Role of Self-generated Odor Cues in Place Cell Representation of Spatial Context
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The importance of the hippocampus in the formation and retrieval of episodic memory has been famously demonstrated in the case of patient H.M. Subsequent studies conducted in animal models have provided considerable insight into the specific functions of the individual components of the hippocampus. In the rodent, the pyramidal neurons of the CA1 and CA3 regions of the hippocampus have typically been associated with the encoding of visuo-spatial cues and their utilization in navigation. These ‘place cells’ fire when the animal is in a specific part of its environment (its place field). However, these cells also encode non-spatial information from other sensory inputs, such as olfaction and audition. This study was conducted to find out how contextual odor cues are represented in the firing of CA1 place cells and whether these cues could drive stable spatial representations. One group of mice was first extensively familiarized to a cylinder containing both visual cues and preserved, self-generated odor cues. Then, after assessing place field stability across a six hour delay, the visual and odor cues were rotated in opposite directions by ninety degrees (counter-rotated). Another group of mice was familiarized only to the visual cues that were subsequently rotated. The next day stability and rotation were re-assessed in a novel cylinder. However, the odor cues of the two groups were switched: the preserved odor cues of the first group were removed, and the odor cues of the second group were now preserved across the three sessions. In a separate experiment, a third group of animals was familiarized only to the odor cues. Firstly, we found that contextual odor cues attenuated rotation with the visual cues, but only following extensive familiarization. Secondly, the removal of familiar odor cues impaired long-term stability of place fields. Third and finally, the self-generated odor cues alone were not sufficient for the generation of stable place fields in a free, open-field exploration paradigm. We therefore conclude that although they are not as dominant as discrete visual cues, highly familiarized odor cues exert a significant effect on the representation of space of the mouse CA1 place cell, illustrating the role of contextually relevant information in navigating an ever-changing world.