Electro-olfactogram Research Articles

Isolation and Function of Endogenous Zinc Nanoparticles in the Olfactory Epithelium

The olfactory sensory neural system contains the highest levels of zinc found throughout the central nervous system. Zinc is one of many metals known to play a role in multiple facets of biology. Metal nanoparticles such as copper, gold, and zinc have been isolated from human and animal blood. The blood isolated zinc nanoparticles (ZnNPs) were unique in their ability to enhance the electrophysiological olfactory sensory neuron response to odorant by about threefold. Characterization of the physiochemical properties of ZnNPs determined that small (1.2±0.3 nm), non‐oxidized, primarily elemental, zinc nanoparticles were capable of olfaction enhancement. At low concentrations, ZnNPs added to odorant display a dose‐dependent, specific, and reversible effect. In conjunction with the enhancement of olfactory sensory neurons in the isolated rodent olfactory epithelium, cognitive processing of the sensory amplification is demonstrated by a significant increase of canine brain excitation in response to odorants resulted from the addition of ZnNPs. The presence of ZnNPs at the olfactory mucosa level has not been demonstrated. To evaluate the potential biological role ZnNPs have in the initial events of olfaction, the presence of ZnNPs in the olfactory and respiratory epithelia was examined by transmission electron microscopy (TEM). Detailed structures of nanoparticles were characterized by selected area diffraction. The physiological effects of nanoparticles were tested ex vivo by electroolfactogram (EOG). Zinc was identified in both the respiratory and olfactory filtrate samples. TEM demonstrated an organization of high homogeneity, crystalline structure, and within nanoparticle size range. The EOG responses to the epithelia filtrates mixed with odorant evoked higher olfactory responses compared to the response induced by odorant alone. These effects are similar to those produced by the engineered ZnNPs mixed with odorant. Calculation of the concentration of endogenous nanoparticles was performed using the relative EOG peaks as a function of engineered zinc concentration for a calibration curve. The calculated concentrations of ZnNPs in the olfactory epithelium and respiratory epithelium filtrates are 5.3×10−3 ± 5×10−4 nM and 2.7×10−3 ± 5×10−4 nM, respectively. The concentrations of ZnNPs in the olfactory epithelium and respiratory epithelia are 10.3 ± 1.0 nM and 7.9 ± 1.5 nM, respectively. The presence of ZnNPs within the olfactory and respiratory epithelia may have physiological significance. To determine the role of ZnNPs in olfaction, many questions still have to be answered. However, the presence of olfaction enhancing ZnNPs within the olfactory epithelium at the initial site of olfactory signaling suggests a physiological role in the initial events of olfaction at the receptor level. The confirmation of ZnNPs presence by TEM in combination with the electrophysiological enhancement of EOG provide support for this hypothesis.Support or Funding InformationSupported by grant from the National Institute of Standards and Technology: 70NANB14H324This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Neural Circuits and Olfactory Responses to Neurotropic Viruses in Trout

Vertebrate olfactory receptors (OR) are directly exposed to microorganisms, such as viruses, due to their direct contact with the external environment. A previous study showed that nasal delivery of rhabdovirus IHNV (Infectious hematopoietic necrosis virus) in fish activate nasal immune responses marked by an increase of chemokine CCL19 and prostaglandin synthase expression in olfactory epithelia (OE), and infiltration of CD8+ cells in the OE.We hypothesize that nasal immune responses are activated by action potential signals generated by activated olfactory receptor (OR) neurons in the OE and olfactory bulb (OB). Moreover, this neural circuit can be traced from a specific OR cell type (crypt cell) in the nose to a specific site in the olfactory bulb. We tested our hypothesis by measuring olfactory responses to live attenuated IHNV virus by electro‐olfactogram (EOG). We also visualized the IHNV neural circuit after activation of specific OR, and consequent internalization of molecular receptor and IHNV mixed with Alexa dextran 488 3000 MW.Our results showed different EOG olfactory responses to live attenuated IHNV and to the medium where the virus was grown (negative control) in rainbow trout. Olfactory responses followed a dose‐response pattern typical of OR. Cross adaptation studies also showed that live attenuated IHNV activates a set of receptors different from those activated by virus‐free supernatants. Recordings of the OB responses by electroencephalogram are under development. Preliminary tracings show fluorescent oval shaped OR in the apical border of the olfactory lamella (putative crypt cells) that extended to the ventral side of the olfactory bulb. This neural circuit differs from those visualized after exposure of trout OE to the food odorant serine.Combined, our results adds evidence for a new olfactory function in trout, which serves as a first layer of pathogen detection in vertebrates.Support or Funding InformationThis material is based upon work supported by the National Science Foundation under Grant No. 1755348This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

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

ABSTRACTThe 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.

Type 2/Th2-driven inflammation impairs olfactory sensory neurogenesis in mouse chronic rhinosinusitis model.

BackgroundChronic rhinosinusitis (CRS) with nasal polyps (CRSwNP) is a chronic inflammatory disease often accompanied by impairment of sense of smell. This symptom has been somewhat overlooked, and its relationship to inflammatory cytokines, tissue compression, neuronal loss, and neurogenesis is still unclear.MethodsIn order to elucidate potential mechanisms leading to CRS in humans, we have established a type 2/T helper type 2 cell (Th2)‐mediated allergic CRS mouse model, based on house dust mite (HDM) and Staphylococcus aureus enterotoxin B (SEB) sensitization. The inflammatory status of the olfactory epithelium (OE) was assessed using histology, biochemistry, and transcriptomics. The sense of smell was evaluated by studying olfactory behavior and recording electro‐olfactograms (EOGs).ResultsAfter 22 weeks, a typical type 2/Th2‐mediated inflammatory profile was obtained, as demonstrated by increased interleukin (IL)‐4, IL‐5, and IL‐13 in the OE. The number of mast cells and eosinophils was increased, and infiltration of these cells into the olfactory mucosa was also observed. In parallel, transcriptomic and histology analyses indicated a decreased number of immature olfactory neurons, possibly due to decreased renewal. However, the number of mature sensory neurons was not affected and neither the EOG nor olfactory behavior was impaired.ConclusionOur mouse model of CRS displayed an allergic response to HDM + SEB administration, including the type 2/Th2 inflammatory profile characteristic of human eosinophilic CRSwNP. Although the sense of smell did not appear to be altered in these conditions, the data reveal the influence of chronic inflammation on olfactory neurogenesis, suggesting that factors unique to humans may be involved in CRSwNP‐associated anosmia.

Effect of Deepwater Horizon Crude Oil Water Accommodated Fraction on Olfactory Function in the Atlantic Stingray, Hypanus sabinus

The Deepwater Horizon oil spill was the largest accidental marine oil spill in history, releasing nearly 5 million barrels of crude oil. Crude oil causes both lethal and sublethal effects on marine organisms, and sensory systems have the potential to be strongly affected. Marine fishes rely upon the effective functioning of their sensory systems for detection of prey, mates, and predators. However, despite the obvious importance of sensory systems, the impact of crude oil exposure upon sensory function remains largely unexplored. Here we show that olfactory organ responses to amino acids are significantly depressed in oil exposed stingrays. We found that the response magnitude of the electro-olfactogram (EOG) to 1 mM amino acids decreased by an average of 45.8% after 48 h of exposure to an oil concentration replicating that measured in coastal areas. Additionally, in oil exposed individuals, the EOG response onset was significantly slower, and the clearing time was protracted. This study is the first to employ an electrophysiological assay to demonstrate crude oil impairment of the olfactory system in a marine fish. We show that stingrays inhabiting an area impacted by an oil spill experience reduced olfactory function, which would detrimentally impact fitness, could lead to premature death, and could cause additional cascading effects through lower trophic levels.

Olfactory training changes electrophysiological responses at the level of the olfactory epithelium.

Olfactory training (OT) has been shown to increase olfactory performance in healthy subjects and patients with post-traumatic or post-infectious olfactory loss. Morphological correlates such as olfactory bulb volume increase and gray matter changes suggest central changes in olfactory brain areas following olfactory exposure. Some evidence from animal studies indicates peripheral changes upon OT whereas no such data exist in humans. This study explores the question whether changes in olfaction following OT are associated with alterations of the electro-olfactogram (EOG) derived from the olfactory epithelium. We compared electrophysiological EOG responses to a pleasant, rose-like odor (phenylethyl alcohol, PEA) and to an unpleasant odor (rotten eggs, H2S) in patients and controls. EOG were recorded in smell impaired patients before and after OT for a period of 4-6 months. EOG recordings following PEA and H2S stimulation were significantly more often obtained in controls than in patients. OT was associated with a significantly higher number of EOG recordings. OT is associated with an increase in EOG responses implicating stimulus-induced plasticity to start at the level of the olfactory epithelium.

Effects Of Nitrite In Goldfish (Carassius Auratus) Olfactory Function

In the last decade, freshwater ecosystems have experienced a dramatic increase of nitrogenous compounds as a result of farming and industrial activity. Aquatic ammonium (NH4+) is naturally transformed to nitrite (NO−2) by nitrifying bacteria. Nitrite is toxic to vertebrates and is known to cause disruption of physiological processes such as ion homeostasis, respiration, and cardiovascular activity. However, little is known about the effects of NO−2 on the olfactory function in teleost, which is critical for the survival of the species. This study aims to determine the physiological effects of sub‐lethal concentrations of nitrite in the olfactory function of goldfish (Carassius auratus). Thus, goldfish were exposed to 6 different NO−2 concentrations (1000 mM, 500 mM, 300 mM, 100 mM, 10 mM and 0 mM.) for 72 h. At the end of the experiment, blood, brain and nose samples were obtained for histological analysis. Additionally, in order to determine the reduction of olfactory function, olfactory responses to several food odorants were recorded by electro‐olfactogram (EOG) after exposure of the nose to several NO−2 concentrations. No mortality was observed in any treatment. Goldfish exposed to the two highest concentrations of NO−2 exhibited metahemoglobemia (darkening in the blood due to irreversible oxidation of hemoglobin). Histological analysis of the olfactory epithelium showed an increase in mucus production and mucus cells. Moreover, the olfactory epithelia of these fish showed hyperplasia of the olfactory sensory neurons. Hyperplasia of epithelium cells occurred in goldfish exposed to high NO−2 concentrations. Preliminary electrophysiology recordings showed a decrease in the olfactory function due to NO−2 treatments. These results suggest that sub‐lethal concentrations of nitrite are neurotoxic and alter the olfactory function in fish.Support or Funding InformationResearch support: Texas State University start‐up funding to Dr. Mar Huertas.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

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.

The mechanisms underlying olfactory deficits in apolipoprotein E–deficient mice: focus on olfactory epithelium and olfactory bulb

Apolipoprotein E (ApoE) is highly expressed in the central nervous system including the olfactory epithelium (OE) and olfactory bulb (OB). ApoE induction is beneficial for Alzheimer's disease (AD) treatment, whereas ApoE deficiency results in impaired olfaction, but the timing and underlying molecular and cellular mechanisms of these effects remain unclear. Uncovering the mechanisms underlying olfactory dysfunction in ApoE-deficient mice might provide a potential avenue for the early diagnosis of AD. We used an ApoE knockout (ApoE−/−) mouse model and a cookie-finding test to reveal an olfactory deficit in 3- to 5-month-old, but not 1- to 2-month-old, ApoE−/− mice. Electrophysiological experiments indicated a significant decline in the electroolfactogram (EOG) amplitude, which was associated with an increase in rise time in ApoE−/− mice. Knockout mice also exhibited compromised olfactory adaptation, as well as a reduced number of mature olfactory sensory neurons in the OE. Local field potential recording in the OB showed that gamma oscillation power was enhanced, which might be attributed to an increase in GABAergic inhibition mediated by parvalbumin-expressing (PV) interneurons. This study demonstrates the critical involvement of ApoE in olfactory information processing in the OE and OB. ApoE deficiency results in olfaction deficits in mice as young as 3 months old, which has implications for AD pathogenesis.

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.

Indistinguishable odour enantiomers: Differences between peripheral and central-nervous electrophysiological responses

The ability of humans to discriminate enantiomeric odour pairs is substance –specific. Current literature suggests that psychophysical discrimination of odour enantiomers mainly depends on the peripheral processing at the level of the olfactory sensory neurons (OSN). To study the influence of central processing in discrimination, we investigated differences in the electrophysiological responses to psychophysically indistinguishable (+)- and (−)- rose oxide enantiomers at peripheral and central-nervous levels in humans. We recorded the electro-olfactogram (EOG) from the olfactory epithelium and the EEG-derived olfactory event-related potentials (OERP). Results from a psychophysical three alternative forced choice test indicated indistinguishability of the two odour enantiomers. In a total of 19 young participants EOG could be recorded in 74 and OERP in 95% of subjects. Significantly different EOG amplitudes and latencies were recorded in response to the 2 stimuli. However, no such differences in amplitude or latency emerged for the OERP. In conclusion, although the pair of enantiomer could be discriminated at a peripheral level this did not lead to a central-nervous/cognitive differentiation of the two stimuli.

Lack of TRPM5-Expressing Microvillous Cells in Mouse Main Olfactory Epithelium Leads to Impaired Odor-Evoked Responses and Olfactory-Guided Behavior in a Challenging Chemical Environment.

The mammalian main olfactory epithelium (MOE) modifies its activities in response to changes in the chemical environment. This process is essential for maintaining the functions of the olfactory system and the upper airway. However, mechanisms involved in this functional maintenance, especially those occurring via paracrine regulatory pathways within the multicellular MOE, are poorly understood. Previously, a population of non-neuronal, transient receptor potential M5-expressing microvillous cells (TRPM5-MCs) was identified in the MOE, and the initial characterization of these cells showed that they are cholinergic and responsive to various xenobiotics including odorants at high concentrations. Here, we investigated the role of TRPM5-MCs in maintaining olfactory function using transcription factor Skn-1a knockout (Skn-1a-/-) mice, which lack TRPM5-MCs in the MOE. Under our standard housing conditions, Skn-1a-/- mice do not differ significantly from control mice in odor-evoked electro-olfactogram (EOG) responses and olfactory-guided behaviors, including finding buried food and preference reactions to socially and sexually relevant odors. However, after a 2-wk exposure to high-concentration odor chemicals and chitin powder, Skn-1a-/- mice exhibited a significant reduction in their odor and pheromone-evoked EOG responses. Consequently, their olfactory-guided behaviors were impaired compared with vehicle-exposed Skn-1a-/- mice. Conversely, the chemical exposure did not induce significant changes in the EOG responses and olfactory behaviors of control mice. Therefore, our physiological and behavioral results indicate that TRPM5-MCs play a protective role in maintaining the olfactory function of the MOE.

Simultaneous Loss of NCKX4 and CNG Channel Desensitization Impairs Olfactory Sensitivity

In vertebrate olfactory sensory neurons (OSNs), Ca²⁺ plays key roles in both mediating and regulating the olfactory response. Ca²⁺ enters OSN cilia during the response through the olfactory cyclic nucleotide-gated (CNG) channel and stimulates a depolarizing chloride current by opening the olfactory Ca²⁺-activated chloride channel to amplify the response. Ca²⁺ also exerts negative regulation on the olfactory transduction cascade, through mechanisms that include reducing the CNG current by desensitizing the CNG channel via Ca²⁺/calmodulin (CaM), to reduce the response. Ca²⁺ is removed from the cilia primarily by the K⁺-dependent Na⁺/Ca²⁺ exchanger 4 (NCKX4), and the removal of Ca²⁺ leads to closure of the chloride channel and response termination. In this study, we investigate how two mechanisms conventionally considered negative regulatory mechanisms of olfactory transduction, Ca²⁺ removal by NCKX4, and desensitization of the CNG channel by Ca²⁺/CaM, interact to regulate the olfactory response. We performed electro-olfactogram (EOG) recordings on the double-mutant mice, <i>NCKX4</i>^−/−<i>;CNGB1</i>^Δ<i>CaM</i>, which are simultaneously lacking NCKX4 (<i>NCKX4</i>^−/−) and Ca²⁺/CaM-mediated CNG channel desensitization (<i>CNGB1</i>^Δ<i>CaM</i>). Despite exhibiting alterations in various response attributes, including termination kinetics and adaption properties, OSNs in either <i>NCKX4</i>^−/− mice or <i>CNGB1</i>^Δ<i>CaM</i> mice show normal resting sensitivity, as determined by their unchanged EOG response amplitude. We found that OSNs in <i>NCKX4</i>^−/−<i>;CNGB1</i>^Δ<i>CaM</i> mice displayed markedly reduced EOG amplitude accompanied by alterations in other response attributes. This study suggests that what are conventionally considered negative regulatory mechanisms of olfactory transduction also play a role in setting the resting sensitivity in OSNs. <b>SIGNIFICANCE STATEMENT</b> Sensory receptor cells maintain high sensitivity at rest. Although the mechanisms responsible for setting the resting sensitivity of sensory receptor cells are not well understood, it has generally been assumed that the sensitivity is set primarily by how effectively the components in the activation cascade of sensory transduction can be stimulated. Our findings in mouse olfactory sensory neurons suggest that mechanisms that are primarily responsible for terminating the olfactory response are also critical for proper resting sensitivity.

Reproductive and metabolic state differences in olfactory responses to amino acids in a mouth brooding African cichlid fish.

Olfaction mediates many crucial life-history behaviors such as prey detection, predator avoidance, migration and reproduction. Olfactory function can also be modulated by an animal's internal physiological and metabolic states. While this is relatively well studied in mammals, little is known about how internal state impacts olfaction in fishes, the largest and most diverse group of vertebrates. Here we apply electro-olfactograms (EOGs) in the African cichlid fish Astatotilapia burtoni to test the hypothesis that olfactory responses to food-related cues (i.e. l-amino acids; alanine and arginine) vary with metabolic, social and reproductive state. Dominant males (reproductively active, reduced feeding) had greater EOG magnitudes in response to amino acids at the same tested concentration than subordinate males (reproductively suppressed, greater feeding and growth rates). Mouth brooding females, which are in a period of starvation while they brood fry in their mouths, had greater EOG magnitudes in response to amino acids at the same tested concentration than both recovering and gravid females that are feeding. Discriminant function analysis on EOG magnitudes also grouped the male (subordinate) and female (recovering, gravid) phenotypes with higher food intake together and distinguished them from brooding females and dominant males. The slope of the initial negative phase of the EOG also showed intra-sexual differences in both sexes. Our results demonstrate that the relationship between olfaction and metabolic state observed in other taxa is conserved to fishes. For the first time, we provide evidence for intra-sexual plasticity in the olfactory response to amino acids that is influenced by fish reproductive, social and metabolic state.

Olfactory sensitivity of the marine flatfish Solea senegalensis to conspecific body fluids.

Chemical communication is better understood in freshwater fish than marine fish. The Senegalese sole (Solea senegalensis) is a marine flatfish wherein one of the problems in aquaculture is the poor reproductive performance of hatchery-bred males. Is chemical communication involved in the reproduction of this species? Urine, intestinal fluid and mucus samples were taken from adult fish (wild-caught and hatchery-bred) over the spawning season (March-May), and assessed for olfactory potency using the electro-olfactogram (EOG). The effect of stimulation of the olfactory system with adult female urine on circulating luteinizing hormone (LH) levels was also tested in males. Intestinal fluid and urine were potent olfactory stimuli for both juvenile and adult conspecifics, evoking large-amplitude, concentration-dependent EOG responses, with thresholds of detection at approximately 1:106 However, the amplitude of the response to urine depended on the sex and state of maturity of both the donor and the receiver. Most olfactory activity could be extracted by C18 solid-phase cartridges. Urine from mature females evoked a slight, but significant, increase in circulating LH levels in mature males 30 min after exposure. Furthermore, the olfactory potency of urine differed between wild-caught and hatchery-bred fish; however, contrary to expectations, urine from wild-caught females was less potent than that from hatchery-bred females. Taken together, these results strongly suggest that faeces- and urine-released odorants are involved in reproduction in the Senegalese sole, and establish a basis for further investigation into pheromonal communication in marine teleosts.

Simultaneous Loss of NCKX4 and CNG Channel Desensitization Impairs Olfactory Sensitivity.

Sensory receptor cells maintain high sensitivity at rest. Although the mechanisms responsible for setting the resting sensitivity of sensory receptor cells are not well understood, it has generally been assumed that the sensitivity is set primarily by how effectively the components in the activation cascade of sensory transduction can be stimulated. Our findings in mouse olfactory sensory neurons suggest that mechanisms that are primarily responsible for terminating the olfactory response are also critical for proper resting sensitivity.

Role of the type I tumor necrosis factor receptor in inflammation-associated olfactory dysfunction.

To understand mechanisms of human olfactory dysfunction in chronic rhinosinusitis, an inducible olfactory inflammation (IOI) model has been utilized to chronically express inflammatory cytokines locally, resulting in neuronal loss, diminished odorant responses, and repressed olfactory regeneration. Knockout of the minor tumor necrosis factor α receptor 2 (TNFR2) was previously shown to partially rescue these olfactory changes. The purpose of current study was to investigate the role of the major TNF receptor, TNFR1, in chronic olfactory inflammation. Two experimental groups of mice were studied: TNFR1 knockout in IOI background and TNFR1 knockout with allergen-induced inflammation. Olfactory function was assayed by electro-olfactogram (EOG), and olfactory tissue was processed for histology and immunohistochemical staining. TNF-α was dramatically induced in IOI-TNFR1 knockout mice, but the olfactory epithelium did not show inflammation. EOG responses were normal after either 2 or 8 weeks of TNF-α expression. Ovalbumin-sensitized TNFR1 knockout mice developed markedly diminished eosinophilic inflammatory infiltration. Genetic deletion of TNFR1 completely blocks TNF-α-induced inflammation and reduces allergen-induced inflammation. Preserved EOG responses suggest a TNFR1-dependent mechanism of TNF-α-induced olfactory neuron dysfunction.

Effects of salinity on olfactory toxicity and behavioral responses of juvenile salmonids from copper

Dissolved copper is one of the more pervasive and toxic constituents of stormwater runoff and is commonly found in stream, estuary, and coastal marine habitats of juvenile salmon. While stormwater runoff does not usually carry copper concentrations high enough to result in acute lethality, they are of concern because sublethal concentrations of copper exposure have been shown to both impair olfactory function and alter behavior in various species in freshwater. To compare these results to other environments that salmon are likely to encounter, experiments were conducted to evaluate the effects of salinity on the impairment of olfactory function and avoidance of copper. Copper concentrations well within the range of those found in urban watersheds, have been shown to diminish or eliminate the olfactory response to the amino acid, l-serine in freshwater using electro-olfactogram (EOG) techniques. The olfactory responses of both freshwater-phase and seawater-phase coho and seawater-phase Chinook salmon, were tested in freshwater or seawater, depending on phase, and freshwater-phase coho at an intermediate salinity of 10‰. Both 10‰ salinity and full strength seawater protected against the effects of 50μg copper/L. In addition to impairing olfactory response, copper has also been shown to alter salmon behavior by causing an avoidance response. To determine whether copper will cause avoidance behavior at different salinities, experiments were conducted using a multi-chambered experimental tank. The circular tank was divided into six segments by water currents so that copper could be contained within one segment yet fish could move freely between them. The presence of individual fish in each of the segments was counted before and after introduction of dissolved copper (<20μg/L) to one of the segments in both freshwater and seawater. To address whether use of preferred habitat is altered by the presence of copper, experiments were also conducted with a submerged structural element. The presence of sub-lethal levels of dissolved copper altered the behavior of juvenile Chinook salmon by inducing an avoidance response in both freshwater and seawater. While increased salinity is protective against loss of olfactory function from dissolved copper, avoidance could potentially affect behaviors beneficial to growth, survival and reproductive success.

Heat Stress Effects on the Kinetics of Olfactory Responses

Temperature is one of the most challenging stressors, affecting human and animal health. Extreme heat stress can damage the olfactory sensory neurons. In this work, we examined the effects of elevated temperature on the kinetic properties of rat olfactory sensory receptors in vitro by the recording of electroolfactogram (EOG) of the olfactory epithelium (OE) removed from the animals before and after the heat stress at 45 ºC for 25 minutes. EOG were evoked by an odorant mixture of ethyl butyrate, eugenol, and (+) and (−) carvone. The biochemical properties of the animal blood related to heat stress were characterized. Exposure of rats to high temperature resulted in a significant increase of body temperature. The mean body temperatures of rats before and immediately after heat stress were 36.7±0.07 ºC and 40.3 ±0.17 ºC, respectively. The kinetic properties of electrical responses to odorant were significantly changed in animals subjected to heat stress. The mean half‐rise (tr)and –decay times (td) of control OE were 328±11 and 660±20 ms, respectively. The rise and decay of EOG peak amplitude after heat stress were much faster, being reduced by 175 %, and 72 %, respectively. Heat stress did not result in a change of IL‐1β; IL‐6, TNF‐α, INF‐γ cytokines level. However, a significant increase of the IL‐10 level was found in rats exposed to heat. The level of lipopolysaccharides (LPS) was significantly increased in the serum of heat‐stressed animals compared to control animals. The concentration of free vesicles in animal blood increased after exposure to heat stress from (1.4±0.2)×106 to (3.8±0.3)×106 vesicles μl−1, indicating a faster conversion of erythrocytes into echinocytes. The kinetic properties found in this work are consistent with the kinetics of cultured mouse olfactory neurons 1. The effect of elevated temperature on the kinetic properties of olfactory receptors is also consistent with those seen in other neurons2–4. Detrimental effects of elevated temperatures on olfaction are shown in other animals5. The change in kinetic properties of olfactory receptors as a result of elevated temperature is consistent with those in other neurons2–4. Cytokines were found to cause neurodegeneration of the olfactory bulb6. We conclude that heat stress causes a strong irreversible modulation of olfactory responses.Support or Funding InformationSupported by grant from the National Institute of Standards and Technology: 70NANB14H324

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.