Neuronal representation of odourants in the olfactory bulb of Xenopus laevis tadpoles

Olfactory experiences dynamically regulate plasticity of dendritic spines in granule cells of Xenopus tadpoles in vivo

Granule cells, rich in dendrites with densely punctated dendritic spines, are the most abundant inhibitory interneurons in the olfactory bulb. The dendritic spines of granule cells undergo remodeling during the development of the nervous system. The morphological plasticity of the spines' response to different olfactory experiences in vivo is not fully known. In initial studies, a single granule cell in Xenopus tadpoles was labeled with GFP plasmids via cell electroporation; then, morphologic changes of the granule cell spines were visualized by in vivo confocal time-lapse imaging. With the help of long-term imaging, the total spine density, dynamics, and stability of four types of dendritic spines (mushroom, stubby, thin and filopodia) were obtained. Morphological analysis demonstrated that odor enrichment produced a remarkable increase in the spine density and stability of large mushroom spine. Then, with the help of short-term imaging, we analyzed the morphological transitions among different spines. We found that transitions between small spines (thin and filopodia) were more easily influenced by odor stimulation or olfactory deprivation. These results indicate that different olfactory experiences can regulate the morphological plasticity of different dendritic spines in the granule cell.

Fear is the mother of invention: anuran embryos exposed to predator cues alter life-history traits, post-hatching behaviour, and neuronal activity patterns

Neurophysiological modifications associated to phenotypic plasticity in response to predators are largely unexplored, and there is a gap of knowledge on how the information encoded in predator cues is processed by prey sensory systems. To explore these issues, we exposed Rana dalmatina embryos to dragonfly chemical cues (kairomones) up to hatching. At different times after hatching (up to 40 days), we recorded morphology and antipredator behaviour of control and embryonic-treated tadpoles as well as their neural olfactory responses, by recording the activity of their mitral neurons before and after exposure to a kairomone solution. Embryonic-treated embryos hatched later and originated smaller hatchlings than control siblings. In addition, embryonic-treated tadpoles showed a stronger antipredator response than controls at 10 (but not at 30) days post-hatching, though the intensity of the contextual response to the kairomone stimulus did not differ between the two groups. Baseline neuronal activity at 30 days post-hatching, as assessed by the frequency of spontaneous excitatory postsynaptic events and by the firing rate of mitral cells, was higher among embryonic-treated tadpoles compared to controls. At the same time, neuronal activity showed a stronger increase among embryonic-treated tadpoles than among controls after a local kairomone perfusion. Hence, a different contextual plasticity between treatments at the neuronal level was not mirrored by the antipredator behavioural response. In conclusion, our experiments demonstrate ontogenetic plasticity in tadpole neuronal activity after embryonic exposure to predator cues, corroborating the evidence that early-life experience can contribute to shaping the phenotype at later life stages.

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.

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.

Olfactory coding with patterns of response latencies

The encoding of odors by spatiotemporal patterns of mitral/tufted (M/T) cells in the vertebrate olfactory bulb has been discussed controversially. Motivated by temporal constraints from behavioral studies, we investigated the information contained in odor-evoked first-spike latencies. Using simultaneous recordings of dozens of M/T cells with a high temporal resolution and quantitative ensemble correlation techniques, we show that latency patterns, and in particular latency rank patterns, are highly odor specific and reproducible. They reliably predict the odor identity as well as the odor concentration on a single-trial basis and on short timescales-in fact, more reliably than patterns of firing rates. Furthermore, we show that latency ranks exhibit a better reproducibility at the level of M/T cells than in olfactory receptor neurons. Our results suggest that the latency patterns of M/T cells contain all the information higher brain centers need to identify odors and their concentrations.

Calcium imaging in the ant Camponotus fellah reveals a conserved odour-similarity space in insects and mammals

Background

Olfactory systems create representations of the chemical world in the animal brain. Recordings of odour-evoked activity in the primary olfactory centres of vertebrates and insects have suggested similar rules for odour processing, in particular through spatial organization of chemical information in their functional units, the glomeruli. Similarity between odour representations can be extracted from across-glomerulus patterns in a wide range of species, from insects to vertebrates, but comparison of odour similarity in such diverse taxa has not been addressed. In the present study, we asked how 11 aliphatic odorants previously tested in honeybees and rats are represented in the antennal lobe of the ant Camponotus fellah, a social insect that relies on olfaction for food search and social communication.

Results

Using calcium imaging of specifically-stained second-order neurons, we show that these odours induce specific activity patterns in the ant antennal lobe. Using multidimensional analysis, we show that clustering of odours is similar in ants, bees and rats. Moreover, odour similarity is highly correlated in all three species.

Conclusion

This suggests the existence of similar coding rules in the neural olfactory spaces of species among which evolutionary divergence happened hundreds of million years ago.

Activity correlation imaging: visualizing function and structure of neuronal populations

For the analysis of neuronal networks it is an important yet unresolved task to relate the neurons' activities to their morphology. Here we introduce activity correlation imaging to simultaneously visualize the activity and morphology of populations of neurons. To this end we first stain the network's neurons using a membrane-permeable [Ca(2+)] indicator (e.g., Fluo-4/AM) and record their activities. We then exploit the recorded temporal activity patterns as a means of intrinsic contrast to visualize individual neurons' dendritic morphology. The result is a high-contrast, multicolor visualization of the neuronal network. Taking the Xenopus olfactory bulb as an example we show the activities of the mitral/tufted cells of the olfactory bulb as well as their projections into the olfactory glomeruli. This method, yielding both functional and structural information of neuronal populations, will open up unprecedented possibilities for the investigation of neuronal networks.

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.

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.

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.

Cell type-specific relationships between spiking and [Ca2+]i in neurons of the Xenopus tadpole olfactory bulb

Multi-neuronal recordings with Ca2+ indicator dyes usually relate [Ca2+]i to action potentials (APs) assuming a stereotypical dependency between the two. However, [Ca2+]i affects and is affected by numerous complex mechanisms that differ from cell type to cell type, from cell compartment to cell compartment. Moreover, [Ca2+]i depends on the specific way a cell is activated. Here we investigate, by combining calcium imaging and on-cell patch clamp recordings, the relationship between APs (spiking) and somatic [Ca2+]i in mitral and granule cells of the olfactory bulb in Xenopus laevis tadpoles. Both cell types exhibit ongoing and odour-modulated [Ca2+]i dynamics. In mitral cells, the occurrence of APs in both spontaneous and odour-evoked situations correlates tightly to step-like [Ca2+]i increases. Moreover, odorant-induced suppression of spontaneous firing couples to a decrease in [Ca2+]i. In contrast, granule cells show a substantial number of uncorrelated events such as increases in [Ca2+]i without APs occurring or APs without any effect upon [Ca2+]i. The correlation between spiking and [Ca2+]i is low, possibly due to somatic NMDAR-mediated and subthreshold voltage-activated Ca2+ entries, and thus does not allow a reliable prediction of APs based on calcium imaging. Taken together, our results demonstrate that the relationship between somatic [Ca2+]i and APs can be cell type specific. Taking [Ca2+]i dynamics as an indicator for spiking activity is thus only reliable if the correlation has been established in the system of interest. When [Ca2+]i and APs are precisely correlated, fast calcium imaging is an extremely valuable tool for determining spatiotemporal patterns of APs in neuronal population.

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 neuroanatomy of the rhinophore of Aplysia punctata

Background

For marine snails, olfaction represents a crucial sensory modality for long-distance reception, as auditory and visual information is limited. The posterior tentacle of Aplysia, the rhinophore, is a chemosensory organ and several behavioural studies showed that the rhinophores can detect pheromones, initiate orientation and locomotion toward food. However the functional neuroanatomy of the rhinophore is not yet clear. Here we apply serotonin-immunohistochemistry and fluorescent markers in combination with confocal microscopy as well as optical recording techniques to elucidate the structure and function of the rhinophore of the sea slug Aplysia punctata.

Results

With anatomical techniques an overview of the neuroanatomical organization of the rhinophore is presented. Labelling with propidium iodide revealed one layer of cell nuclei in the sensory epithelium and densely packed cell nuclei beneath the groove of the rhinophore, which extends to about two third of the total length of the rhinophore. Serotonin immunoreactivity was found within the olfactory glomeruli underneath the epithelium as well as in the rhinophore ganglion. Retrograde tracing from the rhinophore ganglion with 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide (DiA) demonstrated the connection of glomeruli with the ganglion. Around 36 glomeruli (mean diameter 49 μm) were counted in a single rhinophore. Fluorimetric measurements of intracellular Ca²⁺ levels using Fura-2 AM loading revealed Ca²⁺-responses within the rhinophore ganglion to stimulation with amino acids. Bath application of different amino acids revealed differential responses at different positions within the rhinophore ganglion.

Conclusion

Our neuroanatomical study revealed the number and position of glomeruli in the rhinophore and the rhinophore ganglion as processing stage of sensory information. Serotonin-immunoreactive processes were found extensively within the rhinophore, but was not detected within any peripheral cell body. Amino acids were used as olfactory stimuli in optical recordings and induced sensory responses in the rhinophore ganglion. The complexity of changes in intracellular Ca²⁺-levels indicates, that processing of odour information takes place within the rhinophore ganglion. Our neuroanatomical and functional studies of the rhinophore open up a new avenue to analyze the olfactory system in Aplysia.

In situ background estimation in quantitative fluorescence imaging

Fluorescence imaging of bulk-stained tissue is a popular technique for monitoring the activities in a large population of cells. However, a precise quantification of such experiments is often compromised by an ambiguity of background estimation. Although, in single-cell-staining experiments, background can be measured from a neighboring nonstained region, such a region often does not exist in bulk-stained tissue. Here we describe a novel method that overcomes this problem. In contrast to previous methods, we determined the background of a given region of interest (ROI) using the information contained in the temporal dynamics of its individual pixels. Since no information outside the ROI is needed, the method can be used regardless of the staining profile in the surrounding tissue. Moreover, we extend the method to deal with background inhomogeneities within a single ROI, a problem not yet solved by any of the currently available tools. We performed computer simulations to demonstrate the accuracy of our method and give example applications in ratiometric calcium imaging of bulk-stained olfactory bulb slices. Converting the fluorescence signals into [Ca2+] gives resting values consistent with earlier single-cell staining results, and odorant-induced [Ca2+] transients can be quantitatively compared in different cells. Using these examples we show that inaccurate background subtraction introduces large errors (easily in the range of 100%) in the assessment of both resting [Ca2+] and [Ca2+] dynamics. The proposed method allows us to avoid such errors.

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.

Dopamine inhibits mitral/tufted--> granule cell synapses in the frog olfactory bulb

Synaptic interactions between the dendrites of mitral/tufted (MT) and granule cells (GCs) in the olfactory bulb are important for the determination of spatiotemporal firing patterns of MTs, which form an odor representation passed to higher brain centers. These synapses are subject to modulation from several sources originating both within and outside the bulb. We show that dopamine, presumably released by TH-positive local interneurons, reduces synaptic transmission from MTs to GCs. MT neurons express D2-like receptors (D2Rs), and both dopamine and the D2 agonist quinpirole decrease EPSC amplitude at the MT--> GC synapse. D2R activation also increases paired pulse facilitation and decreases the frequency of action potential-independent spontaneous miniature EPSCs in GCs, consistent with an effect on MT glutamate release downstream from Ca2+ influx. Analysis of spike-evoked Ca2+ transients in MT lateral dendrites additionally shows that quinpirole reduces Ca2+ influx preferentially at distal locations, possibly by reducing dendritic excitability via increased transient K+ channel availability. When the OB is activated physiologically by using odor stimuli, blocking D2Rs increases the power of GABA(A)-dependent oscillations in the local field potential. This demonstrates a functional role for the dopaminergic circuit during normal odor-evoked responses and for the modulation of dendritic release and excitability in neuronal circuit function. Regulation of spike invasion of lateral dendrites by transient K+ currents also may provide a mechanism for local outputs of MTs to be controlled dynamically via other neuromodulators or by postsynaptic potentials.

Brominated pyrrole alkaloids from marine Agelas sponges reduce depolarization-induced cellular calcium elevation

Seven pyrrole alkaloids isolated from Agelas sponges were tested for interactions with the cellular calcium homeostasis. Brominated pyrrole alkaloids reduced voltage dependent calcium elevation in PC12 cells. Dibromosceptrin was the most potent alkaloid with a half maximal concentration of 2.8 microM followed by sceptrin (67.5 microM) and oroidin (75.8 microM). 4,5-Dibromopyrrole-2-carboxylic acid reduced calcium elevation at concentrations exceeding 30 microM but did not eliminate calcium elevation at concentrations up to 1 mM. 4-Bromopyrrole-2-carboxylic acid and pyrrole-2-carboxylic acid were not active in this respect. The aminoimidazole group appeared to have a significant effect on voltage dependent calcium elevation shown by the comparison of oroidin with 4,5-dibromopyrrole-2-carboxylic acid. The degree of bromination of the pyrrole moiety is another important factor, as was shown by the comparison of 4,5-dibromopyrrole-2-carboxylic acid with 4-bromopyrrole-2-carboxylic acid, as well as oroidin with hymenidin and dibromosceptrin with sceptrin. The previously reported feeding deterrent activity of brominated pyrrole alkaloids in Agelas sponges against predatory reef fish may partly be explained by a general interaction of these alkaloids with the cellular calcium homeostasis. The chemoreception of bromopyrrole alkaloids in sea water is shown using sensory neurons in the rhinophore of the sea slug Aplysia punctata.

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

Structural and functional investigations were carried out to study olfactory glomeruli in the main olfactory bulb (OB) in tadpoles of the clawed frog, Xenopus laevis. Calcium imaging of odor response patterns of OB neurons revealed that the synapses within the glomeruli are functional. Tracing axons of individual olfactory receptor neurons (ORNs), dendrites of mitral/tufted (M/T) cells and processes of periglomerular interneurons indicate that the glomerular architecture is solely determined by terminal branches of ORN axons and tufts of M/T primary dendrites. The small population of periglomerular neurons forms wide-field arborizations that always extend over many glomeruli, enter the glomeruli, but lack any glomerular tufts. Antibodies to synaptophysin indicate a high density of synapses within glomeruli, which was further confirmed at the ultrastructural level and quantified to approximately 0.5 synaptic sites per microm(2). Combining immunocytochemistry and ultrastructural investigations, we show that glomeruli in Xenopus laevis tadpoles lack any cellular borders. Glomeruli are surrounded neither by periglomerular somata nor by glial processes. Taken together, our results demonstrate that olfactory glomeruli in Xenopus laevis tadpoles (1) are fully functional, (2) are spheroidal neuropil aggregations of terminal tufts of ORNs and tufts of primary dendrites of M/T cells, and (3) are not enwrapped by a border formed by juxtaglomerular cells.