SUSTAINED HYPEREXCITABILITY RECORDED FROM FETAL AND NEONATAL MOUSE HIPPOCAMPAL EXPLANTS (EPILEPSY, DENTATE GYRUS, GRANULAR CELLS, CALMODULIN ANTAGONISTS)

Abstract

Repeated single electric shocks applied to the dentate at 8-60 minute intervals elicited increases in amplitude, frequency, duration and/or complexity of evoked or spontaneous extracellular slow-waves (71%) in CA3/2 areas of organotypic hippocampal explants. Repeated single electric shocks often resulted in development of self-sustained normal or epileptiform discharges that persisted for the duration of the experiment (15 min-10 hours).;When repeated single electric stimuli were applied to cultures perfused with 100-200 uM Ca('++) over 2 hour periods, evoked or spontaneous epileptiform slow-waves were not observed until similar stimuli were applied after return to BSS containing 1.3 mM Ca('++). Raising extracellular K('+) from 5.9 mM(normal) to 8-9 mM resulted in spontaneous slow-wave discharges when none were present prior to perfusion with high K('+) or enhanced evoked responses. Increasing K('+) to 12-14 mM reversibly suppressed all detectable slow-wave electrical activity.;Light-microscopic studies indicated that hippocampal explants can maintain a characteristic subfield structure (CA1-CA3, dentate) for 8 weeks or more. Pyramidal, granular or granular-like neurons resembled in situ counterparts, although dendritic branching patterns were not as elaborate, dendrites frequently showed alternate patterns of constriction and ballooning and some neurons had large, irregular nodulations. Many granular neurons had basilar or multiple dendrites, a characteristic of granular cells in rodents <15 days of age and displaced granular cells in adult reeler mutant mice. Dendritic spines were observed on both neuronal types but were irregularly distributed and not detected at densities reported from comparable tissue in situ.;N-methyl-d-aspartic and kainic acids had excitatory and toxic effects on neurons in hippocampal explants. Concomitant administration of 2-amino-4-phosphonobutyric acid did not block excitatory or toxic effects. However, perfusion with 10-12 mM Mg('++) blocked excitatory but not toxic effects.;Trifluoperazine had long-term attentuating effects on extracellular slow-wave field potentials. These effects were reversible but occurred over hours rather than minutes. In contrast, when other calmodulin antagonists (pimozide, W-7) were tested, the effects were rapidly reversible, as were the effects of chlorpromazine, xylocaine, procaine, GABA and baclofen.