A method for studying responses and habituation to odors in rats

Strawberry Additive Increases Nicotine Vapor Sampling and Systemic Exposure But Does Not Enhance Pavlovian-Based Nicotine Reward in Mice

Nicotine is an addictive drug whose popularity has recently increased, particularly among adolescents, because of the availability of electronic nicotine devices (i.e., "vaping") and nicotine e-liquids containing additives with rich chemosensory properties. Some efforts to understand the role of these additives in nicotine reward suggest that they increase nicotine reward and reinforcement, but the sensory contributions of additives, especially in their vapor forms, are largely untested. Here, to better understand how a fruit-flavored (i.e., strawberry) additive influences nicotine reward and aversion, we used a conditioned place preference (CPP) procedure in which nicotine and a strawberry additive were delivered as a vapor to male and female adolescent mice. We found that nicotine vapor alone can lead to a dose-dependent CPP when using a biased design. The strawberry additive did not produce CPP on its own, and we did not observe an effect of the strawberry additive on nicotine vapor-induced reward. Nevertheless, mice exposed to nicotine plus strawberry additive vapor had higher plasma cotinine concentrations, which did not appear to reflect altered nicotine metabolism. Instead, by directly measuring vapor sampling through respiration monitoring, we uncovered an increase in the amount of sniffing toward strawberry-containing nicotine vapor compared with nicotine vapor alone. Together these data indicate that chemosensory-rich e-liquid additives may enhance the perceived sensory profile of nicotine vapors rather than the reward value per se, which leads to overall increased nicotine exposure.

Odor exploration behavior of the domestic pig (Sus scrofa) as indicator of enriching properties of odors

Introduction and aim

Although the sense of smell in pigs is widely recognized as being highly developed, surprisingly little is known about their sensory ability. This study aimed to (a) identify which non-social odors pigs were able to detect and distinguish between, (b) investigate the types of behavior expressed when exploring odors and, (c) compare pigs' responses to the different odors to evaluate their interest in the odors.

Methods

Growing pigs (N = 192) of crossbred commercial breeds were enrolled in the experiment (32-110 days of age, weighing 64.9 ± 10.1kg). Littermate pairs of opposite sex were tested in test pens with two odor insertion points in the pen wall, 55 cm apart. All pigs were habituated to the test pens and experimenters. Twelve odors were tested (eight essential oils and four synthetic perfumes) in groups of three odors, with each pig pair tested once with one set of three odors (all possible orders of the three odors were tested on 24 pairs in total), always against a non-odor control (demineralized water). In a test, each of the three odors were presented during three trials in a row (a total of 9 trials per test; trial duration: 1 min; inter-trial breaks: 2 min; total test duration: 25 min). Response variables included: duration of sniffing, feeding-related behavior (licking, biting and rooting), agonistic behavior (biting, displacement and pushing) and no approach of the odor or control, recorded throughout each 1-min odor presentation.

Results

All pigs sniffed an odor less when repeatedly presented (LMM: all odors P < 0.05), and significantly longer at the subsequent presentation of a new odor [LMM (3rd vs. 1st presentations): P < 0.001]. Specific odor and odor type (essential oil vs. synthetic perfume) had no significant effect on sniffing duration. Overall, feeding-related behavior and agonistic behavior were expressed significantly more when pigs explored the odor compared with the control insertion point (Paired t-tests: P < 0.001), and specific odor only affected the expression of feeding-related behavior.

Conclusion

Collectively, pigs express sniffing, agonistic, and feeding-related behavior when exploring odors, which suggests that pigs perceive odors of non-social origin as a resource. Odors may thus constitute relevant enrichment material for pigs.

Horse odor exploration behavior is influenced by pregnancy and age

In spite of a highly developed olfactory apparatus of horses, implying a high adaptive value, research on equine olfaction is sparse. Our limited knowledge on equine olfaction poses a risk that horse behavior does not match human expectations, as horses might react fearful when exposed to certain odors, which humans do not consider as frightening. The benefit of acquiring more knowledge of equine olfaction is therefore twofold; (1) it can aid the understanding of horse behavior and hence reduce the risk of dangerous situations, and (2) there may be unexplored potential of using odors in several practical situations where humans interact with horses. This study investigated behavior and olfactory sensitivity of 35 Icelandic horses who were presented with four odors: peppermint, orange, lavender and cedar wood in a Habituation/Dishabituation paradigm. The response variables were sniffing duration per presentation and behavioral reaction (licking, biting, snorting, and backing), and data were analyzed for potential effects of age, sex and pregnancy. Results showed that habituation occurred between successive odor presentations (1st vs. 2nd and 2nd vs. 3rd presentations: P &lt; 0.001), and dishabituation occurred when a new odor was presented (1st vs. 3rd presentations: P &lt; 0.001). Horses were thus able to detect and distinguish between all four odors, but expressed significantly longer sniffing duration when exposed to peppermint (peppermint vs. orange, lavender and cedar wood: P &lt; 0.001). More horses expressed licking when presented to peppermint compared to cedar wood and lavender (P = 0.0068). Pregnant mares sniffed odors less than non-pregnant mares (P = 0.030), young horses (age 0-5 years) sniffed cedar wood for longer than old horses (P = 0.030), whereas sex had no effect (P &gt; 0.050). The results show that horses’ odor exploration behavior and interest in odors varies with age and pregnancy and that horses naïve to the taste of a substrate, may be able to link smell with taste, which has not been described before. These results can aid our understanding of horses’ behavioral reactions to odors, and in the future, it may be possible to relate these to the physiology and health of horses.

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

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

Adult Neural Stem Cell Migration Is Impaired in a Mouse Model of Alzheimer's Disease

Neurogenesis in the adult brain takes place in two neurogenic niches: the ventricular-subventricular zone (V-SVZ) and the subgranular zone. After differentiation, neural precursor cells (neuroblasts) have to move to an adequate position, a process known as neuronal migration. Some studies show that in Alzheimer’s disease, the adult neurogenesis is impaired. Our main aim was to investigate some proteins involved both in the physiopathology of Alzheimer’s disease and in the neuronal migration process using the APP/PS1 Alzheimer’s mouse model. Progenitor migrating cells are accumulated in the V-SVZ of the APP/PS1 mice. Furthermore, we find an increase of Cdh1 levels and a decrease of Cdk5/p35 and cyclin B1, indicating that these cells have an alteration of the cell cycle, which triggers a senescence state. We find less cells in the rostral migratory stream and less mature neurons in the olfactory bulbs from APP/PS1 mice, leading to an impaired odour discriminatory ability compared with WT mice. Alzheimer’s disease mice present a deficit in cell migration from V-SVZ due to a senescent phenotype. Therefore, these results can contribute to a new approach of Alzheimer’s based on senolytic compounds or pro-neurogenic factors.

Olfactory dysfunction in aging and neurodegenerative diseases

Alterations in olfactory functions are proposed to be early biomarkers for neurodegeneration. Many neurodegenerative diseases are age-related, including two of the most common, Parkinson's disease (PD) and Alzheimer's disease (AD). The establishment of biomarkers that promote early risk identification is critical for the implementation of early treatment to postpone or avert pathological development. Olfactory dysfunction (OD) is seen in 90% of early-stage PD patients and 85% of patients with early-stage AD, which makes it an attractive biomarker for early diagnosis of these diseases. Here, we systematically review widely applied smelling tests available for humans as well as olfaction assessments performed in some animal models and the relationships between OD and normal aging, PD, AD, and other conditions. The utility of OD as a biomarker for neurodegenerative disease diagnosis and future research directions are also discussed.

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

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

Targeting Alzheimer's disease with multimodal polypeptide-based nanoconjugates

Alzheimer's disease (AD), the most prevalent form of dementia, remains incurable mainly due to our failings in the search for effective pharmacological strategies. Here, we describe the development of targeted multimodal polypeptide-based nanoconjugates as potential AD treatments. Treatment with polypeptide nanoconjugates bearing propargylamine moieties and bisdemethoxycurcumin or genistein afforded neuroprotection and displayed neurotrophic effects, as evidenced by an increase in dendritic density of pyramidal neurons in organotypic hippocampal culture. The additional conjugation of the Angiopep-2 targeting moiety enhanced nanoconjugate passage through the blood-brain barrier and modulated brain distribution with nanoconjugate accumulation in neurogenic areas, including the olfactory bulb. Nanoconjugate treatment effectively reduced neurotoxic β amyloid aggregate levels and rescued impairments to olfactory memory and object recognition in APP/PS1 transgenic AD model mice. Overall, this study provides a description of a targeted multimodal polyglutamate-based nanoconjugate with neuroprotective and neurotrophic potential for AD treatment.

Inhibition of focal adhesion kinase increases adult olfactory stem cell self-renewal and neuroregeneration through ciliary neurotrophic factor

Constant neuroregeneration in adult olfactory epithelium maintains olfactory function by basal stem cell proliferation and differentiation to replace lost olfactory sensory neurons (OSNs). Understanding the mechanisms regulating this process could reveal potential therapeutic targets for stimulating adult olfactory neurogenesis under pathological conditions and aging. Ciliary neurotrophic factor (CNTF) in astrocytes promotes forebrain neurogenesis but its function in the olfactory system is unknown. Here, we show in mouse olfactory epithelium that CNTF is expressed in horizontal basal cells, olfactory ensheathing cells (OECs) and a small subpopulation of OSNs. CNTF receptor alpha was expressed in Mash1-positive globose basal cells (GBCs) and OECs. Thus, CNTF may affect GBCs in a paracrine manner. CNTF−/− mice did not display altered GBC proliferation or olfactory function, suggesting that CNTF is not involved in basal olfactory renewal or that they developed compensatory mechanisms. Therefore, we tested the effect of increased CNTF in wild type mice. Intranasal instillation of a focal adhesion kinase (FAK) inhibitor, FAK14, upregulated CNTF expression. FAK14 also promoted GBC proliferation, neuronal differentiation and basal stem cell self-renewal but had no effective in CNTF−/− mice, suggesting that FAK inhibition promotes olfactory neuroregeneration through CNTF, making them potential targets to treat sensorineural anosmia due to OSN loss.

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

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

Modulation of olfactory-driven behavior by metabolic signals: role of the piriform cortex

Olfaction is one of the major sensory modalities that regulates food consumption and is in turn regulated by the feeding state. Given that the olfactory bulb has been shown to be a metabolic sensor, we explored whether the anterior piriform cortex (aPCtx)-a higher olfactory cortical processing area-had the same capacity. Using immunocytochemical approaches, we report the localization of Kv1.3 channel, glucose transporter type 4, and the insulin receptor in the lateral olfactory tract and Layers II and III of the aPCtx. In current-clamped superficial pyramidal (SP) cells, we report the presence of two populations of SP cells: glucose responsive and non-glucose responsive. Using varied glucose concentrations and a glycolysis inhibitor, we found that insulin modulation of the instantaneous and spike firing frequency are both glucose dependent and require glucose metabolism. Using a plethysmograph to record sniffing frequency, rats microinjected with insulin failed to discriminate ratiometric enantiomers; considered a difficult task. Microinjection of glucose prevented discrimination of odorants of different chain-lengths, whereas injection of margatoxin increased the rate of habituation to repeated odor stimulation and enhanced discrimination. These data suggest that metabolic signaling pathways that are present in the aPCtx are capable of neuronal modulation and changing complex olfactory behaviors in higher olfactory centers.

Clearing Amyloid-β through PPARγ/ApoE Activation by Genistein is a Treatment of Experimental Alzheimer's Disease

Amyloid-β (Aβ) clearance from brain, which is decreased in Alzheimer's disease, is facilitated by apolipoprotein E (ApoE). ApoE is upregulated by activation of the retinoid X receptor moiety of the RXR/PPARγ dimeric receptor. Genistein, a non-toxic, well-tested, and inexpensive drug activates the other moiety of the receptor PPARγ. Treatment of an Alzheimer's disease mouse model with genistein results in a remarkable and rapid improvement in various parameters of cognition, such as hippocampal learning, recognition memory, implicit memory, and odor discrimination. This is associated with a lowering of Aβ levels in brain, in the number and the area of amyloid plaques (confirmed in vivo by positron emission tomography) as well as in microglial reactivity. Finally, incubation of primary astrocytes with genistein results in a PPARγ-mediated increased release of ApoE. Our results strongly suggest that controlled clinical trials should be performed to test the effect of genistein as treatment of human Alzheimer's disease.

The Habituation/Cross-Habituation Test Revisited: Guidance from Sniffing and Video Tracking

The habituation/cross-habituation test (HaXha) is a spontaneous odor discrimination task that has been used for many decades to evaluate olfactory function in animals. Animals are presented repeatedly with the same odorant after which a new odorant is introduced. The time the animal explores the odor object is measured. An animal is considered to cross-habituate during the novel stimulus trial when the exploration time is higher than the prior trial and indicates the degree of olfactory patency. On the other hand, habituation across the repeated trials involves decreased exploration time and is related to memory patency, especially at long intervals. Classically exploration is timed using a stopwatch when the animal is within 2 cm of the object and aimed toward it. These criteria are intuitive, but it is unclear how they relate to olfactory exploration, that is, sniffing. We used video tracking combined with plethysmography to improve accuracy, avoid observer bias, and propose more robust criteria for exploratory scoring when sniff measures are not available. We also demonstrate that sniff rate combined with proximity is the most direct measure of odorant exploration and provide a robust and sensitive criterion.

An endocannabinoid system is present in the mouse olfactory epithelium but does not modulate olfaction

Endocannabinoids modulate a diverse array of functions including progenitor cell proliferation in the central nervous system, and odorant detection and food intake in the mammalian central olfactory system and larval Xenopus laevis peripheral olfactory system. However, the presence and role of endocannabinoids in the peripheral olfactory epithelium have not been examined in mammals. We found the presence of cannabinoid type 1 (CB1) and cannabinoid type 2 (CB2) receptor protein and mRNA in the olfactory epithelium. Using either immunohistochemistry or calcium imaging we localized CB1 receptors on neurons, glia-like sustentacular cells, microvillous cells and progenitor-like basal cells. To examine the role of endocannabinoids, CB1- and CB2- receptor-deficient (CB1(-/-)/CB2(-/-)) mice were used. The endocannabinoid 2-arachidonylglycerol (2-AG) was present at high levels in both C57BL/6 wildtype and CB1(-/-)/CB2(-/-) mice. 2-AG synthetic and degradative enzymes are expressed in wildtype mice. A small but significant decrease in basal cell and olfactory sensory neuron numbers was observed in CB1(-/-)/CB2(-/-) mice compared to wildtype mice. The decrease in olfactory sensory neurons did not translate to impairment in olfactory-mediated behaviors assessed by the buried food test and habituation/dishabituation test. Collectively, these data indicate the presence of an endocannabinoid system in the mouse olfactory epithelium. However, unlike in tadpoles, endocannabinoids do not modulate olfaction. Further investigation on the role of endocannabinoids in progenitor cell function in the olfactory epithelium is warranted.

Methods to measure olfactory behavior in mice

Mice rely on the sense of olfaction to detect food sources, recognize social and mating partners, and avoid predators. Many behaviors of mice, including learning and memory, social interaction, fear, and anxiety are closely associated with their function of olfaction, and behavior tasks designed to evaluate those brain functions may use odors as cues. Accurate assessment of olfaction is not only essential for the study of olfactory system but also critical for proper interpretation of various mouse behaviors, especially learning and memory, emotionality and affect, and sociality. Here we describe a series of behavior experiments that offer multidimensional and quantitative assessments for mouse olfactory function, including olfactory habituation, discrimination, odor preference, odor detection sensitivity, and olfactory memory, with respect to both social and nonsocial odors.

Galanin promotes neuronal differentiation from neural progenitor cells in vitro and contributes to the generation of new olfactory neurons in the adult mouse brain

Galanin is a pleiotropic neuropeptide widely expressed in the nervous system. It plays a role in many diverse physiological functions - including nociception, cognition and metabolism regulation - and acts as neurotrophic/neuroprotective factor for several neuronal populations. In this article we sought to determine the role of galanin on neural stem cell function and its contribution to the plasticity of the nervous system. Here we show that galanin and its receptors are expressed in neural progenitor cells (NPCs) isolated from the developing striatum. Stimulation with galanin results in upregulation of Bcl-Xl, Bcl-2, Mash-1 and Olig-2 that are part of well known pro-survival/pro-neuronal signalling pathways. Accordingly, treatment with galanin increases the number of neurons upon differentiation from these progenitors. We then show that these effects are recapitulated in NPCs isolated from the adult subventricular zone (SVZ), where galanin increases the total number of neurons and the number of newly-generated neurons upon differentiation in vitro. The significance of these findings is highlighted in the adult brain where loss of galanin leads to a marked decrease in the rate of adult SVZ neurogenesis and a reduction in the number of newly generated cells in the olfactory bulb. Interestingly, Gal-KO mice display normal performances in simple tasks of olfactory detection and discrimination, which points to the existence of a certain degree of redundancy in SVZ neurogenesis. Our findings establish the role of galanin as a modulator of neural stem cell function and support the importance of galanin for brain plasticity and repair.

Towards improved animal models for evaluating social cognition and its disruption in schizophrenia: the CNTRICS initiative

Social cognition refers to processes used to monitor and interpret social signals from others, to decipher their state of mind, emotional status and intentions, and select appropriate social behaviour. Social cognition is sophisticated in humans, being embedded with verbal language and enacted in a complex cultural environment. Its disruption characterises the entire course of schizophrenia and is correlated with poor functional outcome. Further, deficits in social cognition are related to impairment in other cognitive domains, positive symptoms (paranoia and delusions) and negative symptoms (social withdrawal and reduced motivation). In light of the significance and inadequate management of social cognition deficits, there is a need for translatable experimental procedures for their study, and identification of effective pharmacotherapy. No single paradigm captures the multi-dimensional nature of social cognition, and procedures for assessing ability to infer mental states are not well-developed for experimental therapeutic settings. Accordingly, a recent CNTRICS meeting prioritised procedures for measuring a specific construct: "acquisition and recognition of affective (emotional) states", coupled to individual recognition. Two complementary paradigms for refinement were identified: social recognition/preference in rodents, and visual tracking of social scenes in non-human primates (NHPs). Social recognition is disrupted in genetic, developmental or pharmacological disease models for schizophrenia, and performance in both procedures is improved by the neuropeptide oxytocin. The present article surveys a broad range of procedures for studying social cognition in rodents and NHPs, discusses advantages and drawbacks, and focuses on development of social recognition/preference and gaze-following paradigms for improved study of social cognition deficits in schizophrenia and their potential treatment.

Duration and specificity of olfactory nonassociative memory

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

Immunization targeting a minor plaque constituent clears β-amyloid and rescues behavioral deficits in an Alzheimer's disease mouse model

Although anti-human β-amyloid (Aβ) immunotherapy clears brain β-amyloid plaques in Alzheimer's disease (AD), targeting additional brain plaque constituents to promote clearance has not been attempted. Endogenous murine Aβ is a minor Aβ plaque component in amyloid precursor protein (APP) transgenic AD models, which we show is ∼3%-8% of the total accumulated Aβ in various human APP transgenic mice. Murine Aβ codeposits and colocalizes with human Aβ in amyloid plaques, and the two Aβ species coimmunoprecipitate together from brain extracts. In the human APP transgenic mouse model Tg2576, passive immunization for 8 weeks with a murine-Aβ-specific antibody reduced β-amyloid plaque pathology, robustly decreasing both murine and human Aβ levels. The immunized mice additionally showed improvements in two behavioral assays, odor habituation and nesting behavior. We conclude that passive anti-murine Aβ immunization clears Aβ plaque pathology--including the major human Aβ component--and decreases behavioral deficits, arguing that targeting minor endogenous brain plaque constituents can be beneficial, broadening the range of plaque-associated targets for AD therapeutics.

Targeted deletion of ERK5 MAP kinase in the developing nervous system impairs development of GABAergic interneurons in the main olfactory bulb and behavioral discrimination between structurally similar odorants

ERK5 MAP kinase is highly expressed in the developing nervous system and has been implicated in promoting the survival of immature neurons in culture. However, its role in the development and function of the mammalian nervous system has not been established in vivo. Here, we report that conditional deletion of the erk5 gene in mouse neural stem cells during development reduces the number of GABAergic interneurons in the main olfactory bulb (OB). Our data suggest that this is due to a decrease in proliferation and an increase in apoptosis in the subventricular zone and rostral migratory stream of ERK5 mutant mice. Interestingly, ERK5 mutant mice have smaller OB and are impaired in odor discrimination between structurally similar odorants. We conclude that ERK5 is a novel signaling pathway regulating developmental OB neurogenesis and olfactory behavior.

Respiratory and sniffing behaviors throughout adulthood and aging in mice

Orienting responses are physiological and active behavioral reactions evoked by novel stimulus perception and are critical for survival. We explored whether odor orienting responses are impacted throughout both adulthood and normal and pathological aging in mice. Novel odor investigation (including duration and bout numbers) and its subsequent habituation as assayed in the odor habituation task were preserved in adult C57BL/6J mice up to 12 mo of age with <6% variability between age groups in investigation time. Separately, using whole-body plethysmography we found that both spontaneous respiration and odor-evoked sniffing behaviors were strikingly preserved in wildtype (WT) mice up to 26 mo of age. In contrast, mice accumulating amyloid-β protein in the brain by means of overexpressing mutations in the human amyloid precursor protein gene (APP) showed preserved spontaneous respiration up to 12 mo, but starting at 14 mo showed significant differences from WT. Similar to WTs, odor-evoked sniffing was not impacted in APP mice up to 26 mo. These results show that odor-orienting responses are minimally impacted throughout aging in mice, and suggest that the olfactomotor network is mostly spared of insults due to aging.

Reversal of autophagy dysfunction in the TgCRND8 mouse model of Alzheimer's disease ameliorates amyloid pathologies and memory deficits

Autophagy, a major degradative pathway for proteins and organelles, is essential for survival of mature neurons. Extensive autophagic-lysosomal pathology in Alzheimer's disease brain contributes to Alzheimer's disease pathogenesis, although the underlying mechanisms are not well understood. Here, we identified and characterized marked intraneuronal amyloid-β peptide/amyloid and lysosomal system pathology in the Alzheimer's disease mouse model TgCRND8 similar to that previously described in Alzheimer's disease brains. We further establish that the basis for these pathologies involves defective proteolytic clearance of neuronal autophagic substrates including amyloid-β peptide. To establish the pathogenic significance of these abnormalities, we enhanced lysosomal cathepsin activities and rates of autophagic protein turnover in TgCRND8 mice by genetically deleting cystatin B, an endogenous inhibitor of lysosomal cysteine proteases. Cystatin B deletion rescued autophagic-lysosomal pathology, reduced abnormal accumulations of amyloid-β peptide, ubiquitinated proteins and other autophagic substrates within autolysosomes/lysosomes and reduced intraneuronal amyloid-β peptide. The amelioration of lysosomal function in TgCRND8 markedly decreased extracellular amyloid deposition and total brain amyloid-β peptide 40 and 42 levels, and prevented the development of deficits of learning and memory in fear conditioning and olfactory habituation tests. Our findings support the pathogenic significance of autophagic-lysosomal dysfunction in Alzheimer's disease and indicate the potential value of restoring normal autophagy as an innovative therapeutic strategy for Alzheimer's disease.

Pheromones in birds: myth or reality?

Birds are anosmic or at best microsmatic… This misbelief persisted until very recently and has strongly influenced the outcome of communication studies in birds, with olfaction remaining neglected as compared to acoustic and visual channels. However, there is now clear empirical evidence showing that olfaction is perfectly functional in birds and birds use olfactory information in a variety of ethological contexts. Although the existence of pheromones has never been formally demonstrated in this vertebrate class, different groups of birds, such as petrels, auklets and ducks have been shown to produce specific scents that could play a significant role in within-species social interactions. Behavioral experiments have indeed demonstrated that these odors influence the behavior of conspecifics. Additionally, in quail, deprivation of olfactory inputs decreases neuronal activation induced by sexual interactions with a female. It seems therefore well established that birds enjoy a functional sense of smell and a fast growing body of experimental evidence suggests that they use this channel of olfactory communication to control their social life. The unequivocal identification of an avian pheromone is, however, still ahead of us but there are now many exciting opportunities to unravel the behavioral and physiological particularities of chemical communication in birds.

Olfactory dysfunction correlates with amyloid-beta burden in an Alzheimer's disease mouse model

Alzheimer's disease often results in impaired olfactory perceptual acuity-a potential biomarker of the disorder. However, the usefulness of olfactory screens to serve as informative indicators of Alzheimer's is precluded by a lack of knowledge regarding why the disease impacts olfaction. We addressed this question by assaying olfactory perception and amyloid-beta (Abeta) deposition throughout the olfactory system in mice that overexpress a mutated form of the human amyloid-beta precursor protein. Such mice displayed progressive olfactory deficits that mimic those observed clinically-some evident at 3 months of age. Also, at 3 months of age, we observed nonfibrillar Abeta deposition within the olfactory bulb-earlier than deposition within any other brain region. There was also a correlation between olfactory deficits and the spatial-temporal pattern of Abeta deposition. Therefore, nonfibrillar, versus fibrillar, Abeta-related mechanisms likely contribute to early olfactory perceptual loss in Alzheimer's disease. Furthermore, these results present the odor cross-habituation test as a powerful behavioral assay, which reflects Abeta deposition and thus may serve to monitor the efficacy of therapies aimed at reducing Abeta.

Sniffing behavior of mice during performance in odor-guided tasks

Sniffing, a rhythmic inhalation and exhalation of air through the nose, is a behavior thought to play a critical role in shaping how odor information is represented and processed by the nervous system. Although the mouse has become a prominent model for studying olfaction, little is known about sniffing behavior in mice. Here, we characterized mouse sniffing behavior by measuring intranasal pressure transients in behaving mice. Sniffing was monitored during unstructured exploratory behavior and during performance of 3 commonly used olfactory paradigms: a habituation/dishabituation task, a sand digging-based discrimination task, and a nose poke-based discrimination task. We found that respiration frequencies in quiescent mice ranged from 3 to 5 Hz--higher than that reported for rats. During exploration, sniff frequency increased up to approximately 12 Hz and was highly dynamic, with rapid changes in frequency, amplitude, and waveform. Sniffing behavior varied strongly between tasks as well as for different behavioral epochs of each task. For example, mice performing the digging-based task showed little increase in sniff frequency prior to digging, whereas mice performing a nose poke-based task showed robust increases. Mice showed large increases in sniff frequency prior to reward delivery in all tasks. Mice also showed increases in sniff frequency when nose poking in a nonodor-guided task. These results show that mouse sniffing behavior is highly dynamic, varies with behavioral context, and is strongly modulated by olfactory as well as nonolfactory events.

Attraction thresholds and sex discrimination of urinary odorants in male and female aromatase knockout (ArKO) mice

We previously found that both male and female aromatase knockout (ArKO) mice, which cannot synthesize estrogens due to a targeted mutation of the aromatase gene, showed less investigation of volatile body odors from anesthetized conspecifics of both sexes in Y-maze tests. We now ask whether ArKO mice are in fact capable of discriminating between and/or responding to volatile odors. Using habituation/dishabituation tests, we found that gonadectomized ArKO and wild-type (WT) mice of both sexes, which were tested without any sex hormone replacement, reliably distinguished between undiluted volatile urinary odors of either adult males or estrous females versus deionized water as well as between these two urinary odors themselves. However, ArKO mice of both sexes were less motivated than WT controls to investigate same-sex odors when they were presented last in the sequence of stimuli. In a second experiment, we compared the ability of ArKO and WT mice to respond to decreasing concentrations of either male or female urinary odors. We found a clear-cut sex difference in urinary odor attraction thresholds among WT mice: WT males failed to respond to urine dilutions higher than 1:20 by volume, whereas WT females continued to respond to urine dilutions up to 1:80. Male ArKO mice resembled WT females in their ability to respond to lower concentrations of urinary odors, raising the possibility that the observed sex difference among WT mice in urine attraction thresholds results from the perinatal actions of estrogen in the male nervous system. Female ArKO mice failed to show significant dishabituation responses to two (1:20 and 1:80) dilutions of female urine, perhaps, again, because of a reduced motivation to investigate less salient, same-sex urinary odors. Previously observed deficits in the preference of ArKO male and female mice to approach volatile body odors from conspecifics of either sex cannot be attributed to an inability of ArKO subjects to discriminate these odors according to sex but instead may reflect a deficient motivation to approach same-sex odors, especially when their concentration is low.

Selective ablation of olfactory receptor neurons without functional impairment of vomeronasal receptor neurons in OMP-ntr transgenic mice

This study used transgenic mice, in which expression of a bacterial nitroreductase (ntr) gene was linked to the expression of olfactory marker protein (OMP). The nitroreductase enzyme is thus expressed in mature chemosensory neurons of these OMP-ntr transgenic mice, and converts the pro-drug CB1954 to a cytotoxic form, specifically killing these neurons. Systemic injections of the pro-drug led to the ablation of receptor neurons in both the main olfactory and vomeronasal epithelia. Due to the anatomical separation of the epithelia, however, when the pro-drug was administered by intranasal infusion only the receptors of the main olfactory epithelium were destroyed. This procedure resulted in a profound deficit in olfactory investigation and discrimination in a habituation-dishabituation test, whereas the pregnancy blocking effect of male pheromones, which is mediated via the vomeronasal system was unaffected. OMP-ntr mice receiving intranasal infusion of pro-drug had not recovered any significant main olfactory function at 24 days following treatment. This novel technique could potentially be applied to selectively ablate olfactory receptor neurons expressing a particular olfactory receptor by linking its expression to that of the nitroreductase enzyme.

A possible role for imprinted genes in inbreeding avoidance and dispersal from the natal area in mice

The expression of a subset of mammalian genes is subject to parent of origin effects (POE), most of which can be explained by genomic imprinting. Analysis of mutant animals has demonstrated that a number of imprinted genes influence brain development and behaviour. Here we provide evidence for POE on olfactory related behaviour and sensitivity to maternal odour cues. This was investigated by examining the odour preference behaviour of reciprocal cross F(1) mice made by embryo transfer to genetically unrelated foster parents. We determined that both adult males and females show an avoidance of female urinary odours of their genetic maternal but not paternal origin. This was found not to be due to any previous exposure to these odours or due to self-learning, but may be related to direct effects on the olfactory system, as reciprocal F(1) males show differential sensitivity to female odour cues. Currently the most robust theory to explain the evolution of imprinting is the conflict hypothesis that focuses on maternal resource allocation to the developing foetus. Kinship considerations are also likely to be important in the selection of imprinted genes and we discuss our findings within this context, suggesting that imprinted genes act directly on the olfactory system to promote post-weaning dispersal from the natal area.

Sex difference in attraction thresholds for volatile odors from male and estrous female mouse urine

Volatile urinary odors from opposite sex conspecifics contribute to mate recognition in numerous mammalian species, including mice. We used a simple habituation/dishabituation testing procedure to ask whether the capacity to detect and investigate decreasing concentrations of volatile urinary odors is sexually differentiated in mice. Beginning 2 months after gonadectomy and in the absence of any sex steroid treatment, adult, sexually naive male and female CBA x C57Bl/6 F1 hybrid mice received two series of daily tests that involved the presentation of different dilutions of urine from C57Bl/6 males followed by urine from estrous females. Each test session began with three consecutive presentations of deionized water (10 microl on filter paper for 2 min, behind a mesh barrier which prevented direct physical access, in the home cage at 1-min intervals) followed by three presentations of one of five different dilutions of urine (a different dilution on each test day). Males and females showed equivalent, significant habituation/dishabituation responses (low investigation times for successive water presentations; increased investigation of the first urine stimulus, followed by a decline in successive urine investigation times) to both male and female urine/water dilutions of 1:1, 1:10, and 1:20. However, only female mice responded reliably to 1:40 and 1:80 dilutions of both types of urine, pointing to a sex dimorphism in the detection and/or processing of biologically relevant, volatile urinary odors by the main olfactory system.

Conditional ablation of neurones in transgenic mice

Conditional targeted ablation of specific cell populations in living transgenic animals is a very powerful strategy to determine cell functions in vivo. This approach would be of particular value to study the functions of distinct neuronal populations; however, the transgene of choice for conditional cell ablation studies in mice, the herpes simplex virus thymidine kinase gene, cannot be used to ablate neurones as its principal mode of action relies on cell proliferation. Here we report that expression of the E.coli nitroreductase gene (Ntr) and metabolism of the prodrug CB1954 (5-aziridin-1-yl-2-4-dinitrobenzamide) to its cytotoxic derivative can be used to conditionally and acutely ablate specific neuronal populations in vivo. As proof of principal, we have ablated olfactory and vomeronasal receptor neurones by expressing Ntr under the control of the olfactory marker protein (OMP) gene promoter. We demonstrate that following CB1954 administration, olfactory and vomeronasal receptor neurones expressing the transgene were selectively eliminated from the olfactory epithelium (OE), and projections to the olfactory bulb (OB) were lost. The functional efficacy of cell ablation was demonstrated using a highly sensitive behavioural test to show that ablated mice had lost the olfactory ability to discriminate distinct odors and were consequently rendered anosmic. Targeted expression of Ntr to specific neuronal populations using conventional transgenes, as described here, or by "knock-in" gene targeting using embryonic stem cells may be of significant value to address the functions of distinct neuronal populations in vivo.

Frequency-dependent plasticity of potentials evoked by repetitive stimulation of the lateral olfactory tract in rat olfactory cortex slices

Habituation and sensitization were demonstrated in slices of olfactory cortex. One of the habituation properties, its dependence on the frequency of the lateral olfactory tract (LOT) stimulation, was tested. It was used in the frequency ranges (1, 10, 20, 30, 40, 50 and 100 impulses/s) which imitated the spontaneous activity of pyriform cortex (PC). Monosynaptic field potentials (FPs) were recorded in PC. Repeated stimulation with one of the frequencies of this range evoked different types of reactions in the PC populations of neurons. Three groups of neuronal reactions were distinguished. The first group of neurons manifested habituation (H-group), while the second one demonstrated sensitization (S-group). In the third group primary sensitization was observed, which then transformed into a short-term phase, when the amplitude of FPs did not change. After that the habituation was found to develop (S-H-group). In the H-group, it was more difficult to evoke habituation at the frequencies of 20-40/s and the number of stimuli in a trial of repeated stimuli (n) being equal to 10 and 150. In the S-group, it was more difficult to induce sensitization at the frequencies of 30-40/s and n equal to 10 and 150. This data proves that the development of habituation and sensitization depends on two factors at least: on the frequency of stimulation and on the type of excited synapse/cell population. Thus, in the PC neuronal populations, a property of habituation suggested by Thompson and Spencer [Psychol. Rev., 73 (1966) 16-43] (higher stimulation frequency causes more rapid and/or pronounced habituation) is not true for all ranges of frequencies.

An investigation of some temporal aspects of olfactory coding with the model of multi-site electrical stimulation of the olfactory bulb in the rat

Electrical stimulation of the olfactory bulb was used to investigate some temporal aspects of olfactory coding, with reference to respiration. Food-deprived rats implanted with permanent electrodes were trained to use bulbar multi-site stimulation patterns as discriminative stimuli for predicting the nature of an incoming reinforcement. Electrical pulse trains (100 Hz) were periodically delivered in phase with precisely defined moments of the respiratory cycle (during inspiration or expiration). Temporal aspects of olfactory coding were first considered through the measurement of the minimum duration of a stimulus necessary to identify this stimulus. The results showed that a bulbar stimulation lasting for 30 ms (3 pulses), and delivered during inspiration, was clearly identified by the rats. Stimulus identification induced a discriminative respiratory response which could manifest itself as early as the first cycle concomitant with the beginning of stimulation. It was then shown that a bulbar electrical stimulation pattern was identified with the same latency whether it occurred during expiration or during inspiration. Moreover, the perceptive events induced in those two conditions of stimulation were not different enough to be discriminated by the animals. The findings are discussed within the framework of olfactory information processing.

Modulations of EEG activity in the entorhinal cortex and forebrain olfactory areas during odour sampling

With the aim of determining a possible role of the entorhinal cortex (EC) in the processing of olfactory information, its electrical activity was recorded during different types of odour sampling behaviour. The EC spontaneous field potentials (EEGs) were related to those recorded simultaneously from the more rostrally lying olfactory bulb (OB) and prepiriform cortex (PPC), using coherence functions. The coherence values are measures of coupling between two neuronal populations. Auto- and coherence spectra were characterized by a peak in the beta-band (35-36 Hz) and another in the 16-20 Hz band. For the beta-peak of the PPC-EC coherence it was shown that both the maximal values and the median frequency decrease during the first 0.5-1.0 s of a trial in a two-choice odour discrimination task. These transients differed significantly for CS+ and CS- trials. However, no such difference was observed during exploratory sniffing at the same pair of odorants. It was concluded that during sniffing the degree of coupling of the EC with the PPC depends on the behavioural context but not on the quality of the odorants. As regards the 16-20 Hz components it was shown, using bispectral analysis, that these components represent the 1/2 subharmonic of the beta-components. The subharmonic components were enhanced during the initiation of sniffing. This means that the neural networks of OB, PPC and EC have non-linear dynamic properties. These networks show different modes of oscillatory behaviour, characteristic of the restful state and of the active sniffing state. Theoretical implications of these experimental results are discussed. In conclusion, it was demonstrated that modulations with sniffing behaviour are present in the beta-EEG components and their subharmonics in the olfactory brain areas including the EC. This indicates that the latter is involved in processes of the evaluation of olfactory cues in relation to the animal's behaviour.

Class I transplantation antigens in solution in body fluids and in the urine. Individuality signals to the environment

Classical class I transplantation antigens present in solution in the body fluids have been studied. These antigens have been found in a monomeric, soluble form in blood, lymph, and urine, and a major source is the hemopoietic system which gives rise to cells that secrete these molecules into the blood. The cell types most probably involved in their secretion are of the macrophage/dendritic cell lineage. The serum molecule is a heterodimer with a heavy chain of 39,000 mol wt associated noncovalently with beta 2-microglobulin and is present in serum at a concentration between 350 and 390 ng/ml. These molecules have a short half-life of 2.7 h and are excreted into the environment via the kidneys in the urine. In the urine, greater than 90% of the molecules are degraded into smaller fragments. This finding that normal metabolic processes lead to the excretion of classical highly polymorphic class I molecules in the urine provides a direct explanation in molecular terms of the ability of animals to identify individuals on the basis of urinary odor. Since intact class I molecules are unlikely to be the odoriferous component in the urine, two hypotheses have been suggested. Either small fragments of class I molecules are detected or the molecule acts as a carrier that transports volatiles from the serum into the urine where they are released, giving rise to the class I-associated odor.

Response properties of neurones in the rat olfactory bulb to various parameters of odour stimulation

Carbon fibre filled microelectrodes were used for extracellular recording of spikes discharges from 345 neurones of the lateral aspect of the left olfactory bulb of the rat under pentobarbitone anaesthesia. The identity of the neurones was determined by antidromic activation of the cell axons by electrical stimuli applied to the lateral olfactory tract (LOT) using collision blocking and twin stimuli. The bulbar units were classified as mitral (269) or tufted (16) cells; units in a third class were not identified (60). Application of stimuli (21) was made over a period of 15 s while slowly increasing the odorant concentration monitored by a flame ionization device (FID). The stimulating efficiency reached a maximum at a nasal flow of about 8 cm3 s-1. The response type excitation was seen in 12.0% and inhibition in 11.8% of the 7245 odour presentations. Each response type was subdivided into four classes according to the reactions to ramp stimulation. In the majority of cases the spike discharge remained constant or followed the odour concentration for the entire stimulation period. In 18% of the cases, the response adapted quickly towards the spontaneous activity level.

The major histocompatibility complex and the chemosensory recognition of individuality in rats

The present experiments provide the first evidence that congenic strains of rats, which differ only in the MHC, produce discriminably different urinary chemosignals. Urine from adult male PVG and PVG.R1 rats, which differ only in the A region (class 1) of the MHC, was used in a habituation-dishabituation task, with male PVG-RTlu, Wistar albino, and Lister hooded rats as subjects. Urine from PVG males was easily distinguished from that of PVG.R1 males by all three strains. Individual PVG males were not distinguished by their urine odours, but individual PVG.R1 males appeared to have discriminably different odours. A repetition of this experiment indicated that this discrimination may have been due to impurities in the urine. Odours from serum were not sufficient for discrimination between the two strains, nor was the class 1 molecule purified from the urine. Urine with the class 1 molecule removed (remainder fraction) could, however, be used to distinguish between the strains. The chemicals in the urine which give this distinctive odour may be fragments of the class 1 molecule or small molecules associated with the class 1 molecule. The MHC appears to control the odour cues which are used by mammals for individual recognition and may provide an olfactory basis for kin recognition but the mechanism by which the MHC controls these olfactory signals is unknown.

On the ability of rats to discriminate between microstimulations of the olfactory bulb in different locations

An investigation was made into the ability of rats to discriminate between electrical stimulations applied to the mitral cell layer of the olfactory bulb in different locations. Water-deprived rats implanted with permanent electrodes were trained to use single- or multi-site microstimulations as discriminative stimuli for selecting a palatable solution without tasting it in a two-choice test. Spontaneous reactions of the animals to stimulation with sinusoidal currents higher than 3 microA per electrode resembled sensory arousal. All rats were found to discriminate between the effects of concurrent microstimulations applied to bulbar sites separated by 500 micron. Changing the current intensity in the range 4-20 microA had no detectable effect on the discrimination. Discrimination was still possible, with a few exceptions, when electrodes were separated by 250 micron and even when they were closely adjacent. Spatial resolution of discrimination seemed not to vary in different regions along the rostrocaudal axis of the bulb. The discrimination of patterns of simultaneous stimulation at several sites was also investigated. Different multi-site patterns were easily distinguished, even when their respective components were closely adjacent or when some components occupied the same area. The findings are discussed with reference to the concept of spatial coding of odours in the olfactory bulb.