The olfactory network of larval Xenopus laevis regenerates accurately after olfactory nerve transection

Across vertebrate species, the olfactory epithelium (OE) exhibits the uncommon feature of lifelong neuronal turnover. Epithelial stem cells give rise to new neurons that can adequately replace dying olfactory receptor neurons (ORNs) during developmental and adult phases and after lesions. To relay olfactory information from the environment to the brain, the axons of the renewed ORNs must reconnect with the olfactory bulb (OB). In Xenopus laevis larvae, we have previously shown that this process occurs between 3 and 7 weeks after olfactory nerve (ON) transection. In the present study, we show that after 7 weeks of recovery from ON transection, two functionally and spatially distinct glomerular clusters are reformed in the OB, akin to those found in non-transected larvae. We also show that the same odourant response tuning profiles observed in the OB of non-transected larvae are again present after 7 weeks of recovery. Next, we show that characteristic odour-guided behaviour disappears after ON transection but recovers after 7-9 weeks of recovery. Together, our findings demonstrate that the olfactory system of larval X. laevis regenerates with high accuracy after ON transection, leading to the recovery of odour-guided behaviour.

S100Z is expressed in a lateral subpopulation of olfactory receptor neurons in the main olfactory system of Xenopus laevis

In contrast to other S100 protein members, the function of S100 calcium-binding protein Z (S100Z) remains largely uncharacterized. It is expressed in the olfactory epithelium of fish, and it is closely associated with the vomeronasal organ (VNO) in mammals. In this study, we analyzed the expression pattern of S100Z in the olfactory system of the anuran amphibian Xenopus laevis. Using immunohistochemistry in whole mount and slice preparations of the larval olfactory system, we found exclusive S100Z expression in a subpopulation of olfactory receptor neurons (ORNs) of the main olfactory epithelium (MOE). S100Z expression was not co-localized with TP63 and cytokeratin type II, ruling out basal cell and supporting cell identity. The distribution of S100Z-expressing ORNs was laterally biased, and their average number was significantly increased in the lateral half of the olfactory epithelium. The axons of S100Z-positive neurons projected exclusively into the lateral and intermediate glomerular clusters of the main olfactory bulb (OB). Even after metamorphic restructuring of the olfactory system, S100Z expression was restricted to a neuronal subpopulation of the MOE, which was then located in the newly formed middle cavity. An axonal projection into the ventro-lateral OB persisted also in postmetamorphic frogs. In summary, S100Z is exclusively associated with the main olfactory system in the amphibian Xenopus and not with the VNO as in mammals, despite the presence of a separate accessory olfactory system in both classes.

Calcium imaging of adult olfactory epithelium reveals amines as important odor class in fish

The odor space of aquatic organisms is by necessity quite different from that of air-breathing animals. The recognized odor classes in teleost fish include amino acids, bile acids, reproductive hormones, nucleotides, and a limited number of polyamines. Conversely, a significant portion of the fish olfactory receptor repertoire is composed of trace amine-associated receptors, generally assumed to be responsible for detecting amines. Zebrafish possess over one hundred of these receptors, but the responses of olfactory sensory neurons to amines have not been known so far. Here we examined odor responses of zebrafish olfactory epithelial explants at the cellular level, employing calcium imaging. We report that amines elicit strong responses in olfactory sensory neurons, with a time course characteristically different from that of ATP-responsive (basal) cells. A quantitative analysis of the laminar height distribution shows amine-responsive cells undistinguishable from ciliated neurons positive for olfactory marker protein. This distribution is significantly different from those measured for microvillous neurons positive for transient receptor potential channel 2 and basal cells positive for proliferating cell nuclear antigen. Our results suggest amines as an important odor class for teleost fish.

Functional odor map heterogeneity is based on multifaceted glomerular connectivity in larval Xenopus olfactory bulb

Glomeruli are the functional units of the vertebrate olfactory bulb (OB) connecting olfactory receptor neuron (ORN) axons and mitral/tufted cell (MTC) dendrites. In amphibians, these two circuit elements regularly branch and innervate multiple, spatially distinct glomeruli. Using functional multiphoton-microscopy and single-cell tracing, we investigate the impact of this wiring on glomerular module organization and odor representations on multiple levels of the Xenopus laevis OB network. The glomerular odor map to amino acid odorants is neither stereotypic between animals nor chemotopically organized. Among the morphologically heterogeneous group of uni- and multi-glomerular MTCs, MTCs can selectively innervate glomeruli formed by axonal branches of individual ORNs. We conclude that odor map heterogeneity is caused by the coexistence of different intermingled glomerular modules. This demonstrates that organization of the amphibian main olfactory system is not strictly based on uni-glomerular connectivity.

Activation of endothelial NO synthase and P2X7 receptor modification mediates the cholinergic control of ATP-induced interleukin-1β release by mononuclear phagocytes

Objective

The pro-inflammatory cytokine interleukin-1β (IL-1β) plays a central role in host defense against infections. High systemic IL-1β levels, however, promote the pathogenesis of inflammatory disorders. Therefore, mechanisms controlling IL-1β release are of substantial clinical interest. Recently, we identified a cholinergic mechanism inhibiting the ATP-mediated IL-1β release by human monocytes via nicotinic acetylcholine receptor (nAChR) subunits α7, α9 and/or α10. We also discovered novel nAChR agonists that trigger this inhibitory function in monocytic cells without eliciting ionotropic functions at conventional nAChRs. Here, we investigate the ion flux-independent signaling pathway that links nAChR activation to the inhibition of the ATP-sensitive P2X7 receptor (P2X7R).

Methods

Different human and murine mononuclear phagocytes were primed with lipopolysaccharide and stimulated with the P2X7R agonist BzATP in the presence or absence of nAChR agonists, endothelial NO synthase (eNOS) inhibitors, and NO donors. IL-1β was measured in cell culture supernatants. Patch-clamp and intracellular Ca2+ imaging experiments were performed on HEK cells overexpressing human P2X7R or P2X7R with point mutations at cysteine residues in the cytoplasmic C-terminal domain.

Results

The inhibitory effect of nAChR agonists on the BzATP-induced IL-1β release was reversed in the presence of eNOS inhibitors (L-NIO, L-NAME) as well as in U937 cells after silencing of eNOS expression. In peripheral blood mononuclear leukocytes from eNOS gene-deficient mice, the inhibitory effect of nAChR agonists was absent, suggesting that nAChRs signal via eNOS to inhibit the BzATP-induced IL-1β release. Moreover, NO donors (SNAP, S-nitroso-N-acetyl-DL-penicillamine; SIN-1) inhibited the BzATP-induced IL-1β release by mononuclear phagocytes. The BzATP-induced ionotropic activity of the P2X7R was abolished in the presence of SIN-1 in both, Xenopus laevis oocytes and HEK cells over-expressing the human P2X7R. This inhibitory effect of SIN-1 was absent in HEK cells expressing P2X7R, in which C377 was mutated to alanine, indicating the importance of C377 for the regulation of the P2X7R function by protein modification.

Conclusion

We provide first evidence that ion flux-independent, metabotropic signaling of monocytic nAChRs involves eNOS activation and P2X7R modification, resulting in an inhibition of ATP signaling and ATP-mediated IL-1β release. This signaling pathway might be an interesting target for the treatment of inflammatory disorders.

Patterns of tubb2b Promoter-Driven Fluorescence in the Forebrain of Larval Xenopus laevis

Microtubules are essential components of the cytoskeleton of all eukaryotic cells and consist of α- and β-tubulin heterodimers. Several tissue-specific isotypes of α- and β-tubulins, encoded by distinct genes, have been described in vertebrates. In the African clawed frog (Xenopus laevis), class II β-tubulin (tubb2b) is expressed exclusively in neurons, and its promoter is used to establish different transgenic frog lines. However, a thorough investigation of the expression pattern of tubb2b has not been carried out yet. In this study, we describe the expression of tubb2b-dependent Katushka fluorescence in the forebrain of premetamorphic Xenopus laevis at cellular resolution. To determine the exact location of Katushka-positive neurons in the forebrain nuclei and to verify the extent of neuronal Katushka expression, we used a transgenic frog line and performed several additional antibody stainings. We found tubb2b-dependent fluorescence throughout the Xenopus forebrain, but not in all neurons. In the olfactory bulb, tubb2b-dependent fluorescence is present in axonal projections from the olfactory epithelium, cells in the mitral cell layer, and fibers of the extrabulbar system, but not in interneurons. We also detected tubb2b-dependent fluorescence in parts of the basal ganglia, the amygdaloid complex, the pallium, the optic nerve, the preoptic area, and the hypothalamus. In the diencephalon, tubb2b-dependent fluorescence occurred mainly in the prethalamus and thalamus. As in the olfactory system, not all neurons of these forebrain regions exhibited tubb2b-dependent fluorescence. Together, our results present a detailed overview of the distribution of tubb2b-dependent fluorescence in neurons of the forebrain of larval Xenopus laevis and clearly show that tubb2b-dependent fluorescence cannot be used as a pan-neuronal marker.

Distinct interhemispheric connectivity at the level of the olfactory bulb emerges during Xenopus laevis metamorphosis

During metamorphosis, the olfactory system of anuran tadpoles undergoes substantial restructuring. The main olfactory epithelium in the principal nasal cavity of Xenopus laevis tadpoles is associated with aquatic olfaction and transformed into the adult air-nose, while a new adult water-nose emerges in the middle cavity. Impacts of this metamorphic remodeling on odor processing, behavior, and network structure are still unexplored. Here, we used neuronal tracings, calcium imaging, and behavioral experiments to examine the functional connectivity between the epithelium and the main olfactory bulb during metamorphosis. In tadpoles, olfactory receptor neurons in the principal cavity project axons to glomeruli in the ventral main olfactory bulb. These projections are gradually replaced by receptor neuron axons from the newly forming middle cavity epithelium. Despite this reorganization in the ventral bulb, two spatially segregated odor processing streams remain undisrupted and behavioral responses to waterborne odorants are unchanged. Contemporaneously, new receptor neurons in the remodeling principal cavity innervate the emerging dorsal part of the bulb, which displays distinct wiring features. Glomeruli around its midline are innervated from the left and right nasal epithelia. Additionally, postsynaptic projection neurons in the dorsal bulb predominantly connect to multiple glomeruli, while half of projection neurons in the ventral bulb are uni-glomerular. Our results show that the “water system” remains functional despite metamorphic reconstruction. The network differences between the dorsal and ventral olfactory bulb imply a higher degree of odor integration in the dorsal main olfactory bulb. This is possibly connected with the processing of different odorants, airborne vs. waterborne.

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.

Olfaction across the water-air interface in anuran amphibians

Extant anuran amphibians originate from an evolutionary intersection eventually leading to fully terrestrial tetrapods. In many ways, they have to deal with exposure to both terrestrial and aquatic environments: (i) phylogenetically, as derivatives of the first tetrapod group that conquered the terrestrial environment in evolution; (ii) ontogenetically, with a development that includes aquatic and terrestrial stages connected via metamorphic remodeling; and (iii) individually, with common changes in habitat during the life cycle. Our knowledge about the structural organization and function of the amphibian olfactory system and its relevance still lags behind findings on mammals. It is a formidable challenge to reveal underlying general principles of circuity-related, cellular, and molecular properties that are beneficial for an optimized sense of smell in water and air. Recent findings in structural organization coupled with behavioral observations could help to understand the importance of the sense of smell in this evolutionarily important animal group. We describe the structure of the peripheral olfactory organ, the olfactory bulb, and higher olfactory centers on a tissue, cellular, and molecular levels. Differences and similarities between the olfactory systems of anurans and other vertebrates are reviewed. Special emphasis lies on adaptations that are connected to the distinct demands of olfaction in water and air environment. These particular adaptations are discussed in light of evolutionary trends, ontogenetic development, and ecological demands.

Whole-Brain Calcium Imaging in Larval Xenopus

Sensory systems detect environmental stimuli and transform them into electrical activity patterns interpretable by the central nervous system. En route to higher brain centers, the initial sensory input is successively transformed by interposed secondary processing centers. Mapping the neuronal activity patterns at all of those stages is essential to understand sensory information processing. Larval Xenopus laevis is very well-suited for whole-brain imaging of neuronal activity. This is mainly due to its small size, transparency, and the accessibility of both peripheral and central parts of sensory systems. Here we describe a protocol for calcium imaging at several levels of the olfactory system using focal injection of chemical calcium indicator dyes or a Xenopus transgenic line with neuronal GCaMP6s expression. In combination with fast volumetric multiphoton microscopy, the calcium imaging methods described can provide detailed insight into spatiotemporal activity of entire brain regions at different stages of sensory information processing. Although the methods are broadly applicable to the central nervous system, in this work we focus on protocols for calcium imaging of glomeruli in the olfactory bulb and odor-responsive neurons in the olfactory amygdala.

Conservation of Glomerular Organization in the Main Olfactory Bulb of Anuran Larvae

The glomerular array in the olfactory bulb of many vertebrates is segregated into molecularly and anatomically distinct clusters linked to different olfactory functions. In anurans, glomerular clustering is so far only described in Xenopus laevis. We traced olfactory projections to the bulb in tadpoles belonging to six distantly related anuran species in four families (Pipidae, Hylidae, Bufonidae, Dendrobatidae) and found that glomerular clustering is remarkably conserved. The general bauplan consists of four unequally sized glomerular clusters with minor inter-species variation. During metamorphosis, the olfactory system undergoes extensive remodeling. Tracings in metamorphotic and juvenile Dendrobates tinctorius and Xenopus tropicalis suggest a higher degree of variation in the glomerular organization after metamorphosis is complete. Our study highlights, that the anatomical organization of glomeruli in the main olfactory bulb (MOB) is highly conserved, despite an extensive ecomorphological diversification among anuran tadpoles, which suggests underlying developmental constraints.

A nanobody-based fluorescent reporter reveals human α-synuclein in the cell cytosol

Aggregation and spreading of α-Synuclein (αSyn) are hallmarks of several neurodegenerative diseases, thus monitoring human αSyn (hαSyn) in animal models or cell cultures is vital for the field. However, the detection of native hαSyn in such systems is challenging. We show that the nanobody NbSyn87, previously-described to bind hαSyn, also shows cross-reactivity for the proteasomal subunit Rpn10. As such, when the NbSyn87 is expressed in the absence of hαSyn, it is continuously degraded by the proteasome, while it is stabilized when it binds to hαSyn. Here, we exploit this feature to design a new Fluorescent Reporter for hαSyn (FluoReSyn) by fusing NbSyn87 to fluorescent proteins, which results in fluorescence signal fluctuations depending on the presence and amounts of intracellular hαSyn. We characterize this biosensor in cells and tissues to finally reveal the presence of transmittable αSyn in human cerebrospinal fluid, demonstrating the potential of FluoReSyn for clinical research and diagnostics.

Multi-glomerular projection of single olfactory receptor neurons is conserved among amphibians

Individual receptor neurons in the peripheral olfactory organ extend long axons into the olfactory bulb forming synapses with projection neurons in spherical neuropil regions, called glomeruli. Generally, odor map formation and odor processing in all vertebrates is based on the assumption that receptor neuron axons exclusively connect to a single glomerulus without any axonal branching. We comparatively tested this hypothesis in multiple fish and amphibian species (both sexes) by applying sparse cell electroporation to trace single olfactory receptor neuron axons. Sea lamprey (jawless fish) and zebrafish (bony fish) support the unbranched axon concept, with 94% of axons terminating in single glomeruli. Contrastingly, axonal projections of the axolotl (salamander) branch extensively before entering up to six distinct glomeruli. Receptor neuron axons labeled in frog species (Pipidae, Bufonidae, Hylidae, and Dendrobatidae) predominantly bifurcate before entering a glomerulus and 59 and 50% connect to multiple glomeruli in larval and postmetamorphotic animals, respectively. Independent of developmental stage, lifestyle, and adaptations to specific habitats, it seems to be a common feature of amphibian olfactory receptor neuron axons to frequently bifurcate and connect to multiple glomeruli. Our study challenges the unbranched axon concept as a universal vertebrate feature and it is conceivable that also later diverging vertebrates deviate from it. We propose that this unusual wiring logic evolved around the divergence of the terrestrial tetrapod lineage from its aquatic ancestors and could be the basis of an alternative way of odor processing.

Purinergic signalling selectively modulates maintenance but not repair neurogenesis in the zebrafish olfactory epithelium

Olfactory sensory neurons (OSNs) of the vertebrate olfactory epithelium (OE) undergo continuous turnover but also regenerate efficiently when the OE is acutely damaged by traumatic injury. Two distinct pools of neuronal stem/progenitor cells, the globose (GBCs), and horizontal basal cells (HBCs) have been shown to selectively contribute to intrinsic OSN turnover and damage-induced OE regeneration, respectively. For both types of progenitors, their rate of cell divisions and OSN production must match the actual loss of cells to maintain or to re-establish sensory function. However, signals that communicate between neurons or glia cells of the OE and resident neurogenic progenitors remain largely elusive. Here, we investigate the effect of purinergic signaling on cell proliferation and OSN neurogenesis in the zebrafish OE. Purine stimulation elicits transient Ca²⁺ signals in OSNs and distinct non-neuronal cell populations, which are located exclusively in the basal OE and stain positive for the neuronal stem cell marker Sox2. The more apical population of Sox2-positive cells comprises evenly distributed glia-like sustentacular cells (SCs) and spatially restricted GBC-like cells, whereas the more basal population expresses the HBC markers keratin 5 and tumor protein 63 and lines the entire sensory OE. Importantly, exogenous purine stimulation promotes P2 receptor-dependent mitotic activity and OSN generation from sites where GBCs are located but not from HBCs. We hypothesize that purine compounds released from dying OSNs modulate GBC progenitor cell cycling in a dose-dependent manner that is proportional to the number of dying OSNs and, thereby, ensures a constant pool of sensory neurons over time.

An extracellular acidic cleft confers profound H+-sensitivity to epithelial sodium channels containing the δ-subunit in Xenopus laevis

The limited sodium availability of freshwater and terrestrial environments was a major physiological challenge during vertebrate evolution. The epithelial sodium channel (ENaC) is present in the apical membrane of sodium-absorbing vertebrate epithelia and evolved as part of a machinery for efficient sodium conservation. ENaC belongs to the degenerin/ENaC protein family and is the only member that opens without an external stimulus. We hypothesized that ENaC evolved from a proton-activated sodium channel present in ionocytes of freshwater vertebrates and therefore investigated whether such ancestral traits are present in ENaC isoforms of the aquatic pipid frog Xenopus laevis Using whole-cell and single-channel electrophysiology of Xenopus oocytes expressing ENaC isoforms assembled from αβγ- or δβγ-subunit combinations, we demonstrate that Xenopus δβγ-ENaC is profoundly activated by extracellular acidification within biologically relevant ranges (pH 8.0-6.0). This effect was not observed in Xenopus αβγ-ENaC or human ENaC orthologs. We show that protons interfere with allosteric ENaC inhibition by extracellular sodium ions, thereby increasing the probability of channel opening. Using homology modeling of ENaC structure and site-directed mutagenesis, we identified a cleft region within the extracellular loop of the δ-subunit that contains several acidic amino acid residues that confer proton-sensitivity and enable allosteric inhibition by extracellular sodium ions. We propose that Xenopus δβγ-ENaC can serve as a model for investigating ENaC transformation from a proton-activated toward a constitutively-active ion channel. Such transformation might have occurred during the evolution of tetrapod vertebrates to enable bulk sodium absorption during the water-to-land transition.

SLPI Inhibits ATP-Mediated Maturation of IL-1β in Human Monocytic Leukocytes: A Novel Function of an Old Player

Interleukin-1β (IL-1β) is a potent, pro-inflammatory cytokine of the innate immune system that plays an essential role in host defense against infection. However, elevated circulating levels of IL-1β can cause life-threatening systemic inflammation. Hence, mechanisms controlling IL-1β maturation and release are of outstanding clinical interest. Secretory leukocyte protease inhibitor (SLPI), in addition to its well-described anti-protease function, controls the expression of several pro-inflammatory cytokines on the transcriptional level. In the present study, we tested the potential involvement of SLPI in the control of ATP-induced, inflammasome-dependent IL-1β maturation and release. We demonstrated that SLPI dose-dependently inhibits the ATP-mediated inflammasome activation and IL-1β release in human monocytic cells, without affecting the induction of pro-IL-1β mRNA by LPS. In contrast, the ATP-independent IL-1β release induced by the pore forming bacterial toxin nigericin is not impaired, and SLPI does not directly modulate the ion channel function of the human P2X₇ receptor heterologously expressed in Xenopus laevis oocytes. In human monocytic U937 cells, however, SLPI efficiently inhibits ATP-induced ion-currents. Using specific inhibitors and siRNA, we demonstrate that SLPI activates the calcium-independent phospholipase A2β (iPLA2β) and leads to the release of a low molecular mass factor that mediates the inhibition of IL-1β release. Signaling involves nicotinic acetylcholine receptor subunits α7, α9, α10, and Src kinase activation and results in an inhibition of ATP-induced caspase-1 activation. In conclusion, we propose a novel anti-inflammatory mechanism induced by SLPI, which inhibits the ATP-dependent maturation and secretion of IL-1β. This novel signaling pathway might lead to development of therapies that are urgently needed for the prevention and treatment of systemic inflammation.

Incorporation of the δ-subunit into the epithelial sodium channel (ENaC) generates protease-resistant ENaCs in Xenopus laevis

The epithelial sodium channel (ENaC) is a critical regulator of vertebrate electrolyte homeostasis. ENaC is the only constitutively open ion channel in the degenerin/ENaC protein family, and its expression, membrane abundance, and open probability therefore are tightly controlled. The canonical ENaC is composed of three subunits (α, β, and γ), but a fourth δ-subunit may replace α and form atypical δβγ-ENaCs. Using Xenopus laevis as a model, here we found that mRNAs of the α- and δ-subunits are differentially expressed in different tissues and that δ-ENaC predominantly is present in the urogenital tract. Using whole-cell and single-channel electrophysiology of oocytes expressing Xenopus αβγ- or δβγ-ENaC, we demonstrate that the presence of the δ-subunit enhances the amount of current generated by ENaC due to an increased open probability, but also changes current into a transient form. Activity of canonical ENaCs is critically dependent on proteolytic processing of the α- and γ-subunits, and immunoblotting with epitope-tagged ENaC subunits indicated that, unlike α-ENaC, the δ-subunit does not undergo proteolytic maturation by the endogenous protease furin. Furthermore, currents generated by δβγ-ENaC were insensitive to activation by extracellular chymotrypsin, and presence of the δ-subunit prevented cleavage of γ-ENaC at the cell surface. Our findings suggest that subunit composition constitutes an additional level of ENaC regulation, and we propose that the Xenopus δ-ENaC subunit represents a functional example that demonstrates the importance of proteolytic maturation during ENaC evolution.

Dye Electroporation and Imaging of Calcium Signaling in Xenopus Nervous System

Electroporation is an efficient method of transferring charged macromolecules into living cells in order to study their morphology, function, and connectivity within neuronal networks. Labeling cells with fluorophore-coupled macromolecules can be used to trace projections of whole neuronal ensembles, as well as the fine morphology of single cells. Here, we present a protocol to visualize pre- and postsynaptic components of a sensory relay synapse in the brain, using the olfactory system of Xenopus laevis tadpoles as a model. We apply bulk electroporation to trace projections of receptor neurons from the nose to the brain, and single cell electroporation to visualize the morphology of their synaptic target cells, the mitral-tufted cells. Labeling the receptor neurons with a calcium-sensitive dye allows us to record stimulus-induced presynaptic input to the dendrites of the postsynaptic cells via functional calcium imaging.

Functional Reintegration of Sensory Neurons and Transitional Dendritic Reduction of Mitral/Tufted Cells during Injury-Induced Recovery of the Larval Xenopus Olfactory Circuit

Understanding the mechanisms involved in maintaining lifelong neurogenesis has a clear biological and clinical interest. In the present study, we performed olfactory nerve transection on larval Xenopus to induce severe damage to the olfactory circuitry. We surveyed the timing of the degeneration, subsequent rewiring and functional regeneration of the olfactory system following injury. A range of structural labeling techniques and functional calcium imaging were performed on both tissue slices and whole brain preparations. Cell death of olfactory receptor neurons and proliferation of stem cells in the olfactory epithelium were immediately increased following lesion. New olfactory receptor neurons repopulated the olfactory epithelium and once again showed functional responses to natural odorants within 1 week after transection. Reinnervation of the olfactory bulb (OB) by newly formed olfactory receptor neuron axons also began at this time. Additionally, we observed a temporary increase in cell death in the OB and a subsequent loss in OB volume. Mitral/tufted cells, the second order neurons of the olfactory system, largely survived, but transiently lost dendritic tuft complexity. The first odorant-induced responses in the OB were observed 3 weeks after nerve transection and the olfactory network showed signs of major recovery, both structurally and functionally, after 7 weeks.

Surfactant inhibits ATP-induced release of interleukin-1β via nicotinic acetylcholine receptors

Interleukin (IL)-1β is a potent pro-inflammatory cytokine of innate immunity involved in host defense. High systemic IL-1β levels, however, cause life-threatening inflammatory diseases, including systemic inflammatory response syndrome. In response to various danger signals, the pro-form of IL-1β is synthesized and stays in the cytoplasm unless a second signal, such as extracellular ATP, activates the inflammasome, which enables processing and release of mature IL-1β. As pulmonary surfactant is known for its anti-inflammatory properties, we hypothesize that surfactant inhibits ATP-induced release of IL-1β. Lipopolysaccharide-primed monocytic U937 cells were stimulated with an ATP analog in the presence of natural or synthetic surfactant composed of recombinant surfactant protein (rSP)-C, palmitoylphosphatidylglycerol, and dipalmitoylphosphatidylcholine (DPPC). Both surfactant preparations dose-dependently inhibited IL-1β release from U937 cells. DPPC was the active constituent of surfactant, whereas rSP-C and palmitoylphosphatidylglycerol were inactive. DPPC was also effective in primary mononuclear leukocytes isolated from human blood. Experiments with nicotinic antagonists, siRNA technology, and patch-clamp experiments suggested that stimulation of nicotinic acetylcholine receptors (nAChRs) containing subunit α9 results in a complete inhibition of the ion channel function of ATP receptor, P2X7. In conclusion, the surfactant constituent, DPPC, efficiently inhibits ATP-induced inflammasome activation and maturation of IL-1β in human monocytes by a mechanism involving nAChRs.

Coordinated shift of olfactory amino acid responses and V2R expression to an amphibian water nose during metamorphosis

All olfactory receptors identified in teleost fish are expressed in a single sensory surface, whereas mammalian olfactory receptor gene families segregate into different olfactory organs, chief among them the main olfactory epithelium expressing ORs and TAARs, and the vomeronasal organ expressing V1Rs and V2Rs. A transitional stage is embodied by amphibians, with their vomeronasal organ expressing more 'modern', later diverging V2Rs, whereas more 'ancient', earlier diverging V2Rs are expressed in the main olfactory epithelium. During metamorphosis, the main olfactory epithelium of Xenopus tadpoles transforms into an air-filled cavity (principal cavity, air nose), whereas a newly formed cavity (middle cavity) takes over the function of a water nose. We report here that larval expression of ancient V2Rs is gradually lost from the main olfactory epithelium as it transforms into the air nose. Concomitantly, ancient v2r gene expression begins to appear in the basal layers of the newly forming water nose. We observe the same transition for responses to amino acid odorants, consistent with the hypothesis that amino acid responses may be mediated by V2R receptors.

Dual processing of sulfated steroids in the olfactory system of an anuran amphibian

Chemical communication is widespread in amphibians, but if compared to later diverging tetrapods the available functional data is limited. The existing information on the vomeronasal system of anurans is particularly sparse. Amphibians represent a transitional stage in the evolution of the olfactory system. Most species have anatomically separated main and vomeronasal systems, but recent studies have shown that in anurans their molecular separation is still underway. Sulfated steroids function as migratory pheromones in lamprey and have recently been identified as natural vomeronasal stimuli in rodents. Here we identified sulfated steroids as the first known class of vomeronasal stimuli in the amphibian Xenopus laevis. We show that sulfated steroids are detected and concurrently processed by the two distinct olfactory subsystems of larval Xenopus laevis, the main olfactory system and the vomeronasal system. Our data revealed a similar but partially different processing of steroid-induced responses in the two systems. Differences of detection thresholds suggest that the two information channels are not just redundant, but rather signal different information. Furthermore, we found that larval and adult animals excrete multiple sulfated compounds with physical properties consistent with sulfated steroids. Breeding tadpole and frog water including these compounds activated a large subset of sensory neurons that also responded to synthetic steroids, showing that sulfated steroids are likely to convey intraspecific information. Our findings indicate that sulfated steroids are conserved vomeronasal stimuli functioning in phylogenetically distant classes of tetrapods living in aquatic and terrestrial habitats.

Different expression domains for two closely related amphibian TAARs generate a bimodal distribution similar to neuronal responses to amine odors

Olfactory perception is mediated by a multitude of olfactory receptors, whose expression in the sensory surface, the olfactory epithelium, is spatially regulated. A common theme is the segregation of different olfactory receptors in different expression domains, which in turn leads to corresponding segregation in the neuronal responses to different odor groups. The amphibian olfactory receptor gene family of trace amine associated receptors, in short TAARs, is exceedingly small and allows a comprehensive analysis of spatial expression patterns, as well as a comparison with neuronal responses to the expected ligands for this receptor family, amines. Here we report that TAAR4b exhibits a spatial expression pattern characteristically different in two dimensions from that of TAAR4a, its close homolog. Together, these two genes result in a bimodal distribution resembling that of amine responses as visualized by calcium imaging. A stringent quantitative analysis suggests the involvement of additional olfactory receptors in amphibian responses to amine odors.

Metamorphic remodeling of the olfactory organ of the African clawed frog, Xenopus laevis

The amphibian olfactory system undergoes massive remodeling during metamorphosis. The transition from aquatic olfaction in larvae to semiaquatic or airborne olfaction in adults requires anatomical, cellular, and molecular modifications. These changes are particularly pronounced in Pipidae, whose adults have secondarily adapted to an aquatic life style. In the fully aquatic larvae of Xenopus laevis, the main olfactory epithelium specialized for sensing water-borne odorous substances lines the principal olfactory cavity (PC), whereas a separate olfactory epithelium lies in the vomeronasal organ (VNO). During metamorphosis, the epithelium of the PC is rearranged into the adult "air nose," whereas a new olfactory epithelium, the adult "water nose," forms in the emerging middle cavity (MC). Here we performed a stage-by-stage investigation of the anatomical changes of the Xenopus olfactory organ during metamorphosis. We quantified cell death in all olfactory epithelia and found massive cell death in the PC and the VNO, suggesting that the majority of larval sensory neurons is replaced during metamorphosis in both sensory epithelia. The moderate cell death in the MC shows that during the formation of this epithelium some cells are sorted out. Our results show that during MC formation some supporting cells, but not sensory neurons, are relocated from the PC to the MC and that they are eventually eliminated during metamorphosis. Together our findings illustrate the structural and cellular changes of the Xenopus olfactory organ during metamorphosis.

[How we smell and what it means to us: basic principles of the sense of smell]

The origins of the sense of smell lie in the perception of environmental molecules and go back to unicellular organisms such as bacteria. Odors transmit a multitude of information about the chemical composition of our environment. The sense of smell helps people and animals with orientation in space, warns of potential threats, influences the choice of sexual partners, regulates food intake and influences feelings and social behavior in general. The perception of odors begins in sensory neurons residing in the olfactory epithelium that express G protein-coupled receptors, the so-called olfactory receptors. The binding of odor molecules to olfactory receptors initiates a signal transduction cascade that converts olfactory stimuli into electrical signals. These signals are then transmitted to the olfactory bulb, the first relay center in the olfactory pathway, via the axons of the sensory neurons. The olfactory information is processed in the bulb and then transferred to higher olfactory centers via axons of mitral cells, the bulbar projection neurons. This review describes the mechanisms involved in peripheral detection of odorants, outlines the further processing of olfactory information in higher olfactory centers and finally gives an overview of the overall significance of the ability to smell.

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.

Trpc2 is expressed in two olfactory subsystems, the main and the vomeronasal system of larval Xenopus laevis

Complete segregation of the main olfactory epithelium (MOE) and the vomeronasal epithelium is first observed in amphibians. In contrast, teleost fishes possess a single olfactory surface, in which genetic components of the main and vomeronasal olfactory systems are intermingled. The transient receptor potential channel TRPC2, a marker of vomeronasal neurons, is present in the single fish sensory surface, but is already restricted to the vomeronasal epithelium in a terrestrial amphibian, the red-legged salamander (Plethodon shermani). Here we examined the localization of TRPC2 in an aquatic amphibian and cloned the Xenopus laevis trpc2 gene. We show that it is expressed in both the MOE and the vomeronasal epithelium. This is the first description of a broad trpc2 expression in the MOE of a tetrapod. The expression pattern of trpc2 in the MOE is virtually undistinguishable from that of MOE-specific v2rs, indicating that they are co-expressed in the same neuronal subpopulation.

Phospholipase C and diacylglycerol mediate olfactory responses to amino acids in the main olfactory epithelium of an amphibian

The semi-aquatic lifestyle of amphibians represents a unique opportunity to study the molecular driving forces involved in the transition of aquatic to terrestrial olfaction in vertebrates. Most amphibians have anatomically segregated main and vomeronasal olfactory systems, but at the cellular and molecular level the segregation differs from that found in mammals. We have recently shown that amino acid responses in the main olfactory epithelium (MOE) of larval Xenopus laevis segregate into a lateral and a medial processing stream, and that the former is part of a vomeronasal type 2 receptor expression zone in the MOE. We hypothesized that the lateral amino acid responses might be mediated via a vomeronasal-like transduction machinery. Here we report that amino acid-responsive receptor neurons in the lateral MOE employ a phospholipase C (PLC) and diacylglycerol-mediated transduction cascade that is independent of Ca²⁺ store depletion. Furthermore, we found that putative transient receptor potential (TRP) channel blockers inhibit most amino acid-evoked responses in the lateral MOE, suggesting that ion channels belonging to the TRP family may be involved in the signaling pathway. Our data show, for the first time, a widespread PLC- and diacylglycerol-dependent transduction cascade in the MOE of a vertebrate already possessing a vomeronasal organ.

Purinergic receptor-induced Ca2+ signaling in the neuroepithelium of the vomeronasal organ of larval Xenopus laevis

Purinergic signaling has considerable impact on the functioning of the nervous system, including the special senses. Purinergic receptors are expressed in various cell types in the retina, cochlea, taste buds, and the olfactory epithelium. The activation of these receptors by nucleotides, particularly adenosine-5′-triphosphate (ATP) and its breakdown products, has been shown to tune sensory information coding to control the homeostasis and to regulate the cell turnover in these organs. While the purinergic system of the retina, cochlea, and taste buds has been investigated in numerous studies, the available information about purinergic signaling in the olfactory system is rather limited. Using functional calcium imaging, we identified and characterized the purinergic receptors expressed in the vomeronasal organ of larval Xenopus laevis. ATP-evoked activity in supporting and basal cells was not dependent on extracellular Ca²⁺. Depletion of intracellular Ca²⁺ stores disrupted the responses in both cell types. In addition to ATP, supporting cells responded also to uridine-5′-triphosphate (UTP) and adenosine-5′-O-(3-thiotriphosphate) (ATPγS). The response profile of basal cells was considerably broader. In addition to ATP, they were activated by ADP, 2-MeSATP, 2-MeSADP, ATPγS, UTP, and UDP. Together, our findings suggest that supporting cells express P2Y₂/P2Y₄-like purinergic receptors and that basal cells express multiple P2Y receptors. In contrast, vomeronasal receptor neurons were not sensitive to nucleotides, suggesting that they do not express purinergic receptors. Our data provide the basis for further investigations of the physiological role of purinergic signaling in the vomeronasal organ and the olfactory system in general.

Amino acid- vs. peptide-odorants: responses of individual olfactory receptor neurons in an aquatic species

Amino acids are widely used waterborne olfactory stimuli proposed to serve as cues in the search for food. In natural waters the main source of amino acids is the decomposition of proteins. But this process also produces a variety of small peptides as intermediate cleavage products. In the present study we tested whether amino acids actually are the natural and adequate stimuli for the olfactory receptors they bind to. Alternatively, these olfactory receptors could be peptide receptors which also bind amino acids though at lower affinity. Employing calcium imaging in acute slices of the main olfactory epithelium of the fully aquatic larvae of Xenopus laevis we show that amino acids, and not peptides, are more effective waterborne odorants.

Bimodal processing of olfactory information in an amphibian nose: odor responses segregate into a medial and a lateral stream

In contrast to the single sensory surface present in teleost fishes, several spatially segregated subsystems with distinct molecular and functional characteristics define the mammalian olfactory system. However, the evolutionary steps of that transition remain unknown. Here we analyzed the olfactory system of an early diverging tetrapod, the amphibian Xenopus laevis, and report for the first time the existence of two odor-processing streams, sharply segregated in the main olfactory bulb and partially segregated in the olfactory epithelium of pre-metamorphic larvae. A lateral odor-processing stream is formed by microvillous receptor neurons and is characterized by amino acid responses and Gα_o/Gα_i as probable signal transducers, whereas a medial stream formed by ciliated receptor neurons is characterized by responses to alcohols, aldehydes, and ketones, and Gα_olf/cAMP as probable signal transducers. To reveal candidates for the olfactory receptors underlying these two streams, the spatial distribution of 12 genes from four olfactory receptor gene families was determined. Several class II and some class I odorant receptors (ORs) mimic the spatial distribution observed for the medial stream, whereas a trace amine-associated receptor closely parallels the spatial pattern of the lateral odor-processing stream. Other olfactory receptors (some class I odorant receptors and vomeronasal type 1 receptors) and odor responses (to bile acids, amines) were not lateralized, the latter not even in the olfactory bulb, suggesting an incomplete segregation. Thus, the olfactory system of X. laevis exhibits an intermediate stage of segregation and as such appears well suited to investigate the molecular driving forces behind olfactory regionalization.

Thapsigargin-induced Ca2+ increase inhibits TGFβ1-mediated Smad2 transcriptional responses via Ca2+/calmodulin-dependent protein kinase II

Transforming growth factor β (TGFβ) signalling plays important roles in a variety of tissues and cell types. Impaired TGFβ signalling contributes to several pathologies, including cancer, fibrosis as well as neurodegenerative diseases. TGFβ receptor type I-mediated phosphorylation of Smad2, the formation of the Smad2-Smad4 complex and translocation to the nucleus are critical steps of the TGFβ signalling pathway. Here, we demonstrate that thapsigargin-mediated increase of intracellular Ca(2+) concentrations inhibited TGFβ1-induced Smad2 transcriptional activity in the oligodendroglial cell line OLI-neu. We provide evidence that thapsigargin treatment dramatically reduced the nuclear translocation of Smad2 after TGFβ1 treatment but had no effect on its phosphorylation at Ser465/467. Moreover, using Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) inhibitors and a constitutively active CaMKII mutant, we provide evidence that the observed inhibition of TGFβ signalling in OLI-neu cells was strongly dependent on Ca(2+)-mediated CaMKII activation. In summary, this study clearly shows that the TGFβ1-induced Smad2 nuclear translocation is negatively regulated by intracellular Ca(2+) in OLI-neu cells and that increased intracellular Ca(2+) concentrations block Smad2-mediated transcription of TGFβ target genes. These results underline the importance of intracellular Ca(2+) for the regulation of TGFβ signalling.

Endocannabinoid modulation in the olfactory epithelium

Appetite, food intake, and energy balance are closely linked to the endocannabinoid system in the central nervous system. Now, endocannabinoid modulation has been discovered in the peripheral olfactory system of larval Xenopus laevis. The endocannabinoid 2-AG has been shown to influence odorant-detection thresholds according to the hunger state of the animal. Hungry animals have increased 2-AG levels due to enhanced synthesis of 2-AG in sustentacular supporting cells. This renders olfactory receptor neurons, exhibiting CB1 receptors, more sensitive at detecting lower odorant concentrations, which probably helps the animal to locate food. Since taste and vision are also influenced by endocannabinoids, this kind of modulation might boost sensory inputs of food in hungry animals.

Purinergic receptor-mediated Ca signaling in the olfactory bulb and the neurogenic area of the lateral ventricles

Like in other vertebrates, the anterior part of the telencephalon of amphibians mainly consists of the olfactory bulb (OB), but different from higher vertebrates, the lateral telencephalic ventricles of larval Xenopus laevis expand deep into the anterior telencephalon. The neurogenic periventricular zone (PVZ) of the lateral ventricles generates new OB neurons throughout the animal's lifetime. We investigated the ultrastructural organization of the PVZ and found that within a time period of 24 h, 42.54 ± 6.65% of all PVZ cells were actively proliferating. Functional purinergic receptors are widespread in the central nervous system and their activation has been associated with many critical physiological processes, including the regulation of cell proliferation. In the present study we identified and characterized the purinergic system of the OB and the PVZ. ATP and 2MeSATP induced strong [Ca(2+)](i) increases in cells of both regions, which could be attenuated by purinergic antagonists. However, a more thorough pharmacological investigation revealed clear differences between the two brain regions. Cells of the OB almost exclusively express ionotropic P2X purinergic receptor subtypes, whereas PVZ cells express both ionotropic P2X and metabotropic P1 and P2Y receptor subtypes. The P2X receptors expressed in the OB are evidently not involved in the immediate processing of olfactory information.

Purinergic signaling regulates cell proliferation of olfactory epithelium progenitors

In the olfactory epithelium (OE) continuous neurogenesis is maintained throughout life. The OE is in direct contact with the external environment, and its cells are constantly exposed to pathogens and noxious substances. To maintain a functional sense of smell the OE has evolved the ability to permanently replenish olfactory receptor neurons and sustentacular cells lost during natural turnover. A cell population residing in the most basal part of the OE, the so-called basal cells (BCs), keep up this highly regulated genesis of new cells. The population of BCs is thought to include both the stem cells of the OE and various progenitor cells. In recent years a number of regulatory factors that positively and/or negatively regulate the proliferation within the OE have been identified, but a thorough comprehension of the complex interplay of these regulatory factors and the role of the different epithelial cell types is still illusive. Combining labeling techniques, immunohistochemistry, electron microscopy, functional calcium imaging, and a bromo-2'-deoxyuridine incorporation assay, we show for the first time that purinergic receptors are expressed in BCs of the OE of larval Xenopus laevis and that nucleotide-induced Ca(2+) signaling in these cells is involved in the regulation of the cell turnover in the OE. Our data contribute to a better understanding of the regulation of the cell turnover in the OE in particular and also of how the proliferation of neuronal progenitor cells is regulated in general.

Highly specific responses to amine odorants of individual olfactory receptor neurons in situ

The main olfactory system of larval Xenopus laevis is made up of at least two subsystems consisting of subsets of olfactory receptor neurons (ORNs) with different transduction mechanisms. One ORN subset lacks the canonical cAMP transduction pathway and responds to amino acid odorants. The second subset has the cAMP transduction pathway but as yet suitable odorants are unknown. Here we report the identification of amines as proper olfactory stimuli for larval X. laevis using functional Ca(2+) imaging and slice preparations of the olfactory system. The response profiles of individual ORNs to a number of amines were extremely complex and mostly highly specific. The great majority of amine-sensitive ORNs responded also to forskolin, an activator of the olfactory cAMP transduction pathway. Most amine-induced responses could be attenuated by the cyclic nucleotide-gated channel inhibitor LY83583. This confirms that most amine-responsive olfactory receptors (ORs) are coupled to the cAMP-dependent transduction pathway. Furthermore, we show that trace amine-associated receptors (TAARs), which have been shown to act as specific ORs for amines in mammals, are expressed in the olfactory organ of X. laevis. The TAARs expressed in Xenopus cannot, however, explain the complex responses of individual ORNs to amines because there are too few of them. This indicates that, in addition to TAARs, there must be other receptor families involved in the detection of amines.

Nucleotide-induced Ca2+ signaling in sustentacular supporting cells of the olfactory epithelium

Extracellular purines and pyrimidines are important signaling molecules acting via purinergic cell-surface receptors in neurons, glia, and glia-like cells such as sustentacular supporting cells (SCs) of the olfactory epithelium (OE). Here, we thoroughly characterize ATP-induced responses in SCs of the OE using functional Ca2+ imaging. The initial ATP-induced increase of the intracellular Ca2+ concentration [Ca2+]i always occurred in the apical part of SCs and subsequently propagated toward the basal lamina, indicating the occurrence of purinergic receptors in the apical part of SCs. The mean propagation velocity of the Ca2+ signal within SCs was 17.10 +/- 1.02 microm/s. ATP evoked increases in [Ca2+]i in both the presence and absence of extracellular Ca2+. Depletion of the intracellular Ca2+ stores abolished the responses. This shows that the ATP-induced [Ca2+]i increases were in large part, if not entirely, due to the activation of G protein-coupled receptors followed by Ca2+ mobilization from intracellular stores, suggesting an involvement of P2Y receptors. The order of potency of the applied purinergic agonists was UTP > ATP > ATPgammaS (with all others being only weakly active or inactive). The ATP-induced [Ca2+]i increases could be reduced by the purinergic antagonists PPADS and RB2, but not by suramin. Our findings suggest that extracellular nucleotides in the OE activate SCs via P2Y2/P2Y4-like receptors and initiate a characteristic intraepithelial Ca2+ wave.

Improved fluorescent (calcium indicator) dye uptake in brain slices by blocking multidrug resistance transporters

ATP-binding cassette (ABC) transporters are a family of transmembrane proteins that, also known as multidrug resistance proteins, transport a wide variety of substrates across biological membranes in an energy-dependent manner. Recently it has been shown that members of this protein family interfere with fluorescent (calcium indicator) dye uptake in taste buds of rat and in cells in the olfactory epithelium of larval Xenopus laevis, including olfactory receptor neurons. It has, however, not been resolved whether this effect only serves to extrude xenobiotics in sensory taste and olfactory cells, or alternatively, whether it is a more general feature of many central nervous system neurons. In the latter case blocking these transporters would improve fluorescent dye uptake in general. Here we show, by means of cell imaging, that also neurons of the olfactory bulb express multidrug resistance transporters, whereby a marked inhomogeneity among cells in the main and accessory olfactory bulb was observed. Blocking these transporters improved the net uptake of fluorescent dyes not only in cell somata of the olfactory bulb, but especially in fine neuronal structures such as individual dendrites or olfactory glomeruli, which consist of a tangle of tiny neuronal processes. We therefore suggest that the expression of multidrug resistance proteins may be common in cells of the central nervous system, and that the application of specific transport inhibitors could generally improve fluorescent dye uptake in brain slices, thereby improving calcium imaging conditions.

Presynaptic protein distribution and odour mapping in glomeruli of the olfactory bulb of Xenopus laevis tadpoles

The sensory input layer in the olfactory bulb (OB) is typically organized into spheroidal aggregates of dense neuropil called glomeruli. This characteristic compartmentalization of the synaptic neuropil is a typical feature of primary olfactory centres in vertebrates and most advanced invertebrates. In the present work we mapped the location of presynaptic sites in glomeruli across the OB using antibodies to presynaptic vesicle proteins and presynaptic membrane proteins in combination with confocal microscopy. In addition the responses of glomeruli upon mucosal application of amino acid-odorants and forskolin were monitored using functional calcium imaging. We first describe the spatial distribution of glomeruli across the main olfactory bulb (MOB) in premetamorphic Xenopus laevis. Second, we show that the heterogeneous organization of glomeruli along the dorsoventral and mediolateral axes of the MOB is associated with a differential distribution of synaptic vesicle proteins. While antibodies to synaptophysin, syntaxin and SNAP-25 uniformly labelled glomeruli in the whole MOB, intense synaptotagmin staining was present only in glomeruli in the lateral, and to a lesser extent in the intermediate, part of the OB. Interestingly, amino acid-responsive glomeruli were always located in the lateral part of the OB, and glomeruli activated by mucosal forskolin application were exclusively located in the medial part of the OB. This correlation between odour mapping and presynaptic protein distribution is an additional hint on the existence of different subsystems within the main olfactory system in larval Xenopus laevis.

Response profiles to amino acid odorants of olfactory glomeruli in larval Xenopus laevis

Glomeruli in the vertebrate olfactory bulb (OB) appear as anatomically discrete modules receiving direct input from the olfactory epithelium (OE) via axons of olfactory receptor neurons (ORNs). The response profiles with respect to amino acids (AAs) of a large number of ORNs in larval Xenopus laevis have been recently determined and analysed. Here we report on Ca(2+) imaging experiments in a nose-brain preparation of the same species at the same developmental stages. We recorded responses to AAs of glomeruli in the OB and determined the response profiles to AAs of individual glomeruli. We describe the general features of AA-responsive glomeruli and compare their response profiles to AAs with those of ORNs obtained in our previous study. A large number of past studies have focused either on odorant responses in the OE or on odorant-induced responses in the OB. However, a thorough comparison of odorant-induced responses of both stages, ORNs and glomeruli of the same species is as yet lacking. The glomerular response profiles reported herein markedly differ from the previously obtained response profiles of ORNs in that glomeruli clearly have narrower selectivity profiles than ORNs. We discuss possible explanations for the different selectivity profiles of glomeruli and ORNs in the context of the development of the olfactory map.

Cannabinoid action in the olfactory epithelium

The perception of odors is influenced by a variety of neuromodulators, and there is growing evidence that modulation already takes place in the olfactory epithelium. Here we report on cannabinergic actions in the olfactory epithelium of Xenopus laevis tadpoles. First we show that CB1 receptor-specific antagonists AM251, AM281, and LY320135 modulate odor-evoked calcium changes in olfactory receptor neurons. Second, we localize CB1-like immunoreactivity on dendrites of olfactory receptor neurons. Finally, we describe the cannabinergic influence on odor-induced spike-associated currents in individual olfactory receptor neurons. Here we demonstrate that the cannabinergic system has a profound impact on peripheral odor processing and discuss its possible function.

ATP activates both receptor and sustentacular supporting cells in the olfactory epithelium of Xenopus laevis tadpoles

Nucleotides and amino acids are acknowledged categories of water-borne olfactory stimuli. In previous studies it has been shown that larvae of Xenopus laevis are able to sense amino acids. Here we report on the effect of ATP in the olfactory epithelium (OE) of Xenopus laevis tadpoles. First, ATP activates a subpopulation of cells in the OE. The ATP-sensitive subset of cells is almost perfectly disjoint from the subset of amino acid-activated cells. Both responses are not mediated by the well-described cAMP transduction pathway as the two subpopulations of cells do not overlap with a third, forskolin-activated subpopulation. We further show that, in contrast to amino acids, which act exclusively as olfactory stimuli, ATP appears to feature a second role. Surprisingly it activated a large number of sustentacular supporting cells (SCs) and, to a much lower extent, olfactory receptor neurons. The cells of the amino acid- and ATP-responding subsets featured differences in shape, size and position in the OE. The latencies to activation upon stimulus application differed markedly in these subsets. To obtain these results two technical points were important. We used a novel dextran-tetramethylrhodamine-backfilled slice preparation of the OE and we found out that an antibody to calnexin, a known molecular chaperone, also labels SCs. Our findings thus show a strong effect of ATP in the OE and we discuss some of the possible physiological functions of nucleotides in the OE.