Cholinergic modulation of cortical sensory processing: combining functional coritcal imaging and optogenetic approaches. – Sensory Processing

Voltage sensitive dye imaging allows visualizing the spatiotemporal dynamics of cortical activity on the millisecond timescale with a spatial resolution of a few tens of microns. Our project aimed at combining this method with optogenetics in order to record cortical activity while controlling with light the activity of cholinergic neurons of the nucleus basalis magnocellularis.
In parallel to the implementation of the necessary tools to control specifically the activity of cholinergic neurons in vivo, we have combined the voltage sensitive dye imaging with the use of a multivibrissal tactile stimulator, previously developed in the team, thus providing a unique tool to study the propagation and integration of neural information in the primary somatosensory cortex in response to spatially distributed stimulation on the receptive surface. In order to fully benefit from these approaches, we have developed (in collaboration with Gabriel Peyré, CEREMADE) a method for aligning recorded voltage sensitive dye signals with the underlying cytoarchitectural map of the cortical network.

The project Sensory Processing allowed building two setups for voltage sensitive dye imaging of the mouse barrel cortex that combine imaging with a multivibrissal tactile stimulator on the one hand, and with optogenetics on the other hand. We have developed and validated a method for mapping the functional images collected on the anatomical organization of the cortex. We have revealed the spatial distribution of the global direction selectivity to multivibrissal stimulations at the supra-columnar scale. And we explore the impact of cholinergic inputs on the integrative properties of the primary somatosensory cortex. The methods implemented in the framework of this project are applicable to the awake mouse in a configuration where the mouse is habituated to have its head immobilized. To allow the exploration of a richer behavioral repertoire, we are currently working on the development of fiber optic interfaces for visualizing cortical dynamics, or controlling them with patterned optogenetic photostimulation, in the awake freely moving mouse.

We have developed a unique experimental approach combining the most sophisticated tool for controlled tactile stimulation (allowing independent multidirectional deflection of the 24 principal whiskers) and the functional imaging of the cortical representation of these 24 whiskers in the mouse primary somatosensory cortex. Beyond the collected data on the processing of multivibrissal inputs, we will explore other aspects of the cortical function such as the ability to generate a waiting signal linked to the strong spatiotemporal predictability of the sensory inputs (induced by the repeated presentation of complex structured tactile stimuli). We will keep on exploring the neuromodulation of the integrative properties of the somatosensory cortex using the implemented optogenetic tools. Finally, having such methodological facilities will allow us studying how these functional properties of cortex are impaired in pathologies (murine models).

This work gave rise to a peer reviewed publication in «Journal of Neuroscience Methods«. The results obtained on the integration of multiwhisker stimuli that were the subject of a PhD thesis, resulted in another manuscript which will be submitted shortly. They have been presented in several national and international meetings and workshops: two poster presentations at the French Neuroscience symposium, two oral presentations in workshops (NeuroScience Workshop Saclay, Gif sur Yvette - NeWS 2014 and Network dynamics at mesoscopic scales, Marseille 2015), a poster presentation at the Society for Neuroscience meeting (San Diego, USA 2013), and at the more specialized satellite symposium Barrels 26th meeting (San Diego, USA, 2013).

The cerebral cortex is considered as the most integrative structure of the mammalian brain. It processes inputs from multiple brain areas through a complex neuronal network and is involved in many cognitive functions. This structure therefore constitutes a central subject of research in modern Neuroscience which aims to understand the neuronal bases of behaviour.
Because of its high degree of organization, the rodent’s whisker-to-barrel cortex pathway is a model of choice to study the cortical processing of sensory information. The neuronal actors and the cascade of excitatory synaptic events that underlies this sensory pathway have been largely studied using classical anatomical and electrophysiological techniques from in vitro brain slices preparations or in vivo, in anesthetised animals. However, little is known about the neural mechanisms underlying sensory processing and perception in awake behaving animals.
Recent progresses in the technology related to voltage sensitive dye imaging has opened new avenues in this field, allowing the recording of cortical ensemble activity with a temporal resolution reaching the millisecond and a spatial resolution of few tens of micrometers in the awake mouse sensorimotor cortex. Such recordings have revealed a strong influence of the behaviour on the spatiotemporal properties of the cortical responses induced by tactile stimulations.
At the neuronal network level, the basalo-cortical cholinergic fibers, which project broadly to the cerebral cortex and whose impairment affect motivational and arousal states, are good candidates to be involved in this behavioural modulation of the cortical sensory processing. Because they can exert a rapid excitation of specific cortical interneuron populations via ligand-gated ion channels, they could notably contribute to shape the cortical activation induced by tactile stimulation.
In order to study the impact of cholinergic inputs on cortical sensory processing we will focus on the model system of the mouse whisker-to-barrel cortex sensory pathway. The modulation of cortical responses evoked by tactile stimulation will be studied by in vivo voltage sensitive dye imaging and multiple single-unit recordings combined with the activation or inhibition of cholinergic release from basalo-cortical fibers using optogenetic tools. This novel experimental approach combines genetic and optical methods to achieve either activation or silencing of chosen neuronal populations with high spatial and temporal precision. It is based on the genetically targeted selective expression of microbial light-activated proteins (i.e. light-gated cation channel for neuronal excitation, or light drivable proton pump for neuronal silencing). By using Cre-driver mice combined with recombinase-dependent viral vectors, we will drive specifically the expression of such light-activated proteins in basalo-cortical cholinergic neurons, and therefore get the ability to control precisely and specifically their activity.
The realisation of this project will certainly help us to describe the implication of ascending cholinergic neuromodulatory projections on cortical function, and to further the understanding of the mechanisms underlying the cognitive deficits associated with their degeneration.