Étude de la plasticité du cortex strié par l’entremise de la kétamine et de l’adaptation visuelle

Le cortex cérébral est impliqué dans plusieurs fonctions entre autres le traitement des informations sensorielles. Il inclut des zones recevant directement une entrée sensorielle telle que le cortex visuel primaire (V1) qui traite les informations visuelles. Au niveau du V1 des mammifères, chaque neurone présente une combinaison préférentielle de stimuli pour lesquels sa réponse est optimale. Cela dit, chaque attribut de stimulus tel que les fréquences temporelle et spatiale, l’orientation et la direction du mouvement induit une réponse maximale du neurone. Le neurone du V1 est donc sélectif. Cependant, cette sélectivité n’est pas le résultat de l’activité du neurone en question seul, mais plutôt du réseau neuronal dans lequel il est impliqué. L’ensemble des préférences d’un neurone ainsi que le réseau neuronal auquel il appartient demeurent sensiblement inchangés, tant que les facteurs contextuels ne varient que peu ou pas. Toutefois, si les composantes de l’environnement changent de manière imposante, la sélectivité neuronale et l’organisation du réseau original seront modifiées pour induire un nouvel état d’équilibre. C’est la plasticité neuronale. Le but ultime de cette thèse est de comprendre et d’approfondir les connaissances relatives aux mécanismes régissant la sélectivité à l’orientation ainsi que la plasticité dans V1, et ce, par différentes études qui sont organisées, dans cette thèse en trois sections. Les sections (3) et (4) se basent sur une étude pharmacologique qui vise à examiner l’effet de la kétamine sur la sélectivité à l’orientation (section 3) et sur l’adaptation visuelle tout en traitant la connectivité neuronale (section 4). La section (5) vise à examiner l’effet de l’adaptation sur l’affinité des courbes d’accord des neurones. Ce travail a permis d’étudier l’effet de la kétamine et de l’adaptation visuelle sur les propriétés sélectives à l’orientation des neurones ainsi que sur la dynamique des relations fonctionnelles au sein du microcircuit.The cerebral cortex plays a key role in several functions including the processing of sensory information. It contains areas that receive direct sensory input such as the primary visual cortex (V1) which processes visual information. V1 neurons of mammals are selective for several attributes, such as spatial and temporal frequencies, orientation, and direction of motion. Thus, V1 neurons exhibit selectivities. This neuronal selectivity rests in the convergence of clusters of synapses involved in the network. Neural selectivity and networks are formed during the sensitive period of brain development and is present throughout the animal’s life. However, in V1 during postnatal life, the neuronal selectivity and the neural circuitry are further shaped by experience, thus, rendering it plastic. The main objective of the current thesis is to understand the mechanisms involved in the orientation selectivity as well as the neuroplasticity in V1. To this aim, different investigations, organized in this thesis, in three sections, were carried out. The sections (3) and (4) are based on a pharmacological study that aim to examine the effect of ketamine on orientation selectivity (section 3) and on visual adaptation in relation with neural connectivity (section 4). The study presented in the third section (section 5) investigated the effect of adaptation on the cell’s tuning. Here, we disclose the effects of ketamine and visual adaptation on the cell’s tuning properties as well as on the dynamics of functional relationships between neurons in the microcircuit

Are Sensory Neurons in the Cortex Committed to Original Trigger Features?

Sensory cortices are inherently dynamic and exhibit plasticity in response to a variety of stimuli. Few studies have revealed that depending upon the nature of stimuli, excitation of the corresponding sensory region also evokes a response from other neighboring connected areas. It is even more striking, when somatosensory areas undergo reorganization as a result of an intentional disturbance and further explored as a paradigm to understand neuroplasticity. In addition, it has also been proved that drugs too can be used as a model to explore potential plasticity in sensory systems. To this aim, through electrophysiology in cats, we explored that visual neurons, throughout the cortical column, have a tendency to alter their inherent properties even when presented a non-visual stimulus. Furthermore, it was explored in mice, how the application of drugs (serotonin and ketamine) modulates potential plasticity within the visual system. Indeed, we found a shift in orientation tuning of neurons indicated by Gaussian tuning fits in both scenarios. These results together suggest that sensory cortices are capable of adapting to intense experiences by going through a recalibration of corresponding or neighboring sensory area(s) to redirect the sensory function and exhibit remarkable extent of neuroplasticity within the brain

Cortical Plasticity under Ketamine: From Synapse to Map

Sensory systems need to process signals in a highly dynamic way to efficiently respond to variations in the animal’s environment. For instance, several studies showed that the visual system is subject to neuroplasticity since the neurons’ firing changes according to stimulus properties. This dynamic information processing might be supported by a network reorganization. Since antidepressants influence neurotransmission, they can be used to explore synaptic plasticity sustaining cortical map reorganization. To this goal, we investigated in the primary visual cortex (V1 of mouse and cat), the impact of ketamine on neuroplasticity through changes in neuronal orientation selectivity and the functional connectivity between V1 cells, using cross correlation analyses. We found that ketamine affects cortical orientation selectivity and alters the functional connectivity within an assembly. These data clearly highlight the role of the antidepressant drugs in inducing or modeling short-term plasticity in V1 which suggests that cortical processing is optimized and adapted to the properties of the stimulus