Abstract

Spatial learning involves the storage and replay
of temporally ordered spatial information. The hippocampus
is a key brain structure involved in spatial learning in
rats. Temporally ordered spatial memories are encoded and
replayed by the firing rate and phase of hippocampal
pyramidal cells and inhibitory interneurons with respect to
ongoing network theta and ripple oscillations paced by
intra- and extrahippocampal areas. Theta oscillations
(4–7 Hz) may contribute to memory formation, whereas
fast ripple oscillations to temporally compressed forward
and reverse replay of previously stored memories. Different
classes of CA1 excitatory and inhibitory neurons and
medial septal inhibitory neurons have been shown to differentially
phase their activities with respect to theta and
ripples. Understanding how the different hippocampal and
extrahippocampal areas and their neuronal classes interact
during these network oscillations and how they facilitate
the storage and replay of spatiotemporal memories is of
great importance. A computational model of the hippocampal
CA1 microcircuit that uses biophysical representations
of the major cell types, including pyramidal cells
and four types of inhibitory interneurons, is extended.
Inputs to the network come from the entorhinal cortex
(EC), the CA3 Schaffer collaterals and the medial septum.
A biophysical mechanism of spike timing-dependent
plasticity (STDP) is used for learning spatial memory
patterns in the correct order. The model addresses two
important issues: (1) How are the storage and replay (forward
and reverse) of temporally ordered memory patterns
controlled in the CA1 microcircuit during theta and ripples?
(2) What roles do the various types of inhibitory
interneurons play in these processes?