The neocortical microcircuit lies at the heart of the information processing capability of the mamalian neocortex. It is the ultimate goal of LNMC to unravel this neocortical computational capability by deriving the blue print of the cortical microcircuit. This will prove essential for a comprehensive understanding of higher cognitive functions. To this end, the LNMC has a long track record of mapping neocortical microcircuits and their dynamic properties using state-of-the-art multi-pach electropysiology. Major work includes publications on the dynamics of excitatory synapses between layer 5 pyramidal cells [2]; understanding the Martinotti loop [4] and describing how and what kind of layer 5 pyramidal cells connect across the cortical hemispheres [3]. We continue this work in LNMC, mapping the cellular composition of the neocortex layer by layer as well as thier synaptic connetivity and dynamics.
Approach
We study the properties and dynamics of microcircuits by patching multiple neurons in small local clusters. After single cell electrophysiological characterization, we study the connections between these very cells. Our multiple multi-patch clamp setups help us to study all sizes of circuitry – from pairs of cells to large intricate circuits of up to 12 cells. The presence of biocytin in our intracellular solutions ensures that the cells under investigation can be reconstructed in 3D, with subcellular details such as dendritic spines and axonal boutons.
We also use Multi-Electrode Arrays (MEAs) that help us probe wider circuits of cells, for example across neocortical layers and brain areas.
The combination of these major sources of information leads to a deeper study of the cortical neurons and the networks they form.
Layer 5 cell properties
Our lab has a long standing commitment to study the principles of connectivity in neocortical microcircuits. We have found, among many other observations, evidence of Hebbian assemblies at the most elementary level of the cortex [8]. We were also surprised to discover that their connectivity and synaptic weights are highly predictable and constrained by a simple common neighbor rule. Our future directions will include the study of neural microcircuits in many fronts, identifying essential relationships among neurons, aiming at a better understanding of how such interactions influence neural function.
Layer 1 characterization
Layer 1 constitutes the most superficial layer of the neocortex, located right beneath the dura mater.
We are currently emplying patch clamp electrophysiology and 3D reconstructions to describe the structure, function and connectivity between these neurons. Understanding the basic properties of these cells will help us better understand the neuronal circuitry and their contribution to information processing in the brain. Using single cell milti-patch clamp electrophysiology we find that layer 1 cells can be grouped into five firing types, as per the Petilla Interneuron convention. On the other hand, anatomical analyses shows that they can be separated into 6 distinct morphological groups.
Local field potentails (LFPs) and ephaptic coupling
There is a wide variety of forms in which neurons interact with each other. The most commonly conceived forms are through synapses and gap-junctions, but neurons have also been shown to interact through electric fields. The very same electric fields generated by local network activity can affect the activity of individual neurons. We quantified the impact of electric fields onto single neurons [9] and continue to actively investigate the implications of these findings with multi-electrode recordings of intra- and extra-cellular activity in controlled conditions.
Inter-layer connectivity
We combine Multi Electrode Array (MEA) technology with the patch-clamp technique to probe inter-layer connectivity in an acute cortical brain slice. We record in all non-stimulating extracellular sites the field potentials generated in response to a systematical electrical stimulation in each cortical layer at various frequencies, mimicking inputs at different brain regions. Simultaneously, we measure intracellular signal from layer five thick-tufted pyramidal cells. We find a variety of properties such as cortical layers interconnectivity, frequency dependent connectivity, and network plasticity. This project is being mirrored in-silico in the BBP neocortical column.
Cortico-Thalamic Neuronal Circuits
Adding to the complexity of the system is that cortical microcircuits do not function in isolation. They receive input from subcortical regions, each with their own microcircuitry properties. The thalamus is a subcortical brain region that relays sensory and motor information from the peripheral sensory system to the cortex, via functionally distinct subregions. We advance our understanding of microcircuit function characterizing the electrophysiological properties of neuronal sybtypes, characterize their morphologies and anatomical contacts, and identify molecular mechanisms underlying the biophysical and anatomical properties.
Neuron-Glia-Vasculature Characterization
Neuronal cells are also out-numbered by non-neuronal components, such as glial cells, and are surrounded by specialized vasculature and fluid/metabolites control systems. Together, these components create neuron-glia-vasculature (NGV) interactions that affect neuron activity, brain energy metabolism, clearance, and immune responses, among others, all integral components of microcircuit function. The systematic characterization of each component of NGV has received less attention than neurons. New projects in LNMC will explore in particular, how these components interact with each other and change with increasing age.