Abstracts

Rationale: High frequency activity (HFA) is defined as brain activity faster than 100 Hz. It can be subdivided into ripples (100-~200 Hz), which occur during normal function, and fast ripples (~200-500 Hz), which are associated with epileptic tissue. HFA requires the synchronisation of pyramidal cells on a millisecond timescale and the mechanism for this is unclear. One hypothesis is that fast-spiking interneurons create this synchronisation by imposing short temporal windows on their many target pyramidal cells, causing them to fire in synchrony at high frequency. Methods: 300 m horizontal brain slices were prepared from VGAT-Venus A rats (from Drs. Y. Yanagawa, M. Hirabayashi and Y. Kawaguchi, National Institute for Physiological Science, Okazaki, Japan, using pCS2-Venus from Dr. A. Miyawaki), which selectively express a fluorescent protein in interneurons that enables visually targeted recordings. Extracellular fields in hippocampal CA3b stratum pyramidale were recorded simultaneously with single cell interneuron activity under 9mM potassium. Event correlation, on a 10 ms timescale, between HFA troughs and action potential peaks determined the temporal relationship of each interneuron with the oscillation. Event correlations were tested for statistical significance using a Kolmogorov-Smirnov test against a uniform distribution ( =0.05). Results are presented as mean SEM.Results: HFA (frequency 212 9 Hz; power 820 150 V²; n=18 slices), which was superimposed on interictal-like discharges (frequency = 0.8 0.04 Hz), was induced in elevated potassium. 14 out of 19 recorded cells increased firing frequency by 463 87 % during interictal discharges and the remaining 5 fired exclusively during interictal discharges. In all cells, action potential amplitude decreased during interictal discharges to an average of 47 6 %. 7 out of 19 cells showed significant correlation, of which 6 were located in stratum oriens and 1 in stratum pyramidale. The 12 uncorrelated cells were found in all layers of the hippocampus.Conclusions: A subset of interneurons is involved in HFA and is located in stratum oriens and stratum pyramidale. Targeted manipulation of these interneurons has the potential to modulate HFA and epileptic pathophysiology. This work was funded by Epilepsy Research UK.