Abstracts

Rationale: We are interested in properties of network activity that lead to seizures. Multiscale analysis of EEG and in vivo single-unit recordings has motivated much discussion focusing on a period of more orderly neural firing that may precede seizures. We attempted to examine this process at the level of the in vitro slice by analyzing spontaneous EPSCs recorded from pyramidal cells of the neocortex in the presence of the convulsant, 4-aminopyridine (4-AP). Methods: Neocortical slices were obtained from the sensorimotor cortex of five control rats, aged 22-25 d. Whole cell recordings (spontaneous EPSCs) were obtained from layer V pyramidal cells in voltage clamp configuration. Recordings were obtained before and after exposure to 4-AP (100 μM). Spontaneous EPSCs from each animal were divided into three epochs: before exposure to 4-AP (PRE), after exposure to 4-AP but prior to bursting activity (4-AP), and after exposure to 4-AP after bursting had started (BURST). We analyzed 2-minute segments from the PRE and 4-AP groups and either 2- or 10-minute segments from the BURST groups. We compared EPSC amplitude and coefficient of variation (CV) of interevent interval between the three conditions for each cell using ANOVA and post-hoc comparison between groups. Comparison of EPSC frequency between conditions for each cell was performed using the Kolmogorov-Smirnoff test. Significance was set at P<0.05. Results: The addition of 4-AP resulted in increased amplitude and frequency of EPSCs compared to base line recordings (PRE vs 4-AP) in all 5 cells. There was a further increase in frequency during the bursting recordings in all 5 cells (4-AP vs BURST) and in amplitude in 4 out of 5 cells. In 4 out of 5 cells, there was a significant decrease in CV of interevent interval after the addition of 4-AP (PRE vs 4-AP). One cell showed an increase in CV after the addition of 4-AP. Once bursting started, the CV response was variable. In 2 cells, the CV decreased further (4-AP vs BURST) and in 3 cells it increased back toward baseline levels. Conclusions: As expected, 4-AP results in increased excitatory activity in the necortex. However, using EPSCs from single cells to monitor circuit activity, we have demonstrated a reduction in time-dependent variability of EPSCs prior to bursting activity in 4 out of 5 cells. We propose that this reflects more orderly firing in the excitatory cells that synapse on the recorded cells. This supports the concept that the onset of seizure-like activity occurs in an environment of more orderly firing of excitatory cells in the local circuitry. The variable response of CV in the presence of bursting may be the result of combining both interburst and intraburst data in our analysis of this condition.