Spontaneous calcium transients in the immature adult-born neurons of the olfactory bulb

Spontaneous neuronal activity and concomitant intracellular Ca²⁺ signaling are abundant during early perinatal development and are well known for their key role in neuronal proliferation, migration, differentiation and wiring. However, much less is known about the in vivo patterns of spontaneous Ca²⁺ signaling in immature adult-born cells. Here, by using two-photon Ca²⁺ imaging, we analyzed spontaneous in vivo Ca²⁺ signaling in adult-born juxtaglomerular cells of the mouse olfactory bulb over the time period of 5 weeks, from the day of their arrival in the glomerular layer till their stable integration into the preexisting neural network. We show that spontaneous Ca²⁺ transients are ubiquitously present in adult-born cells right after their arrival, require activation of voltage-gated Na⁺ channels and are little sensitive to isoflurane anesthesia. Interestingly, several parameters of this spontaneous activity, such as the area under the curve, the time spent in the active state as well as the fraction of continuously active cells show a bell-shaped dependence on cell’s age, all peaking in 3–4 weeks old cells. This data firmly document the in vivo presence of spontaneous Ca²⁺ signaling during the layer-specific maturation of adult-born neurons in the olfactory bulb and motivate further analyses of the functional role(s) of this activity.     

Adiponectin Enhances the Responsiveness of the Olfactory System

The peptide hormone adiponectin is secreted by adipose tissue and the circulating concentration is reversely correlated with body fat mass; it is considered as starvation signal. The observation that mature sensory neurons of the main olfactory epithelium express the adiponectin receptor 1 has led to the concept that adiponectin may affect the responsiveness of the olfactory system. In fact, electroolfactogram recordings from olfactory epithelium incubated with exogenous adiponectin resulted in large amplitudes upon odor stimulation. To determine whether the responsiveness of the olfactory sensory neurons was enhanced, we have monitored the odorant-induced expression of the immediate early gene Egr1. It was found that in an olfactory epithelium incubated with nasally applied adiponectin the number of Egr1 positive cells was significantly higher compared to controls, suggesting that adiponectin rendered the olfactory neurons more responsive to an odorant stimulus. To analyze whether the augmented responsiveness of sensory neurons was strong enough to elicit a higher neuronal activity in the olfactory bulb, the number of activated periglomerular cells of a distinct glomerulus was determined by monitoring the stimulus-induced expression of c-fos. The studies were performed using the transgenic mOR256-17-IRES-tauGFP mice which allowed to visualize the corresponding glomerulus and to stimulate with a known ligand. The data indicate that upon exposure to 2,3-hexanedione in adiponectin-treated mice the number of activated periglomerular neurons was significantly increased compared to controls. The results of this study indicate that adiponectin increases the responsiveness of the olfactory system, probably due to a higher responsiveness of olfactory sensory neurons.     

In Vivo Study of Dynamics and Stability of Dendritic Spines on Olfactory Bulb Interneurons in Xenopus laevis Tadpoles

Dendritic spines undergo continuous remodeling during development of the nervous system. Their stability is essential for maintaining a functional neuronal circuit. Spine dynamics and stability of cortical excitatory pyramidal neurons have been explored extensively in mammalian animal models. However, little is known about spiny interneurons in non-mammalian vertebrate models. In the present study, neuronal morphology was visualized by single-cell electroporation. Spiny neurons were surveyed in the Xenopus tadpole brain and observed to be widely distributed in the olfactory bulb and telencephalon. DsRed- or PSD95-GFP-expressing spiny interneurons in the olfactory bulb were selected for in vivo time-lapse imaging. Dendritic protrusions were classified as filopodia, thin, stubby, or mushroom spines based on morphology. Dendritic spines on the interneurons were highly dynamic, especially the filopodia and thin spines. The stubby and mushroom spines were relatively more stable, although their stability significantly decreased with longer observation intervals. The 4 spine types exhibited diverse preferences during morphological transitions from one spine type to others. Sensory deprivation induced by severing the olfactory nerve to block the input of mitral/tufted cells had no significant effects on interneuron spine stability. Hence, a new model was established in Xenopus laevis tadpoles to explore dendritic spine dynamics in vivo.     

Interneurons versus efferent neurons: heterogeneity in their neurite outgrowth response to glia from several brain regions

We have studied in vitro the morphology of two populations of dopaminergic neurons from mouse embryos: the periglomerular interneurons from the olfactory bulb (DOBI) and the efferent neurons from the substantia nigra (DENN). The intrinsic potential of both neuronal types has been studied by comparing process outgrowth in a predominantly neuronal environment or in a glial environment that is endogenous or from other brain regions. Both populations exhibit in vitro different characteristics that reflect their phenotype in situ. In addition they greatly differ in their response to glial signals. DOBI maintain a constant stellate morphology with short processes under all culture conditions tested, whereas DENN exhibit a great plasticity and in particular respond to olfactory bulb glia with a striking increase in neurite length. The olfactory bulb glia differs from other brain region glia in two aspects: (a) in addition to type I astrocytes, common to all the glial monolayers that we have studied, it contains a population of fusiform astrocytes (GFAP+) that might represent the superficial glia (Raisman, 1985); and (b) both astrocytes and fusiform cells produce large amounts of laminin that is secreted in a thick extracellular matrix. DENN outgrowth on olfactory bulb glia, however, is not blocked by antilaminin antibodies that block outgrowth on a laminin substrate. Our results demonstrate that two neuronal populations sharing the same neurotransmitter present intrinsic differences in the control of cell shape. The fact that glia harvested from different brain regions supports varying extent of DENN neurite outgrowth suggests a heterogeneity of environmental signals throughout the developing brain.     

Olfactory bulb axonal outgrowth is inhibited by draxin

Olfactory bulb (OB) projection neurons receive sensory input from olfactory receptor neurons and precisely relay it through their axons to the olfactory cortex. Thus, olfactory bulb axonal tracts play an important role in relaying information to the higher order of olfactory structures in the brain. Several classes of axon guidance molecules influence the pathfinding of the olfactory bulb axons. Draxin, a recently identified novel class of repulsive axon guidance protein, is essential for the formation of forebrain commissures and can mediate repulsion of diverse classes of neurons from chickens and mice. In this study, we have investigated the draxin expression pattern in the mouse telencephalon and its guidance functions for OB axonal projection to the telencephalon. We have found that draxin is expressed in the neocortex and septum at E13 and E17.5 when OB projection neurons form the lateral olfactory tract (LOT) rostrocaudally along the ventrolateral side of the telencephalon. Draxin inhibits axonal outgrowth from olfactory bulb explants in vitro and draxin-binding activity in the LOT axons in vivo is detected. The LOT develops normally in draxin−/− mice despite subtle defasciculation in the tract of these mutants. These results suggest that draxin functions as an inhibitory guidance cue for OB axons and indicate its contribution to the formation of the LOT.     

Short-term Effects of Endothelins on Tyrosine Hydroxylase Activity and Expression in the Olfactory Bulb of Normotensive Rats

The olfactory system in rats is part of the limbic region with extensive afferent connections with brain areas involved in the regulation of behaviour and autonomic responses. The existence of the endothelin system and catecholaminergic neurons in the olfactory bulb suggests that endothelins may modulate noradrenergic transmission and diverse olfactory mediated processes. In the present work we studied the effect of endothelin-1 and -3 on neuronal norepinephrine release and the short-term regulation of tyrosine hydroxylase in the olfactory bulb. Results showed that both endothelins increased tyrosine hydroxylase activity through the activation of a non-conventional endothelin G-protein coupled receptor, coupled to the stimulation of protein kinase A and C, as well as Ca²⁺/calmodulin-dependent protein kinase II. On the other hand, neither endothelin-1 nor endothelin-3 modified tyrosine hydroxylase total protein levels, but both peptides increased the phosphorylation of serine residues of the enzyme at sites 19 and 40. Furthermore, endothelins enhanced norepinephrine release in olfactory neurons suggesting that this event may contribute to increased tyrosine hydroxylase activity by reducing the feedback inhibition. Taken together present findings show a clear interaction between the endothelin system, and the catecholaminergic transmission in the olfactory bulb. Additional studies are required to evaluate the physiological functions regulated by endothelins at this brain level.     

The source of spontaneous activity in the main olfactory bulb of the rat

Introduction

In vivo, most neurons in the main olfactory bulb exhibit robust spontaneous activity. This paper tests the hypothesis that spontaneous activity in olfactory receptor neurons drives much of the spontaneous activity in mitral and tufted cells via excitatory synapses.

Methods

Single units were recorded in vivo from the main olfactory bulb of a rat before, during, and after application of lidocaine to the olfactory nerve. The effect of lidocaine on the conduction of action potentials from the olfactory epithelium to the olfactory bulb was assessed by electrically stimulating the olfactory nerve rostral to the application site and monitoring the field potential evoked in the bulb.

Results

Lidocaine caused a significant decrease in the amplitude of the olfactory nerve evoked field potential that was recorded in the olfactory bulb. By contrast, the lidocaine block did not significantly alter the spontaneous activity of single units in the bulb, nor did it alter the field potential evoked by electrical stimulation of the lateral olfactory tract. Lidocaine block also did not change the temporal patters of action potential or their synchronization with respiration.

Conclusions

Spontaneous activity in neurons of the main olfactory bulb is not driven mainly by activity in olfactory receptor neurons despite the extensive convergence onto mitral and tufted cells. These results suggest that spontaneous activity of mitral and tufted is either an inherent property of these cells or is driven by centrifugal inputs to the bulb.     

Olfactory maps and odor images

The understanding of neuronal processing of olfactory stimuli has been furthered by genetic studies and specialized imaging of particular neuronal populations. Selective optical imaging of odor-induced presynaptic and postsynaptic glomerular activity in the olfactory bulb/antennal lobe has visualized odorant-responsive receptor repertoires and shown a more confined odor image at the level of projection neurons compared to their olfactory receptor neuron input. Genetic tracing of projection neurons connected to particular glomeruli has revealed a somewhat dispersed spatial map of termination areas for these neurons both in insects and in vertebrates. Modifications of the glomerular odor map have resulted in altered percepts of the corresponding odors.     

Simultaneous Recording of Calcium Signals from Identified Neurons and Feeding Behavior of Drosophila melanogaster

To study neuronal networks in terms of their function in behavior, we must analyze how neurons operate when each behavioral pattern is generated. Thus, simultaneous recordings of neuronal activity and behavior are essential to correlate brain activity to behavior. For such behavioral analyses, the fruit fly, Drosophila melanogaster, allows us to incorporate genetically encoded calcium indicators such as GCaMP¹, to monitor neuronal activity, and to use sophisticated genetic manipulations for optogenetic or thermogenetic techniques to specifically activate identified neurons^2-5. Use of a thermogenetic technique has led us to find critical neurons for feeding behavior (Flood et al., under revision). As a main part of feeding behavior, a Drosophila adult extends its proboscis for feeding⁶ (proboscis extension response; PER), responding to a sweet stimulus from sensory cells on its proboscis or tarsi. Combining the protocol for PER⁷ with a calcium imaging technique⁸ using GCaMP3.0^{1, 9}, I have established an experimental system, where we can monitor activity of neurons in the feeding center – the suboesophageal ganglion (SOG), simultaneously with behavioral observation of the proboscis. I have designed an apparatus ("Fly brain Live Imaging and Electrophysiology Stage": "FLIES") to accommodate a Drosophila adult, allowing its proboscis to freely move while its brain is exposed to the bath for Ca²⁺ imaging through a water immersion lens. The FLIES is also appropriate for many types of live experiments on fly brains such as electrophysiological recording or time lapse imaging of synaptic morphology. Because the results from live imaging can be directly correlated with the simultaneous PER behavior, this methodology can provide an excellent experimental system to study information processing of neuronal networks, and how this cellular activity is coupled to plastic processes and memory.     

Efficient neuronal in vitro and in vivo differentiation after immunomagnetic purification of mESC derived neuronal precursors

The cellular heterogeneity that is generated during the differentiation of pluripotent stem cells into specific neural subpopulations represents a major obstacle for experimental and clinical progress. To address this problem we developed an optimized strategy for magnetic isolation of PSA-NCAM positive neuronal precursors from embryonic stem cells (ESCs) derived neuronal cultures. PSA-NCAM enrichment at an early step of the in vitro differentiation process increased the number of ES cell derived neurons and reduced cellular diversity. Gene expression analysis revealed that mainly genes involved in neuronal activity were over-represented after purification. In vitro derived PSA-NCAM⁺ enriched precursors were characterized in vivo through grafting into the forebrain of adult mice. While unsorted control cells 40 days post graft gave rise to a mixed population composed of immature precursors, early postmitotic neurons and glial cells, PSA-NCAM⁺ enriched cells differentiated predominantly into NeuN positive cells. Furthermore, PSA-NCAM enriched population showed efficient migration towards the olfactory bulb after transplantation into the rostral migratory stream of the forebrain neurogenic system. Thus, enrichment of neuronal precursors based on PSA-NCAM expression represents a general and straightforward approach to narrow cellular heterogeneity during neuronal differentiation of pluripotent cells.     

Hyperpolarization-activated and cyclic nucleotide-gated channels are differentially expressed in juxtaglomerular cells in the olfactory bulb of mice

In the olfactory bulb, input from olfactory receptor neurons is processed by neuronal networks before it is relayed to higher brain regions. In many neurons, hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels generate and control oscillations of the membrane potential. Oscillations also appear crucial for information processing in the olfactory bulb. Four channel isoforms exist (HCN1–HCN4) that can form homo- or heteromers. Here, we describe the expression pattern of HCN isoforms in the olfactory bulb of mice by using a novel and comprehensive set of antibodies against all four isoforms. HCN isoforms are abundantly expressed in the olfactory bulb. HCN channels can be detected in most cell populations identified by commonly used marker antibodies. The combination of staining with marker and HCN antibodies has revealed at least 17 different staining patterns in juxtaglomerular cells. Furthermore, HCN isoforms give rise to an unexpected wealth of co-expression patterns but are rarely expressed in the same combination and at the same level in two given cell populations. Therefore, heteromeric HCN channels may exist in several cell populations in vivo. Our results suggest that HCN channels play an important role in olfactory information processing. The staining patterns are consistent with the possibility that both homomeric and heteromeric HCN channels are involved in oscillations of the membrane potential of juxtaglomerular cells.     

Roles of odorant receptors in projecting axons in the mouse olfactory system

In the mouse olfactory epithelium, there are about ten million olfactory sensory neurons, each expressing a single type of odorant receptor out of approximately 1000. Olfactory sensory neurons expressing the same odorant receptor converge their axons to a specific set of glomeruli on the olfactory bulb. How odorant receptors play an instructive role in the projection of axons to the olfactory bulb has been one of the major issues of developmental neurobiology. Recent studies revealed previously overlooked roles of odorant receptor-derived cAMP signals in the axonal projection of olfactory sensory neurons; the levels of cAMP and neuronal activity appear to determine the expression levels of axon guidance/sorting molecules and thereby direct the axonal projection of olfactory sensory neurons. These findings provide new insights as to how peripheral inputs instruct neuronal circuit formation in the mammalian brain.     

Shh‐proteoglycan interactions regulate maturation of olfactory glomerular circuitry

The olfactory system relies on precise circuitry connecting olfactory sensory neurons (OSNs) and appropriate relay and processing neurons of the olfactory bulb (OB). In mammals, the exact correspondence between specific olfactory receptor types and individual glomeruli enables a spatially precise map of glomerular activation that corresponds to distinct odors. However, the mechanisms that govern the establishment and maintenance of the glomerular circuitry are largely unknown. Here we show that high levels of Sonic Hedgehog (Shh) signaling at multiple sites enable refinement and maintenance of olfactory glomerular circuitry. Mice expressing a mutant version of Shh (Shh^Ala/Ala), with impaired binding to proteoglycan co‐receptors, exhibit disproportionately small olfactory bulbs containing fewer glomeruli. Notably, in mutant animals the correspondence between individual glomeruli and specific olfactory receptors is lost, as olfactory sensory neurons expressing different olfactory receptors converge on the same glomeruli. These deficits arise at late stages in post‐natal development and continue into adulthood, indicating impaired pruning of erroneous connections within the olfactory bulb. In addition, mature Shh^Ala/Ala mice exhibit decreased proliferation in the subventricular zone (SVZ), with particular reduction in neurogenesis of calbindin‐expressing periglomerular cells. Thus, Shh interactions with proteoglycan co‐receptors function at multiple locations to regulate neurogenesis and precise olfactory connectivity, thereby promoting functional neuronal circuitry. © 2014 Wiley Periodicals, Inc. Develop Neurobiol 74: 1255–1267, 2014     

The Olfactory System as a Model to Study Axonal Growth Patterns and Morphology In Vivo

The olfactory system has the unusual capacity to generate new neurons throughout the lifetime of an organism. Olfactory stem cells in the basal portion of the olfactory epithelium continuously give rise to new sensory neurons that extend their axons into the olfactory bulb, where they face the challenge to integrate into existing circuitry. Because of this particular feature, the olfactory system represents a unique opportunity to monitor axonal wiring and guidance, and to investigate synapse formation. Here we describe a procedure for in vivo labeling of sensory neurons and subsequent visualization of axons in the olfactory system of larvae of the amphibian Xenopus laevis. To stain sensory neurons in the olfactory organ we adopt the electroporation technique. In vivo electroporation is an established technique for delivering fluorophore-coupled dextrans or other macromolecules into living cells. Stained sensory neurons and their axonal processes can then be monitored in the living animal either using confocal laser-scanning or multiphoton microscopy. By reducing the number of labeled cells to few or single cells per animal, single axons can be tracked into the olfactory bulb and their morphological changes can be monitored over weeks by conducting series of in vivo time lapse imaging experiments. While the described protocol exemplifies the labeling and monitoring of olfactory sensory neurons, it can also be adopted to other cell types within the olfactory and other systems.     

Spiking patterns and their functional implications in the antennal lobe of the tobacco hornworm Manduca sexta

Bursting as well as tonic firing patterns have been described in various sensory systems. In the olfactory system, spontaneous bursts have been observed in neurons distributed across several synaptic levels, from the periphery, to the olfactory bulb (OB) and to the olfactory cortex. Several in vitro studies indicate that spontaneous firing patterns may be viewed as "fingerprints" of different types of neurons that exhibit distinct functions in the OB. It is still not known, however, if and how neuronal burstiness is correlated with the coding of natural olfactory stimuli. We thus conducted an in vivo study to probe this question in the OB equivalent structure of insects, the antennal lobe (AL) of the tobacco hornworm Manduca sexta. We found that in the moth's AL, both projection (output) neurons (PNs) and local interneurons (LNs) are spontaneously active, but PNs tend to produce spike bursts while LNs fire more regularly. In addition, we found that the burstiness of PNs is correlated with the strength of their responses to odor stimulation--the more bursting the stronger their responses to odors. Moreover, the burstiness of PNs was also positively correlated with the spontaneous firing rate of these neurons, and pharmacological reduction of bursting resulted in a decrease of the neurons' responsiveness. These results suggest that neuronal burstiness reflects a physiological state of these neurons that is directly linked to their response characteristics.     

Expression of Kv1.3 potassium channels regulates density of cortical interneurons

The Kv1.3 protein is a member of the large family of voltage‐dependent K⁺ subunits (Kv channels), which assemble to form tetrameric membrane‐spanning channels that provide a selective pore for the conductance of K⁺ across the cell membrane. Kv1.3 differs from most other Kv channels in that deletion of Kv1.3 gene produces very striking changes in development and structure of the olfactory bulb, where Kv1.3 is expressed at high levels, resulting in a lower threshold for detection of odors, an increased number of synaptic glomeruli and alterations in the levels of a variety of neuronal signaling molecules. Because Kv1.3 is also expressed in the cerebral cortex, we have now examined the effects of deletion of the Kv1.3 gene on the expression of interneuron populations of the cerebral cortex. Using unbiased stereology we found an increase in the number of parvalbumin (PV) cells in whole cerebral cortex of Kv1.3^?/? mice relative to that in wild‐type mice, and a decrease in the number of calbindin (CB), calretinin (CR), neuropeptide Y (NPY), vasoactive intestinal peptide (VIP), and somatostatin (SOM) interneurons. These changes are accompanied by a decrease in the cortical volume such that the cell density of PV interneurons is significantly increased and that of SOM neurons is decreased in Kv1.3^?/? animals. Our studies suggest that, as in the olfactory bulb, Kv1.3 plays a unique role in neuronal differentiation and/or survival of interneuron populations and that expression of Kv1.3 is required for normal cortical function. © 2013 Wiley Periodicals, Inc. Develop Neurobiol 73:841–855, 2013     

NAD causes dissociation of neural networks into subpopulations of neurons by inhibiting the network synchronous hyperactivity evoked by ammonium ions

The mechanisms of hyperexcitability of neuronal networks by ammonium ions and inhibition of this activity by coenzyme NAD were investigated on mixed neuro-glial cultures of rat hippocampus. Ammonium ions cause activation of silent or spontaneously active neuronal networks inducing a bursting electrical activity of neurons and high-frequency synchronous calcium oscillations. In control conditions NAD completely inhibits spontaneous activity of the neuronal network. NAD added after NH₄Cl disrupts synchronous oscillation in neurons and splits the network into five populations of neurons. In 4% of cells NAD decreased the amplitude of Ca²⁺ oscillations, preserving initial mode of oscillations. In 32% of cells, a transient suppression of the neuronal oscillations was observed: inhibition was followed by restoration of the synchronous periodic activity. In 10% of cells, NAD produced a gradual decrease of Ca²⁺ oscillations down to a complete termination of the initial periodic activity induced by ammonium. Fast and total inhibition of Ca²⁺ oscillations by NAD was observed in two small groups of neurons. First group (A) participated in the initial spontaneous network activity (5% of cells) with a period of 66–100 s. Second group (B), on the contrary, did not participate in the spontaneous activity. This group of neurons began to pulse with a high frequency (with a period of 6–8 s) synchronously with other neurons in the network after the addition of NH₄Cl. Based on the comparison of calcium responses of different cell groups to the depolarization caused by KCl and NH₄Cl and to the application of domoic acid, as well as on the results obtained in experiments with fluorescent antibodies against GAD 65/67, parvalbumin, calretinin, and calbindin, we propose that neurons of populations (A) and (B) may belong to GABAergic neurons containing calbindin and parvalbumin, respectively. Further analysis of specificity of the NAD effect on these neuronal groups may allow identification of the main targets of the ammonium toxic action in the brain. Thus, we have shown that NAD selectively inhibits neuronal activity and high-frequency synchronous Ca²⁺ oscillations in GABAergic neurons containing calcium-binding proteins. The inhibition is accompanied by desynchronization of oscillations and dissociation of neuronal network into several populations.     

X inactivation of the OCNC1 channel gene reveals a role for activity-dependent competition in the olfactory system

The organization of neuronal systems is often dependent on activity and competition between cells. In olfaction, the X-linked OCNC1 channel subunit is subject to random inactivation and is essential for odorant-evoked activity. Reporter-tagged OCNC1 mutant mice permit the visualization of OCNC1-deficient olfactory neurons and their projections. In heterozygous females, X inactivation creates a mosaic with two populations of genetically distinct neurons. OCNC1-deficient neurons are slowly and specifically depleted from the olfactory epithelium and display unusual patterns of projection to the olfactory bulb. Remarkably, this depletion is dependent on odorant exposure and is reversed by odorant deprivation. This suggests that odorants and the activity they evoke are critical for neuronal survival in a competitive environment and implicate evoked activity in the organization and maintenance of the olfactory system.     

Firing pattern and synchronization property analysis in a network model of the olfactory bulb

In the olfactory system, both the temporal spike structure and spatial distribution of neuronal activity are important for processing odor information. In this paper, a biophysically-detailed, spiking neuronal model is used to simulate the activity of olfactory bulb. It is shown that by varying some key parameters such as maximal conductances of Ks and Nap the spike train of single neuron can exhibit various firing patterns. Synchronization in coupled neurons is also investigated as the coupling strength varying in the situation of two neurons and network. It is illustrated that the coupled neurons can exhibit different types of pattern when the coupling strength varies. These results may be instructive to understand information transmission in olfactory system.