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INNOVATIVE METHODOLOGY

Fiberoptic System for Recording Dendritic Calcium Signals in Layer 5 Neocortical Pyramidal Cells in Freely Moving Rats

Physiologisches Institut, Universität Bern, Bern, Switzerland

Submitted 24 January 2007; accepted in final form 13 July 2007

Calcium influx into the dendritic tufts of layer 5 neocortical pyramidal neurons modifies a number of important cellular mechanisms. It can trigger local synaptic plasticity and switch the firing properties from regular to burst firing. Due to methodological limitations, our knowledge about Ca²⁺ spikes in the dendritic tuft stems mostly from in vitro experiments. However, it has been speculated that regenerative Ca²⁺ events in the distal dendrites correlate with distinct behavioral states. Therefore it would be most desirable to be able to record these Ca²⁺ events in vivo, preferably in the behaving animal. Here, we present a novel approach for recording Ca²⁺ signals in the dendrites of populations of layer 5 pyramidal neurons in vivo, which ensures that all recorded fluorescence changes are due to intracellular Ca²⁺ signals in the apical dendrites. The method has two main features: 1) bolus loading of layer 5 with a membrane-permeant Ca²⁺ dye resulting in specific loading of pyramidal cell dendrites in the upper layers and 2) a fiberoptic cable attached to a gradient index lens and a prism reflecting light horizontally at 90° to the angle of the apical dendrites. We demonstrate that the in vivo signal-to-noise ratio recorded with this relatively inexpensive and easy-to-implement fiberoptic-based device is comparable to conventional camera-based imaging systems used in vitro. In addition, the device is flexible and lightweight and can be used for recording Ca²⁺ signals in the distal dendritic tuft of freely behaving animals.

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