Burst firing and spatial coding in subicular principal cells

Abstract

The subiculum is the major output structure of the hippocampal formation and is involved in learning and memory as well as in spatial navigation. Little is known about how the cellular diversity of subicular neurons is related to function. Primed by in vitro studies, which identified distinct bursting patterns in subicular cells, we asked how subicular burst firing is related to spatial coding in vivo. Using high-resolution juxtacellular recordings in freely moving rats, we analyzed the firing patterns of 51 subicular principal neurons and distinguished two populations based on their bursting behavior, i.e. sparsely bursting ([~]80%) and dominantly bursting neurons ([~]20%). Dominantly bursting neurons had significantly higher firing rates than sparsely bursting neurons. Furthermore, the two clusters had distinct spatial properties, sparsely bursting cells showing strong positional tuning and dominantly bursting cells being only weakly tuned. Additionally, the occurrence of bursts in sparsely bursting neurons defined well-defined spatial fields. In contrast, isolated spikes contained less spatial information. We conclude that burst firing distinguishes subicular principal cell types and constitutes a distinct unit encoding spatial information in sparsely bursting spatial cells. Overall, our results demonstrate that burst firing is highly relevant to subicular space coding.