IRREGULAR IN VIVO-LIKE BACKGROUND SYNAPTIC ACTIVITY RECREATED IN IN VITRO NEOCORTICAL SLICES

M. Giugliano, H.-R. Lüscher.
Institute of Physiology, University of Bern.

Estimates of the number of synaptic contacts of a neocortical cell range between 5000 and 60000, 70% of them originating from intracortical areas. Furthermore, neocortical neurons fire spontaneously at a frequency of 5–20Hz in awake animals. These considerations define a scenario in which neurons experience very large synaptic currents (i.e. hundreds of postsynaptic potentials over a ms-time scale). Such an intense background activity induces random-walk fluctuations in the postsynaptic membrane potentials and it is thought to have a profound impact on the neuronal integrative properties, on the response dynamics to external stimuli, as well as on the activity-dependent plasticities. These implications have been never systematically studied in vivo, because of the technical difficulties related to intracellular and patch-clamp recordings in behaving animals. On the other hand, acute neocortical slices are widely employed as a reduced in vitro model, but in spite of the many advantages they do not accurately represent the realistic cortical networks physiology. In particular, since deafferentation causes the lack of background synaptic activity, conclusions obtained in vitro may not transfer to in vivo situations. A substantial contribution at restoring a realistic network input drive may come from a novel application of substrate arrays of microelectrodes (MEAs). We developed an experimental set-up combining standard whole-cell patch-clamp and MEAs, in in vitro rat brain tissue slices (see the figure). By means of a asynchronous multi-site electrical stimulation, delivered via 60 microelectrodes, we could recreate in vivo-like sustained synaptic activity in the neuronal microcircuits, resulting in neuron membrane potential fluctuations and irregular spike emission, similar to those observed in vivo. We present an analysis of single-neuron recordings and discuss research directions, expected to fully exploit the potential of such a novel stimulation paradigm as an advanced tool for network-level neuroscience.