Glomerular Layer Research Articles

Localization of α1-2 Fucose Glycan in the Mouse Olfactory Pathway

Glycoconjugates in the olfactory system play critical roles in neuronal formation, and α1-2 fucose (α1-2Fuc) glycan mediates neurite outgrowth and synaptic plasticity. Histochemical findings of α1-2Fuc glycan in the mouse olfactory system detected using Ulex europaeus agglutinin-I (UEA-I) vary. This study histochemically assessed the main olfactory and vomeronasal pathways in male and female ICR and C57BL/6J mice aged 3-4 months using UEA-I. Ulex europaeus agglutinin-I reacted with most receptor cells arranged mainly at the basal region of the olfactory epithelium. The olfactory nerve layer and glomerular layer of the main olfactory bulb were speckled with positive UEA-I staining, and positive fibers were scattered from the glomerular to the internal plexiform layer. The lateral olfactory tract and rostral migratory stream were also positive for UEA-I. We identified superficial short-axon cells, interneurons of the external plexiform layer, external, middle and internal tufted cells, mitral cells and granule cells as the origins of the UEA-I-positive fibers in the main olfactory bulb. The anterior olfactory nucleus, anterior piriform cortex and olfactory tubercle were negative for UEA-I. Most receptor cells in the vomeronasal epithelium and most glomeruli of the accessory olfactory bulb were positive for UEA-I. Our findings indicated that α1-2Fuc glycan is located within the primary and secondary, but not the ternary, pathways of the main olfactory system, in local circuits of the main olfactory bulb and within the primary, but not secondary, pathway of the vomeronasal system.

Olfactory Sensory Activity Modulates Microglial-Neuronal Interactions during Dopaminergic Cell Loss in the Olfactory Bulb.

The mammalian olfactory bulb (OB) displays robust activity-dependent plasticity throughout life. Dopaminergic (DA) neurons in the glomerular layer (GL) of the OB are particularly plastic, with loss of sensory input rapidly reducing tyrosine hydroxylase (TH) expression and dopamine production, followed by a substantial reduction in DA neuron number. Here, we asked whether microglia participate in activity-dependent elimination of DA neurons in the mouse OB. Interestingly, we found a significant reduction in the number of both DA neurons and their synapses in the OB ipsilateral to the occluded naris (occluded OB) within just 7 days of sensory deprivation. Concomitantly, the volume of the occluded OB decreased, resulting in an increase in microglial density. Microglia in the occluded OB also adopted morphologies consistent with activation. Using in vivo 2-photon imaging and histological analysis we then showed that loss of olfactory input markedly altered microglial-neuronal interactions during the time that DA neurons are being eliminated: both microglial process motility and the frequency of wrapping of DA neuron somata by activated microglia increased significantly in the occluded OB. Furthermore, we found microglia in the occluded OB that had completely engulfed components of DA neurons. Together, our data provide evidence that loss of olfactory input modulates microglial-DA neuron interactions in the OB, thereby suggesting an important role for microglia in the activity-dependent elimination of DA neurons and their synapses.

Glomerular and Mitral-Granule Cell Microcircuits Coordinate Temporal and Spatial Information Processing in the Olfactory Bulb.

The olfactory bulb processes inputs from olfactory receptor neurons (ORNs) through two levels: the glomerular layer at the site of input, and the granule cell level at the site of output to the olfactory cortex. The sequence of action of these two levels has not yet been examined. We analyze this issue using a novel computational framework that is scaled up, in three-dimensions (3D), with realistic representations of the interactions between layers, activated by simulated natural odors, and constrained by experimental and theoretical analyses. We suggest that the postulated functions of glomerular circuits have as their primary role transforming a complex and disorganized input into a contrast-enhanced and normalized representation, but cannot provide for synchronization of the distributed glomerular outputs. By contrast, at the granule cell layer, the dendrodendritic interactions mediate temporal decorrelation, which we show is dependent on the preceding contrast enhancement by the glomerular layer. The results provide the first insights into the successive operations in the olfactory bulb, and demonstrate the significance of the modular organization around glomeruli. This layered organization is especially important for natural odor inputs, because they activate many overlapping glomeruli.

Cell-Type-Specific Modulation of Sensory Responses in Olfactory Bulb Circuits by Serotonergic Projections from the Raphe Nuclei.

Serotonergic neurons in the brainstem raphe nuclei densely innervate the olfactory bulb (OB), where they can modulate the initial representation and processing of olfactory information. Serotonergic modulation of sensory responses among defined OB cell types is poorly characterized in vivo Here, we used cell-type-specific expression of optical reporters to visualize how raphe stimulation alters sensory responses in two classes of GABAergic neurons of the mouse OB glomerular layer, periglomerular (PG) and short axon (SA) cells, as well as mitral/tufted (MT) cells carrying OB output to piriform cortex. In PG and SA cells, brief (1-4 s) raphe stimulation elicited a large increase in the magnitude of responses linked to inhalation of ambient air, as well as modest increases in the magnitude of odorant-evoked responses. Near-identical effects were observed when the optical reporter of glutamatergic transmission iGluSnFR was expressed in PG and SA cells, suggesting enhanced excitatory input to these neurons. In contrast, in MT cells imaged from the dorsal OB, raphe stimulation elicited a strong increase in resting GCaMP fluorescence with only a slight enhancement of inhalation-linked responses to odorant. Finally, optogenetically stimulating raphe serotonergic afferents in the OB had heterogeneous effects on presumptive MT cells recorded extracellularly, with an overall modest increase in resting and odorant-evoked responses during serotonergic afferent stimulation. These results suggest that serotonergic afferents from raphe dynamically modulate olfactory processing through distinct effects on multiple OB targets, and may alter the degree to which OB output is shaped by inhibition during behavior. Modulation of the circuits that process sensory information can profoundly impact how information about the external world is represented and perceived. This study investigates how the serotonergic system modulates the initial processing of olfactory information by the olfactory bulb, an obligatory relay between sensory neurons and cortex. We find that serotonergic projections from the raphe nuclei to the olfactory bulb dramatically enhance the responses of two classes of inhibitory interneurons to sensory input, that this effect is mediated by increased glutamatergic drive onto these neurons, and that serotonergic afferent activation alters the responses of olfactory bulb output neurons in vivo These results elucidate pathways by which neuromodulatory systems can dynamically regulate brain circuits during behavior.

Evidence for a Notch1-mediated transition during olfactory ensheathing cell development.

Olfactory ensheathing cells (OECs) are a unique glial population found in both the peripheral and central nervous system: they ensheath bundles of unmyelinated olfactory axons from their peripheral origin in the olfactory epithelium to their central synaptic targets in the glomerular layer of the olfactory bulb. Like all other peripheral glia (Schwann cells, satellite glia, enteric glia), OECs are derived from the embryonic neural crest. However, in contrast to Schwann cells, whose development has been extensively characterised, relatively little is known about their normal development in vivo. In the Schwann cell lineage, the transition from multipotent Schwann cell precursor to immature Schwann cell is promoted by canonical Notch signalling. Here, in situ hybridisation and immunohistochemistry data from chicken, mouse and human embryos are presented that suggest a canonical Notch‐mediated transition also occurs during OEC development.

Increased olfactory bulb acetylcholine bi-directionally modulates glomerular odor sensitivity.

The glomerular layer of the olfactory bulb (OB) receives heavy cholinergic input from the horizontal limb of the diagonal band of Broca (HDB) and expresses both muscarinic and nicotinic acetylcholine (ACh) receptors. However, the effects of ACh on OB glomerular odor responses remain unknown. Using calcium imaging in transgenic mice expressing the calcium indicator GCaMP2 in the mitral/tufted cells, we investigated the effect of ACh on the glomerular responses to increasing odor concentrations. Using HDB electrical stimulation and in vivo pharmacology, we find that increased OB ACh leads to dynamic, activity-dependent bi-directional modulation of glomerular odor response due to the combinatorial effects of both muscarinic and nicotinic activation. Using pharmacological manipulation to reveal the individual receptor type contributions, we find that m2 muscarinic receptor activation increases glomerular sensitivity to weak odor input whereas nicotinic receptor activation decreases sensitivity to strong input. Overall, we found that ACh in the OB increases glomerular sensitivity to odors and decreases activation thresholds. This effect, along with the decreased responses to strong odor input, reduces the response intensity range of individual glomeruli to increasing concentration making them more similar across the entire concentration range. As a result, odor representations are more similar as concentration increases.

A Transgenic Rat for Specifically Inhibiting Adult Neurogenesis.

The growth of research on adult neurogenesis and the development of new models and tools have greatly advanced our understanding of the function of newborn neurons in recent years. However, there are still significant limitations in the ability to identify the functions of adult neurogenesis in available models. Here we report a transgenic rat (TK rat) that expresses herpes simplex virus thymidine kinase in GFAP+ cells. Upon treating TK rats with the antiviral drug valganciclovir, granule cell neurogenesis can be completely inhibited in adulthood, in both the hippocampus and olfactory bulb. Interestingly, neurogenesis in the glomerular and external plexiform layers of the olfactory bulb was only partially inhibited, suggesting that some adult-born neurons in these regions derive from a distinct precursor population that does not express GFAP. Within the hippocampus, blockade of neurogenesis was rapid and nearly complete within 1 week of starting treatment. Preliminary behavioral analyses indicate that general anxiety levels and patterns of exploration are generally unaffected in neurogenesis-deficient rats. However, neurogenesis-deficient TK rats showed reduced sucrose preference, suggesting deficits in reward-related behaviors. We expect that TK rats will facilitate structural, physiological, and behavioral studies that complement those possible in existing models, broadly enhancing understanding of the function of adult neurogenesis.

CaMKII mediates stimulus specificity in early odor preference learning in rats.

After naturalistic odor preference training, Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) was rapidly phosphorylated in the olfactory bulb, specifically in the odor encoding regions of the glomerular layer and external plexiform layer. Intrabulbar CaMKII antagonist experiments revealed that CaMKII supports short- and long-term preference memory formation. With bulbar PKA activation as the unconditioned stimulus odor preferences could be induced despite CaMKII blockade, but now odor specificity was lost, with odor preference generalizing to an untrained odor. Odor-specific learning was associated with increased membrane-associated AMPA receptors, while nonspecific odor preference was not. Thus CaMKII activation provides a tag to confer stimulus specificity as well as supporting natural odor preference learning.

Feasibility of fluorescence energy transfer system for imaging the renoprotective effects of aliskiren in diabetic mice.

Introduction:We investigated the feasibility of using a fluorescence resonance energy transfer system to image enzymatic activity in order to evaluate the effects of aliskiren (a direct renin inhibitor) on diabetic nephropathy.Materials and methods:First, we induced diabetes in C57BL/6J mice using streptozotocin, then treated them with either aliskiren (25 mg/kg/day) or the angiotensin type 1 receptor blocker valsartan (15 mg/kg/day) for four weeks. Finally, we utilized renin fluorescence resonance energy transfer substrate to assess renin activity.Results:Renin activity was much higher in the kidneys of diabetic mice compared to those of the non-diabetic control mice. While aliskiren inhibited this activity, valsartan did not. We noted that production of reactive oxygen species intensified and the bioavailability of nitric oxide diminished in the glomeruli of diabetic mice. Aliskiren and valsartan significantly ameliorated these effects. They suppressed glomerular production of reactive oxygen species and urinary albumin excretion. In fact, urinary albumin excretion in diabetic mice treated with aliskiren or valsartan was lower than that in untreated diabetic mice. Furthermore, aliskiren and valsartan significantly reduced glomerular permeability by maintaining the glomerular endothelial surface layer.Conclusion:Fluorescence resonance energy transfer could provide a new tool for evaluating tissue and plasma enzymatic activity.

Expression of receptor protein tyrosine phosphatase δ, PTPδ, in mouse central nervous system

Protein tyrosine phosphate δ (PTPδ), one of the receptor type IIa protein tyrosine phosphates, is known for its roles in axon guidance, synapse formation, cell adhesion, and tumor suppression. Alternative splicing of this gene generates at least four (A–D) isoforms; however, the major isoform in vivo is yet to be determined. The protein localization has neither been revealed. We have generated anti-mouse PTPδ-specific monoclonal antibody and analyzed the protein expression in wild-type and Ptpδ knockout mice. Immunoblot analysis of various organs revealed that neuronal tissues express both C-and D-isoforms of PTPδ, whereas non-neuronal tissues express only C-isoform. Immunohistochemistry of wild-type or Ptpδ heterozygous sections showed that olfactory bulb, cerebral cortex, hippocampus, cerebellum, and several nuclei in brain stem exhibit moderate to strong positive signals. These signals were absent in Ptpδ knockout specimens. Higher magnification revealed differences between expression patterns of PTPδ mRNA and its protein product. In hippocampus, weak mRNA expression in CA1 stratum pyramidale but strong immunostaining in the stratum lacunosum moleculare was observed, suggesting the axonal expression of PTPδ in the entorhinal cortical afferents. Olfactory mitral cells exhibited mRNA expression in cell bodies and protein localization in their dendritic fields, glomerular and external plexiform layers. Nissl staining showed that the external plexiform layer was reduced in Ptpδ knockout mice. Golgi-impregnation confirmed the poor dendritic growth of homozygous mitral cells. These results suggest that PTPδ may localize in axons as well as in dendrites to regulate their elaboration in the central nervous system.

Mapping oxygen concentration in the awake mouse brain.

Although critical for brain function, the physiological values of cerebral oxygen concentration have remained elusive because high-resolution measurements have only been performed during anesthesia, which affects two major parameters modulating tissue oxygenation: neuronal activity and blood flow. Using measurements of capillary erythrocyte-associated transients, fluctuations of oxygen partial pressure (Po2) associated with individual erythrocytes, to infer Po2 in the nearby neuropil, we report the first non-invasive micron-scale mapping of cerebral Po2 in awake, resting mice. Interstitial Po2 has similar values in the olfactory bulb glomerular layer and the somatosensory cortex, whereas there are large capillary hematocrit and erythrocyte flux differences. Awake tissue Po2 is about half that under isoflurane anesthesia, and within the cortex, vascular and interstitial Po2 values display layer-specific differences which dramatically contrast with those recorded under anesthesia. Our findings emphasize the importance of measuring energy parameters non-invasively in physiological conditions to precisely quantify and model brain metabolism.

Distributed Features of Vimentin-Containing Neural Precursor Cells in Olfactory Bulb of SOD1G93A Transgenic Mice: a Study about Resource of Endogenous Neural Stem Cells.

No any effective treatments can prevent from the motor neuron degeneration in amyotrophic lateral sclerosis (ALS) at present. In order to modulating the endogenous neural precursor cells (NPCs) to repairing the degenerative motor neurons in ALS, we studied the alteration of endogenous vimentin-containing NPCs in olfactory bulb (OB) at the different stages of SOD1 wlid-type and G93A transgenic mice. The results showed that the vimentin-containing cells (VCCs) were mainly distributed in the glomerular layer (Gl), the accessory OB (AOB), the OB core, the granular cell layer (GRO) and the mitral cell layer (MI)+the internal plexiform layer (IPL) of the OB of adult mice. Almost all VCCs in Gl, OB core and GRO were the GFAP positive cells. Almost all VCCs in AOB were the Oligo-2 positive cells. Fewer VCCs in MI+IPL were the NeuN positive cells. VCCs significantly increased in the OB core and Gl of adult OB at the pre-onset, onset and progression stages of ALS-like G93A transgenic disease, particularly in OB core. All increased VCCs were the GFAP positive cells. Our data suggested that there extensively existed the endogenous vimentin-containing NPCs in the OB of adult mice, which was a potential resource of neural stem cells, they could differentiate into astrocyte, oligodendrocyte and neuron cells, were a potential astrocyte neuroregenerative response in adult OB in the ALS-like disease, were a potential pathway to repair the degenerated motor neurons.

Comparison of glomerular activity patterns by fMRI and wide-field calcium imaging: Implications for principles underlying odor mapping

Functional imaging signals arise from distinct metabolic and hemodynamic events at the neuropil, but how these processes are influenced by pre- and post-synaptic activities need to be understood for quantitative interpretation of stimulus-evoked mapping data. The olfactory bulb (OB) glomeruli, spherical neuropil regions with well-defined neuronal circuitry, can provide insights into this issue. Optical calcium-sensitive fluorescent dye imaging (OICa2+) reflects dynamics of pre-synaptic input to glomeruli, whereas high-resolution functional magnetic resonance imaging (fMRI) using deoxyhemoglobin contrast reveals neuropil function within the glomerular layer where both pre- and post-synaptic activities contribute. We imaged odor-specific activity patterns of the dorsal OB in the same anesthetized rats with fMRI and OICa2+ and then co-registered the respective maps to compare patterns in the same space. Maps by each modality were very reproducible as trial-to-trial patterns for a given odor, overlapping by ~80%. Maps evoked by ethyl butyrate and methyl valerate for a given modality overlapped by ~80%, suggesting activation of similar dorsal glomerular networks by these odors. Comparison of maps generated by both methods for a given odor showed ~70% overlap, indicating similar odor-specific maps by each method. These results suggest that odor-specific glomerular patterns by high-resolution fMRI primarily tracks pre-synaptic input to the OB. Thus combining OICa2+ and fMRI lays the framework for studies of OB processing over a range of spatiotemporal scales, where OICa2+ can feature the fast dynamics of dorsal glomerular clusters and fMRI can map the entire glomerular sheet in the OB.

Gross and Microscopic Anatomy of the Olfactory Bulbs in the Chinchilla (Chinchilla lanigera)

Chinchillas are primarily used as laboratory animals in auditory pathways research, it is necessary to acquire detailed neuroanatomical data on other encephalon segments, in order to assess whether this species might also contribute as olfactory pathways experimental models. Thus, our research focused on providing a gross and microscopic description of the chinchilla’s olfactory bulbs. Five chinchilla encephalon samples were harvested from commercially slaughtered young adults. The pairs of olfactory bulb were isolated by dissection, macroscopically examined while still attached, then detached, fixed in buffered formalin, serially cut and stained using a modified Luxol-Cresyl protocol. Gross and microscopic features were assessed and compared to literature descriptions of related species such as the rat and the rabbit. Chinchillas presented large olfactory bulbs with an average maximal diameter of 3 mm each, positioned in two corresponding deep grooves of the cribriform plate, orally to the anterior limit of the cerebral hemispheres. They decreased in girth ventro-aborally towards their base, which was located on the ventral side of the telencephalon. Histologically, they presented a laminar structure: an olfactory nerve layer, a glomerular layer, an external plexiform layer, a mitral cell layer, an internal plexiform layer and a granule cell layer. The proportionally well developed olfactory bulbs and the existence of the accessory olfactory bulbs demonstrates that C. lanigera is a macrosmatic species. The olfactory bulbs in the chinchilla present the gross anatomy and histological outline concurring with literature rodent descriptions, with certain particularities (accessory bulb topography, olfactory ventricle topography, glomerular size and shape).

Layer-Specific fMRI Responses to Excitatory and Inhibitory Neuronal Activities in the Olfactory Bulb.

Functional magnetic resonance imaging (fMRI) is a noninvasive, in vivo technique widely used to map function of the entire brain, including deep structures, in animals and humans. However, it measures neuronal activity indirectly by way of the vascular response. It is currently unclear how finely the hemodynamic response is regulated within single cortical layers and whether increased inhibitory neuronal activities affect fMRI signal changes. Both laminar specificity and the neural origins of fMRI are important to interpret functional maps properly, which we investigated by activating discrete rat olfactory bulb circuits.

Habituation of glomerular responses in the olfactory bulb following prolonged odor stimulation reflects reduced peripheral input.

Following prolonged odor stimulation, output from olfactory bulb (OB) mitral/tufted (M/T) cells is decreased in response to subsequent olfactory stimulation. Currently, it is unclear if this decrease is a function of adaptation of peripheral olfactory sensory neuron (OSN) responses or reflects depression of bulb circuits. We used wide-field calcium imaging in anesthetized transgenic GCaMP2 mice to compare excitatory glomerular layer odor responses before and after a 30-s odor stimulation. Significant habituation of subsequent glomerular odor responses to both the same and structurally similar odorants was detected with our protocol. To test whether depression of OSN terminals contributed to this habituation, olfactory nerve layer (ON) stimulation was used to drive glomerular layer responses in the absence of peripheral odor activation of the OSNs. Following odor habituation, in contrast to odor-evoked glomerular responses, ON stimulation-evoked glomerular responses were not habituated. The difference in response between odor and electrical stimulation following odor habituation provides evidence that odor response reductions measured in the glomerular layer of the OB are most likely the result of OSN adaptation processes taking place in the periphery.

Harvesting of olfactory ensheathing cells for autologous transplantation into the spinal cord injury. Its complexity in dogs.

Harvesting of olfactory ensheathing cells for autologous transplantation into the spinal cord injury. Its complexity in dogs.

Gender and age related changes in number of dopaminergic neurons in adult human olfactory bulb

IntroductionDopamine is one of the major brain neurotransmitters, and the loss of dopaminergic neurons in basal ganglia cause motor deficits in Parkinson's disease. We proposed that the difficulty in olfaction observed in the elderly may be due to an alteration in the number of dopaminergic neurons. Materials and methodsSections were taken from olfactory bulbs of post-mortem tissue specimens of 13 humans, males and females, aged from 19 to 63 years (≤35 and ≥50 years), with no history of neurological disorders. The tissues were fixed, embedded, cut on a freezing microtome, and prepared for immunohistochemical analysis using tyrosine hydroxylase (TH) and aromatic l-amino acid decarboxylase (AADC) antibodies. The number of positive neurons was counted. ResultsTH- and AADC-positive cells were present in the glomerular layer. There was no significant difference between the numbers of TH- and AADC-positive cells, in males and females, and in young and elderly individuals. The quantitative analysis revealed that the number of TH- and AADC-positive neurons were significantly higher in males than in females (P<0.05). Moreover, there was a significant increase in the number of TH- and AADC-positive neurons in the olfactory bulbs of the elderly compared with young individuals (P<0.05). ConclusionFactors such as gender and age may affect the number of dopaminergic neurons, and there is a correlation between increased dopaminergic neurons and olfactory performance. Moreover, the increase in dopaminergic cells in the olfactory bulb of the elderly may indicate the existence of rostral migratory stream in adult humans.

Deficits of olfactory interneurons in polysialyltransferase- and NCAM-deficient mice.

The neurogenic niche of the anterior subventricular zone (SVZ) persistently generates neuroblasts, which migrate along the rostral migratory stream (RMS) into the olfactory bulb (OB), where they differentiate into granule and periglomerular cells. Loss of the neural cell adhesion molecule NCAM or its post-translational modification polysialic acid (polySia) impairs migration causing accumulations of cells in the proximal RMS and decreased OB volume. Polysialylation of NCAM is implemented by two polysialyltransferases, ST8SIA2 and ST8SIA4, with overlapping functions. Here, we used mice with Ncam1 and polysialyltransferase deletions to analyze how partial or complete loss of polySia synthesis or a combined loss of polySia and NCAM affects the RMS and the interneuron composition in the OB. Numerous calretinin (CR)-positive cells were detected dispersed around the RMS in Ncam1 knockout, St8sia2, St8sia4 double-knockout, and St8sia2, St8sia4, Ncam1 triple-knockout mice, as well as in St8sia2(-/-) but not in St8sia4(-/-) mice. These changes were not reflected by reductions of CR-positive cells in the granule or glomerular layer of the OB. Instead, calbindin-positive periglomerular interneurons were strongly reduced in all polySia-NCAM negative mice and slightly attenuated in St8sia2(-/-) as well as in the St8sia4(-/-) mice, which were devoid of ectopic CR-positive cells along the RMS. Consistent with the early developmental generation of calbindin- as compared with CR-positive OB interneurons, this phenotype was fully developed at postnatal day 5. Together, these results demonstrate that the early development of calbindin-positive periglomerular interneurons depends on the presentation of polySia on NCAM and requires the activity of both polysialyltransferases.

Signaling between periglomerular cells reveals a bimodal role for GABA in modulating glomerular microcircuitry in the olfactory bulb

In the mouse olfactory bulb glomerulus, the GABAergic periglomerular (PG) cells provide a major inhibitory drive within the microcircuit. Here we examine GABAergic synapses between these interneurons. At these synapses, GABA is depolarizing and exerts a bimodal control on excitability. In quiescent cells, activation of GABAA receptors can induce the cells to fire, thereby providing a means for amplification of GABA release in the glomerular microcircuit via GABA-induced GABA release. In contrast, GABA is inhibitory in neurons that are induced to fire tonically. PG-PG interactions are modulated by nicotinic acetylcholine receptors (nAChRs), and our data suggest that changes in intracellular calcium concentrations triggered by nAChR activation can be amplified by GABA release. Our results suggest that bidirectional control of inhibition in PG neurons can allow for modulatory inputs, like the cholinergic inputs from the basal forebrain, to determine threshold set points for filtering out weak olfactory inputs in the glomerular layer of the olfactory bulb via the activation of nAChRs.