Whiskers, barrels and cortical efferent pathways in gap crossing by rats

Rats can readily be trained to jump a gap of around 30 cm in the light and 16 cm in the dark for a food reward. In the light they use vision to estimate the distance to be jumped. In the dark they use their vibrissae at the farthest distances. Bilateral whisker shaving or barrel field lesions reduce the gap crossed in the dark by about 2 cm (Hutson and Masterton, 1986). Information from the barrel fields reaches motor areas via the cortico-cortical, basal ganglia, or cerebellar pathways. The cells of origin of the pontocerebellar pathway are segregated in layer Vb of the barrel field (Mercier et al., 1990). Efferent axons of Vb cells occupy a central position within the basis pedunculi, and terminate on cells in the pontine nuclei (Glickstein et al., 1992). Pontine cells, in turn, project to the cerebellar cortex as mossy fibres. We trained normal rats to cross a gap in the light and in a dark alley that was illuminated with an infra-red source. When the performance was stable we made unilateral lesions in the central region of the basis pedunculi which interrupted connections from the barrel field to the pons whilst leaving cortico-cortical and basal ganglia pathways intact. Whisking was not affected on either side by the lesion and the rats with unilateral peduncle lesions crossed gaps of the same distance as they did pre-operatively. Shaving the whiskers in the side of the face that retains its input to the pontine nuclei reduced the maximal gap jumped in the dark by the same amount as bilateral whisker shaving. Performance in the light was not affected. Re-growth of the shaved whiskers was associated with the recovery of the maximum distance crossed in the dark. In control cases, shaving the whiskers on the other side of the face did not reduce the distance jumped in the dark or in the light. These results suggests that the cerebellum must receive whisker information from the barrel fields from the barrel fields for whisker-guided jumps

Somatosensory cortical input to the striatum

The information from whiskers is processed in layer IV of the cortex by groups of neurones arranged in discrete functional units known as barrels. Each barrel processes input obtained from a single whisker. The barrel cortex can be differentiated into the cytochrome rich barrel centres and the septal cells surrounding the barrels. Previous work in the laboratory had established two cortical inputs to striatum from the barrel cortex. One of these arises from septal cells and is bilateral and composed of thin calibre fibres. The other route involves the barrel centres, is only unilaterally represented and is composed of topographically arranged, thick fibres. Based on these morphological differences, the postsynaptic targets of the two pathways with reference to the two output pathways of the striatum were examined. A method was also developed to examine the physiological consequences of stimulation of the two pathways upon the striatal output cells of the anaesthetised rat in both normal and dopamine-depleted animals. An anatomical study of the cortical input to the GABAergic intemeurones was also undertaken as these cells strongly modulate the output of striatal neurones.The pathways differ in their connectivity, with the bilateral pathway contacting the neurones of the striatopallidal pathway more often than the fibres of the topographic system. The stimulation of the two pathways can depolarise striatal cells and give rise to EPSPs, which can be differentiated based on their rise times. EPSPs in response to whisker pad stimulation have a rapid rise time, while the contralateral cortically derived EPSPs are slower to rise and the spike initiation latency more variable. Both pathways interacted at the level of a single striatal cell and gave rise to a summation of EPSPs at a time interval of 10ms, followed by a period of inhibition, the extent of which was dependent on the order and source of the stimuli. This pattern ofinteraction was not seen in cortical neurones. In dopamine depleted animals both stimuli were also able to depolarise the spiny neurones to their firing threshold. However the EPSPs to whisker pad stimulation were significantly slower to rise compared to control animals and were similar to the rise times of EPSPs in response to contralateral cortical stimulation. The interaction of the two pathways was also affected by the loss of dopamine and the summation of EPSP amplitude observed when stimuli were delivered 1 Oms apart in control animals was no longer present. The anatomical study revealed that GABAergic intemeurones receive convergent cortical input from both motor and sensory cortices and that their pattern of innervation is different from the cortical innervation of striatal output neuronesThe results of this thesis suggest that the two inputs from the barrel cortex differ in their physiological influence on striatal neurones, and that they might convey different aspects of somatosensory information to the striatum. The changes observed in dopamine-lesioned animals indicate that the topographic, ipsilateral pathway is selectively affected by the loss of dopamine suggesting that dopaminedepletion does not have a generalized action that is independent of presynaptic or postsynaptic origins. Rather its effects are specific to the neuronal subtype affected as well to the origin of the synapses. The complex pattern of innervation of striatal intemeurones suggests that these cells play a very important role in striatal physiology and that their modulation by dopamine may serve as a possible explanation for the effects seen after lesion in this stud

Structure and dynamics of the corticothalamic driver pathway in the mouse whisker system

To generate a sensory percept of the environment, the brain needs to analyze and integrate spatially and temporally distributed sensory signals. Consequently, sensation on a neuronal basis is a distributed, non-linear and dynamic process. Following sensory receptor activation the signal travels through many brain regions wherein the pathway is split, loops back onto itself and joins together with others. At each step, neurons dynamically transform and filter the signal. To understand how the brain arrives at a sensory percept, it is therefore essential to determine the neuronal connectivity along the processing chain, the stimulus specificity of responses as well as the input-output transformations at each station. An interesting model system for investigating these dynamical processes is the rodent whisker system. Rodents can solve highly complicated tasks with their whiskers alone, distributed receptors at the follicles require spatial integration and rhythmic movements suggest temporal processing components. The posterior group nucleus of the thalamus (PO) is in a key position of the whisker sensory system. In addition to being part of the ascending paralemniscal pathway it is mainly driven by somatosensory barrel cortex (BC) and projects to many cortical and subcortical areas. Due to its poor excitability by whisker deflections, its function is unclear. The origin of the corticothalamic drive onto PO neurons are ‘thick-tufted’ layer 5B cortical neurons, which have large synaptic terminals in thalamus. One of those synapses alone has a strong influence on postsynaptic target neurons – a very unusual property for cortical synapses. Here, using quantitative anatomy, in vivo electrophysiology and optogenetics I characterize the organization and input-output computations along the BC L5B to PO pathway. Using a dual anterograde tract tracing approach and large scale anatomical reconstructions we demonstrate that BC L5B synaptic boutons divide PO in 4 subregions with different projection parameters. The lateral area (POm lateral) receives most boutons with the highest density. Additionally, L5B neurons innervate two inhibitory nuclei in thalamus and midbrain that both inhibit PO. In all 6 regions we report map specific projections, with different map orientations, showing that somatotopic projections are the rule in these cortico-subcortical projections. Next we investigated the L5B to POm action potential transfer efficacy during spontaneous slow oscillations in anesthesia. Using pharmacology and cell-type specific optogenetics we show that cortical activity is necessary and L5B activation is sufficient to evoke large excitatory postsynaptic potentials (EPSPs) in POm, typical for L5B inputs. Simultaneous cortical local field potential and L5B as well as POm juxtasomal recordings demonstrate that the gain of action potential transmission is high following periods of relative cortical silence, but dynamically decreases during periods of higher cortical activity. Isolation of individual EPSPs allowed us to determine the frequency dependent adaptation of the L5B to POm synapse in vivo. We determined that approximately half of the recorded POm neurons follow a simple rule of EPSP adaptation, suggesting that the subthreshold activity in these neurons originates from a single active L5B input. Using two independent modeling approaches, we determined that on average POm neurons receive 2-3 functional inputs from BC L5B. Finally we investigated how whisker deflection signals reach POm. We found that POm neurons fall into two groups. Approximately one third of the recorded neurons were activated at a relatively short latency by large EPSPs and fired action potentials following whisker stimulation. All neurons had long latency sub- and suprathreshold responses, due to Up-state initiation by the whisker stimulation. POm whisker responses were entirely dependent on cortex and were blocked by optogenetic cortical inactivation. Taken together we quantified the anatomical and physiological properties of the L5B to POm projection. The connection is sparse, parallel, strong and the dominant input for POm spontaneous activity as well as whisker evoked responses. Its gain is dynamically regulated and depends on cortical activity states

Neuronal Population Encoding of Sensory Information in the Rat Barrel Cortex: Local Field Potential Recording and Characterization by an Innovative High-Resolution Brain-Chip Interface

Neuronal networks are at the base of information processing in the brain. They are series of interconnected neurons whose activation defines a recognizable linear pathway. The main goal of studying neural ensembles is to characterize the relationship between the stimulus and the individual or general neuronal responses and the relation amongst the electrical activities of neurons within the network, also understanding how topology and connectivity relates to their function. Many techniques have been developed to study these complex systems: single-cell approaches aim to investigate single neurons and their connections with a limited number of other nerve cells; on the opposite side, low-resolution large-scale approaches, such as functional MRI (Magnetic Resonance Imaging) or electroencephalography (EEG), record signal changes in the brain that are generated by large populations of cells. More recently, multisite recording techniques have been developed to overcome the limitations of previous approaches, allowing to record simultaneously from huge neuronal ensembles with high spatial resolution and in different brain regions, i.e. by using implantable semiconductor chips. Local Field Potentials (LFPs), the part of electrophysiological signals that has frequencies below 500 Hz, capture key integrative synaptic processes that cannot be measured by analyzing the spiking activity of few neurons alone. Several studies have used LFPs to investigate cortical network mechanisms involved in sensory processing, motor planning and higher cognitive processes, like memory and perception. LFPs are also promising signals for steering neuroprosthetic devices and for monitoring neural activity even in human beings, since they are more easily and stably recorded in chronic settings than neuronal spikes. In this work, LFP profiles recorded in the rat barrel cortex through high-resolution CMOS-based needle chips are presented and compared to those obtained by means of conventional Ag/AgCl electrodes inserted into glass micropipettes, which are widely used tools in electrophysiology. The rat barrel cortex is a well-known example of topographic mapping where each of the whiskers on the snout of the animal is mapped onto a specific cortical area, called a barrel. The barrel cortex contains the somatosensory representation of the whiskers and forms an early stage of cortical processing for tactile information, along with the trigeminal ganglion and the thalamus. It is an area of great importance for understanding how the cerebral cortex works, since the cortical columns that form the basic building blocks of the neocortex can be actually seen within the barrel. Moreover, the barrel cortex has served as a test-bed system for several new methodologies, partly because of its unique and instantly identifiable form, and partly because the species that have barrels, i.e. rodents, are the most commonly used laboratory mammal. The barrel cortex, the whiskers that activate it and the intervening neural pathways have been increasingly the subject of focus by a growing number of research groups for quite some time. Nowadays, studies (such this one) are directed not only at understanding the barrel cortex itself but also at investigating issues in related fields using the barrel cortex as a base model. In this study, LFP responses were evoked in the target barrel by repeatedly deflecting the corresponding whisker in a controlled fashion, by means of a specifically designed closed-loop piezoelectric bending system triggered by a custom LabView acquisition software. Evoked LFPs generated in the barrel cortex by many consecutive whiskers' stimulations show large variability in shapes and timings. Moreover, anesthetics can deeply affect the profile of evoked responses. This work presents preliminary results on the variability and the effect of commonly used anesthetics on these signals, by comparing the distributions of evoked responses recorded from rats anesthetized with tiletamine-xylazine, which mainly blocks the excitatory NMDA receptors, and urethane, which conversely affects both the excitatory and inhibitory system, in a complex and balanced way yet preserving the synaptic plasticity. Representative signal shape characteristics (e.g., latencies and amplitude of events) extracted from evoked responses acquired from different cortical layers are analyzed and discussed. Statistical distributions of these parameters are estimated for all the different depths, in order to assess the variability of LFPs generated by individual mechanical stimulations of single whiskers along the entire cortical column. Preliminary results showed a great variability in cortical responses, which varied both in latency and amplitude across layers. We found significant difference in the latency of the first principal peak of the responses: under tiletamine-xylazine anesthetic, the responses or events of the evoked LFPs occurred later than the ones recorded while urethane was administered. Furthermore, the distributions of this parameter in all cortical layers were narrower in case of urethane. This behavior should be attributed to the different effects of these two anesthetics on specific synaptic receptors and thus on the encoding and processing of the sensory input information along the cortical pathway. The role of the ongoing basal activity on the modulation of the evoked response was also investigated. To this aim, spontaneous activity was recorded in different cortical layers of the rat barrel cortex under the two types of anesthesia and analyzed in the statistical context of neuronal avalanches. A neuronal avalanche is a cascade of bursts of activity in neural networks, whose size distribution can be approximated by a power law. The event size distribution of neuronal avalanches in cortical networks has been reported to follow a power law of the type P(s)= s^-a, with exponent a close to 1.5, which represent a reflection of long-range spatial correlations in spontaneous neuronal activity. Since negative LFP peaks (nLFPs) originates from the sum of synchronized Action Potentials (AP) from neurons within the vicinity of the recording electrode, we wondered if it were possible to model single nLFPs recorded in the basal activity traces by means of only one electrode as the result of local neuronal avalanches, and thus we analyzed the size (i.e. the amplitude in uV) distribution of these peaks so as to identify a suitable power-law distribution that could describe also these single-electrode records. Finally, the results of the first ever measurements of evoked LFPs within an entire column of the barrel cortex obtained by means of the latest generation of CMOS-based implantable needles, having 256 recording sites arranged into two different array topologies (i.e. 16 x 16 or 4 x 64, pitches in the x- and y-direction of 15 um and 33 um respectively), are presented and discussed. A propagation dynamics of the LFP can be already recognized in these first cortical profiles. In the next future, the use of these semiconductor devices will help, among other things, to understand how degenerating syndromes like Parkinson or Alzheimer evolve, by coupling detected behaviors and symptoms of the disease to neuronal features. Implantable chips could then be used as 'electroceuticals', a newly coined term that describes one of the most promising branch of bioelectronic medicine: they could help in reverting the course of neurodegenerative diseases, by constituting the basis of neural prostheses that physically supports or even functionally trains impaired neuronal ensembles. High-resolution extraction and identification of neural signals will also help to develop complex brain-machine interfaces, which can allow intelligent prostheses to be finely controlled by their wearers and to provide sophisticated feedbacks to those who have lost part of their body or brain functions

Synaptic plasticity and sensory information processing through the thalamus and the cortex of the rodent barrel field

Tesis doctoral inédita cotutelada por l´Université de Lausanne y la Universidad Autónoma de Madrid, Facultad de Medicina, Departamento de Anatomía, Histología y Neurociencia. Fecha de lectura: 26-06-2017Le traitement de l’information sensorielle est un processus clef dans le cerveau parce qu’il reçoit de nombreux inputs sensoriels. Certains d’entre eux sont pertinents et devraient provoquer une réponse motrice ou sensorielle. La plasticité synaptique dans le système nerveux central est un processus général qui améliore ou réduit les réponses sensorielles selon le circuit temporel des stimuli. L’information sensorielle des vibrisses des rongeurs est envoyée depuis le follicule de la vibrisse jusqu’à la zone contralatérale du thalamus et du thalamus jusqu’au cortex somato-sensoriel (BC). Au cours de cette thèse de doctorat nous avons effectué des enregistrements extra-cellulaires in vivo dans le BC et le thalamus de rats et souris anesthésiés à l’uréthane afin de découvrir les mécanismes de la plasticité synaptique et du traitement sensoriel. Nous avons observé qu’une stimulation répétée à des fréquences auxquelles l’animal explore son environnement provoquait une potentialisation à long terme (LTP). De plus, une stimulation à basse fréquence peut provoquer une LTP ou une dépression à long terme (LTD) selon la concentration intra-cellulaire de Ca2+ pendant la durée de la stimulation. Cette plasticité à long terme dépend de l’activation des récepteurs NMDA et de l’activation des récepteurs cholinergiques muscariniques et nicotiniques. Grâce à une étude optogénétique nous avons pu montrer que le prosencéphale basal (BF), la source principale d’acetylcholine (Ach) vers le cortex, envoyait ses projections de façon organisée. Par conséquent, la facilitation des réponses corticales dépendant de l’Ach se produit demanière très spécifique. Nous avons également découvert que le noyau thalamique postéro-médial (POM) régulait la vibrisse du BC grâce à des neurones GABAergiques situés dans les couches supérieures du cortex. Mot clefs : cortex somato-sensoriel, thalamus, plasticité synaptique, récepteurs NMDA, acetylcholineEl procesamiento de la información sensorial es un proceso clave en el cerebro ya que involucra varios inputs sensoriales. Algunos de ellos son relevantes e inducen respuestas motoras o cognitivas. Además, muchos estímulos irrelevantes alcanzan la via sensorial y deben ser descartados. La plasticidad sináptica en el sistema nervios central es un proceso que aumenta o deprime las respuestas sensoriales según un patrón temporal de estimulación.Mi objetivo principal es estudiar la plasticidad sináptica en la via somatosensorial principalmente en el circuito tálamo-cortex. La información sensorial de las vibrisas de los roedores viaja del folículo de estas a la zona contralateral del tálamo, y desde esta a la corteza de barriles (BC). En esta Tesis Doctoral hicimos registros extracelulares in vivo en la BC y el tálamo en ratas y ratones anestesiados con uretano con el objetivo de conocer los mecanismos de la plasticidad sináptica y el procesamiento sensorial en esta vía. Observamos que una estimulación repetitiva a las frecuencias a las cuales el animal explora su entorno, inducen potenciación a largo plazo (LTP). Además, la estimulación a baja frecuencia pudo inducir LTP o depresión a largo plazo (LTD) dependiendo del la concentración de Ca2+ intracellular durante el periodo de estimulación.Mediante un estudio de optogenética, observamos que el prosencefalo basal (BF), el núcleo que surte principalmente a la corteza de acetilcolina (Ach) manda proyecciones de forma organizada. Encontramos tambien que el núcleo posterior-medial del talamo (POM) regula la respuesta de la corteza de barriles a traves de las neuronas GABAérgicas de la capa I. Palabras clave: corteza somatosensorial, tálamo, plasticidad sináptica, receptores NMDA, acetilcolin