Re-entrant activity in a presubiculum-subiculum circuit generates epileptiform activity in vitro

The retrohippocampal cortices form the transition between neocortex and the hippocampus. Area CA3 of the hippocampus and the entorhinal cortex (EC) of the retrohippocampal region are established as brain regions that generate epileptiform activity. Interictal activity generated in EC consists of a p rimary population burst followed by multiple afterdischarges. The presubicu lum is similar to EC in its six-layered structure, but lacks a columnar cir cuitry that the EC possesses. Isolated presubicular tissue cannot generate afterdischarges and isolated subicular tissue generates no spontaneous acti vity under some conditions. We report epileptiform activity in combined pre subiculum-subiculum slices that consists of synchronous population bursts a nd multiple after discharges. Intracellular and field potential recordings reveal two re-entrant paths for interaction of presubicular and subicular n eurons. We demonstrate a deep presubicular input to subiculum and separate return paths from subicular bursting neurons onto deep and superficial laye r pre-/parasubicular neurons. Recordings from subicular cell apical dendrit es showed repetitive burst firing during sustained depolarizing current inj ection. We conclude that re-entrant activity in a presubiculum-subiculum ci rcuit generates epileptiform activity in both regions. Presubicular inputs to subiculum depolarize apical dendrites which can then burst repetitively. These bursts are transmitted back to the presubiculum. We suggest that ite rations on this circuit act to prolong the dendritic depolarization of subi cular bursting neurons and to entrain the activity across subicular cells r esulting in multiple afterdischarges. (C) 1999 Elsevier Science B.V. All ri ghts reserved.