| Summary: | 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.
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