[Abstract Title]. - Society for Neuroscience

Topic: C.06.f. Circuits and systems
Support: Burroughs Wellcome Fund 1001749
NIH Grant EB005736
NIH Grant MH61604
NIH Grant NS34425
Title: Two-photon imaging of cell-type specific firing patterns and neuronal network
correlations during epileptiform activity in hippocampal slice
Authors: *K. P. LILLIS 1 , M. A. KRAMER 2 , J. MERTZ 1 , J. A. WHITE 3 ;
1 Biomed Engin., 2 Dept. of Mathematics, Boston Univ., Boston, MA; 3 Bioengineering, Univ. of
Utah, Salt Lake City, UT
Abstract: A recently developed laser-scanning strategy allows the simultaneous measurement
from many neurons distributed across a large area with high spatial and temporal resolution.
Here we use this technique, Targeted Path Scanning (TPS), in conjunction with two-photon
excitation of bath-applied, calcium-sensitive dyes, Calcium Green-1 AM or Indo-1 AM, to image
epileptiform activity in the hippocampal formation. With TPS, user-selected neurons separated
by a distance of >2mm could be sampled at rates exceeding 100Hz without sacrificing single cell
resolution. In this study, TPS was employed to record up to four minutes of 4-AP-induced
epileptiform activity. Dozens of cells in the dentate gyrus (DG) and CA3 were sampled at a rate
of 30-50 Hz. The resulting data provided an independent calcium signal for each cell. Cellular
calcium dynamics were evaluated both visually and using statistical techniques to pick out
stereotypical motifs. Preliminary analysis reveals at least two classes of cells in the DG: those
which are active at the beginning of a seizure-like event (SLE) only and those which sustain
firing through the SLE. To better understand the role these cells play in neuronal network
dynamics, multidimensional correlation analyses were also performed. Correlations were
computed between calcium signals recorded from each pair of cells during SLE and used to
establish graph representations of the data. In these graphs, each cell was represented as a node,
and edges were drawn between sufficiently coupled pairs of nodes (i.e., strongly correlated pairs
of cells). To analyze the complicated topological properties of the graphs determined during
SLE, techniques from network analysis were implemented. Two measures of network
connectivity were calculated for each node: in-degree, the sum of correlations for which other
cells lead the node (as determined by peak correlation); and out-degree, the sum of correlations
for which the node leads other cells. Preliminary results suggest that DG cells exhibiting
transient firing during SLE tend to have a higher in-degree than out-degree. Conversely, DG
cells exhibiting prolonged activity during SLE tend to have higher out-degree than in-degree.
Together these results support a hypothesis that, in the latter phase of SLE, a class of cells in the
DG, which sustain high levels of activity throughout the SLE, drive a second class of cells,
which transiently fire at the beginning of the SLE. In coming experiments, we plan to use both
GAD/67 GFP mice and patch-clamp recordings with intracellular dye application to better
categorize these dynamically distinct cell types.