Neurogliaform cells · Contribution of neurogliaform cells to signal flow in the barrel cortex during whisking behaviour

Contribution of neurogliaform cells to signal flow in the barrel cortex during whisking behaviour

The goal of this Marie-Curie postdoc fellowship grant awarded to Szabolcs Olah to work in the laboratory of Carl Petersen at the Brain Mind Institute of the Ecole Polytechnique Federale de Lausanne (EPFL) was to investigate the role of neurogliaform cells in vivo in the neocortex of mice. Previous work in vitro in brain slices defined neurogliaform cells as inhibitory GABAergic neurons that are thought to primarily release GABA into the extracellular space and to modulate cortical function through both ionotropic GABA-A receptors, as well as metabotropic GABA-B receptors. The goal of this project was to investigate the function of neurogliaform cells in the primary somatosensory barrel cortex of head-restrained mice during quantified whisker behavior. Specifically, we proposed: i) to target whole-cell membrane potential recordings to fluorescently labeled neurogliaform cells and correlate membrane potential fluctuations with whisker movement and whisker deflections, ii) to obtain dual whole-cell recordings from a neurogliaform cell and another type of neuron to examine correlations and the impact of action potential firing, and iii) to make optogenetic manipulations of neurogliaform cells to examine their functional role. During the project period from 1st March 2012 until the termination of the project on 30th September, we only partially achieved these goals. We ran into multiple difficulties that ultimately precluded the collection of data relating to neurogliaform cells. We were therefore unable to address any of the three specific goals of the project. However, Szabolcs Olah learned many important in vivo techniques during his time at the EPFL. There was therefore good success in terms of training and this will doubtless be useful in Szabolcs Olah’s career. The technical difficulties in achieving the project goals are described below in further detail. The first technical difficulty arose in identifying the best transgenic mouse line to study. There are currently no mouse lines that specifically label neurogliaform cells and we decided to focus our attention on GABAergic neurons in layer 1. Approximately half of these neurons are thought to be neurogliaform cells. Using a two photon microscope, we therefore targeted recordings to red fluorescing cells in a genetic cross of GAD-Cre mice with a LSL-tdTomato Cre-reporter line. These are technically difficult experiments, but, after some months of training for in vivo imaging and whole-cell recording, Szabolcs Olah began to successfully record from these fluorescently labeled neurons. In the experiments over the first half-year, we worked primarily under anesthesia to help with mechanical stability. In the next step, we began to attempt recordings in awake head-restrained mice in order to compare membrane potential dynamics with behavior. Unfortunately, we experienced considerable difficulty in stabilizing the brain in awake head-restrained mice in such a way that we could also introduce a recording electrode into layer 1. Neurogliaform cells are known to have very small cell bodies and this provided additional technical difficulties in obtaining whole-cell recordings. Eventually, whole-cell recordings from layer 1 GABAergic neurons were obtained from awake mice. We found a diversity of electrophysiological properties and membrane potential fluctuations in these recordings. In order to make sense of this diversity, we aimed to correlate the electrophysiology with anatomical analysis of the axonal and dendritic structure of the recorded neuron. We attempted to do this with biocytin filling, but unfortunately did not succeed in identifying the anatomical subtype of any recorded neuron. We were therefore unable to make conclusions about the function of neurogliaform cells.

Data: CORDIS, © European Union