Naris occlusion alters the electro-olfactogram: evidence for compensatory plasticity in the olfactory system

Attentional processing in the rat dorsal posterior parietal cortex

The human posterior parietal cortex (PPC) is known to support sustained attention. Specifically, top-down attention is generally processed in dorsal regions while bottom-up regulation occurs more ventrally. In rodent models, however, it is still unclear whether the PPC is required for sustained attention, or whether there is a similar functional dissociation between anatomical regions. Consequently, the aim of this study was to investigate the contribution of the rodent dorsal PPC (dPPC) in sustained attention. We used the five-choice serial reaction time task (5CSRTT) and compared rats with neurotoxic dPPC lesions to sham operated rats. We found that rats with dPPC lesions were less accurate and took longer to make correct choices, indicating impaired attention and reduced processing speed. This effect, however, was limited to the first few days of post-operative testing. After an apparent recovery, omissions became elevated in the lesion group, which, in the absence of reduced motivation and mobility, can also be interpreted as impaired attention. In subsequent challenge probes, the lesion group displayed globally elevated latency to make a correct response, indicating reduced processing speed. No differences in premature responses or perseverative responses were observed at any time, demonstrating that dPPC lesions did not affect impulsivity and compulsivity. This pattern of behavior suggests that while intact dPPC supports goal-driven (top-down) modulation of attention, it likely does not play a central role in processing stimulus-driven (bottom-up) attention. Furthermore, compensatory mechanisms can support sustained attention in the absence of a fully functioning dPPC, although this occurs at the expense of processing speed. Our results inform the literature by confirming that rodent PPC is involved in regulating sustained attention and providing preliminary evidence for a functional dissociation between top-down and bottom-up attentional processing.

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.

Sensory deprivation arrests cellular and synaptic development of the night-vision circuitry in the retina

Experience regulates synapse formation and function across sensory circuits. How inhibitory synapses in the mammalian retina are sculpted by visual cues remains unclear. By use of a sensory deprivation paradigm, we find that visual cues regulate maturation of two GABA synapse types (GABAA and GABAC receptor synapses), localized across the axon terminals of rod bipolar cells (RBCs)-second-order retinal neurons integral to the night-vision circuit. Lack of visual cues causes GABAA synapses at RBC terminals to retain an immature receptor configuration with slower response profiles and prevents receptor recruitment at GABAC synapses. Additionally, the organizing protein for both these GABA synapses, LRRTM4, is not clustered at dark-reared RBC synapses. Ultrastructurally, the total number of ribbon-output/inhibitory-input synapses across RBC terminals remains unaltered by sensory deprivation, although ribbon synapse output sites are misarranged when the circuit develops without visual cues. Intrinsic electrophysiological properties of RBCs and expression of chloride transporters across RBC terminals are additionally altered by sensory deprivation. Introduction to normal 12-h light-dark housing conditions facilitates maturation of dark-reared RBC GABA synapses and restoration of intrinsic RBC properties, unveiling a new element of light-dependent retinal cellular and synaptic plasticity.

Intranasal delivery of SARS-CoV-2 spike protein is sufficient to cause olfactory damage, inflammation and olfactory dysfunction in zebrafish

Anosmia, loss of smell, is a prevalent symptom of SARS-CoV-2 infection. Anosmia may be explained by several mechanisms driven by infection of non-neuronal cells and damage in the nasal epithelium rather than direct infection of olfactory sensory neurons (OSNs). Previously, we showed that viral proteins are sufficient to cause neuroimmune responses in the teleost olfactory organ (OO). We hypothesize that SARS-CoV-2 spike (S) protein is sufficient to cause olfactory damage and olfactory dysfunction. Using an adult zebrafish model, we report that intranasally delivered SARS-CoV-2 S RBD mostly binds to the non-sensory epithelium of the olfactory organ and causes severe olfactory histopathology characterized by loss of cilia, hemorrhages and edema. Electrophysiological recordings reveal impaired olfactory function to both food and bile odorants in animals treated intranasally with SARS-CoV-2 S RBD. However, no loss of behavioral preference for food was detected in SARS-CoV-2 S RBD treated fish. Single cell RNA-Seq of the adult zebrafish olfactory organ indicated widespread loss of olfactory receptor expression and inflammatory responses in sustentacular, endothelial, and myeloid cell clusters along with reduced numbers of T_regs. Combined, our results demonstrate that intranasal SARS-CoV-2 S RBD is sufficient to cause structural and functional damage to the zebrafish olfactory system. These findings may have implications for intranasally delivered vaccines against SARS-CoV-2.

A transcriptional rheostat couples past activity to future sensory responses

Animals traversing different environments encounter both stable background stimuli and novel cues, which are thought to be detected by primary sensory neurons and then distinguished by downstream brain circuits. Here, we show that each of the ∼1,000 olfactory sensory neuron (OSN) subtypes in the mouse harbors a distinct transcriptome whose content is precisely determined by interactions between its odorant receptor and the environment. This transcriptional variation is systematically organized to support sensory adaptation: expression levels of more than 70 genes relevant to transforming odors into spikes continuously vary across OSN subtypes, dynamically adjust to new environments over hours, and accurately predict acute OSN-specific odor responses. The sensory periphery therefore separates salient signals from predictable background via a transcriptional rheostat whose moment-to-moment state reflects the past and constrains the future; these findings suggest a general model in which structured transcriptional variation within a cell type reflects individual experience.

Olfactory Deprivation and Enrichment: An Identity of Opposites?

The effects of deprivation and enrichment on the electroolfactogram of mice were studied through the paradigms of unilateral naris occlusion and odor induction, respectively. Deprivation was shown to cause an increase in electroolfactogram amplitudes after 7 days. We also show that unilateral naris occlusion is not detrimental to the gross anatomical appearance or electroolfactogram of either the ipsilateral or contralateral olfactory epithelium even after year-long survival periods, consistent with our previous assumptions. Turning to induction, the increase in olfactory responses after a period of odor enrichment, could not be shown in CD-1 outbred mice for any odorant tried. However, consistent with classical studies, it was evident in C57BL/6J inbred mice, which are initially insensitive to isovaleric acid. As is the case for deprivation, enriching C57BL/6J mice with isovaleric acid causes an increase in their electroolfactogram response to this odorant over time. In several experiments on C57BL/6J mice, the odorant specificity, onset timing, recovery timing, and magnitude of the induction effect were studied. Considered together, the current findings and previous work from the laboratory support the counterintuitive conclusion that both compensatory plasticity in response to deprivation and induction in response to odor enrichment are caused by the same underlying homeostatic mechanism, the purpose of which is to preserve sensory information flow no matter the odorant milieu. This hypothesis, the detailed evidence supporting it, and speculations concerning human odor induction are discussed.

Tests of the chromatographic theory of olfaction with highly soluble odors: a combined electro-olfactogram and computational fluid dynamics study in the mouse

The idea that the vertebrate nasal cavity operates like a gas chromatograph to separate and discriminate odors, referred to herein as the ‘chromatographic theory’ (CT), has a long and interesting history. Though the last decade has seen renewed interest in the notion, its validity remains in question. Here we examine a necessary condition of the theory: a correlation between nasal odor deposition patterns based on mucus solubility and the distribution of olfactory sensory neuron odotypes. Our recent work in the mouse failed to find such a relationship even across large sorption gradients within the olfactory epithelium (OE). However, these studies did not test extremely soluble odorants or low odor concentrations, factors that could explain our inability to find supporting evidence for the CT. The current study combined computational fluid dynamics (CFD) simulations of odor sorption patterns and electro-olfactogram (EOG) measurements of olfactory sensory neuron responses. The odorants tested were at the extremes of mucus solubility and at a range of concentrations. Results showed no relationship between local odor sorption patterns and EOG response maps. Together, results again failed to support a necessary condition of the CT casting further doubt on the viability of this classical odor coding mechanism.

Summary: This paper casts doubt on the classical chromatographic theory of olfaction, showing there is no correlation between olfactory receptor spatial layout and odor solubility patterns, a necessary condition of the theory.

Tests of the sorption and olfactory "fovea" hypotheses in the mouse

The spatial distribution of receptors within sensory epithelia (e.g., retina and skin) is often markedly nonuniform to gain efficiency in information capture and neural processing. By contrast, odors, unlike visual and tactile stimuli, have no obvious spatial dimension. What need then could there be for either nearest-neighbor relationships or nonuniform distributions of receptor cells in the olfactory epithelium (OE)? Adrian (Adrian ED. J Physiol 100: 459-473, 1942; Adrian ED. Br Med Bull 6: 330-332, 1950) provided the only widely debated answer to this question when he posited that the physical properties of odors, such as volatility and water solubility, determine a spatial pattern of stimulation across the OE that could aid odor discrimination. Unfortunately, despite its longevity, few critical tests of the "sorption hypothesis" exist. Here we test the predictions of this hypothesis by mapping mouse OE responses using the electroolfactogram (EOG) and comparing these response "maps" to computational fluid dynamics (CFD) simulations of airflow and odorant sorption patterns in the nasal cavity. CFD simulations were performed for airflow rates corresponding to quiet breathing and sniffing. Consistent with predictions of the sorption hypothesis, water-soluble odorants tended to evoke larger EOG responses in the central portion of the OE than the peripheral portion. However, sorption simulation patterns along individual nasal turbinates for particular odorants did not correlate with their EOG response gradients. Indeed, the most consistent finding was a rostral-greater to caudal-lesser response gradient for all the odorants tested that is unexplained by sorption patterns. The viability of the sorption and related olfactory "fovea" hypotheses are discussed in light of these findings.NEW & NOTEWORTHY Two classical ideas concerning olfaction's receptor-surface two-dimensional organization-the sorption and olfactory fovea hypotheses-were found wanting in this study that afforded unprecedented comparisons between electrophysiological recordings in the mouse olfactory epithelium and computational fluid dynamic simulations of nasal airflow. Alternatively, it is proposed that the olfactory receptor layouts in macrosmatic mammals may be an evolutionary contingent state devoid of the functional significance found in other sensory epithelia like the cochlea and retina.

Anatomical and molecular consequences of Unilateral Naris Closure on two populations of olfactory sensory neurons expressing defined odorant receptors

Mammalian olfactory sensory neurons (OSNs), the primary elements of the olfactory system, are located in the olfactory epithelium lining the nasal cavity. Exposed to the environment, their lifespan is short. Consequently, OSNs are regularly regenerated and several reports show that activity strongly modulates their development and regeneration: the peripheral olfactory system can adjust to the amount of stimulus through compensatory mechanisms. Unilateral naris occlusion (UNO) was frequently used to investigate this mechanism at the entire epithelium level. However, there is little data regarding the effects of UNO at the cellular level, especially on individual neuronal populations expressing a defined odorant receptor. Here, using UNO during the first three postnatal weeks, we analyzed the anatomical and molecular consequences of sensory deprivation in OSNs populations expressing the MOR23 and M71 receptors. The density of MOR23-expressing neurons is decreased in the closed side while UNO does not affect the density of M71-expressing neurons. Using Real Time qPCR on isolated neurons, we observed that UNO modulates the transcript levels for transduction pathway proteins (odorant receptors, CNGA2, PDE1c). The transcripts modulated by UNO will differ between populations depending on the receptor expressed. These results suggest that sensory deprivation will have different effects on different OSNs' populations. As a consequence, early experience will shape the functional properties of OSNs differently depending on the type of odorant receptor they express.

Compensatory plasticity in the olfactory epithelium: age, timing, and reversibility

Like other biological systems, olfaction responds "homeostatically" to enduring change in the stimulus environment. This adaptive mechanism, referred to as compensatory plasticity, has been studied almost exclusively in developing animals. Thus it is unknown if this phenomenon is limited to ontogenesis and irreversible, characteristics common to some other forms of plasticity. Here we explore the effects of odor deprivation on the adult mouse olfactory epithelium (OE) using nasal plugs to eliminate nasal airflow unilaterally. Plugs were in place for 2-6 wk after which electroolfactograms (EOGs) were recorded from the occluded and open sides of the nasal cavity. Mean EOG amplitudes were significantly greater on the occluded than on the open side. The duration of plugging did not affect the results, suggesting that maximal compensation occurs within 2 wk or less. The magnitude of the EOG difference between the open and occluded side in plugged mice was comparable to adults that had undergone surgical naris occlusion as neonates. When plugs were removed after 4 wk followed by 2 wk of recovery, mean EOG amplitudes were not significantly different between the always-open and previously plugged sides of the nasal cavity suggesting that this form of plasticity is reversible. Taken together, these results suggest that compensatory plasticity is a constitutive mechanism of olfactory receptor neurons that allows these cells to recalibrate their stimulus-response relationship to fit the statistics of their current odor environment.

Visualizing olfactory learning functional imaging of experience-induced olfactory bulb changes

The anatomical organization of sensory neuron input allows odor information to be transformed into odorant-specific spatial maps of mitral/tufted cell glomerular activity. In other sensory systems, neuronal representations of sensory stimuli can be reorganized or enhanced following learning or experience. Similarly, several studies have demonstrated both structural and physiological experience-induced changes throughout the olfactory system. As experience-induced changes within this circuit likely serve as an initial site for odor memory formation, the olfactory bulb is an ideal site for optical imaging studies of olfactory learning, as they allow for the visualization of experience-induced changes in the glomerular circuit following learning and how these changes impact of odor representations with the bulb. Presently, optical imaging techniques have been used to visualize experience-induced changes in glomerular odor representations in a variety of paradigms in short-term habituation, chronic odor exposure, and olfactory associative conditioning.

Early survival factor deprivation in the olfactory epithelium enhances activity-driven survival

The neuronal olfactory epithelium undergoes permanent renewal because of environmental aggression. This renewal is partly regulated by factors modulating the level of neuronal apoptosis. Among them, we had previously characterized endothelin as neuroprotective. In this study, we explored the effect of cell survival factor deprivation in the olfactory epithelium by intranasal delivery of endothelin receptors antagonists to rat pups. This treatment induced an overall increase of apoptosis in the olfactory epithelium. The responses to odorants recorded by electroolfactogram were decreased in treated animal, a result consistent with a loss of olfactory sensory neurons (OSNs). However, the treated animal performed better in an olfactory orientation test based on maternal odor compared to non-treated littermates. This improved performance could be due to activity-dependent neuronal survival of OSNs in the context of increased apoptosis level. In order to demonstrate it, we odorized pups with octanal, a known ligand for the rI7 olfactory receptor (Olr226). We quantified the number of OSN expressing rI7 by RT-qPCR and whole mount in situ hybridization. While this number was reduced by the survival factor removal treatment, this reduction was abolished by the presence of its ligand. This improved survival was optimal for low concentration of odorant and was specific for rI7-expressing OSNs. Meanwhile, the number of rI7-expressing OSNs was not affected by the odorization in non-treated littermates; showing that the activity-dependant survival of OSNs did not affect the OSN population during the 10 days of odorization in control conditions. Overall, our study shows that when apoptosis is promoted in the olfactory mucosa, the activity-dependent neuronal plasticity allows faster tuning of the olfactory sensory neuron population toward detection of environmental odorants.

Activity-dependent modulation of odorant receptor gene expression in the mouse olfactory epithelium

Activity plays critical roles in development and maintenance of the olfactory system, which undergoes considerable neurogenesis throughout life. In the mouse olfactory epithelium, each olfactory sensory neuron (OSN) stably expresses a single odorant receptor (OR) type out of a repertoire of ∼1200 and the OSNs with the same OR identity are distributed within one of the few broadly-defined zones. However, it remains elusive whether and how activity modulates such OR expression patterns. Here we addressed this question by investigating OR gene expression via in situ hybridization when sensory experience or neuronal excitability is manipulated. We first examined the expression patterns of fifteen OR genes in mice which underwent neonatal, unilateral naris closure. After four-week occlusion, the cell density in the closed (sensory-deprived) side was significantly lower (for four ORs), similar (for three ORs), or significantly higher (for eight ORs) as compared to that in the open (over-stimulated) side, suggesting that sensory inputs have differential effects on OSNs expressing different OR genes. We next examined the expression patterns of seven OR genes in transgenic mice in which mature OSNs had reduced neuronal excitability. Neuronal silencing led to a significant reduction in the cell density for most OR genes tested and thinner olfactory epithelium with an increased density of apoptotic cells. These results suggest that sensory experience plays important roles in shaping OR gene expression patterns and the neuronal activity is critical for survival of OSNs.

Potential role of transient receptor potential channel M5 in sensing putative pheromones in mouse olfactory sensory neurons

Based on pharmacological studies of chemosensory transduction in transient receptor potential channel M5 (TRPM5) knockout mice it was hypothesized that this channel is involved in transduction for a subset of putative pheromones in mouse olfactory sensory neurons (OSNs). Yet, in the same study an electroolfactogram (EOG) in the mouse olfactory epithelium showed no significant difference in the responses to pheromones (and odors) between wild type and TRPM5 knockout mice. Here we show that the number of OSNs expressing TRPM5 is increased by unilateral naris occlusion. Importantly, EOG experiments show that mice lacking TRPM5 show a decreased response in the occluded epithelia to putative pheromones as opposed to wild type mice that show no change upon unilateral naris occlusion. This evidence indicates that under decreased olfactory sensory input TRPM5 plays a role in mediating putative pheromone transduction. Furthermore, we demonstrate that cyclic nucleotide gated channel A2 knockout (CNGA2-KO) mice that show substantially decreased or absent responses to odors and pheromones also have elevated levels of TRPM5 compared to wild type mice. Taken together, our evidence suggests that TRPM5 plays a role in mediating transduction for putative pheromones under conditions of reduced chemosensory input.

Neural sensitivity to odorants in deprived and normal olfactory bulbs

Early olfactory deprivation in rodents is accompanied by an homeostatic regulation of the synaptic connectivity in the olfactory bulb (OB). However, its consequences in the neural sensitivity and discrimination have not been elucidated. We compared the odorant sensitivity and discrimination in early sensory deprived and normal OBs in anesthetized rats. We show that the deprived OB exhibits an increased sensitivity to different odorants when compared to the normal OB. Our results indicate that early olfactory stimulation enhances discriminability of the olfactory stimuli. We found that deprived olfactory bulbs adjusts the overall excitatory and inhibitory mitral cells (MCs) responses to odorants but the receptive fields become wider than in the normal olfactory bulbs. Taken together, these results suggest that an early natural sensory stimulation sharpens the receptor fields resulting in a larger discrimination capability. These results are consistent with previous evidence that a varied experience with odorants modulates the OB's synaptic connections and increases MCs selectivity.

An electroolfactogram study of odor response patterns from the mouse olfactory epithelium with reference to receptor zones and odor sorptiveness

Olfactory sensory neuron (OSN) responses to odors, measured at the population level, tend to be spatially heterogeneous in the vertebrates that have been studied. These response patterns vary between odors but are similar across subjects for a given stimulus. However, few species have been studied making functional interpretation of these patterns problematic. One proximate explanation for the spatial heterogeneity of odor responses comes from evidence that olfactory receptor (OR) genes in rodents are expressed in OSN populations that are spatially restricted to a few zones in the olfactory epithelium (OE). A long-standing functional explanation for response anisotropy in the OE posits that it is the signature of a supplementary mechanism for quality coding, based on the sorptive properties of odor molecules. These theories are difficult to assess because most mapping studies have utilized few odors, provided little replication, or involved but a single species (rat). In fact, to our knowledge, a detailed olfactory response "map" has not been reported for mouse, the species used in most studies of gene localization. Here we report the results of a study of mouse OE response patterns using the electroolfactogram (EOG). We focused on the medial aspect of olfactory turbinates that are accessible in the midsagittal section. This limited approach still allowed us to test predictions derived from the zonal distribution of OSN types and the sorption hypothesis. In 3 separate experiments, 290 mice were used to record EOGs from a set of standard locations along each of 4 endoturbinates utilizing 11 different odors resulting in over 4,400 separate recordings. Our results confirmed a marked spatial heterogeneity in odor responses that varied with odor, as seen in other species. However, no discontinuities were found in the odor-specific response patterns across the OE as might have been predicted given the existence of classical receptor zones nor did we find clear support for the hypothesis that OE response patterns, presumably a reflection of OSN distribution, have been shaped through natural selection by the relative sorptive properties of odors. We propose that receptor zones may be an epiphenomenon of a contingent evolutionary process. In this formulation, constraints on developmental programs for distributing OSN classes within the OE may be minimally related to the odor ligands of specific class members. Further, we propose that odor sorptiveness, which appears to be correlated with the inherent response patterns in the OE of larger species, may be of minimal effect in mice owing to scaling issues.

The activity-dependent histone variant H2BE modulates the life span of olfactory neurons

We have identified a replication-independent histone variant, Hist2h2be (referred to herein as H2be), which is expressed exclusively by olfactory chemosensory neurons. Levels of H2BE are heterogeneous among olfactory neurons, but stereotyped according to the identity of the co-expressed olfactory receptor (OR). Gain- and loss-of-function experiments demonstrate that changes in H2be expression affect olfactory function and OR representation in the adult olfactory epithelium. We show that H2BE expression is reduced by sensory activity and that it promotes neuronal cell death, such that inactive olfactory neurons display higher levels of the variant and shorter life spans. Post-translational modifications (PTMs) of H2BE differ from those of the canonical H2B, consistent with a role for H2BE in altering transcription. We propose a physiological function for H2be in modulating olfactory neuron population dynamics to adapt the OR repertoire to the environment.

DOI:http://dx.doi.org/10.7554/eLife.00070.001

A hallmark of the nervous systems of all mammals is their capacity to undergo changes in function that are shaped by experience. This phenomenon underlies the ability of our brains to develop properly and to learn, and also enables various sensory systems—including the visual, auditory and olfactory systems—to perform optimally in diverse environments.

In most mammals, a high-functioning olfactory system is essential for carrying out tasks that are crucial for survival, such as finding food, avoiding predators and mating. In general, sensory systems have to decipher only a limited collection of stimuli, but the olfactory system must be able to process information from thousands of distinct odors that are found in a given environment and which may vary dramatically from one environment to the next. Each odor-sensing neuron in the nose of a mammal contains just one kind of odorant receptor protein, although mammalian genomes typically encode 1000 or so different kinds of receptor proteins. This suggests that it might be possible to ‘tune’ the olfactory system to a particular environment by changing the relative numbers of the different types of neurons. Indeed, it is known that the relative abundance of each type of odor-sensing neuron changes with age and experience, and that these changes might be caused by variations in the lifespans of the neurons.

Although our understanding of how these experience-dependent changes are orchestrated at the molecular level is far from complete, it is clear that adjustments in the levels of specific gene products is necessary. But how do experiences alter the levels of gene products to give rise to lasting changes in the brain? One hypothesis is that changes to a structure called chromatin are key to this process: chromatin is an assembly of DNA molecules, which are quite long, and organizing proteins, mostly proteins known as histones, that together form a compact structure that can fit inside the nucleus of a cell.

Santoro and Dulac have now discovered a previously uncharacterized protein called H2BE that is found only in the odor-sensing neurons of mice. H2BE is a variant of a protein called H2B, which is a well-known histone. They found that in odor-sensing neurons, H2BE replaces H2B to an extent that depends on the amount of activity experienced by the neuron: H2BE is nearly undetectable in highly active neurons, but almost completely replaces H2B in neurons that are inactive. Moreover, genetic manipulation showed that the deletion of H2BE significantly extended the lifespan of neurons, whereas elevated levels of H2BE shortened their lifespan. These findings reveal an extraordinary process that involves inactive odor-sensing neurons being depleted relative to active ones over time.

How does H2BE, which differs from H2B by just five amino acids, cause such dramatic changes in neuronal composition? One hint comes from evidence that these amino acids disrupt interactions between chromatin and ‘effector’ proteins, which modulate gene activity. Consistent with this, Santoro and Dulac have found that the replacement of H2B by H2BE strongly alters gene activity, although the precise mechanism by which these alterations regulate neuronal lifespans remains to be determined. Understanding this process in detail, and exploring if similar phenomena are involved in experience-dependent changes elsewhere in the nervous system, are fascinating areas of future research.

DOI:http://dx.doi.org/10.7554/eLife.00070.002

Changes in the neural representation of odorants after olfactory deprivation in the adult mouse olfactory bulb

Olfactory sensory deprivation during development has been shown to induce significant alterations in the neurophysiology of olfactory receptor neurons (ORNs), the primary sensory inputs to the brain's olfactory bulb. Deprivation has also been shown to alter the neurochemistry of the adult olfactory system, but the physiological consequences of these changes are poorly understood. Here we used in vivo synaptopHluorin (spH) imaging to visualize odorant-evoked neurotransmitter release from ORNs in adult transgenic mice that underwent 4 weeks of unilateral olfactory deprivation. Deprivation reduced odorant-evoked spH signals compared with sham-occluded mice. Unexpectedly, this reduction was equivalent between ORNs on the open and plugged sides. Changes in odorant selectivity of glomerular subpopulations of ORNs were also observed, but only in ORNs on the open side of deprived mice. These results suggest that naris occlusion in adult mice produces substantial changes in primary olfactory processing which may reflect not only the decrease in olfactory stimulation on the occluded side but also the alteration of response properties on the intact side. We also observed a modest effect of true sham occlusions that included noseplug insertion and removal, suggesting that conventional noseplug techniques may have physiological effects independent of deprivation per se and thus require more careful controls than has been previously appreciated.

Olfactory input is critical for sustaining odor quality codes in human orbitofrontal cortex

Ongoing sensory input is critical for shaping internal representations of the external world. Conversely, a lack of sensory input can profoundly perturb the formation of these representations. The olfactory system is particularly vulnerable to sensory deprivation, due to the widespread prevalence of allergic, viral, and chronic rhinosinusitis, but how the brain encodes and maintains odor information under such circumstances remains poorly understood. Here we combined functional magnetic resonance imaging (fMRI) with multivariate (pattern-based) analyses and psychophysical approaches to show that a seven-day period of olfactory deprivation induces reversible changes in odor-evoked fMRI activity in piriform cortex and orbitofrontal cortex (OFC). Notably, multivoxel ensemble codes of odor quality in OFC became decorrelated following deprivation, and the magnitude of these changes predicted subsequent olfactory perceptual plasticity. Our findings suggest that transient changes in these key olfactory brain regions are instrumental in sustaining odor perception integrity in the wake of disrupted sensory input.

Postnatal experience modulates functional properties of mouse olfactory sensory neurons

Early experience considerably modulates the organization and function of all sensory systems. In the mammalian olfactory system, deprivation of the sensory inputs via neonatal, unilateral naris closure has been shown to induce structural, molecular and functional changes from the olfactory epithelium to the olfactory bulb and cortex. However, it remains unknown how early experience shapes the functional properties of individual olfactory sensory neurons (OSNs), the primary odor detectors in the nose. To address this question, we examined the odorant response properties of mouse OSNs in both the closed and open nostril after 4 weeks of unilateral naris closure, with age-matched untreated animals as control. Using a patch-clamp technique on genetically tagged OSNs with defined odorant receptors (ORs), we found that sensory deprivation increased the sensitivity of MOR23 neurons in the closed side, whereas overexposure caused the opposite effect in the open side. We next analyzed the response properties, including rise time, decay time, and adaptation, induced by repeated stimulation in MOR23 and M71 neurons. Even though these two types of neuron showed distinct properties with regard to dynamic range and response kinetics, sensory deprivation significantly slowed down the decay phase of odorant-induced transduction events in both types. Using western blotting and antibody staining, we confirmed the upregulation of several signaling proteins in the closed side as compared with the open side. This study suggests that early experience modulates the functional properties of OSNs, probably by modifying the signal transduction cascade.

Studies of olfactory system neural plasticity: the contribution of the unilateral naris occlusion technique

Unilateral naris occlusion has long been the method of choice for effecting stimulus deprivation in studies of olfactory plasticity. A significant body of literature speaks to the myriad consequences of this manipulation on the ipsilateral olfactory pathway. Early experiments emphasized naris occlusion's deleterious and age-critical effects. More recent studies have focused on life-long vulnerability, particularly on neurogenesis, and compensatory responses to deprivation. Despite the abundance of empirical data, a theoretical framework in which to understand the many sequelae of naris occlusion on olfaction has been elusive. This paper focuses on recent data, new theories, and underappreciated caveats related to the use of this technique in studies of olfactory plasticity.

The effects of unilateral naris occlusion on gene expression profiles in mouse olfactory mucosa

Unilateral naris occlusion has been the method of choice for effecting stimulus deprivation in studies of olfactory plasticity. Early experiments emphasized the deleterious effects of this technique on the developing olfactory system while more recent studies have pointed to several apparently "compensatory" responses. However, the evidence for deprivation-induced compensatory processes in olfaction remains fragmentary. High-throughput methods such as microarray analysis can help fill the deficits in our understanding of naris occlusion as a mode of stimulus deprivation. Here we report for young adult mice the effects of early postnatal naris occlusion on the olfactory mucosal transcriptome using microarray analysis with RT-PCR confirmation. The transcripts of key genes involved in olfactory reception, transduction, and transmission were up-regulated in deprived-side olfactory mucosa, with opposite effects in non-deprived-side mucosa, compared to controls. Results support the hypothesis that odor environment triggers a previously unknown homeostatic control mechanism in olfactory receptor neurons designed to maximize information transfer.

Relationship between olfactory function and olfactory neuronal population in C57BL6 mice injected intraperitoneally with 3-methylindole

OBJECTIVE

It is not known how many olfactory receptor neurons should be intact to maintain olfaction in mouse models treated with 3-methylindole. The aim of this study is to investigate the relationship between a simple olfactory test outcome and the olfactory neuronal population.

STUDY DESIGN

Mouse model.

SETTING

Animal laboratory of the Seoul National University Bundang Hospital.

SUBJECTS AND METHODS

Olfactory dysfunction was induced by intraperitoneal injection of 3-methylindole in 38 six-week-old female C57BL6 mice. Olfactory function was evaluated by a food-finding test following 72-hour starvation. The olfactory neuronal population was quantified by olfactory marker protein (OMP) expression.

RESULTS

The average time for finding food was 8.1 seconds in control mice. It was 13.4, 84.4, 90.1, and 111.4 seconds for mice injected with 100, 200, 300, and 400 μg/g of 3-methylindole, respectively. Harvesting the whole olfactory neuroepithelium, densitometric analysis showed significant decrease of OMP in the 300- and 400-μg/g groups as compared with controls (18.8% and 17.5% of relative density, respectively). In the olfactory bulb, there was a significant decrease of OMP in the 200-, 300-, and 400-μg/g groups (44.5%, 37.0%, and 9.0% of relative density, respectively). The food-finding time had a significant reverse correlation with the relative density of OMP both in the olfactory bulb and in the olfactory neuroepithelium.

CONCLUSION

Our study showed that olfactory impairment was correlated with olfactory neuronal population in mice treated with 3-methylindole. The food-finding test would be a useful tool that could be easily performed without special training in the 3-methylindole-treated C57BL6 anosmic mouse model.

Role of sensory activity on chemospecific populations of interneurons in the adult olfactory bulb

The olfactory bulb (OB) retains a remarkable capacity to renew its interneuronal populations throughout the lifespan of animals. Neuronal precursors giving rise to the bulbar interneurons are generated in the subventricular zone and have to migrate long distances before reaching the OB. In the adult OB these neuronal precursors differentiate into distinct neuronal types, including GABAergic cells located in the granule cell layer and a diverse set of neurons in the glomerular layer comprising GABAergic and dopaminergic interneurons, as well as other neuronal subtypes expressing calretinin and calbindin. While the role of sensory activity in the integration and/or survival of newly generated cells in the olfactory system is well established, very little is known about how odorant-induced activity affects fate specification of newborn cells as well as survival and fate maintenance of preexisting neuronal populations generated in adulthood. The present study demonstrates that sensory deprivation diminishes not only the number of newborn cells in the OB, but also reduces the density of granule and periglomerular cells generated before nostril occlusion. It also shows that sensory activity has an important influence on the development and expression of dopaminergic, but not GABAergic, calretinin or calbindin phenotypes. Our data reveal that odorant-induced activity is important for the survival of both newborn and preexisting OB interneurons generated at adulthood and suggests that these chemospecific populations are differentially affected by sensory deprivation.

How does long-term odor deprivation affect the olfactory capacity of adult mice?

Background

Unilateral naris occlusion (UNO) has been the most common method of effecting stimulus deprivation in studies of olfactory plasticity. However, despite the large corpus on the effects of this manipulation, dating back to the 19th century, little is known about its behavioral sequela. Here we report the results of standard olfactory habituation and discrimination studies on adult mice that had undergone perinatal UNO followed by adult contralateral olfactory bulbectomy (bulb-x).

Methods

The olfactory performance of UNO mice was compared to matched controls that had unilateral bulb-x but open nares. Both habituation and discrimination (operant) experiments employed a protocol in which after successful dishabituation or discrimination to dilute individual odors (A = 0.01% isoamyl acetate; B = 0.01% ethyl butyrate; each v/v in mineral oil), mice were challenged with a single odor versus a mixture comparison (A vs. A + B). In a series of tests the volume portion of Odor B in the mixture was systematically decreased until dishabituation or discrimination thresholds were reached.

Results

For the habituation experiment, UNOs (n = 10) and controls (n = 9) dishabituated to a 10% mixture of Odor B in Odor A after being habituated to A alone, while both groups failed to show differential responding to a 2% mixture of B in A. However, the UNO group's increased investigation durations for the 2% mixture approached significance (p < 0.06). A replication of this study (7 controls & 8 UNOs) confirmed that controls did not differentiate Odor A and a 2% mixture of B in A but UNOs did not (p < 0.05). For the discrimination experiment, 4 UNOs and 4 controls were shaped to dig in one of two containers of sand that contained the S+ odor (Odor B) to obtain sugar pellet rewards. As in the habituation experiment, UNOs displayed greater olfactory capacity than controls on this task. Controls and UNOs had an average mixture discrimination threshold of 1.6% (± 0.4) and 0.22% (± 0.102) respectively, a difference that was statistically significant (p < 0.02).

Conclusions

Adult mice relying on an olfactory system deprived of odor by naris occlusion from near the time of birth display enhanced olfactory capacity compared to control mice. This counterintuitive result suggests that UNO is neither an absolute method of deprivation nor does it diminish olfactory capabilities. Enhanced olfactory capacity, as observed in the current study, that is a consequence of deprivation, is consistent with recent molecular and physiological evidence that stimulus deprivation triggers compensatory processes throughout the olfactory system.

Peripheral olfactory sensitivity in rodents after treatment with docetaxel

OBJECTIVES/HYPOTHESIS

Clinical studies have documented that cytotoxic chemotherapy is often associated with body weight loss and decreased enjoyment of food. Besides taste, olfaction plays a role in food intake. We assessed whether systemic chemotherapeutic cancer treatment compromises olfactory function in rats and mice treated with docetaxel (Taxotere; Sanofi-Aventis, Paris, France).

STUDY DESIGN

Randomized, controlled trials on mice and rats.

METHODS

Male mice received a single and male rats either a single, two, or three docetaxel administrations. Olfactory function was tested by means of electroolfactograms (EOGs) from the chemosensory epithelium of the nasal septum and the endoturbinates. We evaluated and compared the magnitude of EOG responses evoked by different odorants recorded at different time points after treatment.

RESULTS

In both animal species, docetaxel administration reduced body weight gain, thus evidencing the general toxic effect of the drug. In both animal species, the olfactory mucosa remained responsive to stimulation of odorants during the whole course of experiment, but treatment revealed regional differences of docetaxel susceptibility and induced marked transitory electrophysiological changes. In mice and rats a significant transitory decrease in EOG response magnitude occurred after a single administration. Unexpectedly, in rats we also observed an increase of the olfactory response following the second administration of the drug.

CONCLUSIONS

Docetaxel exerts a neurotoxic effect on olfactory epithelia of rodents at doses similar to human doses, thus inducing transitory functional alterations. Although moderate, they are consistent with the hypothesis of a dysfunction of olfactory function. Further experiments are needed to elucidate the origin of the electrophysiological effects and their impact on the olfactory perception.

Activity plays a role in eliminating olfactory sensory neurons expressing multiple odorant receptors in the mouse septal organ

A fundamental belief in the field of olfaction is that each olfactory sensory neuron (OSN) expresses only one odorant receptor (OR) type. Here we report that coexpression of multiple receptors in single neurons does occur at a low frequency. This was tested by double in situ hybridization in the septal organ in which greater than 90% of the sensory neurons express one of nine identified ORs. Notably, the coexpression frequency is nearly ten times higher in newborn than in young adult mice, suggesting a reduction of the sensory neurons with multiple ORs during postnatal development. In addition, such reduction is prevented by four-week sensory deprivation or impaired apoptosis. Furthermore, the high coexpression frequency is restored following four-week naris closure performed in young adult mice. The results indicate that activity induced by sensory inputs plays a role in ensuring the one cell-one receptor rule in a subset of olfactory sensory neurons.