Short- and long-term plasticity of the perforant path synapse in hippocampal area CA3.

The direct perforant path (PP) projection to CA3 is a major source of cortical input to the hippocampal region, yet relatively little is known about the basic properties of physiology and plasticity in this pathway. We tested whether PP long-term potentiation (LTP) in CA3 possesses the Hebbian property of associativity; i.e., whether the firing of fibers of different orders can induce PP LTP. We stimulated PP with weak trains of high-frequency stimulation (HFS), which by itself was below the threshold for LTP induction. The identical HFS was effective in inducing LTP when the mossy fiber pathway (MF) was activated simultaneously, thus demonstrating associative plasticity between the two pathways. We also demonstrated associative LTP between PP and recurrent collateral fibers (RC). PP LTP was blocked by the N-methyl-D-aspartate receptor (NMDAR) antagonist 2-amino-5-phosphonovaleric acid in both the associative and homosynaptic induction conditions. Neither MF nor RC fiber HFS alone resulted in permanent changes in PP field excitatory postsynaptic potential (fEPSP) amplitude. However, HFS delivered to either MF or RC alone led to transient heterosynaptic depression of the PP fEPSP. Our results support the conceptual framework that regards CA3 as an autoassociative memory network in which efficient retrieval of previously stored activity patterns is mediated by associative plasticity of the PP synapse.