In mammals, spatial learning and memory depend on neural processing carried out
in the hippocampal formation. Interestingly, extracellular recordings from behaving
animals have shown that cells in this region exhibit spatially modulated activity patterns,
thus providing insights into the neural activity underlying spatial behaviour.
One area within the hippocampal formation, layer II of the medial entorhinal cortex,
houses cells that encode a grid-like map of space using a firing rate code. At
the same time, oscillatory signals at distinct theta (4–12 Hz) and gamma (30–120 Hz)
frequencies are also present in layer II, providing a substrate for a timing code. To understand
how layer II of the medial entorhinal cortex produces these outputs I sought
to characterise the electrical properties and functional computational architecture of
its microcircuitry.
The functionality of any neural circuit depends on the electrical properties of its
constituent cells. Because the grid cells in layer II are likely to be stellate cells, I
used the perforated patch-clamp technique to accurately assess the intrinsic excitable
properties of this cell type. Compared to whole-cell recordings, these recordings indicate
that some intrinsic properties of stellate cells, such as spike clustering, which
is revealed to be robust, are more likely to play a functional role in circuit computation.
Conversely, other intrinsic properties, such as spontaneous membrane potential
fluctuations, which are confirmed to be insufficiently stable to support reliable interference
patterns, are revealed to be less likely than other, more robust electrical
properties to play a direct role in circuit function.
The characteristic connectivity profiles of different cell types are also critical for
circuit function. To investigate cell type-specific connectivity in layer II I used optogenetic
stimulation in combination with in vitro electrophysiology to record synaptic
activity in different cell types while selectively activating distinct subpopulations of
cells with light. Using this method I found that connections between stellate cells are
absent or very rare and that communication between stellate cells is instead mediated
by strong feedback inhibition from fast-spiking interneurons.
Dissecting oscillatory activity in neural circuits may be important for establishing
functionally relevant circuit architecture and dynamics but is difficult in vivo. I accomplished
this in vitro by recapitulating the interacting theta and gamma rhythms
that are observed in vivo with an optogenetic method. I found that locally driving
a subset of neurons in the layer II microcircuit at theta frequency with a light stimiulus produced a nested field rhythm at gamma frequency that was also evident as
rhythmic inhibition onto stellate cells. Critically, these interacting rhythms closely
resembled those recorded from behaving animals. In addition, I found that this thetanested
gamma is sufficiently regular to act as a clock-like reference signal, indicating
its potential role in implementing a timing code. To functionally dissect the circuit
I performed multiple simultaneous whole-cell patch-clamp recordings during circuit
activation. These recordings revealed how feedback interactions between stellate cells
and fast-spiking interneurons underpin the theta-nested gamma rhythm.
Together, these results suggest that feedback inhibition in layer II acts as a common
substrate for theta-nested gamma oscillations and possibly also grid firing fields,
thereby providing a framework for understanding how computations are carried out
in layer II of the medial entorhinal cortex