Download 1050927abstract

In vivo studies of cellular mechanisms underlying memory formation
The hippocampus plays critical roles in remembering where we are and what we have
experienced. How these cognitive processes are implemented by hippocampal circuits
is largely unknown. Previous recordings identified hippocampal neurons that are
active when animals enter a specific location. It is still unclear whether or how such
place activity is formed or altered when animals learn to recognize new environments
or experience any changes in familiar environments. Using whole-cell patch clamp in
freely moving rats, we found that spatially homogenous current injection to a silent
hippocampal neuron leads to spatially tuned subthreshold depolarization and place
fields formation. Cells return to silent states when stimulation is removed. A small
constant current elicits a sharp, narrow-tuned place field, whereas larger current leads
to broader tuning. This result suggests that a place field could be formed by regulating
intrinsic excitability of hippocampal pyramidal neurons. In addition, silent cells show
long-lasting activity in respond to past experience of encountering novel objects. Such
reverberating activity is reminiscent of engram cell activity that reflects storage of the
memory. Using two-photon imaging and whole-cell recordings, we focus on
mechanisms underlying formation of memory to novel objects in the hippocampus
and how these newly formed activity interacts with existing place activity that related
to spatial memory.