Long-term depression of perforant path excitatory postsynaptic potentials following synchronous network bursting in area CA3 of immature hippocampus.

Various forms of synaptic long-term potentiation and depression have been studied in detail in hippocampus and neocortex. Each are produced by specific patterns of synaptic activation and rely on electrical stimulation of afferents for their induction. Few studies have explored the ability of activity produced by cortical networks themselves to generate similar long-term changes in synaptic efficacy. Experiments have shown that periods of synchronized network bursting in both slices and dissociated cultures of hippocampus can lead to persistent network discharges. Conversely, one study has reported that network discharging can disrupt the induction of long-term potentiation in hippocampus. Whether long-term depression can be induced by synchronous network discharging is unknown. In experiments reported here, we examined the long-term effects of synchronized activity within hippocampal CA3 networks on synaptic potentials produced by the converging perforant path. Prior to induction of network bursting, slices were incubated in a perfusate containing picrotoxin and elevated (4 mM) Ca2+ and Mg2+. To induce network discharges, the concentration of both divalent cations were reduced to normal levels (1.5 mM). Following 20 min of network bursting, perforant path synapses, that did not participate in network discharging, underwent a 30% non-decrementing long-term depression. At the same time, synchronized network discharges, that were absent prior to induction, persisted upon return to preinduction conditions. The antagonist of the N-methyl-D-aspartate receptor, D(-)2-amino-5-phosphonovalerate, blocked both long-term depression of perforant path excitatory postsynaptic potentials and persistent network discharging. Results suggest that activity generated by hippocampal networks is able to produce long-term depression of non-coincidentally active synapses.