjunction with chronic odor, the canonical Notch pathway also enhances cholinergic activation of PNs. We present evidence suggesting that this is due to increased acetylcholine release from ORNs. In regulating physiological plasticity, Notch functions solely by the canonical pathway, suggesting that there is no direct connection between morphological and physiological plasticity.