Evidence for dendritic origin of spikes without depolarizing prepotentials in hippocampal neurons during and after seizure.

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Abstract
Repetitive subiculum stimulation results in reversible blockade or attenuation of EPSP''s [excitatory post synaptic potential] and IPSP''s [inhibitory post synaptic potential] recorded from hippocampal neurons and the appearance of rhythmically recurring orthodromic spikes which lack depolarizing prepotentials. The latter are associated with prespike negativities which are the intra-cellular reflections of giant extracellular negative responses that characterize subiculum induced ictal or inter-ictal activated states. Orthodromic discharges without depolarizing prepotentials in activated preparations exhibit variations in rise time and spike duration and generally show markedly prolonged depolarizing after-potentials. During post-activation recovery periods EPSP''s may trigger spikes at a ''firing level'' 6-10 mV higher than orthodromic spikes associated with pre-spike negativities. Orthodromic spikes which arise directly from the baseline are noted during the end stage of the activated state. The data suggest that intense axodendritic synaptic activation of hippocampal neurons facilitates spike generation and propagation in dendrites. Characteristics of spike discharges without depolarizing prepotentials during ictal and inter-ictal periods are attributable to variations in degree of soma invasion and post-spike conductance changes in dendritic as well as soma membrane.