Abstracts

Rationale: EEG spikes and seizures coexist in epileptic patients. These two types of discharge differ in frequency and duration, although the reasons for these differences are entirely unknown. Pharmacological disinhibition of CA3 neurons in organotypic slice cultures produces synchronous network activities that resemble both interictal and ictal discharges. We used electrical recordings and simultaneous calcium imaging to study differences in the calcium dynamics accompanying these two types of network activity. Methods: We used high-speed calcium imaging (up to 250 Hz) of organotypic hippocampal CA3 slice cultures, which were sufficiently thin to permit imaging of the entire CA3 network without confocal or 2 photon technologies. Synchronous activity was initiated by pharmacological disinhibition with 100 uM picrotoxin/ 1uM CGP 55845. Electrical fields were recorded using a tungsten electrode to verify the presence of bursts and seizures. Calcium transients associated with action potentials and paroxysmal depolarizing shifts of the membrane potential were imaged using AM dyes (e.g. Oregon Green Bapta-1, Molecular Probes). Slow glial signals were differentiated from neuronal signals by high-pass temporal pixel filtering. Neuronal calcium signals were mapped onto a Cartesian coordinate system. Propagation of calcium transients was quantified using 3-D bubble plots in which bubble diameter represented the number of neurons whose calcium concentration was increasing; x,y coordinates represented the median x,y positions of all such neurons within the network; and the z axis represented time. These analysis techniques allow us to distinguish neuronal vs. glial contributions to the propagation of activity. Results: We demonstrate distinct patterns of calcium dynamics for these two types of epileptiform discharges. High amplitude, aperiodic, short-duration bursts are accompanied by a large and diffuse increase in neuronal calcium occurring throughout entire extent of the CA3 network. The onset to peak time of this calcium transient is <= 50 ms. Calcium levels drop substantially within 250 ms, but remain elevated above pre-burst levels minimally 2 seconds. In contrast, recurrent, seizure-like activity results in a highly localized, slow-traveling wave of increased neuronal calcium that moves through the pyramidal layer of the slice, with faster frequency changes in calcium occurring locally within this wave. These waves of seizures lack a distinct calcium peak, and activity persists minimally for the duration of the re-entrant activity (> 4 seconds). Conclusions: In organotypic hippocampal networks, seizure-like and interictal-like bursts of network activity generate elevations in neuronal calcium that have markedly different onset kinetics and patterns of spatial propagation. These findings provide a new avenue of investigation into the differences between these two fundamental types of epileptiform discharge. The different calcium dynamics accompanying these forms of activity likely influence long-term plasticity of the CA3 network and contribute to epileptogenesis.