Abstracts

RATIONALE: Non-synaptic mechanisms are implicated in some kinds of neuronal synchronisation during epileptic discharges. Field effects (aka ephaptic interactions) can, in principle, synchronise firing of suitably aligned neurons within a ms timescale, resulting in large population spikes in several models. One way of investigating the role of field effects is to alter extracellular space, and hence resistance, by changing osmolarity; but this treatment also affects ion channels and synaptic potentials. We therefore attempted to short circuit the extracellular currents responsible for field effects using metal wires, in order to assess the impact of field effects. METHODS: We used conventional rat hippocampal slices: maintained in an interface chamber; and incubated in artificial CSF containing 0.2 mM Ca2+ and 5 mM K+. This treatment results in spontaneous, highly synchronised trains of populations spikes, known as field bursts. After the field bursts were established, we applied pairs of 4 mm long ? 325 ?m diameter chlorided silver wires, either bare or teflon-insulated, to the slice surface perpendicular to the CA1 pyramidal layer, 0.5-1.0 mm apart. RESULTS: The bare wires blocked the population spikes, but not the underlying negative DC shift, of the field bursts (n=7); insulated wires did not block population spikes. Population spikes could still be recorded when bare silver wires were in place, but only when evoked by electrical stimulation of the alveus. This antidromic stimulation of CA1 resulted in trains of 5-6 population spikes, many fewer than the "100 typical of a low-Ca2+ field burst. CONCLUSIONS: The persistence of intact field bursts under the insulated wires shows that pressure or interrupted fluid flow cannot explain the loss of population spikes with the bare wires. The persistence of population spikes with alvear stimulation shows that we still were able to record population spikes with the bare wires. We conclude that bare wires can short-circuit field effects and abort population spikes during low-Ca2+ field bursts, and that this provides a viable means to test the roles of field effects in epileptic discharges. Supported by MRC (UK) & Univ. Birmingham