Organization of glomeruli in the main olfactory bulb of Xenopus laevis tadpoles

Predation cues induce predator specific changes in olfactory neurons encoding defensive responses in agile frog tadpoles

Although behavioural defensive responses have been recorded several times in both laboratory and natural habitats, their neural mechanisms have seldom been investigated. To explore how chemical, water-borne cues are conveyed to the forebrain and instruct behavioural responses in anuran larvae, we conditioned newly hatched agile frog tadpoles using predator olfactory cues, specifically either native odonate larvae or alien crayfish kairomones. We expected chronic treatments to influence the basal neuronal activity of the tadpoles' mitral cells and alter their sensory neuronal connections, thereby impacting information processing. Subsequently, these neurons were acutely perfused, and their responses were compared with the defensive behaviour of tadpoles previously conditioned and exposed to the same cues. Tadpoles conditioned with odonate cues differed in both passive and active cell properties compared to those exposed to water (controls) or crayfish cues. The observed upregulation of membrane conductance and increase in both the number of active synapses and receptor density at the postsynaptic site are believed to have enhanced their responsiveness to external stimuli. Odonate cues also affected the resting membrane potential and firing rate of mitral cells during electrophysiological patch-clamp recordings, suggesting a rearrangement of the repertoire of voltage-dependent conductances expressed in cell membranes. These recorded neural changes may modulate the induction of an action potential and transmission of information. Furthermore, the recording of neural activity indicated that the lack of defensive responses towards non-native predators is due to the non-recognition of their olfactory cues.

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.

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.

Axon terminals control endolysosome diffusion to support synaptic remodelling

Endolysosomes present in the presynaptic terminal move by diffusion constrained by F-actin and increase their mobility during the remodelling of synaptic connectivity to support a local degradative activity.

Endolysosomes are acidic organelles formed by the fusion of endosomes with lysosomes. In the presynaptic compartment they contribute to protein homeostasis, the maintenance of vesicle pools and synaptic stability. Here, we evaluated the mobility of endolysosomes found in axon terminals of olfactory sensory neurons of Xenopus tropicalis tadpoles. F-actin restricts the motion of these presynaptic acidic organelles which is characterized by a diffusion coefficient of 6.7 × 10⁻³ μm²·s⁻¹. Local injection of secreted protein acidic and rich in cysteine (SPARC) in the glomerular layer of the olfactory bulb disrupts the structure of synaptic F-actin patches and increases the presence and mobility of endolysosomal organelles found in axon terminals. The increased motion of endolysosomes is localized to the presynaptic compartment and does not promote their access to axonal regions for retrograde transportation to the cell body. Local activation of synaptic degradation mechanisms mediated by SPARC coincides with a loss of the ability of tadpoles to detect waterborne odorants. Together, these observations show that the diffusion of presynaptic endolysosomes increases during conditions of synaptic remodelling to support their local degradative activity.

COVID-19 and Parkinson's disease: Defects in neurogenesis as the potential cause of olfactory system impairments and anosmia

Anosmia, a neuropathogenic condition of loss of smell, has been recognized as a key pathogenic hallmark of the current pandemic SARS-CoV-2 infection responsible for COVID-19. While the anosmia resulting from olfactory bulb (OB) pathology is the prominent clinical characteristic of Parkinson's disease (PD), SARS-CoV-2 infection has been predicted as a potential risk factor for developing Parkinsonism-related symptoms in a significant portion of COVID-19 patients and survivors. SARS-CoV-2 infection appears to alter the dopamine system and induce the loss of dopaminergic neurons that have been known to be the cause of PD. However, the underlying biological basis of anosmia and the potential link between COVID-19 and PD remains obscure. Ample experimental studies in rodents suggest that the occurrence of neural stem cell (NSC) mediated neurogenesis in the olfactory epithelium (OE) and OB is important for olfaction. Though the occurrence of neurogenesis in the human forebrain has been a subject of debate, considerable experimental evidence strongly supports the incidence of neurogenesis in the human OB in adulthood. To note, various viral infections and neuropathogenic conditions including PD with olfactory dysfunctions have been characterized by impaired neurogenesis in OB and OE. Therefore, this article describes and examines the recent reports on SARS-CoV-2 mediated OB dysfunctions and defects in the dopaminergic system responsible for PD. Further, the article emphasizes that COVID-19 and PD associated anosmia could result from the regenerative failure in the replenishment of the dopaminergic neurons in OB and olfactory sensory neurons in OE.

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.

Subpopulations of Projection Neurons in the Olfactory Bulb

Generation of neuronal diversity is a biological strategy widely used in the brain to process complex information. The olfactory bulb is the first relay station of olfactory information in the vertebrate central nervous system. In the olfactory bulb, axons of the olfactory sensory neurons form synapses with dendrites of projection neurons that transmit the olfactory information to the olfactory cortex. Historically, the olfactory bulb projection neurons have been classified into two populations, mitral cells and tufted cells. The somata of these cells are distinctly segregated within the layers of the olfactory bulb; the mitral cells are located in the mitral cell layer while the tufted cells are found in the external plexiform layer. Although mitral and tufted cells share many morphological, biophysical, and molecular characteristics, they differ in soma size, projection patterns of their dendrites and axons, and odor responses. In addition, tufted cells are further subclassified based on the relative depth of their somata location in the external plexiform layer. Evidence suggests that different types of tufted cells have distinct cellular properties and play different roles in olfactory information processing. Therefore, mitral and different types of tufted cells are considered as starting points for parallel pathways of olfactory information processing in the brain. Moreover, recent studies suggest that mitral cells also consist of heterogeneous subpopulations with different cellular properties despite the fact that the mitral cell layer is a single-cell layer. In this review, we first compare the morphology of projection neurons in the olfactory bulb of different vertebrate species. Next, we explore the similarities and differences among subpopulations of projection neurons in the rodent olfactory bulb. We also discuss the timing of neurogenesis as a factor for the generation of projection neuron heterogeneity in the olfactory bulb. Knowledge about the subpopulations of olfactory bulb projection neurons will contribute to a better understanding of the complex olfactory information processing in higher brain regions.

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.

Interhemispheric asymmetry of c-Fos expression in glomeruli and the olfactory tubercle following repeated odor stimulation

Odor adaptation allows the olfactory system to regulate sensitivity to different stimulus intensities, which is essential for preventing saturation of the cell‐transducing machinery and maintaining high sensitivity to persistent and repetitive odor stimuli. Although many studies have investigated the structure and mechanisms of the mammalian olfactory system that responds to chemical sensation, few studies have considered differences in neuronal activation that depend on the manner in which the olfactory system is exposed to odorants, or examined activity patterns of olfactory‐related regions in the brain under different odor exposure conditions. To address these questions, we designed three different odor exposure conditions that mimicked diverse odor environments and analyzed c‐Fos‐expressing cells (c‐Fos+ cells) in the odor columns of the olfactory bulb (OB). We then measured differences in the proportions of c‐Fos‐expressing cell types depending on the odor exposure condition. Surprisingly, under the specific odor condition in which the olfactory system was repeatedly exposed to the odorant for 1 min at 5‐min intervals, one of the lateral odor columns and the ipsilateral hemisphere of the olfactory tubercle had more c‐Fos+ cells than the other three odor columns and the contralateral hemisphere of the olfactory tubercle. However, this interhemispheric asymmetry of c‐Fos expression was not observed in the anterior piriform cortex. To confirm whether the anterior olfactory nucleus pars externa (AONpE), which connects the left and right OB, contributes to this asymmetry, AONpE‐lesioned mice were analyzed under the specific odor exposure condition. Asymmetric c‐Fos expression was not observed in the OB or the olfactory tubercle. These data indicate that the c‐Fos expression patterns of the olfactory‐related regions in the brain are influenced by the odor exposure condition and that asymmetric c‐Fos expression in these regions was observed under a specific odor exposure condition due to synaptic linkage via the AONpE.

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.

Tight temporal coupling between synaptic rewiring of olfactory glomeruli and the emergence of odor-guided behavior in Xenopus tadpoles

Olfactory sensory neurons (OSNs) are chemoreceptors that establish excitatory synapses within glomeruli of the olfactory bulb. OSNs undergo continuous turnover throughout life, causing the constant replacement of their synaptic contacts. Using Xenopus tadpoles as an experimental system to investigate rewiring of glomerular connectivity, we show that novel OSN synapses can transfer information immediately after formation, mediating olfactory-guided behavior. Tadpoles recover the ability to detect amino acids 4 days after bilateral olfactory nerve transection. Restoration of olfactory-guided behavior depends on the efficient reinsertion of OSNs to the olfactory bulb. Presynaptic terminals of incipient synaptic contacts generate calcium transients in response to odors, triggering long lasting depolarization of olfactory glomeruli. The functionality of reconnected terminals relies on well-defined readily releasable and cytoplasmic vesicle pools. The continuous growth of non-compartmentalized axonal processes provides a vesicle reservoir to nascent release sites, which contrasts to the gradual development of cytoplasmic vesicle pools in conventional excitatory synapses. The immediate availability of fully functional synapses upon formation supports an age-independent contribution of OSNs to the generation of odor maps.

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.

Integrating temperature with odor processing in the olfactory bulb

Temperature perception has long been classified as a somesthetic function solely. However, in recent years several studies brought evidence that temperature perception also takes place in the olfactory system of rodents. Temperature has been described as an effective stimulus for sensory neurons of the Grueneberg ganglion located at the entrance of the nose. Here, we investigate whether a neuronal trace of temperature stimulation can be observed in the glomeruli and mitral cells of the olfactory bulb, using calcium imaging and fast line-scanning microscopy. We show in the Xenopus tadpole system that the γ-glomerulus, which receives input from olfactory neurons, is highly sensitive to temperature drops at the olfactory epithelium. We observed that thermo-induced activity in the γ-glomerulus is conveyed to the mitral cells innervating this specific neuropil. Surprisingly, a substantial number of thermosensitive mitral cells were also chemosensitive. Moreover, we report another unique feature of the γ-glomerulus: it receives ipsilateral and contralateral afferents. The latter fibers pass through the contralateral bulb, cross the anterior commissure, and then run to the ipsilateral olfactory bulb, where they target the γ-glomerulus. Temperature drops at the contralateral olfactory epithelium also induced responses in the γ-glomerulus and in mitral cells. Temperature thus appears to be a relevant physiological input to the Xenopus olfactory system. Each olfactory bulb integrates and codes temperature signals originating from receptor neurons of the ipsilateral and contralateral nasal cavities. Finally, temperature and chemical information is processed in shared cellular networks.

Distribution and functional organization of glomeruli in the olfactory bulbs of zebrafish (Danio rerio)

Odor molecules are transduced by thousands of olfactory sensory neurons (OSNs) located in the nasal cavity. Each OSN expresses a single functional odorant receptor protein and projects an axon from the sensory epithelia to an olfactory bulb glomerulus, which is selectively innervated by only one or a few OSN types. We used whole-mount immunocytochemistry to study the neurochemistry and anatomical organization of glomeruli in the zebrafish olfactory system. By employing combinations of antibodies against G-protein α subunits, calcium-binding proteins, and general neuronal markers, we selectively labeled various OSN types, their axonal projections to glomeruli, and the detailed anatomical distributions of individual glomeruli in different regions of the olfactory bulb. In this way we identified ≈140 glomeruli in each olfactory bulb of mature zebrafish. A small subset (27) of these glomeruli was unambiguously identifiable in nearly all animals examined. These units were large and, located mainly in the medial olfactory bulbs. Most glomeruli, however, were comparatively small, anatomically indistinguishable, and located in coarsely circumscribed regions; almost all of these latter glomeruli were innervated by OSNs that were labeled with anti-G(α s/olf) and/or anti-calretinin antibodies. Collectively, our results provide a uniquely detailed description of a vertebrate olfactory system and highlight anatomically distinct parallel neural pathways that mediate early aspects of olfactory processing in the zebrafish.

Glial investment of the adult and developing antennal lobe of Drosophila

In recent years the Drosophila olfactory system, with its unparalleled opportunities for genetic dissection of development and functional organization, has been used to study the development of central olfactory neurons and the molecular basis of olfactory coding. The results of these studies have been interpreted in the absence of a detailed understanding of the steps in maturation of glial cells in the antennal lobe. Here we present a high-resolution study of the glia associated with olfactory glomeruli in adult and developing antennal lobes. The study provides a basis for comparison of findings in Drosophila with those in the moth Manduca sexta that indicate a critical role for glia in antennal lobe development. Using flies expressing GFP under a Nervana2 driver to visualize glia for confocal microscopy, and probing at higher resolution with the electron microscope, we find that glial development in Drosophila differs markedly from that in moths: glial cell bodies remain in a rind around the glomerular neuropil; glial processes ensheathe axon bundles in the nerve layer but likely contribute little to axonal sorting; their processes insinuate between glomeruli only very late and then form only a sparse, open network around each glomerulus; and glial processes invade the synaptic neuropil. Taking our results in the context of previous studies, we conclude that glial cells in the developing Drosophila antennal lobe are unlikely to play a strong role in either axonal sorting or glomerulus stabilization and that in the adult, glial processes do not electrically isolate glomeruli from their neighbors.

Amphibian larvae and zinc sulphate: a suitable model to study the role of brain-derived neurotrophic factor (BDNF) in the neuronal turnover of the olfactory epithelium

The vertebrate olfactory system has fascinated neurobiologists over the last six decades because of its ability to replace its neurons and synaptic connections continuously throughout adult life, under both physiological and pathological conditions. Among the factors that are proposed to be involved in this regenerative potential, brain-derived neurotrophic factor (BDNF) is a candidate for having an important role in the neuronal turnover in the olfactory epithelium (OE) because of its well-documented neurogenic and trophic effects throughout the nervous system. The aim of the present study was to generate a suitable model to study the participation of BDNF in the recovery of the OE after injury in vivo. We developed an experimental design in which the OE of Rhinella arenarum tadpoles could be easily and selectively damaged by immersing the animals in ZnSO(4) solutions of various concentrations for differing time periods. Image analysis of histological sections showed that different combinations of each of these conditions produced statistically different degrees of injury to the olfactory tissue. We also observed that the morphology of the OE was restored within a few days of recovery after ZnSO(4) treatment. Immunohistochemical analysis of BDNF was performed with an antiserum whose specificity was confirmed by Western blotting, and which showed drastic changes in the abundance and distribution pattern of this neurotrophin in the damaged olfactory system. Our results thus suggest that BDNF is involved in the regeneration of the OE of amphibian larvae, and that our approach is suitable for further investigations of this topic.

Odor coding by modules of coherent mitral/tufted cells in the vertebrate olfactory bulb

Odor representation in the olfactory bulb (OB) undergoes a transformation from a combinatorial glomerular map to a distributed mitral/tufted (M/T) cell code. To understand this transformation, we analyzed the odor representation in large populations of individual M/T cells in the Xenopus OB. The spontaneous [Ca(2+)] activities of M/T cells appeared to be irregular, but there were groups of spatially distributed neurons showing synchronized [Ca(2+)] activities. These neurons were always connected to the same glomerulus. Odorants elicited complex spatiotemporal response patterns in M/T cells where nearby neurons generally showed little correlation. But the responses of neurons connected to the same glomerulus were virtually identical, irrespective of whether the responses were excitatory or inhibitory, and independent of the distance between them. Synchronous neurons received correlated EPSCs and were coupled by electrical conductances that could account for the correlated responses. Thus, at the output stage of the OB, odors are represented by modules of distributed and synchronous M/T cells associated with the same glomeruli. This allows for parallel input to higher brain centers.

Cytoarchitecture of the accessory olfactory bulb in the salamander Plethodon shermani

Plethodontid terrestrial salamanders are emerging models in the study of the evolution of chemical communication in vertebrates. Their vomeronasal system is well defined. It comprises sensory neurons in the epithelium of the vomeronasal organ, whose axons form the vomeronasal nerve projecting to the accessory olfactory bulb (AOB), which in turn projects to the vomeronasal amygdala through the accessory olfactory tract. A detailed description of the cellular elements of the urodele AOB is lacking. Neuronal morphology in the AOB was studied by means of biocytin intracellular injections and retrograde tract tracing in the salamander Plethodon shermani. The AOB exhibits the characteristic lamination of olfactory bulbs, except that it displays a mixed periglomerular and mitral somata layer superficially. Mitral cells are the only AOB neurons projecting to the vomeronasal amygdala. Each mitral cell sends multiple axonal branches, generally through both dorsal and ventral portions of the accessory olfactory tract. Some mitral cells additionally send axon collaterals in the white matter immediately ventral to the AOB. AOB interneurons are divided into superficial periglomerular and deep granule cells, each category exhibiting morphological variety. Some neurons in the granule cell layer of the AOB or the region ventral to the AOB have dendritic trees that cover both regions. The present study is the first to highlight the full anatomical extent of single AOB neurons and surprisingly suggests that the ventrolateral telencephalon found below the AOB is part of the salamander vomeronasal system.

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.

Functional regeneration of the olfactory bulb requires reconnection to the olfactory nerve in Xenopus larvae

Larvae of the South African clawed frog (Xenopus laevis) can regenerate the telencephalon, which consists of the olfactory bulb and the cerebrum, after it has been partially removed. Some authors have argued that the telencephalon, once removed, must be reconnected to the olfactory nerve in order to regenerate. However, considerable regeneration has been observed before reconnection. Therefore, we have conducted several experiments to learn whether or not reconnection is a prerequisite for regeneration. We found that the olfactory bulb did not regenerate without reconnection, while the cerebrum regenerated by itself. On the other hand, when the brain was reconnected by the olfactory nerve, both the cerebrum and the olfactory bulb regenerated. Morphological and histological investigation showed that the regenerated telencephalon was identical to the intact one in morphology, types and distributions of cells, and connections between neurons. Froglets with a regenerated telencephalon also recovered olfaction, the primary function of the frog telencephalon. These results suggest that the Xenopus larva requires reconnection of the regenerating brain to the olfactory nerve in order to regenerate the olfactory bulb, and thus the regenerated brain functions, in order to process olfactory information.

The role of sensory network dynamics in generating a motor program

Sensory input plays a major role in controlling motor responses during most behavioral tasks. The vestibular organs in the marine mollusk Clione, the statocysts, react to the external environment and continuously adjust the tail and wing motor neurons to keep the animal oriented vertically. However, we suggested previously that during hunting behavior, the intrinsic dynamics of the statocyst network produce a spatiotemporal pattern that may control the motor system independently of environmental cues. Once the response is triggered externally, the collective activation of the statocyst neurons produces a complex sequential signal. In the behavioral context of hunting, such network dynamics may be the main determinant of an intricate spatial behavior. Here, we show that (1) during fictive hunting, the population activity of the statocyst receptors is correlated positively with wing and tail motor output suggesting causality, (2) that fictive hunting can be evoked by electrical stimulation of the statocyst network, and (3) that removal of even a few individual statocyst receptors critically changes the fictive hunting motor pattern. These results indicate that the intrinsic dynamics of a sensory network, even without its normal cues, can organize a motor program vital for the survival of the animal.

3D atlas describing the ontogenic evolution of the primary olfactory projections in the olfactory bulb of Xenopus laevis

The adult Xenopus presents the unique capability to smell odors both in water and air thanks to two different olfactory pathways. Nevertheless, the tadpole can initially perceive only water-borne odorants, as the olfactory receptor neurons (ORN) that will detect air-borne odorants develop later. Such a phenomenon requires major reorganization processes. Here we focused on the precise description of the neuroanatomical modifications occurring in the olfactory bulb (OB) of the tadpole throughout metamorphosis. Using both carbocyanine dyes and lectin staining, we investigated the evolution of ORN projection patterns into the OB from Stages 47 to 66, thus covering the period of time when all the modifications take place. Although our results confirm previous works (Reiss and Burd [1997] Semin Cell Dev Biol 8:171-179), we showed for the first time that the main olfactory bulb (MOB) is subdivided into seven zones at Stage 47 plus the accessory olfactory bulb (AOB). These seven zones receive fibers dedicated to aquatic olfaction ("aquatic fibers") and are conserved until Stage 66. At Stage 48 the first fibers dedicated to the aerial olfaction constitute a new dorsomedial zone that grows steadily, pushing the seven original zones ventrolaterally. Only the part of the OB receiving aquatic fibers is fragmented, reminiscent of the organization described in fish. This raises the question of whether such an organization in zones constitutes a plesiomorphy or is linked to aquatic olfaction. We generated a 3D atlas at several stages which are representative of the reorganization process. This will be a useful tool for future studies of development and function.

Glial fibrillary acidic protein and vimentin expression in the frog olfactory system during metamorphosis

In the present study, we investigated glial cell organization in the olfactory system of adult and tadpole Xenopus laevis using glial fibrillary acidic protein and vimentin antibodies. Our results showed for the first time that glial fibrillary acidic protein was strongly expressed at the level of the olfactory nerve from tadpole to adult and was likely to be expressed by ensheathing glia. In the olfactory bulb, the nerve layer was stained, and no staining was observed in glomeruli. By contrast, vimentin decorated radial glia in the bulb but faintly stained the olfactory nerve. Interestingly, glial fibrillary acidic protein and vimentin presented complementary staining patterns, with glial fibrillary acidic protein being expressed in the peripheral olfactory system and vimentin being expressed in the central part of the olfactory system.

Individual olfactory sensory neurons project into more than one glomerulus in Xenopus laevis tadpole olfactory bulb

An essential step in the coding of odorants is the way olfactory sensory neurons (OSNs) convey their information to the olfactory bulb. This projection determines how the specificities of OSNs are mapped onto the spatial activity patterns of the olfactory bulb (OB). Despite the fact that virtually nothing is known about how individual OSN axons project to glomeruli, it is generally believed that OSNs always project to one glomerulus each. Our recent findings in tadpoles of Xenopus laevis challenge this view. By injection of a tracer into individual OSNs, we show for the first time that axons typically project into more than one glomerulus and that they do so in a characteristic way. Upon entering the olfactory bulb, an axon bifurcates to give two primary branches. Each of these branches bifurcates again to give two subbranches, thus resulting in four subbranches per OSN. The two subbranches of each primary branch project into two different glomeruli. Variations of this characteristic innervation pattern include the innervation of three and, occasionally, one glomerulus. In any case, and independent of the number of glomeruli innervated by an OSN, each glomerulus receives at least two axonal branches of the same OSN.