Layer-specific phase, coherence and synchrony measures in Mg2+- free media-induced status epilepticus in transverse piriform cortical slices, in vitro

Samantha E. Weston1, Andrew Constanti2, Benjamin J. Whalley1
1University of Reading, Reading, United Kingdom, 2London School of Pharmacy, University of London, London, United Kingdom

The magnesium-free (Mg2+-free) model of status epilepticus is widely used for in vitro epilepsy research. Whilst most investigations of this nature utilise single intra- and/or extra-cellular electrophysiological measures, little is known about such models’ network-level characteristics; a deficiency considering the network-based nature of the epileptic conditions it is used to emulate. Here, we have investigated layer-specific changes to coherence, phase and synchrony in Mg2+-free status epilepticus activity induced in a transverse piriform cortical (PC) brain slice preparation using a multi-electrode array (MEA) electrophysiological method (Weston et al., 2006). Transverse rat PC slices (450 μm; P>40; female; Hampshire and Berkshire; outbred; n = 3) were prepared (Weston et al., 2006), adhered to the MEA surface and perfused with carboxygenated (95% O2/5% CO2) artificial cerebrospinal fluid (aCSF; 23°C). Spontaneous caudo-rostral epileptiform bursting was induced by media exchange to a Mg2+-free aCSF, removing the Mg2+-dependent blockade of NMDA receptors. Spontaneous local field potentials (LFPs) were recorded via 60 electrodes (8x8 array; 30 μm diameter; 200 μm spacing; 50kHz). Data were analysed to produce measures of phase, coherence and synchrony between different slice areas vs epileptiform focus (R) using Neurospec (Halliday et al., 1995) in MATLAB (Table 1). Significance was accepted at P<0.05.

Figure 1 MEA electrode positions relative to PC slice layers.

Bursts consistently arose caudally in PC layer II and spread rostrally, with peak depolarisation remaining within layer II. Calculated coherence, phase and synchrony reflected these observations as indicated by high degrees of intra-layer II significance but reduced extra-layer II values. Moreover, rostral sites also showed reduced measures. These results reveal that Mg2+-free-induced PC bursting propagates in a direction consistent with PC intrinsic fibre connectivity but in the opposite direction (caudo-rostrally) to the majority of normal intrinsic PC information flow (Wilson & Bower, 1992). This finding clearly implicates the utilisation of existing intracortical connectivity in this widely used model of status epilepticus.

Halliday D.M. et al. (1995) Progress in Biophysics and molecular Biology, 64, 237-278
Weston S. et al. (2006) Proc. Brit. Pharm. Soc. (http://www.pa2online.org/)
Wilson M. & Bower J.M. (1992) J. Neurophysiol., 67(4) 981–995.