Contact interactions between epitheliocytes and fibroblasts: Formation of heterotypic cadherin-containing adhesion sites is accompanied by local cytoskeletal reorganization

Contact interactions between different cell types play a number of important roles in development, for example in cell sorting, tissue organization, and ordered migration of cells. The nature of such heterocellular interactions, in contrast to interactions between cells of the same type, remains largely unknown. In this report, we present experimental data examining the dynamics of heterocellular interactions between epitheliocytes and fibroblasts, which express different cadherin cell adhesion molecules and possess different actin cytoskeletal organizations. Our analysis revealed two striking features of heterocellular contact. First, the active free edge of an epitheliocyte reorganizes its actin cytoskeleton after making contact with a fibroblast. Upon contact with the leading edge of a fibroblast, epitheliocytes disassemble their marginal bundle of actin filaments and reassemble actin filaments into a geometric organization more typical of a fibroblast lamella. Second, epitheliocytes and fibroblasts form cell–cell adhesion structures that have an irregular organization and are associated with components of cell adhesion complexes. The structural organization of these adhesions is more closely related to the type of contacts formed between fibroblasts rather than to those between epitheliocytes. Heterotypic epithelio-fibroblastic contacts, like homotypic contacts between fibroblasts, are transient and do not lead to formation of stable contact interactions. We suggest that heterocellular contact interactions in culture may be regarded as models of how tissue systems consisting of epithelia and mesenchyme interact and become organized in vivo.

Global properties of the mapping between local amino acid sequence and local structure in proteins.

Local protein structure prediction efforts have consistently failed to exceed approximately 70% accuracy. We characterize the degeneracy of the mapping from local sequence to local structure responsible for this failure by investigating the extent to which similar sequence segments found in different proteins adopt similar three-dimensional structures. Sequence segments 3-15 residues in length from 154 different protein families are partitioned into neighborhoods containing segments with similar sequences using cluster analysis. The consistency of the sequence-to-structure mapping is assessed by comparing the local structures adopted by sequence segments in the same neighborhood in proteins of known structure. In the 154 families, 45% and 28% of the positions occur in neighborhoods in which one and two local structures predominate, respectively. The sequence patterns that characterize the neighborhoods in the first class probably include virtually all of the short sequence motifs in proteins that consistently occur in a particular local structure. These patterns, many of which occur in transitions between secondary structural elements, are an interesting combination of previously studied and novel motifs. The identification of sequence patterns that consistently occur in one or a small number of local structures in proteins should contribute to the prediction of protein structure from sequence.

Significance of bound water to local chain conformations in protein crystals.

We examine how the polypeptide chain in protein crystal structures exploits the multivalent hydrogen-bonding potential of bound water molecules. This shows that multiple interactions with a single water molecule tend to occur locally along the chain. A distinctive internal-coordinate representation of the local water-binding segments reveals several consensus conformations. The fractional water occupancy of each was found by comparison of the total number of conformations in the database regardless of the presence or absence of bound water. The water molecule appears particularly frequently in type II beta-turn geometries and an N-terminal helix feature. This work constitutes a first step into assessing not only the generality but also the significance of specific water binding in globular proteins.

Trapping of branched DNA in microfabricated structures.

We have observed electrostatic trapping of tribranched DNA molecules undergoing electrophoresis in a microfabricated pseudo-two-dimensional array of posts. Trapping occurs in a unique transport regimen in which the electrophoretic mobility is extremely sensitive to polymer topology. The arrest of branched polymers is explained by considering their center-of-mass motion; in certain conformations, owing to the constraints imposed by the obstacles a molecule cannot advance without the center of mass first moving a short distance backwards. The depth of the resulting local potential well can be much greater than the thermal energy so that escape of an immobilized molecule can be extremely slow. We summarize the expected behavior of the mobility as a function of field strength and topology and point out that the microfabricated arrays are highly suitable for detecting an extremely small number of branched molecules in a very large population of linear molecules.

The star formation history of CALIFA galaxies: Radial structures

We have studied the radial structure of the stellar mass surface density (μ∗) and stellar population age as a function of the total stellar mass and morphology for a sample of 107 galaxies from the CALIFA survey. We applied the fossil record method based on spectral synthesis techniques to recover the star formation history (SFH), resolved in space and time, in spheroidal and disk dominated galaxies with masses from 10^9 to 10^12 M_⊙. We derived the half-mass radius, and we found that galaxies are on average 15% more compact in mass than in light. The ratio of half-mass radius to half-light radius (HLR) shows a dual dependence with galaxy stellar mass; it decreases with increasing mass for disk galaxies, but is almost constant in spheroidal galaxies. In terms of integrated versus spatially resolved properties, we find that the galaxy-averaged stellar population age, stellar extinction, and μ_∗ are well represented by their values at 1 HLR. Negative radial gradients of the stellar population ages are present in most of the galaxies, supporting an inside-out formation. The larger inner (≤1 HLR) age gradients occur in the most massive (10^11 M_⊙) disk galaxies that have the most prominent bulges; shallower age gradients are obtained in spheroids of similar mass. Disk and spheroidal galaxies show negative μ∗ gradients that steepen with stellar mass. In spheroidal galaxies, μ∗ saturates at a critical value (~7 × 10^2 M_⊙/pc^2 at 1 HLR) that is independent of the galaxy mass. Thus, all the massive spheroidal galaxies have similar local μ_∗ at the same distance (in HLR units) from the nucleus. The SFH of the regions beyond 1 HLR are well correlated with their local μ_∗, and follow the same relation as the galaxy-averaged age and μ_∗; this suggests that local stellar mass surface density preserves the SFH of disks. The SFH of bulges are, however, more fundamentally related to the total stellar mass, since the radial structure of the stellar age changes with galaxy mass even though all the spheroid dominated galaxies have similar radial structure in μ_∗. Thus, galaxy mass is a more fundamental property in spheroidal systems, while the local stellar mass surface density is more important in disks.

Quantifying the contamination by old main-sequence stars in young moving groups: the case of the Local Association

Context. The associations and moving groups of young stars are excellent laboratories for investigating stellar formation in the solar neighborhood. Previous results have confirmed that a non-negligible fraction of old main-sequence stars is present in the lists of possible members of young stellar kinematic groups. A detailed study of the properties of these samples is needed to separate the young stars from old main-sequence stars with similar space motion, and identify the origin of these structures. Aims. Our intention is to characterize members of the young moving groups, determine their age distribution, and quantify the contamination by old main-sequence stars, in particular, for the Local Association. Methods. We used stars possible members of the young (~10-650 Myr) moving groups from the literature. To determine the age of the stars, we used several suitable age indicators for young main sequence stars, i.e., X-ray fluxes from the Rosat All-sky Survey database, photometric data from the Tycho-2, Hipparcos, and 2MASS database. We also used spectroscopic data, in particular the equivalent width of the lithium line Li I λ6707.8 Å and H_α, to constrain the range of ages of the stars. Results. By combining photometric and spectroscopic data, we were able to separate the young stars (10-650 Myr) from the old (> 1 Gyr) field ones. We found, in particular, that the Local Association is contaminated by old field stars at the level of ~30%. This value must be considered as the contamination for our particular sample, and not of the entire Local Association. For other young moving groups, it is more difficult to estimate the fraction of old stars among possible members. However, the level of X-ray emission can, at least, help to separate two age populations: stars with <200 Myr and stars older than this. Conclusions. Among the candidate members of the classical moving groups, there is a non-negligible fraction of old field stars that should be taken into account when studying the stellar birthrate in the solar neighborhood. Our results are consistent with a scenario in which the moving groups contain both groups of young stars formed in a recent star-formation episode and old field stars with similar space motion. Only by combining X-ray and optical spectroscopic data is it possible to distinguish between these two age populations.

Propriedades magnéticas de trímeros de FexCo1-x depositados em Pt(111)

Quais propriedades magnéticas são modificadas quando se agrupam átomos de Fe/Co para formar estruturas quasi-2D, se comparadas aos nanofios (quasi-1D) de FexCo1-x? E como estas propriedades reagem com a variação da proporção de Fe/Co nos aglomerados? A fim de responder a estas questões, trímeros de FexCo1-x depositados em Pt(111) são investigados utilizando o método de primeiros princípios Real Space-Linear Muffin-Tin Orbital-Atomic Sphere Approximation (RS-LMTO-ASA) no âmbito da Teoria do Funcional da Densidade (DFT). Diferentes configurações de trímeros triangulares são consideradas, variando-se as posições e a concentração dos átomos de Fe/Co. Neste trabalho, demonstra-se a ocorrência de uma tendência não-linear estritamente decrescente dos momentos orbitais médios como função da concentração de Fe, distinta do encontrado tanto para os nanofios de FexCo1-x (dependência linear) quanto para a monocamada correspondente (dependência não-linear). Os resultados obtidos mostram ainda que os momentos orbitais variam com o ambiente local e com a direção de magnetização, especialmente quando associados aos átomos de Co, em concordância com publicações anteriores. A mudança de dimensionalidade quasi-1D (nanofios) para quasi-2D (trímeros compactos) não afeta o comportamento dos momentos de spin, que permanecem descritos por uma função linear com respeito à proporção de Fe/Co. Ambos o formato e a concentração de Fe nos sistemas apresentam um papel importante nos valores de energia de anisotropia magnética. Em adição, observou-se que o subtrato de Pt opera ativamente na definição das propriedades magnéticas dos aglomerados. Embora todas as configurações lineares e compactas dos aglomerados de FexCo1-x sejam estáveis e exibam interações fortemente ferromagnéticas entre os primeiros vizinhos, nem todas revelaram o ordenamento colinear como estado fundamental, apresentando uma interação de Dzyaloshinskii-Moriya não-desprezível induzida pelo acoplamento spin-órbita. Estes casos específicos são: o trímero triangular de Co puro e o trímero linear (nanofio) de Fe puro, para o qual foi verificado o acoplamento do tipo Ruderman-Kittel-Kasuya-Yosida entre os átomos de Fe constituintes. Os resultados obtidos contribuem para o entendimento de quais mecanismos definem o magnetismo nos trímeros de FexCo1-x/Pt(111), e discutem as questões presentes atualmente na literatura no contexto destes sistemas.

Oxide Semiconductors For Organic Opto-electronic Devices

Development of transparent oxide semiconductors (TOS) from Earth-abundant materials is of great interest for cost-effective thin film device applications, such as solar cells, light emitting diodes (LEDs), touch-sensitive displays, electronic paper, and transparent thin film transistors. The need of inexpensive or high performance electrode might be even greater for organic photovoltaic (OPV), with the goal to harvest renewable energy with inexpensive, lightweight, and cost competitive materials. The natural abundance of zinc and the wide bandgap ($sim$3.3 eV) of its oxide make it an ideal candidate. In this dissertation, I have introduced various concepts on the modulations of various surface, interface and bulk opto-electronic properties of ZnO based semiconductor for charge transport, charge selectivity and optimal device performance. I have categorized transparent semiconductors into two sub groups depending upon their role in a device. Electrodes, usually 200 to 500 nm thick, optimized for good transparency and transporting the charges to the external circuit. Here, the electrical conductivity in parallel direction to thin film, i.e bulk conductivity is important. And contacts, usually 5 to 50 nm thick, are optimized in case of solar cells for providing charge selectivity and asymmetry to manipulate the built in field inside the device for charge separation and collection. Whereas in Organic LEDs (OLEDs), contacts provide optimum energy level alignment at organic oxide interface for improved charge injections. For an optimal solar cell performance, transparent electrodes are designed with maximum transparency in the region of interest to maximize the light to pass through to the absorber layer for photo-generation, plus they are designed for minimum sheet resistance for efficient charge collection and transport. As such there is need for material with high conductivity and transparency. Doping ZnO with some common elements such as B, Al, Ga, In, Ge, Si, and F result in n-type doping with increase in carriers resulting in high conductivity electrode, with better or comparable opto-electronic properties compared to current industry-standard indium tin oxide (ITO). Furthermore, improvement in mobility due to improvement on crystallographic structure also provide alternative path for high conductivity ZnO TCOs. Implementing these two aspects, various studies were done on gallium doped zinc oxide (GZO) transparent electrode, a very promising indium free electrode. The dynamics of the superimposed RF and DC power sputtering was utilized to improve the microstructure during the thin films growth, resulting in GZO electrode with conductivity greater than 4000 S/cm and transparency greater than 90 %. Similarly, various studies on research and development of Indium Zinc Tin Oxide and Indium Zinc Oxide thin films which can be applied to flexible substrates for next generation solar cells application is presented. In these new TCO systems, understanding the role of crystallographic structure ranging from poly-crystalline to amorphous phase and the influence on the charge transport and optical transparency as well as important surface passivation and surface charge transport properties. Implementation of these electrode based on ZnO on opto-electronics devices such as OLED and OPV is complicated due to chemical interaction over time with the organic layer or with ambient. The problem of inefficient charge collection/injection due to poor understanding of interface and/or bulk property of oxide electrode exists at several oxide-organic interfaces. The surface conductivity, the work function, the formation of dipoles and the band-bending at the interfacial sites can positively or negatively impact the device performance. Detailed characterization of the surface composition both before and after various chemicals treatment of various oxide electrode can therefore provide insight into optimization of device performance. Some of the work related to controlling the interfacial chemistry associated with charge transport of transparent electrodes are discussed. Thus, the role of various pre-treatment on poly-crystalline GZO electrode and amorphous indium zinc oxide (IZO) electrode is compared and contrasted. From the study, we have found that removal of defects and self passivating defects caused by accumulation of hydroxides in the surface of both poly-crystalline GZO and amorphous IZO, are critical for improving the surface conductivity and charge transport. Further insight on how these insulating and self-passivating defects cause charge accumulation and recombination in an device is discussed. With recent rapid development of bulk-heterojunction organic photovoltaics active materials, devices employing ZnO and ZnO based electrode provide air stable and cost-competitive alternatives to traditional inorganic photovoltaics. The organic light emitting diodes (OLEDs) have already been commercialized, thus to follow in the footsteps of this technology, OPV devices need further improvement in power conversion efficiency and stable materials resulting in long device lifetimes. Use of low work function metals such as Ca/Al in standard geometry do provide good electrode for electron collection, but serious problems using low work-function metal electrodes originates from the formation of non-conductive metal oxide due to oxidation resulting in rapid device failure. Hence, using low work-function, air stable, conductive metal oxides such as ZnO as electrons collecting electrode and high work-function, air stable metals such as silver for harvesting holes, has been on the rise. Devices with degenerately doped ZnO functioning as transparent conductive electrode, or as charge selective layer in a polymer/fullerene based heterojunction, present useful device structures for investigating the functional mechanisms within OPV devices and a possible pathway towards improved air-stable high efficiency devices. Furthermore, analysis of the physical properties of the ZnO layers with varying thickness, crystallographic structure, surface chemistry and grain size deposited via various techniques such as atomic layer deposition, sputtering and solution-processed ZnO with their respective OPV device performance is discussed. We find similarity and differences in electrode property for good charge injection in OLEDs and good charge collection in OPV devices very insightful in understanding physics behind device failures and successes. In general, self-passivating surface of amorphous TCOs IZO, ZTO and IZTO forms insulating layer that hinders the charge collection. Similarly, we find modulation of the carrier concentration and the mobility in electron transport layer, namely zinc oxide thin films, very important for optimizing device performance.

When Wells Run Dry: Water and Tourism Along the Western Coast of Nicaragua

This dissertation uses a political ecology approach to examine the relationship between tourism development and groundwater in southwest Nicaragua. Tourism in Nicaragua is a booming industry bolstered by ‘unspoiled’ natural beauty, low crime rates, and government incentives. This growth has led to increased infrastructure, revenue, and employment opportunities for many local communities along the Pacific coast. Not surprisingly, it has also brought concomitant issues of deeper poverty, widening gaps between rich and poor, and competition over natural resources. Adequate provisions of freshwater are necessary to sustain the production and reproduction of tourism; however, it remains uncertain if groundwater supplies can keep pace with demand. The objective of this research is to assess water supply availability amidst tourism development in the Playa Gigante area. It addresses the questions: 1) are local groundwater supplies sufficient to sustain the demand for freshwater imposed by increased tourism development? and 2) is there a power relationship between tourism development and control over local freshwater that would prove inequitable to local populations? Integrating the findings of groundwater monitoring, geological mapping, and ethnographic and survey research from a representative stretch of Pacific coastline, this dissertation shows that diminishing recharge and increased groundwater consumption is creating conflict between stakeholders with various levels of knowledge, power, and access. Although national laws are structured to protect the environment and ensure equitable access to groundwater, the current scramble to secure water has powerful implications on social relations and power structures associated with tourism development. This dissertation concludes that marginalization due to environmental degradation is attributable to the nexus of a political promotion of tourism, poorly enforced state water policies, insufficient water research, and climate change. Greater technical attention to hydrological dynamics and collaboration amongst stakeholders are necessary for equitable access to groundwater, environmental sustainability, and profitability of tourism.

Magnetism in graphene nanoislands

We study the magnetic properties of nanometer-sized graphene structures with triangular and hexagonal shapes terminated by zigzag edges. We discuss how the shape of the island, the imbalance in the number of atoms belonging to the two graphene sublattices, the existence of zero-energy states, and the total and local magnetic moment are intimately related. We consider electronic interactions both in a mean-field approximation of the one-orbital Hubbard model and with density functional calculations. Both descriptions yield values for the ground state total spin S consistent with Lieb’s theorem for bipartite lattices. Triangles have a finite S for all sizes whereas hexagons have S=0 and develop local moments above a critical size of ≈1.5  nm.

Electronic structure and transport properties of atomic NiO spinvalves

Ab initio quantum transport calculations show that short NiO chains suspended in Ni nanocontacts present a very strong spin-polarization of the conductance.The generalized gradient approximation we use here predicts a similar polarization of the conductance as the one previously computed with non-local exchange, confirming the robustness of the result. Their use as nanoscopic spinvalves is proposed.

Vibration frequency measurement using a local multithreshold technique

In this paper, we demonstrate the use of a video camera for measuring the frequency of small-amplitude vibration movements. The method is based on image acquisition and multilevel thresholding and it only requires a video camera with high enough acquisition rate, not being necessary the use of targets or auxiliary laser beams. Our proposal is accurate and robust. We demonstrate the technique with a pocket camera recording low-resolution videos with AVI-JPEG compression and measuring different objects that vibrate in parallel or perpendicular direction to the optical sensor. Despite the low resolution and the noise, we are able to measure the main vibration modes of a tuning fork, a loudspeaker and a bridge. Results are successfully compared with design parameters and measurements with alternative devices.

Plasticité du cortex visuel: «homéodynamie» des connexions neuronales et modèle d’effets d’antidépresseurs

Les informations sensorielles sont traitées dans le cortex par des réseaux de neurones co-activés qui forment des assemblées neuronales fonctionnelles. Le traitement visuel dans le cortex est régit par différents aspects des caractéristiques neuronales tels que l’aspect anatomique, électrophysiologique et moléculaire. Au sein du cortex visuel primaire, les neurones sont sélectifs à divers attributs des stimuli tels que l’orientation, la direction, le mouvement et la fréquence spatiale. Chacun de ces attributs conduit à une activité de décharge maximale pour une population neuronale spécifique. Les neurones du cortex visuel ont cependant la capacité de changer leur sélectivité en réponse à une exposition prolongée d’un stimulus approprié appelée apprentissage visuel ou adaptation visuelle à un stimulus non préférentiel. De ce fait, l’objectif principal de cette thèse est d’investiguer les mécanismes neuronaux qui régissent le traitement visuel durant une plasticité induite par adaptation chez des animaux adultes. Ces mécanismes sont traités sous différents aspects : la connectivité neuronale, la sélectivité neuronale, les propriétés électrophysiologiques des neurones et les effets des drogues (sérotonine et fluoxétine). Le modèle testé se base sur les colonnes d’orientation du cortex visuel primaire. La présente thèse est subdivisée en quatre principaux chapitres. Le premier chapitre (A) traite de la réorganisation du cortex visuel primaire suite à une plasticité induite par adaptation visuelle. Le second chapitre (B) examine la connectivité neuronale fonctionnelle en se basant sur des corrélations croisées entre paires neuronales ainsi que sur des corrélations d’activités de populations neuronales. Le troisième chapitre (C) met en liaison les aspects cités précédemment (les effets de l’adaptation visuelle et la connectivité fonctionnelle) aux propriétés électrophysiologiques des neurones (deux classes de neurones sont traitées : les neurones à décharge régulière et les neurones à décharge rapide ou burst). Enfin, le dernier chapitre (D) a pour objectif l’étude de l’effet du couplage de l’adaptation visuelle à l’administration de certaines drogues, notamment la sérotonine et la fluoxétine (inhibiteur sélectif de recapture de la sérotonine). Méthodes En utilisant des enregistrements extracellulaires d’activités neuronales dans le cortex visuel primaire (V1) combinés à un processus d’imagerie cérébrale optique intrinsèque, nous enregistrons l’activité de décharge de populations neuronales et nous examinons l’activité de neurones individuels extraite des signaux multi-unitaires. L’analyse de l’activité cérébrale se base sur différents algorithmes : la distinction des propriétés électrophysiologiques des neurones se fait par calcul de l’intervalle de temps entre la vallée et le pic maximal du potentiel d’action (largeur du potentiel d’action), la sélectivité des neurones est basée sur leur taux de décharge à différents stimuli, et la connectivité fonctionnelle utilise des calculs de corrélations croisées. L’utilisation des drogues se fait par administration locale sur la surface du cortex (après une craniotomie et une durotomie). Résultats et conclusions Dans le premier chapitre, nous démontrons la capacité des neurones à modifier leur sélectivité après une période d’adaptation visuelle à un stimulus particulier, ces changements aboutissent à une réorganisation des cartes corticales suivant un patron spécifique. Nous attribuons ce résultat à la flexibilité de groupes fonctionnels de neurones qui étaient longtemps considérés comme des unités anatomiques rigides. En effet, nous observons une restructuration extensive des domaines d’orientation dans le but de remodeler les colonnes d’orientation où chaque stimulus est représenté de façon égale. Ceci est d’autant plus confirmé dans le second chapitre où dans ce cas, les cartes de connectivité fonctionnelle sont investiguées. En accord avec les résultats énumérés précédemment, les cartes de connectivité montrent également une restructuration massive mais de façon intéressante, les neurones utilisent une stratégie de sommation afin de stabiliser leurs poids de connectivité totaux. Ces dynamiques de connectivité sont examinées dans le troisième chapitre en relation avec les propriétés électrophysiologiques des neurones. En effet, deux modes de décharge neuronale permettent la distinction entre deux classes neuronales. Leurs dynamiques de corrélations distinctes suggèrent que ces deux classes jouent des rôles clés différents dans l’encodage et l’intégration des stimuli visuels au sein d’une population neuronale. Enfin, dans le dernier chapitre, l’adaptation visuelle est combinée avec l’administration de certaines substances, notamment la sérotonine (neurotransmetteur) et la fluoxétine (inhibiteur sélectif de recapture de la sérotonine). Ces deux substances produisent un effet similaire en facilitant l’acquisition des stimuli imposés par adaptation. Lorsqu’un stimulus non optimal est présenté en présence de l’une des deux substances, nous observons une augmentation du taux de décharge des neurones en présentant ce stimulus. Nous présentons un modèle neuronal basé sur cette recherche afin d’expliquer les fluctuations du taux de décharge neuronale en présence ou en absence des drogues. Cette thèse présente de nouvelles perspectives quant à la compréhension de l’adaptation des neurones du cortex visuel primaire adulte dans le but de changer leur sélectivité dans un environnement d’apprentissage. Nous montrons qu’il y a un parfait équilibre entre leurs habiletés plastiques et leur dynamique d’homéostasie.

Climate change in mountain regions: how local communities adapt to extreme events

This paper examines how local communities adapt to climate change and how governance structures can foster or undermine adaptive capacity. Climate change policies, in general, and disaster risk management in mountain regions, in particular, are characterised by their multi-level and multi-sectoral nature during formulation and implementation. The involvement of numerous state and non-state actors at local to national levels produces a variety of networks of interaction and communication. The paper argues that the structure of these relational patterns is critical for understanding adaptive capacity. It thus proposes an expanded concept of adaptive capacity that incorporates (horizontal and vertical) actor integration and communication flow between these actors. The paper further advocates the use of formal social network analysis to assess these relational patterns. Preliminary results from research on adaptation to climate change in a Swiss mountain region vulnerable to floods and other natural hazards illustrate the conceptual and empirical significance of the main arguments.

Plasticité du cortex visuel: «homéodynamie» des connexions neuronales et modèle d’effets d’antidépresseurs

Les informations sensorielles sont traitées dans le cortex par des réseaux de neurones co-activés qui forment des assemblées neuronales fonctionnelles. Le traitement visuel dans le cortex est régit par différents aspects des caractéristiques neuronales tels que l’aspect anatomique, électrophysiologique et moléculaire. Au sein du cortex visuel primaire, les neurones sont sélectifs à divers attributs des stimuli tels que l’orientation, la direction, le mouvement et la fréquence spatiale. Chacun de ces attributs conduit à une activité de décharge maximale pour une population neuronale spécifique. Les neurones du cortex visuel ont cependant la capacité de changer leur sélectivité en réponse à une exposition prolongée d’un stimulus approprié appelée apprentissage visuel ou adaptation visuelle à un stimulus non préférentiel. De ce fait, l’objectif principal de cette thèse est d’investiguer les mécanismes neuronaux qui régissent le traitement visuel durant une plasticité induite par adaptation chez des animaux adultes. Ces mécanismes sont traités sous différents aspects : la connectivité neuronale, la sélectivité neuronale, les propriétés électrophysiologiques des neurones et les effets des drogues (sérotonine et fluoxétine). Le modèle testé se base sur les colonnes d’orientation du cortex visuel primaire. La présente thèse est subdivisée en quatre principaux chapitres. Le premier chapitre (A) traite de la réorganisation du cortex visuel primaire suite à une plasticité induite par adaptation visuelle. Le second chapitre (B) examine la connectivité neuronale fonctionnelle en se basant sur des corrélations croisées entre paires neuronales ainsi que sur des corrélations d’activités de populations neuronales. Le troisième chapitre (C) met en liaison les aspects cités précédemment (les effets de l’adaptation visuelle et la connectivité fonctionnelle) aux propriétés électrophysiologiques des neurones (deux classes de neurones sont traitées : les neurones à décharge régulière et les neurones à décharge rapide ou burst). Enfin, le dernier chapitre (D) a pour objectif l’étude de l’effet du couplage de l’adaptation visuelle à l’administration de certaines drogues, notamment la sérotonine et la fluoxétine (inhibiteur sélectif de recapture de la sérotonine). Méthodes En utilisant des enregistrements extracellulaires d’activités neuronales dans le cortex visuel primaire (V1) combinés à un processus d’imagerie cérébrale optique intrinsèque, nous enregistrons l’activité de décharge de populations neuronales et nous examinons l’activité de neurones individuels extraite des signaux multi-unitaires. L’analyse de l’activité cérébrale se base sur différents algorithmes : la distinction des propriétés électrophysiologiques des neurones se fait par calcul de l’intervalle de temps entre la vallée et le pic maximal du potentiel d’action (largeur du potentiel d’action), la sélectivité des neurones est basée sur leur taux de décharge à différents stimuli, et la connectivité fonctionnelle utilise des calculs de corrélations croisées. L’utilisation des drogues se fait par administration locale sur la surface du cortex (après une craniotomie et une durotomie). Résultats et conclusions Dans le premier chapitre, nous démontrons la capacité des neurones à modifier leur sélectivité après une période d’adaptation visuelle à un stimulus particulier, ces changements aboutissent à une réorganisation des cartes corticales suivant un patron spécifique. Nous attribuons ce résultat à la flexibilité de groupes fonctionnels de neurones qui étaient longtemps considérés comme des unités anatomiques rigides. En effet, nous observons une restructuration extensive des domaines d’orientation dans le but de remodeler les colonnes d’orientation où chaque stimulus est représenté de façon égale. Ceci est d’autant plus confirmé dans le second chapitre où dans ce cas, les cartes de connectivité fonctionnelle sont investiguées. En accord avec les résultats énumérés précédemment, les cartes de connectivité montrent également une restructuration massive mais de façon intéressante, les neurones utilisent une stratégie de sommation afin de stabiliser leurs poids de connectivité totaux. Ces dynamiques de connectivité sont examinées dans le troisième chapitre en relation avec les propriétés électrophysiologiques des neurones. En effet, deux modes de décharge neuronale permettent la distinction entre deux classes neuronales. Leurs dynamiques de corrélations distinctes suggèrent que ces deux classes jouent des rôles clés différents dans l’encodage et l’intégration des stimuli visuels au sein d’une population neuronale. Enfin, dans le dernier chapitre, l’adaptation visuelle est combinée avec l’administration de certaines substances, notamment la sérotonine (neurotransmetteur) et la fluoxétine (inhibiteur sélectif de recapture de la sérotonine). Ces deux substances produisent un effet similaire en facilitant l’acquisition des stimuli imposés par adaptation. Lorsqu’un stimulus non optimal est présenté en présence de l’une des deux substances, nous observons une augmentation du taux de décharge des neurones en présentant ce stimulus. Nous présentons un modèle neuronal basé sur cette recherche afin d’expliquer les fluctuations du taux de décharge neuronale en présence ou en absence des drogues. Cette thèse présente de nouvelles perspectives quant à la compréhension de l’adaptation des neurones du cortex visuel primaire adulte dans le but de changer leur sélectivité dans un environnement d’apprentissage. Nous montrons qu’il y a un parfait équilibre entre leurs habiletés plastiques et leur dynamique d’homéostasie.