Optogenetic Probing of Cortical Synaptic Circuits Measured Through in Vivo Whole-Cell Recordings in the Mouse Barrel Cortex

Although neocortex underlies higher-order brain functions, little is known about the synaptic interactions that drive neocortical microcircuit function in vivo. The neocortex is spontaneously active in vivo so I explored how such spontaneous network activity affects in vivo the interactions between excitatory and inhibitory neurons in the barrel cortex. To this aim, I have focused on the layer 2/3 microcircuit of the C2 barrel column driving sensory processing for the C2 whisker, and combined in vivo whole-cell patch-clamp recordings in anesthetised and awake head-restrained mice with optogenetic stimulation, that allows controlling the activity of genetically defined neurons with light. I expressed channelrhodopsin-2 (ChR2), a light-gated cation channel to evoke action potentials in excitatory neurons with brief light pulses. The impact of this optogenetic stimulus was measured on the membrane potential dynamics of their postsynaptic target neurons. The optogenetic stimulus drove reliable action potentials in ChR2-expressing excitatory neurons, independent of the state of ongoing spontaneous cortical activity. In contrast, I found a marked modulation of light-evoked postsynaptic responses in ChR2-non-expressing excitatory neurons by cortical activity. During spontaneously hyperpolarised periods (DOWN state), optogenetic stimulation resulted in depolarisations of postsynaptic excitatory neurons, while during depolarised periods (UP state), the response was significantly smaller and shorter. The optogenetic stimulus drove the membrane potential of postsynaptic excitatory layer 2/3 neurons towards reversal potentials, which were hyperpolarised relative to action potential threshold, leading to a reduction in firing rates. Targeting whole-cell recordings to GFP-labelled GABAergic layer 2/3 neurons with two-photon microscopy revealed that both fast-spiking and non-fast-spiking inhibitory postsynaptic neurons were rapidly driven to fire action potentials, preferentially from UP states. Such rapid activation of inhibitory neurons in the UP state is likely to be responsible for decreasing firing rate in excitatory neurons. As a result, optogenetic stimulation of a given set of excitatory neurons results in a state-dependent recruitment of inhibitory neurons that limit the spread of excitation within the layer 2/3 microcircuit. The data presented here indicates that a powerful inhibitory loop within the cortical microcircuit rapidly balances excitation with inhibition and enforces sparse coding of layer 2/3 excitatory neurons.

In vivo measurement of excitatory synaptic transmission between identified neurons in layer 2/3 mouse barrel cortex

Aurélie Pala

The neocortex is the most distinctive feature of the mammalian brain and it is considered to be the substrate of high-order cognitive functions. The nature and the arrangement of the diverse neuronal elements constituting its multiple areas have received longstanding attention. Progressively such anatomical and functional investigations are undertaken in the context of an intact living being. It offers the possibility of examining the role of particular areas, networks or even individual neurons during various behavioral states. Synaptic connectivity and synaptic transmission have been traditionally investigated in reduced preparations. Typically, electrophysiological and optical techniques have been used to control and record the propagation of electrical activity between two or more neurons in acute brain slices in vitro. The purpose of this thesis is the investigation of synaptic connectivity and synaptic transmission within the intact neocortex of the living mouse. Here I took advantage of the recent development of optogenetics, in combination with electrophysiology and two-photon microscopy to systematically and directly record synaptic transmission between a single excitatory neuron and two main types of GABAergic neurons in layer 2/3 of the mouse barrel cortex in vivo. Overall, I discovered stronger excitatory connections onto GABAergic neurons than onto excitatory neurons, irrespective of the absolute or relative locations of the pre- and postsynaptic neurons somas. I further revealed that parvalbumin-expressing (PV) and somatostatin-expressing (Sst) GABAergic neurons received excitatory inputs that were similar in magnitude, but were more reliable and faster in PV neurons than in Sst neurons. Exploring postsynaptic responses to multiple presynaptic action potentials elicited at high frequency, I found a strong short-term facilitation accompanied by significant input summation in Sst neurons, but little short-term dynamics with no summation in PV neurons. Lastly, I compared the amplitude of single action potential-evoked postsynaptic responses as a function of neocortical activity level and found that it was unchanged in both neuron types. Overall, the results of this thesis provide new insights into the functioning of microcircuits in vivo while confirming many findings from reduced preparations. In the future, it will be interesting to extend these initial in vivo measurements to other neuron and synapse types, particularly in awake animals engaged in different behavioral states.

EPFL2014

Membrane potential dynamics of GABAergic neurons in the barrel cortex of behaving mice

Michael Avermann, Ferenc Mátyás, Carl Petersen

Computations in cortical circuits are mediated by synaptic interactions between excitatory and inhibitory neurons, and yet we know little about their activity in awake animals. Here, through single and dual whole-cell recordings combined with two-photon microscopy in the barrel cortex of behaving mice, we directly compare the synaptically driven membrane potential dynamics of inhibitory and excitatory layer 2/3 neurons. We find that inhibitory neurons depolarize synchronously with excitatory neurons, but they are much more active with differential contributions of two classes of inhibitory neurons during different brain states. Fast-spiking GABAergic neurons dominate during quiet wakefulness, but during active wakefulness Non-fast-spiking GABAergic neurons depolarize, firing action potentials at increased rates. Sparse uncorrelated action potential firing in excitatory neurons is driven by fast, large, and cell-specific depolarization. In contrast, inhibitory neurons fire correlated action potentials at much higher frequencies driven by slower, smaller, and broadly synchronized depolarization.

Elsevier2010

In vivo measurement of excitatory synaptic transmission between identified neurons in layer 2/3 mouse barrel cortex

Aurélie Pala

EPFL2014