GIANT DEPOLARIZING POTENTIALS - THE SEPTAL POLE OF THE HIPPOCAMPUS PACES THE ACTIVITY OF THE DEVELOPING INTACT SEPTOHIPPOCAMPAL COMPLEX IN-VITRO

In neonatal hippocampal slices, recurrent spontaneous giant depolarizi ng potentials (GDPs) provide neuronal synchronized firing and Ca2+ osc illations. To investigate the possible role of GDPs in the synchroniza tion of neuronal activity in intact neonatal limbic structures, we use d multiple simultaneous electrophysiological recordings in the recentl y described preparation of intact neonatal septohippocampal complex in vitro. Combined whole-cell (in single or pairs of cells) and extracel lular field recordings (one to five simultaneous recording sites) from the CA3 hippocampal region and various parts of the septum indicated that spontaneous GDPs, which can be initiated anywhere along the longi tudinal hippocampal axis, are most often initiated in the septal poles of hippocampus and propagate to medial septum and temporal poles of b oth hippocampi simultaneously. GDPs were abolished in the medial septu m but not in the hippocampus after surgical separation of both structu res, suggesting hippocampal origin of GDPs. The preferential septotemp oral orientation of GDP propagation observed in the intact hippocampus was associated with a corresponding gradient of GDP frequency in isol ated portions of hippocampus. Accordingly, most GDPs propagated in the septotemporal direction in both septal and temporal hippocampal isola ted halves, and whereas GDP frequency remained similar in the septal p art of hippocampus after its surgical isolation, it progressively decr eased in more temporally isolated portions of the hippocampus. Because GDPs provide most of the synaptic drive of neonatal neurons, they may modulate the development of neuronal connections in the immature limb ic system.