Propagation of synchronous epileptiform events from subiculum backward into area CA1 of rat brain slices

The hippocampal trisynaptic pathway is comprised of superficial entorhinal afferents (part of the perforant path) to dentate granule cells, dentate mo ssy fiber inputs to CA3 pyramidal neurons, and CA3 cell projections to CAI pyramidal neurons. This CA1 output is among others to the subiculum, and bo th CAI and subiculum project to the entorhinal cortex to close the loop. Sm aller circuits involving fewer hippocampal and parahippocampal regions have also been described. We present morphological and electrophysiological evi dence from rat brain slices for a projection from subiculum back into area CAI. Axons of neurobiotin-labeled subicular pyramidal neurons were visualiz ed in the apical dendritic region of CA1. Spontaneous activity in isolated subiculum-CA1 slices was produced by bathing slices in reduced magnesium me dia. Events in CA I always followed events in proximal subiculum. Disruptio n of this subiculum-CA1 circuit with a radially oriented knife cut in the a pical dendritic region between subiculum and CAI eliminated afterdischarges in subicular and CAL events, but did not de-synchronize the two regions. F ull transections between CA1 and subiculum were necessary to functionally i solate the two regions. Only subiculum remained spontaneously active. We co nclude that a subiculum-CA1 circuit supports afterdischarges in both region s and synchronizes their activity. This circuit may serve to maintain a lev el of depolarization in subicular and CAI pyramidal neurons well beyond the duration of excitatory synaptic potentials resulting from activation of th e trisynaptic circuitry. (C) 2001 Elsevier Science BN. All rights reserved.