Intracellular currents of interictal penicillin spikes: evidence from neuromagnetic mapping

To analyze the net intracellular current produced by interictal spikes, we mapped the extracranial magnetic fields of the rat brain following application of penicillin to the right or left medial cingulate cortex. Averaged interictal spikes in both the magnetoencephalogram (MEG) and in the electrocorticogram (ECoG) were composed of 4 temporal components, a biphasic spike and slow wave. Magnetic field maps for each of these components indicated a source at the location of penicillin application, with intracellular currents oriented perpendicular to the surface of the cingulate cortex, along the axis of the major pyramidal cells. The polarity of the magnetic fields for each of the components was reversed between the two cingulate groups, reflecting the respective orientation of pyramidal cells between the juxtaposed faces of the medial cingulate cortex. This neuromagnetic study of net intracellular current complements and extends the analysis of extracellular currents within the penicillin focus obtained using laminar electrodes. These data also demonstrate how animal neuromagnetometry may provide an empirical foundation for the neurogenesis of the MEG and a new unique method for the non-invasive study of population cell physiology.