Stereological quantification of olfactory receptor neurons in mice

Low signaling efficiency from receptor to effector in olfactory transduction: A quantified ligand-triggered GPCR pathway

G protein-coupled receptor (GPCR) signaling is ubiquitous. As an archetype of this signaling motif, rod phototransduction has provided many fundamental, quantitative details, including a dogma that one active GPCR molecule activates a substantial number of downstream G protein/enzyme effector complexes. However, rod phototransduction is light-activated, whereas GPCR pathways are predominantly ligand-activated. Here, we report a detailed study of the ligand-triggered GPCR pathway in mammalian olfactory transduction, finding that an odorant-receptor molecule when (one-time) complexed with its most effective odorants produces on average much less than one downstream effector. Further experiments gave a nominal success probability of tentatively ∼10-4 (more conservatively, ∼10-2 to ∼10-5). This picture is potentially more generally representative of GPCR signaling than is rod phototransduction, constituting a paradigm shift.

A genetic platform for functionally profiling odorant receptors in olfactory cilia ex vivo

The molecular basis for odor perception in humans remains enigmatic because of the difficulty in studying odorant receptors (ORs) outside their native environment. Efforts toward OR expression and functional profiling have been met with limited success because of the poor efficiency of their cell surface expression in vitro. Structures protruding from the surface of olfactory sensory neurons called cilia contain all of the components of the olfactory signal transduction machinery and can be placed in an ex vivo plate assay to rapidly measure odor-specific responses. Here, we describe an approach using cilia isolated from the olfactory sensory neurons of mice expressing two human ORs, OR1A1 and OR5AN1, that showed 10- to 100-fold more sensitivity to ligands as compared to previous assays. A single mouse can produce enough olfactory cilia for up to 4000 384-well assay wells, and isolated cilia can be stored frozen and thus preserved. This pipeline offers a sensitive and highly scalable ex vivo odor-screening platform that has the potential to decode human olfaction.

Longitudinal imaging of individual olfactory sensory neurons in situ

Olfactory sensory neurons are found deep within the nasal cavity at a spatially restricted sheet of sensory epithelium. Due to their location behind the nasal turbinates, accessing these cells for physiological measurements in living animals is challenging, and until recently, not possible. As a further complication, damage to the overlying bone on the dorsal surface of the snout disrupts the negative pressure distribution throughout the nasal cavities, which fundamentally alters how odorants are delivered to the sensory epithelium and the inherent mechanosensory properties of olfactory sensory neurons in live animals. The approach described here circumvents these limitations and allows for optical access to olfactory sensory neurons in mice across time scales ranging from days to months.

The scaling of olfaction: Moths have relatively more olfactory surface area than mammals

Body size affects nearly every aspect of locomotion and sensing, but little is known how body size influences olfaction. One reason for this missing link is that olfaction differs fundamentally from vision and hearing in that molecules are advected by fluid before depositing on olfactory sensors. This critical role of fluid flow in olfaction leads to complexities and trade-offs. For example, a greater density of hairs and sensory neurons may lead to greater collection, but can also lead to reduced flow through hairs and additional weight and drag due to a larger olfactory organ. In this study, we report the surface area and sensory neuron density in olfactory organs of 95 species of moths and mammals. We find that approximately 12-14 percent of an olfactory system's surface area is devoted to chemosensors. Furthermore, total olfactory surface area and olfactory sensing surface area scale with body mass to the 0.49 and 0.38 powers respectively, indicating that moths have a higher proportion of olfactory surface area than mammals. The density of olfactory neurons appears to be near the limit, at 10,000 to 100,000 neurons per square mm across both insects and mammals. This study demonstrates the need for future work detailing how scaling of olfaction and other senses vary across taxa.

Modification of the Peripheral Olfactory System by Electronic Cigarettes

Electronic cigarettes (e-cigs) are used by millions of adolescents and adults worldwide. Commercial e-liquids typically contain flavorants, propylene glycol, and vegetable glycerin with or without nicotine. These chemical constituents are detected and evaluated by chemosensory systems to guide and modulate vaping behavior and product choices of e-cig users. The flavorants in e-liquids are marketing tools. They evoke sensory percepts of appealing flavors through activation of chemical sensory systems to promote the initiation and sustained use of e-cigs. The vast majority of flavorants in e-liquids are volatile odorants, and as such, the olfactory system plays a dominant role in perceiving these molecules that enter the nasal cavity either orthonasally or retronasally during vaping. In addition to flavorants, e-cig aerosol contains a variety of by-products generated through heating the e-liquids, including odorous irritants, toxicants, and heavy metals. These harmful substances can directly and adversely impact the main olfactory epithelium (MOE). In this article, we first discuss the olfactory contribution to e-cig flavor perception. We then provide information on MOE cell types and their major functions in olfaction and epithelial maintenance. Olfactory detection of flavorants, nicotine, and odorous irritants and toxicants are also discussed. Finally, we discuss the cumulated data on modification of the MOE by flavorant exposure and toxicological impacts of formaldehyde, acrolein, and heavy metals. Together, the information presented in this overview may provide insight into how e-cig exposure may modify the olfactory system and adversely impact human health through the alteration of the chemosensory factor driving e-cig use behavior and product selections. © 2021 American Physiological Society. Compr Physiol 11:2621-2644, 2021.

Principles of odor coding in vertebrates and artificial chemosensory systems

The biological olfactory system is the sensory system responsible for the detection of the chemical composition of the environment. Several attempts to mimic biological olfactory systems have led to various artificial olfactory systems using different technical approaches. Here we provide a parallel description of biological olfactory systems and their technical counterparts. We start with a presentation of the input to the systems, the stimuli, and treat the interface between the external world and the environment where receptor neurons or artificial chemosensors reside. We then delineate the functions of receptor neurons and chemosensors as well as their overall I-O relationships. Up to this point, our account of the systems goes along similar lines. The next processing steps differ considerably: while in biology the processing step following the receptor neurons is the "integration" and "processing" of receptor neuron outputs in the olfactory bulb, this step has various realizations in electronic noses. For a long period of time, the signal processing stages beyond the olfactory bulb, i.e., the higher olfactory centers were little studied. Only recently there has been a marked growth of studies tackling the information processing in these centers. In electronic noses, a third stage of processing has virtually never been considered. In this review, we provide an up-to-date overview of the current knowledge of both fields and, for the first time, attempt to tie them together. We hope it will be a breeding ground for better information, communication, and data exchange between very related but so far little connected fields.

Maturation of the Olfactory Sensory Neuron and Its Cilia

Olfactory sensory neurons (OSNs) are bipolar neurons, unusual because they turn over continuously and have a multiciliated dendrite. The extensive changes in gene expression accompanying OSN differentiation in mice are largely known, especially the transcriptional regulators responsible for altering gene expression, revealing much about how differentiation proceeds. Basal progenitor cells of the olfactory epithelium transition into nascent OSNs marked by Cxcr4 expression and the initial extension of basal and apical neurites. Nascent OSNs become immature OSNs within 24-48 h. Immature OSN differentiation requires about a week and at least 2 stages. Early-stage immature OSNs initiate expression of genes encoding key transcriptional regulators and structural proteins necessary for further neuritogenesis. Late-stage immature OSNs begin expressing genes encoding proteins important for energy production and neuronal homeostasis that carry over into mature OSNs. The transition to maturity depends on massive expression of one allele of one odorant receptor gene, and this results in expression of the last 8% of genes expressed by mature OSNs. Many of these genes encode proteins necessary for mature function of axons and synapses or for completing the elaboration of non-motile cilia, which began extending from the newly formed dendritic knobs of immature OSNs. The cilia from adjoining OSNs form a meshwork in the olfactory mucus and are the site of olfactory transduction. Immature OSNs also have a primary cilium, but its role is unknown, unlike the critical role in proliferation and differentiation played by the primary cilium of the olfactory epithelium's horizontal basal cell.

Sex differences in main olfactory system pathways involved in psychosexual function

We summarize literature from animal and human studies assessing sex differences in the ability of the main olfactory system to detect and process sex-specific olfactory signals ("pheromones") that control the expression of psychosexual functions in males and females. A case is made in non primate mammals for an obligatory role of pheromonal signaling via the main olfactory system (in addition to the vomeronasal-accessory olfactory system) in mate recognition and sexual arousal, with male-specific as well as female-specific pheromones subserving these functions in the opposite sex. Although the case for an obligatory role of pheromones in mate recognition and mating among old world primates, including humans, is weaker, we review the current literature assessing the role of putative human pheromones (eg, AND, EST, "copulin"), detected by the main olfactory system, in promoting mate choice and mating in men and women. Based on animal studies, we hypothesize that sexually dimorphic effects of putative human pheromones are mediated via main olfactory inputs to the medial amygdala which, in turn, transmits olfactory information to sites in the hypothalamus that regulate reproduction.

Sequential Maturation of Olfactory Sensory Neurons in the Mature Olfactory Epithelium

The formation of the olfactory nerve and olfactory bulb (OB) glomeruli begins embryonically in mice. However, the development of the olfactory system continues throughout life with the addition of new olfactory sensory neurons (OSNs) in the olfactory epithelium (OE). Much attention has been given to the perinatal innervation of the OB by OSN axons, but in the young adult the process of OSN maturation and axon targeting to the OB remains controversial. To address this gap in understanding, we used BrdU to label late-born OSNs in young adult mice at postnatal day 25 (P25-born OSNs) and timed their molecular maturation following basal cell division. We show that OSNs in young adults undergo a sequential molecular development with the expression of GAP 43 (growth-associated protein 43) > AC3 (adenylyl cyclase 3) > OMP (olfactory marker protein), consecutively, in a time frame of ∼8 d. To assess OSN axon development, we implemented an in vivo fate-mapping strategy to label P25-born OSNs with ZsGreen. Using sampling intervals of 24 h, we demonstrate the progressive extension of OSN axons in the OE, through the foramen of the cribriform plate, and onto the surface of the OB. OSN axons reached the OB and began to target and robustly innervate specific glomeruli ∼10 d following basal cell division, a time point at which OMP expression becomes evident. Our data demonstrate a sequential process of correlated axon extension and molecular maturation that is similar to that seen in the neonate, but on a slightly longer timescale and with regional differences in the OE.

Sequential Maturation of Olfactory Sensory Neurons in the Mature Olfactory Epithelium

The formation of the olfactory nerve and olfactory bulb (OB) glomeruli begins embryonically in mice. However, the development of the olfactory system continues throughout life with the addition of new olfactory sensory neurons (OSNs) in the olfactory epithelium (OE). Much attention has been given to the perinatal innervation of the OB by OSN axons, but in the young adult the process of OSN maturation and axon targeting to the OB remains controversial. To address this gap in understanding, we used BrdU to label late-born OSNs in young adult mice at postnatal day 25 (P25-born OSNs) and timed their molecular maturation following basal cell division. We show that OSNs in young adults undergo a sequential molecular development with the expression of GAP 43 (growth-associated protein 43) > AC3 (adenylyl cyclase 3) > OMP (olfactory marker protein), consecutively, in a time frame of ∼8 d. To assess OSN axon development, we implemented an in vivo fate-mapping strategy to label P25-born OSNs with ZsGreen. Using sampling intervals of 24 h, we demonstrate the progressive extension of OSN axons in the OE, through the foramen of the cribriform plate, and onto the surface of the OB. OSN axons reached the OB and began to target and robustly innervate specific glomeruli ∼10 d following basal cell division, a time point at which OMP expression becomes evident. Our data demonstrate a sequential process of correlated axon extension and molecular maturation that is similar to that seen in the neonate, but on a slightly longer timescale and with regional differences in the OE.

Osteopontin attenuates early brain injury through regulating autophagy-apoptosis interaction after subarachnoid hemorrhage in rats

Aim

To determine the effect of osteopontin (OPN) on autophagy and autophagy‐apoptosis interactions after SAH.

Methods

The endovascular perforation model of SAH or sham surgery was performed in a total of 86 Sprague‐Dawley male rats. The temporal expressions of endogenous OPN and autophagy‐related proteins (Beclin 1, ATG5, LC3 II to I ratio) were measured in sham and SAH rats at different time points (3, 6, 12, 24, and 72 hours). Rats were randomly divided into three groups: Sham, SAH + Vehicle (PBS, phosphate‐buffered saline), and SAH + rOPN (5 μg/rat recombinant OPN). Neurobehavioral tests were performed 24 hours after SAH, followed by the collection of brain samples for assessment of autophagy and apoptosis proteins. These tests assessed whether an autophagy‐apoptosis relationship existed on the histological level in the brain.

Results

Endogenous OPN and autophagy‐related proteins all increased after SAH. rOPN administration improved neurological dysfunction, increased the expression of autophagy‐related proteins (Beclin 1, ATG5, LC3 II to I ratio) and antiapoptotic protein Bcl‐2, while decreasing the expression of proapoptotic proteins (cleaved Caspase‐3 and Bax). rOPN also regulated autophagy‐apoptosis interactions 24 hours after SAH.

Conclusion

rOPN attenuates early brain injury and inhibits neuronal apoptosis by activating autophagy and regulating autophagy‐apoptosis interactions.

Scaling Principles of Distributed Circuits

Identifying shared quantitative features of a neural circuit across species is important for 3 reasons. Often expressed in the form of power laws and called scaling relationships [1, 2], they reveal organizational principles of circuits, make insights gleaned from model systems widely applicable, and explain circuit performance and function, e.g., visual circuits [3, 4]. The visual circuit is topographic [5, 6], wherein retinal neurons target and activate predictable spatial loci in primary visual cortex. The brain, however, contains many circuits, where neuronal targets and activity are unpredictable and distributed throughout the circuit, e.g., olfactory circuits, in which glomeruli (or mitral cells) in the olfactory bulb synapse with neurons distributed throughout the piriform cortex [7-10]. It is unknown whether such circuits, which we term distributed circuits, are scalable. To determine whether distributed circuits scale, we obtained quantitative descriptions of the olfactory bulb and piriform cortex in six mammals using stereology techniques and light microscopy. Two conserved features provide evidence of scalability. First, the number of piriform neurons n and bulb glomeruli g scale as n∼g^3/2. Second, the average number of synapses between a bulb glomerulus and piriform neuron is invariant at one. Using theory and modeling, we show that these two features preserve the discriminatory ability and precision of odor information across the olfactory circuit. As both abilities depend on circuit size, manipulating size provides evolution with a way to adapt a species to its niche without designing developmental programs de novo. These principles might apply to other distributed circuits like the hippocampus.

A transcriptomic atlas of mammalian olfactory mucosae reveals an evolutionary influence on food odor detection in humans

The mammalian olfactory system displays species-specific adaptations to different ecological niches. To investigate the evolutionary dynamics of olfactory sensory neuron (OSN) subtypes across mammalian evolution, we applied RNA sequencing of whole olfactory mucosa samples from mouse, rat, dog, marmoset, macaque, and human. We find that OSN subtypes, representative of all known mouse chemosensory receptor gene families, are present in all analyzed species. Further, we show that OSN subtypes expressing canonical olfactory receptors are distributed across a large dynamic range and that homologous subtypes can be either highly abundant across all species or species/order specific. Highly abundant mouse and human OSN subtypes detect odorants with similar sensory profiles and sense ecologically relevant odorants, such as mouse semiochemicals or human key food odorants. Together, our results allow for a better understanding of the evolution of mammalian olfaction in mammals and provide insights into the possible functions of highly abundant OSN subtypes.

Effects of various lengths of hypoglossal nerve resection on motoneuron survival

We employed stereological analyses for whole quantification of hypoglossal (XII) motoneurons in adult rats that received varying degrees of resection of the XII nerve. Various lengths of nerve gaps (0.0-13.3 mm) were made at the main trunk of the unilateral XII nerve, and the total number of XII neurons on the injured and uninjured sides was counted 12 weeks after nerve resection. The stereologically estimated total number of XII neurons decreased after various lengths of nerve resection, and survival rates ranged from 34.4% to 87.1%. Statistically significant negative correlations were observed between increasing length of the resected nerve and decreasing XII neuron survival. It was concluded that the total number of XII neurons decreased after nerve resection and that survival rates of XII neurons were related to distances between resected nerve stamps.

A resource for the Drosophila antennal lobe provided by the connectome of glomerulus VA1v

Using FIB-SEM we report the entire synaptic connectome of glomerulus VA1v of the right antennal lobe in Drosophila melanogaster. Within the glomerulus we densely reconstructed all neurons, including hitherto elusive local interneurons. The fruitless-positive, sexually dimorphic VA1v included >11,140 presynaptic sites with ~38,050 postsynaptic dendrites. These connected input olfactory receptor neurons (ORNs, 51 ipsilateral, 56 contralateral), output projection neurons (18 PNs), and local interneurons (56 of >150 previously reported LNs). ORNs are predominantly presynaptic and PNs predominantly postsynaptic; newly reported LN circuits are largely an equal mixture and confer extensive synaptic reciprocity, except the newly reported LN2V with input from ORNs and outputs mostly to monoglomerular PNs, however. PNs were more numerous than previously reported from genetic screens, suggesting that the latter failed to reach saturation. We report a matrix of 192 bodies each having >50 connections; these form 88% of the glomerulus' pre/postsynaptic sites.

Tailoring Formulations for Intranasal Nose-to-Brain Delivery: A Review on Architecture, Physico-Chemical Characteristics and Mucociliary Clearance of the Nasal Olfactory Mucosa

The blood-brain barrier and the blood-cerebrospinal fluid barrier are major obstacles in central nervous system (CNS) drug delivery, since they block most molecules from entering the brain. Alternative drug delivery routes like intraparenchymal or intrathecal are invasive methods with a remaining risk of infections. In contrast, nose-to-brain delivery is a minimally invasive drug administration pathway, which bypasses the blood-brain barrier as the drug is directed from the nasal cavity to the brain. In particular, the skull base located at the roof of the nasal cavity is in close vicinity to the CNS. This area is covered with olfactory mucosa. To design and tailor suitable formulations for nose-to-brain drug delivery, the architecture, structure and physico-chemical characteristics of the mucosa are important criteria. Hence, here we review the state-of-the-art knowledge about the characteristics of the nasal and, in particular, the olfactory mucosa needed for a rational design of intranasal formulations and dosage forms. Also, the information is suitable for the development of systemic or local intranasal drug delivery as well as for intranasal vaccinations.

Differences in peripheral sensory input to the olfactory bulb between male and female mice

Female mammals generally have a superior sense of smell than males, but the biological basis of this difference is unknown. Here, we demonstrate sexually dimorphic neural coding of odorants by olfactory sensory neurons (OSNs), primary sensory neurons that physically contact odor molecules in the nose and provide the initial sensory input to the brain’s olfactory bulb. We performed in vivo optical neurophysiology to visualize odorant-evoked OSN synaptic output into olfactory bub glomeruli in unmanipulated (gonad-intact) adult mice from both sexes, and found that in females odorant presentation evoked more rapid OSN signaling over a broader range of OSNs than in males. These spatiotemporal differences enhanced the contrast between the neural representations of chemically related odorants in females compared to males during stimulus presentation. Removing circulating sex hormones makes these signals slower and less discriminable in females, while in males they become faster and more discriminable, suggesting opposite roles for gonadal hormones in influencing male and female olfactory function. These results demonstrate that the famous sex difference in olfactory abilities likely originates in the primary sensory neurons, and suggest that hormonal modulation of the peripheral olfactory system could underlie differences in how males and females experience the olfactory world.

Variation in olfactory neuron repertoires is genetically controlled and environmentally modulated

The mouse olfactory sensory neuron (OSN) repertoire is composed of 10 million cells and each expresses one olfactory receptor (OR) gene from a pool of over 1000. Thus, the nose is sub-stratified into more than a thousand OSN subtypes. Here, we employ and validate an RNA-sequencing-based method to quantify the abundance of all OSN subtypes in parallel, and investigate the genetic and environmental factors that contribute to neuronal diversity. We find that the OSN subtype distribution is stereotyped in genetically identical mice, but varies extensively between different strains. Further, we identify cis-acting genetic variation as the greatest component influencing OSN composition and demonstrate independence from OR function. However, we show that olfactory stimulation with particular odorants results in modulation of dozens of OSN subtypes in a subtle but reproducible, specific and time-dependent manner. Together, these mechanisms generate a highly individualized olfactory sensory system by promoting neuronal diversity.

DOI:http://dx.doi.org/10.7554/eLife.21476.001

Smells are simply chemicals in the air that are recognized by nerves in our nose. Each nerve has a receptor that can identify a limited number of chemicals, and the nerve then relays this information to the brain. Animals have hundreds to thousands of different types of these nerves meaning that they can detect a wide array of smells.

Smell receptors are proteins, and the genes that encode these proteins can be very different in two unrelated people. This could partly explain, for example, why some people find certain odors intense and unpleasant while others do not. However, having different genes for smell receptors does not by itself completely explain why some people are more sensitive than others to particular smells. The amounts of each nerve type in the nose might also differ between people and have an effect, but to date it has not been possible to accurately count them all.

Ibarra-Soria et al. have now devised a new method to essentially count the number of each nerve type in the noses of mice from different breeds. The method makes use of a technique called RNA-sequencing, which can reveal which genes are active at any one time, and thus show how many nerves are producing each type of smell receptor. Ibarra-Soria et al. learned that different breeds of mice had remarkably different compositions of nerves in their noses. Further analysis revealed that this was due to changes to the DNA code near to the genes that encode the smell receptor.

Next, Ibarra-Soria et al. sought to find out how the amount of each nerve type is controlled by giving mice water with different smells for weeks and looking how this affected their noses. These experiments revealed that a small number of the nerve types became more or less common after exposure to a smell. The altered nerves were directly involved in recognizing the smells, proving that the very act of smelling can change the make-up of nerves in a mouse’s nose.

These results confirm that the diversity in the nose of each individual is not only dictated by the types of receptors found in there, but also by the number of each nerve type. The next challenge is to understand better how these differences change the way people perceive smells.

DOI:http://dx.doi.org/10.7554/eLife.21476.002

Stereological assessment of the total number of hypoglossal neurons after repeated crush injuries to the hypoglossal nerve in adult rats

OBJECTIVE

Retrograde neuronal cell death does not occur in mature motoneurons following the axonal injury of peripheral nerves. However, a previous study suggested that retrograde neuronal cell death does occur in adult rats after the creation of double lesions on the hypoglossal (XII) nerve based on a substantial decrease in the number of XII neurons. Using stereological methods, we examined neuronal apoptosis in XII neurons and the total number of XII neurons following repeated crush injuries to the XII nerve.

METHODS

The right XII nerve of adult rats was crushed three times at one-week intervals with a brain aneurysm clip. At 4 weeks after the final crush, the total numbers of XII neurons on the injured right and uninjured left sides were estimated stereologically.

RESULTS

After repeated crush injuries, no apoptosis was evident in XII neurons as indicated by immunostaining for cleaved caspase-3. Moreover, immunohistochemistry for the vesicular acetylcholine transporter revealed axonal elongation in the tongue 4 weeks after repeated crush injuries. At 4 weeks, the total numbers of XII neurons were 7800 ± 290 on the injured right side and 8000 ± 230 on the uninjured left side, and no significant difference was evident between the injured and uninjured sides.

CONCLUSION

Neuronal cell death does not occur in XII neurons and the total number of XII neurons does not decrease after repeated crush injuries of the XII nerve in adult rats.

Olfactory Sensitivity for the Mammalian Blood Odor Component Trans-4,5-epoxy-(E)-2-decenal in CD-1 Mice

Using a conditioning paradigm and an automated olfactometer, we investigated the olfactory sensitivity of CD-1 mice for the mammalian blood odor component trans-4,5-epoxy-(E)-2-decenal. We found that two of the animals significantly discriminated concentrations down to 3.0 ppt (parts per trillion) from the solvent, and three animals even successfully detected dilutions as low as 0.3 ppt. Intraspecific comparisons between the olfactory detection thresholds obtained here with those obtained in earlier studies with other odorants show that mice are extraordinarily sensitive to this blood odor component. Interspecific comparisons of olfactory detection thresholds show that human subjects are even more sensitive to trans-4,5-epoxy-(E)-2-decenal than the mice tested here. Both intra- and inter-specific comparisons suggest that neither neuroanatomical properties such as the size of the olfactory epithelium, the total number of olfactory receptor neurons, or the size of olfactory brain structures, nor genetic properties such as the number of functional olfactory receptor genes or the proportion of functional relative to the total number of olfactory receptor genes allow us to reliably predict a species' olfactory sensitivity. In contrast, the results support the notion that the behavioral relevance of an odorant rather than neuroanatomical or genetic properties may determine a species' olfactory sensitivity.

Electroacupuncture Produces the Sustained Motor Improvement in 6-Hydroxydopamine-Lesioned Mice

Clinical and research evidence has shown that electroacupuncture (EA) promotes recovery of motor function in patients with Parkinson’s disease (PD). However, the “efficacy span” of EA treatment, especially the long-term effect of EA that is thought to last after the cessation of EA treatment, has not been investigated. The present study thus investigated and compared the effect of EA during and after chronic EA application on motor activity and dopamine lesions in a 6-hydroxydopamine (6-OHDA)-lesioned mouse model of PD. Chronic EA treatment (30 min a day, 6 days a week for 2 or 4 weeks) significantly attenuated motor deficiency and reduced dopamine neuron degeneration. Remarkably, EA showed a long-lasting effect after the cessation of EA stimulation. At 2 and 4 weeks after the termination of EA, EA continued to improve motor function in 6-OHDA-lesioned mice. Consistent with sustained behavioral effects, EA induced an enduring increase in the dopamine turnover ratio in the striatum 2 weeks after the cessation of EA treatment. Here we demonstrated that the therapeutic effect of EA outlasted the duration of EA application. During a relatively long period of time after the completion of EA treatment, EA is able to continue to improve motor function and enhance dopamine availability in 6-OHDA-lesioned PD mice.

Hierarchical deconstruction of mouse olfactory sensory neurons: from whole mucosa to single-cell RNA-seq

The mouse olfactory mucosa is a complex chemosensory tissue composed of multiple cell types, neuronal and non-neuronal. We have here applied RNA-seq hierarchically, in three steps of decreasing cellular heterogeneity: starting with crude tissue samples dissected from the nose, proceeding to flow-cytometrically sorted pools of mature olfactory sensory neurons (OSNs), and finally arriving at single mature OSNs. We show that 98.9% of intact olfactory receptor (OR) genes are expressed in mature OSNs. We uncover a hitherto unknown bipartition among mature OSNs. We find that 19 of 21 single mature OSNs each express a single intact OR gene abundantly, consistent with the one neuron-one receptor rule. For the 9 single OSNs where the two alleles of the abundantly expressed OR gene exhibit single-nucleotide polymorphisms, we demonstrate that monoallelic expression of the abundantly expressed OR gene is extremely tight. The remaining two single mature OSNs lack OR gene expression but express Trpc2 and Gucy1b2. We establish these two cells as a neuronal cell type that is fundamentally distinct from canonical, OR-expressing OSNs and that is defined by the differential, higher expression of 55 genes. We propose this tiered experimental approach as a paradigm to unravel gene expression in other cellularly heterogeneous systems.

Genome-Wide Screen Reveals Rhythmic Regulation of Genes Involved in Odor Processing in the Olfactory Epithelium

Odor discrimination behavior displays circadian fluctuations in mice, indicating that mammalian olfactory function is under control of the circadian system. This is further supported by the facts that odor discrimination rhythms depend on the presence of clock genes and that olfactory tissues contain autonomous circadian clocks. However, the molecular link between circadian function and olfactory processing is still unknown. To elucidate the molecular mechanisms underlying this link, we focused on the olfactory epithelium (OE), the primary target of odors and the site of the initial events in olfactory processing. We asked whether olfactory sensory neurons (OSNs) within the OE possess an autonomous circadian clock and whether olfactory pathways are under circadian control. Employing clock gene-driven bioluminescence reporter assays and time-dependent immunohistochemistry on OE samples, we found robust circadian rhythms of core clock genes and their proteins in OSNs, suggesting that the OE indeed contains an autonomous circadian clock. Furthermore, we performed a circadian transcriptome analysis and identified several OSN-specific components that are under circadian control, including those with putative roles in circadian olfactory processing, such as KIRREL2-an established factor involved in short-term OSN activation. The spatiotemporal expression patterns of our candidate proteins suggest that they are involved in short-term anabolic processes to rhythmically prepare the cell for peak performances and to promote circadian function of OSNs.

Comparison of unbiased estimation of neuronal number in the rat hippocampus with different staining methods

BACKGROUND

NeuN and Nissl staining (toluidine blue, cresyl violet staining) are routinely used methods in unbiased stereological estimation of the total number of hippocampal neurons.

NEW METHOD

In the present study, we stained serial frozen coronal sections from 5 normal adult male Sprague-Dawley rat brains with different methods, measured the deformation of hippocampal area in brain sections and estimated the total number of hippocampal neurons using the optical fractionator.

RESULTS

The deformation in x, y-axis was not obviously different with different staining protocols, but shrinkage in z-axis was significant after staining (p < 0.001). NeuN staining produced significant higher estimate number than cresyl violet staining by 24% (p = 0.002), however, NeuN and Cresyl Violet staining showed a high degree of correlation in quantification of total neuronal numbers and both methods are suitable for unbiased stereological estimation.

COMPARISON WITH EXISTING METHOD (S)

NeuN is more reliable but if time is limited or the number of animals used in experiments is high, cresyl violet staining may be a feasible method.

CONCLUSIONS

Compared with previous estimates of the neurons number in rat hippocampus, our present data is reliable and the stereological analysis based on our system is a cost-effective unbiased method for estimation of neuron number.

Linear correlation between the number of olfactory sensory neurons expressing a given mouse odorant receptor gene and the total volume of the corresponding glomeruli in the olfactory bulb

Chemosensory specificity in the main olfactory system of the mouse relies on the expression of ∼1,100 odorant receptor (OR) genes across millions of olfactory sensory neurons (OSNs) in the main olfactory epithelium (MOE), and on the coalescence of OSN axons into ∼3,600 glomeruli in the olfactory bulb. A traditional approach for visualizing OSNs and their axons consists of tagging an OR gene genetically with an axonal marker that is cotranslated with the OR by virtue of an internal ribosome entry site (IRES). Here we report full cell counts for 15 gene‐targeted strains of the OR‐IRES‐marker design coexpressing a fluorescent protein. These strains represent 11 targeted OR genes, a 1% sample of the OR gene repertoire. We took an empirical, “count every cell” strategy: we counted all fluorescent cell profiles with a nuclear profile within the cytoplasm, on all serial coronal sections under a confocal microscope, a total of 685,673 cells in 56 mice at postnatal day 21. We then applied a strain‐specific Abercrombie correction to these OSN counts in order to obtain a closer approximation of the true OSN numbers. We found a 17‐fold range in the average (corrected) OSN number across these 11 OR genes. In the same series of coronal sections, we then determined the total volume of the glomeruli (TGV) formed by coalescence of the fluorescent axons. We found a strong linear correlation between OSN number and TGV, suggesting that TGV can be used as a surrogate measurement for estimating OSN numbers in these gene‐targeted strains. J. Comp. Neurol. 524:199–209, 2016. © 2015 Wiley Periodicals, Inc.

Stereological estimation of olfactory receptor neurons in rats

The total number of olfactory receptor neurons (ORNs) in the rat main olfactory epithelium (MOE) was estimated using stereological sampling. Skulls and noses of newborn (postnatal day 0), young adult (8 weeks), and adult (6 months) rats were decalcified, embedded in paraffin, cut into 10-μm thick sections serially at 100-μm intervals, and processed for immunohistochemistry for olfactory marker protein (OMP), a specific marker of mature ORNs with fiber connections to the olfactory bulb. The number of OMP (+) receptor neurons was measured using an optical fractionator with stereological software (Stereoinvestigator). The total number of OMP (+) receptor neurons in the unilateral MOE was 0.47 × 10(6) in newborns and 21 × 10(6) in young adults and adults. Our previous study showed that the corresponding unilateral total number of neurons was 5.2 × 10(6) in young adult mice. Accordingly, we concluded that rats had 4 times more OMP (+) receptor neurons than mice at the adult stage and that the number of these neurons increased approximately 45 times between birth and maturity.