Intrinsic connectivity of the rat subiculum: II. Properties of synchronousspontaneous activity and a demonstration of multiple generator regions

Brain structures that can generate epileptiform activity possess excitatory interconnections among principal cells and a subset of these neurons that can be spontaneously active ("pacemaker" cells). We describe electrophysiol ogical evidence for excitatory interactions among rat subicular neurons. Su biculum was isolated from presubiculum, CA1, and entorhinal cortex in ventr al horizontal slices. Nominally zero magnesium perfusate, picrotoxin (100 m uM), or NMDA (20 muM) was used to induce spontaneous firing in subicular ne urons. Synchronous population activity and the spread of population events from one end of subiculum to the other in isolated subicular subslices indi cate that subicular pyramidal neurons are coupled together by excitatory sy napses. Both electrophysiological classes of subicular pyramidal cells (bur sting and regular spiking) exhibited synchronous activity, indicating that both cell classes are targets of local excitatory inputs. Burst firing neur ons were active in the absence of synchronous activity in field recordings, indicating that these cells may serve as pacemaker neurons for the generat ion of epileptiform activity in subiculum. Epileptiform events could origin ate at either proximal or distal segments of the subiculum from ventral hor izontal slices. In some slices, events originated in both proximal and dist al locations and propagated to the other location. Finally, propagation was supported over axonal paths through the cell layer and in the apical dendr itic zone. We conclude that subicular burst firing and regular spiking neur ons are coupled by means of glutamatergic synapses. These connections may s erve to distribute activity driven by topographically organized inputs and to synchronize subicular cell activity. (C) 2001 Wiley-Liss, Inc.