The blood–brain barrier (BBB) is a semipermeable interface that prevents the non-selective transport into the central nervous system. It controls the delivery of macromolecules fueling the brain metabolism and the immunological surveillance. The BBB permeability is locally regulated depending on the physiological requirements, maintaining the tissue homeostasis and influencing pathological conditions. Given its relevance in vertebrate CNS, it is surprising that little is known about the BBB in Amphibians, some of which are capable of adult CNS regeneration. The BBB size threshold of the anuran Xenopus laevis (African clawed toad), as well as two urodele species, Ambystoma mexicanum (axolotl) and Pleurodeles waltl (Iberian ribbed newt), was evaluated under physiological conditions through the use of synthetic tracers. We detected important differences between the analyzed species. Xenopus exhibited a BBB with characteristics more similar to those observed in mammals, whereas the BBB of axolotl was found to be permeable to the 1 kDa tracer. The permeability of the 1 kDa tracer measured in Pleurodeles showed values in between axolotl and Xenopus vesseks. We confirmed that these differences are species-specific and not related to metamorphosis. In line with these results, the tight junction protein Claudin-5 was absent in axolotl, intermediate in Pleurodeles and showed full-coverage in Xenopus vessels. Interestingly, electron microscopy analysis and the retention pattern of the larger tracers (3 and 70 kDa) demonstrated that axolotl endothelial cells exhibit higher rates of macropinocytosis, a non-regulated type of transcellular transport. Our study demonstrated that, under physiological conditions, the blood–brain barrier exhibited species-specific variations, including permeability threshold, blood vessel coverage, and macropinocytosis rate. Future studies are needed to test whether the higher permeability observed in salamanders could have metabolic and immunological consequences contributing to their remarkable regenerative capacity.

Gaze stabilization and locomotion rely often on an accurate sensory detection of head movements by the vestibular system. A functional relationship between vestibular sensitivity, locomotor skills and semicircular canal morphology has been suspected in vertebrates as an adaptation to eco-physiological and species-specific needs, but has only partially and indirectly documented. However, evaluating the vestibulo-ocular reflexive activity and the locomotor efficiency simultaneously with the rotational sensor geometry remains absent from the literature. From such a perspective, this study attempted to provide a simultaneous quantification of the vestibulo-ocular response, the swimming efficiency and the canal morphology in the salamander axolotl and the frog Xenopus laevis, two amphibian species with comparable lifestyle and identical locomotor and vestibular systems at larval stages. Animals were studied at an equivalent developmental period: the late pre-metamorphic stage where the hindlimbs start to differentiate. Larval axolotl demonstrated an angular vestibulo-ocular reflex (aVOR) with a gain ~83% lower than Xenopus. Like in Xenopus at earlier stages, the aVOR gain increased in axolotl indicating a later functional onset. The morphological comparison of the semicircular canals of both species revealed that the horizontal canal in axolotl was thinner, less curved and less coplanar to the horizontal plane compared to Xenopus. Additionally, the ampulla of Xenopus was rounder and less elongated than in axolotl. All these parameters are critical for endolymph flow and consequently for the capacity of semicircular canals to perceive head motion. Interestingly, axolotl demonstrated a reduced swimming activity, more episodic than Xenopus, resulting in less frequent exposure to important head accelerations. Altogether, our results provide correlative evidences for a clear functional link between semicircular canal morphology, vestibular sensitivity, influencing aVOR performance, but also locomotor capacity in two comparable species, representative of anuran and salamander amphibians. This study, even preliminary, should open the pathway for further and more complete demonstrations of this functional relationship, that seems to be commonly shared during the evolution.

To generate a coherent visual percept, information from both eyes must be appropriately transmitted into the brain, where binocular integration forms the substrate for visuomotor behaviors. To establish the anatomical substrate for binocular integration, the presence of bilateral eyes and interaction of both optic nerves during retinotectal development play a key role. However, the extent to which embryonic monocularly derived visual circuits can convey visuomotor behaviors is unknown. In this study, we assessed the retinotectal anatomy and visuomotor performance of embryonically generated one-eyed tadpoles. In one-eyed animals, the axons of retinal ganglion cells from the singular remaining eye exhibited striking irregularities in their central projections in the brain, generating a non-canonical ipsilateral retinotectal projection. This data is indicative of impaired pathfinding abilities. We further show that these novel projections are correlated with an impairment of behavioral compensation for the loss of one eye.

BackgroundEnergetic aspects of neuronal activity have become a major focus of interest given the fact that the brain among all organs dominates the oxygen consumption. At variance with the importance of neuroenergetics, the knowledge about how electrical activity and metabolism is correlated in defined neuronal populations is still rather scarce.ResultsWe have estimated the ATP consumption in the two physiologically well characterized populations of frog central vestibular neurons, with tonic and phasic firing patterns, respectively. These two distinct groups of neurons jointly process head/body movements detected by semicircular canal and otolith organs in the inner ear. The ATP consumption for maintenance of the resting membrane potential (Vr) and postsynaptic action potential (AP) generation was calculated based on the wealth of previously reported morpho-physiological features of these two neuronal types. Accordingly, tonic vestibular neurons require less ATP across the physiological activity range for these major processes, than phasic vestibular neurons, despite the considerably higher firing rates of the former subtype. However, since both neuronal subtypes are indispensable for the encoding and processing of the entire head/body motion dynamics, the higher energy demand of phasic neurons represents an obvious and necessary price to pay. Although phasic and tonic neurons form the respective core elements of the frequency-tuned vestibular pathways, both cellular components are cross-linked through feedforward and feedback side loops. The prominent influence of inhibitory tonic neurons in shaping the highly transient firing pattern of phasic neurons is cost-intensive and contributes to energy consumption for electrical activity in addition to the already extensive energy costs of signal processing by the very leaky phasic vestibular neurons.ConclusionDespite the sparse production of action potentials by phasic vestibular neurons, the computation by this neuronal type dominates the ATP expense for processing head/body movements, which might have contributed to the late evolutionary arrival of this central neuronal element, dedicated to the encoding of highly dynamic motion profiles.

EDITORIAL article Front. Neurol., 31 March 2023Sec. Neuro-Otology Volume 14 - 2023 | https://doi.org/10.3389/fneur.2023.1191086

Neural replicas of the spinal motor commands that drive locomotion have become increasingly recognized as an intrinsic neural mechanism for producing gaze-stabilizing eye movements that counteract the perturbing effects of self-generated head/body motion. By pre-empting reactive signaling by motion-detecting vestibular sensors, such locomotor efference copies (ECs) provide estimates of the sensory consequences of behavioral action. Initially demonstrated in amphibian larvae during spontaneous fictive swimming in deafferented in vitro preparations, direct evidence for a contribution of locomotor ECs to gaze stabilization now extends to the ancestral lamprey and to tetrapod adult frogs and mice. Supporting behavioral evidence also exists for other mammals, including humans, therefore further indicating the mechanism's conservation during vertebrate evolution. The relationship between feedforward ECs and vestibular sensory feedback in ocular movement control is variable, ranging from additive to the former supplanting the latter, depending on vestibular sensing ability, and the intensity and regularity of rhythmic locomotor movements.

The vestibular system in the inner ear plays a central role in sensorimotor control by informing the brain about the orientation and acceleration of the head. However, most experiments in neurophysiology are performed using head-fixed configurations, depriving animals of vestibular inputs. To overcome this limitation, we decorated the utricular otolith of the vestibular system in larval zebrafish with paramagnetic nanoparticles. This procedure effectively endowed the animal with magneto-sensitive capacities: applied magnetic field gradients induced forces on the otoliths, resulting in robust behavioral responses comparable to those evoked by rotating the animal by up to 25°. We recorded the whole-brain neuronal response to this fictive motion stimulation using light-sheet functional imaging. Experiments performed in unilaterally injected fish revealed the activation of a commissural inhibition between the brain hemispheres. This magnetic-based stimulation technique for larval zebrafish opens new perspectives to functionally dissect the neural circuits underlying vestibular processing and to develop multisensory virtual environments, including vestibular feedback.

Gaze stabilization relies on bilateral mirror-symmetric vestibular endorgans, central circuits, and extraocular motor effectors. Embryonic removal of one inner ear before the formation of these structures was used to evaluate the extent to which motor outputs in the presence of a singular inner ear can develop. Near-congenital one-eared tadpoles subjected to separate or combinatorial visuo-vestibular motion stimulation exhibited comparable eye movements, though smaller in gain to controls, whereas isolated visuo-motor responses were unaltered. Surprisingly, vestibulo-ocular reflexes were robust during off-direction motion toward the missing ear in most cases and often attenuated during on-direction motion. This bidirectional plasticity of signal encoding appears to occur at the expense of vestibular reflexes during motion in the normally preferential activation direction of the singular ear. Consequently, formation of central vestibulo-motor circuits in one-eared animals likely relies on multi-neuronal homeostatic strategies, including enhanced afferent fiber activity in the attempt to adjust bilateral sensorimotor transformations.

Locomotion in vertebrates is accompanied by retinal image-stabilizing eye movements that derive from sensory-motor transformations and predictive locomotor efference copies. During development, concurrent maturation of locomotor and ocular motor proficiency depends on the structural and neuronal capacity of the motion detection systems, the propulsive elements and the computational capability for signal integration. In developing Xenopus larvae, we demonstrate an interactive plasticity of predictive locomotor efference copies and multi-sensory motion signals to constantly elicit dynamically adequate eye movements during swimming. During ontogeny, the neuronal integration of vestibulo- and spino-ocular reflex components progressively alters as locomotion parameters change. In young larvae, spino-ocular motor coupling attenuates concurrent angular vestibulo-ocular reflexes, while older larvae express eye movements that derive from a combination of the two components. This integrative switch depends on the locomotor pattern generator frequency, represents a stage-independent gating mechanism, and appears during ontogeny when the swim frequency naturally declines with larval age.

COPYRIGHT © 2022 Strupp, Straka and Oh. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. Editorial: Insights in neuro-otology: 2021 and 2022

Oxygen in vertebrates is generally provided through respiratory organs and blood vessels. This protocol describes transcardial injection, vascular distribution, and accumulation of phototrophic microalgae in the brain of Xenopus laevis tadpoles. Following tissue isolation, oxygen dynamics and neuronal activity are recorded in semi-intact whole-head preparations. Illumination of such microalgae-filled prepara-tions triggers the photosynthetic production of oxygen in the brain that, under hyp-oxic conditions, rescues neuronal activity. This technology is potentially able to ameliorate consequences of hypoxia under pathological conditions.For complete details on the use and execution of this protocol, please refer to O & BULL; zugur et al. (2021).

Vertebrate locomotion presents a major challenge for maintaining visual acuity due to head movements resulting from the intimate biomechanical coupling with the propulsive musculoskeletal system. Retinal image stabilization has been traditionally ascribed to the transformation of motion-related sensory feedback into counteracting ocular motor commands. However, extensive exploration of spontaneously active semi-intact and isolated brain/spinal cord preparations of the amphibian Xenopus laevis, have revealed that efference copies (ECs) of the spinal motor program that generates axial- or limb-based propulsion directly drive compensatory eye movements. During fictive locomotion in larvae, ascending ECs from rostral spinal central pattern generating (CPG) circuitry are relayed through a defined ascending pathway to the mid- and hindbrain ocular motor nuclei to produce conjugate eye rotations during tail-based undulatory swimming in the intact animal. In post-metamorphic adult frogs, this spinal rhythmic command switches to a bilaterally-synchronous burst pattern that is appropriate for generating convergent eye movements required for maintaining image stability during limb kick-based rectilinear forward propulsion. The transition between these two fundamentally different coupling patterns is underpinned by the emergence of altered trajectories in spino-ocular motor coupling pathways that occur gradually during metamorphosis, providing a goal-specific, morpho-functional plasticity that ensures retinal image stability irrespective of locomotor mode. Although the functional impact of predictive ECs produced by the locomotory CPG matches the spatio-temporal specificity of reactive sensory-motor responses, rather than contributing additively to image stabilization, horizontal vestibulo-ocular reflexes (VORs) are selectively suppressed during intense locomotor CPG activity. This is achieved at least in part by an EC-mediated attenuation of mechano-electrical encoding at the vestibular sensory periphery. Thus, locomotor ECs and their potential suppressive impact on vestibular sensory-motor processing, both of which have now been reported in other vertebrates including humans, appear to play an important role in the maintenance of stable vision during active body displacements.

Usher syndrome (USH) is the most common form of monogenic deaf-blindness. Loss of vision is untreatable and, so far, there are no suitable animal models for testing therapeutic strategies. By introducing a human mutation into the harmonin-encoding USH1C gene in pigs, we generated the first translational animal model for USH type 1 with characteristic hearing defect, vestibular dysfunction and visual impairment. Changes in photoreceptor architecture, quantitative motion analysis and electroretinography were characteristics of the reduced retinal virtue in USH1C pigs. Primary cells from those animals and USH1C patients showed significantly elongated primary cilia, compared to wild-type, confirming the nature of USH as a true and general ciliopathy and proving the therapeutic capacity of gene supplementation and gene repair approaches.

Vertebrate locomotion presents a major challenge for maintaining visual acuity due to head movements resulting from the intimate biomechanical coupling with the propulsive musculoskeletal system. Retinal image stabilization has been traditionally ascribed to the transformation of motion-related sensory feedback into counteracting ocular motor commands. However, extensive exploration of spontaneously active semi-intact and isolated brain/spinal cord preparations of the amphibian Xenopus laevis, have revealed that efference copies (ECs) of the spinal motor program that generates axial- or limb-based propulsion directly drive compensatory eye movements. During fictive locomotion in larvae, ascending ECs from rostral spinal central pattern generating (CPG) circuitry are relayed through a defined ascending pathway to the mid- and hindbrain ocular motor nuclei to produce conjugate eye rotations during tail-based undulatory swimming in the intact animal. In post-metamorphic adult frogs, this spinal rhythmic command switches to a bilaterally-synchronous burst pattern that is appropriate for generating convergent eye movements required for maintaining image stability during limb kick-based rectilinear forward propulsion. The transition between these two fundamentally different coupling patterns is underpinned by the emergence of altered trajectories in spino-ocular motor coupling pathways that occur gradually during metamorphosis, providing a goal-specific, morpho-functional plasticity that ensures retinal image stability irrespective of locomotor mode. Although the functional impact of predictive ECs produced by the locomotory CPG matches the spatio-temporal specificity of reactive sensory-motor responses, rather than contributing additively to image stabilization, horizontal vestibulo-ocular reflexes (VORs) are selectively suppressed during intense locomotor CPG activity. This is achieved at least in part by an EC-mediated attenuation of mechano-electrical encoding at the vestibular sensory periphery. Thus, locomotor ECs and their potential suppressive impact on vestibular sensory-motor processing, both of which have now been reported in other vertebrates including humans, appear to play an important role in the maintenance of stable vision during active body displacements.

Visual image motion-driven ocular motor behaviors such as the optokinetic reflex (OKR) provide sensory feedback for optimizing gaze stability during head/body motion. The performance of this visuo-motor reflex is subject to plastic alterations depending on requirements imposed by specific eco-physiological or developmental circumstances. While visuo-motor plasticity can be experimentally induced by various combinations of motion-related stimuli, the extent to which such evoked behavioral alterations contribute to the behavioral demands of an environment remains often obscure. Here, we used isolated preparations of Xenopus laevis tadpoles to assess the extent and ontogenetic dependency of visuo-motor plasticity during prolonged visual image motion. While a reliable attenuation of large OKR amplitudes can be induced already in young larvae, a robust response magnitude-dependent bidirectional plasticity is present only at older developmental stages. The possibility of older larvae to faithfully enhance small OKR amplitudes coincides with the developmental maturation of inferior olivary-Purkinje cell signal integration. This conclusion was supported by the loss of behavioral plasticity following transection of the climbing fiber pathway and by the immunohistochemical demonstration of a considerable volumetric extension of the Purkinje cell dendritic area between the two tested stages. The bidirectional behavioral alterations with different developmental onsets might functionally serve to standardize the motor output, comparable to the known differential adaptability of vestibulo-ocular reflexes in these animals. This homeostatic plasticity potentially equilibrates the working range of ocular motor behaviors during altered visuo-vestibular conditions or prolonged head/body motion to fine-tune resultant eye movements.

n analysis of fossil specimens of the inner ear helps to refine the timeframe of a key transition in vertebrate evolution — when our mammal-like ancestors began to regulate and maintain a high body temperature.