The hippocampus is involved in the formation of memories for the events we experience every day (episodic memory) and in our capacity to locate ourselves and navigate in familiar environments without the nee d of a map (spatial navigation). For a long time these two functions of the hippocampus were separated. Recent work however suggests that there is a link between these two functions in the mammalian brain. Our aim is to understand the mechanisms of spatial navigation both at the cellular and network level. This could help us understand episodic memory formation. To answer this question we perform extracellular recordings of hippocampal cells activity. Extracellular recordings can only record the spiking output of neurons but not the intracellular mechanisms leading to that spiking. This is why I recently contributed together with my colleagues Dr. Albert Lee and Dr. Michael Brecht to the development of a new technique allowing reliable intracellular recordings of hippocampal neurons in freely behaving animals (Lee*, Epsztein* and Brecht 2009; Epsztein*, Lee* et al., 2010; Epsztein et al., 2011). Intracellular recordings allow us to study the mechanisms of spatial coding in great detail. Indeed we do not only record the output of cells in form of action potentials but also their inputs in form of synaptic potential and intrinsic properties. We can also characterize the morphology of the recorded cell and stimulate or silence a given cell through direct current injection. Because intracellular recordings are difficult to perform in freely behaving animals we also perform such recordings in head fixed animals navigating virtual reality environments.
* equally contributing
Intracellular recordings in freely behaving animals
A Top view of the maze. Animal position in the maze is determined by light emitting diodes. B. Trajectory of the animal in the environment (blue line) during the intracellular recording of a CA1 pyramidal cell. Red dots indicate animal position when the cell fires spikes. Most of the spikes occur when the animal is in the lower right part of the maze (grey shaded area). C. Membrane potential (black trace) and instantaneous speed of the animal (green trace) during three successive laps around the maze (green path in B). The cell fires at high frequency each time the animal crosses the grey shaded area (grey bars under the trace). This neuron is thus a place cell coding for the grey shaded area of this particular maze. D. Average membrane potential of the same cell over all laps around the maze in one direction plotted against the linearized position of the animal in the maze. We see a large bump or hill in the membrane potential at the location for the cell’s place field but not elsewhere in the maze. E. Same as in D for spiking activity.
Dr. D. Fricker, Centre de Neurophysique, Physiologie et Pathologie – CNRS UMR 8119, Paris
Pr. F. Bartolomei, Service de Neurophysiologie Clinique APHM, La Timone, Marseille
Dr. Bruno Poucet, Laboratoire de Neurosciences cognitives, Marseille