Flexible categorization in the mouse olfactory bulb

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Activity-dependent changes in cholinergic innervation of the mouse olfactory bulb

The interplay between olfactory activity and cholinergic modulation remains to be fully understood. This report examines the pattern of cholinergic innervation throughout the murine main olfactory bulb across different developmental stages and in naris-occluded animals. To visualize the pattern of cholinergic innervation, we used a transgenic mouse model, which expresses a fusion of the microtubule-associated protein, tau, with green fluorescence protein (GFP) under the control of the choline acetyltransferase (ChAT) promoter. This tau-GFP fusion product allows for a remarkably vivid and clear visualization of cholinergic innervation in the main olfactory bulb (MOB). Interestingly, we find an uneven distribution of GFP label in the adult glomerular layer (GL), where anterior, medial, and lateral glomerular regions of the bulb receive relatively heavier cholinergic innervation than other regions. In contrast to previous reports, we find a marked change in the pattern of cholinergic innervation to the GL following unilateral naris occlusion between the ipsilateral and contralateral bulbs in adult animals.     

Cholinergic modulation of dopaminergic neurons in the mouse olfactory bulb

Considerable evidence exists for an extrinsic cholinergic influence in the maturation and function of the main olfactory bulb. In this study, we addressed the muscarinic modulation of dopaminergic neurons in this structure. We used different patch-clamp techniques to characterize the diverse roles of muscarinic agonists on identified dopaminergic neurons in a transgenic animal model expressing a reporter protein (green fluorescent protein) under the tyrosine hydroxylase promoter. Bath application of acetylcholine (1 mM) in slices and in enzymatically dissociated cells reduced the spontaneous firing of dopaminergic neurons recorded in cell-attached mode. In whole-cell configuration no effect of the agonist was observed, unless using the perforated patch technique, thus suggesting the involvement of a diffusible second messenger. The effect was mediated by metabotropic receptors as it was blocked by atropine and mimicked by the m2 agonist oxotremorine (10 muM). The reduction of periglomerular cell firing by muscarinic activation results from a membrane-potential hyperpolarization caused by activation of a potassium conductance. This modulation of dopaminergic interneurons may be important in the processing of sensory information and may be relevant to understand the mechanisms underlying the olfactory dysfunctions occurring in neurodegenerative diseases affecting the dopaminergic and/or cholinergic systems.     

Structural and immunohistological modifications in olfactory bulb of the staggerer mutant mouse.

In the present study, we describe the structural and cytological changes observed in staggerer mutant olfactory bulbs, as compared to normal mice. On the basis of photonic and ultrastructural observations we tried to define the alterations induced by the mutation: i.e. a reduction of bulb size, a reduction in the volume of three out of the six architectonic layers (glomerular, external and internal plexiform), a reduction of glomeruli size, a loss of half the mitral cells and a slight decrease in juxtaglomerular interneuron number. In staggerer, an hypertrophy of glial ensheathing cell processes was especially evident at the level of each glomerulus, whereas the density of the astrocyte network was weaker in the granular layer and the nerve layer not apparently impaired. An immunofluorescent labelling study combined with confocal scanning microscopy was performed in order to identify the cellular type and the differentiation degree of the various elements. Antibodies anti-GFAP, a protein present in both ensheathing cells and astrocytes, and anti-OMP, the specific maturation protein of the nerve layer, were used for that purpose. Data confirmed the reality of the gliosis and the persistence of the sensory component in the mutant. All the structural alterations described in staggerer olfactory bulb were in close agreement with the functional troubles previously recorded. Our results are discussed in connection with the present knowledge on embryonal origin, fetal development and adult cellular renewal of the olfactory bulb.     

Postnatal developmental expression of calbindin, calretinin and parvalbumin in mouse main olfactory bulb

The distribution of calbindin, calretinin and parvalbumin during the development of the mouse main olfactory bulb (MOB) was studied using immunohistochemistry techniques. The results are as follows:(1) calbindin-immunoreactive profiles were mainly located in the glomerular layer, and few large calbindin-immunoreactive cells were found in the subependymal layer of postnatal day 10 (P10) to postnatal day 40 (P40) mice; (2) no calbindin was detected in the mitral cell layer at any stage; (3) calretinin-immunoreactive profiles were present in all layers of the main olfactory bulb at all stages, especially in the olfactory nerve layer, glomerular layer and granule cell layer; (4) parvalbumin-immunoreactive profiles were mainly located in the external plexiform layer (except for P10 mice); (5) weakly stained parvalbumin-immunoreactive profiles were present in the glomerular layer at all stages; and (6) no parvalbumin was detected in the mitral cell layer at any stage.     

Representation of odorants by receptor neuron input to the mouse olfactory bulb.

To visualize odorant representations by receptor neuron input to the mouse olfactory bulb, we loaded receptor neurons with calcium-sensitive dye and imaged odorant-evoked responses from their axon terminals. Fluorescence increases reflected activation of receptor neuron populations converging onto individual glomeruli. We report several findings. First, five glomeruli were identifiable across animals based on their location and odorant responsiveness; all five showed complex response specificities. Second, maps of input were chemotopically organized at near-threshold concentrations but, at moderate concentrations, involved many widely distributed glomeruli. Third, the dynamic range of input to a glomerulus was greater than that reported for individual receptor neurons. Finally, odorant activation slopes could differ across glomeruli, and for different odorants activating the same glomerulus. These results imply a high degree of complexity in odorant representations at the level of olfactory bulb input.     

BDNF promoter-mediated beta-galactosidase expression in the olfactory epithelium and bulb

The neurotrophin brain-derived neurotrophic factor (BDNF) has been implicated in the generation and differentiation of new olfactory sensory neurons (OSNs) and in the regulation of branching of OSN axons in their target glomeruli. However, previous reports of BDNF mRNA and protein expression in olfactory epithelium and olfactory bulb (OB) have been inconsistent, raising questions on the proposed roles for BDNF. Here, we report on beta-galactosidase (beta-gal) expression in adult gene-targeted mice where the BDNF promoter drives expression of the Escherichia coli lacZ gene (BDNF(lacZneo) mice). We find that beta-gal is expressed in a small subset of OSNs with axons that reach the olfactory nerve layers throughout the OB. In the OB, we find expression of beta-gal in gamma-aminobutyric acidergic but not dopaminergic periglomerular cells and external tufted cells and in interneurons located in the mitral cell layer. Our results are inconsistent with the regulation of generation and differentiation of new OSNs elicited by the release of BDNF from horizontal basal cells. The results are consistent with a role for BDNF in competitive branching of OSN axons within the glomeruli of the OB.     

A sexually dimorphic group of atypical glomeruli in the mouse olfactory bulb

Atypical glomeruli (AtG) are clearly distinguishable from typical ones because of their strong cholinergic innervation. AtG are located in defined positions in the caudal half of the main olfactory bulb of rodents. The AtG partially overlap with other specialized olfactory subsystems, such as the modified glomerular complex, which is close to the accessory olfactory bulb. So far, possible sex differences in these specialised olfactory systems have not been investigated. In this work we have identified AtG in the mouse by means of acetylcholinesterase histochemistry and compared the number and size of these glomeruli between the sexes and also between the two strains that demonstrate intraglomerular synaptic differences, i.e. BALB/c and CD-1 mice. First, we divided the AtG into three types according to their position (I, rostral-most; II, around the accessory olfactory bulb; III, caudal-most) or their reactivity to acetylcholinesterase histochemistry (AtG type II being the least reactive glomeruli). ANOVA analyses revealed differences in the maximum diameter of glomeruli among the three types, but not in their sectional areas, indicating that all three types have different shapes. Moreover, both morphoplanimetric parameters were seen to be different between the two strains studied and also between the sexes: male mice and BALB/c animals had the largest glomeruli. The number of AtG was also significantly different between the sexes and strains, although these factors presented a strong interaction. Thus, the males had higher numbers of AtG in the CD-1 strain whereas in the BALB/c mice males demonstrated fewer AtG than females. These differences in number were largely due to AtG type II. The present work is evidence that AtG type II is a sexually dimorphic group of specialized glomeruli located in the main olfactory bulb.     

Functional topography of connections linking mirror-symmetric maps in the mouse olfactory bulb

In rodents, each main olfactory bulb contains two mirror-symmetric glomerular maps, a feature not found in the initial topographic maps of other sensory systems. Targeting tracer injections to identified glomeruli revealed that isofunctional odor columns-translaminar assemblies connected to a given glomerulus-were specifically and reciprocally interconnected through a mutually inhibitory circuit with exquisite topographic specificity. Thus, instead of containing two mirror-symmetric maps, we propose that the olfactory bulb contains a single integrated map in which isofunctional odor columns are connected through an intrabulbar link, analogous to the specific horizontal connections linking iso-orientation columns in primary visual cortex.     

达乌尔黄鼠犁鼻器和副嗅球的组织结构及嗅球c-Fos表达的季节变化

啮齿动物的犁鼻器和副嗅球与社会通讯和生殖行为有关,主嗅球影响其觅食行为。达乌尔黄鼠（Spermophilus dauricus）是一种具有较低社会行为的储脂类冬眠动物。本研究用组织学和免疫组织化学方法探究了其犁鼻器和副嗅球的结构特点及嗅球神经元活动对季节变化的适应。结果发现,达乌尔黄鼠犁鼻器具有较大的血管,犁鼻器管腔外侧为非感觉性的呼吸上皮（Respiratory epithelium,RE）,内侧为感觉上皮（Sensory epithelium,SE）,RE较SE薄,靠近管腔处为假复层柱状上皮。选取犁鼻器中间部位比较,发现SE的厚度、长度及感觉细胞密度均无性别差异。副嗅球位于主嗅球后方背内侧,由6层细胞构成。侧嗅束穿过副嗅球,位于颗粒细胞层之上。雄性达乌尔黄鼠较雌性有更长的僧帽细胞层和颗粒细胞层。春季（3月）和冬季（1月）达乌尔黄鼠主嗅球的嗅小球层、僧帽细胞层和颗粒细胞层的c-Fos-ir神经元密度显著低于夏季（7月）和秋季（10月）,且冬季外网织层的c-Fos-ir神经元密度显著低于夏季和秋季,说明达乌尔黄鼠在冬季和春季的嗅觉神经活动较弱,呈现出对冬眠的生理性适应。这些结果丰富了动物犁鼻器和副嗅球的形态学资料,并有助于理解冬眠动物嗅觉系统对季节变化和冬眠的适应。     

V-ATPase expression in the mouse olfactory epithelium

The vacuolar proton-pumping ATPase (V-ATPase) is responsible for the acidification of intracellular organelles and for the pH regulation of extracellular compartments. Because of the potential role of the latter process in olfaction, we examined the expression of V-ATPase in mouse olfactory epithelial (OE) cells. We report that V-ATPase is present in this epithelium, where we detected subunits ATP6V1A (the 70-kDa "A" subunit) and ATP6V1E1 (the ubiquitous 31-kDa "E" subunit isoform) in epithelial cells, nerve fiber cells, and Bowman's glands by immunocytochemistry. We also located both isoforms of the 56-kDa B subunit, ATP6V1B1 ("B1," typically expressed in epithelia specialized in regulated transepithelial proton transport) and ATP6V1B2 ("B2") in the OE. B1 localizes to the microvilli of the apical plasma membrane of sustentacular cells and to the lateral membrane in a subset of olfactory sensory cells, which also express carbonic anhydrase type IV, whereas B2 expression is stronger in the subapical domain of sustentacular cells. V-ATPase expression in mouse OE was further confirmed by immunoblotting. These findings suggest that V-ATPase may be involved in proton secretion in the OE and, as such, may be important for the pH homeostasis of the neuroepithelial mucous layer and/or for signal transduction in CO₂ detection. proton secretion; vacuolar H⁺-ATPase; immunofluorescence; pH homeostasis; olfaction     

Acid-sensing ion channels in mouse olfactory bulb M/T neurons

The olfactory bulb contains the first synaptic relay in the olfactory pathway, the sensory system in which odorants are detected enabling these chemical stimuli to be transformed into electrical signals and, ultimately, the perception of odor. Acid-sensing ion channels (ASICs), a family of proton-gated cation channels, are widely expressed in neurons of the central nervous system. However, no direct electrophysiological and pharmacological characterizations of ASICs in olfactory bulb neurons have been described. Using a combination of whole-cell patch-clamp recordings and biochemical and molecular biological analyses, we demonstrated that functional ASICs exist in mouse olfactory bulb mitral/tufted (M/T) neurons and mainly consist of homomeric ASIC1a and heteromeric ASIC1a/2a channels. ASIC activation depolarized cultured M/T neurons and increased their intracellular calcium concentration. Thus, ASIC activation may play an important role in normal olfactory function.     

New automated fluorometric peptide microassay for carnosine in mouse olfactory bulb

A procedure for assaying peptides at the picomole level in tissue extracts has been developed and used to measure the dipeptide carnosine in mouse olfactory bulb. In this procedure the tissue extract is reacted with 2-methoxy-2,4-diphenyl-3(2H)-furanone (MDPF), and the resultant fluorophors are resolved on a high performance reverse-phase column. Quantitation is performed in a filter fluorometer equipped with a flow cell. Carnosine was found to be present at a level of 1.93 ± 0.44 nmol/mg of tissue (mean + SD of 11 samples), in agreement with previous findings by other methods.