Neural network activity in rodent sensory cortex assessed with novel calcium recording techniques

The neocortex is one of the most fascinating structures of the brain. It consists of 6 horizontal layers, containing a highly complex microcircuitry composed of a number of different specifically interconnected cell types. Elaborate calculations are performed within the networks of the neocortex that enable intricate behaviors. Neuroscientists have employed various techniques to probe different levels of cortical organisation. Namely functional mapping on a macroscopic level has been conducted by means of functional magnetic resonance imaging (fMRI), a number of electrophysiological approaches as well as optical methods such as voltage sensitive dye (VSD) imaging. On the level of single neurons, patch-clamp experiments have elucidated synaptic connectivity and compartment specific calculations. However, the link of these two different levels, a microcircuit comprised of tens to thousands of cells, has been challenging to investigate mainly due to technical limitations. In this thesis I present two different approaches using optical methods in association with fluorescent calcium indicators to investigate neuronal network dynamics at this intermediate scale in the primary sensory cortex of the rodent brain. In the first project, genetically encoded calcium indicators (GECI) were expressed in the sensory barrel cortex of mice. A signature of whisking-related behavior in the supragranular layers 2 and 3 (L2/3) was uncovered by means of two-photon calcium imaging in awake headfixed mice. Populations of ~ 35 neurons were simultaneously imaged. While the large fraction of cells was down-modulation during whisking (59%), a small fraction of cells (7%) was robustly activated, in line with papers published during the course of this dissertation [1, 2]. This pattern of activity persisted during sensory air-puff stimulation, which induced whisking behavior and thus led to a net decrease of activity, with only very few cells responding to the sensory stimulation. The use of the GECI YC3.60 allowed to revisit the same cells over months and thus prove the stability of their whisking-related modulation. This suggested the contribution of distinct cell types and gave rise to the hypothesis that up-modulated cells correspond to interneurons that cause down-modulation in a large part of the pyramidal population. I identified interneurons by means of transgenic mice expressing tdTomato in GABAergic cells and post hoc immunohistochemistry. Potential interneurons were heterogeneously distributed over all classes of modulation and importantly, only contributed one third of the up-modulated population. Morphological evidence further supported the contribution of pyramidal cells to this cell group. The particular interneuron subtypes parvalbumin positive (PV+) and to a large extent also calretinin positive (CR+) cells were down-modulated. Furthermore, I discovered a large fraction of inhibitory L1 cells that were up-modulated during whisking. These cells could potentially correspond to neurogliaform cells and their influence on L2/3 population activity needs to be further evaluated. To evaluate the source of modulatory input-information into S1, I eliminated the re-afference