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Conditioning by Subthreshold Synaptic Input Changes the Characteristic Firing Pattern of CA3 Hippocampal Neurons
Saray Soldado-Magraner1 and Federico Brandalise2 and Suraj Honnuraiah1 and Michael Pfeiffer1 and Urs Gerber2 and Rodney Douglas1
1Institute of Neuroinformatics, University of Zurich and ETH Zurich 2Brain Research Institute, University of Zurich and ETH Zurich

The pattern of action potential discharge of cortical neurons in response to step input currents is generally assumed to be sufficiently static that they can be used as a basis for the classification of neuron type. We find that this assumption is not true of neurons in the CA3 field of rat hippocampus. Instead, they change their responses very significantly following only a short period of low frequency sub-threshold stimulation of their afferents. Cluster analysis of the discharge patterns before and after conditioning revealed systematic transitions between discharge types. These transitions were generally towards adapting and intrinsic burst behaviors, irrespective of the initial pattern exhibited by the cell. These results could be reproduced by a conductance based electrophysiological model, in which conditioning by stimulation results in an increase of calcium conductances and muscarinic potassium conductances. We conclude that CA3 neurons are able to adapt their conductance profile to the statistics of ongoing activity in their embedding circuits, suggesting that local rules within individual neurons may support collective circuit homeostasis. Consequently, the response to step currents should not be considered a reliable feature for the classification of at least these neuronal types.