Volume-conducted epileptiform events between adjacent necortical slices in an interface tissue chamber

“Far-field” artifacts are presumed to contribute negligibly to the field potential activity recorded from brain slices maintained in vitro. While performing paired intracellular and field potential recordings from rat neocortical slices superfused with medium containing 4-aminopyridine + GABA receptor antagonists, we identified: (i) epileptiform discharges characterized by concomitant field oscillations (amplitude = 2.6-6.4 mV) and intracellular depolarizations as well as (ii) smaller amplitude (0.3-1.3 mV) field epileptiform events that were not associated with any intracellular activity. By placing an additional extracellular recording electrode into adjacent slices, we discovered that large amplitude, epileptiform discharges were generated concomitant to those seen in the first slice at the field potential level only. In addition, we found in these slices small amplitude, field discharges that were synchronous with those recorded intracellularly in the original slice. Analysis of the changes in field potential amplitude over space demonstrated that this parameter was reduced by approximately 60% when the recording electrode was moved from the slice generating the epileptiform activity to the bathing medium and further decreased in a quasi-linear mode when recordings were obtained from an adjacent slice. In conclusion, these observations indicate that brain slices can, under appropriate conditions, produce field potentials that are of amplitude sufficient for being recorded from other slices in the tissue chamber. These findings suggest that caution should be taken in assuming that field potential activity seen in an in vitro brain slice is generated within the recorded tissue.