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Olfactory wiring logic in amphibians challenges the basic assumptions of the unbranched axon concept
No full textOlfactory wiring logic in amphibians challenges the basic assumptions of the unbranched axon concept
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The Olfactory System as a Model to Study Axonal Growth Patterns and Morphology In Vivo
No full textThe 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
Olfactory Wiring Logic in Amphibians Challenges the Basic Assumptions of the Unbranched Axon Concept
No full textOlfactory receptor neurons extend axons into the olfactory bulb, where they face the challenge to integrate into existing circuitry. The consensus view is that in vertebrates individual receptor neurons project unbranched axons into one specific glomerulus of the olfactory bulb. Wereport here that, strikingly different from the generally assumed wiring principle in vertebrate olfactory systems, axons of single receptor neurons of Xenopus laevis regularly bifurcate and project into more than one glomerulus. Specifically, the innervation of multiple glomeruli is present in all ontogenetic stages of this species, from the larva to the postmetamorphic frog. Also, we show that this unexpected wiring pattern is not restricted to axons of immature receptor neurons, but that it is also a feature of mature neurons of both the main and accessory olfactory system. This glomerular innervation pattern is unique among vertebrates investigated so far and represents a new olfactory wiring strategy
cAMP-independent olfactory transduction of amino acids in Xenopus laevis tadpoles
No full textWhether odorants are transduced by only one or more than one second messenger has been a longstanding question in olfactory research. In a previous study we started to address this question mainly by using calcium imaging in the olfactory bulb. Here, we present direct evidence for our earlier conclusions using the calcium imaging technique in the mucosa slice. The above question can now unambiguously be answered. We show that some olfactory receptor neurons (ORNs) respond to stimulation with amino acids with an increase of the intracellular calcium concentration [Ca2+](i). In order to see whether or not these responses were mediated by the cAMP transduction pathway we applied forskolin or the membrane-permeant cAMP analogue pCPT-cAMP to the olfactory epithelium. The ensemble of ORNs that was activated by amino acids markedly differed from the ensemble of neurons activated by forskolin or pCPT-cANIP. Less than 6% of the responding ORNs showed a response to both amino acids and the pharmacological agents activating the cANIP transduction pathway. We conclude that ORNs of Xenopus laevis tadpoles have both cAMP-dependent and cAMP-independent olfactory transduction pathways and that most amino acids are transduced in a cAMP-independent way
Cascades of response vectors of olfactory receptor neurons in Xenopus laevis tadpoles
No full textOlfactory receptor neurons (ORNs) of Xenopus laevis tadpoles respond to water-born stimuli such as amino acids. Their sensitivity spectra with respect to amino acids have recently been shown to become more selective over ontogenetic stages [Manzini & Schild (2004) J. Gen. Physiol., 123, 99-107]. In this paper, we undertake a theoretical analysis of this data set and determine the correlational relationships among odorant responses represented as binary response vectors. We first show that, on the one hand, the number of 204 ORN classes (out of 283 recorded ORNs) cannot be explained by a random expression pattern of olfactory receptors (ORs). On the other hand, this number does not appear to be reconcilable with the idea that individual ORNs express one type of OR each. The covariance matrix of stimulus responses shows that the responses to some stimuli are correlated to those of others. Furthermore, the response vectors show positive as well as negative correlations among each other. While the positive correlations can partly be explained by the differing response frequencies to the odorants used, the negative ones cannot. Finally, we analyse the similarity among responses using the Hamming distance as a distance measure, the result being that most response vectors differ from others by small Hamming distances. Such vectors are shown to form pattern cascades, possibly reflecting a decreasing number of ORs being expressed over ontogenetic stages
The Amphibian Olfactory System as a Model to Study Axonal Growth and Synaptogenesis In Vivo
No full textSulfated steroids are chemosensory stimuli in both the main and accessory olfactory system of an amphibian
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