Prof. Dr. Petersen (CH)

Prof. Dr. Carl C. H. Brain Mind Institute,
Ecole Polytechnique Fédérale de Lausanne

 

 Whole-cell recordings from GABAergic neurons in the barrel cortex of awake mice

The neocortex is composed of an intricate network of synaptically connected excitatory glutamatergic and inhibitory GABAergic neurons. Remarkable progress has been made over the last decades in determining the synaptic connectivity of local neocortical microcircuits through anatomical and electrophysiological studies. The synaptic connectivity of the rodent barrel cortex, responsible for processing whisker information (recently reviewed by Petersen, 2007), in particular has begun to be studied in quantitative detail (Bureau et al., 2004, 2006; Feldmeyer et al., 1999, 2002, 2005, 2006; Helmstaedter et al., 2008, 2009a,b,c; Petersen and Sakmann, 2000, 2001; Schubert et al., 2001, 2003, 2006; Shepherd et al., 2003, 2005; Shepherd and Svoboda, 2005; Silver et al., 2003; Brasier and Feldman, 2008; Frick et al., 2008; Lefort et al., 2009; Xu & Callaway, 2009). In order to understand how these barrel cortex synaptic circuits function during behaviour, it is of critical importance to record neuronal activity from awake rodents, whilst quantifying whisker-related behaviour. Whole-cell recordings from excitatory pyramidal neurons in awake rodents have already begun to reveal basic principals of cortical function (Crochet & Petersen, 2006; Lee et al., 2006; Poulet & Petersen, 2008). However, no equivalent recordings from GABAergic neurons have yet been reported and we therefore lack functional information on a critical population of neocortical neurons. Here, we plan to obtain whole-cell recordings in awake head-restrained mice focusing on specific populations of GABAergic neurons labelled with GFP and visualised through two-photon microscopy.

            Our goal is to target three major classes of GABAergic neurons, which are predominantly labelled in different transgenic mouse lines. Fast-spiking parvalbumin-expressing GABAergic neurons will be targeted in G42 mice (Chattopadhyaya et al., 2004) and PV-GFP BAC-transgenic mice (Meyer et al., 2002). These cells are likely to correspond primarily to ‘Basket Cells’. Somatostatin-expressing neurons are labelled with GFP in GIN mice (Oliva et al., 2000), and through targeting our recordings to GFP-expressing neurons in this mouse we hope to record from ‘Martinotti’-like cells. In addition, we will record from GFP-labelled neurons in a BAC-transgenic mice in which VIP-expressing ‘Bipolar’ neurons are labelled (GENSAT mouse; BAC address: RP23-25A8). Through our whole-cell recordings we will first characterise the basic intrinsic physiological properties of these different classes of neurons. Next we will examine their spontaneous activity in the absence of whisker-related sensorimotor activity. Finally, we will investigate the membrane potential dynamics of these neurons during active whisker movement, during whisker deflection and during whisker-object contact. In collaboration with Prof. Cornelius Schwarz, we will also develop behavioural paradigms for linking psychometric and neurometric performance.

            Previous characterisations of neocortical GABAergic neurons in vitro in brain slices have revealed important differences in the sources and short-term dynamics of synaptic inputs (Reyes et al., 1998; Gibson et al., 1999; Gupta et al., 2000; Yoshimura et al., 2005; Cruikshank et al., 2007; Kapfer et al., 2007; Silberberg & Markram, 2007; Xu & Callaway, 2009; Helmstaedter et al. 2008, 2009a,b,c) onto the different classes of GABAergic neurons and in their responses to a variety of neuromodulators (Gulledge et al., 2007; Fanselow et al., 2008). It is therefore possible that the different classes of GABAergic neurons will behave in radically different ways during different whisker-related behaviours and the goal of this project is to examine this in detail within the limits of the currently available GFP mouse lines.

            Importantly, during the whole-cell recording, we will fill the neurons with biocytin allowing detailed post hoc anatomical study, which can be correlated with the physiological response properties of the neurons. The anatomical part of this work will be carried out by the group of Prof. Jochen Staiger (see separate proposal included in this collaborative grant application).

            The goal is to obtain a detailed description of the structure and function of several important classes of GABAergic neurons found in the mouse barrel cortex. Such knowledge will form an essential step towards understanding how the neocortex processes sensory information at the level of individual neurons and their synaptic interactions.